WO1993019388A1

WO1993019388A1 - Electromagnetic induction well-logging device

Info

Publication number: WO1993019388A1
Application number: PCT/US1992/008021
Authority: WO
Original assignee: Inco Limited; J.W. Fishers Mfg., Inc.
Priority date: 1992-03-23
Filing date: 1992-09-22
Publication date: 1993-09-30
Also published as: ZA932022B; AU2673492A

Abstract

An electromagnetic induction well-logging system for measuring the combined effect of electrical conductivity and magnetic susceptibility in a highly conductive and highly magnetic orebody, using a transmitting and receiving coil (6) in a borehole probe which operates on a time domain system to produce and then sense eddy currents in the surrounding ore. A timing circuit (24) generates a series of two identical timing pulses between successive primary electric pulses of the time domain system. A differentiating means (32) receives the signals from the coil and timing circuit, and compares the signals to provide a relative measurement of combined conductivity and susceptibility of the orebody.

Description

DESCRIPTION TFtrrRn AGNEπc INDUCπON WF1.T.-T nπcpjG DEVICE

Technical Field

This invention relates to geophysical exploration within a borehole, wherein induced currents are measured to reveal the electrical conductivity and magnetic susceptibility of the surrounding formations. The conductivity and susceptibility data are then used to determine the composition of the orebody along the length of the borehole.

Background Art The measurement of conductivity and magnetic susceptibility in boreholes are well known methods for differentiating among various mineral bodies within a borehole. Measurement devices generally consist of a transmitting antenna coil and a receiving antenna coil. The coils are contained within a probe which is lowered into the borehole and connected to the surface by a cable. Electrical currents flowing in the transmitting coil create a primary magnetic field around the coil. When the current in the transmitting coil is varied (either sinusoidally, as in frequency domain systems, or abruptly shut off, as in time domain systems), the magnetic field around the coil changes in strength. This changing magnetic field induces voltages, causing secondary current flow in the surrounding rock. The secondary current, in turn, generates a secondary magnetic field which is sensed at the receiving coil as induced voltage signals. The secondary currents, secondary magnetic field, and induced voltage signals are proportional to the conductivity and susceptibility of the rock. Systems using these principles are described in U.S. Patent N^δ 3,090,910 to Moran.

When choosing operating parameters for a particular type of surrounding material to be explored using the frequency domain system it is necessary to calculate the induction number p of the system as a whole. p = (2nσμ,fτ~ Vz Where a = conductivity of the material μ = magnetic susceptibility of the material = frequency of electromagnetic transmission and r = receiver - transmitter coil separation

At low induction numbers, that is when p < < 1, the secondary magnetic field due to induced current flow is entirely out-of-phase with respect to the primary magnetic field and thus provides a proportional measurement standard for conductivity of the material. Likewise, at low induction numbers, the magnetic susceptibility of the material produces a secondary magnetic field in response to the primary magnetic field, which is entirely in-phase with the primary field. Thus, at low induction numbers, the conductivity and susceptibility may be distinguished as separate components, albeit through the use of complex systems and analysis. These measurements can be used to distinguish ore from surrounding rock, as well as different types of ore. However, this complex wave separation need not be done for certain orebodies, which exhibit high conductivity accompanied by high magnetic susceptibility.

Secondary magnetic fields sensed at the receiving coil must then be acted upon to obtain real conductivity data. A typical method, such as that explained in U.S. Patent No. 4,544,892 to Kaufman et al., involves providing gating pulses during the transmitting signal off-time to pass only induced voltage signals. These induced signals are then compared to a reference signal and passed through a filter to recover the in-phase component of the secondary magnetic field. The principle drawback of this system is the overly complex nature of the filtering mechanism required to isolate the resulting secondary waveform in its in-phase component.

The principle drawback of all of the existing systems lies in their complexity, and associated costs in money and manpower. These considerations Disclosure of Invention Accordingly, the present invention provides a simple, inexpensive borehole probe which provides immediate, usable data to an operator working in the mine. These parameters are critical, for instance, for a blasting engineer working by himself in a production hole who needs immediate data as to ore-rock contacts to minimize dilution in the mined ore.

The invention has accomplished the above in a borehole probe which utilizes a simple three pulse time domain system to provide a combined induced voltage signal resulting from both the conductivity and susceptibility of the ore. Such a system is especially advantageous in ore bodies which have both high conductivity and high susceptibility, such as in nickel-copper sulfide ores.

In the novel three pulse system of the invention, the initial transmit pulse T induces eddy currents, which in turn induce voltage signals in the receiver. The voltage signals are then sent to the sampling circuit. A short timing pulse Gl is sent to the sampling circuit at the precise time at which the eddy currents would be affecting the analyzed wave shape of the induced voltage signal. A second short timing pulse G2 is sent to the sampling circuit after the effects of the eddy currents have dissipated, but before the next transmit pulse is sent. The magnitude of the wave induced is then compared to the off-time response of the instances marked by pulses Gl and G2 to give a precise voltage measurement, which can then be translated to conductivity plus susceptibility, and eventually to the ore make-up by calibration with known assays.

