AU2009100027A4 - Electromagnetic Survey for Highly Conductive Ore Bodies - Google Patents

Description

16/01 2009 FRI 16:02 16/0 209 FI 1:02 FAX 61 2 8231 1099 FB RICE CO Ij0/2 Q 005/021

I

AUSTRALIA

FB RICE CO Patent and Trade M-.rk Attorneys Patents Act 1990 BHP BILLITON INNOVATION PTY LTD COMPLETE SPECIFICATION INNOVATION PATENT Invention Title: Electromagnetic Survey for Highly Conductive Ore Bodies The following statement is a full description of this invention including the best method of performing it known to us:- COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:02 FAX 61 2 8231 1099 FB RICE CO R006/021 2 O 0\ Title ELECTROMAGNETIC SURVEY FOR HIGHLY CONDUCTIVE ORE

BODIES

Technical Field O 10 This invention concerns electromagnet (EM) surveys for identifying highly 0conductive ore bodies. In particular, but not exclusively, the invention concerns airborne surveys. In a further aspect the invention concerns a method for conducting 0 OEM surveys.

Background Electromagnetic (EM) survey systems are used in searching for mineral deposits.

These systems may be ground or airborne systems, but in both cases they have a transmitter which generates a pulsed or oscillating electromagnetic field, and which induces electrical currents in conductive materials including the mineral deposits which are the target of the survey.

These systems also have a receiver to detect the electromagnetic field due to the currents induced in the target. Commonly the receiver is sensitive to the vertical component of the magnetic field, but some systems measure a horizontal component, or two or three orthogonal components. Less commonly a total field sensor is used which effectively measures the component in the direction of the Earth's field.

Inevitably the receiver is also sensitive to the transmitted electromagnetic field and to the fields due to currents induced in other conductive bodies, such as weathered surface layers, graphitic conductors and barren or low grade mineralisation.

Investigations in Geophysics No. 3, Electromagnetic Methods in Applied Geophysics, volume 2, Application, Parts A and B, edited by M.N. Nabighian and published by the Society of Exploration Geophysicists (incorporated herein by reference) describes the techniques and interpretation of EM survey systems.

In searching for new mineral deposits it has been found to be advantageous to use airborne sensing techniques as these allow large areas to be surveyed efficiently and COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:03 FAX 61 2 8231 1099 FB RICE CO [007/021 3 quickly. Airborne EM technologies have been developed and used for many years and in the main involve the transmitter and receiver being transported by a single aircraft.

One of the difficulties introduced by airborne operation is that the receiver and transmitter are moving and inevitably move relative to each other, if only by small amounts. This movement changes the magnitude of the transmitted electromagnetic field at the receiver, and this variation can interfere with detection of the field due to the target.

Two approaches are used to overcome this problem: First, time domain systems using a pulsed transmitter discard the 'on-time' receiver signal while the transmitter is being pulsed, and use only the 'off-time' receiver signal which as a result is uncontaminated with the transmitter pulse.

Second, frequency domain systems such as DIGHEM (used by Fugro Airborne Surveys) and GEM2A (used by Geophex) and the AEROTEM time domain systems (used by Aeroquest) connect the transmitter and receiver rigidly together to eliminate relative motion between them.

The penetration of the transmitted electromagnetic field into the ground is limited by the'skin depth' effect which is proportional to the reciprocal of the frequency of the signal. Operating at lower frequencies is therefore an advantage for detection of buried deposits, and the systems typically operate at frequencies of 10 100,000 Hz, with frequencies in the range 10 100 Hz being desirable for deeper penetration.

It is a characteristic of EM survey systems that the currents induced in a conductive body are generally proportional to the reciprocal of the cube of the distance between the transmitter and the body, and that the magnitude of the electromagnetic field due to those currents, detected at the receiver, is proportional to the reciprocal of the distance between the body and the receiver. As a result these systems can not usually detect conductive bodies more than several hundred meters below the ground.

Detection to greater depth is desirable for making these systems generally useful for mineral surveys.

