CA2166919C - Apparatus for measuring resistivity of an earth formation using delta-sigma digital signal generation and sigma-delta digital detection system - Google Patents
Apparatus for measuring resistivity of an earth formation using delta-sigma digital signal generation and sigma-delta digital detection systemInfo
- Publication number
- CA2166919C CA2166919C CA002166919A CA2166919A CA2166919C CA 2166919 C CA2166919 C CA 2166919C CA 002166919 A CA002166919 A CA 002166919A CA 2166919 A CA2166919 A CA 2166919A CA 2166919 C CA2166919 C CA 2166919C
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- CA
- Canada
- Prior art keywords
- digital
- current
- output
- coupled
- frequency
- Prior art date
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
- G01V3/18—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging
- G01V3/20—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with propagation of electric current
- G01V3/24—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with propagation of electric current using ac
Abstract
An apparatus for measuring resistivity of an earth formation. The apparatus includes at least one source of measuring current. The source includes a delta-sigma modulator and a serial bit source. The serial bit source represents the magnitude of the measure current at spaced apart time intervals. The apparatus includes a sensor responsive to formation current resulting from interaction of the measure current with the earth formation. The sensor is coupled to a sigma-delta modulator and a digital filter.
The output of the digital filter corresponds to the magnitude of the measure current detected by the sensor.
The output of the digital filter corresponds to the magnitude of the measure current detected by the sensor.
Description
APPARATIJ~ FOR MFA.~IJRTNC~T RF~T~TTVTTY OF AN FARTT-T F()RMATT()N
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L Fi~ 1 of th.o Tnvention The present invention relates to the field of electrical resistivity tools, which are used to measure certain properties of earth formations pe~ t~d by boreholes. More specifically, the present invention relates to a system for digitally processing signals in electrical resistivity tools to improve the accuracy of measurements made by the tools.
T)i~r~ inn of th~ o1ev~nt Art F.1~tril ~1 resi~ivi~y tools are used to make measurements of electrical resi~ivily of earth formations penetrated by boreholes. Electrical resistivity measurements can be used for, among other things, estim~ting content of various types of fluids which can be contained in pore spaces in the earth formations.
Electrical resistivity tools known in the art include galvanic devices. Galvanicdevices typically comprise electrodes placed on an in~ ting exterior surface of the tool.
All the electrodes on the tool typically make electrical contact with the earth formation through a conductive fluid which fills the borehole.Some of the electrodes are connPcted to circuits in the tool which gellel~t~ electrical current. Other electrodes are connected to different circuits in the tool which measure voltage dirrerellces and current flow m~gnih1de~. Measurements of voltage difference and current flow can be related to the electrical resistivity of the earth formations.
A galvanic instrument known in the art is called a dual laterolog tool. The duallaterolog tool co~ lises electrodes which emit ~ current, and focusing electrodes which emit focusing ~;ul-ell~s used to constrain, or focus, flow of the measuring current in a pred~ ~l pattern. By focusing the measure current in a predetermined pattern, 216691~
mea~ulclll.;ll~ of resistivity can be related more precisely, for example, to thin vertical section~ of the earth formation. Other pre~ r. .~ patterns for focusing the measuring current can enable measurement of formation resis~ivily at a radial li~t~n~e closer to the borehole, which can be useful for estim~ting movement of fluid from the borehole into the pore space in the formation. A description of the typical electrode al~ gelllent and current focusing paLl~llls of the dual laterolog tool can be found for example in "Introduction to Wireline Log Analysis", by Ed L. Bigelow, Atlas Wireline Senices, Houston, TX, 1992 (p. 59).
The tool desclil)ed in the Bigelow reference, for example, ~imlllt~n~ously makestwo dirrclcll~y focused lll~UlClllClll~ of resistivity using the same set of electrodes. The siml11t~nPous measurements are performed by using measuring and focusing currentsystems operating at two dirrelclll frequencies. Each of the measuring and focusing systems operates at a dirrelcnl one of the two frequencies.
In order for the dual laterolog tool to accurately record the measurements made by each focusing and measuring system, the signals generated and detected by each frequency system must, to the greatest extent possible, be ~lcvellled from hll~lrerillg with each other.
Methods are known in the art for reducing hll~lrelcllce between measuring and focusing systems opel~ g at dirrelcl~t frequencies. One method known in the art is to provide an analog b~n-lp~s filter having a very narrow bandwidth to an input of the voltage and current measuring circuits in the tool corresponding to each mea~urclllclll system frequency. Analog b~n-ll)a~s filters reject passage of electrical current at fre4u~ ies other than within a so-called frequency pa~sb~m1. One of the limitations of analog b~ filters is tbat they can pass some current at frequencies other than within the frequency passband. A plurality of measurement systems operating at different frequencies may not be sufficiently isolated from each other by using analog bandpass filters to plcvelll h~l~lr~lcllce between respective systems.
Another method known in the art for reducing inle,relc~lce between dirr~le frequency measulclllent systems is to provide a current source for each measulclllcn system having as nearly as possible only one frequency component. This type of source is called a monochromatic current source. By providing a subst~nti~lly monochromatic current source for each measurement system, detections of voltages and ~;ullcnl~ in a particular measurement system which are not at the frequency of that particular measurement system can be reduced.
A system for providing a subst~nti~lly monochromatic current source is known in the art and is described, for example, in U. S. patent number 4,499,421 issued to Sinclair. The system described in the Sinclair '421 patent comprises a pair of digital latches and a precision resistor nclwol~ to generate a stair-step approximation of a sinusoidal waveform. The stair-step approximation output from the resistor network is then con-lucted to an amplifier having an analog low-pass filter. The analog low-pass filter reduces the m~gnihlde of the "stair-steps" since they have a much higher effective frequency than the sinusoidal signal. In addition to the limitations of analog filters as previously descril ed herein, the system disclosed in the Sinclair '421 patent has a further limit~tion in that the system in the Sinclair patent uses a precision resistor network to accomplish the digital-to-analog conversion. Some of the limitations of precision resistor l,c~wol~ used in digital-to-analog collvc,~ioll are described, for example, in U. S. patent number 5,357,252 issued to T~7.ills, et al. The T~-17.ills '252 patent states that the "resistive-divider" technique of data conversion, which includes the digital-to-analog conversion of the signal generator disclosed in the Sinclair '421 patent, can be difficult because the lc~ ive-divider technique requires using high precision analog components which may be diffirlllt to form, particularly in a system intended to be used in the limited space provided inside a resistivity tool used in boreholes. A resistivity tool colllp~isillg a plurality of dirr~ ~nl measulel,lell~ and focusing systems which operate at dirrel~
frequencies, using a plurality of signal generators similar to the one disclosed in the Sinclair '421 patent, is impractical.
It is known in the art to provide an analog-to-digital converter responsive to arange of frequencies to reduce signal distortion which can be present in analog signal processing circuits. The Led~ius '252 patent, for example, discloses an analog-to-digital converter responsive to a plurality of frequencies defining a usable range called the S bandwidth. The analog-to-digital CO~ .~r in the '252 patent could be used in a multiple frequency lc~ ivily tool if each measuring circuit for each dirr~lelll frequency could be connected to a col-vellel similar to the co-~ve,ler disclosed in the '252 patent and combined with a narrow bandwidth ar~alog filter. However, a limitation on the use of the coll~ tcl of the '252 patent in a multiple frequency resistivity tool is the need to include analog components in a filter stage of the col~vc.ler, as shown at 83 and 84 of Figure 5 in the '252 patent. The converter disclosed in the '252 patent was intended to have a bandwidth comprising a relatively wide range of frequencies in order to be useful, for t;A~ll~l-, in digital telephony. Tnr~ lin~ the analog components of the '252 patent in the output stage of the Collvt;ll~l as disclosed in the '252 patent would likely allow inte,rerellce between the dirrercl.l frequency measurement systems when used in a multiple frequency resistivity tool.
A fur~her limh~ti-)n on using the co~vc~lel disclosed in the '252 patent is that the disclosed co..vc,~, does not elimin~te the need for the narrow bandwidth analog b~nl1p~
filter provided at the input of the analog-to-digital converter. The limitations of using analog b~n-lp~s filters in the measuring circuit of a multiple frequency l~si~livily tool, as previously ~ c~1sse~1, would still apply if the co--~ .~r in the '252 patent were used in a resistivity tool.
It is an object of the present invention to provide a resislivily measuring toolhaving a plurality of fully digital measurement circuits, each circuit capable of o~ela~ g at a dirrelclll predetermined frequency, in order to provide mi~ h~le~relcllce between individual measurement systems.
2166!119 It is a further object of the present invention to provide a resistivity mP~llring tool having a plurality of monochromatic current sources each of which is fully digitally synthesized in order to ",illi",i,P generation of spurious frequencies in the individual llled~iUl'C ~;UllCllk;.
~;:IJMMARY OF TT-TF, TNVF,NTTON
The present invention is a tool for measuring the resistivity of an earth formation comprising at least one measuring current source. The at least one source includes a sigma-delta modulator and a serial bit source corresponding to a digital representation of a predeterminPd measure current waveform. The present invention also comprises at least one measure current sensor coupled to a delta-sigma modulator and a digital filter which ge~ dl~s a digital output corresponding to an amplitude of said measure current at said at least one sensor.
In a plcrcllcd embodiment of the invention the tool comprises a measure current circuit in which a first measure current source gelleldlcs a monochromatic sinusoidal signal having a first frequency and also comprises a first focusing current circuit which Opeldlt~S at the first frequency. The tool of the prcr~llcd embodiment further comprises second and third measure current sources which gelleldle monochromatic sinusoidal signals having second and third frcq lenriPs and second and third focusing current sources Opeld~ , at the second and third frequencies. The plcrcllcd embodiment of the invention includes measure current sensors responsive to each measure current frequency.
