AU2007202492B2 - Airborne Electromagnetic Time Domain System, Computer Product and Method - Google Patents

Description

P/00/011 28/5/91 Regulation 3.2 AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Name of Applicants: Edward Beverly Morrison Petr Valentinovich Kuzmin Pavel Tishin Actual Inventors Edward Beverly Morrison Petr Valentinovich Kuzmin Pavel Tishin Address for service is: WRAY & ASSOCIATES Level 4, The Quadrant 1 William Street Perth, WA 6000 Attorney code: WR Invention Title: Airborne Electromagnetic Time Domain System, Computer Product and Method The following statement is a full description of this invention, including the best method of performing it known to me: 1 -2 Airborne Electromagnetic Time Domain System, Computer Product and Method Field of the Invention This invention relates in general to the field of airborne geological mapping. This invention further relates to an apparatus for conduction geological surveying using an electromagnetic time domain method. Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Furthermore, throughout the specification, unless the context requires otherwise, the word "include" or variations such as "includes" or "including", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Background of the Invention Each document, reference, patent application or patent cited in this text is expressly incorporated herein in their entirety by reference, which means that it should be read and considered by the reader as part of this text. That the document, reference, patent application, or patent cited in this text is not repeated in this text is merely for reasons of conciseness. The following discussion of the background to the invention is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge of the person skilled in the art in any jurisdiction as at the priority date of the invention. Time Domain Electromagnetic (TDEM) surveying is a rapidly developing area of geophysical surveying. It encompasses ground based and - 3/1 airborne applications. TDEM geological mapping involves equations for calculating the value of electromagnetic fields that are time dependent. Geological data is then inferred from the electromagnetic field data based on resistivity factors. The TDEM method was originally designed for exploration of conductive ore bodies buried in resistive bedrock, but at the present time it is also used extensively in general geological mapping, in hydrogeology, in environmental investigations etc. The method involves generating periodic magnetic field pulses penetrating below the Earth surface. Turning off this magnetic field at the end of each pulse causes an appearance of eddy currents in geological space. These currents then gradually decay and change their disposition and direction depending on electrical resistivity and geometry of geological bodies. The electromagnetic fields of these eddy currents (also called transient or secondary fields) are then measured above the Earth surface and used for mapping and geological interpretation. The common technical means to generate magnetic field pulses is a transmitter generally consisting of a loop of wire or a multi-turn coil connected to the output of an electrical current pulse generator or transmitter driver. The typical size of a transmitter coil is a few meters in diameter for an airborne device and up to hundreds of meters for ground systems. Generally, the bigger the transmitter coil diameter the stronger its magnetic moment, which then results in deeper and more accurate investigations. An additional multi-turn coil or an x-y-z coil system usually serves as a receiver or sensor for the secondary electromagnetic field. Magnetometers are also applicable for this purpose. Received signals are digitised by an analog to digital converter (ADC) and processed and stored by computer. The advantage of airborne TDEM systems is the speed with which ground that can be covered in geological surveying. However, there are - 3/2 a number of technical problems in designing airborne TDEM systems based on prior art. The transmitted electromagnetic fields generally generate eddy currents not only in the Earth but also in the proximate metallic parts including those of the system and the aircraft body. The secondary fields of these currents behave as a noise due to typical instability of the system geometry and thermal changes in conductors. This noise impacts the survey data by generally decreasing their reliability for extrapolating geological data therefrom. The most common way to minimize this noise is by keeping the receiver at an adequate distance from the transmitter driver. The result of this spaced apart relationship between the transmitter driver and the receiver is -4 that the secondary fields of the eddy currents in the Earth are comparable with secondary fields of local metal parts and therefore noise level is negligible. This type of solution is used in the TDEM systems branded "GEOTEM" and "MEGATEM" of (FUGRO AIRBONE SURVEYS LTD) 5 GEOTERREX PTY. LTD. This particular solution includes a bird towed behind a fixed-wing aircraft on a tow cable approximately 130 meters long. Another prior art TDEM system consists of a helicopter towed system manufactured by T.H.E.M Geophysics Inc. This system uses a helium 10 balloon to keep its sensor suspended at a distance apart from the transmitter system. One of the disadvantages of these prior art solutions is that there is relatively poor horizontal resolution of the system due to the relatively long 15 distance between transmitter coil and receiver sensor. Another disadvantage is difficulties of system mechanical management in start/landing and in flight manoeuvres. Another prior art method currently used to minimise this kind of noise is 20 to cancel the transmitter primary field localised in metal parts of the system using special coils producing in this local area a magnetic field having opposite direction to the main field of the transmitter coils. This technology is used in the AEROTEMTM branded solution of Aeroquest Ltd. in order to minimise the secondary fields in the metal parts of the transmitter electronics, 25 which instead they locate in the towed bird. This solution requires a high level of system mechanical rigidity. In turn, it leads to heavier frame construction. The heavier frame results in a number of disadvantages. In particular the heavier frame makes transportation of the bird difficult. The production costs and fuel costs associated with manufacturing and use of the AEROTEMTM 30 solution are also relatively high.

