Method and apparatus for simultaneously collecting spatially and temporally varying magnetic survey data

• US 5,444,374 A
• Filed: 02/05/1993
• Issued: 08/22/1995
• Est. Priority Date: 06/06/1990
• Status: Expired due to Term

• Current Assignee: Gap Geophysics Pty., Limited
• Original Assignee: University of New England
• Inventors: Lee, Stephen J., Cattach, Malcolm K., Stanley, John
• Primary Examiner(s): O'Shea, Sandra L.
• Assistant Examiner(s): PATIDAR, JAY M
• Application Number:
  US07/955,875
• Time in Patent Office: 928 Days
• Field of Search: 324/334, 324/330, 324/331, 324/335, 324/345, 324/350, 324/359
• US Class Current: 324/334
• 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
• CPC Class Codes: G01V 3/15 specially adapted for use during transport, e.g. by a person, vehicle or boat

Abstract

A method and a device for implementing the method, for geophysical surveying of an area (1) including continuous measurement of the earth'"'"'s magnetic and electromagnetic field using a detector (6) held at a constant height (E) above the ground surface, in the order of one meter, while traversing in a criss-cross manner over the area (1) at speeds below a maximum speed (S), in the order of 10 km/h, and utilizing measured signal frequencies below a cutoff (F), being approximately the maximum speed (S) divided by twice the detector height (E), to indicate spatial variations in the measured field. Traversal distance can be measured on board the host vehicle and a real time output supplied. Simultaneously, using the same detector (6) and artificially induced magnetic fields generated by an inductive (2b and/or galvanic source (2a)), temporally varying fields having frequencies above the cutoff (F) can be measured and simply filtered out of the detector'"'"'s (6) output for separate processing and storage. Temporally varying fields below the cutoff (F) can be detected by a synchronized stationary detector inputting to the processing equipment (10) so as to subtract such signals from the signals used to measure spatial field variations.

Claims

1. A method for subterranean investigation wherein spatially varying magnetic and temporally varying electromagnetic fields being determined the method comprising the steps of:

traversing a survey area with a magnetic detector at a predetermined approximately constant detector height E at traversing speeds not exceeding a predetermined maximum speed S, thereby defining a cut off frequency F for signals representing said spatially varying magnetic field where

space="preserve" listing-type="equation">F=S/2E;

using stationary field generating means to generate an artificially induced magnetic field in said survey area, the induced field having a frequency greater than said cut off frequency; and

separating signals produced by the magnetic detector into two components, a first component comprising frequencies less than or equal to said cut off frequency and representing said spatially varying magnetic field, and a second component comprising frequencies greater than said cut off frequency resulting from the artificially induced temporally varying electromagnetic field.

2. The method of claim 1 wherein:

E is approximately 1 meter; and

S is approximately 2 meters per second.

3. The method of claim 1 additionally comprising the step of:

providing a stationary magnetic sensor measuring low frequency temporal changes in said fields, the stationary sensor having an output signal which is subtracted from the first signal component so as to significantly reduce or remove from the first signal component the effect of any low frequency temporal field changes that occur during the investigation.

4. The method of claim 1 wherein the first signal component is resampled at points corresponding to regular predetermined traversing distances which are approximately equal to, or less than, said detector height.

5. An apparatus for subterranean investigation comprising:

a vehicle for traversing a survey area at a controllable speed;

a mobile magnetic detector means on board the vehicle and held at a predetermined approximately constant detector height above ground level for producing a signal comprising a first signal component corresponding to spatially varying magnetic fields and a second signal component corresponding to temporally varying electromagnetic fields;

a stationary magnetic detector means arranged to produce a signal representing temporal magnetic and electromagnetic field changes; and

signal processing means arranged to receive the signal from the stationary magnetic detector means and the mobile magnetic detector means, the processing means being further arranged to subtract the signal produced by the stationary detector means from the signal produced by the mobile detector means, and to then separate said first and second signal components produced by the mobile detector means and to record each of these components, the first and second signal components being separated by a cut off frequency determined by a predetermined maximum traversing speed and said detector height.

6. The apparatus of claim 5 wherein the vehicle is adapted to be operated up to a maximum traversing speed S, and the high frequency cut off is adapted to be set at a frequency F such that

7. The apparatus of claim 6;

further including an on board device for measuring traversed distance, and for continually providing a traversed distance reference signal output to the signal processing means.

8. The apparatus of claim 6, further comprising on-board processing equipment for receiving the separated signals and said traversed distance signal and adapted to enhance, process and save the separated information in storage means in real time.

