System and method for airborne electromagnetic surveying

Abstract

A system and method of surveying involves traversing a transmitter supported on or by an aircraft over the ground where the transmitter has a current loop that is subdivided into two or more electrically parallel subloops lying side by side across a common plane where the subdivided loops having lower inductance than an undivided loop of the same total area. The transmitter may be driven by a current having a waveform which in one half period, has at least three steps so that the transmitter will transmit an electromagnetic field with a frequency spectrum comprising in substance only odd harmonics of equal amplitude, up to a bandwidth of the transmitter, where the multistep waveform has more high frequency content or higher bandwidth than a square wave with the same slew rate and period.

Description

TECHNICAL FIELD A system and method for airborne electromagnetic surveying is disclosed. The system and method may be used for example for detecting and mapping subsurface mineral or water resources over a large area. BACKGROUND ART Airborne electromagnetic surveying for mapping subsurface mineral and water resources involves traversing an electromagnetic transmitter supported by an aircraft (fixed wing or helicopter) over a large area of land. The transmitter is in the form of a conductor wound in a single or multi turn loop. Figure 1 illustrates a transmitter T supported by a fixed wing aircraft F used for airborne electromagnetic surveying. The inductance L, of the transmitter T is proportional to the number of turns in series times the square root of the area of the transmitter T: L oc                                                    (1) where Ns is the number of turns in the inductor, and A is the area of the transmitter T. So, for a given area A of the transmitter, the inductance L is minimised by forming the transmitter T to have a single loop. Far from the airborne transmitter T, the magnetic field is proportional to the magnetic dipole moment of the associated transmitter loop and inversely proportional to the cube of the distance. The dipole moment is equal to: m = NSIOA                                 (2) where Ns is the number of turns connected in series, Io, and A is the loop area. High transmitter moments are essential to maximize weak signals from conductive or deep geology. Good vertical resolution of subsurface geology comes from early delay time measurements taken immediately after the transmitter current is switched off. However, the earth response immediately following a linear switch-off ramp is smeared out over a time scale proportional to the duration of the ramp. The vertical resolution in meters is: Resolution = 0.55^2pAt / / z                            (3) where At is the ramp time, p is the resistivity of the ground, and / 7=4tt10-7 H / m is the permeability. That is, shorter ramp times At gives better vertical resolution. The electromagnetic waves generated by the pulsed current in the airborne transmitter T induce eddy currents in the ground. These eddy currents in turn generate secondary electromagnetic fields which can be detected by a receiver towed behind or below the aircraft. The airborne transmitter T is fed with a pulsed current. Typically, the current pulse ramps from zero to maximum absolute amplitude for example between 600-1800 Amps within a predetermined ramp time At, for example between about 2 ps to about 5000 ps. The current pulse can be characterised by one or more features including: its waveform in the time domain; its frequency spectrum; duty cycle, and number of steps or levels in the time domain wave form. Throughout this specification and claims the expression “duty cycle” is to be understood as a measure of the ON time of the transmitter current over a half period of the time domain waveform; and “number of steps” denotes the number of times the current transitions from one level to another in a half period. A common current wave form for the transmitter is a one-step square wave, shown in Figs 2a and 3a in the time and frequency domains respectively. In the time domain, and referring to Fig 2a, this current waveform transitions or ramps from zero to a nominal maximum (or minimum) amplitude in a single step. The amplitude remains at the maximum (or minimum) for the remainder of the half period. In the theoretical square wave, the transition is instantaneous. In practice however, the transition is in the form of a ramp with a finite and determinable slew rate and ramp time At. The square wave has a well known frequency spectrum consisting of an infinite number of odd integer harmonics of progressively reducing amplitude. A common receiver type is an induction coil which measures the electromagnetic field. In this document, electromagnetic field is defined as the change in the magnetic field in units Tesla per second, which amplifies high frequency harmonics. Thus, Fig 3a shows that the spectra of the transmitted electromagnetic field have odd integer harmonics of equal amplitude, for frequencies up to the 3dE3 bandwidth of the example transmitter. In Fig 3a, the transmitter current is switched off from a steady state value to