GB2148012A - Induced magnetic field borehole surveying method and probe - Google Patents

Abstract

An alternating primary magnetic field is established in the region under a loop laid out on the surface of the earth to induce currents in any conducting orebody in that region and establish a secondary alternating magnetic field. Information about the location of the orebody is obtained by measuring, at different locations down a borehole in the vicinity of the surface loop, the instantaneous values of the magnetic field at those locations in at least three linearly independent directions, and establishing from those measurements the polarisation ellipse of the magnetic field at each location. A borehole probe 30 for affecting such measurements comprises at least three magnetic field detectors 31, 32, 33, each responsive to the magnetic field in a respective direction. The detectors each comprise a plurality of pickup coils A, B, C, wound on respective ferrimagnetic cores. The pickup coils are mounted as a linear array within the probe. <IMAGE>

Description

SPECIFICATION Induced magnetic field borehole surveying method and probe This invention concerns geophysical surveying using induced electromagnetic fields. In particular it concerns an improved EM field technique for locating conducting orebodies and a borehole probe for use in that technique.

During the last ten years, there has been an increased awareness among geophysical surveyors of the value of down-hole exploration techniques as complementary to surface geophysical exploration. Included in such techniques are those which measure the value of a magnetic field which has been established by induced electrical currents in conductive orebodies. Down-hole EM field techniques are particularly useful in Australia, where conductive surface layers are an extra problem in surface geophysical exploration, but they have also been found to be of significant value in other regions.

As a consequence of this awareness, a number of borehole probes have been developed to detect induced alternating fields. One of the most successful of these has been the "SIROTEM" (trade mark) borehole probe which originated from the Division of Mineral Physics of the Commonwealth Scientific and Industrial Research Organization.

The usual method of establishing the induced fields which are to be investigated is to cause an alternating current to flow in a large, horizontal loop of wire which is laid out on the surface of the earth. The primary alternating magnetic field which is then established induces currents to flow in any conductive orebody which is located within the influence of the field. The induced currents establish a secondary field.

The "SIROTEM" probe, like others that have been developed, measures the decay with time of an induced secondary field when the current through the surface loop is switched off. It is known, therefore, as a "time-domain" instrument.

Most currently-available commercial magnetic field detection probes measure only that component of the induced secondary field which is parallel to the axis of the borehole at the location of the probe. All such "single axis" probes suffer from a lack of coupling when the conductive orebody has certain geometrical shapes. In particular, if a single axis probe is used with a relatively small surface loop (for example, a square loop measuring about 100 metres by 100 metres) in the vicinity of a steeply dipping conductive orebody, it is possible for the probe to indicate no anomaly in the magnetic field. This possibility arises because the direction of the secondary field (that is, the field associated with currents induced in the orebody) may be at right angles to the borehole over that section of borehole which is closest to the orebody.A single axis detector in that part of the borehole would be unable to sense the presence of the orebody.

The inadequacy of single axis probes has been recognised by geophysicists, but the general consensus of opinion in the field of geophysical instrumentation has been that it is not practical to devise sensitive field-detecting elements which can produce a signal in response to horizontal components of a magnetic field and which, in addition, can be mounted in a probe which is to be used in boreholes having a diameter of only 60 mm (known as "BQ" diameter boreholes).

One attempt to produce a horizontal field detecting element for a borehole probe has been described by M. H. Worthington, A.

Kuckes and M. Oristaglio in their paper entitled "A borehole induction procedure for investigating electrical conductivity structure within the broad vicinity of a hole", which appeared in Geophysics, Vol 46, No. 1, pages 65-67, January 1981. The horizontal field induction coil structure developed by Worthington et al consisted of two generally Ushaped mu-metal frames. Each frame had a narrow and short base member of the "U" and was adapted to be fitted into an elongate probe with the long arms of the "U" parallel to the axis of the probe. The two frames were mounted with their base members adjacent to each other and a single pickup coil of 7000 turns was wound around both of the bases.

When used in a probe, this arrangement of mu-metal frames apparently enabled the magnetic flux over a wide area to be effectively focused into the region containing the pickup coil.

Worthington et al used two such coil structures, located one above the other and with the axes of the pickup coils at right angles to each other, to construct a double axis borehole probe which they used in a borehole at the centre of the surface loop. Any departure from a zero reading of horizontal magnetic field in the borehole was interpreted as indicative of some lateral variation in geological structure. It will be noted, however, that the use of the Worthington et al probe requires the establishment of a surface loop which is symmetrical about the borehole--which means that the surface loop has to be physically re-laid each time the probe is used in a new borehole.

