US2345608A - Geophysical prospecting - Google Patents

Description

April 4, 1944. F. w. LEE

I GEOPHYSICAL PROSPECTING Filed May '7, 1940 2 Sheets-Sheet 2 Fig 7 Feder/'Ck W Zee //2 Ven for Patented Apr. 4, 1944 UNITED STATES PATENT vOFFICE 1s claims. (ci. 11s-isz) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) The invention disclosed herein may be used by or for the Government of the United States without payment of any royalty therefor.

'I'his invention relates to geophysical prospecting or surveying and aims generally to improve the same.

In particular this invention enables the determination of the contour, shape, position and/or composition of geologic bodies, hereafter referred to as geologic structure, and more particularly provides a method of and means for making such determinations by electrical resistivity, electrical dielectric and magnetic permeability measurements obtained at the surface of the ground and other accessible points, such as shafts, stopes, drifts and drill holes. The invention differentiates ground which may be electrically isotropic or anisotropic in regard to its resistivity, dielectric constant, and magnetic permeability, hereafter referred to as electrical properties. By anisotropic properties of geological structure is meant that the material has different resistivities and electrical properties in different directions. This invention concerns itself in dening geologic structure by separating and using the directional reslstivities and electrical properties as further to be described.

In the accompanying drawings, exemplifying the principles and application of the present invention:

Fig. 1 is a perspective view showing a suitable ground coniiguration of current electrodes Ci-Cz, preferably equally spaced at opposite sides of a station point Po, and .showing a suitable location of potential electrodes relative thereto;

' Fig. 2 is a diagrammatic representation of the current flow and transverse potential values under isotropic conditions or under anisotropic conditions symmetrical longitudinally and transversely of the line of centers of current electrodes;

Fig. 3 is a similar diagram, for anisotropic conditions symmetrical only about the plane transverse to the center line of current electrodes;

Figs. 4, 5 and 6 are similar diagrams for different conditions;

Fig. '7 is a circuit layout for application to the ground conflguration; and

Fig. 8 is a three dimensional diagrammatic illustration showing subterranean anisotropic conditions.

A simple case where an isotropic geologic body overlies an anisotropic one is found in glaciated districts Where the glacial till covers the hard A rock outcrops las for example in northern Michigan. Such hard-rock outcrops are often banded in composition and tilted, which makes the ground anisotropic in character. deflne'these diterentially conducting bands constitutes the nature of this invention, even when covered with other isotropic or anisotropic material.v

It is well known in the art that by applying a potential to spaced connections on the ground an indication may be obtained which relates to or is a function of the electrical resistivity and other electrical constants of the ground. See F. W. Lee, U. S. Patent No. 1,951,760, granted March 20, 1934.

In explaining the present invention, it is desirable to lirst consider the electrical resistivity which for convenience may be divided into two component parts, one being called the normal resistivity as lwould exist in an isotropic medium, the other the transverse resistivity as would be' caused by the non-isotropic portion of the medium. For simplicity of measurement and mathematical analysis the two resistivity components are separated in space by a right-angle relationship,-such that Where p=apparent ground resistivity; pn=normal component of the apparent ground resistivity; pt=transver component of the apparent resistivity; z'=\/-l (i is the operator square root of m. inus one). Thus the absolute value of p=.\/pn'-|p:2 and the direction is arc tan Q While it is possible to compute pn and pt between spaced potential and current contacts on the ground, this invention discloses a, method for diierentiating the two in such a way that each may be measured and interpreted separately. Furthermore it, divides the ground in such a manner that .the change of symmetry in the transverse resistivities discloses the changes of ground structure. In the above mentioned case where the ground is covered with glacial till the sand and gravel is of an isotropic character for which pn would have a value and pt the transverse resistivity would be zero.

Referring to Fig. 1, showing a suitable ground configuration, current of suitable nature is applied through the ground electrodes CiCz, preferably equally spaced at opposite sides of the central or station electrode Po, as shown. 'I'he dotted line electrodes PiPz, which are considered The ability to as located on a line with CiCz, may be used to determine the normal resistivity component, as explained in my above mentioned patent and my copending application entitled sistivity or impedisivity measuring, Serial No. 200,948, iiled April 8, 1938 patented March 31., 1942, Patent No. 2,277,707. To determine the .transverse resistivity particularly "contemplated in the present invention, and newly coordinated thereby with the normal resistivity to give com' plex data as set forth, the new coordination of transversely positioned electrodes Pa, ,P4, preferably equally spaced on opposite sides of the stationvpoint Po and at right angles to lthe line of centers of the current electrodes CiCz, is used. This arrangement divides the ground by two partitioning planes, passing vertically into the ground through CiCz and through PaPr, respectively. Measurements on both sides of the vertical plane CrCz, by means, for example, of potential electrodes P3P4, in accordance with the present invention, enables determination of the anisotropic character changes on each side of this plane, as indicated by the transverse resistivity of the geologic structure.

