AU2012380303A1

AU2012380303A1 - Receiver coil assembly with air and ferromagnetic cored sensors for geophysical surveying

Info

Publication number: AU2012380303A1
Application number: AU2012380303A
Authority: AU
Inventors: Jack Dodds
Original assignee: Geotech Airborne Ltd
Current assignee: Geotech Airborne Ltd
Priority date: 2012-05-17
Filing date: 2012-05-17
Publication date: 2014-12-11
Also published as: RU2014150933A; BR112014028500A2; WO2013170340A1; CN104641260A; EP2850464A1; EP2850464A4; CA2873849A1; IN2014DN10512A

Abstract

A receiver coil assembly for performing geophysical surveys, including a hollow outer shell defining a continuous internal passage that forms a loop; a multiturn receiver air coil extending around the continuous internal passage; and a first cored coil comprising multiturn solenoid windings about a ferromagnetic core, the first cored coil located adjacent a region of the air coil within the internal passage and having a sensing axis in a different direction than a sensing axis of the air coil.

Description

WO 2013/170340 PCT/CA2012/050322 RECEIVER COIL ASSEMBLY WITH AIR AND FERROMAGNETIC CORED SENSORS FOR GEOPHYSICAL SURVEYING BACKGROUND 5 [0001] Embodiments of the described invention relate to the field of airborne geological mapping and receiver systems used for such mapping. [0002] Active source electromagnetic surveying such as time domain electromagnetic (TDEM) surveying is a rapidly developing area of geophysical surveying. It encompasses ground based and airborne applications. TDEM 10 geological mapping involves measuring the magnetic response of the earth to a primary magnetic field transmitted by the survey system. The relation between the transmitted field and the response is used to calculate the electrical resistivity structure of the earth, from which geological information is inferred. [0003] An example of a TDEM surveying system and method is shown in 15 U.S. Patent No. 7,157,914. [0004] Natural source electromagnetic surveying has traditionally been performed using ground based techniques such as the magnetotelluric (MT) technique. Recently, airborne surveying using natural source electromagnetic techniques has become practical. In these techniques, two or more components 20 of naturally occurring random fluctuations of the electromagnetic field of the earth are measured (possibly at different locations), and the frequency dependent transfer functions between the measured components are calculated. As in active source methods, the transfer functions are interpreted in terms of the electrical resistivity structure of the earth, from which geological information 25 is inferred. [0005] An example of a natural source electromagnetic surveying system is shown in U.S. Patent No. 6,876,202 [0006] An active source electromagnetic survey system has a transmitter and a receiver, while a natural source system has only a receiver. Typically a 30 transmitter includes a coil of one or more turns of electrical wire. When an electric current is passed through the wire, a magnetic field is transmitted. In TDEM surveying, a pulsed current of alternating polarity is used, and the 1 WO 2013/170340 PCT/CA2012/050322 response of the earth is sensed in the "off" time between transmitter current pulses. [0007] A receiver or sensor typically includes of one or more multiturn coils of electrical wire. In the presence of a changing magnetic field, an electrical 5 voltage appears across the terminals of each coil. This voltage can be amplified and recorded. Coils may have different orientations, making them sensitive to variations in magnetic field components having different directions. Other things being equal, the sensitivity and noise floor of a receiver coil improve as the coil is scaled up in size. The signals used in natural source systems are typically 10 weaker, requiring larger receiver coils, compared to active source systems. [0008] The response to movement and vibration of receivers used in active and natural source surveying systems is a significant noise source, especially in a turbulent airborne environment, becoming increasingly important as the signal frequency decreases below 100 Hz. A major contribution to this type of noise is 15 caused by the motion of the receiver coil(s) relative to the static geomagnetic field. Motion or vibration that changes the total geomagnetic flux passing through a receiver coil causes a electrical voltage to appear across the terminals of the coil. In the case of a rigid receiver coil, this can be caused by rotation of the coil. No receiver coil is perfectly rigid, so flexing of the coil also contributes 20 to such voltages. These voltages are a type of noise that interferes with the desired signal. Techniques for reduction of noise are important. [0009] In some electromagnetic survey systems, the receiver is sensitive to changes in one component of the magnetic field, typically a nominally vertical component. Receivers that independently measure changes in two or three 25 substantially orthogonal components of the magnetic field provide improved geological information, but are bulkier than single axis receivers. [0010] Improved receiver systems for airborne geophysical survey systems are desirable. SUMMARY 30 [0011] According to one example embodiment there is provided a receiver coil assembly for performing geophysical surveys. The receiver coil assembly includes a hollow outer shell defining a continuous internal passage that forms a loop; a multiturn receiver air coil extending around the continuous internal 2 WO 2013/170340 PCT/CA2012/050322 passage; and a first cored coil comprising multiturn solenoid windings about a ferromagnetic core, the first cored coil located adjacent a region of the air coil within the internal passage and having a sensing axis in a different direction than a sensing axis of the air coil. 5 [0012] According to another example embodiment is a receiver coil assembly for performing geophysical surveys, including a multiturn air coil receiver defining a loop; at least one receiver coil having a ferromagnetic core and a solenoid winding, supported immediately adjacent a region of the air coil, with a long axis of the core being substantially parallel to turns of the adjacent 10 region of the air coil. [0013] According to an example embodiment is a receiver coil assembly for performing geophysical surveys, including an outer shell including a tubular outer portion enclosing a space defining a polygonal or circular loop; a multiturn air coil receiver extending around and within the tubular outer portion; a first 15 cored coil receiver comprising one or more multiturn solenoid windings about one or more ferromagnetic cores, located in the outer shell, each core being positioned substantially parallel to the segments of the air coil receiver adjacent to it; and a second cored coil receiver comprising one or more multiturn solenoid windings about one or more ferromagnetic cores, located in the outer shell, each 20 core being positioned substantially parallel to the segments of the air coil receiver adjacent to it; with the two cored coil receivers positioned so that the air coil receiver and the two cored coil receivers each sense changes in a different component of a magnetic field. [0014] According to another example embodiment is a receiver coil 25 assembly as described in the previous paragraph, in which the cored coil receivers are attached to the turns of the air coil receiver, thereby mitigating noise which could be induced (especially in the air coil) by relative motion between the cored coils and the air coils in the presence of an external magnetic field. 30 [0015] Accoording to another example embodiment is a receiver coil assembly as described in the previous paragraph, in which the cored coil receivers are embedded within the turns of the air coil, such that the turns of the air coil lie substantially parallel to the axes of the adjacent cores and are distributed symmetrically around two or more sides of the adjacent cores, 3 WO 2013/170340 PCT/CA2012/050322 thereby mitigating noise which could be induced (especially in the air coil) by rotation of the assembly in an external magnetic field. BRIEF DESCRIPTION OF THE DRAWINGS 5 [0016] Figure 1 is a perspective diagrammatic view of an airborne electromagnetic survey system according to an example embodiment of the invention. [0017] Figure 2 is an illustrative view of a receiver system that can be used in the airborne electromagnetic survey system of Figure 1. 