It is an object of the present invention to provide a method and probe which overcomes the major disadvantages of single axis probes.

The first aspect of this objective is achieved by making field measurements in a borehole, using a method in which the "polarisation ellipse" of the magnetic field that is established when the primary and secondary magnetic fields interact, is measured in the bore hole while alternating current is flowing in the surface loop to produce the primary field.

The second aspect of this objective is achieved by providing a probe which contains at least three linearly independent field detecting elements (that is, the detectors are mounted so that their directions of maximum sensitivity are neither coplanar nor colinear. If only three detectors are used, it will be most convenient if their directions of maximum sensitivity are orthogonal to each other).

In more detail, according to the present invention, a frequency-domain method of borehole geophysical surveying comprises the steps of a) passing an alternating current through a substantially horizontal loop laid out on the surface of the earth, to thereby establish a primary field within the earth in a region in the vicinity of the surface loop; b) measuring the strength of the alternating magnetic field at a location in the borehole in at least three linearly independent directions; c) establishing, from said field strength measurements, the polarisation ellipse of the alternating magnetic field at said location in the borehole; and d) repeating steps (b) and (c) at different locations in the borehole until a sufficient number of polarisation ellipses have been established.

The geophysicist in charge of the survey will normally determine the spacing between locations in the borehole and decide when a sufficient number of polarisation ellipses have been measured.

The establishment of the polarisation ellipses will normally be effected at the surface, using amplified signals from the borehole which are representative of the field strengths in the linearly independent directions. The ellipses may be established before the probe is moved to a new location, or the information needed to establish each polarisation ellipse may be stored, using any suitable means, so that the polarisation ellipses can be established at a time subsequent to the descent and/or ascent of the borehole by the probe. If the polarisation ellipses are established using stored information, it will be clear that step (d) must be modified, to refer only to repetition of step (b) until the required number of locations in the borehole have been sampled in part before step (c) has been performed.

The frequency of the alternating current may be any suitable value. Typically, audiofrequency alternating current will be used, and a vertical "scan" of a borehole will be effected at several frequencies before the probe is moved to another borehole. It will also be normal practice, when maximum information is to be obtained from the induced field measurements, to measure the orientation of each of the probe axes in space with respect to north and the vertical by means of a suitable compass and inclinometer at each point in the borehole where measurements are made. This enables the consequent determination of the orientation of the axes of the polarisation ellipse. By a partial reconstruction of the secondary field from the knowledge of the orientation of the ellipse axes, a good indication of the direction of the orebody may be obtained.

According to the second aspect of the present invention, a borehole probe for use in induced field geophysical surveying comprises at least three magnetic field detectors and is further characterised in that: a) each said detector is adapted to generate a signal in response to a magnetic field in a respective direction, the respective directions of the detectors being linearly independent; and b) each said detector comprises a plurality of pickup coils, each said coil being wound on a respective core of a ferrimagnetic ferrite material, each coil having an axis of symmetry, the axes of symmetry of respective coils of each detector being parallel to each other, the respective coils of each detector being connected in series with each other and the coils of all said detectors being formed into a linear array within the probe.

Preferably, three detectors are included in the probe, with their magnetic field response directions being mutually orthogonal. In this form of probe, it is also preferable that the three sets of coils constituting the three magnetic detectors are disposed so that the coils from one detector are evenly distributed amongst each of the other detector coil sets.

That is, with the exception of the two end coils, each coil of a particular detector has as its nearest neighbour one coil from each of the other two detectors. This ensures that each detector has a uniform sensitivity and directional pattern with respect to the other detectors, so that the final results from the computed data will have an accurate omnidirectional character.

It will be appreciated that any number (greater than 3) of magnetic field directions may be monitored by the probe, provided that qualitative values of the relative strengths of the monitored magnetic fields can be used to establish the polarisation ellipse of magnetic field.

Each coil of the probe will normally be provided with an electrostatic shield to minimise the capacitive coupling of the coil with the wall of the borehole, and the connections between coils will be balanced to minimise common-mode signals.

An amplifier will normally be included as a component of the probe, to amplify the signal generated by each detector before its transmission to the surface equipment being used in conjunction with the borehole survey.

A compass and an inclinometer will also be normal components of the probe.

Embodiments of the invention will now be described with reference to the accompanying drawings.

Brief description of the drawings Figure 1 illustrates, schematically, a crosssection of the earth containing a conductive orebody and a borehole, and includes the surface geophysical exploration equipment used to establish the field polarisation ellipses at locations within the borehole.