zero with a ramp time of 1 ms, so the 3dB bandwidth is approximately 0.5 / (1 ms) = 500 Hz. Waveforms with shorter ramp times will have a larger bandwidth. In this document, the 3dB bandwidth is the frequency range where the harmonics of the transmitted electromagnetic spectrum are attenuated less than 3dB. The square wave will induce a relatively simple ground response that is easy to interpret and allows the longest measurement time. But a disadvantage of a square wave current pules is that it has a relatively large current transition A / with a correspondingly long ramp time At =L AI / V, and consequently small bandwidth. Here L is the inductance of the transmitter loop and V is the voltage of the transmitter electronics during the ramp. With a view to reducing the ramp times, several different waveforms have been considered. These include a two-step bipolar pulse wave with 50% duty cycle (shown in Figs 2b and 3b), and the Cox waveform (shown in Figs 2e and 3e). Referring to Fig 2b, the two-step bipolar pulse wave transitions from zero to a maximum in a single step at which it remains for a quarter of its period then transitions in a single step back to zero where it remains for the next quarter of its period. For the second half of the wave period this repeats but with opposite polarity, (i.e., the next transition is from zero to a minimum period then from the minimum back to zero). Each transition or step has substantially the same slew rate (and thus same voltage 1 / ) as the steps of the square wave. Fig 3b shows that the two step 50% duty cycle bipolar pulse wave has a frequency spectrum of the same form as the square wave, but transition times are twice as quick. The Cox wave form transitions in four steps, in one half period as shown in Fig 2e. The first step is from the zero to a maximum. The second step is from the maximum back to zero. The third step is from zero to a minimum. The fourth step is from the minimum back to zero. Each step is of the same absolute magnitude and has the same ramp time, but with opposite slew rate. In the Cox waveform the period for which the current is at a maximum or minimum is the same. The period for which the current is at zero between step is also the same but shorter than the period for the maximum or minimum. The Cox waveform does provide shorter ramp times than the square wave but, as shown in Fig 3e its frequency spectrum is irregular and includes harmonics with a near zero value. This is disadvantageous for frequency domain processing of the response. Figs 3c and Fig 3d show that the bipolar waveforms with 40% and 20% duty cycle also have irregular frequency spectrums. Of all the possible waveforms with 2 steps per half period, only the 50% duty cycle waveform has a regular frequency spectrum the same as a square wave. As mentioned above, ramp times for various known airborne electromagnetic transmitter loops typically range between about 2 ps to about 5000 ps. Previous efforts by the industry to achieve a quick turn OFF time have come at a cost of lower transmitter moment, or noisy dual waveforms. The above references to the background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the disclosed system and method to any particular form of the system and method. SUMMARY OF THE DISCLOSURE In one aspect there is disclosed a method of surveying the ground for subsurface mineral or water resources comprising; traversing a transmitter supported on or by an aircraft over the ground wherein the transmitter comprises a current loop subdivided into two or more electrically parallel subloops lying side by side across a common plane of, the aircraft or, a transmitter support carried by the aircraft; the subdivided loops having lower inductance than an undivided loop of the same total area; applying a current to the transmitter the current having a multilevel waveform wherein the transmitter transmits an electromagnetic wave toward the ground; and supporting one or more receivers on or by the vehicle for receiving electromagnetic waves generated by decay of currents induced in subsurface mineral or water resources by the electromagnetic waves transmitted by the transmitter. In one embodiment the one or more receivers include an antenna attached by a tether to the aircraft wherein the antenna is capable of being deployed from the aircraft when in flight and towed by the aircraft. In one embodiment the one or more receivers include a magnetometer fixed to a part of a body of the aircraft. In one embodiment applying a current to the transmitter includes applying a current having a waveform which has, at least three current steps per half period, and transmitting an electromagnetic field with a frequency spectrum comprising in substance only odd harmonics of equal amplitude, upto a bandwidth of the transmitter. In one embodiment each current step has substantially the same absolute slew rate. In one embodiment applying a current to the transmitter includes applying a current having a waveform which has in at least one full period: at least 4 different levels at which the current remains constant for selected periods of time, and a plurality of current ramps; and transmitting an electromagnetic field with a