AU662667B2 - A method for measuring magnetic induced polarisation using mobile sensing means - Google Patents

Description

RATENTS ACT 129 I" n M I T. V 'VrTV 9P V~ nT V~ T n A T T ON E-9% A STMNARD PA2&=M

ORIGINAL

Name of Applicant: M.I.M. EXPLORATION PTY. LTD.o A.C.N. 009 681. 118 44 4* *4 4 4 4 444* *444 44 4 444* 44 4 4 4* 4 44 4 44 44 4 444 *4 4* *4 4 4 4 4444 4 44 44 4 44 4 4 4 4 4* 444*44 4 4 Actual Inventor: Address for Service: Stuart Nicholas SHEARD SHELSTON WATERS Clarence Street SYDNEY NSW 2000 "IA METHOD FOR MEASURING MAGNETIC INDUCED POL~ARISATION USING MOBILE SENSING MEANSI' Provisional Application No: PL6642 dated 4th 3aanuary 1993 Invention Title: Details of Associated The following atatement is a full description of this invention, including the beat method of performing it known to us:-.

2 The present invention relates to the geophysical exploration method known as magnetic induced polarisation and in particular to a method for measuring magnetic induced polarisation using mobile sensing means.

S The invention has been developed primarily for use with mineral exploration and will be described hereinafter with reference to that application. However, it will be appreciated that the invention is not limited to that particular field of use and is also suitable for other geop:ysical surveys such as determining the location of sub-surface boundaries.

Hitherto, both galvanic and induced polarisation techniques have been used for the purposes of mineral exploration. The disadvantages of both techniques are 15 well known.

In particular, galvanic methods measure the

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S. resistivity of the land mass being surveyed and as such j "deposits of sulphides, carbonatious material and ground water are indistinguishable. Moreover, the accuracy of I j 20 this method is very low and if a conductive surface layer S is present the depth of the survey will be severely limited, In order to overcome some of these problems the magnetic induced polarisation (MIP) method has been used, This method involves the transmission of a current through spaced apart electrodes in the land mass being surveyed and the subsequent measurement of the induced magnetic field at the surface. This method is, however, iI' 3 very slow, expensive and in some cases environmentally damaging. The sensing means is generally carried on a vehicle to a measurement site and subsequently unloaded and prepared for taking a measurement. Thereafter, the sensing means is loaded onto the vehicle and transported to the next measurement site.

An alternative measurement technique used in conjunction with the MIP method is to use a coil type sensor which provides a signal indicative of the time rate of change of the induced magnetic field. This type of sensor is particularly disadvantageous due to, amongst other things, the limitations of the available information that can be extracted from the data obtained.

The expense of the MIP method is in part due to the large number of measurements which are required to allow meaningful results to be obtained. Furthermore, the j transmitting electrodes are generally spaced apart by about 2000 metres which then requires that many separate surveys be conducted to cover a given land mass.

20 Additionally, the surveys are only conducted over approximately 60% of the distance between the electrodes which further exacerbates the abovementioned problems.

It is an object of the present invention, at least .t in its preferred embodiment, to overcome or substantially *,4 25 ameliorate at least some of these deficiencies of the prior art.

According to the invention there is provided a method for measuring magnetic induced polarisation using I 4 mobile sensing means having a sensing ptrtion, said method including the steps of: selecting a land mass; placing at least two longitudinally spaced apart electrodes in said land mass; electrically connecting the electrodes for supplying a time varying current through said land mass; and progressing said sensing means across said land mass in a direction transverse to the longitudinal axis of the electrodes for obtaining signals indicative of the magnetic field strength, wherein at least said sensor t portion is maintained at least about three metres above I said land mass.

15 Preferably, said sensing means is mounted to an airborne craft. More preferably, said sensing portion is i mounted externally to said craft and more preferably at y the end of a boom extending from said craft.

It is also preferred that said sensing means is I 4 20 progressed across said land mass at a rate of at least about 10 metres per second and obtains at least about S' of said signals per second.

Preferably also, said signals are processed for removing the component due to the earth's magnetic field.

More preferably, said signals are obtained in synchronism with said time varying current.

In a preferred form, said time varying current includes a square wave, modified square wave or a A L"4-i- I I

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sinusoidal wave having a period of at least about one sixteenth of a second. More preferably, the modified square wave includes two rising and two falling edges per period.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a crows section of a land mass which is being surveyed in accordance with the present invention; Figure 2 is a modified square wave for use with the invention; and Figure 3 is an example of the results obtained following use of the invention.

Referring to Figure 1, a land mass I has been 4415 prepared for a survey and includes two longitudinally if" spaced apart electrodes 2 and 3 embedded therein which are electrically connected by way of cabling 4 and a too 4current source S to provide a time varying current through land mass 1. A sensing means in the form of a magnetometer (not shown) is mounted to an aircraft 7 and is sequentially progressed along a plurality of substantially parallel transversely extending paths 8 for *ht obtaining signals indicativi of the total magnetic field strength. Aircraft 7 includes a flexible clAble or boom 4 4# 4 4* 25 7a which supports at its free end a sensor portion 6 which supplies to the magnetometer a signal indicative of the magnetic field strength. Sensor portion 6 is maintained at least three metres above land mass I and i;- 6 more preferably at least 15 metres above land mass 1.