Figures 2 and 3 show the field polarisation ellipses at borehole locations A and B of Fig.

1, respectively.

Figure 4 is a schematic diagram of the probe of the present invention.

Detailed Description of illustrated embodiments In the arrangement shown schematically in Fig. 1, a surface loop 11 is laid out on the surface of the earth above a region of prospective mineralisation or orebody 1 3. A nonvertical borehole 10 is located alongside the surface loop 11. An alternating current generator 1 2 supplies a sinusoidal alternating current to loop 11 to induce an alternating primary magnetic field within the earth. The instantaneous position of some of the lines of force of the primary magnetic field are designated Hp.Eddy currents induced in the orebody 1 3 (which, for this example, will be taken to be a massive, steeply dipping leadzinc sulphide orebody) create a secondary magnetic field in the region of the orebody 1 3. Some of the instantaneous positions of the lines of force of the secondary magnetic field are shown by dashed lines designated Hs.

At the moment when the instantaneous fields are as shown in Fig. 1, the primary and secondary field vectors at location A in the borehole 10 are as shown, respectively, by arrows Hp and Hs in Fig. 2. The resultant instantaneous field at location A is thus shown by the vector Hr in Fig. 2. In the course of a cycle of the current in loop 11, the primary and secondary fields at location A will vary so that the resultant field vector Hr in Fig. 2 traces out the area of an ellipse 20. This is the field polarisation ellipse at location A in borehole 10. Similarly, ellipse 21 of Fig. 3 is the field polarisation ellipse at location B of borehole 1 0.

In the absence of the conductive orebody, there would be no secondary field at locations A and B, so that the field polarisation ellipses at each location would be a straight line (which is, of course, a special case of an ellipse). Thus the presence of a field polarisation ellipse at locations A and B of the type illustrated in Figs. 2 and 3 indicates the presence of a secondary field at those locations and, consequently, the presence of a conductive orebody within the region of influence of the primary field established by the current flowing in surface loop 11.

Further information about the location and shape of the orebody 1 3 can be obtained by measuring the shape and orientation of the field polarisation ellipses at a series of locations within the borehole 10, and by repeating this measurement using different frequencies of the alternating current in surface loop 11. Logging at several frequencies and observing the field polarisation ellipses can provide a geophysicist with information about the conductivity-thickness product of the orebody 1 3, and by changing the position of the surface loop 11 and repeating the measurements, additional directional information about the orebody may be obtained. Repeating the measurements also provides a means of discriminating multiple orebodies.

Geophysicists will appreciate that: The field polarisation ellipse is a planar figure, and a knowledge of six linearly independent quantities is required for its complete determination with respect to the coordinate axes of the probe. A sufficient set of quantities is provided by the measurement of the amplitude and phase of the resultant magnetic field vector in three orthogonal directions. However, an in-probe compass and inclinometer are required to enable a linear transformation to be made of the polarisation ellipse in the probe coordinate system to the surface exploration grid coordinates.

It will also be appreciated by those skilled in this art that in the borehole illustrated in Fig. 1, a single axis probe capable of sensing magnetic field in a direction parallel to the axis of the borehole, using either time-domain or frequency domain methods (that is, using the techniques known hitherto) would fail to detect an anomaly because the induced secondary field is too weak to be detected (when the measurements are made remote from the orebody 13) or is directly transverse the borehole.

A probe, constructed in accordance with the apparatus aspect of the present invention, is shown schematically in Fig. 4. In this probe, three groups of small pickup coils are shown mounted as a linear array within a cylindrical sheath 30 which has an outside diameter sufficient to allow it to be inserted down a borehole. The mountings of the coils are not shown. Each group of pickup coils constitutes a magnetic field detector. The coils comprising the group 33A, 33B and 33C are mounted with their axes of symmetry perpendicular to the plane of the drawing. Thus these coils detect variations in the magnetic field which is perpendicular to the plane of the drawing. The coils 32A, 32B and 32C are mounted to respond to variations in the magnetic field which is horizontally in the plane of the drawing.The coils 31A, 31B and 31C are a group of coils mounted with their axes of symmetry vertical (as shown in Fig. 4), and are thus adapted to produce an output signal when variations occur in the magnetic field which is vertically in the plane of the drawing.

Any suitable number of coils may be used in a groups of coils, but each group should contain the same number of coils. Each coil is electrostatically shielded, and the coils in each group are connected in series.

