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SUB-AUDIO MAGNETICS (SAM)

A high definition technique has been developed for simultaneously mapping electrical and magnetic characteristics of the ground. Each of the parameters may be measured at sub-metre intervals while continuously traversing either on foot or in a vehicle. Electric current, typically in the sub-audio frequency range of 3-200Hz, is induced in the ground

by either galvanic or electromagnetic means. An optically pumped total field magnetometer continuously measures the natural and synthetic field changes from DC to the highest transmitted frequency. Real-time spectral analysis of the magnetic field

changes allows a suite of geophysical parameters to be derived.

Sub-Audio Magnetics (SAM).doc Last Modified: 24/07/2003 at 7:51 PM

SUB-AUDIO MAGNETICS (SAM)

A High Resolution Technique for Simultaneously Mapping

Electrical and Magnetic Properties1

M.K. Cattach G.W. Boyd J.M. Stanley

S.J. Lee

Geophysical Research Institute Normandy Poseidon Group University of New England PO Box 7175 Armidale NSW 2351 100 Hutt Street Australia Adelaide SA 5000

Australia

Abstract

A high definition technique has been developed for simultaneously mapping electrical and magnetic characteristics of the ground. Each of the parameters may be measured at sub-metre intervals while continuously traversing either on foot or in a vehicle. Electric current, typically in the sub-audio frequency range of 3-200Hz is induced in the ground by either galvanic or electromagnetic means. An optically pumped total field magnetometer continuously measures the natural and synthetic field changes from DC to the highest transmitted frequency. Real-time spectral analysis of the magnetic field changes allows a suite of geophysical parameters to be derived. The geophysical measurements so obtained include, but are not limited to: • Total Field, High Definition Magnetics (HDM) • Total Field, Magnetometric Resistivity (TFMMR) • Total Field, Magnetometric Induced Polarization (TFMMIP) • Total Field, Electromagnetics (TFEM). Because of its inherent cost-efficiency and superior spatial resolution, the SAM technique has the potential to significantly enhance geophysical mapping for mineral exploration, archaeological investigation and environmental studies of salinity, industrial waste and chemical contamination in soils.

Key words: Sub-Audio Magnetics, SAM, HDM, TFMMR, TFMMIP, magnetometer, high-definition, geological mapping, environmental geophysics.

1This paper was presented at the 10th National ASEG conference, 20th-25th Feb., 1994. It was published in Exploration Geophysics (1993) 24, 387-400.

Cattach, Stanley, Lee & Boyd Sub-Audio Magnetics (SAM)

Sub-Audio Magnetics (SAM).doc Last Modified: 24/07/2003 at 7:51 PM 2

Introduction Recent advances in microprocessor technology have had a great impact on geophysical instrumentation. In particular, these developments have resulted in instruments which feature faster sampling rates, enhanced data acquisition and signal processing capability and improved sensitivity. When combined with navigation facilities such as GPS, higher quality surveys have become more economically viable and consequently, the exploration industry is becoming more aware of the benefits of higher spatial resolution in their surveys. One area of geophysics which has greatly benefited from technological advances is magnetics. The last decade has seen the development of rapid sampling, portable magnetometers incorporating optically pumped sensors. The main advantage of these systems is that, unlike proton precession magnetometers, data can be recorded automatically at sample intervals as small as 5cm while the operators are moving at considerable speed. High Definition MagneticsTM has not only improved the quality of magnetic surveys for conventional applications, but due to the enhanced capabilities, many new applications have also been recognized (Stanley and Cattach, 1990). The quality of exploration data is often assessed by the precision with which measurements are made. However, it is the interpretability of the data that is of greatest concern to the explorationist. In practice, the quality of exploration data is dependent upon the measurement spatial resolution rather than instrument resolution. The cost of data acquisition usually dictates that measurement intervals be selected to sample the predicted "signal" anomaly of interest. In fact, it is the higher frequency "noise" profile that must be adequately sampled if the signal is to be properly interpreted. Experience with high definition magnetic surveys has demonstrated that many conventional surveys have been wasted as a result of the false economy associated with compromised data sampling. Unfortunately, the explorationist may never become aware of the information that was overlooked by undersampling. While exploration expenditure wasted in this way remains the responsibility of the practitioner, those involved in instrumentation research need to improve the cost- efficiency in acquiring high spatial resolution data. With respect to a number of parameters, Sub-Audio Magnetics achieves this. Improvements in survey efficiency have not been uniform across all geophysical techniques. Higher TM Trade Mark of the Geophysical Research Institute.

