AU2016203396B2 - Magnetometer signal sampling within time-domain EM transmitters and method - Google Patents

Abstract

A time-domain airborne electromagnetic (AEM) system (200) for measuring a signal related to a magnetic field during a survey. The AEM system includes a 5 transmitter coil system (202) configured to generate a primary electromagnetic field that penetrates into the earth; a receiver coil system (212) configured to record a secondary electromagnetic field generated by the earth as a response of the primary electromagnetic field; and a magnetometer sensor (220) located coplanar with the transmitter coil system (202) and configured to record signals related to an external 0 magnetic field. A sampling window (416) is associated with the magnetometer sensor (220) and the sampling window (416) is calculated based on (i) various functional windows (410, 412, 414) characterizing the magnetometer sensor and (ii) current pulses (402A, 402C) applied to the transmitter coil system during the survey. Fig. 2 2/8 FIG. 2 200 250 25225 225 z Y 224x 222 212A 212 220 202A , 202

Description

2/8

FIG. 2 200

250 25225

225 z Y

224x 222 212A

212

220 202A

, 202

Magnetometer Signal Sampling within Time-Domain EM Transmitters and

Method

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority and benefit from U.S. Provisional

Patent Application No. 62/167,351, filed on May 28, 2015, the entire disclosure of

which is incorporated herein by reference.

BACKGROUND TECHNICAL FIELD

[0002] Embodiments of the subject matter disclosed herein generally relate

to methods and systems and, more particularly, to mechanisms and techniques for

selectively sampling magnetometer data for obtaining information that is usable for

determining the underground properties of a survey area.

[0002a] Reference to any prior art in the specification is not an

acknowledgement or suggestion that this prior art forms part of the common general

knowledge in any jurisdiction or that this prior art could reasonably be expected to be

combined with any other piece of prior art by a skilled person in the art.

DISCUSSION OF THE BACKGROUND

[0003] Electromagnetic (EM) surveying is a method of geophysical

exploration to determine the properties of a portion of the earth's subsurface,

information that is especially helpful in the oil and gas industry (e.g., for generating

an image of the subsurface that helps the exploration engineer to correctly drill an la exploration well). EM surveys may be based on a controlled source that sends EM energy waves into the earth. By measuring the associated secondary fields with an

EM receiver, it is possible to estimate the depth and/or composition of the

subsurface features. These features may be associated with subterranean

hydrocarbon deposits.

[0004] An airborne EM survey system 100 generally includes, as illustrated in

Figure 1, a transmitter 102 for generating a primary electromagnetic field 104 that is

directed toward the earth. When the primary EM field 104 enters the ground 108, it

induces eddy currents 106 inside the earth. These eddy currents 106 generate a

secondary electromagnetic field or ground response 110. An EM receiver 112

measures the response 110 of the ground. Transmitter 102 and receiver 112 may

be connected to an aircraft 114 through a connecting mechanism 115 so that a large

area of the ground is swept. Aircraft 114 may be manned or unmanned, having or

not a propulsion mechanism, and capable of flying by any known means. Receiver

112 may be located concentric with transmitter 102. The currents induced in the

ground are a function of the earth's conductivity and of course, the transmitter

characteristics. By processing and interpreting the received signals, it is possible to

study and estimate the distribution of conductivity in the subsurface. The distribution

of conductivity is associated with the various layers 116 and 118 making up the

subsurface, which is implicitly indicative of the location of oil and gas reservoirs,

and/or other resources of interest for the mining industry.

[0005] EM systems can be either frequency-domain or time-domain. Both

types of systems are based on Faraday's Law of electromagnetic induction, which

states that a time-varying magnetic field will produce an electric field. For time

domain systems, a time-varying field (usually a magnetic field) is created by a

current that may be pulsed. The change in the transmitted current induces an

electrical current in the ground that persists after the primary field is turned off.

Typical time-domain receiver coils measure the rate of change of this secondary

field. The time-domain transmitter current waveform repeats itself periodically and can be transformed to the frequency domain where each harmonic has a specific amplitude and phase. In frequency-domain systems, the time-varying transmitter signal is a sinusoidal waveform of constant frequency, inducing electrical currents in the ground of the same frequency. The transmitter coil is excited to generate different frequencies during the geophysical survey.

[0006] Another type of receiver that may be used for measuring the results of

the electromagnetic induction is the magnetometer or the gradient magnetometer.