frequency spectrum comprising in substance only odd harmonics of equal amplitude, up to a bandwidth of the transmitter. In one embodiment each current ramp has substantially the same absolute slew rate. In a second aspect there is disclosed current waveform for a current applied to an electromagnetic transmitter, the current waveform comprising: at least one cycle where the current waveform has at least 4 different levels at which the current remains constant for selected periods of time, and a plurality of current ramps; and wherein the frequency spectrum of the current waveform in the at least one cycle has a normalised frequency spectrum substantially identical to that of a square wave. In a third aspect there is disclosed a multistep current waveform for a current applied to an electromagnetic transmitter, the multistep current waveform comprising: at least one cycle where the current waveform, in one half period, has at least three steps; and when applied to a transmitter, transmits an electromagnetic field with a frequency spectrum comprising in substance only odd harmonics of equal amplitude, up to a bandwidth of the transmitter wherein the multistep waveform has more high frequency content or higher bandwidth than a 50% duty cycle a square wave with the same, slew rate and period. In a fourth aspect there is disclosed method of processing the ground response measured by a receiver and induced an electromagnetic field produced by applying to a transmitter a current waveform of the second or third aspects, the method comprising transforming receiver voltage into that which would be obtained with a perfect square wave by using this formula: vsqcn = isq(n ■ ven Kf) where V(f) is the Fourier transform of the receiver voltage, 1(f) is the Fourier transform of the transmitter current Isq(f) is the Fourier transform of an ideal square wave transmitter current, and f is frequency, wherein the spectrum of the transmitted waveform has amplitudes that are substantially identical to that of a square wave and only phases of the spectrum need to be adjusted to align with the perfect square wave, and whereby substantially no noise is added. BRIEF DESCRIPTION OF THE DRAWINGS Notwithstanding any other forms which may fall within the scope of the System and Method as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to becoming drawings in which: Figure 1 is a schematic representation of a prior art transmitter supported on a fixed wing aircraft; Figures 2a, 2b, 2c, 2d, and 2e show the time domain waveforms of 5 prior art transmitter currents; Figures 3a, 3b, 3c, 3d, and 3e show the frequency spectra of the transmitted electromagnetic field, for each of the transmitter currents shown in Figs 2a, 2b, 2c, 2d and 2e respectively; Figure 4 is a representation in plan view of an embodiment of the disclosed transmitter supported on a fixed wing aircraft; Figure 5 is a perspective view of the transmitter and aircraft shown in Figure 4 with the addition of a tethered trailing receiver; Figures 6a and 6b, show aircraft transmitter current waveforms for two prior art transmitter currents, identical to Figures 2a and 2b; Figures 6c, 6d, and 6e show three new transmitter currents in accordance with the present disclosure; Figures 7a, 7b, 7c, 7d, and 7e show the frequency spectra of the transmitted electromagnetic field, for each of the transmitter currents shown in Figs 6a, 6b, 6c, 6d and 6e respectively; Figures 8a, 8b, 8c, 8d, 8e, 8f, 8g, and 8h show eight new transmitter current waveforms in accordance with embodiments of the present disclosure; Figure 9 is a circuit diagram for a circuit capable of producing a transmitter current waveform in accordance with employment in the present disclosure; Figure 10 is the transmitter current simulated in Pspice using the circuit shown in Figure 9; Figure 11 is the frequency spectrum of the transmitted electromagnetic field for the transmitter current shown in Figure 10; and Figures 12a-12d illustrates, in the time domain the effect of a prior art processing technique on actual transmitter and measured ground response waveforms for a 25 Hz transmitter signal. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS Specific embodiments of the disclosed System and Method will now be described by way of example only. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the disclosed System and Method. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to pertaining to System and Method In the drawings, it should be understood that like reference numbers refer to like parts. An embodiment of one aspect of the disclosed system and method involves modifying the electromagnetic transmitter (hereinafter, for simplicity, also referred to as “transmitter”) supported by an aircraft in a manner to assist in reducing the ramp time of the transmitter current. In this embodiment this is achieved by forming the transmitter to comprise a single endless loop current path subdivided into N electrically parallel subloops where N is an integer > 2, while maintaining a substantially constant transmitter loop area in comparison to the undivided loop. The transmitter loop area is maintained substantially constant by physically arranging the subloops to