precision data from electrical methods such as Resistivity and Induced Polarization (IP) has been achieved from real-time signal processing enhancements. However, apart from the employment of multi-conductor cables by receivers such as the Scintrex IPR-12 (Scintrex, 1993) which allow multiple input mode of operation, survey speed has not significantly improved. The reason for this is the logistical constraint imposed by the requirement for grounded potential electrodes. It may be argued that due to that constraint, the speed at which Electrical Induced Polarization (EIP) surveys are conducted can never be significantly increased. Electromagnetic methods such as Magnetometric Resistivity (MMR) and Magnetic Induced Polarization (MIP) have also benefited from improved signal processing capability. These methods involve measuring the horizontal component of the magnetic field produced by current flow between two electrodes. A major difficulty in improving the efficiency of these techniques is the requirement to precisely orient and level the sensor prior to taking measurements. Recognizing the need for a cost-effective method which would provide detailed electrical information, the authors have developed a new, high-definition technique for mapping parameters related to the electrical characteristics of the ground. The technique employs a transmitter operating at fundamental frequencies in the sub-audio frequency range of 5-20Hz and the project has therefore been entitled "Sub-Audio Magnetics" (SAM). This paper defines the SAM concept and describes the results of some of the preliminary feasibility studies.

The Sub-Audio Magnetics Technique Sub-Audio Magnetics2 is a high definition technique which has been developed for simultaneously mapping the electrical and magnetic characteristics of the ground. Each of the parameters may be measured at sub-metre intervals while continuously traversing on foot or in a vehicle. Electric current, typically in the sub-audio frequency range of 5 to 200Hz is induced in the ground by either galvanic or electromagnetic means. An optically pumped total field magnetometer continuously measures the natural and synthetic field changes from DC to the highest transmitted frequency. Real-time spectral analysis of the magnetic field changes allows a suite of geophysical parameters to be derived. The geophysical measurements so obtained include, but are not limited to:

2SAM is the subject of International Patent No.WO 91/19210.



• Total Field, High Definition, spatially varying

Magnetics (HDM) • Total Field, Magnetometric Resistivity

(TFMMR) • Total Field, Magnetometric Induced

Polarization (TFMMIP) • Total Field, Electromagnetics (TFEM). The method uses a constant current transmitter to produce a low frequency energizing field which is applied either to the ground through distant electrodes as for conventional gradient array resistivity surveys or through a loop. The electromagnetic signal is measured simultaneously with the earth's spatially varying magnetic field using a total field, rapid sampling magnetometer. The combined signals are sampled at a fast enough rate to adequately sample the full spectrum of the artificial waveform. The signals are spectrally distinct and with the aid of Digital Signal Processing (DSP) chip technology, they may be separated and processed in real-time. The benefit of having this ability is the efficient, concurrent, high definition mapping of parameters related to the electrical characteristics of the ground as well as the spatially varying magnetic field.

Survey Speed

The maximum survey speed is determined by the frequency of the transmitted signal, the sampling speed of the magnetometer, the elevation of the sensor above ground and the required spatial resolution. Assuming that the nearest magnetic source is at ground level, the shortest significant wavelength in a magnetic profile will be equal to twice the sensor elevation (Stanley et al, 1992). If we consider that the magnetic sensor is operated at a height h above the ground surface and moved at a maximum velocity V, then it has been determined that the maximum frequency f Max in the magnetic field of spatial origin will be given approximately by the relationship

f VhMax = 2

For example, the rate at which the spatially varying magnetic field changes for a magnetic sensor traversing at 2.0 m/sec (7.2km/hr) at an elevation of 1m above ground, will have a 40db cut off at about 1Hz. Clearly, in order to be able to separate the applied signal from the spatially varying magnetic field, the transmitter frequency must be greater than f Max . Naturally occurring temporal changes in the

magnetic field may be removed by reference to a synchronized stationary magnetic sensor as for conventional magnetic surveying.