10 [0018] Figure 3 is a block diagram representation of a receiver coil orientation sensing system of Figure 2. [0019] Figure 4 is a block diagram representation of a noise reduction module used with the receiver coil assembly of Fig. 2. [0020] Figure 5A is a view of alternative receiver coil system that can be 15 used in the airborne electromagnetic survey system of Figure 1. This is a plan view, with the upper half of the outer shell removed to shown the inner shell and coil assembly. [0021] Figure 5B is a cutaway side view of the alternative receiver coil system shown in Figure 5A, taken along the lines A-A of Figure 5A. 20 [0022] Figure 6 is a diagram of a solenoid coil assembly with a ferromagnetic core, which is a component of the receiver coil system shown in Figure 5. [0023] Figure 7 is a diagram of an alternative receiver coil system using three square coils suspended in an external shell in the form of a regular 25 octahedron. [0024] Figure 8 is a diagram of a receiver coil system, according to a further example embodiment, using an air coil receiver suspended in an external shell, and cored coil receivers attached to the turns of the air coil. [0025] Figure 9 is a cross-section IX-IX of one embodiment of the receiver 30 coil system of Figure 8 showing the cored coil attached to the turns of the air 4 WO 2013/170340 PCT/CA2012/050322 coil, with the entire assembly suspended by elastic members from the outer shell. [0026] Figure 10 is a cross-section IX-IX of the receiver coil system of Figure 8 showing an alternative arrangement in which the turns of the air coil 5 are positioned symmetrically around the cored coil, with the entire assembly suspended by elastic members from the outer shell. [0027] Figure 11 is a cross-section IX-IX of the recevier coil assembly of Figure 8 showing an alternative arrangement in which the elastic suspension is a double suspension. 10 [0028] Figure 12 is a side view of a multi-segmented cored coil that could be used in the receiver coil assembly of Figures 8-11. [0029] Figure 12A is an enlarged partial view of a section of the cored coil of Figure 12. DETAILED DESCRIPTION 15 [0030] In one example embodiment, the receiver coil system includes a substantially rigid assembly carrying three coils of electrical wire having mutually orthogonal axes. These coils sense changes in three independent components of the magnetic field, which provides more information than a single axis receiver. In addition, the assembly carries an orientation sensing system, including 20 angular accelerometers, a three axis fluxgate magnetometer and two axis tilt sensors. The rigid assembly is elastically suspended within a non-metallic enclosing outer shell which protects it from air flow and is in turn suspended directly or indirectly from a towing aircraft. The elastic suspension attenuates motion and vibration transmitted to the rigid assembly from the outer shell. 25 [0031] In this first example embodiment, a processing system accepts the outputs of the orientation sensing system. It uses them to calculate, and subtract from each of the receiver coil outputs, the noise which is caused by rotational motion of the receiver coils in the static geomagnetic field. It also uses them to combine the three receiver coil outputs to correct errors in each 30 receiver output which result from static departures of the receiver coil assembly from its nominal orientation. 5 WO 2013/170340 PCT/CA2012/050322 [0032] Alternatively, in the first example embodiment, the output of the fluxgate magnetometer may be used to combine the three receiver coil outputs to resolve a signal which would be sensed by a receiver coil oriented parallel to the geomagnetic field vector. In this resolved signal, noise due to rotation in the 5 geomagnetic field is minimized, and changes caused by departures of the receiver coil assembly from its nominal orientation are eliminated. [0033] In a second example embodiment, the receiver includes a semi rigid assembly in the shape of a polygonal or circular loop having two perpendicular diameters, in a nominally horizontal plane. The outer polygonal 10 or circular part of the assembly includes a multiturn coil of electrical wire, while multiturn solenoid windings with rod shaped ferromagnetic cores are positioned on the diameters and cross in the middle of the assembly. The assembly is partially enclosed by and elastically suspended at multiple points from a similarly shaped inner shell. The inner shell also carries six or more accelerometers 15 positioned around its edge and oriented to sense rotations about three independent axes. The inner shell is fully enclosed by, and elastically suspended at multiple points from, a similarly shaped outer shell which protects the inner shell and semi-rigid assembly from air flow and is in turn suspended directly or indirectly from the towing aircraft. The elastic suspensions attenuate motion 20 and vibration transmitted to the semi-rigid assembly from the outer shell. [0034] In the second example embodiment, the multi-point suspensions distribute inertial loads uniformly, reducing the flexing of the semi-rigid assembly and inner shell. This improves their effective rigidity, or allows the equivalent rigidity to be achieved with less material. The use of ferromagnetic 25 cores for the two coils having nominally horizontal axes reduces the size of the receiver in the nominally vertical direction. [0035] In the second example embodiment, a processing system accepts the outputs of the accelerometers. It uses them in an adaptive noise cancellation algorithm to remove noise from each of the receiver coil outputs 30 which is caused by motion of the receiver coils in the geomagnetic field. It also processes the dc component of the outputs of some of the accelerometers (those oriented with horizontal sensitive directions) to sense the tilt of the receiver coil system, and combines the three receiver coil outputs to correct errors in each receiver output which result from the static tilt of the receiver coil assembly 6 WO 2013/170340 PCT/CA2012/050322 relative to its nominal orientation. Optionally, heading information from a navigation system or other sensors may be used to additionally correct for departures from nominal heading. [0036] A multi-turn coil serves as a receiver for changes in the magnetic 5 field, measuring the magnetic field time derivative dB/dt. In the case of an active source TDEM system, the receiver coil is used to measure the time decay of eddy currents in geological structures during the OFF time following a transmitter pulse. In the case of a natural source system, the receiver senses random fluctuations of the natural electromagnetic field, which are affected by 10 geological structures. Coil voltages are digitized by a known analog to digital converter (ADC) and processed and stored by a computer. Processing and storage may take place during the acquisition of the data, or at a later time. [0037] Among other things, horizontal or vertical rotational motion of the receiver coil can introduce noise into the measurements made by the receiver 15 system. For example, rotation of a vertical axis receiver coil about a horizontal axis can induce noise due to the movement of the receiver coil relative to the geomagnetic field. The effect of the noise tends to increase as the frequency decreases below 100 Hz, so introduction of this noise can provide a lower limit on the usable frequency range of the system. This in turn can place limits on 20 the penetration depth provided by the survey system. [0038] Departures of the receiver coil from its nominal attitude can introduce errors into the measurements. For example, tilting the axis of a horizontal axis receiver coil will cause it to respond to changes in the vertical magnetic field, in addition to the intended horizontal field, which may lead to 25 errors in interpretation of the results. [0039] Example embodiments are described herein for a multiple axis receiver coil system, and for de-noising such a receiver coil system to mitigate against noise and errors introduced through dynamic and static horizontal or vertical rotation of the receiver coil system. 