Electromagnetic subsoil prospecting process using an asymptotic low frequency range
AssigneeCompagnie Generale de Geophysique
InventorsJean-Pierre Rocroi, Andre Cecchini
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Abstract
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.
Claims32
A sub-soil prospecting method, comprising the steps of:
- (a) placing above ground a magnetic transmitter dipole and a magnetic receiver dipole spaced at a distance from each other in a prospecting area;
- (b) energizing the transmitter dipole with alternating current of frequency below a predetermined low frequency;
- (c) sensing at an output of the receiver dipole a signal representing a detected magnetic field, said predetermined low frequency being a maximum frequency of an asymptotic range in which the magnetic field represented by the receiver dipole output signal is linearly related to the energizing frequency;
- and
- determining a quantity related to the ratio of the magnitude of said sensed signal to the value of said energizing frequency.
A sub-soil prospecting method, comprising the steps of:
- (a) placing a magnetic transmitter dipole and a magnetic receiver dipole at respective locations above ground spaced at a distance from each other in a prospecting area;
- (b) energizing the magnetic transmitter dipole with a low frequency alternating current;
- (c) sensing at an output of the magnetic receiver dipole a signal representing a detected magnetic field;
- and
- (d) repeating steps (b) and (c) while varying the frequency of said alternating current until reaching a low-frequency asymptotic range in which the detected magnetic field represented by the receiver dipole output signal is substantially linearly related to the alternating current energizing frequency;
- and
- for each repetition of steps (b) and (c), determining a quantity related to the ratio of the magnitude of said sensed signal to the value of said energizing frequency.
Dependencies are inferred from the claim text.