Now referring to Fig. 2, representing isotropic conditions, it will be seen that with the electrode configuration .shown the transverse current for small values of Po-Pa and Ply-P4 owing in the direction P3-P4 would be zero, since the field would be symmetrical about the Ps-P4 axis as well as about the Pi--Pa axis. With measurements of increased separations of C1--Cz, Po-Pr, Po-Pz, Po-Pa, Po-P4 appreciable parts of the current will flow deeper and deeper in the terrain and current will eventually begin to ow through the underlying bed rock. If the bed rock is anisotropic, then potential will begin to appear between Po-Ps and Po-Pi and by noting the separation when this occurs it is possible to compute the depth of the isotropic overburden.

In practicing the present invention the point Po, Fig. 1, is called a station point at which measurements are made at successive depths by increasing the distances between the other points of the conguration while maintaining the relative symmetry. The station points are preferably located at spaced distances along a line of traverse. Such spaced distances are preferably chosen so as to not omit, any portion of ground and preferably so as preliminarily to eliminate the infrequently found positions at which the transverse observations become zero. Such positions may be found when the material AB, which has anisotropic electrical properties, passes perpendicularly between Ci-Cz at Pu of Fig. l or when it passes parallel to CiCz as shown in Fig. 3. 'I'he more frequently found and preliminariiy more useful positions are shown in Figs. 4, 5 and 6.

In Fig. 4 the transverse components of potential are equal but in opposite direction. In Fig. 5 the transverse components of potential are equal and in the same direction and in Fig. 6 they are unequal depending upon the position of AE in reference to symmetry about the line C1C2 and the distance from Po. After a conducting materialv AB has been identied as for example in Fig. 6, or Fig. 4, the point P is adjusted until an arrangement similar to Figure is found, in which P0P: and P0P4 are, nearly equal in value indicating that Po is substantially electrically centered over the vein (AB). By then varying the azimuth of C102 relative to AB, the two positions of zero indication when AB lies parallel or normal to Ci-Cz (giving a diagram slmrilarto Fig. 2) serve to 'further delineate the structure.

Fig. 8 shows a hypothetical cross-section of the earth in which thematerial AB sought for is dipping to the left. The isotropic overburden D covers this material. Adjacent to AB is material C which differs materially from the conductivity of AB. The stream lilies of current E are indicated on the top of the ground on which measurements are made at the spaced electrical configuration of distances a and a.

Referring again to Figs. 4 lt will be -observed that the direction of vectors will reverselfA-B'lies on the other side of Po, or if the polarities of CiCz are reversed; from Fig. 5, it will be apparent that if the position of strike is displaced upwardly toward P3, parallel to the direction shown, the vector directions of Pa, P4 will remain the same, but their values will become unequal; and from Figs. 4, 5 and 6 it is apparent that when, and only when both P3 and P4 are on the same side of the position of strike, will the vector directions be opposed.

Furthermore',"if the configuration, relative to anomaly AB, lig.V 5, is turned about P0 as a center, the values between PoPs and PuPi will gradually decrease, being zero when AB coincides with eitherrPaP4 or C1Cz, and the vector directions shown willreverse from quadrant to quadrant, the vector values being equal to one another so long as Po is electrically centered over AB.

From the foregoing analysis it will be appreciated that the similarity, or opposition, of the vector directions relative to each other, their relative values and their absolute directions and values, in various combinations, will evidence denite position of strike and dip.

In the foregoing only the ohmic resistivity or direct. current impedisivity was considered; in the alternating current case as explained in my above mentioned copending application, the impedisivity involves not only a real part corresponding to ohmic resistivity but also an imaginary part com..

, posed of a time vector operator which is a function of frequency, dielectric constant, and permeability of the material. A similar analysis is made in the alternating current case, using transverse impedisivity instead of resistivity, the phase-shift, or time vector operator, due to the dielectric and magnetic permeability constants of the structure, being determined by means .of a phase shifter as explained in my said copending application.

