Method for locating discontinuities in the electrical conductivity of the sub-soil using a plurality of magnetic detectors in a predetermined spatial arrangement

• US 4,095,169 A
• Filed: 03/09/1976
• Issued: 06/13/1978
• Est. Priority Date: 04/07/1975
• Status: Expired due to Term

• Current Assignee: General Electro-Magnetic Prospecting Incorporated
• Original Assignee: General Electro-Magnetic Prospecting Incorporated
• Inventors: Muse, Louis Marcel
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US05/665,212
• Time in Patent Office: 826 Days
• Field of Search: 324/3, 324/4, 324/8, 324/43 R
• US Class Current: 324/345
• CPC Class Codes: G01V 3/082 operating with fields produced by spontaneous potentials, e.g. electrochemical or produced by telluric currents G01V3/26 takes precedence

Abstract

A method and apparatus for investigating the electro-magnetic characteristics of the sub-soil. At least four and normally five directional magnetic field detectors placed on the ground. A first set of two detectors is spaced apart and oriented in the same general direction, a second set of two detectors is also spaced apart and oriented in a second direction transverse to the first direction, and the fifth detector is positioned on the ground transversely to the first four detectors. The output signals generated by the detectors, which are characteristic of portions of the sub-soil underlying the detectors, are simultaneously recorded for subsequent analysis to identify sub-soil characteristics, discontinuities, mineral deposits and the like.

Claims

1. A method for obtaining an indication of the electro-magnetic characteristics of the sub-soil underlying a ground surface comprising the steps of:

a. placing on the ground in a substantially horizontal plane at least four directional magnetic field detectors in two sets wherein each set comprises a spaced pair of detectors oriented in the same direction with the detectors of each set oriented at right angles to the detectors of the other set;

b. simultaneously monitoring the output signal from each of said detectors over a period of time, each of said output signals being made up of a plurality of spectral components;

c. electronically processing said output signals by characterizing each of said spectral components as being of the form H = H_O e t/τ

where t is time and τ

is a time-constant to derive for each of said spectral components in each of said output signals a magnetic field amplitude value H_O ;

d. combining amplitude values corresponding to the same spectral component from each of said output signals to obtain a non-variable physical parameter associated with said spectral component which is related to the electrical conductivity of the sub-soil beneath the detectors and is independent of any phase shift existing between the output signals from the pair of detectors in each of said sets.

2. The method of claim 1 comprising the further steps of deriving a plurality of additional non-variable physical parameters each being associated with a different one of said spectral components;

and evaluating the group of parameters so obtained to determine the geophysical structure of the sub-soil beneath said detectors.

3. The method of claim 1 wherein said parameter reflects the difference between the amplitude values corresponding to the two detectors in each set.

4. The method of claim 1 wherein said detectors are deployed in a rectangular arrangement with the detectors of each pair lying along opposite sides of a rectangle.

5. The method of claim 4 wherein the detectors are arranged in a square pattern with the detectors of each pair lying along opposite sides of a square and the distance between the detectors of each pair is the same.

6. The process of claim 1 comprising the further steps of placing on the ground a fifth detector oriented in a substantially vertical direction;

monitoring the output signal therefrom and utilizing said output signal in the derivation of said physical parameter.

7. A process according to claim 1 wherein the detectors have substantially identical response curves.

8. A process according to claim 1 wherein the detectors are induction detectors having a very large number of thin wire turns.

9. A process according to claim 8 comprising the further step of utilizing additional elements such as condensers to create resonance effects which are favorable to an increase in sensitivity in one rea of the spectrum.

10. A process according to claim 9 including the step of placing a detector into a closed container of metal having a high magnetic permeability to enable the zero calibration of instruments connected with the detector.

11. A method for obtaining an indication of electro-magnetic characteristics of a sub-soil underlying a ground surface comprising the steps of:

a. defining with respect to a reference point on the ground surface three mutually perpendicular X, Y and Z directions, directions X and Y being in a generally horizontal plane and direction Z being generally transverse to the ground surface;

b. providing at least four directional magnetic field detectors;

c. positioning a first and a second detector relative to the ground surface area and orienting said first and second detectors in X and Y directions, respectively;

d. positioning a third and a fourth detector relative to the ground surface area in spaced-apart and substantially parallel relationship with respect to the first and second detectors, respectively, and orienting the third and fourth detectors in substantially the same direction as the first and second detectors, respectively;

e. generating with each of the detectors output signals responsive to a magnetic field of the sub-soil underlying the detector, each of said output signals further comprising a plurality of spectral components;

f. electronically processing each of said output signals by characterizing each of said spectral components as being of real increasing exponential form to derive for each of said spectral components of each of said output signals an amplitude value;

g. combining amplitude values associated with corresponding spectral components from each of said output signals to produce a non-variable physical parameter which is indicative of the electrical conductivity of the sub-soil beneath said detectors.

12. A method according to claim 11 including the step of positioning at least a fifth detector relative to the ground surface area and orienting the axis of the fifth detector substantially parallel to direction Z.

13. A method according to claim 12 wherein the step of positioning the fifth connector comprises the step of positioning the fifth detector between the first, second, third and fourth detectors.

14. A method according to claim 13 wherein the step of positioning the fifth detector comprises the step of substantially centering the fifth detector with respect to the first through the fourth detectors.

15. A method according to claim 11 wherein the step of positioning the detectors includes the step of positioning the detectors substantially equi-distantly from the reference point.