This type of sensor is called herein a magnetometer sensor 120. The magnetometer

sensors are used in the geophysical industry for measuring the local magnetic field

from the earth (only amplitude, only direction or both amplitude and direction; thus, a

magnetometer sensor can output a scalar or a vector or one or more components of

a vector). However, there are always issues with obtaining correct readings when

the magnetometer sensors are placed within or near large or rapidly time-varying

electromagnetic fields 104, such as those in the frequency or time-domain airborne

electromagnetic (AEM) systems, one of which being illustrated in Figure 1.

[0007] When performing electromagnetic induction surveys, it is also

desirable to carry equipment or sensors which measure the local magnetic field of

the earth, but not the primary magnetic field from the transmitter or the secondary

magnetic fields induced by the transmitter. Those surveys that measure the passive

magnetic field of the earth are referred to as magnetic field surveys. These surveys

are also useful for oil and gas and mineral exploration.

[0008] The time-domain AEM systems are posing an enormous challenge to

the magnetometer sensors for the following reasons. When the time-domain

transmitters 102 fire a pulse of current to create a large electromagnetic field 104, the magnetometer sensors may be overloaded and/or lose their lock on the ambient magnetic field. The repeated pulsing of the transmitter allows for no usable data to be collected because the magnetometer sensors are continually losing lock on the field. However, if the ambient magnetic field is perturbed with a large electromagnetic field, like the one produced by the typical transmitter 104, the associated sensor loses the lock on the ambient magnetic field and no reliable output can be detected.

[0009] Thus, there is a need to have a time-domain AEM system that

includes a magnetometer sensor capable of generating valuable information about

the geophysics of the surveyed area.

SUMMARY

[0009a] According to a first aspect of the invention there is provided a time

domain airborne electromagnetic (AEM) system for measuring a signal related to a

magnetic field during a survey, the AEM system comprising: a transmitter coil system

configured to generate a primary electromagnetic field that penetrates into the earth;

a receiver coil system configured to record a secondary electromagnetic field

generated by the earth as a response of the primary electromagnetic field; and a

magnetometer sensor located coplanar with the transmitter coil system and

configured to record signals related to an external magnetic field, wherein a sampling

window is associated with the magnetometer sensor and the sampling window is

calculated based on (i) various functional windows characterizing the magnetometer

sensor and (ii) current pulses applied to the transmitter coil system during the

survey.

[0009b] According to a second aspect of the invention there is provided a

method for collecting magnetometer data with a magnetometer sensor during an

airborne electromagnetic survey, the method comprising: receiving various functional

time windows characterizing the magnetometer sensor; receiving activation

characteristics of a transmitter coil system that generates a primary electromagnetic

field that penetrates the earth; calculating a sampling window of the magnetometer

sensor based on the various functional time windows of the magnetometer sensor

and the activation characteristics of the transmitter coil system; collecting the

magnetometer data during the sampling window with the magnetometer sensor while

flying a time-domain airborne electromagnetic (AEM) system, wherein the AEM

system includes the transmitter coil system and the magnetometer sensor; and

generating an image of a surveyed subsurface based on the magnetometer data

collected with the magnetometer sensor during the sampling window.

[0010] According to one embodiment, there is a time-domain airborne

electromagnetic (AEM) system for measuring a signal related to a magnetic field

during a survey. The AEM system includes a transmitter coil system configured to

generate a primary electromagnetic field that penetrates into the earth, a receiver

coil system configured to record a secondary electromagnetic field generated by the

earth as a response of the primary electromagnetic field, and a magnetometer

sensor located coplanar with the transmitter coil system and configured to record

signals related to an external magnetic field. A sampling window is associated with

the magnetometer sensor and the sampling window is calculated based on (i)

various functional windows characterizing the magnetometer sensor and (ii) current

pulses applied to the transmitter coil system during the survey.

5a

[0011] According to another embodiment, there is a method for collecting

magnetometer data with a magnetometer sensor during an airborne electromagnetic

survey. The method includes a step of receiving various functional time windows

characterizing the magnetometer sensor; a step of receiving activation

characteristics of a transmitter coil system that generates a primary electromagnetic

field that penetrates the earth; a step of calculating a sampling window of the

magnetometer sensor based on the various functional time windows of the

magnetometer sensor and the activation characteristics of the transmitter coil

system; a step of collecting the magnetometer data during the sampling window with

the magnetometer sensor while flying a time-domain airborne electromagnetic (AEM)

system, wherein the AEM system includes the transmitter coil system (202) and the

magnetometer sensor (220); and a step of generating an image of a surveyed

subsurface based on the magnetometer data collected with the magnetometer

sensor during the sampling window.