-5 More importantly, because of the need for a rigid frame having a relatively significant weight, a frame with a generally smaller transmitter coil diameter is selected resulting in a lower transmitter dipole moment. This generally results in insufficient transmitter dipole moment to make deeper 5 measurements. Another problem with the prior art solutions is that they do not easily permit exploiting optimal system geometry, that is the receiver in the centre of the transmitter coil. A relatively large voltage is induced in the receiver coil by 10 each of the magnetic field pulses. But this relatively high voltage in tum renders the receiver preamp saturated and therefore inoperative during system measurement time for a short period after this pulse. This is an important and necessary time for making measurements of the Earth's response. 15 As a result, the solution of existing systems is to place the system receiver at a distance away from the transmitter where the transmitted pulse is much lower since the strength of this field diminishes as the inverse cube of the distance. However, this then results in a departure from the optimal 20 system geometry. In the case of the AEROTEM TM system, the method of dealing with this large voltage pulse while maintaining optimal system geometry, i.e. receiver in centre of transmitter coil, is to place the receiver coil inside a 25 bucking coil carrying the anti-phased transmitter current so as to cancel a large part of the voltage pulse induced in the receiver coil during the transmitter "ON TIME" while not substantially affecting the reception of the secondary field from the Earth. 30 This approach works well to solve the problem of this on-time voltage pulse problem, however, the process of accurately bucking this signal again mandates the rigid geometry of all parts including the receiver coil. This rigid -6 mounting precludes the proper vibration isolation of the receiver coil thus unwanted mechanical vibration influences the receiver coil so as to induce electrical interference thereby reducing sensitivity. Another technical problem is how to produce maximum magnetic moment in the transmitter coil using minimum weight, size and electrical power. In the above-mentioned systems a significant part of the total weight is used for the structure and power sources. Another problem is the air drag of the bird during flight. Complicated support structures with large effective surface areas create excessive drag. This limits possible flight speed increasing survey cost. Another limitation of the previously mentioned systems is the limitation on the maximum transmitter diameter and therefore obtainable dipole moment. A maximum diameter for these systems is generally attained relatively quickly because the rigidity criterion mandates significant weight of the structure. This stiffness factor forces this type of design to reach the maximum allowable weight for helicopter use before a desirable diameter is attained. Summary of the Invention In accordance with a first aspect of the present invention, there is provided an airborne time domain electromagnetic surveying system comprising a tow assembly for connection to an aircraft, the tow assembly comprising: (a) a transmitter section including a generally flexible transmitter support frame and a transmitter coil, wherein the transmitter coil is housed in the transmitter support frame; and (b) a receiver section including a receiver support frame and a sensor loop, wherein the sensor loop is elastically suspended inside a shell that is elastically suspended inside the receiver support frame for vibration reduction.