It is a characteristic of very conductive targets that the currents induced in them closely follow the transmitted field, and so are relatively large during the 'on-time' and relatively small during the 'off-time'. For a target with time constant t, measured with COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:04 FAX 61 2 8231 1099 FB RICE CO R008/021 4 0 O a system with on-time 8 c, the ratio of the on-time to off-time responses approaches Therefore it is a disadvantage of time domain systems not to measure et in the 'on-time', and rigid systems measuring in the 'on-time' have an advantage for these targets.

For very conductive targets, known as 'perfect' conductors, the response due to the target is indistinguishable from a scaled version of the transmitter signal. To detect these conductors the magnitude of the received waveform during the on-time must be O 10 compared with the direct primary signal from the transmitter. Otherwise only the 0variation within the on-time response can be detected, and this is of similar magnitude O to the off-time response.

0 The currents induced in conductive bodies are proportional to the magnitude of the transmitted electromagnetic field, which is usually quantified as the maximum magnetic dipole moment of the transmitter. The transmitter usually consists of a coil or multi-turn loop of electrical conductor where the dipole moment is the product of the current in the conductor, the number of turns, and the area inside the coil.

The transmitter moment is most easily increased by increasing the diameter of the transmitter loop, but in doing this it is difficult to maintain the rigidity of the system so that performance then becomes limited by relative motion between the transmitter and receiver, without increasing the weight of the system beyond the payload capacity of an aircraft.

The AEROTEM II system has been described by Balch et al. (The Leading Edge, 2003, 22(6), pp. 562-566). In that system the maximum signal at the receiver due to the transmitter, is 109 nT/s. This is reduced to 103 nT/s at the receiver by bucking coils, and the noise is further reduced to 1 nT/s in post processing deconvolution with the transmitter current waveform. We note that one part in 106 variation in the primary signal at the centre of the transmitter loop would arise from a 2.5 gm change in the transmitter loop radius, or a few 4m displacement from the centre of the transmitter loop. This is indicative of the rigidity required in this type of system.

In a non-rigid system, motion of the receiver relative to the transmitter loop, and distortion of the transmitter loop, will change the magnitude of the received primary signal. For a transmitter loop of 10m radius, for example, the following distortions will lead to variation of the magnitude of the primary signal by one part in 106: COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:04 FAX 61 2 8231 1099 FB RICE CO Q009/021 In-plane displacement of the receiver by 8 mm Axial displacement of the receiver by 6 mm Rotation of the receiver by 1 mRad Expansion of the loop radius by 10 Om In-plane distortion of the loop by 7 mm Increasing the diameter of the transmitter loop, while maintaining the number of turns and maximum current in the transmitter waveform, is beneficial in that it results in a larger transmitter moment (proportional to the square of the radius), and at the same time decreases the primary field at the receiver position (proportional to the reciprocal of the radius). Overall the required dynamic range decreases as the third power of the radius. However increase in radius increases the weight of the system and simultaneously and additionally makes rigidity more difficult to achieve.

Disclosure of the Invention The invention is an electromagnetic survey system for identifying conductive ore body targets, comprising: a transmitter to transmit a pulsed or oscillating electromagnetic field which induces electrical currents in conductive materials including ore body targets, a receiver sensitive to the transmitted electromagnetic field, including any modification to the field due to an ore body target, monitoring means to determine the magnitude of a directly coupled transmitter primary signal at the receiver, and a processor to process the received, and monitored, signals and to distinguish the effect of a conductive ore body target.

This system is able to measure the 'on-time' responses in an airborne electromagnetic survey system with very high transmitter moment. Rather than using a completely rigid system, the magnitude of the directly coupled transmitter primary signal is continuously monitored using means separate to the receiver so that variations due to 'perfect conductor' ore bodies can be detected.

The monitoring means are designed so that their sensitivity to the effect of the conductive ore body target, compared with their sensitivity to the primary signal, is relatively less, and the difference in relative sensitivity allows the response of ore body target to be separated.

COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:05 FAX 61 2 8231 1099 FB RICE CO 010/021 6 0 0 The system may be able to take advantage of using a large diameter, large moment, airborne time-domain electromagnetic system, since it does not necessarily rely on Srigid coupling between transmitter and receiver.