RRTF.F nF,.~(~RTPTT()N OF TT-TF, T~RAWTNC'T.S
Figure 1 shows the tool according to the present invention deployed in a borehole.
Figure 2 shows a resistivity measuring tool according to the present invention.
Figure 3 shows a system for adjusting the amount of focusing current.
Figure 4 shows a measuring signal gellelatol according to the present invention.Figure 5 shows a voltage measuring circuit according to the present invention.
Figure 6 shows an ~ltern~tive embodiment of a resistivity measuring tool.
nF~RTpTTON OF TT-TF PRF.FFRRF.T) FMT~OnTMFNT
Figure 1 shows a resistivity logging tool 10 as it is typically used in a borehole 1 pellelld~ g an earth formation 3. The tool 10 is typically connrcted to one end of a cable 33 colll~lisillg at least one in~ ted electrical conductor (not shown). The cable 33 can be extrn l~ into the borehole 1 by means of a surface logging unit 2. The cable 33 carries electrir~l power from the surface unit 2 to the tool 10, and can lldl~lllil signals from the tool 10 to the surface unit 2. The surface unit 2 includes equipment (not shown se~ ly) for receiving and i,lt~ ting signals tr~n~mitt~-1 by the tool 10. The surface unit also inrlll~les e lui~ lll (not shown SæP5~IAIe1Y) for tl~ iLI;I~g control signals to the tool 10.
Figure 2 shows a functional diagram of the tool 10 according to the present invention. The tool 10 comprises a sonde 12 having a plurality of electrodes 14, 16, 16A, 18, 18A, 20, 20A, 22, 22A, 23, 23A disposed on an exterior in~ tin~ surface (not shown separately) of the sonde 12. The purposes of the individual electrodes will be further explained. The tool 10 also comprises various circuits, shown combined on a circuit assembly 11 disposed inside the sonde 12, which measure voltage drops ofmr~llrin~ ;ullellls passing through the earth formation 3. The purposes of the various circuits on the assembly 11 will be explained further. The measuring ~;ullclll~ are introduced into the formation (shown as 3 in Figure 1) ~dj~cPnt to the borehole 1 by other circuits disposed on the assembly 11.
The circuit assembly 11 is shown in more detail in Figure 2 as a functional block diagram including lc~l~sellLdlive connections of the various circuits disposed on the assembly 11 to the dirrelenl electrodes, as will be further explained.
The circuit assembly 11 comprises a formation voltage measuring circuit 34 conn~octecl at one input terminal to monitor electrodes 16 and 16A through a resistive divider 27, and at the other input terminal to a ground electrode G located at the earth' s surface, the connection to the other input tPrmin~l being made through the conductor (not shown) in the cable 33. The formation voltage measuring circuit 34 measures a voltage occllrring between the electrodes 16, 16A on the sonde 12 and the ground electrode G.
The measurement of the voltage made by the formation voltage mcasu~ g circuit 34 is provided as a digital word at terminal D34A of the formation voltage measuring circuit 34. The voltage measured by the formation voltage measuring circuit 34 is related to e~i~livily of the ear~ formation 3 adjacent to the tool 10. The voltage measured by the circuit 34 lepLese~ a potential dirÇerellce reslllting from a current of known magnitlllle flowing through the formation 3 between a source electrode 14 and electrodes 20 and 20A. The current of known m~gnitllde is gelle~ d by a measure current source 24, as will be further explained. The digital word present at the terminal D34A can be con-lucted to a central processor 51, the operation of which will be further explained.
The formation voltage measuring circuit 34, which will be explained in greater detail, can be responsive to voltages at each of three dirÇe~nl frequencies to enable substantially ~imlllt~nPous measul~lllenl in three dirrelelllly focused measure current systems. In the present embo~im~nt, the frequencies of the measure current systems typically are 32, 128 and 512 Hz.
The measure current source 24, which in the present embodiment can include three, single-frequency monochlull~lic current sources each opel~lhlg at one of the three previously described frequencies, is connPctPd at one output I~lllPinal to the source electrode 14, and at the other output terminal to electrodes 20 and 20A (the ~yllullt;llical connection to electrode 20 is not shown in Figure 2 for clarity of the illustration). The llle~ul~ current source 24 provides the current with which the voltage drop through the earth formation 3 is measured by the formation voltage measuring circuit 34, as previously explained herein.
A bucking voltage measuring circuit 24A, responsive to the same three fr~lenriPs as the frequencies of the measure current source 24, is connected through a phase m~trllPd L~ rolllæL 25 across pairs of monitor electrodes 16, 18; and 16A, 18A.
21G~gl~
The bucking voltage Illf ~ p circuit 24A lllca~ul~s a voltage drop between the monitor electrodes 16 and 18, and symmetrically about the source electrode 14 makes the same lll~ultllælll b~tw~ell electrodes 16A and 18A. A digital word l~lcsen~ g the voltage drop measured by the bucking voltage measuring circuit 24A is provided at tellllillal D24A on the bucking circuit 24A and is conrl~cted to the central processor 51. If the voltage drop across the monitor electrodes 16, 18 (or symmetrically 16A, 18A) is non-zero, the pl~Jcessor 51 can be plogl~ llled to adjust the current output from the measure source 24 by ch~n~in~ the value of a digital control word con~ cted to terminal D24 on the source 24 from the processor 51. The means by which the processor 51 adjusts the output of the source 24 will be further explained. By adjusting the current output from the current source 24 to m~int~in subst~nti~lly zero voltage drop between the monitor electrodes 16, 18 and 16A, 18A, the processor 51 substantially "~i"~ .c a predetellllhled focusing pattern of the measuring current near the wellbore 1. Because the measure current is ~ul~ ially m~int~in~d within the predetermined focusing pattern, the voltage drop measuled by the formation voltage m~ ring circuit 34 can be more directly related to resistivity of the formation 3. It is known in the art to provide a single analog circuit which provides the same function as the combined opeMtion of the bucking measuring circuit 24, the measure current source 24 and the measure current adjll~tm~nt feature of the processor 51, however the present embodiment is directed to a fully digital resistivity tool.
The m~gnit~ e of the measure current ~el~ldt~d by the measure source 24 is itself lllea~ul~d by a current ",~ circuit 26 which is responsive to each of the same three mea~u Clll~ frequencies as is the formation voltage mP~ ring circuit 34. The current measuring circuit 26 comprises a voltage measuring circuit (which will be explained in more detail), of s~bst~nti~lly the same design as the formation voltage measuring circuit 34, connected across a shunt resistor 29 interposed in the measure current path b~tw~ell the electrode 14 and the measure current source 24. Current flowing across the shunt resistor 29 gel~ela~es a voltage drop proportional to the current flow across the shunt 216691~
resistor 29. The voltage measured across the shunt resistor 29 therefore is proportional to the m~gnihlde of the measure current ~ l by the llle~ulc current source 24. The m~a~urelllent made by the second measuring circuit 26 is provided as a digital word on tçrmin~l D26A which is con~ ctçd to the processor 51. The measurement made by the second m~lrin~ circuit 26 which is pr~ollional to current m~gnit~lde can be combined with the voltage drop measurement made by the formation voltage measuring circuit 34 to de~llllille the resi~LiviLy of the earth formation 3.
Three focus current sources 28, 30 and 32, each opelaling at a different one of the three previously described measurement system frequencies, are conn~cted syl-llll~-l- ir~lly about the source electrode 14 to focusing electrodes 20 and 20A; 22 and 22A; and 23 and 23A, these electrodes being disposed on the sonde 12 at axially spaced apart locations from the source electrode 14. Each of the focus current sources 28, 30, 32 is connected to the electrodes in a dirrelclll configuration so as to cause focusing current from each source to flow in a different path. Each of the three dirrelc frequency mP~cllrin~ ~;ullcllL~ ~llc~lxlhlg to one of the focusing ~;Ull~llki can therefore constrained to a dirr~l~lll predetermined focusing pattern in the borehole 1 and the earth formation 3 adjacent to the borehole 1. For example, a first focusing source 28, which u~ ates at a first frequency, is conn~cted at one output to all three focusing electrodes 20, 22, and 23, and symmetrically about the source electrode 14 to electrodes 20A, 22A
and 23A (although the symm~tric connections are not shown in Figure 2 for clarity of illustration). The other output of the first focusing source 28 is conn~cted to the cable 33 armor. The first focusing source 28 provides focusing to the measuring current having the greatest radial depth of col~lldillL because the focusing current from the first source 28 is col~L~ led to flow ~ull~L~llially entirely radially uuL~al-l from the sonde 12 before dispersing in the earth formation 3 and le~ g to the cable 33 armor.
A second focusing source 30, opeldling at a second frequency, is typically conn~cted across electrodes 20 and 22 (and symmetrically 20A and 22A, but the symm~trical connections again are not shown for clarity) to provide focusing to the . .
second frequency llle~ul~ current having a reduced radial depth of constraint relative to the first frequency measuring current focused by the first focusing source 28. The focusing pattern of the second focusing source 30 enables dispersion of the measuring current in the formation 3 at a shallower radial ~ t~nre from the sonde 12 than does the first focusing source 28 pattern, because the second focusing current is returned to the electrodes 22, 22A on the sonde rather than on the cable 33 armor. I~ ...,ing the focusing current to the electrodes 22, 22A enables dispersion of the second frequency measure current radially closer to the sonde 12, so that a relatively shallower radial lllea~ul~ of formation l~ livily can be made by the second than can be made by the first measure current.