30 [0040] For the purposes of explaining one example embodiment, Figure 1 shows a schematic view of an airborne TDEM survey system 100 that includes a transmitter coil 104 and a receiver coil assembly or system 102. The TDEM survey system 100 can be carried by an aircraft 28 such as an airplane, 7 WO 2013/170340 PCT/CA2012/050322 helicopter, balloon or airship, for example. In at least some example embodiments, the transmitter coil 104 and receiver coil system 102 are part of a tow assembly 12 that is towed by the aircraft 28. In the example embodiment shown in Figure 1, the transmitter coil 104 and receiver coil system 102 are 5 substantially concentric, with the transmitter coil 104 being mounted to a frame 20 that is suspended from multiple support cables or ropes 16 that are each attached to a unique point on the circumference of the transmitter coil frame at one end and to a common tow cable 15 at the other end. In one example embodiment the transmitter coil frame 20 is a simple polygonal frame that 10 approximates a circle and is formed from a plurality of tubular segments that define a continuous internal passage in which the transmitter coil 104 extends. In some example embodiments, the ropes 16 include at least one suspension cable or rope that supports the receiver coil system 102. The receiver coil system may in some example embodiments be centrally positioned by a series of 15 radially extending cables or ropes 14 that extend to the transmitter coil frame 20. In one example embodiment, when in use the transmitter coil 104 is horizontally positioned with a substantially vertical dipole axis, and the receiver coil system 102 is located at a center of the transmitter coil 104. [0041] The tow assembly configuration shown in Figure 1 is merely one 20 example of many possible physical configurations that the TDEM survey system 100 can have - for example, in some embodiments the receiver coils system 102 can be physically supported separately from the transmitter coil 104 rather than being part of the same tow assembly. [0042] Figure 2 illustrates the receiver coil system 102 in greater detail. 25 Also shown in Figure 2 is a controller 106 that is included in the TDEM survey system 100, and which is coupled to both the transmitter coil 104 and the receiver coil system 102. The controller 106 includes, among other things, one or more analog to digital converters for converting data received from the receiver coil system 102, a transmitter driver for driving the transmitter coil 104, 30 and a computer for controlling the overall operation of the TDEM survey system 100 and processing the data received through the components of the TDEM survey system 100. The controller 106 can also include an altimeter system for tracking the absolute and relative altitude of the TDEM survey system 100. In one example embodiment, the controller 106 is located within a body of the 8 WO 2013/170340 PCT/CA2012/050322 aircraft. In some example embodiments some of the functions of the controller 106 are performed at a location remote from the aircraft that is carrying the transmitter coil 104 and a receiver coil system 102. [0043] In one example embodiment, the receiver coil system 102 includes 5 a fully enclosing outer shell 101. Within the shell, an elastic suspension 103 supports a rigid receiver coil assembly. The rigid assembly includes three substantially planar coils that are substantially orthogonal to each other. For example, in its nominal orientation, a first or Z-axis coil 112 has a dipole axis that runs along a Z-axis, a second or X-axis coil 114 has a dipole axis oriented in 10 a X-axis direction, and a third or Y-axis coil 116 has a dipole axis that is oriented along a Y-axis direction. As indicated by the X-Y-Z reference coordinates 120 shown in Figure 2, the Z-axis corresponds to vertical, the X-axis extends horizontally in the direction of travel and the Y-Axis extends horizontally transverse to the direction of travel. During operation, the Z, X and Y axis coils 15 112, 114, 116 of the receiver coil system 102 move relative to the reference coordinate system 120, and example embodiments are directed to removing noise introduced by such movement. [0044] In one non-limiting example embodiment, the Z, X and Y receiver coils 112, 114 and 116 each are air-core coils having 100 turns of approximately 20 1 square meter each turn, however many other numbers of coil turns and coil size could alternatively be used. [0045] Output voltages from the Z-coil 112, the X-coil 114 and the Y-coil 116 are provided through a connection box 108 to the controller 106. The rigid receiver coil assembly also includes a coil orientation sensing system 110 that 25 collects angular attitude and heading information about the rigid assembly for controller 106. [0046] As shown in Figure 3, in an example embodiment, the receiver coil orientation sensing system 110 senses the orientation and rotation rates of the rigid receiver coil assembly. Sensing system 110 includes three angular 30 accelerometers 310, 311, 312, with sensitive rotational axes parallel to the axes of receiver coils 112, 114 and 116. The bandwidth of the angular accelerometers is 100 Hz or more, so that it includes at least the lower end of the range of frequencies which is sensed by the receiver coils 112, 114, and 9 WO 2013/170340 PCT/CA2012/050322 116. Sensing system 110 includes a three-axis fluxgate magnetometer 315 with sensitive axes of receiver coils 112, 114 and 116, which measures the magnitude and direction of the geomagnetic field relative to the receiver coil axes. Sensing system 110 includes tilt sensors 313 and 314, which measure 5 the tilt of the rigid receiver coil assembly relative to the z (vertical) axis as shown in reference coordinates 120 (Fig. 2). The bandwidth of the fluxgate and the tilt sensors may be substantially less than the bandwidth of the receiver coils and the accelerometers. The orientation sensing system can also include other orientation sensing equipment such as multiple GPS receivers operating in 10 differential carrier phase mode, linear accelerometers, or gyroscope based sensors to measure the angular motion rates of the receiver system. Example embodiments can include all types of the sensors identified here, or a subset. [0047] With reference to Figure 4, the controller 106 includes a noise reduction module 300 (which could for example be implemented by a suitably 15 configured computer) for de-noising and correcting the receiver coil outputs received through analog to digital converters 302 from Z, X and Y receiver coils 112, 114, 116 in dependence on the information received through analog to digital converters 304 from the Z, X and Y angular accelerometers 310, 311 and 312, the tilt sensors 313 and 314, and the three axis fluxgate magnetometer 20 315. [0048] Within the noise reduction module 300, the angular accelerometer outputs are processed using known techniques to determine instantaneous angular rotation rates of the receiver coil assembly about each of its axes. These rotation rates are combined with the geomagnetic field measured by the 25 fluxgate 315 to predict the resulting voltages across each receiver coil outputs, which are then subtracted from each of the digitized receiver outputs to mitigate the effect of angular motion of the receiver coils system 102 in the geomagnetic field. In an additional processing step, the tilt sensor 313, 314 and fluxgate magnetometer 315 outputs are processed to determine a matrix which rotates 30 vectors in the moving frame of reference defined by the sensitive axes of receiver coils 112, 114, and 116 into the fixed frame of reference 120. This matrix is then used to combine the outputs of receiver coils 112, 114 and 116 to correct the output signal of each coil for departures from its nominal orientation. 10 WO 2013/170340 PCT/CA2012/050322 [0049] In some example embodiments, the voltages received from the receiver coils 112, 114 and 116 are digitized at a 50 to 200 kHz sampling rate, and coil orientation sensor system 110 outputs are sampled at 100 to 300 Hz. In some example embodiments, the noise reduction module 300 processes the 5 digitized angular accelerometer outputs using known low pass filtering and numerical integration techniques to estimate the angular rotation rate pseudo vector at each sample interval. For more accurate integration of the rotation rates, known techniques based on the representation of rotations by quaternions may be used. Once the angular rate estimate at each sample interval has been 10 found, the predicted voltage across each receiver coil output is given by vs=(SxQ) * B where vs is the voltage across the coil with sensitive direction S; S is a vector representing the response of the coil, with magnitude equal to the effective area of the coil and direction the same as the sensitive direction (axis) of the coil; x 15 denotes the vector cross product (outer product); A is the angular rotation rate pseudo-vector, derived as described above from the angular accelerometer outputs; * denotes the dot product (inner product) and B is the geomagnetic field vector as measured by the fluxgate magnetometer. The vectors and pseudo-vector are expressed in the coordinate system of the receiver coil 20 assembly. Accordingly, the denoising module 300 processes the digitized orientation sensor outputs for each sample interval according to the above formula, yielding a motion noise estimate time series for each receiver coil. This time series is resampled using known techniques to obtain the sample rate of the digitized receiver coil