WO2007045963A2 - Method and apparatus for conducting electromagnetic exploration - Google Patents

Method and apparatus for conducting electromagnetic exploration Download PDF

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Publication number
WO2007045963A2
WO2007045963A2 PCT/IB2006/002892 IB2006002892W WO2007045963A2 WO 2007045963 A2 WO2007045963 A2 WO 2007045963A2 IB 2006002892 W IB2006002892 W IB 2006002892W WO 2007045963 A2 WO2007045963 A2 WO 2007045963A2
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Prior art keywords
primary
nulling
coils
field
coil
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PCT/IB2006/002892
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French (fr)
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WO2007045963A3 (en
Inventor
David Bruce Dickson
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Anglo Operations Limited
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Priority to CA2626195A priority Critical patent/CA2626195C/en
Publication of WO2007045963A2 publication Critical patent/WO2007045963A2/en
Publication of WO2007045963A3 publication Critical patent/WO2007045963A3/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
    • G01V3/08—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices
    • G01V3/10—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices using induction coils
    • G01V3/104—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices using induction coils using several coupled or uncoupled coils
    • G01V3/105—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices using induction coils using several coupled or uncoupled coils forming directly coupled primary and secondary coils or loops
    • G01V3/107—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices using induction coils using several coupled or uncoupled coils forming directly coupled primary and secondary coils or loops using compensating coil or loop arrangements
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
    • G01V3/15—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for use during transport, e.g. by a person, vehicle or boat
    • G01V3/165—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for use during transport, e.g. by a person, vehicle or boat operating with magnetic or electric fields produced or modified by the object or by the detecting device