The SAM technique may prove to have a wide range of applications including mapping, profiling and downhole logging. As an initial priority, the feasibility studies have targeted its role as a high definition electrical mapping tool which would complement high definition magnetics. In this mode, the survey configuration closely parallels Magnetometric Resistivity (MMR) (Jakosky, 1940; Edwards, 1974) and Magnetic Induced Polarization (MIP) (Siegel, 1974). In order to distinguish the SAM technique from conventional MMR and MIP, the terms 'Total Field Magnetometric Resistivity' (TFMMR) and 'Total Field Magnetometric Induced Polarization' (TFMMIP) have been adopted. While development to date has concentrated on TFMMR / TFMMIP, ample evidence has been acquired that the SAM method will be equally effective in mapping other galvanic and electromagnetic properties.

TFMMR / TFMMIP The TFMMR / TFMMIP methods are variants of MMR / MIP. The field procedures are similar to those employed for MMR / MIP and a brief review of these methods is therefore in order. The MMR technique as described by Edwards (1974), is based on the measurement of the low level (<5 gamma), low frequency (1-5Hz) magnetic fields associated with noninductive current flow in the earth. The field techniques are also similar to those used in the gradient array resistivity method. The major difference between the methods is that the potential electrodes used in resistivity techniques are replaced by a highly sensitive, component magnetometer. Edwards (ibid) describes a major technical difference between the MMR method and the gradient array resistivity method with respect to the positioning of the current electrodes. In gradient array resistivity surveying, the current flow should be across strike, so that the structure looks like a set of resistors in series. In contrast, for MMR, the electrodes are positioned so that current flow is along the strike of the structure. The structure will then appear as a set of resistors in parallel which will carry different currents. Current will flow preferentially along conductors. As the magnetic field due to this current flow is essentially horizontal at the ground surface, the amplitude of the horizontal component of the magnetic field, which is at right angles to the line joining the current electrodes, is usually measured. The MMR anomaly is defined as the difference between the measured values and the 'normal' values, which are the theoretical values expected at the surface of a homogeneous earth.



The Magnetic Induced Polarization method (MIP) was first described mathematically by Siegel (1974) and more recently summarized by Siegel and Howland-Rose (1990). MIP is essentially an extension of the MMR method whereby parameters related to induced polarization are also determined. Consequently, MMR is one of the basic quantities measured in MIP. The precise parameters measured during an MIP survey depend upon whether time domain or frequency domain equipment is being used. In frequency domain, the parameters may include: • Percentage Frequency Effect (PFE) - which is

the percentage change in the observed magnetic field with a change in transmitter frequency.

• Phase Angle between the observed magnetic

field and the current applied to the ground. • Relative Phase Shift (RPS) - the phase angle

between two harmonically related components of the applied current.

• Magnetometric Resistivity In time domain, the parameters are : • Chargeability which is determined by

integrating under the decay curve, and • Magnetometric Resistivity A fundamental difference between TFMMR / TFMMIP and conventional MMR / MIP is that the Total Magnetic Intensity is measured instead of the horizontal component. The magnetic field due to current flowing in the cable supplying the electrodes is commonly referred to in the literature as the Primary field. In the survey area, the Primary field has only a vertical component. Therefore, by measuring the horizontal magnetic field as in MMR / MIP, only the magnetic field due to subsurface current will be observed. However, the Primary magnetic field will have a significant affect on total field measurements. Consequently, in TFMMR / TFMMIP surveys, a correction needs to be made for the Primary field. For this reason, the position of the cable is required to be known and is generally laid out in a rectangular shape around the survey area. A typical TFMMR / TFMMIP survey layout is shown in Figure 1.

Components of the SAM Signal The total magnetic field ( )HT measured by the SAM receiver is comprised of two major spectrally distinct components:

Figure 1. Layout of a typical TFMMR/TFMMIP survey showing the ideal electrode locations and survey orientation relative to the direction of geological strike.

• The Spatially-varying magnetic field (HS )

which will have a maximum frequency component as defined previously.

• The low frequency Modulation (HMod ) due to

the transmitted signal.