Frequency Separation One approach is for the processor to separate the waveforms on the basis of frequency. The primary field coupling to the receiver is purely geometric, and therefore independent of the frequency of the transmitted signal. On the other hand O 10 the 'skin depth', a measure of the distance the EM field penetrates the Earth, is 0proportional to the reciprocal of frequency. At sufficiently high frequencies there will Obe insufficient penetration for the field to be effected by the 'perfect conductor' target, O and its effect at this frequency will be negligible.

The injection of a continuous high frequency pilot signal onto the transmitter loop, and detection of the magnitude of this signal, allows use of the magnitude of this signal to determine magnitude of the primary signal before the effect of conductive ore bodies.

A separate pilot signal receiver coil may be used, rigidly attached to the main receiver coil, but with higher bandwidth. This allows the higher frequency signal to be detected with invariant gain.

It may be advantageous to inject a number of frequencies as this would allow differences in the conductance of the surface to be assessed.

The waveforms used in time-domain electromagnetic systems are generally broadband and include many harmonics of the operating frequency, and the invention may make use of one or more of these harmonics in the place of injected monotone frequencies.

It is a disadvantage of this approach that variations in near surface conductivity can be removed, however these will usually be of lower time constant and detectable in the off-time response. In this arrangement variations in the near surface conductivity, and topography and survey height lead to variations in the magnitude of the near-surface response and so can be a source of noise in the corrected signal.

Gradient Separation COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:06 FAX 61 2 8231 1099 FB RICE CO Q011/021 7 0 O A second approach is to make use of the relative proximity of the transmitter loop, and distance of the target conductors to differentiate their signals.

To a first approximation, spatial gradients, including higher order gradients such as curvature, of the field measured at the receiver due to the target conductors will be negligible, whereas the gradients of the primary field are large. Therefore measurements of spatial gradients provide measurements of the magnitude of the primary signal at the receiver location after the effects of system distortions. For this O 10 technique to be effective the gradient measurements must be made by sensors which 0 _are fixed rigidly to the receiver. The dimensions of this part of the system are N relatively small, and do not increase with the transmitter loop dimensions, so a rigid o sensor group for the receiver is easier to manufacture, and is lighter, than a rigid Ci transmitter.

As an example, if the system distortion was limited to axial changes in the receiver position, then measurement of the axial gradient of the received field would allow correction of the primary signal for this change.

If changes in loop radius also needed to be compensated, measuring the second axial gradients of the signal will allow this.

Signals from additional sensors, such as accelerometers, may also be combined in this processing.

In this approach a high frequency pilot signal may be transmitted before the spatial gradients of the field are sensed by the at least one sensor.

In another aspect the invention is a method for conducting electromagnetic surveys to identify conductive ore body targets, comprising the steps of: transmitting a pulsed or oscillating electromagnetic field which induces electrical currents in conductive materials including ore body targets, receiving the transmitted electromagnetic field at a receiver, including any modification to the field due to an ore body target, determining the magnitude of a directly coupled transmitted primary signal at the receiver, and processing the received, and monitored, signals and distinguishing the effect of a conductive ore body target.

COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:06 FAX 61 2 8231 1099 FB RICE CO R012/021 8 Brief Description of the Drawings An example of the invention will now be described with reference to the accompanying drawings, in which: Fig. I is a diagram of a helicopter towed system.

Fig. 2 is a diagram of the transmitter waveform.

Fig. 3(a) is a diagram of a first arrangement suitable for the frequency separation approach. Fig. 3(b) is a diagram of a second arrangement suitable for the frequency separation approach.

Fig. 4 is a diagram of an arrangement suitable for the gradient separation approach.

Best Modes of the Invention Referring first to Fig. I, an example of the invention is a helicopter towed system consisting of a multi-turn transmitter of eight circular turns 12, each 20 m in diameter, of cylindrical aluminium tubes or pipes carrying a maximum current of 250 A.

The transmitter waveform is a half sine pulse of width 10ms followed by an 'off-time' of 10 ms, and then this is repeated with opposite polarity for an operating frequency of 25 Hz; see Fig. 2.