Definitions

  • THIS invention relates to a method and apparatus for conducting electromagnetic exploration, i.e. geophysical survey.
  • the known AeroTEM system makes use of a transmitter consisting of a flat primary coil or loop which is fed with current in one direction to produce a primary field in one axial direction.
  • a flat nulling or bucking coil of smaller diameter is arranged at the centre of the primary coil and is fed with current in the opposite direction to produce a nulling field opposed axially to the primary field, thereby to null the primary field at the axis and in the axial direction.
  • nulling of the primary field in the axial direction alone enables the receiver to detect the secondary field in that direction only because components of the primary field in other directions still swamp the receiver. It is accordingly not possible with the known system to perform accurate detection of the secondary field in a three-dimensional or vectorial sense.
  • accuracy of the known system is sensitive to various perturbations, for example axial, radial or rotational displacement of the nulling coil relative to the primary coil. This in turn means that the mechanical coil structure must be robust and stiff, thereby detracting from its aerodynamics. This may be particularly problematical in situations where the coil structure is to be conveyed by an aircraft such as a helicopter.
  • the present invention seeks to provide an improved system.
  • a method of conducting electromagnetic exploration of the earth's surface comprising the steps of providing at least one primary coil, powering the coil to generate a primary electromagnetic field, exposing the earth's surface to the primary field, providing a receiver to detect a secondary field generated by the earth as a result of currents generated therein by the primary field, providing a plurality of spaced apart nulling coils and powering the nulling coils in a manner to null the primary field in a three- dimensional volume surrounding the receiver.
  • the nulling coils may be powered by the same current as the primary coil(s), by arranging them in series with the primary coil. It is however within the scope of the invention for the current feed to the nulling coils to be controlled, eg by appropriate current shunts, such that specific nulling coil(s) are driven by a fraction only of the primary current, i.e. the current fed to the primary coil(s).
  • the nulling coils are arranged coaxially, on the axis of the primary coil(s), in a vertically spaced, stacked relationship. There may be multiple primary coils arranged coaxially in vertically spaced relationship.
  • apparatus for conducting electromagnetic exploration comprising at least one primary coil, means for powering the coil to generate a primary electromagnetic field and for exposing the earth's surface to the primary field, a receiver to detect a secondary field generated by the earth as a result of currents generated therein by the primary field, a plurality of spaced apart nulling coils and means for powering the nulling coils in a manner to null the primary field in a three-dimensional volume surrounding the receiver, thereby enabling the receiver to detect the secondary field vectorially.
  • a nulling coil apparatus comprising a plurality of nulling coils arranged in a predetermined configuration relative to a receiver and to one or more primary coils producing a primary field, and means for powering the nulling coils such that in combination they produce a field serving to null the primary field in a volume surrounding the receiver, thereby enabling the receiver to detect, vectorially, a secondary field generated by the earth in response to exposure thereof the primary field.
  • Figure 1 diagrammatically illustrates a first embodiment of the invention
  • Figure 2 diagrammatically illustrates a second embodiment of the invention.
  • the apparatus 10 seen in Figure 1 includes a single, flat primary coil 12 and a plurality, in this case two, flat nulling or bucking coils 14 arranged in a vertically spaced, stacked relationship, coaxially with the primary coil.
  • the primary coil is powered with current in the sense indicated by the arrow 16 while the nulling coils 14 are powered with current in the opposite sense indicated by the arrow 18.
  • the primary field produced by the primary coil 12 is indicated by the arrow 20.
  • the nulling field produced by the nulling coils 14 is indicated by the arrow 22.
  • the geometry of the coils and their powering is such that the field 20 at least approximately cancels the field 22 in a volume at the common axis of the coils.
  • the nulling coils may have a single turn or multiple turns.
  • the coil radius and vertical coil separation will be selected in order to provide optimal nulling of the primary field 20 over the nulled volume.
  • nulling coil arrangement consists of two single turn nulling coils 14 of diameter 1.813m and spaced apart vertically by 0.886m
  • Z-bucking or nulling i.e. nulling in a vertical, axial sense
  • nulled volume had a limited radial extent.
  • nulled volume was of greater radial extent.
  • Figure 2 illustrates an embodiment which includes a pair of stacked primary coils 12 and, again, a pair of stacked nulling coils 14.
  • the primary coils are arranged as a Helmholtz pair in which the combined magnetic field, along the common axis of the coils, is constant.
  • vertical nulling of the primary field can be carried out particularly effectively by arranging the nulling coils as an opposing Helmholtz pair, i.e. with a number of turns scaled in proportion to the ratio of the radii of the primary and nulling coils, and with current flowing in the nulling coils in a direction opposite to that in which it flows in the primary coils.
  • the diameter of the primary coils was selected to be 7.07m, each primary coil having eight turns, in order to provide a dipole moment similar to that of the 10mm diameter, single primary coil arrangement of Figure 1, for comparison purposes.
  • the table below gives optimal configurations and geometry for the nulling coils.
  • This table shows that Z-bucking or nulling, i.e. nulling in the vertical or axial sense, is effective for single turn as well as two-, three- and four-turn nulling coils, and that R-bucking or nulling, i.e. nulling in the radial sense, is effective for the multiple turn nulling coil configurations.
  • the configurations with multiple turns show nulled volumes of increasing radial extent with increasing numbers of turns.
  • nulling coils in these versions are of smaller diameter than in the corresponding versions described above with reference to the Figure 1 embodiment.
  • axial perturbation or displacement is more readily accommodated than with the embodiment of Figure 1 while the system is also tolerant of radial and rotational perturbation.
  • optimised configurations can provide effective nulling in the vertical or axial sense, making both embodiments particularly suitable for use with an axially positioned, vertically mounted receiver/detector.
  • the analyses also indicate that with both embodiments it is possible to provide a nulled volume of significant radial extent with a controlled radial field gradient. Both embodiments are reasonably tolerant of radial and rotational perturbations while the embodiment of Figure 2 is more tolerant of axial perturbations than the embodiment of Figure 1.
  • the nulling coils in both embodiments may, as indicated above, be powered in series with the primary coil(s).
  • suitable current shunts or other controllers can be used to feed fractions of the primary current to individual nulling coils.