[0011a] By way of clarification and for avoidance of doubt, as used herein and

except where the context requires otherwise, the term "comprise" and variations of

the term, such as "comprising", "comprises" and "comprised", are not intended to

exclude further additions, components, integers or steps.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings, which are incorporated in and constitute

a part of the specification, illustrate one or more embodiments and, together with the

description, explain these embodiments. In the drawings:

[0013] Figure 1 is a schematic diagram of an EM acquisition system;

5b

[0014] Figure 2 illustrates an AEM system equipped with a magnetometer

sensor;

[0015] Figures 3A and 3B illustrate a voltage applied to a transmitter coil

system and voltages being generated in a receiver coil system;

[00161 Figure 4 illustrates the various functional windows exhibited by a

magnetometer sensor when in the presence of alarge magnetic field;

[0017] Figure 5 illustrates calculating a sampling window for the

magnetometer sensor for which the recorded data is usable in a geological survey;

[0018] Figure 6 illustrates a method for calculating the sampling window for a

magnetometer sensor; and

[0019] Figure 7 is a schematic diagram of a control device.

DETAILED DESCRIPTION

[0020] The following description of the embodiments refers to the accompanying

drawings. The same reference numbers in different drawings identify the same or

similar elements. The following detailed description does not limit the invention.

Instead, the scope of the invention is defined by the appended claims. The following

embodiments are discussed, for simplicity, with regard to the terminology and structure

of a time-domain AEM system. However, the embodiments to be discussed next are

not limited to geophysics, they may be applied to other fields.

[0021] Reference throughout the specification to "one embodiment" or "an

embodiment" means that a particular feature, structure or characteristic described in

connection with an embodiment is included in at least one embodiment of the subject

matter disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an

embodiment" in various places throughout the specification is not necessarily referring

to the same embodiment. Further, the particular features, structures or characteristics

may be combined in any suitable manner in one or more embodiments.

[0022] According to an embodiment, a magnetometer sensor is placed coplanar

and within an area defined by a transmitter coil of a time-domain AEM system for

measuring a magnetic field or a quantity related to the magnetic field. Although the

transmitter coil generates large magnetic fields that will make the magnetometer sensor

inoperable for a given time interval, a novel method for still extracting useful information

from the magnetometer sensor is now discussed.

[0023] To better understand the novel method, a time-domain AEM system is

discussed with reference to Figure 2, the magnetic fields generated by a transmitter coil

of the time-domain AEM system are discussed with reference to Figures 3A and 3B,

the response of a magnetometer sensor in alarge magnetic field is discussed with

reference to Figure 4 and the solution proposed by the novel method is discussed with

reference to Figures 5 and 6.

[0024] Figure 2 shows a time-domain AEM system 200 that includes a

transmitter coil system 202 and a receiver coil system 212. Each of these systems

may include one or more coils (202A and 212A) for generating/measuring EM fields.

The receiver coil system may include one or more coils 212A for each Cartesian

direction so that EM fields along one or more axes can be measured. Figure 2 shows

the transmitter coil system being coplanar with the receiver coil system. The term

coplanarr" is understood herein that the coils (i.e., coils with their axis parallel to the

vertical (z) axis of the earth reference frame) of each system are in the same plane

within a given error. More specifically, if the radius of the transmitter coil is in the range

of tens of meters or less and the radius of the receiver coil is in the range of meters or

less, then the transmitter and receiver Z coils (only the Z coils are shown in Figure 2)

are considered to be coplanar when located so that a difference of their vertical coordinates is less than 5 m. This number is exemplary and those skilled in the art would understand that the term coplanar may also be defined as including those coils that although having different vertical coordinates, they are exposed to substantially the same vertical magnetic flux. Those skilled in the art would also understand that although the receiver coil system is connected with plural links 222 (e.g., ropes, etc.) to the transmitter coil system, due to the weight of each system it is likely that the receiver coil system has a slightly different Z coordinate than the transmitter coil system.

However, even with this difference due to the weight of the elements, the two coil

systems are considered to be "coplanar."