Y In the method of making ground observations exemplied in Fig. 7, both direct current and alternating current may be applied to the same ground contacts and the alternating characteristics of the ground compared to the direct current permitting vdiierentiations of asymmetric material based on direct measurements between PoPi and P0P: and transverse measurements between PoPa and P0P4.

Thus the vector magnitude, in either the direct current or fluctuating current case, indicates and enables determination of the position of the underlying strata and aids in identifying the composition of the material; and the phase shift, in the fluctuating current case, gives further evidence of the composition of the materials of the structure. It will be appreciated by those skilled in the art that a uctuating current may be an alternating current of simple sinusoidal or other continuous or discontinuous form, alone, or suvperposed on a direct current component, as may 4, 5 and 6: from Fig;`

- fluctuating current cases a best suit the conditions of use, although for most generally sinusoidally varied alternating current is preferred.

To obtain a complete identification of isotropic and anisotropic material the potential electrodes P1 and Pn,

with my Patent No. 1,951,760-are employed as well as the transversely positioned electrodes Pa-Po--P4, the symmetry of which entirely eliminates normal impedisivlty, thus giving evidence oi' the transverse resistivity uninuenced by the isotropic factors,

By using both Pi-Pn and Pa-P4 with the same Ci--Po-4-Cz configuration, it is possible for the rst time to obtain complete geophysical evidence giving knowledge of the character and depth of isotropic overburden and contact with and position of anisotropic stratified and banded underlying material.

To summarize, by establishing a configuration exemplified in Figs. 1 and 8; (1) with relatively short distances values a, a1, conditions at relatively shallow depths can be determined. These will usually be isotropic, as where a homogeneous overburden overlies the rock formations, zero values being determined for the transverse impedisivities between Po and Pa and between Po and P4, the character of isotropic material being indicated by values appearing between P1 and Po and P2 and Po.

(2) By increasing the distances a and al, greater depth of survey is accomplished, and when repetitions result in the appearance of values between Po and Pa or between Po and P4, or both, then it is indicated that contact has been established with anisotropic material at the depth of survey functionally corresponding to the a, a1 distances in use.

(3) By then varying the azimuthal direction of line Ci-Cz the direction of strike may be determined as above explained.

Y (4) At this same time the-relative values appearing between Po and P1, and between Po and l Pz evidence the direction andangle of dip, if

gentle enough to lie within the range of both observations, without repeating the set-up of the configuration at another location, as would be necessary if only Po, Pa, P4 were used.

(5) For steep dips, not within the range of Po-Pi, and Po-Pz observations, an accurate indication can be obtained by moving the configuration longitudinally in the direction of dip indicated by item 3, above, and repeating items 1 and 2 in the new location to determine the depth of the anisotropic structure thereat.

(6) During items 1 and 2, and before the orientation of item 3 is reached, the phase-shift factor of `impedisivity determined by applying fiuctuating current to the configuration, serves to give evidence of the character of the anisotropic material.

(7) Also, at any time evidence lof the asymmetric locations of sink-holes, faults, dikes, etc., with reference to the partitioning plane along Ci-Czmay be determined by the directions and values of the potentials appearing between Pu and Pa or Po and P4 or both; and at the same time evidence of the locations thereof relative to the plane along P3-P4 may be determined by the relative values of potentials appearing between Po and P1, or Po and Pz. Thus the anomalies are seggregated into quadrants with only a single set up of the configuration.

(8) By repeated set-ups and measuring of the preferably with electrode Po-giving data as to the normal impedisivity, in accordance change in values and direction of the potentials of the transverse system at various values of a and a1, it is possible to determine the change of strike through geological unconformities. 'Ihus if one system of bedding having a definite `strike and dip overlies another system of bedding having a different strike and dip, or stratified in another direction, this may be determined.

While it will be apparent from the foregoing description vthat various arrangements may be used for applying and measuring the configuration currents and potentials, Fig. '7 shows for purposes of illustration, the manner of employing the arrangement disclosed in my copending application for "Electrical resistivity or impedisivity measuring, Ser. No. for this purpose. Y.