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  • Physics & Mathematics (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Remote Sensing (AREA)
  • Electromagnetism (AREA)
  • Environmental & Geological Engineering (AREA)
  • Geology (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Geophysics (AREA)
  • Geophysics And Detection Of Objects (AREA)

Abstract

The invention concerns a method and apparatus (10) for conducting electromagnetic exploration of the earth's surface. At least one primary coil (12) is provided. The primary coil(s) is or are powered to generate a primary electromagnetic field and the earth's surface is exposed to this primary field. A receiver is provided to detect a secondary field generated by the earth as a result of currents generated therein by the primary field. Spaced apart nulling coils (14) are powered in such a manner as to null the primary field in a three-dimensional volume surrounding the receiver, thereby enabling the receiver to detect the secondary field vectorially.

Description

"METHOD AND APPARATUS FOR CONDUCTING ELECTROMAGNETIC EXPLORATION"
BACKGROUND TO THE INVENTION
THIS invention relates to a method and apparatus for conducting electromagnetic exploration, i.e. geophysical survey.
It is known in electromagnetic exploration systems to make use of a high power transmitter which generates a primary, time-varying electromagnetic field by means of a transmitter loop. The primary field excited currents in the earth which in turn generate a secondary field. The secondary field detected by a receiver can be used in analysis of, for instance, the earth's composition. It is recognised that the sensitivity and dynamic range of the system are limited by the ratio between the primary and secondary fields detected by the receiver.
Exposure of the receiver to the primary field interferes with the accuracy and accordingly the usefulness of the detected signal. It is accordingly recognised that it is desirable to null or buck the primary field at the receiver in order to limit the interference. For instance, the known AeroTEM system makes use of a transmitter consisting of a flat primary coil or loop which is fed with current in one direction to produce a primary field in one axial direction. A flat nulling or bucking coil of smaller diameter is arranged at the centre of the primary coil and is fed with current in the opposite direction to produce a nulling field opposed axially to the primary field, thereby to null the primary field at the axis and in the axial direction. However, nulling of the primary field in the axial direction alone enables the receiver to detect the secondary field in that direction only because components of the primary field in other directions still swamp the receiver. It is accordingly not possible with the known system to perform accurate detection of the secondary field in a three-dimensional or vectorial sense. In addition, it has been shown that such the accuracy of the known system is sensitive to various perturbations, for example axial, radial or rotational displacement of the nulling coil relative to the primary coil. This in turn means that the mechanical coil structure must be robust and stiff, thereby detracting from its aerodynamics. This may be particularly problematical in situations where the coil structure is to be conveyed by an aircraft such as a helicopter.
The present invention seeks to provide an improved system.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided a method of conducting electromagnetic exploration of the earth's surface, the method comprising the steps of providing at least one primary coil, powering the coil to generate a primary electromagnetic field, exposing the earth's surface to the primary field, providing a receiver to detect a secondary field generated by the earth as a result of currents generated therein by the primary field, providing a plurality of spaced apart nulling coils and powering the nulling coils in a manner to null the primary field in a three- dimensional volume surrounding the receiver.
Nulling the primary field in a three-dimensional volume surrounding the receiver enables the receiver to detect the secondary field vectorially.
The nulling coils may be powered by the same current as the primary coil(s), by arranging them in series with the primary coil. It is however within the scope of the invention for the current feed to the nulling coils to be controlled, eg by appropriate current shunts, such that specific nulling coil(s) are driven by a fraction only of the primary current, i.e. the current fed to the primary coil(s).
In one particular configuration envisaged by the invention the nulling coils are arranged coaxially, on the axis of the primary coil(s), in a vertically spaced, stacked relationship. There may be multiple primary coils arranged coaxially in vertically spaced relationship.
According to another aspect of the invention there is provided apparatus for conducting electromagnetic exploration, the apparatus comprising at least one primary coil, means for powering the coil to generate a primary electromagnetic field and for exposing the earth's surface to the primary field, a receiver to detect a secondary field generated by the earth as a result of currents generated therein by the primary field, a plurality of spaced apart nulling coils and means for powering the nulling coils in a manner to null the primary field in a three-dimensional volume surrounding the receiver, thereby enabling the receiver to detect the secondary field vectorially.
Still further according to the invention there is provided a nulling coil apparatus comprising a plurality of nulling coils arranged in a predetermined configuration relative to a receiver and to one or more primary coils producing a primary field, and means for powering the nulling coils such that in combination they produce a field serving to null the primary field in a volume surrounding the receiver, thereby enabling the receiver to detect, vectorially, a secondary field generated by the earth in response to exposure thereof the primary field.