outputs, scaled to account for the gains of the various 25 analog to digital converters, and subtracted from the receiver coil time series outputs. A skilled practitioner could achieve similar results using variations of this process. [0050] In some example embodiments, the tilt sensor outputs are used to define the tilt of the receiver coil assembly relative to the z (vertical) axis of 30 coordinate system 120. Given the tilt angles, the output of the fluxgate magnetometer 315 can be used to resolve the horizontal component of the geomagnetic field, giving the magnetic heading, thereby completely defining the orientation of the receiver coil assembly. To mitigate errors caused by the effect of acceleration on the tilt sensors, in some example embodiments, the long term 11 WO 2013/170340 PCT/CA2012/050322 attitude and heading determined from the fluxgate magnetometer 315 and tilt sensors 313 and 314 are combined using known integration techniques with short-term orientation changes determined from the outputs of the angular accelerometers 310, 311, 312. The attitude and heading are processed using 5 known techniques to determine a matrix which rotates vectors in the moving frame of reference defined by the sensitive axes of receiver coils 112, 114, and 116 into the fixed frame of reference 120. This matrix is then used to combine the outputs of receiver coils 112, 114 and 116 to correct the output signal of each coil for departures from its nominal orientation. 10 [0051] In some example embodiments, the angular compensation module 300 determines the angles between the geomagnetic field and the axes of the three receiver coils 112, 114, and 116 and combines the digitized voltages from the receiver coils to calculate the signal that would be measured by a receiver coil with its axis aligned with the geomagnetic field. The signal measured by a 15 coil so aligned is insensitive to small changes in the coil orientation, which mitigates the effect of rotations of the receiver coil system. More specifically, the output from coil 112 is multiplied by the cosine of the angle between the coil axis and the geomagnetic field, similarly for coils 114 and 116, and the sum of these three contributions is output by module 300. The fluxgate magnetometer 20 output yields the required cosines directly, by dividing each of the three components by the magnitude of the geomagnetic field vector. In an alternative embodiment using GPS receivers instead of a fluxgate magnetometer, the direction of the geomagnetic field is calculated from the geographic position of the survey location using standard formulas (e.g. those 25 known as the "International Geomagnetic Reference Field") for the orientation of the geomagnetic field. This is then combined with the attitude and heading measurements of the receiver coil system determined from the GPS receivers to calculate the required angles. [0052] Although the three receiver coils 112, 114 and 116 in the presently 30 described embodiment have been described as being orthogonal to each other and generally oriented along Z, X and Y axis, the three receiver coils could be positioned at non-orthogonal angles relative to each other, so long as the relative angles are known, and the processing of the information received from the coils and orientation sensors adjusted accordingly. 12 WO 2013/170340 PCT/CA2012/050322 [0053] In some example embodiments, the de-noising and orientation correction described above can also be applied to natural source airborne survey systems, including for example audio frequency magnetic (AFMAG) airborne systems that measure the earth's response to naturally occurring events such as 5 lightening strikes. [0054] The receiver coil system shown in Figure 1 can fully implement a three axis, noise reduction and orientation correction capability as described above. There are a number of ways in which it can be modified so as to more conveniently and economically exploit those principles for specific applications. 10 These modifications can be used to increase the effective rigidity, and reduce the size, the number and cost of sensors, and the strength of the motions which are to be compensated. These modifications are illustrated in an example embodiment described below. [0055] Referring to the example embodiment in Figure 5A and 5B, there is 15 shown an alternative embodiment of a receiver coil assembly 102' that can be used in survey system 100 in place of receiver coil system 102 discussed above. The receiver coil assembly 102' includes an outer tubular frame or shell 220 housing an inner frame or shell 240 in which an inner coil assembly 242 is positioned. In the plan view of Figure 5A, an upper half of the tubular outer 20 shell 220 is removed to show the inner shell 240 and inner coil assembly 242. In some example embodiments, an upper half of the tubular outer shell 220 is releasably secured to a lower half to allow the halves to be separated for servicing of the inner shell 240 and inner coil assembly 242. In the illustrated embodiment, the tubular outer shell 220 includes an outer polygonal portion 250 25 with a central X or t -shaped portion 252. Polygonal portion 250 is octagonal in Figure 5A, and approximates a circle, however it could take other forms - for example it could be circular or square or have more or fewer sides than eight. The semi-rigid outer shell 220 has a nominally vertical extent which is substantially less than a horizontal extent thereof, giving the shell a small 30 vertical profile. [0056] The inner coil assembly 242 includes multiturn air core loop 211 ("air coil" receiver loop) with a nominally vertical axis ("Z coil"), and two solenoid coils 212A and 212B (referred to generically using reference 212 herein) with ferromagnetic cores ("cored coils") with mutually orthogonal, 13 WO 2013/170340 PCT/CA2012/050322 nominally horizontal axes. The air coil receiver loop 211 is housed primarily within the polygonal portion 250 of outer shell 220, and the cored coils 212A, 212B are housed primarily with the central X or t shaped portion 252. Accordingly, in an example embodiment, the outer shell 220 is a semi-rigid shell 5 having a tubular outer portion 250 defining a polygonal or circular loop and a cross-shaped portion 252 having a first tubular cross member sxtending across a first diameter having a first tubular cross member extending across a first diameter of the tubular outer portion 250 and a second tubular cross member extending across a second diameter of the tubular outer portion 250 that is 10 perpendicular to the first diameter. The outer portion 250 and the cross-shaped portion 252 are in a nominally horizontal plane. The outer polygonal or circular portion 250 of each shell includes a multi-turn air coil receiver 211 of electrical wire, while coils 212A and 212B made up of multiturn solenoid windings with rod shaped ferromagnetic cores are each positioned in a respective cross member. 15 The inner shell 240 has a shape that corresponds to that of the outer shell 220. The inner coil assembly 242 of Figs 5A and 5B is intended to be rigid so its rotational motion has only three degrees of freedom. Because it is constructed of components which are narrow (in contrast to the structure of Figure 1), the inner coil assembly 242 will flex in response to movement and vibration. To 20 minimize this flexing, the inner coil assembly 242 is suspended from the inner shell 240 by highly compliant sets of elastomeric cords 209. The cords 209 are positioned so as to support the inner coil assembly 242 (and in particular each of the air coil receiver or loop 211, and cored coils 212A and 212B) at multiple points so that inertial forces are applied uniformly to the inner coil assembly 25 242, thereby reducing the resulting bending moments on components of the assembly 242. This increases the effective rigidity of the receiver coil assembly 242. The compliance of the cords 209 is chosen, considering the mass of the receiver coil assembly 242 components, so that motion and vibration transmitted by the cords 209 to the assembly 242 from the inner shell 240 is 30 attenuated in the frequency range of the magnetic signals being sensed. [0057] An example of one of the cored coils 212A, 212B is shown in more detail in Figure 6, and consist of a rigid plastic tube 221, a winding 222 that extends along the plastic tube 221 in two separate parts 222A and 222B connected in series by a wire 225, and a rod shaped ferromagnetic core 244. 