In one embodiment, current source 5 supplies a current waveform 9 as illustrated in Figure 2 which has a period of one second. In other embodiments of the invention the period of the current waveform is varied to suit the perceived geophysical characteristics of the land mass being surveyed, although in practice the shortest period would generally be no shorter than one sixteenth of a second.

Waveform 9 is a modified square wave which includes two rising and falling edges 10 and 11 respectively per period. Alternative waveforms include a standard square wave or a sinusoidal waveform.

The amplitude of the current waveform is preferably in the range of about 1 amp to 30 amps, although much larger currents can be used if required. In general, the current is limited in magnitude by the capacity of the generator associated with current source Aircraft 7 can be either a fixed wing craft or a helicopter or the like given that the craft's major requirements are to maintain a relatively constant speed during the survey as well as following the paths specified. Alternative modes of transportation include ultra light craft, remote controlled craft and land based vehicles having sensor portion 6 maintained in a raised configuration by way of an aerial or the like.

Sensor portion 6 is pieferably mounted externally to aircraft 7 and more preferably via boom 7a such that Ii Ii QI I 71 L I-1 ii- -7the sensor portion is maintained at the desired height without having to bring the aircraft in close proximity to the land mass.

It has been found that if sensor portion 6 is maintained at least about 3 metres above the ground the magnetic noise is greatly reduced and useful measurements can be obtained. Where airborne craft are used, sensor 6 is maintained at least 15 metres above ground level and more preferably at about 80 metres. The upper limit for useful measurements has been found to be about 150 metres where the signal to noise ratio dramatically decreases due to the height degradation of the magnetic signal.

Electrodes 2 and 3 are longitudinally spaced apart by a distance which is at least 2 km and preferably i at least 6 km. In some embodiments a spacing of 10 km is achievable. Consequently, much larger areas are surveyed per electrode placement. That is, the mobility of the survey is increased as cabling 4 and current source 5 are not required to be moved as frequently to survey a given f 20 area.

Additionally, due to the height at which sensor 6 ,4 is maintained, useful measurements can be made in both longitudinal and transverse directions across land mass 1 lt,, to a greater extent than was previously possible. For it example it is possible to obtain useful data at a distance of L/2 each side of electrodes 2 and 3 and in some instances at even greater distances. This transverse extent of measurement will be limited by the

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8 geophysical characteristics of land mass 1 along with the magnitude of the current being supplied. It is also possible to obtain useful measurements along the entire longitudinal length of land mass 1 which falls between electrodes 2 and 3.

The increased mobility of the invention is coupled with the use of airborne craft to allow rapid data aquisition. Accordingly, not only ic land mass 1 of much greater area than could have been previously surveyed with a single placement of electrodes, the time taken to gather the required data is reduced.

Sensor 6 preferably includes a fast sampling magnetometer which measures the total magnetic field, although in other embodiments only one or more components of the field are measured. Preferably, however, the three orthoganal components of the field are measured.

Magnetometers capabe of making up to 400 samples per second are available and are suitable for use in I conjunction with the invention. However, magnetometers 20 capable of greater sampling speeds can also be used. In other embodiments of the invention less than 400 samples 1i per second are taken. For example, useful results are I' obtainable where samples are obtained only about every twentieth of a second.

In embodiments where the total magnetic field is measured, the resultant data can be plotted against time to produce a waveform such as that shown in Figure 3.

Waveform 12 includes a number of peaks and troughs which

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9 generally follow the modified square wave form of waveform 9. The exponential curvature is due to the capacitive effects of different materials in land mass 1.

As shown in Figure 3, waveform 12 is constructed from samples taken every twentieth of a second and illustrates the characteristic charging and discharging properties of land mass 1 corresponding to the rising and falling edges of waveform 9 respectively.

In this particular example aircraft 7 progressed transversely across land mass 1 at about 15 ms 1 and as such the distance between sample points is about 0.75 metres. If an increase in resolution is required a larger number of samples can be taken.

Waveform 12 also includes a component due to the 9. 4 15 earth's magnetic field which is approximated by broken I line 13. If required this component can be removed from waveform 11 to enable an isolated analysis of the induced i magnetic field.

The peak induced magnetic field H is used to calculate the apparent resistivity of land mass 1 at a number of locations. Furthermore, the charge and discharge portions 14 and 15 of waveform 12 are used to *calculate the chargeability or capacitive characteristics of land mass 1 given that the magnetic field is (2,g 25 proportional to the current from which it is produced.

