Telluric sounding method in which recorded variations in the telluric field with time are converted into amplitude-versus-frequency spectra

Claim 1

IN A METHOD FOR THE GEOPHYSICAL EXPLORATION OF THE UNDERGROUND IN A GIVEN AREA, THE STEPS OF: (A) AT A FIRST STATION WHERE THE ELECTRICAL RESISTIVITY VARIATION WITH DEPTH OF SUBSURFACE FORMATIONS IS KNOWN, MEASURING AND RECORDING OVER A GIVEN PERIOD OF TIME VALUES REPRESENTING VARIATIONS OCCURRING IN AT LEAST A COMPONENT OF THE ELECTRIC TELLURIC FIELD AT SAID FIRST STATION, (B) OVER THE SAME GIVEN PERIOD OF TIME AT A SECOND STATION HAVING A KNOWN LOCATION WITH RESPECT TO SAID FIRST STATION, MEASURING AND RECORDING VALUES REPRESTING VARIATIONS IN AT LEAST THE SAME COMPONENT OF THE TELLURIC ELECTRIC FIELD AT SAID SECOND STATION, (C) ANALYZING SAID RECORDED VALUES TO DERIVE AMPLITUDE-VERSUS-FREQUENCY SPECTRA AT EACH OF SAID STATIONS DURING SAID PERIOD OF TIME, (D) AND COMPARING SELECTED PEAKS REPRESENTING THE SAME FREQUENCY IN SAID DERIVED FREQUENCY SPECTRA FROM SAID FIRST AND SECOND STATIONS TO RELATE SAID KNOWN ELECTRICAL RESISTIVITY VARIATION WITH DEPTH AT SAID FIRST STATION TO PREDICT THE ELECTRICAL RESISTIVITY VARIATION WITH DEPTH AT SAID SECOND STATION.