14 WO 2013/170340 PCT/CA2012/050322 The core 244 is shown schematically in Figure 6, as a dotted line. It may for example have an approximately circular or square cross-section which is able to fit within the plastic tube 221, having a width comparable to the inner diameter of the plastic tube 221. The core 244 is fixed inside the plastic tube 221 and is 5 at least long enough to occupy the tube 221 inside both sections 222A and 222B of the winding 222. The plastic tube 221 has two notches 223 at its ends, which hook over the Z coil loop 211. One cored coil 212A hooks over and is attached to the Z coil loop 211 from below and the other cored coil 212B hooks over and is attached to the Z-coil loop 211from above, so that the cored coils 10 212A, 212B make contact and cross over each other at the center of the receiver coil assembly 102'. The spaced apart coil sections 222A, 222B on each cored coil 212A,212B is located on opposite sides of the center of the receiver coil assembly 102'. In an example embodiment, the ferromagnetic core 244 is constructed of a material that has minimal magnetostriction so as to reduce 15 noise generated by flexing of the core. In the case of a TDEM system, the core 244is processed to minimize its tendency to acquire remnant magnetization in response to the transmitted field. In other example embodiments, the mechanical details of the cored coils 212A, 212B and their installation may differ from the example embodiment. 20 [0058] Referring again to Figure 5A, the inner shell 240 is constructed of standard non-metallic pipe sections 202 and 204, elbows 203, tees 201, and a cross 205. In some example embodiments, after assembly, part of the upper surface of the inner shell 240 is cut away to allow the suspension cords 209 and the inner coil assembly 242 to be installed. This produces a light and relatively 25 rigid structure. Other construction techniques and materials are possible. For example, the function of the inner shell 240 could be implemented by one or more skeletal structures or rods which would contribute to the isolation of the inner coil assembly 242 from the motion of the outer shell 220. [0059] In one non-limiting example embodiment, used in an active source 30 TDEM system, the overall diameter of the outer shell 220 is 1.3 m and the diameter of the tubular components of the outer shell (the diameters and the segments of the tubular polygonal perimeter) is 0.16 m. The diameter of the tubular components of the inner shell is 60 mm. The diameter of the plastic tubes 221 is 16 mm. Other dimensions can be used in other embodiments - for 15 WO 2013/170340 PCT/CA2012/050322 example, the outer shell 220 could have a diameter greater or less than 1.3 m and its tubular components could have a diameter of more or less than 0.16 m. [0060] While the inner shell 240 is more rigid than the inner coil assembly 242, it will flex to some extent, and that flexing will be transmitted to some 5 extent to the inner coil assembly 242. To minimize the flexing, and attenuate motion and vibration, the inner shell 240 is suspended from the outer shell 220 by highly compliant sets of elastomeric cords 219. The cords 219 are chosen based on the same considerations as for the inner coil assembly suspension cords 209 described above. 10 [0061] In an example embodiment the motion of the inner shell 242 is sensed by receiver coil orientation sensing system, including for example two axis accelerometers 218, one of which is attached to the inner shell 242 near the end of one of the cored coils 212A, and the other of which is attached to the inner shell 242 near the end of the other of the cored coils 212B. Each 15 accelerometer 218 has one sensitive axis which is axial (i.e. nominally vertical) and another which is tangential (i.e. parallel to the nearest segment of the Z-coil loop 211). [0062] To the extent that the inner shell 242 is rigid, its rotational acceleration can be measured by taking the difference between appropriately 20 selected pairs of outputs of accelerometers 218 located at opposite ends of a diameter. To the extent that the inner shell 242 and the inner coil assembly 242 are rigid, and the suspension cords 209 are linear and elastic, there is for any specified frequency and axis of rotation, a transfer function that relates the inner receiver coil assembly 242 rotation to the inner shell 242 rotation. In turn, there 25 is a transfer function (which depends on the geomagnetic field) that relates noise generated in the receiver coils 211, 212A, 212B (by rotation in the geomagnetic field) to the inner receiver coil assembly 242 rotation. It follows that there are composite transfer functions that relate the accelerometer 218 outputs to the noise generated in each receiver coil 211, 212A, 212B by rotation. 30 Known techniques of adaptive noise cancellation are used to discover these transfer functions, track their changes as system parameters change, and subtract the noise from the receiver coil outputs. 16 WO 2013/170340 PCT/CA2012/050322 [0063] In this example embodiment of Figures 5A and 5B, analog to digital converter 304 differs from that previously discussed in respect of Figure 4 in that it has an input for each accelerometer in the two axis accelerometers 218. Each input is digitized and resampled to produce a sample rate the equal to that of 5 the analog to digital converter 302. Each accelerometer derived time series is processed using finite impulse response digital operators, one for each receiver coil channel, to yield noise cancellation time series. The noise cancellation time series are subtracted from their corresponding receiver coil derived time series. The coefficients of the finite impulse response operators are continuously 10 adjusted to cancel the noise in the receiver coils time series using known adaptive noise cancellation techniques (e.g. see B. Widrow et al., "Adaptive Noise Cancelling: Principles and Applications", Proc. IEEE, vol. 63, pp. 1692 1716, Dec. 1975. ) This type of "time domain" processing is most appropriate for active source, TDEM applications. 15 [0064] In another example embodiment, all the digital time series derived from the accelerometer 218 outputs and the receiver coil 211, 212A, 212B voltages are divided into overlapping time windows and known techniques are used to calculate complex Fourier transforms of the time series segments in each time window. At each frequency of interest, the Fourier transforms for a 20 group of consecutive time windows is processed to calculate a covariance matrix. The covariance matrix is used to remove from the Fourier transforms of each receiver coil output, the component that is correlated with the accelerometer outputs. A practitioner skilled in the art will recognize that this type of "frequency domain" noise cancellation is equivalent to the "time domain" 25 processing described in the previous paragraph, and is appropriate for natural source applications. [0065] The inner receiver coil assembly 242 and inner shell 240 are not rigid, so that flexing and vibration will also contribute to the noise in the receiver coil 211, 212A, 212B outputs. To the extent that the flexing and vibration are 30 correlated with the accelerometer outputs, adaptive noise cancellation techniques will adjust the calculated transfer functions so that this noise component will be partially cancelled. [0066] In the example embodiment of Figure 5A and 5B, the accelerometers 218 with horizontal axes can be used to measure the tilt of the 17 WO 2013/170340 PCT/CA2012/050322 inner shell 240. The coefficients of the adaptive noise cancellation process described above can optionally processed to determine the direction of the geomagnetic field. A set of coefficients correlating rotation rate to noise in the receiver coil outputs, can be solved to find a rotational axis which causes 5 minimum noise. The direction of this axis is an estimate of the geomagnetic field direction, and its horizontal component is an estimate of heading. The tilt and heading information can be used as described earlier to correct the output signal of each coil for departures from its nominal orientation. Alternatively, another sources of heading information such as GPS or a fluxgate magnetometer 10 could be used. [0067] The example embodiment of Figure 5A and 5B provides a low profile 3-axis coil assembly and may be especially useful in the case of a multiple axis receiver system that is to be towed from a fixed wing aircraft. For takeoff and landing of a fixed wing survey aircraft, the towed vehicle ("bird") containing 15 the receiver coil assembly must be stowed below the aircraft fuselage. Its vertical dimension is limited because it must clear the ground during takeoff and landing. The configuration of Figures 5A and 5B minimizes the vertical dimension of the receiver coil system. In such an embodiment, it may be preferable to use a different shape for the outer loop, such as a rectangular 20 shape, instead of the octagonal shape shown, without changing the principle of the technique. [0068] The example embodiment of Figure 5A and 5B applies the same principles as the embodiment of Figure 2. The combination of a Z-axis loop 211 with cored coils 212A, 212B for the horizontal axes allows the receiver assembly 25 to be smaller in the vertical direction and lighter. The use of a double suspension with multipoint support of the inner coil assembly 242 and inner shell 240 improves the effective rigidity of the assembly and offers enhanced (two stage) attenuation of motion at the receiver coil. The use of adaptive noise cancellation techniques allows for the use of only one type of motion sensor, and 30 the electromagnetic noise produced by some motion sensors (especially fluxgate magnetometers) is eliminated. Mounting the attitude or motion sensors on the inner shell 240 instead of the inner coil assembly 242 increases the amplitude of accelerometer outputs, so that the noise specification of the accelerometers is less demanding. 