-7 In accordance with a second aspect of the present invention, there is provided a tow assembly for an airborne electromagnetic surveying system, comprising: a transmitter support frame which is circular or approximates a circle; a transmitter coil for generating an earthbound primary magnetic field, the transmitter support frame bearing the transmitter coil; connecting cables and a tow cable for suspending the transmitter support frame from an aircraft, the connecting cables each having a first end suspending a respective circumferential portion of the transmitter support frame and an opposite end connected to the tow cable; a plurality of tension cables that extend from a central hub to respective portions of the transmitter support frame; a tubular receiver support frame forming a polygonal or circular ring that is supported by at least some of the tension cables; a shell located within the receiver support frame; and a sensor coil suspended by elastic members within the shell, the shell being suspended by further elastic members from the receiver support frame. In accordance with a third aspect of the present invention, there is provided a coil assembly for a tow assembly of an airborne electromagnetic survey system, comprising: a tubular support frame defining an internal passage; a shell suspended by first elastic members within the internal passage of the support frame; and -8/1 a sensor coil suspended by second elastic members from the shell within the intemal passage of the support frame. Preferably, the tubular receiver support frame forms a polygonal or circular ring. Preferably, the support frame comprises a plurality of straight tubular sections interconnected by elbow sections that collectively define the internal passage in which the shell is suspended. Preferably, the shell comprises a plurality of straight sections interconnected by elbow sections. The present invention seeks to provide a TDEM system that provides improved sensor resolution to at least some extent. Brief Description of the Drawings The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates the apparatus of an embodiment of the present invention in an airborne position, in this case towed from a helicopter. Figure 2 illustrates the tow assembly of the embodiment of the present invention in a perspective view. Figure 3 illustrates the tow assembly of the embodiment of the present invention in an elevation view. Figure 4 illustrates the tow assembly of the embodiment of the present invention in a top view thereof, and further showing a bottom view of the receiver section of the tow assembly.