The receiver 20 is located at the centre of the transmitter loop and is encircled by a 'bucking coil' 30 of one turn and -2.5 m diameter which is rigidly attached to the receiver coil (shown exploded). The bucking coil is connected in series with the transmitter loop, but in opposition to it so that the primary field at the receiver is zero.

The current in the transmitter loop 12 is monitored to high precision and recorded.

The receiver 20 consists of a multi-turn coil of aluminium wire 1.2 m in diameter and weighing -20 kg. The noise of this receiver is 6.3 pT/s/rtHz above 1 Hz. A data recorder 40 mounted in the helicopter 10 is used to record the received signal.

Frequency Separation For the frequency separation approach, an oscillator producing a sinusoidal pilot signal at say 20 kHz, (much higher frequency than the transmitter waveform) is connected across the transmitter loop and bucking coil.

COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:07 FAX 61 2 8231 1099 FB RICE CO 013/021 9 0 0 In a first configuration this pilot frequency is detected in the receiver coil by a tuned amplifier 48 and second data recording system 50 separate to the main recorder see Fig. The magnitude of this signal is used to determine magnitude of the primary waveform before the effect of conductive ore bodies.

In a second configuration a pilot signal receiver coil 54 is rigidly attached to, or wound with, the main receiver coil 20, and attached to a separate high quality preamplifier 56 and data acquisition system 50; see Fig. 3(b).

0 It is important that the sensitivity of the receiver to the injected signal does not vary. It ois therefore undesirable to use a frequency close to, or above, a resonance in the Ci receiver coil. For this reason it is desirable to use a separate pilot signal receiver coil, rigidly attached to the main receiver coil, but with higher bandwidth to allow the higher frequency signal to be detected with invariant gain.

It is also important that the injected frequency is transmitted with the same 'geometry' as the transmitter waveform. This can be affected by stray capacitance in the structure of the transmitter, if the pilot frequency is too high.

Gradient Separation For the gradient separation technique the receiver coil 20 is rigidly attached to additional gradient coils mounted on nested spherical shells 60 (see Fig. 4) with each shell supporting a double coil first derivative sensor, and a triple coil second derivative sensor. Each of these derivative sensors is accurately manufactured to minimise its sensitivity to the primary signal and to lower order gradients. This requires close control of the diameter of each turn in the windings and the number of turns in each coil, and also minimising stray area between the leads connecting to the coils, and rigidity in these connections. Where possible, twisted pair, or twisted quadrupolar wiring connections are used.

A high frequency signal (say 10 kHz) is injected continuously into the transmitter and this signal is selectively monitored by the derivative sensors. Each derivative sensor is connected to a high quality preamplifier 62 and analog to digital converter 64, and is separately recorded.

A processing unit 66 combines the signals from the main receiver coil 20, the transmitter current, and the derivative sensors to correct the main receiver signal for COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:08 FAX 61 2 8231 1099 FB RICE CO 014/021

O

O variations in the direct coupling between the transmitter loop and the receiver coil; such as axial distance between transmitter and receiver coils, and changes in etransmitter loop radius. This leaves variations due to variations in conductivity of the Earth observable. In variations, signals from additional sensors, such as accelerometers, are also combined in this processing.

Although the invention has been described with reference to a particular example, it should be appreciated that it could be exemplified in many other forms and in combination with other features not mentioned above.

0 OFor instance, most airborne EM survey systems use induction coils as receivers. These o are generally the most suitable receivers at the frequencies used by these systems. A sensor for the first spatial gradient can consist of a pair of identical coils, spaced apart along the derivative direction, and connected in opposition. A sensor for the second spatial derivative can use four coils, spaced equidistant, and connected with the inner pair in opposition to the outer pair. Alternatively the inner spacing can be varied (reduced is most likely), while the outer spacing remains equal. Alternatively the second spatial derivative coil can consist of three coils, equidistant along the derivative direction, with the centre having the same number of turns, but twice the area of the outer coils. This arrTangement can be built onto a spherical shell for high rigidity.

An example of the invention using gradient separation measures the first and second spatial derivatives of the signal in three orthogonal directions. It is recognised that additional gradients may need to be measured to allow compensation for a greater variety of distortions of the system. While formulae can be derived mathematically for compensation with the gradient signals, it is also possible to derive compensation formulae from calibration measurements made at high altitude (away from any effects of target conductors) or at lower altitude over deep salt water, using regression techniques or neural networks.

