In generating seismic waves in earth a generator having a generally elongate, tubular expandable and contractible outer wall portion closed at its upper and lower ends by rigid strength members having separate input and return pressure lines is placed in a well bore, a hydraulic coupling pressure is...http://www.google.ca/patents/US4252210?utm_source=gb-gplus-sharePatent US4252210 - Methods of and apparatus for generating seismic waves