- 8/2 Figure 5 illustrates the structure of the transmitter section of the tow assembly in a partial cut-away view of a joint section thereof. Figure 5a illustrates the structure of the transmitter section of the tow assembly in a partial view thereof at a joint section. Figure 5b illustrates the structure of the receiver section in a cut away view thereof. Figure 5c is a further cut-away view of the receiver section. Figure 6 is a view of the stabilizer section of the tow assembly, in accordance with one embodiment thereof. Figure 7 is a chart illustrating the survey data generated by the tow assembly of the embodiment of the present invention in operation. Figure 8 is a system resource chart illustrating the resources of the system of the embodiment of the present invention. Figure 9 is a program resource chart that illustrates the resources of the computer product of the embodiment of the present invention. In the drawings, one embodiment of the invention is illustrated by way of example. It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding, and are not intended as a definition of the limits of the invention. Best Mode(s) for Carryinq Out the Invention Detailed Description of the Preferred Embodiment The embodiment of the present invention consists of an airborne TDEM survey system 10. The TDEM survey system 10 includes an aircraft 12 and a tow assembly 14. Fig. 1 illustrates an aircraft 12 that is a helicopter, - 8/3 however, other aircraft such as airplanes having desirable take-off and landing attributes from a geological survey perspective could also be used. It should be understood that in one aspect of the present invention the tow assembly 14 is separate from the aircraft 12 but then attached thereto by a suitable attachment means. Provided that the flexible frame discussed below is provided, the tow assembly 14 could be integrated with an aircraft 12 to produce a geological surveying aircraft including a tow assembly 14 in accordance with the present invention. The tow assembly 14 of the present invention generally includes a flexible frame 15, as illustrated in Fig 2. The flexible frame includes a transmitter section 16 and a receiver section 18. In accordance with the present invention, the receiver section 18 is in most implementations substantially disposed in the center of the transmitter section 16. This generally provides the optimal geometry referred to above. One aspect of the present invention is the ease in which the tow assembly 14 can be assembled, disassembled and therefore transported from one location to another. Another aspect of the present invention is that the -9 flexible frame 15 overall can be adjusted in terms of its size to suit for particular applications. To this end, the transmitter section 16, in a particular implementation of 5 the present invention, as shown in Fig. 4, consists of a substantially octagonal support frame 20. The support frame 20 consists of a plurality of substantially tube sections 22. As best shown in Figure 5a, the various tube sections at the corners are interconnected by means of elbow sections 24. 10 The tube sections 22 can consist of a single piece, or multiple pieces that can be interconnected. The tubing used in the present invention consists of composite material tubing such as fiberglass or Kevlar. Alternatively, the components (described below) of the support frame 20 can be made of carbon fiber for increased strength, preferably with non-conductive areas 15 along the length of one or more of the components to avoid the anomaly that would be caused by a complete conductive loop. One embodiment of the support frame 20 of the present invention described consists of tube sections 22 and elbow sections 27 whereby adding 20 additional tube sections 22 or multiple pieces together providing one of the tube sections 22, as well as additional elbow sections 24, provides a support frame 20 having a greater surface area. It should be understood that tube sections 22 and elbow sections 24 can be added or removed to increase or decrease the surface area. 25 While the support frame 20 shown in the Figures has an octagonal shape, it should be understood that the present invention also contemplates support frames 15 having other polygonal shapes, although a polygonal shape approximating a circular shape is generally preferred. It should be 30 understood that the modular pieces together providing the support frame 20 can be modified to provide a support frame 20 having a substantially circular profile. Also, in applications of the present invention where transportation and - 10 adjustment of the size of the flexible frame 15 is not required, the support frame can be provided in a single unitary construction, as opposed to the modular construction described above. 5 It should be understood that the construction of the support frame 20 described herein enables a relatively large surface area while the'support frame 20 of the present invention is also relatively lightweight. By way of example only, it was found that the construction described herein easily permitted an increase of the transmitter loop diameter (or more than) up to 26 10 meters while permitting maneuvering of the aircraft 10 with the tow assembly 14 in tow. The support frame 20, as best shown in Fig. 3, is suspended using rope 26 from its corners (in the polygonal construction thereof). In a circular 15 construction of the support frame 20, the support frame 20 would be suspended by rope at substantially equidistant points along the circumference thereof. The rope 26 is then attached to a central tow cable in a manner that is 20 known. The support frame 20 bears a known multi-turn transmitter coil 28 so as to provide the transmitter function of the transmitter section 16. In the embodiment of the invention shown in Fig. 3, the transmitter coil 28 is strung 25 along the bottom of the support frame 20 by attaching the transmitter coil from multiple points along the support frame 20 by a suitable form of attachment. Alternatively, the transmitter coil 28 can be disposed inside the support frame 20. 30 In another aspect of the support frame construction of the present invention, the invention also provides flexibility in the ability to make changes - 11 in receiver loop turns and loop area, and also by adding receiver coils in other axes, without change to the disclosed tow assembly 14 configuration. In accordance with the embodiment of present invention, a known electronic transmitter driver 32 that feeds the transmitter coil 28 is installed in the aircraft 12. The transmitter driver 32 is connected to the transmitter coil 28 as illustrated in Figure 8. This connection is generally provided by wiring the transmitter coil 28 to the transmitter driver 32 along the central tow cable and at least one of the ropes 26 supporting the support frame 20. The flexible frame 20 also includes a stabilizer as shown in Fig. 1. The stabilizer 36, as best shown in Fig. 6, generally has a stabilizer frame 37 that supports an aerodynamically shaped stabilizer tube 38. The stabilizer 34 is generally made of plastic and is connected to the support frame 20 at a point by means of a suitable attachment. In an embodiment of the present invention, as best shown in Fig. 4, a series of tension ropes 40 are attached to the support frame 20 at various points and then connected to a central hub 42. In the particular embodiment of the support frame 20 shown in Fig. 4, having an octagonal shape, the tension ropes 40 are attached to the corners of the support frame 20. As best shown in Fig. 4, the receiver section 18 also consists of a plurality of interconnected receiver tube sections 44 together providing a receiver frame 45. These receiver tube sections 44 are also made of plastic and are similar in construction to the tube sections 22 and elbow sections 24 that provide the structure of the support frame 20 in the particular embodiment thereof described herein. The tube sections 44 generally provide, however, a receiver section 18 having a much smaller surface area than that of the receiver section 18 or support frame 20. As best shown in Figure 5a, the - 12 various receiver tube sections 44 are interconnected by means of receiver elbow sections 46. Much as in the case of the support frame 20, the receiver frame 45 has 5 a modular construction whereby additional receiver tube sections 44 and receiver elbow sections 46 may be added to provide a receiver frame 45 having a greater or lesser surface area. Arso similarly, the receiver frame 45 can in accordance with the present invention be provided in accordance with alternate polygonal structures or in fact a circular structure. In addition, a 10 unitary construction as opposed to a modular construction may be desirable. In accordance with one embodiment of the present invention, the receiver frame 45 is mounted on the tension ropes 40 by leading the tension ropes 40 through a series of loops 48 disposed on the receiver frame 45 as 15 best shown in Fig. 4. The receiver frame 45 is provided with a sensor coil 50. In accordance with an embodiment of the present invention, the sensor coil 50 is disposed inside a shell 52 disposed inside the receiver frame 45, as shown in Figs. 5b 20 and Sc. The shell 52 consists of plastic tubing similar to the tubing the receiver tube sections 44 and receiver elbow sections 46, but having a smaller circumference. In addition, the shell 52 is elastically suspended using a series of 25 elastics 54 (one shown only) attached to points 54 along the inner wall of the receiver frame 45 tubing and elastically supporting the shell 52. The sensor coil 50, in turn, is elastically supported by a series of elastics 54 (one shown only) attached to points 56 along the inner wall of the shell 52. 30 The elastic suspension of the sensor coil 50 inside the shell 52 minimizes the effect of vibration.

