Bucking circuit for annulling a magnetic field

• US 9,297,922 B2
• Filed: 04/23/2013
• Issued: 03/29/2016
• Est. Priority Date: 03/21/2013
• Status: Active Grant

• Current Assignee: Vale SA (Caixa de Previdência Dos Funcionários Do Banco Do Brasil)
• Original Assignee: Vale SA (Caixa de Previdência Dos Funcionários Do Banco Do Brasil)
• Inventors: West, Gordon Fox, Walker, Peter Whyte, Polzer, Benjamin David
• Primary Examiner(s): Hollington, Jermele M
• Assistant Examiner(s): MCANDREW, CHRISTOPHER P
• Application Number:
  US13/868,803
• Publication Number:
  US 20140285206A1
• Time in Patent Office: 1,071 Days
• Field of Search: 324/333
• US Class Current: 1/1
• CPC Class Codes: G01V 3/107 using compensating coil or loop arrangements
• CPC Class Codes: G01V 3/165 operating with magnetic or electric fields produced or modified by the object or by the detecting device with electromagnetic waves G01V3/17
• CPC Class Codes: G01V 3/18 specially adapted for well-logging
• CPC Class Codes: G01V 3/26 operating with magnetic or electric fields produced or modified either by the surrounding earth formation or by the detecting device with electromagnetic waves G01V3/30
• CPC Class Codes: G01V 3/28 using induction coils
• CPC Class Codes: G01V 3/30 operating with electromagnetic waves
• CPC Class Codes: G01V 3/38 Processing data, e.g. for analysis, for interpretation, for correction

Abstract

A method and apparatus is provided for bucking a magnetic field of known geometry and time variation by means of a plurality of bucking loops. It utilizes multiple loops, each of which is energized by an electric current that creates a magnetic field of the known time variation. The multi-loop field forms a bucking magnetic field that better opposes the spatial variation in the known magnetic field over a volume than can the magnetic field from a single loop. The present invention is useful in electromagnetic measurements, where the magnetic field of a controlled source transmitter must be annulled at a magnetic field sensor. It is particularly useful for cases where the magnetic sensor may move relative to the transmitter, such as in certain airborne electromagnetic measurements.

Claims

1. A bucked transmitter, comprising:

a transmitting electrical circuit comprising a transmitter loop;

a transmitter loop framework to support the transmitter loop, the transmitter loop framework being substantially rigid;

a transmitter adapted to energize the transmitting electrical circuit with a transmitting electric current having a known waveform so as to form a primary magnetic field;

a plurality of radially separated, substantially planar bucking loops, each bucking loop being dimensionally smaller than the transmitter loop and each bucking loop having a distinct radius less than that of the transmitter loop, arranged coaxially along an axis substantially parallel to a local direction of the primary magnetic field and energized with current by a current controller;

a substantially rigid support to which each of the bucking loops is affixed, the support being attached to the transmitter loop framework, wherein the bucking loops are stably located with respect to geometrical aspects of the primary magnetic field;

a nulling axis which extends bi-directionally in a perpendicular direction from a plane parallel to the bucking loops to a termination point at each end of the nulling axis;

wherein

a bucking magnetic field is formed within a bucked volume by current in the bucking loops, the bucked volume being substantially centered on the nulling axis,

a plurality of the termination points are located where the bucking magnetic field fails to substantially annul the primary magnetic field, and

the bucking magnetic field is substantially in a direction opposite to, and is substantially equal in magnitude with, the primary magnetic field, such that the bucking magnetic field substantially annuls the primary magnetic field within the bucked volume.

2. The bucked transmitter according to claim 1, wherein the plurality bucking loops comprise a first bucking loop and a second bucking loop, the first and second bucking loops being substantially circular.

3. The bucked transmitter according to claim 2, wherein the first bucking loop and the second bucking loop are arranged in a common plane, the first bucking loop being geometrically smaller than the second bucking loop.

4. The bucked transmitter according to claim 3, wherein the common plane comprises the plane of the transmitter loop, the magnetic moments of the first bucking loop and the transmitter loop are substantially parallel in the same direction, and the magnetic moments of the second bucking loop and the transmitter loop are substantially opposed.

5. The bucked transmitter according to claim 2, wherein the first and second bucking loops are each disposed with the same effective radius, number of turns and with magnetic moments in the same direction, wherein each of the bucking loops is offset from the transmitter plane in the direction of the common axis.