As shown in Fig. 7, the resistivity or impedislvity measuring device comprises threeunits, corresponding to the three units of my said application disclosure. The upper, or current-input unit is connected to apply a predetermined direct or fluctuating current to the current electrodes C102. The lower, or potential measuring unit is arranged to be connected to measure either the normal impedisivity between PiPn and PaPo, or the transverse impedisivity between PaPo and P4Po, preferably by suitable switching means, as the triple-pole double throw switch shown; and, as fully discussed in my above-mentioned application, preferably makes these measurements by balancing` out the picked up potentials to prevent flow of current through the potential electrodes,

200,948, filed April 8, 1938,

thus avoiding the disturbances of the electric field distribution in the earth which are always produced when currents are drawn from the potential electrodes, and enabling the ground resistivitles or impedisivities to be measured by the relative magnitudes and signs of the balancing potentials. The center unit, as in my copending application, is the calibrating unit enabling field calibration of the potential measuring unit directly from the current input unit.

This device, as more fully explained in said co pending application, Where its novelty is claimed per se', enables determination of the impedisivities both in the normal and in the transverse directions as regards quantity and sign, which is evidenced as direction in the direct current case, and as phase shift in the fluctuating current case, and is to be so interpreted in the appended claims.

Furthermore, where certain of the claims, for brevity, employ the term "Lee configuration" this term designates the configuration shown in Figs. 1 and 8 of the drawings; Where they employ the term direct impedisivity, this designates the values appearing between Po-l-Pi and between Pri-Pz; while the term transverse impedisivity designates the values appearing between Po-Ps and Pia-P4; the term depth determining dimensions designates the size of the configuration as determined by the dimensions a, a1; and the term point of origin or origin designates the station point Po. l

While I have described preferred embodiments of the several cooperating features of my invention, it is to be understood that these embodiments are but illustrative, and not restrictive of my invention.

I claim as my invention:

1. A method in geophysical surveying, comprising the steps of (a) establishing a flow of electric current through the earth between two points, (b) measuring potentials from a third point intermediate the first two points to other points'on a line transverse to the line connecting the first two points, (c) determining the relative magnitude and sign of the transversely measured potentials which reflect and thus indicate the presence and location of striise of subterranean strata, and (d) repeating steps a, b and c in varis ous orientations of current ow near the position at which a subterranean strike is indicated, until equality of magnitude, with opposite sign if magnitude is other than zero, establishes accurately the direction of strike.

2. A method comprising steps a, t and c oi claim l, in which the relation of current to electrode spacing is maintained in a predetermined ratio throughout a seris of determinations.

3. A method comprising steps a, b and c of claim l, which includes further determining slope of the underlying strata by taking further potential measurements from the intermediate point to further points on the line of the irst two points, and in which the relation of current to electrode spacing is maintained in a predetermined ratio throughout a series of determinations.

4. In geophysical surveying, the step of determining factors of position of an underlying structure by applying current to the terrain at the ends of a base line and measuring the impedisivity of the terrain transverse to the approximate center eof said base line.

5. In geophysical surveyingl the step of determining factors of position of an underlying structure by applying current to the. terrain at the ends of a base line and measuring the impedisivity of the terrain symmetrically transverse to the approximate center of said base line.

6. In geophysical surveying, the step.of determining factors of position of an underlying structure by applying current to the terrain at the ends of a base line and measuring the impedisiv ity of the terrain at right angles to the approximate center of said base line.

7. The method of determining the presence and azimuthal direction of strike of a subter ranean strata in geophysical surveying which comprises selecting a. point of origin for the survey, causing a current to flow through the earth from points on a substantially straight line passing through the point of origin, and measuring the transverse impedisivity between the origin and points spaced transversely on opposite sides thereof, and determining from the said measured impedisivities the approximate position and direction of strike of the subterranean strata..

8. The method of determining the presence and azimuthal direction of strike of a subterranean strata in geophysical surveying which comprises selecting a point of origin for the survey, causing a current to now through the earth from points on a substantially'straght line passing through the point of vorigin and equally spaced therefrom, and measuring the transverse impedisivity between the originV and points equally spaced transversely on opposite sides thereof on a line at right angles to the first straight line, and determining from the said measured impedisivities the approximate position and Idirection of strike of the subterranean strata.