[0025] Time-domain AEM system 200 further includes a magnetometer sensor

220 located on one of the links 222, for example, at half distance between the

transmitter and receiver coil systems along a radial direction. Figure 2 shows a center

C of the receiver and/or transmitter coils 202A and 212. Note that magnetometer

sensor 220, in the embodiment illustrated in Figure 2, is not placed in the center C. In

other words, magnetometer sensor 220 is placed off-center in the time-domain AEM

system 200. In one embodiment, more than one magnetometer sensors may be

placed, for example, an additional magnetometer sensor 224 may be placed opposite,

relative to the center C, to sensor 222. If two or more magnetometer sensors are

located on the links 222, a gradient of the magnetic field may also be measured, in

addition to the actual magnetic field. The number of links 222 may vary from survey to

survey. In addition to employing a magnetometer coplanar with the transmitter loop, a

magnetometer could be placed not coplanar, but substantially coaxial (above or below)

the transmitter loop so as to allow measurement or calculation of a vertical gradient.

For example, a magnetometer could be placed at location 224 and another one 225 may be placed along linkage 215 so as to create a vertical offset between the magnetometers allowing measurement of a vertical gradient. The magnetometer 225 along linkage 215 would employ the same or similar sampling as the magnetometer at

224. Figure 2 also shows a clock system 250, a processor 252 and a memory storage

unit 254.

[0026] Figures 3A-3B illustrate the voltages generated in the transmitter coil and

the receiver coils as a result of applying a given current shape to the transmitter coil.

More specifically, by applying a certain current (e.g., half of a sinusoid) to the

transmitter coil, a voltage 330 is generated, which induces a magnetic field in the

receiver coils. As a result of the EM induction, a voltage is induced into the receiver

coils. Figure 3A shows voltages 332, 334 and 336 that correspond to each coil of the

receiver coil system (each coil is oriented along a different axis, and for this reason, the

amplitude of the measured signals is different). Transmitter current 330 is applied

again, after a period T, but with a reverse polarity. Other currents may be applied, as

for example, a sinusoid 330 followed by a square signal 340 as shown in Figure 3B.

Figure 3B also shows responses 340 and 342 from the receiver coil due to the square

signal 340. Those skilled in the art would know that other types of signals may be

applied to the transmitter coil system.

[0027] Figure 4 shows the various states of a magnetometer sensor when

exposed to a magnetic field produced by a current 402 applied to a transmitter coil.

While the response depends on the size of the magnetometer sensor, the type of

magnetometer sensor, the distance between the sensor and the coil, the shielding of

the sensor, etc., common to some magnetometer sensors placed in a large

electromagnetic field are the three phases illustrated in the figure, i.e., the loss of signal lock window 410, the sweeping for signal window 412, and the valid signal window 414.

Note that the valid signal window is present only if the pulse frequency provides

sufficient time. A rapid repetition rate, for example starting the next pulse before the

end of window 412, will preclude this phase. This is illustrated in Figure 4, where there

is insufficient time between the small square pulse 404 and the following negative large

sinusoid pulse 403. Also note that these windows 410, 412 and 414 are functional

windows associated with the magnetometer sensor, and will vary from sensor to sensor

depending on the size and shape of the electromagnetic field that the sensor is

exposed to.

[0028] Figure 4 shows the current 402 having three regions, a first region 402A

in which the current is large and varying, which also results in a large and varying

magnetic field. This large and varying magnetic field causes the magnetometer sensor

to lose the signal lock, which happens during the loss of signal lock window 410. The

first region 402A is followed by a second region 402B, during which no current is

applied to the transmitter, which means that no electromagnetic field is generated. The

time duration of the second region 402B is selected by the operator of the transmitter

coil and may last a given time interval that is traditionally in the order of a few

milliseconds to many milliseconds, but can be longer for a particular application. In one

application, the given time interval is between 3 and 30 ms. In another application, this

time interval may be seconds long. The duration of the entire waveform depicted in