A third focusing source 32, op~ g at a third frequency, can be connected between electrodes 22 and 23 (and symmetrically to 22A and 23A with symmetrical connections again not shown for clarity of illustration)) to provide even shallower radial constraint of the third frequency m~uring current than the second focusing source 30 does for the second frequency measuring current. The shallower radial constraint of the measuring current provided by the third focusing source 32 results from the focusing electrodes 22, 23 and æA, 23A being spaced at a greater axial ~ t~nre from the source electrode 14 than the focusing electrodes 20, 22 (and ~yllllll~lfically, 20A, 22A) used for e",i~ the focusing current at the second frequency. The longer axial spacing of the electrodes 22, 23 used for the third frequency focusing current enables the third frequency measuring current to disperse in the earth formation 3 at an even shallower Mdial ~ t~n~e from the sonde 12 than does the second frequency m~ lring current.Output levels of the focus sources 28, 30, 32 can be controlled by digital wordsinput to tellllillals D28, D30 and D32, respectively, from the processor 51.
Focusing current nlc~ulillg circuits 30A and 32A can be conn~cte~ across shunt resistors 31 and 35, respectively, to provide measurement of the total amount of focus current ge~ t~d by the first source 28, and the second and third sources 30, 32,e~eclively . Focusing current llleasu~ are provided as digital words on l~ll linals D30A and D32A, lc~e~ /ely, and are conducted to the processor 51. As will be further c~lailled, focusing current m~ les can be adjusted in response to dirrclclll values of formation 3 resistivity so as to control the dynamic range of signal at the input of the formation voltage measuring circuit 34 and the current measulillg circuit 26.
Figure 3 shows a flow chart of a typical measurement sequence provided by the tool (shown as 10 in Figure 1). The focusing cullclll~ are measured by circuits 30A and 32A, as generally shown at 38. If the cullcllL~ from the sources 28, 30, 32 are d~ ~d to be too low or too high to m~int~in a minimum predclcllllilled signal level at the inputs of the formation voltage measuring circuit 34 and the current mr.~llring circuit 26 as shown at number 40, adjustments are made under control of the processor 51 to the output levels of the focus current sources 28, 30 and 32 either to increase as shown at 42A, or to decrease as shown at 42B, the level of the focus cullclll~. The means by which the processor 51 changes the output level of the sources 28, 30, 32 will be further explained. Changes in the amount of required focus current can occur, for C~ , if the fluid filling the borehole 1 is particularly conductive or resistive, or the earth formation 3 is particularly conductive or resistive. When the focus CUllC~ are determined to be of the correct m~gnih--le, voltage drop and current m~gnitllde of the measure ~;UllClll~ are made, as shown generally at 44.
The current sources according to the present invention can be better understood by lcrclling to Figure 4. The source, shown as 28 in Figure 4, can also correspond to any of the other sources shown in Figure 2 such as the focus current sources 28, 30 and 32, or the measure current source 24.
The output m~gnihlde of the source 28 is controlled by a rcrclcllce level digital word con~lurt~ to an input t~rmin~l 58 of a reference digital to analog collvcllcl (DAC) 54 forming part of the source 28. The input termin~l 58 of the reference DAC 54 shown in Figure 4 corresponds to any one of the digital word input terminals D24, D28, D30 or D32 of the sources 24, 28, 30 or 32, respectively, shown in Figure 2. The input trrmin~l 58 is conn~t~d to the corresponding terminal on the processor 51. The digital ~166919 word present at the input terminal 58 is loaded into the reference DAC 54 when a load co~ l is applied to a command terminal 56 on the lcrclcl~ce DAC 54 by the processor 51. A load ~""",~".1 is gcneldlcd by the processor 51 when the processor 51 calculates that the value of the digital word is to be changed. As previously explained, the value of the digital word can be changed by the processor 51 in response to measurements provided by the focus measuring circuits 30A, 32A, and the formation voltage 34 and current measuring 26 circuits.
The lcfercllce DAC 54 is conn~,cted to a precision voltage lerelellce source 52.The precision voltage ler~lell~ source 52 gCllCldlC~ a subst~nti~lly constant voltage direct current (DC). The subst~nti~lly constant voltage DC from the lèrclèllce source 52 X accuracy and repeatability of conversion of the digital word present at the input tcllllillal into an output level control signal by the rcfercllce DAC 54.
The output of the reference DAC 54 typically is a substantially constant voltageDC which is con(1~1cted to a delta sigma modulator 64 at a rèrelèllce input terminal 64A.
As previously explained, the m~gnitllde of the DC voltage output from the rcfclcllce DAC 54 is dcl~. lllil~1 by the reference digital word con~ cted to the input lCllllindl 58.
A serial bit stream 62 is gellcldlcd by a memory control and address generator 66 sequentially activating a digital memory 60. The serial bit stream 62 is conn~cted to a digital input termin~l 64B on the modulator 64. The serial bit stream 62 contains a series of numbers corresponding in m~gnitllde to the i~xl~ ous output m~gnitllde of thesources 28 at spaced apart time intervals. In the present invention the waveform is typically sinusoidal. The serial bit stream 62 also contains il~llllation about the frequency of the current output from the source 28. In the present embodiment the frequency typically is 32, 128 or 512 Hz. The output from the modulator 64 is conducted to a circuit which can be a power amplifier or a source electrode driver as shown at 68, and thence to electrodes such as the focusing electrodes shown as 22 and 22A in Figure 2.
The output of the modulator 64 comprises analog signals proportional in m~gnitl~(le to the m~gnitl1(1e of the reference voltage con~1ctecl from the l~r~,rellce DAC
54. The modulator 64 ge~ d~s output samples at a rate which is delellllilled by the frequency of a system clock 70 conn~cte(l to the modulator 64. The clock 70 frequency is much higher than the output frequency of the source 28. In the present embodiment the system clock 70 frequency typically is 1.024 MHz. The high frequency samples from the CollV~ l 64 are filtered into the correct output waveform in a low-pass filter forming part of the amplifier 68. Because the output frequency of the samples gelleldt~d by the modulator 64 is much higher than the operating frequency of the current source 28, the filter col~ ollellL~ which form part of the amplifier 68 can have cut-off frequencies well above the opeldLhlg frequency of the source 28. In the present embodiment, the filter fulll~illg part of the amplifier 68, can have a cut-off frequency of 256 KHz, which is one-fourth the frequency of the system clock 70, but is 500 times the highest opeldlhlg frequency of the source 28 (which is 512 Hz). The filter components forming part of the amplifier 68 can therefore be designed to have gradual "roll-off" characteristics, which greatly simplifies the design of the filter components.
In the present embodiment of the invention, the source 28 can comprise an integrated module, shown as 50 in Figure 4, including the voltage lc;r~lc;llce 52, the reference DAC 54, the modulator 64, the memory 60, and the memory controller 66.The module 50 can be progr~mm~, by insertion of applupliate bit stream data into the memory 60 during construction or servicing of the module 50, to generate a predetermined signal. In the present embodiment of the invention the pred~llllhled signal can be a monochromatic sinusoid having a frequency of 32, 128 or 512 Hz.
The processor 51 can be programmed to respond to inputs, provided as digital words to input terminals D26, D30A, D32A, and D34A on the processor 51, from themeasuring circuits 26, 30A, 32A and 34, respectively. Signals representing resistivity of the forrnation 3, which are g~ 1 in the processor 51 in response to measurements gell~ldt~d by the formation voltage 34 and second 26 measuring circuits, as previously 216691~
explained, can be ~ ~ to the cable 33 in the form of digital words con-luct.od from ell,linal 32X on the processor 51.
Referring now to Figure 5, the voltage measuring circuit 34 according to the present invention will be explained in more detail. An analog signal input from a sensor, which in the present embodiment can be one or more of the electrodes on the tool 10, is co~ c~P~l to a proglallllllable gain amplifier (PGA) 72. The PGA 72 can be controlled by the central processor 51 so as to m~int~in an output signal level which remains within the resolution range of a sigma-delta modulator 74, to which the output of the PGA 72 is con~ ctçd. The modulator 74 converts the signal from the PGA 72 into a serial bit stream at a rate determined by the frequency of the system clock 70, to which the modulator 74 is operatively conn~cted The serial bit stream output of the modulator repleselll~ the m~gni~ le of the signal input from the PGA 72 sampled at spaced apart time intervals. The serial bit stream is composed essentially of digital "ones" and "zeroes" having individual durations proportional to the cycle time of the system clock 70. The serial bit stream has a ~lu~llily of digital "ones" in any period of time composed of a plurality of clock 70 cycles, which is proportional to the m~gnitude of the analog signal con~uct~l to the delta-sigma modulator 74.
The output of the modulator 74 is con-luctç-l to a digital bandpass filter 76. The digital ban-lp~s filter 76, among other things, removes high frequency components of the output of the modulator 74 which are introduced into the signal as a result ~ligiti7~tion of the signal in the modulator 74. Some of the high frequency components in the output of the modulator 74 are referred to as "qll~nti7~tion noise".
The output of the filter 76 can be resampled into a parallel digital output by processing in a mixer 78. The parallel digital output of the mixer 78 co~ lises multiple-bit binary digital words representing the m~gnitllde of the analog signal entering the measuring circuit 34 sampled at spaced-apart time intervals.
The mixer 78 is adapted to resample the serial bit stream conducted from the filter 76 at a lower frequency than the frequency of the clock 70. The output of the mixer 78 is con-lucted to a digital low-pass filter. The digital low-pass filter 80 attenuates signal co~ o~ above the frequency of the particular measure current being detected. Theparallel digital output from the mixer 78 is gen~ ed at a predel~llllhled sample rate, which pl~r~lably is at least four times the particular frequency being detPcted, to pl~eve ~ cing of the output being processed in the low-pass filter 80.