Other features of the invention will appear from the description given below and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings. In the drawings:
Figure 1 diagrammatically illustrates a first embodiment of the invention; and
Figure 2 diagrammatically illustrates a second embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The apparatus 10 seen in Figure 1 includes a single, flat primary coil 12 and a plurality, in this case two, flat nulling or bucking coils 14 arranged in a vertically spaced, stacked relationship, coaxially with the primary coil. The primary coil is powered with current in the sense indicated by the arrow 16 while the nulling coils 14 are powered with current in the opposite sense indicated by the arrow 18. The primary field produced by the primary coil 12 is indicated by the arrow 20. The nulling field produced by the nulling coils 14 is indicated by the arrow 22. As described below in more detail, the geometry of the coils and their powering is such that the field 20 at least approximately cancels the field 22 in a volume at the common axis of the coils.
The nulling coils may have a single turn or multiple turns. In addition it is possible to control the field curvature of the nulling field produced by the nulling coils by varying their vertical separation and radius. In practice, for a given number of turns in the nulling coils, the coil radius and vertical coil separation will be selected in order to provide optimal nulling of the primary field 20 over the nulled volume. In a practical arrangement use was made of an eight-turn primary coil 12 with a nominal diameter of 10m. The table below gives optimal arrangements, derived mathematically, for the nulling coils.
Figure imgf000006_0001
From this table it can be seen that where the nulling coil arrangement consists of two single turn nulling coils 14 of diameter 1.813m and spaced apart vertically by 0.886m, Z-bucking or nulling, i.e. nulling in a vertical, axial sense, was very effective but the nulled volume had a limited radial extent. At the other end of the scale, with three-turn nulling coils of diameter 6.28m separated vertically by 2.226m, there was again good nulling in the axial sense but in this case the nulled volume was of greater radial extent.
Stacked nulling coils arranged according to the table were subjected to perturbation analyses in which the effect of radial, rotational and axial perturbations applied to the single and multiple turn nulling coil configurations were investigated. These analyses also illustrated that the diameter of the axially nulled volume is considerably greater, for each version, and that radial perturbations, i.e. displacements have a less pronounced detrimental effect than in the case of the known AeroTEM system referred to above. While the system is also tolerant of rotational perturbations, it is quite sensitive to axial perturbations.
Figure 2 illustrates an embodiment which includes a pair of stacked primary coils 12 and, again, a pair of stacked nulling coils 14. The primary coils are arranged as a Helmholtz pair in which the combined magnetic field, along the common axis of the coils, is constant. With this configuration of primary coils, vertical nulling of the primary field can be carried out particularly effectively by arranging the nulling coils as an opposing Helmholtz pair, i.e. with a number of turns scaled in proportion to the ratio of the radii of the primary and nulling coils, and with current flowing in the nulling coils in a direction opposite to that in which it flows in the primary coils.
The diameter of the primary coils was selected to be 7.07m, each primary coil having eight turns, in order to provide a dipole moment similar to that of the 10mm diameter, single primary coil arrangement of Figure 1, for comparison purposes. For this primary coil configuration, the table below gives optimal configurations and geometry for the nulling coils.
Figure imgf000007_0001
This table shows that Z-bucking or nulling, i.e. nulling in the vertical or axial sense, is effective for single turn as well as two-, three- and four-turn nulling coils, and that R-bucking or nulling, i.e. nulling in the radial sense, is effective for the multiple turn nulling coil configurations. The configurations with multiple turns show nulled volumes of increasing radial extent with increasing numbers of turns.
It is however noted that the nulling coils in these versions are of smaller diameter than in the corresponding versions described above with reference to the Figure 1 embodiment. In addition, it is shown that axial perturbation or displacement is more readily accommodated than with the embodiment of Figure 1 while the system is also tolerant of radial and rotational perturbation.
As illustrated above, the use of multiple nulling coils in particular, optimised configurations can provide effective nulling in the vertical or axial sense, making both embodiments particularly suitable for use with an axially positioned, vertically mounted receiver/detector. The analyses also indicate that with both embodiments it is possible to provide a nulled volume of significant radial extent with a controlled radial field gradient. Both embodiments are reasonably tolerant of radial and rotational perturbations while the embodiment of Figure 2 is more tolerant of axial perturbations than the embodiment of Figure 1.
It is perceived that the embodiment of Figure 1 , being vertically more compact, will be better suited to airborne conveyance while the embodiment of Figure 2, being vertically bulkier and hence more difficult to incorporate in an aerodynamic structure, will be better suited to terrestrial i.e. surface-conveyance.
The nulling coils in both embodiments may, as indicated above, be powered in series with the primary coil(s). Alternatively, suitable current shunts or other controllers can be used to feed fractions of the primary current to individual nulling coils.