9. A method according to claim 8, which fury ther includes determining accuratelyV the said azimuthal direction of strike by repeating the steps of claim 8 at locations on lines substantially normal to the approximate direction' of strike until substantially equal and opposite transverse impedisivity measurements are obtained.

the

audace i0. The method of determining, with a sevenelectrode configuration applied to the terrain, the depth of anisotropic material in geophysical surveyng; which comprises employing one electrode as a point of origin, locating four other electrodes along a base line passing through said point of origin, connecting two of said other electrodes to apply electrical energy to said terrain and the other two to pick up potentials from said terrain which reflect and thus indicate the impedisivity of the terrain embraced in said. configuration, locating two electrodes at opposite sides of said base line along a. line transverse to said base line and connecting said last two electrodes to pick up potentials from said terrain which reflect and thus indicate the transverse impedisivity of the terrain embraced 1n said configuration, and repeatedly performing said steps with different depth determining dimensions of the configuration applied to the terrain to determine the depth at which indications of transverse impedisivity appear.

ll. The method of determining, with a flveelectrode connguration applied to the terrain, the direction of strike of anisotropic material in geophysical surveying; which comprises employing one electrode as a point of origin, locating two other electrodes along a baseline passing through said point of origin, connecting said other electrodes to apply electrical energy to said terrain, locating two electrodes at opposite sides of said base line along a line transverse to said base line and connecting said last two electrodes to pick up potentials from said terrain which reect and thus indicate the transverse impedisivity of the terrain embraced in said configuration, and varying the azimuthal orientation of said conguration about its point of origin while maintaining depth determining dimensions of said conguration such that indications of transverse impedisivity are obtained.

l2. The method of determining the direction and dip of gently sloping anisotropic material in geophysical surveying; which comprises establishing by the method of claim 11 the azimuthal orientation of the configuration therein deiined corresponding to symmetry with respect to the direction of strike, and picking up potentials aligned with the base line of said conguration which reflect and determine the direct impedisivities of the coniiguration so oriented as an indication of the direction and angle of dip.

13. The improved conguration for geophysical surveying comprising a point of origin Po, current electrodes C102 positioned on a first line passing through said point of origin and on opposite sides of and substantially equidistant from said point of origin, and means for determining transverse impedisivities comprising potential electrodes P3P.; positioned at opposite sides of the point of origin on a line passing through said point of origin and transverse to said first line.

14. The improved configuration for geophysical surveying comprising a. point of origin P0, current electrodes CiCz positioned on a first line passing through said point of origin, and means for determining direct and transverse impedisivities comprising potential electrodes P1P: positioned at opposite sides of the point of origin on said rst line and other potential electrodes PaP4 positioned at opposite sides of the point of origin on a line passing through said point of origin and transverse to said rst line.

15. A method in geophysical surveying. which comprises establishing a flow of electric current through the earth between two points, picking up potentials from a third point intermediate the rst two to other points on a line transverse to the line connecting the first two points, balancing out the picked up potentials to prevent disturbance of the electric ield established in the earth by the flow of current between said ilrst two points, and determining the relative magnitude and sign of the balancing potentials which reilect and thus indicate the presence and location of strike of subterranean strata.

16.I A method in geophysical surveying, which comprises establishing rent through the earth between two points, picking up potentials from a third point intermediate the rst two to other points on a. line transverse to the line connecting the rst two points, balancing out the picked up potentials to prevent disturbance of the electric ield established in the earth by the flow of current between said iirst two points. and determining the relative magmtude and phase of the balancing potentials which reect and thus indicate the presence and location of strike of subterranean strata.

17. A method in geophysical surveying, which comprises establishing a iiow of direct current through the earth between two points, picking up potentials from a lthird point intermediate the rst two to other points on a line transverse to the line connecting the nrst two points. balancing out the picked up potentials to prevent disturbance pf the electric iieid established in the earth by the flow of current between said rst two points, and determining the 'relative magnitude and direction of the balancing potentials which reiect and thus indicate the presence and location of strike of subterranean strata.

a ilow of alternating cur- 18. In geophysical surveying, the method which comprises applying to the terrain a seven-electrode coniiguration comprising a station electrode Po, two current electrodes C1 C2 equidistant from the station electrode Po and lying on a base line passing through the station electrode P0, two normal-potential electrodes P1 and P2 equidistant from the station electrode Po and also lying on the said base line, and two transverse potential electrodes P3 P4 lying on a line at right angles to said base line and passingthrough said station electrode, orienting said conguration in such direction that no potential diierence exists between the transverse potentials obtained at the transverse potential measuring electrodes P3 and P4, thereby positioning the current base line CiCz of the configuration in alignment with the direction of slope of underlying strata, and measuring the potential differences between the normalpotential measuring electrode pairs P0P; and which reect and thus indicate, in such orientation, the true direction of slope of the underlying strata.

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