Figures 3, 4, and 5 is a fraction of a second based on harmonic frequencies of ambient

noise (power transmission lines), for example 1/90, 1/30, 1/15 of a second in a 60 Hz

environment. During this period, as no electromagnetic field is generated by the

transmitter, the magnetometer sensor and associated electronics enters the sweeping for signal window 412, during which the sensor attempts to regain signal lock by sweeping for a valid magnetic field. Again, the duration of this window depends on the characteristics of each magnetometer sensor and the associated electromagnetic field that it has been exposed to. Once a valid magnetic field is detected, the magnetometer sensor enters a valid signal phase, which is illustrated in the figure as a valid signal window 414. During this window, the magnetometer sensor is locked onto the magnetic field and producing accurate readings. Note that the electromagnetic field generated by the current in the first region 402A is assumed to be a magnetic field that overwhelms the sensor or disrupts the standard operation of the sensor. The excitation current for the transmitter has, in this embodiment, a third region 402C during which a current is applied and generates a magnetic field that again can overwhelm the magnetometer sensor. This means that the magnetometer sensor enters another loss of signal lock window 410, as illustrated in Figure 4. The shape of the current 402 can vary from survey to survey, i.e., it can have any shape that is appropriate for the given survey. The specific shape shown in Figure 4 has a sinusoid component 403 and a square component 404. Any other current shape may be used. After this, the various windows 410, 412 and 414 of the magnetometer sensor and also the various regions of the current 402 are repeated (with an opposite polarity for the regions of the current).

The polarity may be the same or opposite depending on the survey.

[0029] Based on the above observations, the inventors of this application have

concluded that, for example, for the embodiment (timing diagram) illustrated in Figure

4, it would be advantageous to deduce (calculate or measure) a timing for the

magnetometer sensor during which the sensor is locked, and to apply a sampling

window for the magnetometer sensor during the valid signal window 414. In other words, once known, a timing can be deduced that will allow sufficient time for the sensor to recover, regain lock, and become stable before the next current pulse is fired.

This process can take a number of milliseconds after the end of the current pulse 402A.

Once the recovery process has taken place and data is considered to be acceptable,

the sampling of the data can take place for a number of milliseconds leading up to the

next pulse 402C being fired.

[0030] A timing illustration is shown in Figure 5, where magnetometer sensor is

sampled during sampling window 416, which is calculated to start after the end of the

sweeping for signal window 412 and just before the third region 402C when the next

pulse of current is fired in the transmitter. Figure 5 also illustrates a multipulse system

(i.e., a system that has at least two different pulses 402A and 402C). However, those

skilled in the art would know that this method can be applied to any time domain

waveform. The length of the sampling period is known as the sampling gate or window

416, and it can be adjusted to suit various magnetometer sensors, and systems as

required, and to obtain optimal results.

[0031] Thus, according to the above method, by understanding the time interval

the magnetometer sensors become overwhelmed and lose lock, and the time actually

needed to regain their lock on the ambient field, and by excluding that time interval

from the measured data, it is possible to regain a usable measurement from within the

strong fields produced by the transmitter coils, allowing the magnetometer sensors to

be placed in closer proximity to the transmitter loop, and advantageously closer to the

measurement target in the ground. In one embodiment, the magnetometer sensors are

placed in the same plane as the transmitter loop. Therefore, according to an embodiment, there is a method to selectively sample the magnetometer data to ignore the undesirable effects of the sensor losing lock on the ambient magnetic field.

[0032] The method is illustrated in Figure 6 and includes a step 600 of

determining the phases/windows of the magnetometer sensor discussed with regard to

Figure 4. Note that the time length of these windows vary from sensor to sensor and

even the number of windows may vary from sensor to sensor, but some magnetometer

sensors include at least a loss of signal lock window, a sweeping for signal window and

a valid signal window. This step may be performed in a controlled environment in

which external magnetic fields, similar to those to be generated by the transmitter coil

system, are applied to the magnetometer sensor. A time length of each of the above

windows is measured. This data may also be obtained from the manufacturer of the

magnetometer sensor, or it may be theoretically calculated based on various

mathematical models of the magnetometer sensor, or determined experimentally or

experientially by practitioners. In one embodiment, the above noted windows may

change based on the intensity, or rate of change of the current applied to the

transmitter coil system. If this is the case, this step needs to be performed by the

operator of the survey for accurately determining the lengths of the windows 410, 412,

and 414.

[0033] In step 602, the details of the survey are received/calculated. For

example, activation characteristics of the transmitter coil system, as the type of pulses

to be applied to the transmitter coil system and the lengths of each region 402A, 402B

and 402C of the current applied to the transmitter coil system need to be known.