The transmitted pulse is nominally a square wave, but the self- induction of d e coil produces significant oscillation in the turn-off. As the self- induction of the coil depends to some extent on the properties of the surrounding rock, the transmitted waveform will vary with changes in host rock conductivity and/or susceptibility. This variability in the transmitted waveform may complicate the problem of quantitatively calibrating the system. The response can be approximated by the step-function transit response of a small coil in a uniform half-space of conductivity σ and susceptibility μ. The voltage induced in a coincident receiving coil is given approximately by

_t5/2 where M is the power of the transmitter, r is the radius of the coil, and t is the time after the initial transmit pulse.

This represents a rapidly decaying pulse decay whose amplitude is proportional to μ and σ both to the 3/2 power.

The present system measures the difference in voltages between readings at t_lf and j (i.e., Gl and G2). This difference is directly proportional to o³'² and μ ¹². This system, therefore, even at low induction numbers, responds to both σ and μ. In general, however, the conductivity response will dominate because the range of conductivity between ore and non-ore will be greater.

The voltage measurements may be revealed on a meter located near the operator to provide immediate and meaningful data and may be set to eliminate all but the most intense responses.

Since only a simple two-pulse gating system is used, expensive data analysis costs are avoided. In addition, no filtering means is used to separate the in- phase and quadrature components of the induced signal. However, since it is contemplated that the present device will be used in conjunction with highly conductive and highly magnetic ores, a combined signal is desirable, again allowing the simple to give way to the complex prior art devices. An example of a highly conductive, highly magnetic ore body is the nickel-copper sulfide system. Table 1 provides conductivity and susceptibility data for typical ore minerals and host rocks in such an ore body. Table 2 shows the responses generated by the associated ore types. Note how only the desired nickel and nickel- copper ores exhibit both very high conduαivity and very high magnetic susceptibility. The ores would thus be readily distinguishable to the operator when compared to the undesirable magnetite and surrounding rock.

TABLE 1

^* estimated ^** unknown TABLE 2

It has been surprisingly found that the inexpensive three pulse system gives results faster and as accurate as the complex, multi-channel prior art devices for certain purposes. One embodiment of the invention which has been found to give superior results when used with a nickel-copper sulfide system comprises a transmit pulse of 120 μ sec duration at 100 msec intervals, and two timing 20 μ sec timing pulses. It is contemplated that similar pulsing arrangements will be effective with similarly conductive and magnetic ore bodies and such arrangements are within the purview of this invention.

Brief Description of the Drawings

FIG. 1 is a schematic diagram of the well-logging device of the claimed invention;

FIG. 2 is a longitudinal partial cross-sectional view of the probe of the claimed device;

FIG. 3 is a schematic profile of the transmit and timing pulses of the claimed device; and

FIG. 4 is a circuit diagram of an embodiment of the claimed device.

Modes for Carrying Out The Invention and Industrial Applicability With reference to FIG. 1, the well-logging device of the present invention comprises a cylindrical sensor probe 1 connected at its tail end by a cable 2 to a control box 3 which remains at the surface with the system operator. The probe 1, shown in FIG. 2, should be a suitable diameter so as to comfortably house the necessary internal parts and to fit within operating distance of the borehole walls 4. In a preferred arrangement for a borehole of 5.1-21 cm diameter, the probe is 5.1 cm in diameter, with tube length of 61 cm and overall length of 68.5 cm. The probe 1 houses a cylindrical combined transmitter and sensor (receiving) coil 6 disposed towards the nose end 9 of the probe 1 and oriented perpendicular to the length of the borehole 5. In this case, the coil 6 is a simple 56-turn coil having 3.49 cm diameter. The sensor coil 6, probe electronics board 7, and ballast weight 8 are located in the front section 10 of the probe 1. The probe is waterproof and has double O-ring seals 11 at both ends 9, 12. The rear compartment 13 houses the waterproof cable connectors 14. The cable 2 is communicated to the electronics board 7 in the front section 10 through a waterproof feedthrough bolt 16 situated within a waterproof baffle 17, which functions to keep the front section 10 of the probe free of water. The ballast 8 ensures that the probe has negative buoyancy and will sink nose down should water be encountered in the borehole.

As an electromagnetic induction system, the claimed device requires no physical connection to the tested wall surface. However, it is beneficial if the probe is kept accurately centered within the borehole, so as to take advantage of the 360° orientation of the coil. While readings may be taken with the probe close to or touching a wall, such readings may not be reproducible if the ore concentration does not exhibit radial symmetry.

The cable 2 transfers power and timing control from the control box 3 to the coil 6, and sends the amplified return signals from the coil 6 to the control box 3 for processing. The cable 2 preferably has waterproof, four-pin staggered connectors at both ends to provide for ease of connection and interchangeability. The cable is marked at regular intervals to provide the operator with data regarding the depth of the probe. Alternatively, an automated cable length counter may be employed, with the data therefrom fed directly to the processor.

The control box 3 remains at the surface of the terrain and may be advantageously worn on a belt around the operator's waist. In its preferred arrangement, the control box is waterproof and contains all the necessary controls, connectors and indicators for complete operation of the system. A rechargeable battery pack within the control box provides for ease of operation in remote locales. The meter is calibrated by standard methods using rock and ore samples of known composition.