18 WO 2013/170340 PCT/CA2012/050322 [0069] In some applications, the example embodiment of Figure 5A and 5B with its reduced rigidity of the inner coil assembly 242 affects the accuracy of the noise removal. The sensors do not allow the direction of the geomagnetic field to be directly sensed. The cored coils 212 may generate noise as a result of 5 the magnetostrictive property of the core, and in TDEM applications, remnant magnetization of the core in response to the field of the transmitter may affect the measurements. Some example embodiments may therefore have some features like the embodiment of Figure 1, combined with other features from Figure 5A and 5B, as appropriate for a specific application. Furthermore, some 10 embodiments may achieve some of the benefits of the embodiment of Figure 5A/5B with a reduced set of sensors. Accelerometers could be mounted directly to the receiver coils 211 and 212A, 212B instead of the inner shell 240. A reduced set of accelerometers could allow significant noise reduction using adaptive noise cancellation techniques. A single (Z-axis) receiver coil system 15 could use motion sensors with adaptive noise cancellation to reduce noise. [0070] Even without motion sensors of any kind, three axis data acquired in flight can be processed using the approximation that the receiver coil is horizontal and that its bearing is the same as the aircraft heading or track direction. For greater accuracy, the aircraft can cover the survey area by flying 20 on a series of parallel lines, with each line being flown in the direction opposite to the flight direction of the immediately adjacent lines. The departure of the sensor from it nominal orientation will cause consistent differences between the results measured on one line and the immediately adjacent lines flown in the opposite direction. The differences can be analyzed to infer the receiver tilt and 25 heading, relative to the flight direction, which minimizes these differences, and the data can be corrected for this inferred orientation. [0071] In one example embodiment shown in Figure 7, a three axis receiver coil system 230 for a natural source survey system includes three square coils with a width of approximately 3 m, in a semi-rigid skeletal assembly 30 231 having the form of a regular octahedron, suspended from and enclosed in a outer shell 232 of similar form. Many configurations of motion sensors are possible. In one example embodiment, a pair of accelerometers is located at each of the six vertices 233 on the receiver coil assembly, with their sensitive axes perpendicular to each other and to a line from the vertex to the center of 19 WO 2013/170340 PCT/CA2012/050322 the octahedron. Adaptive noise cancellation techniques are used, with the accelerometer outputs as noise samples, to remove noise from the receiver coil outputs. In addition, the orientation of each coil relative to the vertical is determined from the dc component of the accelerometer outputs. 5 [0072] In one example embodiment, a single receiver coil is used. It is 8 m across and similar in form to the embodiment of Figure 5A/5B, but without the two cored coils or the segments of the shells (located along diameters of the Z coil) that enclose them. Pairs of accelerometers are mounted at three or more vertices of the loop, with their sensitive axes aligned in vertical and radial 10 directions, so that they will respond to rigid rotations and also to flexing motions of the receiver coil assembly. Adaptive noise cancellation techniques are used, with the accelerometer outputs as noise samples, to remove noise from the receiver coil outputs. In addition, the orientation of the coil relative to the vertical is determined from the dc component of the accelerometer outputs. 15 [0073] Further example embodiments will be described with reference to Figures 8-12. The example embodiment of a tow assembly 190 shown in Figure 8 comprises an aerodynamic outer frame or shell 125 with an internal passageway 124 housing a large vertical sensing axis air coil 116 (illustrated by 20 dashed lines) as described in U.S. Patent Application 12/910,386 (US 2011/0115489A1 published 2011-05-19), along with ferromagnetic cored coils 151, 152, 153, 154 (illustrated by dotted lines) which are sensitive to magnetic fields in two nominally horizontal directions. [0074] Although several configurations are possible for shell 125, in the 25 illustrated example of Figure 8 the internal passageway 124 extends around the central open area 122 and air can pass through the central open area 122. The rectangular receiver coil frame 125 is formed by a pair of parallel tubular side frame members 130, 132 interconnected by front and back parallel tubular frame members 134, 136. In the illustrated embodiment, the tubular side frame 30 members 130, 132 are longer than the front and back parallel tubular frame members 134, 136, and vertical stabilizing fins are positioned near the back or trailing end of the rectangular receiver coil frame 125 to assist in keeping the 20 WO 2013/170340 PCT/CA2012/050322 frame oriented in a consistent direction during flight with the receiver air coil 116 in a nominally horizontal orientation with its axis vertically oriented. [0075] The cored coils 151, 152 are housed in the tubular side frame members 130, 132, respectively, and are oriented so that they are sensitive to 5 the magnetic field component which is substantially along the direction of flight, while the cored coils 153, 154 are housed in the front and back tubular frame members 134 and 136 and are sensitive to the magnetic field component which is substantially at right angles to the direction of flight. Accordingly, the cored coils 151, 152, 153 and 154 are commonly housed with the air coil 116 in the 10 internal passageway 124, with the air coil 116 extending around the entire loop formed by the internal passageway 124, and each of the cored coils 151, 152, 153 and 154 being located in a respective side or region of the internal passageway 124 immediately adjacent and generally parallel to a respective region of the air coil 116. The long axis of the core of each of the cored coils 15 151, 152, 153 and 154 is parallel to the adjacent turns of the respective region of the air coil 116.By way of non-limiting example, the cored coils 151, 152, 153, 154 could be 2.5 m long, the tubular side frame members 130 and 132 could be 4 m long, while the tubular front and back frame members 134, 136 could be 3 m long. 20 [0076] Cored coils 151, 152, 153, 154 , may each, for example be formed from a single core and solenoid winding as shown in Figure 6. However, the air coil winding 116 may be flexible (while the core material is rigid and brittle). To accommodate flexibility of the air coil 116 region to which it is adjacent, each of the cored coils 151, 152, 153, 154 may itself consist of multiple segments. 