GB1569565A

GB1569565A - Method of and apparatus for testing an electrical network

Info

Publication number: GB1569565A
Application number: GB4749675A
Authority: GB
Original assignee: KALAMOS CO Ltd
Current assignee: KALAMOS CO Ltd
Priority date: 1976-11-18
Filing date: 1976-11-18
Publication date: 1980-06-18

Description

(54) METHOD OF, AND APPARATUS FOR TESTING AN ELECTRICAL NETWORK (71) We, THE KALAMOS COMPANY LIMITED, A British Company of 325, Upper Elmers End Road, Beckenham BR3 3 QP, Kent and DAVID ELLIOTT, A British Subject of 5, Church Street, Belper, Derby, England, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be described in the following statement: The present invention relates to a method of, and apparatus for testing an electrical network such as, for example, a geophone or string or geophones.

It is frequently desired to test an electrical network to ascertain information as to the dynamic behaviour of the electrical network in response to an electrical stimulus. This information can either be detailed information as to the dynamic performance of the network or information as to whether the dynamic characteristics of the network fall within a predetermined range of values. One of the difficulties which is encountered in testing such networks is that they often contain damping components and the presence of these damping components can tend to mask some dynamic characteristic of the network which is of interest. These damping components can be either electrical, i.e. electrical resistances, or mechanical in operation, i.e. where the network contains electro mechanical components, and it is frequently impossible or at the least inconvenient to disconnect these components from the network before testing takes place.

According to the present invention there is provided a method of testing an electrical network having an electrical ouput port and a component which will damp the electrical response, at said output port, of the network, in which the time dependent electrical response of the network to the application of a predetermined electrical stimulus is measured, comprising the steps of applying said electrical stimulus and simultaneously causing an electric current to flow via the port, through the network, which current has a magnitude and polarity appropriate to reduce by a substantial amount, or to cancel, the effect, on the measured time dependent response, of said damping component and measuring said response via said port.

The invention also provides an apparatus for testing an electrical network, the network having an electrical output port and a component which will damp the electrical response at said output port to an electrical stimulus applied to the network, the apparatus comprising means for applying such electrical stimulus to the network, means, having a pair of input terminals for correction across said part, for measuring the time dependent electrical response at said output port to said electrical stimulus and means for monitoring said electrical response at said output part to such electrical stimulus and for applying a further electrical stimulus to said network via said input terminals, the arrangement being such that, in use, the magnitude and polarity of said further electrical stimulus are appropriate to reduce by a substantial amount, or to cancel, the effect of said damping component on said response.

In the embodiments of the invention later described in detail, the output port of the network under test is connected to act as a feedback loop from the output of an amplifier to a summing junction at the amplifier input. The first electrical stimulus is applied to this summing junction, as is also the further electrical stimulus which is intended to reduce by a substantial amount or substantially to cancel the effect of the damping component on the network response. A particularly convenient way to derive this further electrical stimulus is to monitor the output of the amplifier using an inverting amplifier and to deliver the output of the inverting amplifier to the summing junction via a resistor of appropriate value. The characteristics of this inverting amplifier and the value of the resistor are chosen so that the magnitude of the further electrical stimulus is appropriate for its purpose and the inverting nature of the inverting amplifier ensures that the polarity of the signal fed back to the summing junction from the inverting amplifier is correct.

In one embodiment of the invention, which is particularly useful. for testing geophones either individually or a string, the first stimulus which is applied to the summing junction is an alternating current having a frequency substantially equal to the resonant frequency of the network, as a parameter which is frequently of interest in testing geophones is the impedance of the geophone at its resonant frequency since this can readily provide an indication as to whether the geophone is open circuit or has a stuck bobbin. As geophones often have substantial lengths of cable attached to them which are inconvenient to disconnect, the alternating current is combined with a direct current to provide a DC offset since by using this technique and appropriately sampling the output of the amplifier, the effects of the DC resistance of the interconnecting cable can be eliminated.

In a second embodiment, which is also particularly suitable for testing geophones, the above described feedback arrangement is also used and the stimulus applied to the summing junction is a direct current which is changed by means of a switch between two step values.

The response of a geophone or string to such a step change of the direct current applied to the summing junction is in the form of a damped edsinusoid. The amplitude of the first (maximum) peak of this response and the second (minimum) peak of this response can yield useful information as to the performance of the geophone or string as can also the relative magnitudes of these two peaks. In practice, it is convenient for the value of the direct current for the purposes of the test to be reduced from a predetermined level to zero as this ensures that the oscillation of the geophone armature or armatures will take place about their normal equilibrium positions.

