Method and means for investigating the distribution of electrical conductivity in the ground

• US 3,936,728 A
• Filed: 11/29/1973
• Issued: 02/03/1976
• Est. Priority Date: 11/29/1973
• Status: Expired due to Term

• Current Assignee: Federal Business Development Bank
• Original Assignee: Mcphar Geophysics Ltd.
• Inventors: Ghosh, Mrinal K., Hallof, Philip G.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US05/420,019
• Time in Patent Office: 796 Days
• Field of Search: 324/3, 324/4, 324/6
• US Class Current: 324/335
• CPC Class Codes: G01V 3/08 operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices with electromagnetic waves G01V3/12

Abstract

The invention relates to the obtaining of highly diagnostic information concerning the distribution of electrical conductivity (resistivity) in the ground between two points by creating an electromagnetic field whose frequency can be varied over a desired range at one of the points and detecting and comparing at the other of the points, at a plurality of transmitted frequencies, components of the magnetic and/or electric field which are in phase and out of phase with a reference signal derived electromagnetically at the second point from the transmitted field whereby no cable or other physical connection between the two points are required. The invention also relates to apparatus by means of which the electromagnetic field is created, the reference signal electromagnetically derived and the in phase and out of phase magnetic and/or electric field components detected.

Claims

1. A method of investigating the distribution of electrical conductivity in the ground for the purpose of delineating geophysical structures comprising creating an electromagnetic field at a transmitting point and measuring at a receiving point at a finite distance from said transmitter point at a series of frequencies within the frequency range of from about 5 cycles per second to about 43,000 cycles per second one or more of the magnetic induction field and the electric induction field components created by said electromagnetic field measured both in phase and out of phase with a reference signal derived electromagnetically, adjacent the receiving point, from the created electromagnetic field, and having a random phase relation thereto such measurements providing a source of information for delineating any discontinuities in the electrical conductivity in the ground.

2. A method as claim in claim 1 in which the measured component at the receiving point is the magnetic field detected by a receiver coil having minimum coupling with the created electromagnetic field measured both in phase and in quadrature with the derived reference signal

3. A method as claimed in claim 1 in which measurements are made at the receiving point of the magnetic field detected by a receiver coil having

a. minimum coupling, and

b. maximum coupling with the created electromagnetic field measured both in phase and in quadrature phase with the derived reference signal.

4. A method as claimed in claim 3 in which measurement is also made of the electric field at the component at the receiver point orthogonal to a line joining the transmitting and receiving points measured both in phase and in quadrature phase with the derived reference signal.

10. A method as claimed in claim 1 in which said reference signal is derived from an auxiliary coil located adjacent to said receiving point.

11. A method as claimed in claim 1 in which said reference signal is derived from the source of measurement of the orthogonal electric field.

5. A method of investigating the distribution of electrical conductivity in the ground for the purpose of delineating geophysical structures comprising creating an electromagnetic field at a transmitting point and measuring at a receiving point at a plurality of difference frequencies within the range of from about 5 cycles per second to 43,000 cycles per second, the components both in phase and out of phase with a reference signal having a random phase relation to and derived from the created electromagnetic field adjacent said receiving point, of the magnetic induction field detected by a receiver coil arranged at the receiving point

a. for minimum coupling, and

b. for maximum coupling with the created electromagnetic field, and of the electric induction field at the receiving point orthogonal to a line joining the transmitting and receiving points, such measurements providing a source of information for delineating any discontinuities in the electrical conductivity in the ground.

6. A method as claimed in claim 5 in which the amplitude ratio and the phase difference of at least one of

a. the magnetic field detected by minimum receiver coil coupling and the magnetic field detected by maximum receiver coil coupling, and

b. the magnetic field detected by minimum receiver coil coupling and the orthogonal electrical field,

are compared at the plurality of frequencies at which said measurements are carried out to determine and discontinuity in the electrical conductivity in the ground.

7. A method as claimed in claim 6 in which the amplitude ratio and phase difference of both (a) and (b) are determined and compared.

8. A method as claimed in claim 6 in which said created electromagnetic field is created by energizing a transmitter coil having its turns lying generally in a horizontal plane and its axis generally vertical and said receiver coil is oriented with its axis coincidental with the line joining the transmitting and receiving points for minimum coupling and with its axis perpendicular to said line and parallel to the transmitter coil axis for maximum coupling.

9. A method as claimed in claim 6 in which said transmitter coil is oriented with its turns in a vertical plane and said receiver coil, for maximum coupling, is oriented either with its axis parallel to the transmitter coil axis or with its turns lying in the plane of a line aligned with said transmitter coil axis and joining the transmitting receiving points and its axis vertical, and said receiver coil, for minimum coupling, is oriented either with its axis coaxial with said transmitter coil axis or with its turns lying in a plane containing a line joining the transmitter and receiving points perpendicular to the transmitter coil axis and with its axis perpendicular to said line.

12. Apparatus for use in the investigation of the distribution of electrical conductivity in the ground comprising a transmitter coil, a transmitter power source for energizing said transmitter coil at a plurality of frequencies within a frequency range of from about 5 to about 43,000 cycles per second, a receiver coil adapted to be oriented at a receiving point for maximum and minimum coupling with said transmitter coil, cable means adapted to be strung at said receiving point orthogonally across a line joining the center of said transmitting coil and said receiving point and having means for grounding the ends thereof, and means for delivering to said receiver unit a reference signal derived electromagnetically adjacent to said receiving unit, and having a random phase relation to the transmitter coil energization, said receiver unit being adapted to measure the components of the magnetic induction field detected by said receiver coil when oriented for maximum and minimum coupling which are in phase and out of phase with said reference signal, and the components of said orthogonal electric induction field which are in phase and out of phase with said reference signal.

13. Apparatus as claimed in claim 12 in which said means for delivering said reference signal to said receiver unit comprises a connection from said cable means to said receiver unit.

14. Apparatus as claimed in claim 12 in which said means for delivering said reference signal to said receiver unit comprises an auxiliary coil for location adjacent said receiving point and connection from said auxiliary coil to said receiver unit.

15. Apparatus as claimed in claim 12 having means for tuning said transmitter coil to transmit a selected plurality of frequencies and means to correspondingly tune said receiver coil and cable means to detect the transmitted magnetic and electric field components respectively at said plurality of frequencies.