It is recognised that other magnetic field sensors such as SQUIDs and fluxgate magnetometers can alternately be used as sensors. Another sensor used in electromagnetic surveys is the caesium vapour magnetometer, in particular the TM-6 system (Gap Geophysics), and these sensors could also be used as sensors in the present invention.

COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:08 FAX 61 2 8231 1099 FB RICE CO 1h0 1.5/.0 21.

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O A higher frequency pilot signal can be injected into the transmitted waveform for the purpose of monitoring the spatial derivatives. This allows narrow band detection of the derivative signals and thereby increases sensitivity and allows use of smaller S ~5 derivative sensor coils.

Multiple frequency signal injection can be used so that variations in the surface 17 conductivity, which have a residual effect on estimation of the direct primary signal, ocan be estimated. It is recognised that the transmitted signal is broadband and selected frequencies in the transmitted signal could also be used for monitoring the spatial 0derivatives. It is also possible to use the entire broadband transmitted signal directly.

0 O It is recognised that different connection of the coils used for the second spatial derivative can be used to monitor the first spatial derivative, and also the primary signal. These different connections can be implemented using filter networks where high frequency pilot signals are used for the purpose of monitoring the spatial derivatives.

As the separation of the system from the ground surface is not infinite (typically survey heights from 30 m to 100 m are used), there will likely be some remnant effect of the conductive Earth on this correction using gradients. A calibration technique using high altitude, and low altitude flights over deep salt water can be used to calibrate and correct for this residual effect.

It is recognised that it is beneficial to measure three components of the received field, and a set of orthogonal coils can be used to measure the three components of the received field.

It is recognised that inertial sensors such as gyroscopes and accelerometers can be used to monitor motion and relative motion of components of the survey system, such as the receiver assembly, and portions of the transmitter loop, and that such measurements can aid the determination of the direct primary signal. Accelerometer pairs, directed radially and vertically, may be rigidly attached at several positions on the transmitter loop, and three pairs of accelerometers may be rigidly attached to the receiver assembly to allow acceleration and rotation to be measured in three orthogonal axes. The use of accelerometers is particularly of use in monitoring distortions of the transmitter loop, both in-plane and out-of-plane, and relative rotation of the receiver and transmitter loop, that vary in time.

COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16

Claims (4)

1. An electromagnetic survey system for identifying conductive ore body targets, comprising: a transmitter to transmit a pulsed or oscillating electromagnetic field which induces electrical currents in conductive materials including ore body targets, a receiver sensitive to the electromagnetic field, including any modification to the field due to the an ore body target, monitoring means to determine the magnitude of a directly coupled transmitter primary signal at the receiver, and a processor to process the received, and monitored, signals and to distinguish the effect of a conductive ore body target.

2. An electromagnetic survey system according to claim 1, wherein the monitoring means are designed so that their sensitivity to the effect of the conductive ore body target, compared with their sensitivity to the primary signal, is relatively less.

3. An electromagnetic survey system according to claim I or 2, wherein the system uses a large diameter, large moment, airborne time-domain electromagnetic system.

4. A method for conducting electromagnetic surveys to identify conductive ore body targets, comprising the steps of: transmitting a pulsed or oscillating electromagnetic field which induces electrical currents in conductive materials including ore body targets, receiving the transmitted electromagnetic field at a receiver, including any modification to the field due to an ore body target, monitoring the magnitude of a directly coupled transmitted primary signal at the receiver, and processing the received and monitored signals and distinguishing the effect of a conductive ore body target. COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16 16/01 2009 FRI 16:10 FAX 61 2 8231 1099 FB RICE CO I017/021 13 An electromagnetic survey system for identifying conductive ore body targets or a method for conducting electromagnetic surveys to identify conductive ore body targets as substantially herein described with reference to the accompanying drawings. DATED this fifteenth day of January 2009 Patent Attorneys for the Applicant: F.B. RICE CO. COMS ID No: ARCS-220212 Received by IP Australia: Time 16:10 Date 2009-01-16