The total field is then: H H HT S Mod= +

HS and HMod may be separated by a filter. Furthermore, the modulation (HMod ) will consist of: • The Primary field (HP ) which is due to current

flow in the wires supplying the electrodes. • The Ground field (HG ) which has been

defined as the magnetic field due to current flowing through the ground.

The Normalized TFMMR (HNorm) anomaly will then be defined by:

H H H HNorm Mod P N= − −

where HN is the Normal magnetic field and is defined as the magnetic field expected over a homogeneous earth.

The Primary Magnetic Field The Primary magnetic field HP is defined as that due to current flowing in the cable feeding the electrodes. The magnetic field H due to electric current flowing in a long straight wire is directly proportional to the current I in the wire and inversely proportional to the distance r from the wire:



H ∝ Ir

According to Ampere's law, a current I flowing in any path can be considered as many tiny (infinitesimal) current elements, such as in the wire in Figure 2. If ∆l represents any infinitesimal length along which the current is flowing, then the magnetic field ∆H at any point P in space, due to this element of current is given by

∆ ∆H I r= ×( )l r1

24π

where r is the distance from the element ∆l to the point P, and r1 is the unit vector in the direction of r .

The magnitude of ∆H is

∆∆H I lr

=sinθπ4 2

Figure 2 Ampere’s Law – A current I through a length of conductor ∆l create a magnetizing field ∆H at a point P (After Telford et al., 1990).

The magnetic field due to the current flowing through the cable supplying the electrodes can therefore be calculated at any point in the survey area by integrating along the length of the cable. In the absence of significant topographic relief, the Primary

magnetic field in the survey area can be assumed to be vertical at all measurement points.

The Normal Magnetic Field The magnetic field at the surface caused by current flowing into grounded electrodes is described by Keller and Frischnecht (1966). The Normal magnetic field is the same as the field which would be produced by the same current flowing in a wire extending vertically downward from the earth's surface to infinity. The components of the magnetic field at a point P(x,y) for a single electrode located at (0,0) can again be calculated from Ampere's law with the result:

H Iyx y

H Ixx y

H

x

y

z

=+

=+

=

4

4

0

2 2

2 2

π

π

( )

( )

Consequently, the Normal magnetic field has only horizontal components and can be calculated for two electrodes by vector addition.

Determination of Total Field Corrections Because a total field magnetometer is being used, only the component of an artificially induced magnetic field, in the direction of the earth's magnetic field, is measured. Consequently, to make corrections for both the Primary and Normal magnetic fields, their components in the direction of the earth's magnetic field must be calculated. In order to map the components of the Primary and Normal fields onto the direction of the earth's magnetic field, it is first necessary to define in local grid coordinates, the unit vector representing the direction of the earth's magnetic field. In Figure 3, FE is a unit vector representing the direction of the earth's magnetic field. The x , y and z axes define the local grid coordinate system. I is the inclination of the earth's magnetic field and the declinationD is the angle between FE and grid north. It can be shown that

F i j kE E E EX Y Z= + + where



X D IY D IZ I

E

E

E

===

cos cossin cossin

and i , j and k are unit vectors in the x , y and z directions respectively.

Figure 3 Elements of the Earth’s magnetic field with respect to a local coordinate system.

The amplitude of the applied magnetic field in the survey area is insignificant compared to the magnitude of the earth's magnetic field. Consequently, the vector addition of the applied field to the earth's magnetic field will have little effect on its direction. The influence that the applied field will have on total field measurements can therefore be approximated by projecting the applied field onto the direction of the earth's magnetic field. This can be performed by calculating the scalar product of the applied field with the unit vector defining the direction of the earth's magnetic field. With respect to Figure 4, let FA be an applied magnetic field defined as follows:

F i j kA A A AX Y Z= + + Then

F F F FA E A E⋅ = cosθ where FA is the applied field, and FE is the unit vector in the direction of the earth's magnetic field. Since FA cosθ is the projection of FA on FE , then

F F FA E E AF F⋅ = × (projection of on ) E

It can also be shown that

F FA E A E A E A EX X Y Y Z Z⋅ = + + .

Figure 4 Projection of an applied field FA onto a unit vector defining the direction of the Earth’s magnetic field FE.

