Patent GB-624436-A (GB)
Title: Improvements in or relating to methods and apparatus for earth-testing
IPC: G01V3/04
Filed: 1946-06-18
Abstract: 624,436. Prospecting for minerals. GEOPHYSICAL PROSPECTING CO., Ltd., O'SULLIVAN, B. J. C., and TEMPLETON, J. C. June 18, 1946, No. 18302. [Class 37] Apparatus for measuring the resistivity of a mass comprises at least two current electrodes adapted to make two separated connections with the mass, a source of power connected to these electrodes, means for adjusting and measuring the current passing through the electrodes, and a pair of electrodes adapted to make connection with the mass at selected pairs of points, and means for measuring the potential difference between them. In the arrangement shown in Fig. 1, for geophysical measurements in which the apparent ground or specific resistance is measured at any desired depth, two current electrodes C1, C2 are spaced apart from the station S and inserted into the ground. Two potential electrodes P1, P2 are inserted into the ground in the straight line joining C1, C2, the distances C1, P1 ; P1, P2 and P2, C2 being equal. The apparent ground resistivity at a depth a is then =2#aV/I, where V is the potential difference between the electrodes P1, P2 and I is the current flowing between the electrodes C1, C2. This current is supplied by a generator G through a rheostat R and ammeter A. The current I is adjusted so as to be proportional to the distance a so that a/I is constant and the voltmeter V may be graduated in terms of apparent resistivity. In a modification, Fig. 2, the potential electrodes P3, P4 are arranged between the current electrodes C3, C4 at a small constant distance m apart and the distance between the electrodes C3, C4 being more than twice the mean distance b of the electrodes P3, P4 from the nearer current electrode C3. The apparent resistivity is then =2#b<SP>2</SP>V/I. The current I is made proportional to b<SP>2</SP>, so that the voltage V indicates the resistivity directly. The current applied to the electrodes C1, C2 may be alternating current of low frequency, say from 10 to a few hundred cycles per second, but is preferably a squaretopped wave as obtained by rapid reversal or interruption of a D.C. supply. An apparatus for effecting this is shown in Fig. 3 in which the current electrodes C5, C6 are supplied from a generator G through a commutator 12 and the potential electrodes P5, P6 are connected to the voltmeter 18 through a commutator 17. The two commutators 12, 17 are mounted on the same shaft 21 driven by a motor 22, the generator G being driven either from the same motor or from a separate motor 19. The generator G may give 500 volts and current up to half an ampere. The commutator 17 permits the use of a D.C. voltmeter 18. The connections to the commutator 12 may alternatively be so arranged that a D.C. ammeter 16 may be employed. Instead of the commutator 17 a dry metal rectifier or an electronic valve may be employed. An A.C. generator may be directly connected to the current electrodes C5, C6 or to a rotary convertor or rotary transformer. A hand-driven generator may be employed and may be arranged to provide a constant voltage. The commutators may be hand-driven and provided with speedgoverning control.