lie side by side across a common plane. That is, the subloops are arranged with substantially no overlapping area. So, if the N subloops have respective areas Ai, A2, ....An then the sum of these areas is substantially the same as the overall area A of the single endless loop transmitter T of the prior art transmitter of Fig 1. The common plane is substantially parallel to a plane of the ground over which the transmitter is traversed. In practise the common plane is the plane of, the aircraft or, a transmitter support carried by the aircraft. So, for a fixed wing aircraft this is a plane defined by the aircraft wings. In the event a transmitter support is used, the support may for example be in the form of a frame made of a non-conducting material such as fibre glass or plastics materials which is tethered to an aircraft. Such an aircraft may be a helicopter or other vertical take-off aircraft. A practical realisation of this is shown in Figs 4 and 5 which illustrate a fixed wing aircraft 10 supporting a transmitter 12 that is formed as a single endless loop subdivided into two electrically parallel subloops (most evident in Fig 4). A first of the parallel subloops 14 is that starting from the positive symbol “+” on the fuselage of the aircraft and moving in a clockwise direction to the negative symbol thereby traversing in sequence the nose 16 of the aircraft, the starboard wing 18 and the tail 20. A second of the parallel subloops 22 is that starting from the positive symbol “+” on the fuselage of the aircraft and moving in an anticlockwise direction to the negative symbol thereby traversing in sequence the tail 20, the port wing 24 and the nose 16. It should also be noted that the overall area A of the transmitter 12 is substantially the same as for the prior art transmitter T depicted in Figure 1. If / 0 is defined as the current in the loop, V is the voltage of the transmitter electronics during the ramp, and Lo as the inductance of a simple loop, then for a subdivided loop with Np parallel subloops o      the current provided by transmitter electronics is:           I = NPIO o     the inductance presented to transmitter electronics is:      L = L0 / Np o and the ramp time is:              At = LI / V = L0I0 / (NpV^ (4) The ramp time is thus inversely proportional to the number of subloops in parallel Np, and proportional to the overall inductance of the transmitter 12. Also, the inductance L in comparison to a simple (single) loop is lowered from that of the simple loop Lo . Specifically, the inductance is lowered by a factor of the square of the number of subloops. Further the inductance of the subloops is minimised by forming them as single turn subloops. For a fixed wing aircraft this aspect of the disclosed system and method may in practice be limited to the provision of a transmitter 12 having only two parallel subloops. The limitations are by and large imposed by the typical geometry of a fixed wing aircraft and the provision of fuel tanks in the wings. However, other embodiments the system and method may be implemented by the use of transmitters with more than two parallel subloops when suspended from a helicopter, or other vertical take-off aircraft such as drones. The aircraft 10 is also provided in with one or more receivers for receiving the ground response to the electromagnetic waves and corresponding electromagnetic field generated by the transmitter 12. The response may be in the form of an electromagnetic field produced by electrically conductive ground strata, or a magnetic field produced by magnetic strata. The number and type of receivers used is dependent on the type of strata desired to be detected and mapped. For example, a receiver in the form of antenna 26 (see Fig 4) can be deployed from the aircraft 10 when airborne for receiving or sensing electromagnetic fields generated by the decay of induced currents in the ground strata. Here the receiver is able to detect conductive geology. The receiver can be tethered to the aircraft and deployed when the aircraft is airborne. The tether 28 may have a length sufficient to space the receiver about 100m from the transmitter 12. Additionally, or alternately another receiver in the form of a magnetometer 32 may be supported by the aircraft 10 for the purposes of measuring magnetic geology. The magnetometer may be attached by a rigid coupling to the body of the aircraft 10. In one example magnetometer can bolted to the tail 20 of the aircraft, and thus may be only a few meters away from the transmitter 12. In one example, a caesium vapour magnetometer may be used to measure the magnetic field from magnetic minerals. The magnetometer can only measure when the transmitter current is near zero, otherwise there will be too much noise from interference. Therefore, in a broad and general sense the disclosed method of surveying the ground for subsurface mineral or water resources comprises: traversing a transmitter 12 supported on or by an aircraft 10 over the ground wherein the transmitter 12 comprises two or more electrically parallel loops 14 and 22 lying side by side in and across a common plane; applying a pulsed current to the transmitter whereby the transmitter transmits and an