Because, the Primary Field has only a vertical component, its influence on total field measurements is affected by the inclination I of the earth's magnetic field. The Primary Field for a typical TFMMR / TFMMIP array is shown in Figure 5. The Inclination and Declination used to generate the image were 60° and 25° respectively. The corresponding Normal magnetic field expected over a homogeneous earth is shown in Figure 6. Because the Normal field has only horizontal components, its shape is influenced greatly by the declination D . The Normal Field is asymmetric for all declinations other than 0°.

Feasibility Studies A series of field trials have been conducted over various targets in order to determine whether the SAM methodology was technically feasible as well as to gain an insight into the quality of information obtainable from the method. Specific objectives were to quantify signal strengths, instrument resolution and noise characteristics in order to define the required specifications of a SAM receiver. The feasibility studies conducted to date have employed the following instrumentation:


Figure 5 The Primary magnetic field resulting from current flowing through cables feeding the electrodes at positions C1 and C2.

Figure 6 The Normal field due to current flowing through a homogeneous earth.


Magnetometer - Geophysical Technology Model TM-4. The TM-4 was used as a prototype SAM receiver. It is a sophisticated magnetometer system which has been developed by the Geophysical Research Institute over the last 15 years and is manufactured by Geophysical Technology Pty Ltd. The TM-4 consists of a data acquisition and detector control system and one or more atomic resonance type magnetic sensors. The TM-4 has been designed for deployment from a number of survey platforms including hand-held or All-Terrain-Vehicles (ATVs). The system can be configured to include differential GPS and navigation as well as digital compensation for heading, pitch and roll interference from the vehicle. In its simplest hand-held mode of operation, the TM-4 uses an in-built cotton-thread odometer to trigger the data logger to take readings. Samples may be recorded at a minimum of 0.05m intervals although in practice, sample intervals of 0.2m to 0.5m intervals are commonly used for geotechnical or mineral exploration applications. The TM-4 operating system is a proprietary, preemptive, multi-tasking operating system designed specifically for high speed data acquisition. The CPU is a 32 bit Motorola 68030 operating at a clock frequency of 12.5MHz. The standard memory has capacity for 100,000 data points which is usually sufficient for a full day of high definition magnetic surveying. The TM-4 frequency counter is implemented through Programmable Logic Array technology. It is a period counter using a 20MHz reference clock and is capable of converting the Larmor frequency produced by an optically pumped sensor into digital magnetic field readings at selectable rates of nominally 50, 100, 200 or 400 samples per second. Theoretical data resolutions at these sample rates are 0.007, 0.02, 0.07 and 0.15nT respectively. In its hand-held mode of operation, the TM-4 normally requires two operators, one of whom holds the sensor. This enables the sensor to be maintained at a distance from any magnetic interference produced by the control electronics. As the surveys are conducted whilst the operators are continuously moving, production rates on foot of greater than 20km per day are commonly achievable. A more detailed description of the TM-4 magnetometer controller is provided by Geophysical Technology Pty Ltd (1990). A Scintrex CS-2 optically pumped caesium vapour magnetometer sensor was used as the transducer. These sensors are characterised by sensitivities in the range of a few pT and also display high gradient tolerance (40000nT/m) (Scintrex, 1993). The

information bandwidth is in the order of 500Hz but is ultimately dependent on the sample rate and noise characteristics of the frequency counter. Transmitter - Zonge GGT-10 The Zonge GGT-10 Transmitter is a solid state IP/EM constant current transmitter capable of DC to 10KHz output frequency, using time or frequency domain and pulse EM waveforms. The transmitter is capable of outputting 0.2 to 20.0 Amps in both time and frequency domain modes of operation (Zonge Engineering and Research Organization, 1990).