AlL~ iv~ly, the voltage mr~ circuit 34 can be responsive to a plurality of frequenri~s by addition of other mixers 78A, 78B and digital low-pass filters 80A, 80B, each mixer and filter pair being responsive to a dirrel~ frequency. In the present embodiment of the invention, the measuring circuit 34 comprises three mixers 78, 78A, 78B and three digital low pass filters 80, 80A, 80B so as to be responsive to the three operating frequencies of the tool 10, which typically are 32, 128 and 512 Hz. The outputs of the filters 80, 80A, 80B comprise digital words l~pleselllillg m~gnitllcles of voltages at each of the three dirrerelll frequencies measured by the formation voltage measuring circuit 34 sampled at spaced-apart time intervals.
In the present embodiment of the invention, the functions of the modulator 74 and the mixer 78 can be combined into a single module comprising, for example, an analog-to-digital collv~l~r made by Chesapeake Sciences Corp. and sold under model designation dsm-501, and a serial progl~lllllable digital filter made by Harris Semiconductor Corp. and sold under model design~tion HSP43214.
Output of the digital low pass filter 80, 80A, 80B can be directed to buffers B1, B2, B3 rOl~ g part of the central processor (shown as 51 in Figure 2) where the digital words representing the voltage measurements can be stored until the measurements are required to be used by the processor 51, or are to be tr~n~mitted to the surface unit 2.
While the present embodiment of the invention is directed to a galvanic resistivity tool having three O~ldtillg frequenriPs, it is contemplated that the present invention could also colllplise a galvanic resistivity tool having four or more operating frequencies and current mP~llring systems to enable, for example, ~ h~lly sensitive resistivity measurements.
IJ~TN(T T~F.T TA-~ST(~MA T)T(~TTTAT ~STC~TNAT (~TFNFRATTON ANT) ~T(~TMA-T~FT TA
T~T('TTTAT T~FTF(~TT()N SYSTFM
RA~K(~TR()lJNn (lF TT-TF TNVFNTTON
L Fi~ 1 of th.o Tnvention The present invention relates to the field of electrical resistivity tools, which are used to measure certain properties of earth formations pe~ t~d by boreholes. More specifically, the present invention relates to a system for digitally processing signals in electrical resistivity tools to improve the accuracy of measurements made by the tools.
T)i~r~ inn of th~ o1ev~nt Art F.1~tril ~1 resi~ivi~y tools are used to make measurements of electrical resi~ivily of earth formations penetrated by boreholes. Electrical resistivity measurements can be used for, among other things, estim~ting content of various types of fluids which can be contained in pore spaces in the earth formations.
Electrical resistivity tools known in the art include galvanic devices. Galvanicdevices typically comprise electrodes placed on an in~ ting exterior surface of the tool.
All the electrodes on the tool typically make electrical contact with the earth formation through a conductive fluid which fills the borehole.Some of the electrodes are connPcted to circuits in the tool which gellel~t~ electrical current. Other electrodes are connected to different circuits in the tool which measure voltage dirrerellces and current flow m~gnih1de~. Measurements of voltage difference and current flow can be related to the electrical resistivity of the earth formations.
A galvanic instrument known in the art is called a dual laterolog tool. The duallaterolog tool co~ lises electrodes which emit ~ current, and focusing electrodes which emit focusing ~;ul-ell~s used to constrain, or focus, flow of the measuring current in a pred~ ~l pattern. By focusing the measure current in a predetermined pattern, 216691~
mea~ulclll.;ll~ of resistivity can be related more precisely, for example, to thin vertical section~ of the earth formation. Other pre~ r. .~ patterns for focusing the measuring current can enable measurement of formation resis~ivily at a radial li~t~n~e closer to the borehole, which can be useful for estim~ting movement of fluid from the borehole into the pore space in the formation. A description of the typical electrode al~ gelllent and current focusing paLl~llls of the dual laterolog tool can be found for example in "Introduction to Wireline Log Analysis", by Ed L. Bigelow, Atlas Wireline Senices, Houston, TX, 1992 (p. 59).
The tool desclil)ed in the Bigelow reference, for example, ~imlllt~n~ously makestwo dirrclcll~y focused lll~UlClllClll~ of resistivity using the same set of electrodes. The siml11t~nPous measurements are performed by using measuring and focusing currentsystems operating at two dirrelclll frequencies. Each of the measuring and focusing systems operates at a dirrelcnl one of the two frequencies.
In order for the dual laterolog tool to accurately record the measurements made by each focusing and measuring system, the signals generated and detected by each frequency system must, to the greatest extent possible, be ~lcvellled from hll~lrerillg with each other.
Methods are known in the art for reducing hll~lrelcllce between measuring and focusing systems opel~ g at dirrelcl~t frequencies. One method known in the art is to provide an analog b~n-lp~s filter having a very narrow bandwidth to an input of the voltage and current measuring circuits in the tool corresponding to each mea~urclllclll system frequency. Analog b~n-ll)a~s filters reject passage of electrical current at fre4u~ ies other than within a so-called frequency pa~sb~m1. One of the limitations of analog b~ filters is tbat they can pass some current at frequencies other than within the frequency passband. A plurality of measurement systems operating at different frequencies may not be sufficiently isolated from each other by using analog bandpass filters to plcvelll h~l~lr~lcllce between respective systems.
Another method known in the art for reducing inle,relc~lce between dirr~le frequency measulclllent systems is to provide a current source for each measulclllcn system having as nearly as possible only one frequency component. This type of source is called a monochromatic current source. By providing a subst~nti~lly monochromatic current source for each measurement system, detections of voltages and ~;ullcnl~ in a particular measurement system which are not at the frequency of that particular measurement system can be reduced.
A system for providing a subst~nti~lly monochromatic current source is known in the art and is described, for example, in U. S. patent number 4,499,421 issued to Sinclair. The system described in the Sinclair '421 patent comprises a pair of digital latches and a precision resistor nclwol~ to generate a stair-step approximation of a sinusoidal waveform. The stair-step approximation output from the resistor network is then con-lucted to an amplifier having an analog low-pass filter. The analog low-pass filter reduces the m~gnihlde of the "stair-steps" since they have a much higher effective frequency than the sinusoidal signal. In addition to the limitations of analog filters as previously descril ed herein, the system disclosed in the Sinclair '421 patent has a further limit~tion in that the system in the Sinclair patent uses a precision resistor network to accomplish the digital-to-analog conversion. Some of the limitations of precision resistor l,c~wol~ used in digital-to-analog collvc,~ioll are described, for example, in U. S. patent number 5,357,252 issued to T~7.ills, et al. The T~-17.ills '252 patent states that the "resistive-divider" technique of data conversion, which includes the digital-to-analog conversion of the signal generator disclosed in the Sinclair '421 patent, can be difficult because the lc~ ive-divider technique requires using high precision analog components which may be diffirlllt to form, particularly in a system intended to be used in the limited space provided inside a resistivity tool used in boreholes. A resistivity tool colllp~isillg a plurality of dirr~ ~nl measulel,lell~ and focusing systems which operate at dirrel~
frequencies, using a plurality of signal generators similar to the one disclosed in the Sinclair '421 patent, is impractical.
It is known in the art to provide an analog-to-digital converter responsive to arange of frequencies to reduce signal distortion which can be present in analog signal processing circuits. The Led~ius '252 patent, for example, discloses an analog-to-digital converter responsive to a plurality of frequencies defining a usable range called the S bandwidth. The analog-to-digital CO~ .~r in the '252 patent could be used in a multiple frequency lc~ ivily tool if each measuring circuit for each dirr~lelll frequency could be connected to a col-vellel similar to the co-~ve,ler disclosed in the '252 patent and combined with a narrow bandwidth ar~alog filter. However, a limitation on the use of the coll~ tcl of the '252 patent in a multiple frequency resistivity tool is the need to include analog components in a filter stage of the col~vc.ler, as shown at 83 and 84 of Figure 5 in the '252 patent. The converter disclosed in the '252 patent was intended to have a bandwidth comprising a relatively wide range of frequencies in order to be useful, for t;A~ll~l-, in digital telephony. Tnr~ lin~ the analog components of the '252 patent in the output stage of the Collvt;ll~l as disclosed in the '252 patent would likely allow inte,rerellce between the dirrercl.l frequency measurement systems when used in a multiple frequency resistivity tool.
A fur~her limh~ti-)n on using the co~vc~lel disclosed in the '252 patent is that the disclosed co..vc,~, does not elimin~te the need for the narrow bandwidth analog b~nl1p~
filter provided at the input of the analog-to-digital converter. The limitations of using analog b~n-lp~s filters in the measuring circuit of a multiple frequency l~si~livily tool, as previously ~ c~1sse~1, would still apply if the co--~ .~r in the '252 patent were used in a resistivity tool.
It is an object of the present invention to provide a resislivily measuring toolhaving a plurality of fully digital measurement circuits, each circuit capable of o~ela~ g at a dirrelclll predetermined frequency, in order to provide mi~ h~le~relcllce between individual measurement systems.
2166!119 It is a further object of the present invention to provide a resistivity mP~llring tool having a plurality of monochromatic current sources each of which is fully digitally synthesized in order to ",illi",i,P generation of spurious frequencies in the individual llled~iUl'C ~;UllCllk;.
~;:IJMMARY OF TT-TF, TNVF,NTTON
The present invention is a tool for measuring the resistivity of an earth formation comprising at least one measuring current source. The at least one source includes a sigma-delta modulator and a serial bit source corresponding to a digital representation of a predeterminPd measure current waveform. The present invention also comprises at least one measure current sensor coupled to a delta-sigma modulator and a digital filter which ge~ dl~s a digital output corresponding to an amplitude of said measure current at said at least one sensor.
In a plcrcllcd embodiment of the invention the tool comprises a measure current circuit in which a first measure current source gelleldlcs a monochromatic sinusoidal signal having a first frequency and also comprises a first focusing current circuit which Opeldlt~S at the first frequency. The tool of the prcr~llcd embodiment further comprises second and third measure current sources which gelleldle monochromatic sinusoidal signals having second and third frcq lenriPs and second and third focusing current sources Opeld~ , at the second and third frequencies. The plcrcllcd embodiment of the invention includes measure current sensors responsive to each measure current frequency.