Based on this information and the information from step 600, in step 604, a length and

a starting point of the sampling window 416 can be calculated so that the sampling windows span between the end of the sweeping for signal window 412 and the beginning of the next pulse 402C. The beginning and the length of the sampling window 416 or the beginning and the end of the sampling window are calculated in this step. In one embodiment, the (time) length of the sampling window can be smaller than the length of the valid signal window 414. In one embodiment, the length of the sampling window 416 is equal to the length of the valid signal window 414.

[0034] In step 606, the magnetometer sensor, which is placed coplanar with the

transmitter coil system of a time-domain AEM system, is collecting EM data and only

the data recorded during the sampling window 416 is used for the purpose of imagining

in step 608 the survey subsurface. During this step, the time-domain AEM system is

flown (i.e., airborne) above the subsurface of interest and the transmitter coil system is

driven in a time-domain manner, i.e., with one or more pulses that change in time as

illustrated, for example, in Figures 3A and 3B. In this way, the present method allows

data recorded by the magnetometer sensor to be used for imaging the subsurface,

although the large magnetic field produced by the transmitter coil system seriously

perturbs the magnetometer sensor ability to accurately measure and store the data.

[0035] The method discussed above may include, optionally, one or more of the

following steps: recording electromagnetic data with the receiver coil system 212,

where the step of generating an image includes using both the magnetometer data and

the electromagnetic data for generating the image. The method may further include

flying the magnetometer sensor 220 coplanar and within an area defined by the

transmitter coil system 202. The various functional time windows discussed above

include the loss of signal lock window 410, the sweeping for signal window 412, and

the valid signal window 412. The activation characteristics of the transmitter coil system include a current pulse, e.g., 402C. The sampling window may be calculated to start after the sweeping for signal window 412 and to end prior to the current pulse

402C. In one application, the sampling window is calculated to fall within the valid

signal window 412. In one application, the method includes configuring the acquisition

system to record the magnetometer data only during the sampling window or recording

the magnetometer data continuously during the airborne survey and removing, during

processing, all data not within the sampling window

[0036] There are a couple of ways to calculate/create the sampling window 416

discussed above. For example, according to an embodiment, it is possible to create

o the sampling window based on the timing of the magnetic field created by the

transmitter coil system and the timing of the various windows of the magnetometer

sensor. More specifically, the timing of the sampling window 416 is calculated based

on a reference to a synchronous clock signal generated within the AEM system's

clocking architecture. This approach will use an experimental method to determine the

best position and timing of the sampling window. In one application, an iterative

validation of the sampling may be used. Also, this method requires that the AEM

system illustrated in Figure 2 includes at least the clock system 250, the processor 252

and the memory storage unit 254.

[0037] Another approach will be to monitor and apply the magnetometer

sensor's signals such as the "Loss of Lock"signal, and the "Hemisphere Switching"

signal. In this respect, some magnetometer sensors provide auxiliary signals that allow

user logic (possible to be implemented in processor 252) to determine if the output

signal from the sensor is in a valid state or not. These signals can be a "Loss of Lock"

signal ora "Hemisphere Switching" signal. If these signals are informing the user that the output signal is not valid, then the sampling window can be dynamically adjusted until these auxiliary signals indicate that the output is in a valid and known state.

[0038] Another approach would be to use a signal processing algorithm

designed to reject the erroneous signals being measured during the time periods not

contained within the appropriate sampling window. User logic can be added that

monitors the signals for erroneous signal changes (impossible for a typical ambient

magnetic field to exhibit). Once detected, the sampling window can be closed until a

stable value is once again present, at which point the sampling window can be re

opened and valid data collected.

[0039] The sampling technique discussed above with regard to Figure 6 allows

the magnetometer sensors to be used within the area enclosed by the transmitter loop,

thus closer to the earth, for a higher resolution measurement, traditionally a place

considered unusable without this technique. Although the magnetometer sensors may

be placed farther away from the transmitter, such that they are not in the plane of or

inside of the transmitter loop, locating the sensors a distance above the transmitter loop

on the tow cable or within the structural rope cone of the transmitter, or passively or

actively bucking the primary EM field to shield the magnetometer sensor from these

effects, is not as efficient as the methods discussed above. The concept of bucking the

EM field is known in the art, and uses two or more sets of transmitter coils located in

the same plane but receiving the current in opposite directions for cancelling the

generated magnetic field at a given spatial location, as disclosed, for example, in U.S.

patent no. 9,297,922. Note that the embodiment illustrated in Figure 2 does not need a

bucking arrangement for using the magnetometer sensor as discussed above.