Field Procedure In conventional hand-held TM-4 surveys, data measurements are normally initiated at preset intervals by strobe pulses which are generated by an in-built odometer. For the SAM trials, the TM-4 was operated in continuous recording mode wherein the magnetic field measurements are recorded at the maximum selected sample rate. Distance waypoints were automatically recorded every 1.0m along the profiles using the inbuilt odometer. The sample rate used for the trials was nominally 200 samples per second. The survey was conducted at an average walking speed of about 1.5m/sec which resulted in approximately 90000 measurements per 600m profile. Due to the memory limitations, the data were dumped at the end of each profile for subsequent processing. Under these conditions, production rates were restricted to about 12 line km per survey crew per day. The transmitter was set to produce a 100% duty cycle (frequency domain) 8Hz square wave. This frequency was selected firstly because it was readily available from the transmitter but also because it provided a good frequency separation from that due to the spatially varying magnetic field. At the time of the trials, there was no synchronization available between the transmitter and the TM-4. Results The data presented in this paper were recorded over a lead-silver-zinc mineralization typical of the massive sulphide orebodies at Broken Hill, NSW. The figures were selected to illustrate the quality of the data collected, the relative signal amplitudes and the steps required to reduce the data. Figure 7 is a 10 sec data record displaying the artificially induced modulation superimposed on the spatially varying magnetic field. The data were sampled at a rate of approximately 200 samples / second.



Figure 8(A) is an example of the raw data recorded for a single traverse across the conductor. As can be seen from the figure, the modulation due to the transmitted signal is readily visible. The data were lowpass filtered with a cutoff frequency of 5Hz to obtain the spatially varying magnetic field shown in Figure 8(B). Subtraction of the data in Figure 8(B) from the data in Figure 8(A) resulted in the profile in Figure 8(C). The "pinching out" of the envelope signifies polarity reversals relative to the polarity of the transmitted signal. The high amplitude modulation between 4500mE and 4800mE is due to the influence of the Primary magnetic field generated by current flowing through the cable feeding the electrodes. In this example, the current wires were laid out to the west of the profile. Because of the lack of synchronization with the transmitter and the non-constant sample interval typical of a period counter, it was not possible use conventional IP data processing techniques to determine TFMMR and TFMMIP parameters. The signal amplitudes were determined by taking the median values of the positive and negative halves of the received signal and averaging them. This was done because of the median's immunity to noise compared to averaging techniques. The data thus obtained are shown in Figure 9(A). The appropriate Primary and Normal fields were calculated and are shown in Figures 9(B) and 9(C) respectively. Subtraction of both the Primary and Normal fields from the data in Figure 9(A) resulted in the data in Figure 9(D). In this case, the TFMMR anomaly is dipolar and centred roughly over the conductor which is located at 4800mE and dipping slightly to the west. An attempt was made to determine whether any IP information was evident in the data. As there was no synchronization between the transmitter and the TM-4 used at the time of the survey, it was not possible to determine the phase angle between the observed magnetic field and the current applied to the ground. As an alternative, the spectral components of the waveform were examined. The amplitude spectrum for a typical 1024 point section of the waveform shown in Figure 8(C) is shown in Figure 10. The amplitudes have been normalized by division by the amplitude of the fundamental frequency. As can be seen from the figure, there are distinct peaks in the spectrum at the fundamental frequency (8Hz) as well as the odd harmonics. All harmonics up to the 23rd are clearly visible in the spectrum although those above the 13th have been inadequately sampled and appear as aliased frequencies. A peak at 50Hz is due to mains power interference.

Theoretically, the amplitudes of the harmonics are inversely proportional to the harmonic number. Amplitude and phase relationships of the harmonics are often used as a measure of the IP response in conventional EIP and MIP surveys. These simple relationships are not appropriate for TFMMIP data due to the influence of the Primary field. However, as a first attempt to determine whether any IP information was present in the data, the Relative Amplitude of the fundamental frequency and the 3rd harmonic were calculated for a running 1024 point window along the profile. The Relative Amplitude RA is defined by

RA AA

=×3 3

1

where A A1 3and are the amplitudes of the fundamental and third harmonic respectively. The result of this exercise is shown in Figure 11. As can be seen from the figure, a significant anomaly corresponding with the MMR anomaly is present at 4800E. Peaks also occur at three locations to the east of the main anomaly. These peaks correspond to minor TFMMR anomalies. The Relative Amplitude data is severely influenced by the effect of the Primary field and needs to be corrected for. However, without precise synchronization between the transmitter and the TM-4, this was not possible for the data presented here. The prototype instrumentation used in the feasibility studies has not provided proof that TFMMIP data will be a diagnostic measure of the Induced Polarization phenomenon. However, the results are encouraging because close examination of the SAM signal in the various case studies conducted to date, has revealed variable phase relationships between the Primary field HP and the Ground field HG . Precision measurements of these relationships with planned, improved instrumentation will enable these features to be studied in much greater detail in the future.