electromagnetic wave toward the ground; and supporting one or more receivers 26, 32 on or by the aircraft 10 for receiving electromagnetic waves or fields generated by the ground response to the electromagnetic waves transmitted by the transmitter 12. An embodiment of a second aspect of the disclosed system and method involves providing a current waveform for driving the transmitter 12 which reduces the ramp time but provides a ground response that is relatively simple to process. It is possible to reduce the transmitter ramp time by replacing large current steps with smaller but more frequent current steps Al and shorter ramp times At. The ramp time of a multistep waveform is: L ■ A / LIrms M = „ «-= V Vy / n (5) where n is the number of current steps per half cycle. However, many multi-step transmitter waveforms will produce a relatively complex ground response, with multiple switching events per half cycle and low-amplitude effects during the on-time resulting from deviations from a perfect square transmitted waveform shape. It is therefore advantageous to transform the receiver voltage of systems using multistep transmitters such as those disclosed herein, into that which would be obtained with a perfect square wave by using known techniques such as by application of an appropriate formula such as, but not limited to: vsq(f) = isq(n-y(n (6) where V(f) is the Fourier transform of the receiver voltage, / (f)is the Fourier transform of the transmitter current, Isq(f) is the Fourier transform of an ideal square wave transmitter current, and f is frequency. An inverse Fourier transform of Vsq(f) then generates the time domain response with a single decay for a full 20 ms half period (for 25 Hz base frequency). The single decay per half period is easy to interpret and gives the maximum information of conductivity at different depths. The effect of this receiver voltage transformation in the time domain is shown in Figs 12a-12d as previously demonstrated by Lane R. Lane, A. Green, C. Golding, M. Owers, P. Pik, C. Plunkett, D. Sattel, B. Thorn, “An example of 3D conductivity mapping using the TEMPEST airborne electromagnetic system”, Exploration Geophysics 31, p162-172 (2000). Fig 12a shows prior art of a transmitter current waveform with 2 steps per half cycle. The ground response that is measured (i.e., non-transformed) at the receiver is shown in Fig 12b. Fig 12c shows the ideal square wave transmitter current and Fig 12d illustrates the transformed time domain response by application of the transformation formula (6). It will also be appreciated by those skilled in the art that the above transformation formula may be enhanced to correct for jitter and drift and parasitic response. This transformation does not add noise because the spectrum of the transmitted waveform has amplitudes that are substantially identical to that of a square wave, and only the phases need to be adjusted to align the spectrum with that of the square wave To allow improved, i.e., reduced, ramp times and increase bandwidth, the Applicants propose a transmitter current waveform having, in one half period, at least three steps, each step being of substantially the same duration and substantially the same absolute slew rate \dl / dt\ (and thus same voltage I / ); and where the Fourier transform the current waveform has a normalised frequency spectrum substantially identical to that of a square wave. This combination provides transmitter waveforms with current steps that are smaller than that of the square wave or the 50% duty cycle bipolar pulse shown in Figs 2a and 2b; and yet the proposed waveform has the same rms current; and with a frequency spectrum comprising in essence only odd harmonics of progressive decreasing amplitude, i.e., substantially identical to that of a square wave. Through extensive analysis a multitude of different waveforms have been found that meet these criteria and are therefore suitable for driving the transmitter 12. Figs 6a and 6b are graphs showing, in the time domain, the transmitter current wave form for the prior art square wave (wave form 34) and the 50% duty cycle bipolar pulse (wave form 36). Fig 6c, 6d, and 6e show three embodiments of transmitter current waveforms 38, 40 and 42 respectively, in accordance with of the present disclosure. In Figures 6a-6e the vertical axis is transmitter current and the horizontal axis is time. The currents for all waveforms in Figures 6a-6e have approximately the same rms current and thus the induced magnetic field has substantially the same energy. Each of the new waveforms 38, 40 and 42 in Figs 6c, 6d and 6e have smaller current steps A / and shorter ramp times At than the prior art waveforms. In addition, and significantly each of waveforms 38, 40, and 42 have a frequency spectra 38f, 40f, and 42f that are substantially the same as that of a square wave, as shown in Figures 7a-7e. Current waveform 38 of Fig. 6c is a four step waveform, where each current step is 400 A over a ramp time At3s of 0.5 ms. The four steps of this waveform, recalling that the steps are those changes in a half period or cycle of the waveform, the steps are identified as S1, S2, S3 and S4. The second half period of waveform 38 is simply