Preferably, the output signal from each group of coils is supplied to a down-hole amplifier 34. Typically the amplifier 34 is a balanced input balanced output amplifier which multiplies each of its input signals 50 times. The main purpose of the amplifier 34, however, is not to provide signal gain but to ensure that the signal pairs to the surface equipment are kept at low impedance to minimise the possibility of capacitive pickup of stray signals.

A cable 35, which is passed over a sheave wheel at the surface, is used to lower and raise the probe in the borehole. Cable 35 supports the outer sheath 30 of the probe.

Sheath 30 is made from a non-conducting material-typically polycarbonate.

The test the present inventive concept, a probe was constructed as illustrated in Fig. 4, except that each magnetic field detector in the probe comprised ten pickup coils, connected in series. Each coil was wound with 4000 turns of 43 A.W.G. enamelled wire on spools made by fixing bakelite (trade mark) washers on paper bakelite tube with Araldite (trade mark) adhesive. The start and finish of each coil was brought out in a Teflon (trade mark) insulated 7/0048 tail to ensure robust interconnections between coils. The magnetic core of each coil was a ferrite material Fl 4 supplied by Neosid Pty. Ltd. of Sydney, Australia.

Each core was a rod of circular cross-section, of diameter 0.25 inch (6.2 mm) and length 0.75 inch (18.65 mm). Thirty-eight coils were made and the thirty coils which were closest matched were used for the probe assembly.

The coils had a mean inductance (with core) of 474.1 6 millihenrys (standard deviation 2.93 mH) and a mean resistance of 978.32 ohms (standard deviation 8.55 ohms).

The down-hole amplifier was constructed using low noise type OP 075 operational amplifiers which were obtained from Precision Monolithics Pty. Ltd. All other electronic equipment used comprised "off the shelf" instruments.

Equipment at the surface of the borehole was used to measure the real and imaginary part of each component signal, received from one of the groups of coils in the probe, with respect to a reference signal which was derived from the current flowing in the transmit loop. Mathematical procedures were developed to facilitate the computation of (a) the lengths of the major and minor axes of the polarisation ellipse, (b) the direction angles of each axis with respect to the coordinate axes of the probe, and (c) an electrical phase angle.

With information from an inclinometer and/or a compass, the orientation of the polarisation ellipse in space could be determined. The mathematical computations were effected with a Hewlett-Packard HP67 programmable calculator, using a program that was written to perform the calculation of the major and minor axes of the field polarisation ellipses. A larger and faster computer, with additional facilities, would have enabled the results to be processed more rapidly and the borehole logs to be plotted automatically, on-site.

Geophysicists will appreciate that the use of information about the polarisation ellipses has a number of advantages over the use of data obtained from a single axis probe. Among these advantages are: a) the ratio of the minor axis to the major axis of the polarisation ellipse always gives positive anomalies; b) the polarisation ellipse is unaffected by curvature of the field; c) because of the absence of geometric artifacts, small surface loops can be used, including loops which are small enough to be used as a vertical loop, which is especially useful when coupling into a steeply dipping, narrow conductive zone and when avoiding coupling to the horizontally layered conductive overburden; and d) unambiguous information about the secondary field is obtained.

Some of these advantages will be apparent in the following examples of tests made with the prototype probe at several sites in Australia where an orebody is known to exist and the orebody is well mapped.

At Peelwood in New South Wales, a borehole had been drilled in a vulcanogenic environment in the eastern Lachlan Foldbelt. This hole intersects six metres of a massive sulphide mineralisation which dips at 45 degrees at a depth of 180 metres. A frequency-domain log of the borehole with a single axis probe revealed only a trace of an anomaly.

The log obtained using the prototype of the present invention clearly showed the presence of a prominent, positive anomaly which starts to rise above the background signal some 30 metres from the peak of the anomaly.

At a site in Zeehan, Tasmania, there is a thin but massive sulphide mineralisation within a sideritic-dolomite in a vulcanoclastic environment. A borehole has been drilled which intersects the sulphide orebody below 100 metres. Using a 100 metres by 100 metres surface loop, offset from the collar of the borehole by 1 80 metres, the borehole was logged (a) using a frequency domain single axis probe, (b) a time domain single axis probe (a "SIROTEM" probe), and (c) the prototype probe of the present invention. All three vertical logs showed the presence of the mineralisation. However, the log of the single axis frequency domain probe showed only a small negative anomaly at the location of the mineralisation, superimposed on a large phase shift extending from 90 metres to 110 metres, which was an artifact of the geometry of the primary field.The log obtained using the time domain probe was very complex, but knowing the presence of the orebody enabled a negative anomaly around 110 metres to be correlated with the intersection of the mineralisation. The prototype of the probe of the present invention gave a narrow, but prominent, positive anomaly at 110 metres.