One of the principal reasons for wishing to eliminate the effect of the damping resistors, which are almost invariably connected across geophones is that if one applies a current to a geophone with the damping resistor still in circuit part of the current bypasses the geophone itself and flows through the damping resistor instead. Unfortunately, whereas the resistance of such damping resistors is usually relatively insensitive to temperature, the DC resistances of the geophones and connecting cables are strongly temperature-dependant so that the proportion of the known current actually passing through the geophones themselves changes markedly with temperature. It is thus difficult to determine the reactive component of the impedance at resonance of the geophones where even small changes in ambient temperature are encountered unless the effect of the damping resistors is eliminated.

According to a further basic aspect of the present invention there is provided a method of testing a geophone or string af geophones which comprises the steps of: taking a geophone or string of geophones; causing an electric current to flow through the geophone or string of geophones from one terminal to another terminal connected to the geophone or string of geophones, which current comprises a first alternating current component having a frequency substantially equal to the resonant frequency of the geophone or string of geophones, a second direct current component and a third current component which is arranged to cancel the effect of the part of the first and second components which flows through the effective damping resistance of the geophone or string of geophones whereby a substantially constant alternating current having a direct current offset is caused to flow through the geophone or geophones; and measuring the voltage produced across the geophone or terminals.

This further basic aspect of the invention also provides an apparatus for testing a geophone or string of geophones including a pair of terminals means for applying to the geophone or string of geophones via said terminals an electric current, which current comprises a first alternating current component having a frequency substantially equal to the resonant frequency of the geophone or string of geophones, a second direct current component and a third current component which is arranged to cancel the effect of that part of the first and second components which flows through the effective damping resistance of the geophone or string of geophones whereby a substantially constant alternating current having a direct current offset is, in use, caused to flow through the geophone or geophones and means for measuring the voltage thus produced across the terminals.

By "effective damping resistance," there is to be understood that part of the resistance existing between the terminals of the geophone or string which is attributable to the damping resistor or resistors associated with the geophone or string of geophones.

By setting the frequency of the alternating current at the resonant frequency of the geophone or string of geophones and choosing the value of the second direct current to be equal to substantially one half of the peak to peak value of the first alternating current, it is, as mentioned earlier, possible to eliminate the effect of the resistance of the cables connected to the geophone or interconnecting the geophones of the string. The amplitude of those peaks of the voltage produced across the geophone or string of geophones which are opposite in polarity to the second direct current component is proportional to the difference between the AC impedance of the geophone or string of geophones at resonance and the DC resistance thereof, this difference being equal to the reactive component at resonance. In order to eliminate the effect of temperature changes causing differences in the proportions of the applied current flowing through the geophone or geophones and the damping resistor or resistors, the third current is arranged to have a magnitude and polarity so as to eliminate that potential difference which would appear across the geophone or string of geophones as a result of the first and second components flowing through the damping resistor or resistors so that the part of the applied current which flows through the geophone or geophones is substantially constant.

The invention will be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic circuit diagram of test apparatus embodying the invention; Figure 2 is a schematic circuit diagram of a further embodiment of test apparatus according to the invention; and Figure 3 illustrates waveforms occurring in the circuit of Figure 2.

In Figure 1 the test apparatus is shown set up for measuring the impedance of at least one geophone at its resonant frequency and also for providing an indication of the resonance frequency of the geophones. A voltage controlled oscillator which produces a sine wave output is connected via a unity-gain buffer amplifier 2 to an adjustable-gain amplifier 3, the output of which is connected via resistor 6 to a virtual earth summing point 8 at the input 6f DC coupled operational amplifier 4.

Also connected to the output of amplifier 3 is a sample and hold unit 5 which is arranged to sample the output of amplifier 3 at the negative peaks of the output of amplifier 3 and t6 hold the value sampled for the remainder of the period of-each cycle of the waveform,The output of the sample and hold unit 5 is connected via a resistor 7-to the summing point 8, the'gain of the sample and hold unit 5 and the values of the resistors 6 and 7 being chosen so that the oscillating voltage appearing at the summing point 8 is off-set positive to earth an amount corresponding to one half the peak to peak value of the- oscillating voltage.