The chargeability of land mass 1 can be established from a single portion 14 or 15, or alternatively, by averaging adjacent or subsequent portions. That is, both i l 'i i lu~ 10 portions 14 and 15 are usable, although it has been found that portions 14 generally provide more accurate results dimi to reduced noise levels.

The data obtained allows magnetic mapping and the mapping of apparent resistivity and apparent chargeability for a given land mass. These results can then be interpreted by known methods.

In the above described embodiment reference has been limited to time domain measurements, however, the method according to the invention is equally applicable to frequency danain measurements of magnetic induced polarisation. For example, current source 5 can be configured to produce a compound sinusoidal current waveform composed of two or more different frequencies.

15 Sensor portion 6 subsequently supplies signals indicative t of the magnetic field strength to the magnetometer, whereby those signals can be decomposed to provide the field strength at those different frequencies. The phase difference between the transmitted and received signals is then used to calculate th' desired characteristics of land mass 1. It is possible, for instance, to determine f one or more of the following: magnetometric resistivity; (ii) apparent capacitive effect from the phase shift in the secondary magnetic field; (iii) apparent capacitive effect from the phase shift in the harmonics from the secondary magnetic field; and, r -1 11 (iv) the earth's total magnetic field.

Frequency domain measurements are preferred for use with the method according to the invention due to the signified advantages associated with this mode of operation. These advantages include: a higher signal to noise ratio due to measurements being made during the on-time of the transmitted signal; (ii) the frequency of the signal facilitates the collection of a large number of samples in a given time; and (iii) the primary waveform is known and can be removed from the received signal.

The resolution achievable in a transverse direction across land mass 1 is limited by the velocity of a:t :aft 7 and the rate that samples are provided by sensor portion 6. In a longitudinal direction, however, the resolution is limited by the distance between adjacent paths and the accuracy of the navigation system used to guide aircraft 7 along those paths.

In other embodiments aircraft 7 sequentially follows a plurality of longitudinally extending parallel S" "paths between the electrodes to obtain the required measurements.

Preferably, the magnetometer and current source are synchronised so that once the measurements have been obtained either the starting point of the charge and discharge portions of waveform 12 can be accurately i 12 determined or alternatively, (ii) when using frequency domain opezation, the phase difforence between the transmitted and received signal can be accurately determined. In one particular embodiment both the magnetometer and current source are provided with a signal obtained from the Global Positioning System (GPS) and are effectly synchronised to within zI0 9 seconds.

Alternative synchronism configurations are available such as a timing transmission from source S to a complementary receiver associated with the magnetometer.

It will also be appreciated by those skilled in the art that any electrode configuration may be employed, from cl.ose spaced dipoles of 100m or less to large scale of 10km or more. Much larger surveys aro envisaged for the larger electrode spacing which would be more compatible with airborne surveys.

Although the invention has been described with reference to a specific example it will be appreciated by thooe skilled in the art that the invention mny be embodied in many other forms.

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Claims (8)

1. 2. A method according to claim I wherein said sensing means is mounted to an airborne craft and is progressed transversely across said land-mass.
2. 3. A method according to claim 2 wherein said sensing portion is mounted externally to said craft.
3. 4. A method according to claim 3 whereini said sensing portion is mounted to said aircraft via a boom. A method according to any one of the preceding claims wherein said sensing means is progressed across said land mass at a rate of at least about 10 metres per second and obtains at least about 20 of said signals per second.
4. 6. A method adcording to any one Of the preceding gaid land. uwtus claims wherein said signals are processed for removing the component due to the earth's magnetic fie~ld.
5. 7. A method according to any one of the preceding claims whe~rein said signals are obtained in synchronism with said time varying current.
6. 8. A method according to any one of the preceding claims wherein said time varying current includes one of the following: squarewave; modified squarewavel and sinusoidal wave.
7. 9. A method according to claim 8 wherein said wave has 0 4 a period of at least about one sixteenth of a second. A method according to claim 1 wherein said time varying current is in the form of a modified sqi'arewave and includes two rising and two falling edges per period.
8. 11. A method for measuring magnetic induced p polarisation using mobile sensing means substantially as *1 hereindescribed with reference to the accompanying drawings. t DATED this 24th Day of Decombar, 1993 V1, M.I.M. EXPLORATION PTY. LTD. Attorney-. IAN T. ERNST Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS The following statement is a full description of this invention, including tho beet method of performing it known to ust- II ABSTRAC A land mass has been prepared for a survey and includes two longitudinally spaced apart electrodes (2,3) which are electrically connected by way of cabling (4) and a current source to provide a time varying r-irrent through land mass A sensing means in the form of magnetometer is mounted to an aircraft and is sequentially progressed along a plurality of subatantially parallel transversely extending paths (8) for obtaining signals indicative of the total magnetic field strength. Aircraft includes a flexible cable or boom (7a) which supports on its free end a sensor portion which supplies a signal indicative of the magnetic field strength. Sensor portion is maintained at least three meters above land amss and more preferably at least fifteen meters above land mass 4t