RRTF.F nF,.~(~RTPTT()N OF TT-TF, T~RAWTNC'T.S
Figure 1 shows the tool according to the present invention deployed in a borehole.
Figure 2 shows a resistivity measuring tool according to the present invention.
Figure 3 shows a system for adjusting the amount of focusing current.
Figure 4 shows a measuring signal gellelatol according to the present invention.Figure 5 shows a voltage measuring circuit according to the present invention.
Figure 6 shows an ~ltern~tive embodiment of a resistivity measuring tool.
nF~RTpTTON OF TT-TF PRF.FFRRF.T) FMT~OnTMFNT
Figure 1 shows a resistivity logging tool 10 as it is typically used in a borehole 1 pellelld~ g an earth formation 3. The tool 10 is typically connrcted to one end of a cable 33 colll~lisillg at least one in~ ted electrical conductor (not shown). The cable 33 can be extrn l~ into the borehole 1 by means of a surface logging unit 2. The cable 33 carries electrir~l power from the surface unit 2 to the tool 10, and can lldl~lllil signals from the tool 10 to the surface unit 2. The surface unit 2 includes equipment (not shown se~ ly) for receiving and i,lt~ ting signals tr~n~mitt~-1 by the tool 10. The surface unit also inrlll~les e lui~ lll (not shown SæP5~IAIe1Y) for tl~ iLI;I~g control signals to the tool 10.
Figure 2 shows a functional diagram of the tool 10 according to the present invention. The tool 10 comprises a sonde 12 having a plurality of electrodes 14, 16, 16A, 18, 18A, 20, 20A, 22, 22A, 23, 23A disposed on an exterior in~ tin~ surface (not shown separately) of the sonde 12. The purposes of the individual electrodes will be further explained. The tool 10 also comprises various circuits, shown combined on a circuit assembly 11 disposed inside the sonde 12, which measure voltage drops ofmr~llrin~ ;ullellls passing through the earth formation 3. The purposes of the various circuits on the assembly 11 will be explained further. The measuring ~;ullclll~ are introduced into the formation (shown as 3 in Figure 1) ~dj~cPnt to the borehole 1 by other circuits disposed on the assembly 11.
The circuit assembly 11 is shown in more detail in Figure 2 as a functional block diagram including lc~l~sellLdlive connections of the various circuits disposed on the assembly 11 to the dirrelenl electrodes, as will be further explained.
The circuit assembly 11 comprises a formation voltage measuring circuit 34 conn~octecl at one input terminal to monitor electrodes 16 and 16A through a resistive divider 27, and at the other input terminal to a ground electrode G located at the earth' s surface, the connection to the other input tPrmin~l being made through the conductor (not shown) in the cable 33. The formation voltage measuring circuit 34 measures a voltage occllrring between the electrodes 16, 16A on the sonde 12 and the ground electrode G.
The measurement of the voltage made by the formation voltage mcasu~ g circuit 34 is provided as a digital word at terminal D34A of the formation voltage measuring circuit 34. The voltage measured by the formation voltage measuring circuit 34 is related to e~i~livily of the ear~ formation 3 adjacent to the tool 10. The voltage measured by the circuit 34 lepLese~ a potential dirÇerellce reslllting from a current of known magnitlllle flowing through the formation 3 between a source electrode 14 and electrodes 20 and 20A. The current of known m~gnitllde is gelle~ d by a measure current source 24, as will be further explained. The digital word present at the terminal D34A can be con-lucted to a central processor 51, the operation of which will be further explained.
The formation voltage measuring circuit 34, which will be explained in greater detail, can be responsive to voltages at each of three dirÇe~nl frequencies to enable substantially ~imlllt~nPous measul~lllenl in three dirrelelllly focused measure current systems. In the present embo~im~nt, the frequencies of the measure current systems typically are 32, 128 and 512 Hz.
The measure current source 24, which in the present embodiment can include three, single-frequency monochlull~lic current sources each opel~lhlg at one of the three previously described frequencies, is connPctPd at one output I~lllPinal to the source electrode 14, and at the other output terminal to electrodes 20 and 20A (the ~yllullt;llical connection to electrode 20 is not shown in Figure 2 for clarity of the illustration). The llle~ul~ current source 24 provides the current with which the voltage drop through the earth formation 3 is measured by the formation voltage measuring circuit 34, as previously explained herein.
A bucking voltage measuring circuit 24A, responsive to the same three fr~lenriPs as the frequencies of the measure current source 24, is connected through a phase m~trllPd L~ rolllæL 25 across pairs of monitor electrodes 16, 18; and 16A, 18A.
21G~gl~
The bucking voltage Illf ~ p circuit 24A lllca~ul~s a voltage drop between the monitor electrodes 16 and 18, and symmetrically about the source electrode 14 makes the same lll~ultllælll b~tw~ell electrodes 16A and 18A. A digital word l~lcsen~ g the voltage drop measured by the bucking voltage measuring circuit 24A is provided at tellllillal D24A on the bucking circuit 24A and is conrl~cted to the central processor 51. If the voltage drop across the monitor electrodes 16, 18 (or symmetrically 16A, 18A) is non-zero, the pl~Jcessor 51 can be plogl~ llled to adjust the current output from the measure source 24 by ch~n~in~ the value of a digital control word con~ cted to terminal D24 on the source 24 from the processor 51. The means by which the processor 51 adjusts the output of the source 24 will be further explained. By adjusting the current output from the current source 24 to m~int~in subst~nti~lly zero voltage drop between the monitor electrodes 16, 18 and 16A, 18A, the processor 51 substantially "~i"~ .c a predetellllhled focusing pattern of the measuring current near the wellbore 1. Because the measure current is ~ul~ ially m~int~in~d within the predetermined focusing pattern, the voltage drop measuled by the formation voltage m~ ring circuit 34 can be more directly related to resistivity of the formation 3. It is known in the art to provide a single analog circuit which provides the same function as the combined opeMtion of the bucking measuring circuit 24, the measure current source 24 and the measure current adjll~tm~nt feature of the processor 51, however the present embodiment is directed to a fully digital resistivity tool.
The m~gnit~ e of the measure current ~el~ldt~d by the measure source 24 is itself lllea~ul~d by a current ",~ circuit 26 which is responsive to each of the same three mea~u Clll~ frequencies as is the formation voltage mP~ ring circuit 34. The current measuring circuit 26 comprises a voltage measuring circuit (which will be explained in more detail), of s~bst~nti~lly the same design as the formation voltage measuring circuit 34, connected across a shunt resistor 29 interposed in the measure current path b~tw~ell the electrode 14 and the measure current source 24. Current flowing across the shunt resistor 29 gel~ela~es a voltage drop proportional to the current flow across the shunt 216691~
resistor 29. The voltage measured across the shunt resistor 29 therefore is proportional to the m~gnihlde of the measure current ~ l by the llle~ulc current source 24. The m~a~urelllent made by the second measuring circuit 26 is provided as a digital word on tçrmin~l D26A which is con~ ctçd to the processor 51. The measurement made by the second m~lrin~ circuit 26 which is pr~ollional to current m~gnit~lde can be combined with the voltage drop measurement made by the formation voltage measuring circuit 34 to de~llllille the resi~LiviLy of the earth formation 3.
Three focus current sources 28, 30 and 32, each opelaling at a different one of the three previously described measurement system frequencies, are conn~cted syl-llll~-l- ir~lly about the source electrode 14 to focusing electrodes 20 and 20A; 22 and 22A; and 23 and 23A, these electrodes being disposed on the sonde 12 at axially spaced apart locations from the source electrode 14. Each of the focus current sources 28, 30, 32 is connected to the electrodes in a dirrelclll configuration so as to cause focusing current from each source to flow in a different path. Each of the three dirrelc frequency mP~cllrin~ ~;ullcllL~ ~llc~lxlhlg to one of the focusing ~;Ull~llki can therefore constrained to a dirr~l~lll predetermined focusing pattern in the borehole 1 and the earth formation 3 adjacent to the borehole 1. For example, a first focusing source 28, which u~ ates at a first frequency, is conn~cted at one output to all three focusing electrodes 20, 22, and 23, and symmetrically about the source electrode 14 to electrodes 20A, 22A
and 23A (although the symm~tric connections are not shown in Figure 2 for clarity of illustration). The other output of the first focusing source 28 is conn~cted to the cable 33 armor. The first focusing source 28 provides focusing to the measuring current having the greatest radial depth of col~lldillL because the focusing current from the first source 28 is col~L~ led to flow ~ull~L~llially entirely radially uuL~al-l from the sonde 12 before dispersing in the earth formation 3 and le~ g to the cable 33 armor.
A second focusing source 30, opeldling at a second frequency, is typically conn~cted across electrodes 20 and 22 (and symmetrically 20A and 22A, but the symm~trical connections again are not shown for clarity) to provide focusing to the . .
second frequency llle~ul~ current having a reduced radial depth of constraint relative to the first frequency measuring current focused by the first focusing source 28. The focusing pattern of the second focusing source 30 enables dispersion of the measuring current in the formation 3 at a shallower radial ~ t~nre from the sonde 12 than does the first focusing source 28 pattern, because the second focusing current is returned to the electrodes 22, 22A on the sonde rather than on the cable 33 armor. I~ ...,ing the focusing current to the electrodes 22, 22A enables dispersion of the second frequency measure current radially closer to the sonde 12, so that a relatively shallower radial lllea~ul~ of formation l~ livily can be made by the second than can be made by the first measure current.