Connected as the feedback loop of the amplifier 4 is the geophone or string of geophones G, the total effective damping resistance of the geophone or string G being represented schematically by RD. Also connected to the output of the amplifier 4 is a DC coupled operatiotiai ,ffinp-lifier or inverter 9 which is arranged to have a gain of minus one. Connected to the output of the inverter 9 is a variable resistor Rx, whose other terminal is connected to the virtual earth summing point 8. The resistor Rx is adjusted to have a value equal to the total-effective damping resistance RD, of the geophone or-string G and since the amplifier 9 has a gain of minus one it will be appreciated that by arranging the resistor Rx to have this value, if one considers the virtual earth summing point 8, the current flowing through Rx will cancel out the current flowing through RD so that the voltage which appears at the output of amplifier=4 is proportional only to the impedance of the geophone or string of geophones G and does-not depend on the damping resistance RD. The resistance Rx may be obtained by measurement or from the geophone string specification. A sample and hold unit 18 is also connected tb the output of amplifier 4, this sample and hold unit 18 being arranged to sample the output of amplifier 4 at each positive peak of the output of amplifier 3 and tub hold the sampled value between sampling instants. The output of sample and - h6ld- 'unit 18 is connected to an operational amplifier 19, this output is in turn connected to a meter M. It is, of course, possible to connect a digital volt-meter to line 20 if a digital display of the sampled voltage is required.

It will be appreciated that since the sample and hdld unit 18 samples the=output of amplifier 4 corresponding to the positive peaks of the output of-amplifier 3, the sampled voltage is related to the AC impedance Z of the geophone or string Gby the relation V -= Z-k wherelis readily determined from a knowledge of the peak to peak output voltage of amplifier3 and the values of the resistors 6 and 7. The geophone or string G is, because of the virtual earth configuration of amplifier 4, driven by a constant current equal to I, which is unaffected by changes in ambient temperature.

For the measurement of the AC impedance Z of the geophone or string G to be valid-it is necessary for the current I to have a frequency equal to the resonant frequency of the geophone or string G. For this purpose, the amplifier 4 is connected in the feedback loop of the voltage control oscillator 1. A differential amplifier 14 has one of its inputs connected to the output of amplifier 4 and its other input connected to the output of an operational amplifier 10. A resistor 12 is connected between the output of amplifier 3' and inverting input of amplifier 10 and a feedback resistor 13 is connected between the output of amplifier 10 and its inverting input. Also connected to the inverting input of amplifier 10 is resistor 11 which is connected to the output of sample and hold unit 5. The amplifier 14 is arranged so that the voltage appearing at the output thereof-is equal to that component of the output of amplifier 4 which is contributed by the total series DC resistance of the geophone or string G, but not by the damping resistance RD. This is so that the output of differential amplifier 14 has substantially zero DC offset, for facilitating phase comparison, and is strongly dependent on the phase-shift introduced by the AC impedance of the geophone or string G when the frequency of the applied AC voltage deviates from the resonant frequency of the geophone or string G. The output of amplifier 14 is compared by phase comparator 15 with the output of voltage controlled oscillator 1 to produce an error voltage which is fed back via amplifier 15 to the voltage control input of voltage controlled oscillator 1. A phase lock adjustment control 17 is also connected to the input of amplifier 16. The above described arrangement ensures that the frequency of the output of voltage controlled oscillator 1 is substantially equal to the resonant frequency of the geophone or string G.

In order to measure the resonant frequency of the geophone or string G, a phase locked loop multiplier 22 is connected to the output of voltage controlled oscillator 1.

Combined phase comparator and voltage controlled oscillator 23 provides an output which is 100 times the frequency of the output of voltage controlled oscillator 1, the output of the circuit 23 being fed back to one of the comparator inputs via two divide by 10 counters 24,25 connected in cascade. The output appearing at line 26 of circuit 23 can be connected to a digital counter to give a digital display of the resonant frequency of the geophone or string G.

The use of the phase locked loop multiplier 22 enables high resolution of the measurement of the resonant frequency without requiring extremely long sampling times.

In order to ensure that the apparatus is set up so that the resonant frequency of the geophone or string G under test is within the lock-range of the voltage controlled oscillator 1, the lines 21 and 27 may be connected to the Y and X axis inputs of an oscilloscope and the Lissajous figure thus produced observed.

