Compagnie Generale De Geophysique

Jean-Pierre C Rocroi

Francois V Gole

Andre Cecchini

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. As used herein, 'numbered' may be taken to mean digitalized and 'registered' may be taken to mean recorded or memorized. 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 samples or elements of the numerical registration or digital record which correspond temporally to the successive time intervals, followed by a smoothing digital filtering which provides a smoothed numerical or digital representation of the transient phenomenon, cleared of a substantial part of the surrounding noise.

Method for geophysical prospecting using impulsine currents

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.

Method of geophysical prospection using transient currents

The invention concerns electrical prospecting of the subsoil. According to the invention, electrical prospecting is performed with either continuous or alternating current by using a drilling tubing as at leasts one of two lineal exciter electrodes. The profile of apparent resistivity thus obtained gives a very definite indication of the presence of deep anomalies, even those situated under high resistivity layers or masks, which is frequently the case in the search for deposits. Comparing the real resistivity profile with the theoretical curve obtained from a preliminary model, the extent of the deposit can be determined. The invention applies principally to petroleum, mineral or geothermal exploration.

Electric prospecting of the subsoil with lineal electrodes

In a sub-soil prospecting process a vertical magnetic field transmitter dipole and a receiver dipole are placed on the ground, away from each other. The receiver dipole is oriented to detect a radial horizontal magnetic field relative to the transmitter dipole. At low frequencies, the magnetic field detected assumes asymptotic values, and by measuring these values an apparent alternating current resistivity of the sub-soil is deduced. By varying the transmitter-receiver distance, and by simultaneously using a direct current electric prospecting process, a sub-soil model is determined which is less ambiguous than those supplied by electric or induction prospecting alone.

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Massy, France

Jean-Pierre C Rocroi