A third focusing source 32, op~ g at a third frequency, can be connected between electrodes 22 and 23 (and symmetrically to 22A and 23A with symmetrical connections again not shown for clarity of illustration)) to provide even shallower radial constraint of the third frequency m~uring current than the second focusing source 30 does for the second frequency measuring current. The shallower radial constraint of the measuring current provided by the third focusing source 32 results from the focusing electrodes 22, 23 and æA, 23A being spaced at a greater axial ~ t~nre from the source electrode 14 than the focusing electrodes 20, 22 (and ~yllllll~lfically, 20A, 22A) used for e",i~ the focusing current at the second frequency. The longer axial spacing of the electrodes 22, 23 used for the third frequency focusing current enables the third frequency measuring current to disperse in the earth formation 3 at an even shallower Mdial ~ t~n~e from the sonde 12 than does the second frequency m~ lring current.Output levels of the focus sources 28, 30, 32 can be controlled by digital wordsinput to tellllillals D28, D30 and D32, respectively, from the processor 51.
Focusing current nlc~ulillg circuits 30A and 32A can be conn~cte~ across shunt resistors 31 and 35, respectively, to provide measurement of the total amount of focus current ge~ t~d by the first source 28, and the second and third sources 30, 32,e~eclively . Focusing current llleasu~ are provided as digital words on l~ll linals D30A and D32A, lc~e~ /ely, and are conducted to the processor 51. As will be further c~lailled, focusing current m~ les can be adjusted in response to dirrclclll values of formation 3 resistivity so as to control the dynamic range of signal at the input of the formation voltage measuring circuit 34 and the current measulillg circuit 26.
Figure 3 shows a flow chart of a typical measurement sequence provided by the tool (shown as 10 in Figure 1). The focusing cullclll~ are measured by circuits 30A and 32A, as generally shown at 38. If the cullcllL~ from the sources 28, 30, 32 are d~ ~d to be too low or too high to m~int~in a minimum predclcllllilled signal level at the inputs of the formation voltage measuring circuit 34 and the current mr.~llring circuit 26 as shown at number 40, adjustments are made under control of the processor 51 to the output levels of the focus current sources 28, 30 and 32 either to increase as shown at 42A, or to decrease as shown at 42B, the level of the focus cullclll~. The means by which the processor 51 changes the output level of the sources 28, 30, 32 will be further explained. Changes in the amount of required focus current can occur, for C~ , if the fluid filling the borehole 1 is particularly conductive or resistive, or the earth formation 3 is particularly conductive or resistive. When the focus CUllC~ are determined to be of the correct m~gnih--le, voltage drop and current m~gnitllde of the measure ~;UllClll~ are made, as shown generally at 44.
The current sources according to the present invention can be better understood by lcrclling to Figure 4. The source, shown as 28 in Figure 4, can also correspond to any of the other sources shown in Figure 2 such as the focus current sources 28, 30 and 32, or the measure current source 24.
The output m~gnihlde of the source 28 is controlled by a rcrclcllce level digital word con~lurt~ to an input t~rmin~l 58 of a reference digital to analog collvcllcl (DAC) 54 forming part of the source 28. The input termin~l 58 of the reference DAC 54 shown in Figure 4 corresponds to any one of the digital word input terminals D24, D28, D30 or D32 of the sources 24, 28, 30 or 32, respectively, shown in Figure 2. The input trrmin~l 58 is conn~t~d to the corresponding terminal on the processor 51. The digital ~166919 word present at the input terminal 58 is loaded into the reference DAC 54 when a load co~ l is applied to a command terminal 56 on the lcrclcl~ce DAC 54 by the processor 51. A load ~""",~".1 is gcneldlcd by the processor 51 when the processor 51 calculates that the value of the digital word is to be changed. As previously explained, the value of the digital word can be changed by the processor 51 in response to measurements provided by the focus measuring circuits 30A, 32A, and the formation voltage 34 and current measuring 26 circuits.
The lcfercllce DAC 54 is conn~,cted to a precision voltage lerelellce source 52.The precision voltage ler~lell~ source 52 gCllCldlC~ a subst~nti~lly constant voltage direct current (DC). The subst~nti~lly constant voltage DC from the lèrclèllce source 52 X accuracy and repeatability of conversion of the digital word present at the input tcllllillal into an output level control signal by the rcfercllce DAC 54.
The output of the reference DAC 54 typically is a substantially constant voltageDC which is con(1~1cted to a delta sigma modulator 64 at a rèrelèllce input terminal 64A.
As previously explained, the m~gnitllde of the DC voltage output from the rcfclcllce DAC 54 is dcl~. lllil~1 by the reference digital word con~ cted to the input lCllllindl 58.
A serial bit stream 62 is gellcldlcd by a memory control and address generator 66 sequentially activating a digital memory 60. The serial bit stream 62 is conn~cted to a digital input termin~l 64B on the modulator 64. The serial bit stream 62 contains a series of numbers corresponding in m~gnitllde to the i~xl~ ous output m~gnitllde of thesources 28 at spaced apart time intervals. In the present invention the waveform is typically sinusoidal. The serial bit stream 62 also contains il~llllation about the frequency of the current output from the source 28. In the present embodiment the frequency typically is 32, 128 or 512 Hz. The output from the modulator 64 is conducted to a circuit which can be a power amplifier or a source electrode driver as shown at 68, and thence to electrodes such as the focusing electrodes shown as 22 and 22A in Figure 2.
The output of the modulator 64 comprises analog signals proportional in m~gnitl~(le to the m~gnitl1(1e of the reference voltage con~1ctecl from the l~r~,rellce DAC
54. The modulator 64 ge~ d~s output samples at a rate which is delellllilled by the frequency of a system clock 70 conn~cte(l to the modulator 64. The clock 70 frequency is much higher than the output frequency of the source 28. In the present embodiment the system clock 70 frequency typically is 1.024 MHz. The high frequency samples from the CollV~ l 64 are filtered into the correct output waveform in a low-pass filter forming part of the amplifier 68. Because the output frequency of the samples gelleldt~d by the modulator 64 is much higher than the operating frequency of the current source 28, the filter col~ ollellL~ which form part of the amplifier 68 can have cut-off frequencies well above the opeldLhlg frequency of the source 28. In the present embodiment, the filter fulll~illg part of the amplifier 68, can have a cut-off frequency of 256 KHz, which is one-fourth the frequency of the system clock 70, but is 500 times the highest opeldlhlg frequency of the source 28 (which is 512 Hz). The filter components forming part of the amplifier 68 can therefore be designed to have gradual "roll-off" characteristics, which greatly simplifies the design of the filter components.
In the present embodiment of the invention, the source 28 can comprise an integrated module, shown as 50 in Figure 4, including the voltage lc;r~lc;llce 52, the reference DAC 54, the modulator 64, the memory 60, and the memory controller 66.The module 50 can be progr~mm~, by insertion of applupliate bit stream data into the memory 60 during construction or servicing of the module 50, to generate a predetermined signal. In the present embodiment of the invention the pred~llllhled signal can be a monochromatic sinusoid having a frequency of 32, 128 or 512 Hz.
The processor 51 can be programmed to respond to inputs, provided as digital words to input terminals D26, D30A, D32A, and D34A on the processor 51, from themeasuring circuits 26, 30A, 32A and 34, respectively. Signals representing resistivity of the forrnation 3, which are g~ 1 in the processor 51 in response to measurements gell~ldt~d by the formation voltage 34 and second 26 measuring circuits, as previously 216691~
explained, can be ~ ~ to the cable 33 in the form of digital words con-luct.od from ell,linal 32X on the processor 51.
Referring now to Figure 5, the voltage measuring circuit 34 according to the present invention will be explained in more detail. An analog signal input from a sensor, which in the present embodiment can be one or more of the electrodes on the tool 10, is co~ c~P~l to a proglallllllable gain amplifier (PGA) 72. The PGA 72 can be controlled by the central processor 51 so as to m~int~in an output signal level which remains within the resolution range of a sigma-delta modulator 74, to which the output of the PGA 72 is con~ ctçd. The modulator 74 converts the signal from the PGA 72 into a serial bit stream at a rate determined by the frequency of the system clock 70, to which the modulator 74 is operatively conn~cted The serial bit stream output of the modulator repleselll~ the m~gni~ le of the signal input from the PGA 72 sampled at spaced apart time intervals. The serial bit stream is composed essentially of digital "ones" and "zeroes" having individual durations proportional to the cycle time of the system clock 70. The serial bit stream has a ~lu~llily of digital "ones" in any period of time composed of a plurality of clock 70 cycles, which is proportional to the m~gnitude of the analog signal con~uct~l to the delta-sigma modulator 74.
The output of the modulator 74 is con-luctç-l to a digital bandpass filter 76. The digital ban-lp~s filter 76, among other things, removes high frequency components of the output of the modulator 74 which are introduced into the signal as a result ~ligiti7~tion of the signal in the modulator 74. Some of the high frequency components in the output of the modulator 74 are referred to as "qll~nti7~tion noise".
The output of the filter 76 can be resampled into a parallel digital output by processing in a mixer 78. The parallel digital output of the mixer 78 co~ lises multiple-bit binary digital words representing the m~gnitllde of the analog signal entering the measuring circuit 34 sampled at spaced-apart time intervals.
The mixer 78 is adapted to resample the serial bit stream conducted from the filter 76 at a lower frequency than the frequency of the clock 70. The output of the mixer 78 is con-lucted to a digital low-pass filter. The digital low-pass filter 80 attenuates signal co~ o~ above the frequency of the particular measure current being detected. Theparallel digital output from the mixer 78 is gen~ ed at a predel~llllhled sample rate, which pl~r~lably is at least four times the particular frequency being detPcted, to pl~eve ~ cing of the output being processed in the low-pass filter 80.
