Method of geophysical prospection using transient currents

Abstract

In a method of geophysical prospection a transmitter dipole and a receiver dipole which is sensitive to an electromagnetic field are positioned in cooperation with the subsoil. An impulsive excitation current is applied repeatedly to the transmitter dipole, the excitation current comprising a group of pulses of alternate polarity, each followed by zero current. The transient signal, which is produced thereby at the output of the receiver dipole, is repeatedly detected, and the transient signals are processed to obtain a figurative representation of the transient phenomenon which facilitates geophysical interpretation. Time intervals which are consecutive to excitation pulses and simultaneously with the excitation zero current are defined, these time intervals being of a predetermined duration, chosen to substantially cover all the useful parts of the transient phenomenon. The transient signal is detected as a function of time at least during the time intervals and is sampled, numbered and registered, temporally marked. A processing operation is carried out which commences with a preprocessing in which an unprocessed numerical representation of the transient phenomenon as a function of time is subjected to statistical evaluation of elements of the numerical registration which correspond temporally to the successive time intervals, followed by a smoothing digital filtering which provides a smoothed numerical representation of the transient phenomenon, cleared of a substantial part of the surrounding noise.