- 13 In one particular embodiment of the present invention, the sensor coil 50 output is connected to a non linear preamplifier 63 mounted in a box on the shell 52 outer surface (not shown). This is illustrated in Figure 8. 5 The result of the above is that metallic parts except wires and the preamplifier 63 are generally concentrated in the aircraft 12 far enough from field generating and the sensitive components of the flexible frame 12. This results in relatively small parasitic eddy currents whereby useful signals dominate. 10 A further result of the tow assembly construction described above, is that the two assembly consists generally of the tubular fiberglass parts described above whereby generally more than a half of the bird weight belongs to transmitter coil wires. 15 Generally a transmitter coil 30 having relatively thick wires with low resistance that can reach higher intensity of the transmitting magnetic field is used. Of course, the overall weight must not exceed values that would otherwise unduly burden the aircraft 12 or negatively affect maneuverability. 20 In addition to the fiberglass or carbon fiber tubing, the tow assembly 14 uses the ropes discussed above. This reduces the need for additional plastic or metal spokes. The ropes reduce air drag and allows for higher flight speed. 25 As best illustrated in Figure 8, the system of the present invention also includes a signal-processing computer 58. The computer 58 includes a known analog to digital converter device (ADC) 60. The output of the preamplifier is connected in sequence to a known amplifier 62, low pass filter 64 and then the ADC 60, in a manner that is known. The ADC converts the 30 analog data produced by the sensor coil 50 and preamplifier in combination to produce digital data for digital data conversion as described below.

