Magnetic exploration with reduction of magnetic data to the equator

• US 4,570,122 A
• Filed: 03/30/1983
• Issued: 02/11/1986
• Est. Priority Date: 03/30/1983
• Status: Expired due to Fees

• Current Assignee: Mobil Oil Corporation (Exxon Mobil Corporation)
• Original Assignee: Mobil Oil Corporation (Exxon Mobil Corporation)
• Inventors: Leu, Lei-Kuang
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US06/480,192
• Time in Patent Office: 1,049 Days
• Field of Search: 324/331, 324/345
• US Class Current: 324/345
• CPC Class Codes: G01V 3/087 the earth magnetic field being modified by the objects or geological structures
• CPC Class Codes: G01V 3/40 specially adapted for measuring magnetic field characteristics of the earth

Abstract

A method of reducing magnetic data recorded at latitudes close to the magnetic equator to the equator is disclosed. The result corrects magnetic field anomalies recorded with respect to geologic anomalies to more closely reflect the shape of the geologic anomalies. In the preferred embodiment, reduction-to-the-equator is carried out in the wave number domain using an operator ##EQU1## .

Claims

1. The method of geophysical exploration of a magnetic anomaly caused by a geologic anomaly in the subsurface of the earth comprising:

measuring the magnetic intensity at spaced locations on the surface of the earth over said geologic anomaly;

correcting the measurements of magnetic intensity to values of magnetic intensity for locations on the magnetic equator of the earth; and

plotting the corrected measurements of magnetic intensity, representative of the shape of said geologic anomaly.

2. The method of claim 1 wherein said step of correcting the measurements of magnetic intensity to values of magnetic intensity for locations on the magnetic equator of the earth comprises the steps of:

selecting a transfer function for conversion of said measurements of said magnetic intensity to values which said measurements would have had if recorded with respect to a similar geologic anomaly located at the magnetic equator of the earth.

3. The method of claim 2 wherein said step of correcting the measurements of magnetic intensity is performed in the wave number domain, wherein said measurements are Fourier-transformed into wave number domain versions of said measurements, prior to application of said transfer function thereto.

4. The method of claim 2 wherein said transfer function H_E (u,v) is defined as:

##EQU14## where l=cos (I) cos (D)=L;

m=cos (I) sin (D)=M;

n=sin (I)=N;

s=(u² +v²)^1/2 ;

I=inclination of the earth'"'"'s magnetic field; and

D=declination of the earth'"'"'s magnetic field.

5. The method of reduction to the equator of anomalies in the magnetic field of the earth comprising the steps of:

measuring the intensity of the earth'"'"'s magnetic field at spaced locations in a rectangular array on the surface of the earth;

calculating Fourier-transformed versions of the measured values;

computation of a transfer function in the wave number domain calculated using values for the earth'"'"'s magnetic field at the locations at which said measurements are made;

application of said transfer function to said Fourier-transformed versions of said measurements; and

application of the inverse Fourier transform to the data having been operated upon according to said transfer function, whereby said measured values are corrected to reflect the actual shape of the geologic body producing said magnetic anomalies.

6. The method of claim 5 wherein the transfer function H_E (u,v) applied to said Fourier transformed data takes the form ##EQU15## where l=cos (I) cos (D)=L;

m=cos (I) sin (D)=M;

n=sin (I)=N;

s=(u² +v²)^1/2 ;

I=inclination of the earth'"'"'s magnetic field; and

D=declination of the earth'"'"'s magnetic field.

7. Process for exploration of the subsurface configuration of the earth comprising the steps of:

measuring the earth'"'"'s magnetic field at spaced locations on its surface;

examining the measured values for the earth'"'"'s magnetic field thus recorded to locate anomalies in the earth'"'"'s magnetic field, said anomalies generally corresponding to geologic anomalies in the earth'"'"'s subterranean structure; and

where said locations are generally within about 20°

latitude from the magnetic equator of the earth, reducing said values to the equator, said step of reducing comprising;

calculation of a transfer function whereby said values measured for one of said magnetic anomalies are corrected to reflect the inclination and declination of the earth'"'"'s magnetic field at the location of said geologic anomaly; and

applying said transfer function to said magnetic anomaly values, whereby said magnetic anomaly values are reduced to correspond to the values which would have been recorded had said geologic anomaly been located in the vicinity of the earth'"'"'s magnetic equator.

8. The method of claim 7 wherein said transfer function is in the wave number domain and said application of said transfer function to said values is performed in the wave number domain, said values having been operated upon by a Fourier transform process prior to application thereto of said transfer function.

9. The method of claim 8 further comprising the step of generating a map of the subterranean structure of the earth based on said reduced magnetic anomaly measurements, whereby magnetic maxima and minima in said map correspond to the locations of said geologic anomalies in the earth'"'"'s subterranean structure.

10. The method of claim 8 wherein said transfer function H_e (u,v) has the form ##EQU16## where l=cos (I) cos (D)=L;

m=cos (I) sin (D)=M;

n=sin (I)=N;

s=(u² +v²)^1/2 ;

I=inclination of the earth'"'"'s magnetic field; and

D=declination of the earth'"'"'s magnetic field.