25 [0077] By way of example, Figures 12 and 12A illustrate an example of a multiple segment cored coil that could be used to implement any of coils 151, 152, 153 and 154, with the segments being flexibly attached end to end. As shown in cross-section Figure 12, for example, the coil core 160, the coil former tube 162, and the winding 161 each consist of three segments. The segments of 30 the winding 161 may be connected in series. Flexible collars 167 connect the coil former tube 162 segments and keep the axes of the segments aligned, while allowing bending at the connections. Each core 160 segment is supported and positioned radially by exactly two elastomeric rings 168 so that bending stresses on the coil former tube 162 segments are not transmitted to the core 160 21 WO 2013/170340 PCT/CA2012/050322 segments. The core 160 segments are positioned axially by elastomeric spacers 169 which permit bending while preventing axial motion between the adjacent ends of the core segments. Such a configuration can minimize noise caused by relative motion between adjacent segments, The flexible attachment assembly 5 provided by collar 167 and spacer 169 allows bending while maintaining a fixed distance between the ends of the core segments and maintaining the alignment so that the extended axes of the core segments intersect at the point midway between them. [0078] The use of segmented core coils can also have additional benefits in 10 some applications - for example, it may be desirable in some applications to reduce the weight of the solenoid cored coil receivers 151, 152, 153, 154. At the same time, the sensitivity of a solenoid cored coil receiver is strongly dependent on the length of the core, and thus a narrow, long core is desirable, however such a core is susceptible to saturation by the geomagnetic field. A 15 multi-segmented cored receiver such as shown in Figures 12 and 12A can address these concerns in some applications, with the gaps between adjacent core segments optimized to obtain maximum sensitivity without excessive magnetization by the geomagnetic field. By way of non-limiting example, the core segments could be 0.8 m long and 0.01 m in diameter, with a gap of 0.03 20 m between adjacent segments. [0079] It will be appreciated that one cored coil is sufficient to provide sensitivity in each of two nominally horizontal directions, so that for example in Figure 8, cored coils 151 and 153 could be omitted. Nevertheless, the use of multiple cored coils for each sensitive direction may reduce the noise level and 25 provide for a more symmetrical structure. In some embodiments, there may be a requirement for sensitivity in just one nominally horizontal direction, so that some of the cored coils 151, 152, 153, 154 may be omitted. Vibration of coils in the ambient (usually geomagnetic) field is a major source of noise in magnetic sensors. Motion of a ferromagmetic core in the vicinity of the air coil receiver is 30 also a source of noise due to the magnetic field of the ferromagnetic core which is induced by the geomagnetic field. Figure 9 shows an example embodiment assembly 190 in which the cored coils are each independently attached to their respective region of air coil 116. In particular, Figure 9 shows a cross-section of tubular side frame member 132 and the cored coil 152 housed therein attached 22 WO 2013/170340 PCT/CA2012/050322 to its respective region of the air coil 116. Cored coils 151, 153 and 154 are similarly attached to their respective regions of air coil 116, and the following description of coil 152 also applies to coils 151, 153 and 154. In the embodiment of Figure 9, the combined cored coil 152/ air coil 116 assembly is suspended at 5 spaced apart locations along the length of the cored coil 152 by elastic members 32 from the outer shell 125. This isolates both the cored and air coils from vibration of the shell 125, while preventing relative motion between the cored coil 152 and the air coil 116. As noted above, in an example embodiment the cored coil 152 can includes , for example, core 160, enclosed in a coil former 10 tube 162 consisting of glass reinforced epoxy or other non-conductive material upon which is wound coil 161. In an example embodiment the cored coil 152 is supported in an outer tube 163 which in turn is secured by a fastener or fasteners such as glue or cable ties to an upper surface of the air coil 116. In the illustrated example, the air coil 116 includes one or more turns 165 of coil 15 wire enclosed within and supported by an outer tube 166. The cored coil outer tube 163 is secured along its length by a fastener or fasteners such as glue or cable ties to an upper surface of the outer tube 166 that houses the air coil, such that the cored coil 152 is rigidly fixed relative to the air coil 116 In the embodiment of Figure 9, the air coil outer tube 166 is suspended elastically by 20 elastic members 32 from the outer shell 125, and in turn supports the cored coil 152. [0080] In the example embodiment of Figure 9, the weight of the solenoid cored coil 152 is supported by the elastic suspension members 32, so it is desirable to minimize the weight. The use of a multi-segmented cored coil as 25 discussed above and as shown in Figures 12 and 12A can mitigate weight issues. [0081] Even when the cored coil 152 is rigidly fixed relative to the air coil 116, rotation of the rigid combined cored coil/air coil assembly can in some 30 applications induce noise in the air coil 116 due to the changing magnetization of the core 160. This effect can be reduced by placing the turns 165 (or groups of turns 165) of the air coil 116 symmetrically around the core 160, as shown in the alternative embodiment of Figure 10. In, particular, in the embodiment of Figure 10, the cored coil 152 is located within the center of the turns 165 of the 23 WO 2013/170340 PCT/CA2012/050322 air coil 116, and the air coil 116 is supported within outer tube 166 which in turn is supported by elastic members 32 from shell 125. [0082] The example embodiments depicted in Figures 9 and 10 use a single stage elastic suspension. The same principles can be used with a double 5 stage elastic suspension as shown in the example embodiment of Figure 11. In Figure 11, the combined cored coil/air coil assembly is the same as shown in Figure 10, however the suspension members 32 are supported by a tubular inner shell or rgid member 74, which in turn is suspended by outer suspension members 76 from the outer shell 125 . 10 [0083] In other example embodiments, the outer tubular frame 125 is a shape other than rectangular - for example, the tubular frame could be octagonal or other polygonal shape such as shown in Figure 5A, with the cored coils being located instead at respective locations in the same passageway as the air coil rather than in cross shaped portion 252 (which can be omitted). In 15 such an embodiment, the air coil 116 will be octagonal in shape, with the octagon lying in a nominally horizontal plane, such that each turn of the coil follows the perimeter of the octagon; the tubular outer shell 250 forms a continuous internal passageway which closes back on itself in an octagonal shape and contains the air coil; an elastic double suspension suspends the air 20 coil from the outer shell; and at least two cored coils are attached to the air coil on two sides of the octagon which are substantially at right angles to each other. In at least some example embodiment embodiments, cored coils could be attached to the air coil in each of the eight sides of the octagonal shell, with the signals from the cored coils in the first, second, fifth and sixth sides providing 25 one horizontal signal and the signals from the cored coils in the third, fourth, seventh and eighth sides providing another horizontal signal. Such a configuration could in some applications provide greater sensitivity than just two orthogonal coils. [0084] The shape of the loop formed by the outer shell could take a 30 number of different configurations, and the location and spacing of cored coils along the length of the air coil could also take a number of configurations. In another example embodiment, the outer tubular frame 125 and the air coil housed therein is circular, with one or more cored coils extending parallel to one or more respective regions of the air coil 116. 24 WO 2013/170340 PCT/CA2012/050322 [0085] It will be appreciated that the principles demonstrated in the example embodiments of Figures 8 -12 can be realized in many different ways and in different combinations. Features from the embodiments of Figures 1-7 can be combined with and applied to the embodiments of Figuires 8-12 and vice 5 versa. In some example embodiments, the receiver assemblies described herein may be used in applications other than in towed airborne assemblies, including for example receiver assemblies fixed to an aircraft or on land based or marine receiver assemblies. [0086] The specific sensors used in the example embodiments described 10 could be replaced with other types of sensors. In some embodiments, the rotation motion sensed by a pair of accelerometers oriented in the same direction but located at opposite sides of a receiver coil assembly could be detected by a single angular rate sensor. In some embodiments, the geomagnetic field sensed by a fluxgate magnetometer could instead be 15 calculated from known models of the Earth's field (such as the International Geomagnetic Reference Field model) using direction information derived from two GPS receivers located on the receiver system, or (more approximately) from track bearing information derived from a single GPS receiver located on the towing aircraft. 20 25