Future Developments Having used an unmodified TM-4 magnetometer to record data at 200 samples per second, we have been able to conduct research and development on data processing strategies. The objective was to define the hardware enhancements and processing strategy that best extracts the parameters of interest. When these are done, a SAM processor board will be added to the TM-4 to perform real-time processing, allowing the data volume to be reduced before recording. A digital processing board which includes the facility for synchronizing the TM-4 with the transmitter is being developed for this purpose.


Figure 7 A 10s time sample (15m traverse length) illustrating the artificially induced modulation superimposed on the spatially varying magnetic field.

Figure 8 (A) A sample SAM raw data profile HT recorded over a conductor. The component signals were separated by digital filtering to produce: (B) The spatially varying magnetic field data HS and (C) The SAM Signal HMod.



Figure 9 (A) Raw TFMMR data HMod determined from the data in Figure 8. (B) The calculated Primary field HP (C) The calculated Normal field HN (D) The normalised MMR data HNorm.



Figure 10 Normalised Amplitude Spectrum of a 1024 pt SAM signal showing the fundamental frequency and the odd harmonics. The harmonics above the 11th have been folded back about the Nyquist frequency due to aliasing.

Figure 11 Relative Amplitude of the 3rd harmonic and the fundamental frequency calculated from the data in Figure 8.


Conclusion Several SAM case studies have now been conducted over various types of geological targets using the TFMMR / TFMMIP configuration. Confidentiality of the exploration targets requires a temporary delay in the publication of these results. However, in each case, the results have confirmed that SAM performs in practice as was predicted from theory. Signal-to-noise ratios in the parameters are high and sub-metre measurement intervals in the parameters are achievable at continuous walking speeds of traverse. The higher spatial resolution (up to 100 times greater than conventional electrical methods) has proven capable of providing significantly more diagnostic information than has been previously possible using conventional electrical and electromagnetic methods. The high spatial resolution provides detailed electrical mapping with definition equal to that obtained in High Definition Magnetic mapping. With SAM technology, each of several electrical and magnetic properties of the ground may be simultaneously measured with the one, man-portable instrument.

Acknowledgements The Sub-Audio-Magnetics technique is a joint venture development of the Geophysical Research Institute, University of New England and the Normandy Poseidon Group.

References Edwards, R.N., 1974. The Magnetometric Resistivity Method and its Application to the Mapping of a Fault. Can J. Earth Sci., 11, 1136-1156. Geophysical Technology Pty Ltd (1990). Technical Brief - Model TM-4. Magnetometer and Image Processing System. Jakosky, J.J., 1940. Exploration Geophysics, Los Angeles Times-Mirror Press. Keller, G.V. and Frischnecht, F.C., 1966. Electrical Methods in Geophysical Prospecting. Pergamon Press. Scintrex, (1993). CS-2 Cesium Magnetometer (Technical Manual). Siegel, H.O., 1974. 'The Magnetic Induced Polarization method'. Geophysics 39, 321-339. Siegel, H.O. and Howland-Rose, A.W. (1990). 'Magnetic Induced Polarization Method' in Induced Polarization - Applications and Case Studies, Investigations in Geophysics No. 4. Society of Exploration Geophysicists. Stanley, J.M. and Cattach, M.K. (1990). 'The Use of High Definition Mapping in Engineering Site Investigation'. Exploration Geophysics 21, 91-103.

Stanley, J.M., Sertsrivanit, S. and Clark, P.J. (1992). 'Magnetic Exploration Beneath a Near-Surface Magnetic Noise Source'. Exploration Geophysics 23, 323-326. Telford W.M., Geldardt, L.P. and Sheriff, R.E. (1990). Applied Geophysics - Second Edition. Cambridge University Press. Zonge Engineering and Research Organization, (1990). GGT-10 Manual.

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Sub-Audio Magnetics SAM - Home - Gap Geophysics Australia · 2013-10-21 · sub-metre intervals while continuously traversing either on foot or in a vehicle. Electric current, typically