GB2071327A

GB2071327A - Improvements in Electromagnetic Induction Systems for Geophysical Exploration and Conductor Location

Info

Publication number: GB2071327A
Application number: GB7941143A
Authority: GB
Original assignee: Corbyn J A
Current assignee: Corbyn J A
Priority date: 1979-11-29
Filing date: 1979-11-29
Publication date: 1981-09-16

Abstract

Ground effects in pulse induction metal detectors are eliminated by comparing the actual response with a simulated response of the ground. The gated, amplified and synchronously detected voltage D at the output terminals of a receiver coil due to a pulsed primary magnetic field A produced by a transmit coil is fed to one input of a differential amplifier to the other input of which is applied the simulated signal and which is derived from an RC ground effect eliminator (Fig. 3) which fits a predetermined functional form E to the receiver coil output, which form corresponds to that due to background magnetic material. Any output signal F from the differential amplifier indicates a metal. The transmit and receiver coils are constructed to minimise coupling therebetween due to magnetic viscosity effects and the receiver coils form a gradiometer to reduce noise levels. The simulator may use exponential functions or reciprocal of one varying linearly with time. <IMAGE>

Description

SPECIFICATION Improvements in Electromagnetic induction Systems for Geophysical Exploration and Conductor Location This invention relates to the elimination of ground effects in electromagnetic induction systems of the transient type.

Pulse induction metal detectors have hitherto been unsuitable for use in the vicinity of minerals which display magnetic viscosity in that such minerals become magnetized by the primary magnetic field and the decay of this magnetization produces signals similar to those of the conductive targets being sought. The principal minerals present in the ground which cause magnetic viscoscity effects are magnetite and maghemite. These magnetic viscoscity effects are also termed ground effects.

According to the invention the received signal is analysed and a predetermined functional form is fitted to it. The fitted function is then compared with the original signal so that differences between them can be displayed, these differences being significant if the received signal is not completely due to the assumed magnetic viscoscity effects. Experimental work has shown the functional form of the magnetic viscoscity effects is invarient with respect to the amount of magnetically viscous mineral in the ground causing the effect. The functional form of the effect of a conductor is dependent on the physical size of that conductor.

A particular embodiment of the invention will now be described with particular reference to Figure 1 which shows a general schematic of a pulse induction metal detector incorporating magnetic viscoscity effects rejection. Figure 2 shows waveforms associated with Figure 1.

In the pulse induction metal detector described alternating pulses of current are generated and passed through transmit coils. This current is cut off sharply at the end of the transmit period so that the magnetic field due to transmit coils will be as at B in Fig. 2. A typical waveform for transmit coil current is shown at A in Fig. 2. The received signal is passed to a gated amplifier which is grounded except during a receive period which is arranged to occur when the primary transmit current is off. The received signal then passes to a synchronous detector to give a voltage as at D in Fig. 2. The detected signal then passes to a ground effect eliminator, amplifier and display unit.

The ground effect eliminator analyses the input signal to it and fits a predetermined functional form. The fitted function is then compared with the input signal and any significant difference is amplified and displayed. An alternating primary magnetic field is used to avoid magnetic polarization of the ground and so stabilize operation of the ground effect eliminator.

A particular embodiment of the ground effect eliminator is shown in Fig. 3. Switches S1, S2, S3, S4 are digitally controlled analog switches of CMOS or other type. At the begining of the receive period S1 and S2 are closed briefly so that capacitors C1 and C2 can be charged through the much larger capacitor C3 and the amplifier Ova1. Potentiometer P 1 determines the distribution of the input signal between C1 and C2.

At the end of the receive period S3 is briefly closed to charge C3 to a potential corresponding to the end of the receive period. During the receive period C1 and C2 decay through adjustable resistances P2 and P3 towards the potential stored on the much larger capacitor C3.

Amplifier OA2 is used as a buffer to prevent the differencing circuict based on OA3 from loading capacitors C1, C2, C3. S4 is in the grounded position except during the receive period to reduce the amount of noise passed to subsequent stages.

The voltage at the input to OA2 approximately follows the form (1P)e~Vrl+pe-8T2 Equation 1 where T=C1R, and T2=C2R2.

In Fig. 2 waveform E shows the signal due to metal and mineral and the fitted ground effect.

The difference between the signal due to metal and mineral together and the fitted ground effect as output by OA3 is shown at F in Fig. 2. The quantity P in Equation 1 is controlled by P1 and the time constants T1 and T2 are controlled by P2 and P3, these three parameters can be adjusted to suit a particular ground.

The following functional forms are also suitable for simulating the ground effects N SPIe -tj5j Equation 2 where N=1,2,3etc.

1 Equation 3 1+kt where k is a constant.

These functional forms can be simulated by suitable standard methods. Equation 2 can be simulated by an extension of the above described system for simulating Equation 1.

Design of the transmit and receive coil systems should preferably be done to reduce the ground effects in relation to the effects of the conductors being located, Fig. 4 shows a design which achieves this by having transmit coils 1 and 2 coaxial with receive coils 3 and 4 so that coupling between the transmit and receive coils by way of magnetic viscoscity effects is minimized. 5 is the ground containing minerals showing magnetic viscoscity.

Coil 1 is wound opposite to and in series with coil 2 to form the transmit coils and coils 3 and 4 are the same size with the same number of turns and they form a gradiometer receive array to minimize the effects of operating the metal detector in a changing magnetic field. Coils 1 and 2 do not need to have the same number of turns.

To simplify design of the current pulse generator separate coils were used for the two polarities of primary magnetic field. The coil 2 is not aiways needed and satisfactory operation can be achieved without it.

Claims

1. The elimination of unwanted responses in a pulse induction metal detector or other conductor locator of the transient type by simulation of the unwanted response and comparison of the simulated response to the actual response.

2. The use of exponential functions in the simulation of the unwanted response of claim 1.

3. The use of a function of the form of that of the reciprocal of a function varying linearly with time in the simulation of the unwanted response of claim 1.

4. Use of separate transmit and receive coils in a pulse induction metal detector to reduce ground effects by reducing the coupling between the transmit and receive coils due to magnetic viscoscity.

5. Use of oscillating primary magnetic field pulses in a pulse induction metal detector to avoid magnetic polarization of the ground.

6. The use of a gradiometer receive coil array in a pulse induction metal detector to reduce noise levels in the detector due to movement of the detector relative to a magnetic field or to reduce the effects of an unwanted time varying magnetic field.