When the same borehole at Zeehan was logged again with the same probes but with the surface loop offset from the collar of the borehole by 11 2 metres in the opposite direction, all three probes yielded logs which gave good, positive anomalies which indicated the intersection of the mineralisation. There was a similar intrinsic sensitivity for each probe.

However, the log obtained with the prototype probe of the present invention was unaffected by curvature of the primary field, whereas the curvature of the field did affect the logs obtained using the other two probes.

A borehole at the Flying Doctor test site at Broken Hill in New South Wales is known as hole 3071. Borehole 3071 passes beneath the bulk of a massive lead-zinc sulphide mineralisation in a high grade metamorphic sequence. A surface loop measuring 100 metres by 100 metres was laid over the mineralisation, but offset 100 metres along strike from the surface projection of the borehole. Using a single axis frequency domain probe, the phase log of the borehole failed to show the presence of the expected anomaly, but a large phase shift was observed between 1 40 and 1 60 metres due to curvature of the primary field (a consequence of using a relatively small surface loop). The prototype probe of the present invention, however, showed a double humped anomaly in the projection zone of the known mineralisation, at several frequencies of the sinusoidal current in the surface loop.

It has been found that the probe of the present invention works well when the frequency of the current in the surface loop is in the range from 110 Hz to 1 760 Hz. However, the present invention may be used when the current in the surface loop is outside this (relatively convenient) range of frequencies.

Claims (11)

1. A frequency-domain method of borehole geophysical surveying comprising the steps of a) passing an alternating current through a substantially horizontal loop laid out on the surface of the earth, to thereby establish a primary magnetic field within the earth in a region in the vicinity of the surface loop; b) measuring the strength of the alternating magnetic field at a location in the borehole in at least three linearly independent directions; c) establishing, from said field strength measurements, the polarisation ellipse of the alternating magnetic field at said location in the borehole; and d) repeating steps (b) and (c) at different locations in the borehole.

2. A frequency-domain method of geophysical surveying comprising the steps of a) passing an alternating current through a substantially horizontal loop laid out on the surface of the earth, to thereby establish a primary magnetic field within the earth in a region in the vicinity of the surface loop; b) obtaining signals indicative of the strength of the alternating magnetic field at a location in the borehole in at least three independent directions; c) storing said signals; d) repeating step (b) at different locations in the borehole; and e) subsequently establishing, from the stored signals, the polarisation ellipse of the alternating magnetic field at each location in the borehole.

3. A method as defined in claim 1 or claim 2, including the step of measuring, at each location in the borehole, the orientation relative to north and the vertical of said at least three independent directions.

4. A method as defined in any preceding claim, performed at a plurality of frequencies of said alternating current at each location in said borehole.

5. A method as defined in any preceding claim, in which said at least three independent directions are three mutually orthogonal directions.

6. A borehole probe for use in induced field, frequency domain, geophysical surveying, said probe comprising at least three magnetic field detectors, and further characterised in that: a) each said detector is adapted to generate a signal in response to a magnetic field in a respective direction, the respective directions of the detectors being linearly independent; and b) each said detector comprises a plurality of pickup coils, each said coil being wound on a respective core of a ferrimagnetic ferrite material, each coil having an axis of symmetry, the axes of symmetry of respective coils of each detector being parallel to each other, the respective coils of each detector being connected in series with each other and the coils of all said detectors being formed into a linear array within the probe.

7. A borehole probe as defined in claim 6, in which there are three magnetic field detectors and the respective response directions of the detectors are mutually orthogonal.

8. A borehole probe as defined in claim 6 or claim 7, in which the pickup coils of each magnetic detector are distributed evenly within the linear array among the pickup coils of the other detectors of said probe.

9. A borehole probe as defined in any one of claims 6, 7 and 8, including a compass and an inclinometer.

10. A borehole probe as defined in any one of claims 6 to 9, in which each said coil is electrostatically shielded.

11. A borehole probe as defined in any one of claims 6 to 10, including an amplifier to amplify the signal produced by each magnetic field detector, and a cable connecting the output of said amplifier to signal analysis equipment or signal storage equipment.

1 2. A frequency domain method of borehole geographical surveying substantially as hereinbefore described with reference to the accompanying drawings.

1 3. A borehole probe substantially as hereinbefore described with reference to the accompanying drawings.