the negative of the first half, as indeed is the case for all of the waveforms. The current waveform 40 in Fig 6d has eight steps, each of having a magnitude of 280 A and a ramp time At40 of 0.35 ms. The current waveform 42 has 16 steps, each of having a magnitude of 200 A and a ramp time At42 of 0.25 ms. While the embodiments of the disclosed waveforms are described above in relation to their number of steps, they may also be categorised by the number of different levels in the waveform in a full period / cycle. The number of levels is a characterisation of the number of distinctly different current levels of the waveform in the full period. For example, looking at waveform 38 it has the different current levels at different times of 0 A, 400A, 800A, -400 A and -800 A. Figures 7a-7e shows the frequency spectra 34f, 36f, 38f, 40f, and 42f of each of the wave forms 34, 36, 38, 40 and 42 respectively shown in Figs 6a-6e. From this it can be seen that the frequency spectra 38f, 40f, and 42f of each of waveforms 38, 40 and 42 are almost identical to a square wave at low frequencies, albeit with difference phases. The only difference apparent in the spectra is the wider bandwidth of the high order waveforms due to the shorter ramp times. To date Applicant has found the following waveforms that have at least three steps per half period (or at least 4 levels) and waveform with frequency spectra substantially identical to that of the square wave: •  3 step (4 level) shown in Fig 8a •  4 step (5 level) shown in Fig 6c •  5 step (4 level) shown in Fig 8b •  7 step (6 level) shown in Fig 8c •  8 step (5 level) shown in Fig 6d •  9 step (6 level) shown in Fig 8d •  10 step (7 level) shown in Fig 8e •   11 step (6 level) shown in Fig 8f •  13 step (8 level) shown in Fig 8g •  16 step (9 level) shown in Fig 6e •  18 step (9 level) shown in Fig 8h Combining the multistep waveforms (Equation 4) with new transmitter loops (Equation 5) could give a much shorter ramp time. The ramp time At is inversely proportional to the number of sub-loops in parallel and the square root of the number the current steps n: 1 At oc----■= Npy / n (7) Using Np=2 parallel sub-loops and a waveform with n =18 current steps per half cycle would allow the ramp time to be decreased by a factor of 6 compared to the 50% duty cycle bipolar pulse of Fig 2b with / Vp=1 and n =2. Embodiments of the disclosed waveforms may be produced using electronic circuits which comprise a combination of H-bridge or Half-H-bridge circuits and switching circuits. An example of a circuit 50 for producing a four step (5 level) transmitter current waveform is shown in Fig 9. The circuit 50 in Fig 9 comprises a standard H-bridge circuit 52 with a switching circuit 54 having two diode switches combined with a clamping circuit 56 and a protection diode 58. Figure 10 shows a simulated transmitter current waveform 60 using the commercial simulation circuit software Pspice by Cadence Design Systems. The waveform 60 may be considered to be a practical manifestation of the waveform 38 shown in Fig 6c, and has a ramp time At = 18 |is. The frequency spectrum of the transmitted electromagnetic field for waveform 60 is shown in Fig 11. As can be seen, the spectrum closely follows that of a square wave, composed of odd harmonics of equal amplitude. As to be expected for a real-world waveform which can not exactly replicate an ideal waveform there is a variation or distortion in the amplitude of the harmonics from the ideal square wave. In Fig 11 this variance is about 10%. This distortion has very little effect on noise generated by this multistep waveform, as the noise increases by the square of the distortion, namely in this instance by 1%. This 4 step transmitter current waveform 60 also has a period P1 of about 9 milliseconds within its half cycle where the transmitter current is 0, i.e., the transmitter is turned OFF. This makes this waveform particularly well suited for enabling with a caesium vapour magnetometer for measure the magnetic field from magnetic minerals. In this regard a caesium vapour magnetometer will typically take 1 millisecond to recover after the transmitter current is switched OFF. The magnetometer 32 should measure for at least 2 milliseconds to get a good signal to noise ratio. Thus, the transmitter current should be kept near zero for a least 3 milliseconds at some section in the waveform, which is well exceeded by the 9 milliseconds OFF time of the waveform 60. It should be appreciated that other magnetometer technologies may have different recovery times. While several exemplary embodiments have been presented in the foregoing detailed 5 description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiments of the System and Method are only examples, and are not intended to limit the scope, applicability, or configuration of the present disclosure in anyway. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the 10 disclosed System and Method. In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” and variations such as “comprises” or “comprising” are used in an inclusive sense, i.e., to 15 specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the system and method as disclosed herein.