[0040] There are many possible implementations of the geophysical system

discussed above. An electromagnetic geophysical system may include many other

peripheral sensors to determine the position or orientation or state of the

electromagnetic measurement, such as Global Positioning System (GPS), radar or

laser altimeter, gyroscopes or inclinometers measuring transmitter or sensor positions,

thermometers, etc.) or other sensors measuring other geophysical data (such as radar

or laser for topography, gravity or gradiometers sensors, spectrometer sensors, etc.).

Consequently, there are also many different methods to record, process, combine and

control all of these signals and sensors.

[0041] The method discussed above with regard to Figure 6 may be

implemented in a processing device. An example of a processing device capable of

carrying out operations in accordance with the embodiments discussed above is

illustrated in Figure 7. Such processing device may be located on the carrier 214, in a

research facility, distributed at multiple sites, etc. Hardware, firmware, software or a

combination thereof may be used to perform the various steps and operations

described herein.

[0042] The exemplary processing device 700 suitable for performing the

activities described in the exemplary embodiments may include server 701. Such a

server 701 may include a central processor unit (CPU) 702 coupled to a random

access memory (RAM) 704 and/or to a read-only memory (ROM) 706. The ROM 706

may also be other types of storage media to store programs, such as programmable

ROM (PROM), erasable PROM (EPROM), etc. Processor 702 may communicate with

other internal and external components through input/output (1/O) circuitry 708 and

bussing 710, to provide control signals and the like. For example, processor 702 may communicate with the various EM receivers, transmitter, magnetometer sensor, etc.

Processor 702 carries out a variety of functions as are known in the art, as dictated by

software and/or firmware instructions.

[0043] Server 701 may also include one or more data storage devices, including

disk drives 712, CD-ROM drives 714, and other hardware capable of reading and/or

storing information, such as a DVD, etc. In one embodiment, software for carrying out

the above-discussed steps may be stored and distributed on a CD-ROM 716,

removable media 718 or other form of media capable of storing information. The

storage media may be inserted into, and read by, devices such as the CD-ROM drive

714, disk drive 712, etc. Server 701 may be coupled to a display 720, which may be

any type of known display or presentation screen, such as LCD, plasma display,

cathode ray tube (CRT), etc. A user input interface 722 is provided, including one or

more user interface mechanisms such as a mouse, keyboard, microphone, touch pad,

touch screen, voice-recognition system, etc.

[0044] Server 701 may be coupled to other computing devices, such as the

equipment of the carrier, via alink or network. The server may be part of alarger

network configuration as in a global area network (GAN) such as the Internet 728,

which allows ultimate connection to the various landline and/or mobile devices

involved in the survey.

[0045] As also will be appreciated by one skilled in the art, the exemplary

embodiments may be embodied in a wireless communication device, a

telecommunication network, as a method or in a computer program product.

Accordingly, the exemplary embodiments may take the form of an entirely hardware

embodiment or an embodiment combining hardware and software aspects. Further, the exemplary embodiments may take the form of a computer program product stored on a computer-readable storage medium having computer-readable instructions embodied in the medium. Any suitable computer-readable medium may be utilized, including hard disks, CD-ROMs, digital versatile discs (DVD), optical storage devices or magnetic storage devices such as a floppy disk or magnetic tape. Other non-limiting examples of computer-readable media include flash-type memories or other known types of memories.

[0046] This written description uses examples of the subject matter disclosed

to enable any person skilled in the art to practice the same, including making and

using any devices or systems and performing any incorporated methods. For

greater clarity, the figures used to help describe the invention are simplified to

illustrate key features. For example, figures are not to scale and certain elements

may be disproportionate in size and/or location. Furthermore, it is anticipated that

the shape of various components may be different when reduced to practice, for

example. The patentable scope of the subject matter is defined by the claims, and

may include other examples that occur to those skilled in the art. Such other

examples are intended to be within the scope of the claims. Those skilled in the art

would appreciate that features from any embodiments may be combined to generate

a new embodiment.

[0047] The disclosed embodiments provide a method and EM system capable

of recording magnetic fields or signals related to magnetic fields with at least one

magnetometer sensor placed coplanar with a transmitter coil system. It should be

understood that this description is not intended to limit the invention. On the

contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0048] Although the features and elements of the present exemplary

embodiments are described in the embodiments in particular combinations, each

feature or element can be used alone without the other features and elements of the

embodiments or in various combinations with or without other features and elements

disclosed herein.