9. A method of processing survey data collected for subterranean investigation the survey data representing spatially varying magnetic and temporally varying electromagnetic fields which have been simultaneously recorded by traversing a survey area with a roving magnetic detector at a predetermined approximately constant detector height at traversing speeds not exceeding a predetermined maximum speed, thereby defining a cut off frequency for signals representing said spatially varying magnetic field, the processing method comprising the steps of:

providing supplementary data measured by a stationary magnetic sensor indicating temporal changes in said fields having frequencies below said cut off frequency;

subtracting the supplementary data from the signal representing the survey data so as to significantly reduce or remove from the signal representing the spatially varying magnetic field the effects of any temporal field changes that occur during the investigation;

separating a signal representing the survey data into two frequency components, a first component comprising frequencies less than or equal to a cut off frequency determined by the traversing speed and detector height and representing said spatially varying magnetic field and the second component comprising frequencies greater than said cut off frequency and representing the temporally varying electromagnetic field; and

separately processing each of the frequency components to determine the subterranean characteristics of the survey area.

10. The method of claim 9, wherein the cut off frequency F is defined by

11. The method of claim 10 wherein:

E is approximately 1 meter; and

S is approximately 2 meters per second.

12. The method of claim 10 wherein the first signal component is resampled at points representing regular predetermined traversing distances which are approximately equal to, or less than, said detector height.

13. An apparatus for subterranean investigation comprising:

a portable magnetic detector for outputting a signal comprising a first signal component having a frequency range primarily below a predetermined cut off frequency and a second signal component having a frequency range primarily above the predetermined cut off frequency; and

signal processing means arranged to receive the signal from the portable magnetic detector, the processing means being arranged to separate said first and second signal components and to record each component, the selected cut off frequency F being determined by a predetermined maximum traversing speed S and operating height E of the portable detector, wherein

space="preserve" listing-type="equation">F=S/2E, and

the first signal component representing a spatially varying component of the earth'"'"'s magnetic field and the second component resulting from a field artificially induced into a survey area and having a frequency greater than the cut off frequency.

14. The apparatus of claim 13 further including a traversed distance measuring means which continually provides a traversed distance reference signal output to the signal processing means.

15. The apparatus of claim 14 further comprising on-board processing equipment arranged to receive the separated first and second signal components and said traversed distance signal and to enhance, process and save the separated information in storage means in real time.

16. An apparatus for subterranean investigation comprising:

a vehicle for traversing a survey area at a controllable speed,

a mobile magnetic detector means on board the vehicle and held at a predetermined approximately constant detector height above ground level for producing a signal comprising a first signal component corresponding to spatially varying magnetic fields and a second signal component corresponding to temporally varying electromagnetic fields; and

signal processing means arranged to receive the signal from the mobile magnetic detector means, the processing means being further arranged to separate said first and second components and to record each component, the first signal component having an upper cut off frequency F determined by the vehicle speed S and detector height E where

space="preserve" listing-type="equation">F=S/2E

and the second signal component resulting from a field artificially induced into the survey area and having a frequency greater than the cut off frequency F.

17. The apparatus of claim 16 further comprising a stationary magnetic detector means for measuring temporal magnetic and electromagnetic field changes, and processing devices for subtracting the signals measured by the stationary detector from the first signal component.

18. The apparatus of claim 16, further including an on board device for measuring traversed distance, and for continually providing a traversed distance reference signal output to the processing means.

19. The apparatus of claim 16, further comprising on-board processing equipment for receiving the separated signals and said traversed distance signal and adapted to enhance, process and save the separated information in storage means in real time.

20. A method of processing survey data collected for subterranean investigation the survey data representing spatially varying magnetic and temporally varying electromagnetic fields which have been simultaneously recorded by traversing a survey area with a roving magnetic detector at a predetermined approximately constant detector height E at traversing speeds not exceeding a predetermined maximum speed S thereby defining a cut off frequency F for signals representing said spatially varying magnetic field, where F=S/2E, and where a stationary field generator being used to generate an artificially induced temporally varying electromagnetic field in the survey area, the induced field having a frequency greater than the cut off frequency, the processing method comprising the steps of:

separating a signal representing the survey data into two frequency components, a first component comprising frequencies less than or equal to the cut off frequency and representing said spatially varying magnetic field and the second component comprising frequencies greater than said cut off frequency and resulting from the artificially induced temporally varying electromagnetic field; and

separately processing each of the frequency components to determine the subterranean characteristics of the survey area.

21. The method of claim 20, wherein:

E is approximately 1 meter; and

S is approximately 2 meters per second.

22. The method of claim 20 comprising the additional steps of:

providing supplementary data measured by a stationary magnetic sensor, the supplementary data indicating temporal changes in said fields having frequencies below said cut off frequency; and

subtracting the supplementary data from the signal representing the survey data, so as to significantly reduce or remove from the signal data representing the spatially varying magnetic field and the effects of any temporal field changes that occur during the investigation.

23. The method of claim 20 wherein the first signal component is resampled at points representing regular predetermined traversing distances which are approximately equal to, or less than, said detector height.