Claims

1.
A method of conducting electromagnetic exploration of the earth's surface, the method comprising the steps of providing at least one primary coil, powering the coil to generate a primary electromagnetic field, exposing the earth's surface to the primary field, providing a receiver to detect a secondary field generated by the earth as a result of currents generated therein by the primary field, providing a plurality of spaced apart nulling coils and powering the nulling coils in a manner to null the primary field in a three-dimensional volume surrounding the receiver.
2.
A method according to claim 1 wherein the nulling coils are arranged in series with the primary coil(s).
3.
A method according to claim 1 wherein current feed to the nulling coils is controlled such that one or more specific nulling coils is or are powered by a fraction only of the current supplied to the primary coil(s).
4.
A method according to claim 3 wherein current feed to the nulling coils is controlled by appropriate current shunts.
5.
A method according to any one of the preceding claims wherein the nulling coils are arranged coaxially, on the axis of the primary coil(s), in a vertically spaced relationship.
6.
A method according to claim 5 wherein two nulling coils are arranged coaxially on the axis of the primary coil(s).
7.
A method according to claim 6 wherein two primary coils are arranged in a vertically spaced, coaxial relationship.
8.
A method according to claim 7 wherein the primary coils are arranged as a
Helmholtz pair and the nulling coils are arranged as an opposing Helmholtz pair.
9.
A method according to any one of the preceding claims wherein each nulling coil has multiple turns.
10.
An apparatus for conducting electromagnetic exploration of the earth's surface, the apparatus comprising at least one primary coil, means for powering the coil to generate a primary electromagnetic field and for exposing the earth's surface to the primary field, a receiver to detect a secondary field generated by the earth as a result of currents generated therein by the primary field, a plurality of spaced apart nulling coils and means for powering the nulling coils in a manner to null the primary field in a three-dimensional volume surrounding the receiver.
11.
An apparatus according to claim 10 wherein the nulling coils are arranged in series with the primary coil(s).
12.
An apparatus according to claim 10 comprising means for controlling current feed to the nulling coils such that one or more specific nulling coils is or are powered by a fraction only of the current supplied to the primary coil(s).
13.
An apparatus according to any one of claims 10 to 12 comprising nulling coils are arranged coaxially, on the axis of the primary coil(s), in a vertically spaced relationship.
14.
An apparatus according to claim 13 comprising two nulling coils are arranged coaxially on the axis of the primary coil(s).
15.
An apparatus according to claim 14 comprising two primary coils arranged in a vertically spaced, coaxial relationship.
16.
An apparatus according to claim 15 wherein the primary coils are arranged as a Helmholtz pair and the nulling coils are arranged as an opposing Helmholtz pair.
17.
An apparatus according to any one of claims 10 to 16 wherein each nulling coil has multiple turns.
18.
A nulling coil apparatus for use in electromagnetic exploration of the earth's surface, the apparatus comprising a plurality of nulling coils arranged in a predetermined configuration relative to a receiver and to one or more primary coils producing a primary field, and means for powering the nulling coils such that in combination they produce a field serving to null the primary field in a volume surrounding the receiver, thereby enabling the receiver to detect, vectorially, a secondary field generated by the earth in response to exposure thereof the primary field.
PCT/IB2006/002892 2005-10-17 2006-10-17 Method and apparatus for conducting electromagnetic exploration WO2007045963A2 (en)