Claims

1. A method of surveying the ground for subsurface mineral or water resources comprising;traversing a transmitter supported on or by an aircraft over the ground wherein the transmitter comprises a current loop subdivided into two or more electrically parallel subloops lying side by side across a common plane of, the aircraft or, a transmitter support carried by the aircraft; the subdivided loops having lower inductance than an undivided loop of the same total area;applying a current to the transmitter the current having a multilevel waveform wherein the transmitter transmits an electromagnetic wave toward the ground; andsupporting one or more receivers on the aircraft for receiving electromagnetic waves generated by a decay of currents induced in subsurface mineral or water resources by the electromagnetic waves transmitted by the transmitter.

2. The method according to claim 1 wherein the one or more receivers include an antenna attached by a tether to the aircraft wherein the antenna is capable of being deployed from the aircraft when in flight and towed by the aircraft.

3. The method according to claim 1 or 2 wherein the one or more receivers include a magnetometer fixed to a part of a body of the aircraft.

4. The method according to any one of claims 1 to 3 wherein applying a current to the transmitter includes applying a current having a waveform which has, at least three current steps per half period, and transmitting an electromagnetic field with a frequency spectrum comprising in substance only odd harmonics of equal amplitude, up to a bandwidth of the transmitter.

5. The method according to claim 4 wherein each current step has substantially the same absolute slew rate.

6. The method according to any one of claims 1 to 3 wherein applying a current to the transmitter includes applying a current having a waveform which has in at least one full period: at least 4 different levels at which the current remains constant for selected periods of time, and a plurality of current ramps; and transmitting an electromagnetic15field with a frequency spectrum comprising in substance only odd harmonics of equal amplitude, up to a bandwidth of the transmitter.

7. The method according to claim 6 wherein each current ramp has substantially the same absolute slew rate.

8. A current waveform for a current applied to an electromagnetic transmitter, the current waveform comprising:at least one cycle where the current waveform has at least 4 different levels at which the current remains constant for selected periods of time, and a plurality of current ramps; and wherein the frequency spectrum of the current waveform in the at least one cycle has a normalised frequency spectrum substantially identical to that of a square wave.

9. A multistep current waveform for a current applied to an electromagnetic transmitter, the multistep current waveform comprising:at least one cycle where the current waveform, in one half period, has at least three steps; and when applied to a transmitter, transmits an electromagnetic field with a frequency spectrum comprising in substance only odd harmonics of equal amplitude, up to a bandwidth of the transmitter, wherein the multistep current waveform has more high frequency content or higher bandwidth than a square wave with the same slew rate and period.

10. A method of processing the ground response measured by a receiver and induced byan electromagnetic field produced by applying to a transmitter a current waveform ofclaim 8 or 9, the method comprising transforming receiver voltage into that which wouldbe obtained with a perfect square wave by application of a formula:V;q(D =isq(D-v(D i (Dwhere ( ) is the Fourier transform of the receiver voltage, ( ) is the Fourier transform of the transmitter current Isq(f)) is the Fourier transform of an ideal square wave transmitter current, and is frequency, wherein the spectrum of the transmitted waveform has amplitudes that are substantially identical to that of a square wave and only phases of the spectrum need to be adjusted to align with the perfect square wave, and whereby substantially no noise is added.1611. A method of surveying the ground for subsurface mineral or water resources according to any one of claims 1-7 wherein the current applied to the transmitter comprises the current waveform according to claim 8 or 9.17

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