[0049] This written description uses examples of the subject matter disclosed to

enable any person skilled in the art to practice the same, including making and using

any devices or systems and performing any incorporated methods. The patentable

scope of the subject matter is defined by the claims, and may include other examples

that occur to those skilled in the art. Such other examples are intended to be within the

scope of the claims.

Claims (20)

WHAT IS CLAIMED IS:

1. A time-domain airborne electromagnetic (AEM) system for measuring a

signal related to a magnetic field during a survey, the AEM system comprising:

a transmitter coil system configured to generate a primary electromagnetic

field that penetrates into the earth;

a receiver coil system configured to record a secondary electromagnetic field

generated by the earth as a response of the primary electromagnetic field; and

a magnetometer sensor located coplanar with the transmitter coil system and

configured to record signals related to an external magnetic field,

wherein a sampling window is associated with the magnetometer sensor and

the sampling window is calculated based on (i) various functional windows

characterizing the magnetometer sensor and (ii) current pulses applied to the

transmitter coil system during the survey.

2. The AEM system of Claim 1, wherein the functional windows include a loss

of signal lock window, a sweeping for signal window and a valid signal window.

3. The AEM system of Claim 2, wherein the sampling window is equal to or

smaller than the valid signal window.

4. The AEM system of Claim 2, wherein the sampling window matches in time

the valid signal window.

5. The AEM system of Claim 2, wherein the sampling window starts after the

sweeping for signal window and before a new pulse of current is applied to the

transmitter coil system.

6. The AEM system of Claim 1, wherein the magnetometer sensor is located

on a link connecting the transmitter coil system to the receiver coil system.

7. The AEM system of Claim 1, wherein the magnetometer sensor, the

transmitter coil system and the receiver coil system are coplanar.

8. The AEM system of Claim 1, wherein the transmitter coil system is free of a

bucking circuit.

9. The AEM system of Claim 1, wherein the magnetometer sensor is located

in the middle between the transmitter and receiver coil systems, along a radial

direction.

10. The AEM system of Claim 1, further comprising:

another magnetometer sensor located opposite the magnetometer sensor,

relative to a central point (C) of the transmitter coil system, and the two

magnetometer sensors measure a gradient of the magnetic field.

11. The AEM system of Claim 1, further comprising: another magnetometer sensor located substantially above or below the magnetometer sensor to measure a vertical gradient of the magnetic field.

12. A method for collecting magnetometer data with a magnetometer sensor

during an airborne electromagnetic survey, the method comprising:

receiving various functional time windows characterizing the magnetometer

sensor;

receiving activation characteristics of a transmitter coil system that generates

a primary electromagnetic field that penetrates the earth;

calculating a sampling window of the magnetometer sensor based on the

various functional time windows of the magnetometer sensor and the activation

characteristics of the transmitter coil system;

collecting the magnetometer data during the sampling window with the

magnetometer sensor while flying a time-domain airborne electromagnetic (AEM)

system, wherein the AEM system includes the transmitter coil system and the

magnetometer sensor; and

generating an image of a surveyed subsurface based on the magnetometer

data collected with the magnetometer sensor during the sampling window.

13. The method of Claim 12, further comprising:

recording electromagnetic data with a receiver coil system configured to

record a secondary electromagnetic field generated by the earth as a response of

the primary electromagnetic field.

14. The method of Claim 13, wherein the step of generating an image

comprises:

using both the magnetometer data and the electromagnetic data for

generating the image.

15. The method of Claim 12, further comprising:

flying the magnetometer sensor coplanar and within an area defined by the

transmitter coil system.

16. The method of Claim 12, wherein the various functional time windows

include a loss of signal lock window, a sweeping for signal window and a valid signal

window.

17. The method of Claim 16, wherein the activation characteristics of the

transmitter coil system include a current pulse.

18. The method of Claim 17, wherein the sampling window is calculated to

start after the sweeping for signal window and to end prior to the current pulse.

19. The method of Claim 17, wherein the sampling window is calculated to fall

within the valid signal window.

20. The method of Claim 12, further comprising: recording the magnetometer data continuously during the airborne survey; and removing, during processing, all data not within the sampling window.