AlL~ iv~ly, the voltage mr~ circuit 34 can be responsive to a plurality of frequenri~s by addition of other mixers 78A, 78B and digital low-pass filters 80A, 80B, each mixer and filter pair being responsive to a dirrel~ frequency. In the present embodiment of the invention, the measuring circuit 34 comprises three mixers 78, 78A, 78B and three digital low pass filters 80, 80A, 80B so as to be responsive to the three operating frequencies of the tool 10, which typically are 32, 128 and 512 Hz. The outputs of the filters 80, 80A, 80B comprise digital words l~pleselllillg m~gnitllcles of voltages at each of the three dirrerelll frequencies measured by the formation voltage measuring circuit 34 sampled at spaced-apart time intervals.
In the present embodiment of the invention, the functions of the modulator 74 and the mixer 78 can be combined into a single module comprising, for example, an analog-to-digital collv~l~r made by Chesapeake Sciences Corp. and sold under model designation dsm-501, and a serial progl~lllllable digital filter made by Harris Semiconductor Corp. and sold under model design~tion HSP43214.
Output of the digital low pass filter 80, 80A, 80B can be directed to buffers B1, B2, B3 rOl~ g part of the central processor (shown as 51 in Figure 2) where the digital words representing the voltage measurements can be stored until the measurements are required to be used by the processor 51, or are to be tr~n~mitted to the surface unit 2.
While the present embodiment of the invention is directed to a galvanic resistivity tool having three O~ldtillg frequenriPs, it is contemplated that the present invention could also colllplise a galvanic resistivity tool having four or more operating frequencies and current mP~llring systems to enable, for example, ~ h~lly sensitive resistivity measurements.
2~ 66919 T)F~(~RTPTTON C)F AN AT TFRNATTVF FMROnTMFNT
An al~ IA1ivc resistivity m~a~llring tool lOA comprising a measuring circuit anda current source accoldillg to the present invention is shown in Figure 6. The resistivity tool lOA in Figure 6 is an induction measuring device comprising a tran.~mhter coil 100 S disposed within a sonde 12A similar to the sonde 12 in Figure 2. The l1A~ e1 coil 100 is conn~-cted to a measure current source 24B which can be subst~nti~lly the same type as the source shown in detail as 24 in Figure 4. Current flows in the Ll~ r coil 100 and induces eddy eullcllL~ in the ear~ fc,llllation 3 which flow substantially coaxially around the tool lOA. The eddy eullcllL~ in the formation 3 themselves induce voltages in a receiver coil 102 disposed within the sonde 12A at an axially spaced apart location from the I~An~ 1 coil 100. The magnitude of the voltages in~ ced in the receiver coil 102 is related to the lC~;~Livily of the earth formation 3. A voltage m~lring circuit 34B
which can be subst~nti~lly the same type as the circuit shown as 34 in Figure 2 is connPcted to the receiver coil 102 to measure the m~gnill~de of the voltages inrluced in the receiver coil 102. Measurements made by the voltage measulillg circuit 34B can be d to the surface unit 2 for hllel~lc~lion.
The induction tool lOA typically operates at a much higher frequency than the laterolog tool (shown as 10 in Figure 2), because the m~nitllde of the voltages in-luced in the receiver coils 102 is generally proportional to the frequency of the current in the Il,.~ er coil 100. The frequency in the present embodiment can be within a rangefrom 10 kHz to about 150 kHz. The source 24B and the mcasulillg circuit 34B can be progr~mm~cl to gellcld~ and receive, lc~;livcly, any of the individual frequencies used in the induction tool lOA by applopliate selection of mixers (shown as 78 in Figure 5) and digital filters (shown as 80 in Figure 5) for the m~a~llrin~ circuit 34B, and by appr~lidtely progr~mming the memory 60 in the source 24B to gelleldle a serial bit stream lcl~lcsellldlive of a 10-150 kHz sinusoidal signal.
The tool lOA shown in Figure 6 is shown as having only one ll,.~ el coil 100 and one receiver coil 102. For reasons well known in the art, a practical induction logging tool lOA can have a plurality of receiver coils (not shown)and tr~n~mitter coils (not shown) disposed within the sonde 12A at different axially spaced apart locations.
Each of the plurality of coils can be responsive to a different frequency. The tool 10 shown in Figure 2 and the tool lOA shown in Figure 6 are not intended to be exclusive l~lesel~ ions of the invention described herein. The scope of the invention should be limited only by the claims appended hereto.
An al~ IA1ivc resistivity m~a~llring tool lOA comprising a measuring circuit anda current source accoldillg to the present invention is shown in Figure 6. The resistivity tool lOA in Figure 6 is an induction measuring device comprising a tran.~mhter coil 100 S disposed within a sonde 12A similar to the sonde 12 in Figure 2. The l1A~ e1 coil 100 is conn~-cted to a measure current source 24B which can be subst~nti~lly the same type as the source shown in detail as 24 in Figure 4. Current flows in the Ll~ r coil 100 and induces eddy eullcllL~ in the ear~ fc,llllation 3 which flow substantially coaxially around the tool lOA. The eddy eullcllL~ in the formation 3 themselves induce voltages in a receiver coil 102 disposed within the sonde 12A at an axially spaced apart location from the I~An~ 1 coil 100. The magnitude of the voltages in~ ced in the receiver coil 102 is related to the lC~;~Livily of the earth formation 3. A voltage m~lring circuit 34B
which can be subst~nti~lly the same type as the circuit shown as 34 in Figure 2 is connPcted to the receiver coil 102 to measure the m~gnill~de of the voltages inrluced in the receiver coil 102. Measurements made by the voltage measulillg circuit 34B can be d to the surface unit 2 for hllel~lc~lion.
The induction tool lOA typically operates at a much higher frequency than the laterolog tool (shown as 10 in Figure 2), because the m~nitllde of the voltages in-luced in the receiver coils 102 is generally proportional to the frequency of the current in the Il,.~ er coil 100. The frequency in the present embodiment can be within a rangefrom 10 kHz to about 150 kHz. The source 24B and the mcasulillg circuit 34B can be progr~mm~cl to gellcld~ and receive, lc~;livcly, any of the individual frequencies used in the induction tool lOA by applopliate selection of mixers (shown as 78 in Figure 5) and digital filters (shown as 80 in Figure 5) for the m~a~llrin~ circuit 34B, and by appr~lidtely progr~mming the memory 60 in the source 24B to gelleldle a serial bit stream lcl~lcsellldlive of a 10-150 kHz sinusoidal signal.
The tool lOA shown in Figure 6 is shown as having only one ll,.~ el coil 100 and one receiver coil 102. For reasons well known in the art, a practical induction logging tool lOA can have a plurality of receiver coils (not shown)and tr~n~mitter coils (not shown) disposed within the sonde 12A at different axially spaced apart locations.
Each of the plurality of coils can be responsive to a different frequency. The tool 10 shown in Figure 2 and the tool lOA shown in Figure 6 are not intended to be exclusive l~lesel~ ions of the invention described herein. The scope of the invention should be limited only by the claims appended hereto.
Claims (17)
1. An apparatus for measuring resistivity of an earth formation penetrated by a borehole, comprising:
a sonde adapted to traverse said borehole;
at least one source of measuring current including a delta sigma modulator and a serial bit source coupled to said modulator, said bit source providing a digital representation of a magnitude of said measuring current sampled at spaced apart time intervals, said source of measuring current coupled to an emitter disposed on said sonde;
at least one sensor disposed on said sonde, said sensor generating a signal in response to formation current resulting from interaction of said measure current with said earth formation; and a sigma delta modulator coupled to said at least one sensor, said modulator coupled to a digital filter, an output of said filter comprising digital words corresponding to amplitude of said signal generated by said at least one sensor, said words sampled at spaced apart time intervals.
a sonde adapted to traverse said borehole;
at least one source of measuring current including a delta sigma modulator and a serial bit source coupled to said modulator, said bit source providing a digital representation of a magnitude of said measuring current sampled at spaced apart time intervals, said source of measuring current coupled to an emitter disposed on said sonde;
at least one sensor disposed on said sonde, said sensor generating a signal in response to formation current resulting from interaction of said measure current with said earth formation; and a sigma delta modulator coupled to said at least one sensor, said modulator coupled to a digital filter, an output of said filter comprising digital words corresponding to amplitude of said signal generated by said at least one sensor, said words sampled at spaced apart time intervals.
2. The apparatus as defined in claim 1 further comprising a plurality of said sources of measuring current, each one of said sources generating a substantially monochromatic sinusoidal signal at a frequency different from the frequency of the other ones of said sources of measuring current.
3. The apparatus as defined in claim 2 further comprising a plurality of sensors, each one of said sensors coupled to a sigma-delta modulator and a digital filter responsive to a different one of said frequencies of said sources of measuring current so that magnitudes of signal generated by each one of said sensors in response to formation current resulting from interaction of said formation with each one of said measuring current at each of said frequencies can be determined.
4. The apparatus as defined in claim 3 wherein said emitter comprises first electrodes disposed on said sonde, first electrodes disposed on said sonde.
5. The apparatus as defined in claim 3 wherein said plurality of sensors comprises second electrodes disposed on said sonde.
6. The apparatus as defined in claim 1 wherein said emitter comprises an induction transmitter coil.
7. The apparatus as defined in claim 1 wherein said sensor comprises an induction receiver coil.
8. The apparatus as defined in claim 1 further comprising: a digital mixer coupled o an output of said digital filter; and a digital low-pass filter coupled to an output of said digital mixer, said low-pass filter having a cut-off frequency corresponding to an output of said mixer, said digital mixer cooperative with said modulator and said digital filter to generate a digital output corresponding to a magnitude of said signal at said frequency of each one of said measuring current.