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WO2010022515A1 (en) * 2008-08-29 2010-03-04 Geo Equipment Manufacturing Ltd. Bucking coil and b-field measurement system and apparatus for time domain electromagnetic measurements
WO2014146185A3 (en) * 2013-03-21 2014-11-20 Vale S.A. Bucking circuit for annulling a magnetic field
CN111290029A (en) * 2020-03-27 2020-06-16 吉林大学 Non-coplanar Bucking compensated dragging type electromagnetic device and manufacturing method thereof
CN113534266A (en) * 2021-07-08 2021-10-22 吉林大学 Multi-receiving coil spread spectrum aviation electromagnetic exploration device containing compensation ring Z component
CN114624782A (en) * 2022-01-27 2022-06-14 湖南继善高科技有限公司 Primary field compensation type electromagnetic detection device

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DE102011088406A1 (en) * 2011-12-13 2013-06-13 Robert Bosch Gmbh metal sensor
CN104483709A (en) * 2014-12-08 2015-04-01 袁锡章 Handheld corresponding material universal detector

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Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2010022515A1 (en) * 2008-08-29 2010-03-04 Geo Equipment Manufacturing Ltd. Bucking coil and b-field measurement system and apparatus for time domain electromagnetic measurements
EP2324366A1 (en) * 2008-08-29 2011-05-25 Geotech Airborne Limited Bucking coil and b-field measurement system and apparatus for time domain electromagnetic measurements
EP2324366A4 (en) * 2008-08-29 2011-10-12 Geotech Airborne Ltd Bucking coil and b-field measurement system and apparatus for time domain electromagnetic measurements
US8400157B2 (en) 2008-08-29 2013-03-19 Geotech Airborne Limited Bucking coil and B-field measurement system and apparatus for time domain electromagnetic measurements
US8786286B2 (en) 2008-08-29 2014-07-22 Geotech Airborne Limited Bucking coil and B-field measurement system and apparatus for time domain electromagnetic measurements
WO2014146185A3 (en) * 2013-03-21 2014-11-20 Vale S.A. Bucking circuit for annulling a magnetic field
CN105452905A (en) * 2013-03-21 2016-03-30 淡水河谷公司 Bucking circuit for annulling a magnetic field
AU2014234970B2 (en) * 2013-03-21 2017-08-03 Vale S.A. Bucking circuit for annulling a magnetic field
RU2661996C2 (en) * 2013-03-21 2018-07-23 Вале С.А. Bucking circuit for annulling magnetic field
CN111290029A (en) * 2020-03-27 2020-06-16 吉林大学 Non-coplanar Bucking compensated dragging type electromagnetic device and manufacturing method thereof
CN113534266A (en) * 2021-07-08 2021-10-22 吉林大学 Multi-receiving coil spread spectrum aviation electromagnetic exploration device containing compensation ring Z component
CN114624782A (en) * 2022-01-27 2022-06-14 湖南继善高科技有限公司 Primary field compensation type electromagnetic detection device

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CA2626195C (en) 2015-05-26
CN101292176A (en) 2008-10-22
WO2007045963A3 (en) 2008-01-03
ZA200803410B (en) 2009-07-29
CA2626195A1 (en) 2007-04-26

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