The operation of the claimed invention will now be described with reference to FIGS. 3 and 4. When power in the control box 3 is turned on, battery power 18 feeds the power supply 20 and voltage regulation circuits 22, which in turn produce the necessary voltages to power the system. When the power comes up, the timing circuits 24 in the control box produce a series of three pulses T, Gl, G2 at a fundamental frequency of 100 Hz which are used to control the operation of the system. It is estimated that the pulse may contain frequencies up to 10,000 Hz.

The first pulse generated is the transmit pulse T, a 120 μsec pulse occurring every 100 msec. The transmit pulse T goes directiy to the probe electronics and activates the pulse driver circuit 26. The pulse driver circuit produces a 5 amp, 120 μsec wide pulse to the sensor coil 6. The 5.1 cm diameter coil 6 produces an intense magnetic field which lasts for 120 μsec. During this pulse time, any metals or detectable minerals that are within the generated magnetic field have eddy currents induced in them as a result of this magnetic field.

At the end of the 120 μsec pulse time, the magnetic field collapses causing a reverse kickback (counter emf) followed by a damped wave. Any eddy currents generated will affect the kickback and damped wave. The more concentrated the metal or mineral target, the greater the resultant eddy currents will be, which in turn results in a greater kickback and damped wave.

The sensor coil 6 receives the kickback and damped wave signal from the collapsing field and sends it to the pre-amp/driver 28, where it is amplified and sent via the cable 2 to the control box 3. The signal arrives at the sampling circuit 30 and is gated in by pulses Gl and G2 from the timing circuit 24. As shown in FIG. 3, a 20 μsec pulse Gl occurs precisely at the time that any eddy currents would be affecting the waveshape. G2, also a 20 μsec pulse, occurs after eddy currents would affect the wave shape and before the commencement of the next transmit pulse T.

The specified pulsing pattern has been found to be especially effective when used in the exploration of nickel-copper sulfide ores. It is expected that the pattern may be adjusted to suit different types of orebodies.

The output of the sampling circuit 30 feeds a differential amp 32 which compares the signal at the times of the pulses Gl and G2. Without eddy currents, the signal would be basically d e same and the output of the differential amp would be zero. If eddy currents are present, the two signals would be different and the output of the differential amp would be positive. The differential amp output increases in relation to increasing eddy currents.

The differential signals received may be sent to a digital data logger 34, such as the DL55 by Geonics Limited of Canada, which stores the signals digitally for continuing depths of the borehole. The data logger is connected to a computer which translates the stored data into ore composition, e.g., nickel concentration, by comparing the data with the conductivity or susceptibility of known assays. The ore composition is then plotted against the borehole depth to generate a concentration profile.

While this data storage arrangement may be used, it is also advantageous for the operator to have in proximity a visual voltage meter 36 and/or an audio device 38 for providing immediately usable information during mining operations, such as placing charges for blasting. The analog voltage meter 36 and audio device 38 may be advantageously set to give discernable readings for only the highest readings, which would provide accurate information as to the location of desirable ores.

Claims

1. An electromagnetic induction well-logging system for use in a highly conductive and highly magnetic orebody, comprising:

a transmitting coil and a receiving coil residing within a borehole probe which transmitting coil transmits a series of primary electric pulses generated by a pulse driver circuit to create a primary magnetic field around d e receiving coil to induce eddy currents in the surrounding ore and which receiving coil receives secondary magnetic fields generated by d e eddy currents, the secondary magnetic fields causing induced voltage signals to flow in the coil, the magnitude of die induced voltage signals being proportional to the combined magnitude of the conductivity and susceptibility of the ore; and

a timing circuit which generates a series of two identical timing pulses between successive primary electric pulses, the first timing pulse occurring at precisely the time mat the secondary magnetic fields are received by the receiving coil, and the second timing pulse occurring after the secondary magnetic fields have ceased being received by the receiving coil; and

a differentiating means which receives the signals received by die receiving coil and receives the first and second timing pulse signals, and compares the receiving coil signal received at die instant of the first timing pulse with the receiving coil signal received at die instant of the second timing pulse to thereby provide a relative measurement of the combined conductivity and susceptibility of the orebody.

2. The electromagnetic induction well-logging system of claim 1, wherein the orebody comprises nickel and copper sulfides.

3. The electromagnetic induction well-logging system of claim 1, wherein the transmitting coil and the receiving coil are contained in a coincident transmitting-receiving uncoil.

4. The electromagnetic induction well-logging system of claim 1, wherein the primary electric pulse has a duration of approximately 120 μ seconds and occurs approximately every 100 m seconds, and the timing pulses have duration of 20 μ seconds.

5. The electromagnetic induction well-logging system of claim 1, further comprising a signal generating means responsive to a desired threshold measurement of the combined conductivity and susceptibility of die orebody.

6. The electromagnetic induction well-logging system of claim 5, wherein the signal generating means produces an audible signal.

7. The electromagnetic induction well-logging system of claim 5, wherein the signal generating means produces a visual signal.