9. The apparatus as defined in claim 2 further comprising: a plurality of digital mixers coupled to said digital filter, each of said mixers having a different output frequency; and a plurality of digital low-pass filters, each of said digital low-pass filters coupled to an output of one of said plurality of mixers, each of said low pass filters having a different cut-off frequency corresponding to said output frequency of one of said plurality of digital mixers, each one of said digital mixers cooperative with said modulator and said digital filter to generate a digital output corresponding to a magnitude of said signal at a different corresponding one of said frequencies of said sources of measuring current.
10. The apparatus as defined in claim 1 wherein said emitter comprises an electrode.
11. An apparatus for measuring resistivity of an earth formation penetrated by a borehole, comprising:
a sonde adapted to traverse said borehole;
at least one source of measuring current including a sigma-delta modulator and a serial bit source coupled to said modulator, said bit source providing a digital representation of a magnitude of said measuring current sampled at spaced apart time intervals, said source of measuring current coupled to an emitter disposed on said sonde;
at least one sensor disposed on said sonde, said sensor generating a signal in response to a current resulting from interaction of said measure current with said earth formation; and means for measuring a magnitude of said signal coupled to said at least one sensor.
a sonde adapted to traverse said borehole;
at least one source of measuring current including a sigma-delta modulator and a serial bit source coupled to said modulator, said bit source providing a digital representation of a magnitude of said measuring current sampled at spaced apart time intervals, said source of measuring current coupled to an emitter disposed on said sonde;
at least one sensor disposed on said sonde, said sensor generating a signal in response to a current resulting from interaction of said measure current with said earth formation; and means for measuring a magnitude of said signal coupled to said at least one sensor.
12. The apparatus as defined in claim 11 further comprising a plurality of said sources of measuring current, each one of said sources generating a substantially monochromatic sinusoidal signal at a frequency different from the other ones of said sources.
13. The apparatus as defined in claim 12 wherein said emitter comprises first electrodes disposed on said sonde.
14. The apparatus as defined in claim 11 wherein said emitter comprises an induction transmitter coil.
15. The apparatus as defined in claim 11 wherein said sensor comprises an induction receiver coil.
16. An apparatus for measuring resistivity of an earth formation penetrated by a borehole, comprising:
a sonde adapted to traverse said borehole;
at least one source of measuring current coupled to an emitter disposed on said sonde, said source co substantially a substantially monochromatic sinusoidal alternating current;
at least one sensor disposed on said sonde, said sensor generating a signal in response to a current resulting from interaction of said measure current with said formation; and a sigma-delta modulator coupled to said at least one sensor, an output of said modulator coupled to a digital filter, an output of said filter comprising digital words corresponding to amplitude of said signal sampled at spaced apart time intervals.
a sonde adapted to traverse said borehole;
at least one source of measuring current coupled to an emitter disposed on said sonde, said source co substantially a substantially monochromatic sinusoidal alternating current;
at least one sensor disposed on said sonde, said sensor generating a signal in response to a current resulting from interaction of said measure current with said formation; and a sigma-delta modulator coupled to said at least one sensor, an output of said modulator coupled to a digital filter, an output of said filter comprising digital words corresponding to amplitude of said signal sampled at spaced apart time intervals.
17. The apparatus as defined in claim 16 further comprising: a digital mixer coupled to an output of said digital filter; and a digital low-pass filter coupled to an output of said digital mixer, said low-pass filter having a cut-off frequency corresponding to an output frequency of of said mixer, said digital mixer cooperative with said modulator and said digital filter to generate a digital output corresponding to a magnitude of said signal at a frequency of said measuring current.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/373,409 US5585727A (en) | 1995-01-17 | 1995-01-17 | Apparatus for measuring resistivity of an earth formation using delta-sigma digital signal generation and sigma-delta digital detection system |
| US08/373,409 | 1995-01-17 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2166919A1 CA2166919A1 (en) | 1996-07-18 |
| CA2166919C true CA2166919C (en) | 1999-09-28 |
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002166919A Expired - Fee Related CA2166919C (en) | 1995-01-17 | 1996-01-10 | Apparatus for measuring resistivity of an earth formation using delta-sigma digital signal generation and sigma-delta digital detection system |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US5585727A (en) |
| CA (1) | CA2166919C (en) |
| GB (1) | GB2297165B (en) |
| NL (1) | NL1002114C2 (en) |
| NO (1) | NO310845B1 (en) |
Families Citing this family (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6166540A (en) | 1997-06-30 | 2000-12-26 | Wollin Ventures, Inc. | Method of resistivity well logging utilizing nuclear magnetic resonance |
| CA2304310C (en) * | 1997-10-08 | 2004-11-30 | Shell Canada Limited | Resistivity log correction method |
| US6816100B1 (en) | 1999-03-12 | 2004-11-09 | The Regents Of The University Of California | Analog-to-digital converters with common-mode rejection dynamic element matching, including as used in delta-sigma modulators |
| US6211807B1 (en) | 1999-05-26 | 2001-04-03 | Geometrics | System using spread spectrum modulation for locating underground objects |
| US6892137B2 (en) * | 2003-04-29 | 2005-05-10 | Pathfinder Energy Services, Inc. | Adjustment for frequency dispersion effects in electromagnetic logging data |
| US7042225B2 (en) * | 2003-12-12 | 2006-05-09 | Schlumberger Technology Corporation | Apparatus and methods for induction-SFL logging |
| US8050865B2 (en) * | 2008-10-31 | 2011-11-01 | Baker Hughes Incorporated | System and method for measuring resistivity parameters of an earth formation |
| US8614578B2 (en) * | 2009-06-18 | 2013-12-24 | Schlumberger Technology Corporation | Attenuation of electromagnetic signals passing through conductive material |
| CA2878855A1 (en) * | 2012-07-12 | 2014-01-16 | Halliburton Energy Services, Inc. | System and method to improve accuracy of galvanic tool measurements |
| US20170023695A1 (en) * | 2014-04-14 | 2017-01-26 | Halliburton Energy Services Inc. | Generator for laterolog tool |
| EP3175082A4 (en) * | 2014-07-31 | 2018-03-28 | Halliburton Energy Services, Inc. | Metamaterial electromagnetic sensors for well logging measurements |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3328679A (en) * | 1964-11-13 | 1967-06-27 | Schlumberger Technology Corp | Electromagnetic well logging systems with means for modulating the detected signals |
| US4361808A (en) * | 1980-03-17 | 1982-11-30 | Exxon Production Research Co. | Dielectric constant well logging with current and voltage electrodes |
| US4720681A (en) * | 1981-06-08 | 1988-01-19 | Schlumberger Technology Corporation | Digital induction logging tool |
| US4455529A (en) * | 1981-06-08 | 1984-06-19 | Schlumberger Technology Corporation | Digital induction logging tool including means for measuring phase quadrature components in a phase sensitive detector |
| US4499422A (en) * | 1981-06-08 | 1985-02-12 | Schlumberger Technology Corporation | Digital induction logging tool including means for compensating for phase shift errors |
| US4499421A (en) * | 1981-06-08 | 1985-02-12 | Schlumberger Technology Corporation | Digital induction logging system including means for generating a plurality of transmitter frequencies |
| US4439831A (en) * | 1981-06-08 | 1984-03-27 | Schlumberger Technical Corporation | Digital induction logging system including autocalibration |
| US4481472A (en) * | 1981-08-19 | 1984-11-06 | Schlumberger Technology Corporation | Pulsed induction logging for determining conductivity and invaded zone properties |
| US4451790A (en) * | 1981-11-10 | 1984-05-29 | Halliburton Company | Spontaneous potential log apparatus with randomly occurring noise cancellation |
| US5187661A (en) * | 1989-07-21 | 1993-02-16 | Halliburton Logging Services, Inc. | Method of determining invaded formation properties including resistivity dielectric constant and zone diameter |
| US5065099A (en) * | 1990-02-02 | 1991-11-12 | Halliburton Logging Services, Inc. | Coil array for a high resolution induction logging tool and method of logging an earth formation |
| US5083124A (en) * | 1990-04-17 | 1992-01-21 | Teleco Oilfield Services Inc. | Nuclear logging tool electronics including programmable gain amplifier and peak detection circuits |
| US5264795A (en) * | 1990-06-18 | 1993-11-23 | The Charles Machine Works, Inc. | System transmitting and receiving digital and analog information for use in locating concealed conductors |
| US5065098A (en) * | 1990-06-18 | 1991-11-12 | The Charles Machine Works, Inc. | System for locating concealed underground objects using digital filtering |
| US5329448A (en) * | 1991-08-07 | 1994-07-12 | Schlumberger Technology Corporation | Method and apparatus for determining horizontal conductivity and vertical conductivity of earth formations |
| US5357252A (en) * | 1993-03-22 | 1994-10-18 | Motorola, Inc. | Sigma-delta modulator with improved tone rejection and method therefor |
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1995
- 1995-01-17 US US08/373,409 patent/US5585727A/en not_active Expired - Fee Related
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1996
- 1996-01-10 CA CA002166919A patent/CA2166919C/en not_active Expired - Fee Related
- 1996-01-12 NO NO19960138A patent/NO310845B1/en not_active IP Right Cessation
- 1996-01-16 GB GB9600835A patent/GB2297165B/en not_active Expired - Fee Related
- 1996-01-17 NL NL1002114A patent/NL1002114C2/en not_active IP Right Cessation
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| GB9600835D0 (en) | 1996-03-20 |
| GB2297165B (en) | 1998-06-10 |
| NO310845B1 (en) | 2001-09-03 |
| CA2166919A1 (en) | 1996-07-18 |
| GB2297165A (en) | 1996-07-24 |
| NL1002114C2 (en) | 1998-10-14 |
| NO960138D0 (en) | 1996-01-12 |
| NO960138L (en) | 1996-07-18 |
| US5585727A (en) | 1996-12-17 |
| NL1002114A1 (en) | 1996-07-17 |
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