BUCKING CIRCUIT FOR ANNULLING A MAGNETIC FIELD

• US 20140285206A1
• Filed: 04/23/2013
• Published: 09/25/2014
• 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: Polzer, Benjamin David, WEST, Gordon Fox, Walker, Peter Whyte
• Granted Patent: US 9,297,922 B2
• Time in Patent Office: Days
• US Class Current: 324/333
• 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 said transmitter loop, said transmitter loop framework being substantially rigid;

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

a plurality of separate, substantially planar bucking loops, each being dimensionally smaller than said transmitter loop, arranged coaxially along an axis substantially parallel to the local direction of said primary magnetic field and energized with current by a current controller;

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

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

wherein

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

said termination points are located where the bucking magnetic field fails to substantially annul said primary magnetic field,

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

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

3. The bucked transmitter according to claim 1 wherein said first bucking loop and said second bucking loop are arranged in a common plane, said first bucking loop being geometrically smaller than said second bucking loop.

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

5. The bucked transmitter according to claim 2 wherein said 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 said bucking loops is offset from the transmitter plane in the direction of the common axis.

6. The bucked transmitter according to claim 5 wherein said bucking loops are coaxial with said transmitter loop, said 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 said first bucking loop, said second bucking loop are substantially in parallel and are opposed to the magnetic moment of said transmitter loop.

7. The bucked transmitter of claim 1, wherein the electrical connections supplying electrical current to the 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 a bucking loop and its 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 comprising a current sensor and data recorder, wherein the current in said transmitter loop is measured and recorded.

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

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

13. An electromagnetic measurement apparatus, comprising:

the bucked transmitter of claim 1;

a magnetic field sensor;

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

a data recorder disposed to record data from said receiver;

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

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

14. The electromagnetic measurement apparatus of claim 13, comprising means to measure the geometry of said magnetic field sensor with respect to said transmitter loop and said bucking loops, whereby said geometrical data are recorded so as to permit 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, comprising:

a carrier to transport said electromagnetic measurement apparatus,

a means of transporting said carrier,

a sensor to detect the position of said carrier, and

a means of recording said 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 adapted for helicopter borne geophysical surveying, wherein said apparatus is suspended on a tow cable below a helicopter and the plane of the transmitter loop is substantially horizontal.

18. The electromagnetic measurement apparatus of claim 13 comprising an orientation sensor, whereby the orientation of the magnetic field sensor is measured and recorded.

19. The electromagnetic measurement apparatus of claim 13 comprising an orientation sensor, whereby the orientation of the transmitter loop is measured and recorded.