Claims

1. A receiver coil assembly for performing geophysical surveys, comprising: a hollow outer shell defining a continuous internal passage that forms a loop; 5 a multiturn receiver air coil extending around the continuous internal passage; and a first cored coil comprising multiturn solenoid windings about a ferromagnetic core, the first cored coil located adjacent a region of the air coil within the internal passage and having a sensing axis in a different direction 10 than a sensing axis of the air coil.

2. The receiver coil assembly of claim 1 wherein the first cored coil and the air coil are elastically suspended within the internal passage.

3. The receiver coil assembly of claim 1 or 2 wherein windings of the air coil in the region extend substantially parallel to the sensing axis of the first cored 15 coil.

4. The receiver coil assembly of claim 3 wherein the windings of the air coil in the region are symmetrically positioned around the core of the first cored coil.

5. The receiver coil assembly of claim 4 wherein common elastic members suspend both the first cored coil and the region of the air coil within the internal 20 passage.

6. The receiver coil assembly of claim 5 wherein the common elastic members are attached to an intermediate member which is suspended by further elastic members within the internal passage.

7. The receiver coil assembly of claim 1, 2 or 3 wherein the first cored coil is 25 attached to one side of the air coil.

8. The receiver coil assembly of any one of claims 1 to 7 wherein the first cored coil comprises multiple core segments positioned end to end with gaps between adjacent core segments. 26 WO 2013/170340 PCT/CA2012/050322

9. The receiver coil assembly of claim 8 wherein the core segments are connected by flexible connectors.

10. The receiver coil assembly of any one of claims 1 to 9 comprising a second cored coil comprising multiturn solenoid windings about a ferromagnetic core, 5 the second cored coil located adjacent a second region of the air coil within the internal passage and having a sensing axis in a different direction than the sensing axis of the air coil and the sensing axis of the first cored coil.

11. The receiver coil assembly of claim 10 wherein sensing axes of the three receiver coils are mutually orthogonal. 10

12. The receiver coil assembly of claim 10 or 11 comprising additional cored coils each comprising multiturn solenoid windings about a ferromagnetic core, each additional cored coil being located adjacent a further respective region of the air coil within the internal passage, such that the signals from one or more groups of the coils can be combined to increase the sensitivity of the receiver 15 coil assembly.

13. The receiver coil assembly of claim 10 or 11 comprising third and fourth cored coils each comprising multiturn solenoid windings about a ferromagnetic core, the third cored coil being located adjacent a third region of the air coil within the internal passage and having a sensing axis parallel to the sensing axis 20 of the first cored coil, the forth cored coil being located adjacent a forth region of the air coil within the internal passage and having a sensing axis parallel to the sensing axis of the second first cored coil.

14. The receiver coil assembly of any one of claims 1 to 13 wherein the continuous internal passage is polygonal. 25

15. The receiver coil assembly of any one of claims 1 to 14 wherein the receiver coils are configured to sense changes in magnetic components resulting from naturally occurring random fluctuations of the magnetic field.

16. A receiver coil assembly for performing geophysical surveys, comprising: a multiturn air coil receiver defining a loop; 27 WO 2013/170340 PCT/CA2012/050322 at least one receiver cored coil having a ferromagnetic core and a solenoid winding, supported immediately adjacent a region of the air coil, with a long axis of the core being substantially parallel to turns of the adjacent region of the air coil. 5

17. The receiver coils assembly of claim 16 in which the cored coil comprises multiple core segments positioned end to end with gaps between adjacent core segments.

18. The receiver coil assembly of claim 16 or 17 wherein the air coil and the cored coil are supported by an elastic suspension which reduces vibration 10 transmitted to the coils.

19. The receiver coil of assembly claim 18 in which the cored coil is attached to the air coil.

20. The receiver coil assembly of claim 19 in which the turns of the air coil parallel to the long axis of the adjacent cored coil are distributed symmetrically 15 about the cored coil.

21. The receiver coil assembly of any one of claims 18 to 20 wherein the elastic suspension extends to a rigid member which is elastically suspended by a further elastic suspension.

22. The receiver coil assembly of any one of claims 16 to 21 comprising an 20 outer shell that surrounds and supports the air coil and cored coil.

23. The receiver coil assembly of claim 22 wherein the outer shell is in the form of a polygonal or circular tube that closes back on itself, creating a continuous internal passageway in which the receiver coils are contained.

24. The receiver coil assembly of any one of claims 1 to 23 wherein the 25 receiver coil assembly is incorporated into a tow assembly adapted to be towed by an aircraft. 28