- 14 The signal from the sensor coil 50, which is proportional to dB/dt, goes through the amplifier 62 and low pass filter 62. The ADC 60 continuously converts the signal to digits. The computer 58 includes a microprocessor (not shown) and is linked to a memory. A computer program 66 is installed on the 5 computer 58 for analyzing the digital data to produce the survey data illustrated in Fig. 7. The computer program can produce arbitrary output waveforms including square, trapezoidal and triangular waveforms in order to meet the particular survey requirements. The computer program 66 also permits pulse repetition rate to be dynamically altered to lower repetition rates 10 being more suitable for very conductive targets or higher for less conductive targets. Figure 9 illustrates the resources of this computer program. The sensor coil 50 parameters define the necessary sensitivity so that the signal does not exceed the input range of the non linear preamplifier. 15 The preamplifier 63 is a differential amplifier with a specially designed, fast recovery, non-linear gain. In relation to the TDEM process, the differential amplifier has a high linear gain of the signal within a set range equal to the expected measurement signal level with the pulse off and turns 20 the amplified signal to unity gain when the signal exceeds this limit during the "on" pulse. In that way the preamplifier limits output voltage during "ON TIME" pulse and provides low distortion and has fast recovery and high gain during off time. 25 This in turn allows the sensor coil 50 to be placed in the optimal position in the center of the transmitting section 16 without the need for any bucking of the primary transmitted pulse. This then allows the use merely of vibration isolation of the sensor coil 50 (as described above) thus increasing our signal to noise ratio. 30 By using this non-linear preamplifier method over the bucking method, a transmitter loop diameter and corresponding size of the support frame, as -15 well as the number of loop turns can be selected to suit particular geological targets simply and on site. Alteration of these parameters in the context of a bucking system is generally discouraged because the bucking system would lose effectiveness in the advent of such alteration. Thus the bucking method is generally less flexible than embodiments of the present invention. In another aspect of the invention, the support frame 20 is also adapted to measure the signal during the on-time so as to provide in- phase information. This has been found to improve survey data, for example, in the case of ore bodies of relatively high conductance, for example, nickel. This is achieved by taking signal off of the transmitter coil 28, or alternatively a separate receiver coil is looped tightly to the transmitter coil for this purpose. In another aspect of the invention a current measuring unit (not shown) is added to the system of the present invention. The current measuring unit measures the residual currents circulating in the transmitter coil 28 during the "OFF" interval thereby enabling the system to minimize distortions caused by these residual currents to the earth response to the electromagnetic field pulse. This is especially important in the time immediately after the transmit pulse when current leakage and current oscillations may exist for a short time. These currents cause errors in the received signals. One implementation of the current measuring unit consists of an air-core transformer and preamplifier which is then connected to an AD converter. The transformer is preferably designed like a Rogowski coil which includes wide dynamic and frequency ranges, high stability and linearity of its characteristics and easy calibration. The primary winding of the transformer is connected in serial with the transmitter coil so that the current flowing through the coil generates emf=M *dl/dt at the secondary winding of the transformer. The signal-processing computer 58 is connected to the transformer and therefore sample signal therefrom much as the receiver signal and uses this data for further correction of the receiver signal. In one particular -16 implementation thereof the current measuring unit is housed in a box (not shown) and is mounted on the tow cable. Other modifications are possible. For example, additional receiver coils oriented in the X - axis and/or the Y - axis can be added. The use of a mechanically flexible relationship between the transmitter coil and the receiver coil. This simplifies and greatly reduces the necessary weight of the support structure as well as allowing the user to use a much larger loop diameter thus giving the system higher dipole moment. The ability to rotate the entire structure 90 degrees so that the transmitter file in the X-axis direction thus allowing for better detection of vertical conductive bodies. Modifications and variations such as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.

Claims (9)

1. An airborne time domain electromagnetic surveying system comprising a tow assembly for connection to an aircraft, the tow assembly comprising: (a) a transmitter section including a generally flexible transmitter 5 support frame and a transmitter coil, wherein the transmitter coil is housed in the transmitter support frame; and (b) a receiver section including a receiver support frame and a sensor loop, wherein the sensor loop is elastically suspended inside a shell that is elastically suspended inside the receiver support frame for 10 vibration reduction.

2. A tow assembly for an airborne electromagnetic surveying system, comprising: a transmitter support frame which is circular or approximates a circle; a transmitter coil for generating an earthbound primary magnetic field, the 15 transmitter support frame bearing the transmitter coil; connecting cables and a tow cable for suspending the transmitter support frame from an aircraft, the connecting cables each having a first end suspending a respective circumferential portion of the transmitter support frame and an opposite end connected to the tow cable; 20 a plurality of tension cables that extend from a central hub to respective portions of the transmitter support frame; a tubular receiver support frame forming a polygonal or circular ring that is supported by at least some of the tension cables; a shell located within the receiver support frame; and 25 a sensor coil suspended by elastic members within the shell, the shell being suspended by further elastic members from the receiver support frame. -18

3. A coil assembly for a tow assembly of an airborne electromagnetic survey system, comprising: a tubular support frame defining an internal passage; a shell suspended by first elastic members within the internal passage of 5 the support frame; and a sensor coil suspended by second elastic members from the shell within the internal passage of the support frame.

4. The coil assembly of claim 3, wherein the tubular receiver support frame forms a polygonal or circular ring. 10

5. The coil assembly of claim 4, wherein the support frame comprises a plurality of straight tubular sections interconnected by elbow sections that collectively define the internal passage in which the shell is suspended.

6. The coil assembly of any one of claims 3 to 5, wherein the shell comprises a plurality of straight sections interconnected by elbow sections. 15

7. An airborne time domain electromagnetic surveying system substantially as hereinbefore described with reference to the accompanying drawings.

8. A tow assembly for an airborne electromagnetic surveying system substantially as hereinbefore described with reference to the accompanying drawings. 20

9. A coil assembly for a tow assembly of an airborne electromagnetic survey system substantially as hereinbefore described with reference to the accompanying drawings.