6. The bucked transmitter according to claim 5, wherein the bucking loops are coaxial with the transmitter loop, the first and second bucking loops being offset from the plane of the transmitter loop by equal distances and in opposite directions, and wherein the magnetic moments of the first bucking loop, the second bucking loop are substantially in parallel and are opposed to the magnetic moment of the transmitter loop.

7. The bucked transmitter of claim 1, wherein the electrical connections supplying electrical current to the bucking loops are selected from the group consisting of:

bifilar wires, coaxial wires and combinations thereof.

8. The bucked transmitter of claim 1, wherein the transmitter loop and the bucking loops are disposed to form a series electrical circuit, whereby the current controller for the bucking loops is the transmitter.

9. The bucked transmitter of claim 1, wherein each of the plurality of bucking loops and the current controller form a first electrical circuit and the transmitter loop and the transmitter form a second electrical circuit.

10. The bucked transmitter of claim 1, further comprising a current sensor and data recorder, wherein the current in the transmitter loop is measured and recorded.

11. The bucked transmitter of claim 1, further comprising a current sensor and data recorder, wherein the current in each of the plurality of bucking loops is measured and recorded.

12. The bucked transmitter according to claim 1, further comprising a sensing means responding to the geometry of the bucking loops and a data recorder, wherein the sensing means is disposed to respond to the shape and positions of the bucking loops, and the data recorder records data output by the sensing means.

13. An electromagnetic measurement apparatus, comprising:

a bucked transmitter, comprising;

a transmitting electrical circuit comprising a transmitter loop;

a transmitter loop framework to support the transmitter loop, the transmitter loop framework being substantially rigid;

a transmitter adapted to energize the transmitting electrical circuit with a transmitting electric current having a known waveform so as to form a primary magnetic field;

a plurality of radially separated, substantially planar bucking loops, each bucking loop being dimensionally smaller than the transmitter loop and each bucking loop having a distinct radius less than that of the transmitter loop, arranged coaxially along an axis substantially parallel to a local direction of the primary magnetic field and energized with current by a current controller;

a substantially rigid support to which each of the bucking loops is affixed, the support being attached to the transmitter loop framework, wherein the bucking loops are stably located with respect to geometrical aspects of the primary magnetic field;

a nulling axis which extends bi-directionally in a perpendicular direction from a plane parallel to the bucking loops to a termination point at each end of the nulling axis;

wherein

a bucking magnetic field is formed within a bucked volume by current in the bucking loops, the bucked volume being substantially centered on the nulling axis,

a plurality of termination points are located where the bucking magnetic field fails to substantially annul the primary magnetic field, and

the bucking magnetic field is substantially in a direction opposite to, and is substantially equal in magnitude with, the primary magnetic field, such that the bucking magnetic field substantially annuls the primary magnetic field within the bucked volume;

a magnetic field sensor;

a receiver adapted to controlling the magnetic field sensor so as to permit the output of the sensor to be measured and recorded;

a data recorder disposed to record data from the receiver;

a sensor support frame comprising a mechanical support apparatus adapted to support and limit the motion of the magnetic field sensor to the bucked volume of the bucked transmitter, whereby the sensor support frame is affixed to the transmitter loop framework of the bucked transmitter; and

wherein the location of the bucked volume is substantially fixed relative to the mechanical support apparatus.

14. The electromagnetic measurement apparatus of claim 13, further comprising means to measure the geometry of the magnetic field sensor with respect to the transmitter loop and the bucking loops, whereby the geometrical data are recorded so as to permit a calculation of the magnetic field absent bucking, the primary magnetic field, the bucking magnetic field, and combinations thereof, at the magnetic field sensor.

15. The electromagnetic measurement apparatus of claim 13, further comprising:

a carrier to transport the electromagnetic measurement apparatus;

a means of transporting the carrier;

a sensor to detect the position of the carrier; and

a means of recording the positions.

16. The electromagnetic measurement apparatus of claim 13, wherein the means of transporting the carrier is selected from the group consisting of aircraft, airships, dirigibles, helicopters, towed birds, ground vehicles, towed trailers, barges, ships, boats, submersible vehicles and combinations thereof.

17. The electromagnetic measurement apparatus of claim 13, wherein the electromagnetic measurement apparatus is adapted for helicopter borne geophysical surveying, and is suspended on a tow cable below a helicopter and a plane of the transmitter loop is substantially horizontal.

18. The electromagnetic measurement apparatus of claim 13, further comprising an orientation sensor operable to measure and record an orientation of the magnetic field sensor.

19. The electromagnetic measurement apparatus of claim 13, further comprising an orientation sensor operable to measure and record an orientation of the transmitter loop.