Once the dynamic resistance of the geophone or string G has been measured, the geophone or string can be tagged with the measured value; any subsequent change, determined during field testing, in the measured value of the dynamic resistance is indicative of a fault in the geophone or one of the geophones of the string. For such field testing the circuit shown in the Figure can be used, although for this purpose the circuit may be simplified by replacing the circuitry within the dotted block 31 with a simple oscillator 30 having an adjustable or fixed output frequency set nominally at the resonant frequency of the geophone or string G. The oscillator 30 may entirely replace the circuitry 31, in a field test apparatus; alternatively a switch can be placed at the input of amplifier 2 to select the output of VCOI or the oscillator 30.

Figure 2 shows a further embodiment of the invention which uses a rather different technique to obtain information concerning the network under consideration but uses the same principle as the Figure 1 embodiment, i.e. that of forcing a current through the network while applying a stimulus thereto with the value of the forced current being such as to cancel the undesired effect on the measurement being performed of damping components within the network.

In geophones, the electrical response is determined largely by a damped resonant system of mechanical components although the embodiment could equally well be used where the resonant behaviour is defined solely by electrical components.

Thus, the embodiment of Figure 2 and its operation will be explained with reference to the testing of geophones although it will be appreciated that the embodiment could equally well be applied to the testing of various other analogous electrical networks.

In Figure 2 the geophone or string of geophones G and the associated damping resistor or resistors, designated collectively by the effective damping resistance RD are connected in the feedback loop around a DC coupled operational amplifier 101. The output of operational amplifier 101 is also delivered to the input of a DC coupled inverting operational amplifier 102 which has a gain of - 1 and a further feedback loop is provided between the output of inverting amplifier 102 and the virtual earth summing point at the inverting input of amplifier 101 via a resistor Rx. This resistor Rx is adjusted to have a value equal to the components of the DC resistance across the terminals of the geophone or string G which is contributed by the associated damping resistor or resistors RD. If the inverting amplifier 102 and feedback loop comprising resistor Rx were absent and a voltage or current used to excite the geophone or string G, part of the current caused to flow would not flow through the geophone coil or coils but rather would flow through the damping resistor or resistors RD. As explained earlier, the differing temperature coefficients of the geophones themselves and the damping resistors would thus make such a test rather unreliable. The provision of the inverting amplifier 102 and the feedback loop comprising resistor Rx causes a current to flow which in effect cancels the effect of the pertinence of the damping resistor or resistors RD associated with the geophone or string under test.

The embodiment of Figure 2 operates to test the response of the geophone or string G to a stepped input. This, as is conventional in the art, is achieved by closing a switch S1 so causing a current to flow through the coil of the or each geophone which causes the geophone armature to be displaced from its equilibrium position. When the armature has settled down in this displaced position the switch S1 is then opened and the armature performs a damp sinusoidal motion about its equilibrium position. This motion can be observed by monitoring the waveform, for example, at point A in Figure 1 and Figure 3 illustrates this waveform. Thus at time T = 0 the switch S1 is closed and the armature executes a damped sinusoidal motion about the displaced position the displaced position being proportional to the current flowing via S1, this motion being reflected by the damped sinusoidal waveform occurring in the period T = 0 to T = T1 in Figure 3. Once the geophone armature, and hence the current flowing through the geophone or string has settled down in a position displaced from the equilibrium position at a time T2, the switch S1 is opened and the armature executes a damped sinusoidal motion about its original equilibrium position and this again is reflected by the voltage appearing at point A. As is well known, various portions of the waveform occurring as the armature returns to its equilibrium position after the switch opens at time T2 can yield a great deal of information about the geophone under test. The three parameters which are usually of interest are the magnitude V1 of the first, positive peak and the magnitude V2 of the second, negative peak of the waveform and also the time Tx between the waveform increasing through zero towards the first peak and decreasing through zero towards the second peak. Briefly, two useful relationships are: Damping co-efficient, b = sin tan -1 1 12 Resonant frequency, fo = 1/2 TX cos sin 1b It shoud be noted that the damping co-efficient here represents the internal damping of the geophone or string rather than this damping combined with that provided by the external damping resistor or resistors RD since the effect of the external damping resistor or resistors is, in effect, cancelled by the presence of the inverting amplifier 102 and resistor Rx. Thus it is possible to test the geophone or string G and eliminate from the results of the test the effects of the damping resistor or resistors RD without having physically to disconnect the resistors RD for the test.

The switch S1 need not, of course, be a mechanical switch but could also be an electronic switch such as a field defect transistor or transmission gate and the associated resistor 103 is preferably adjustable to ensure that the current used to displace the geophone armature or armatures is insufficient to cause the armature or armatures to settle in positions where they are in contact with the stops which limit their motion.

As will be apparent from the above mentioned equations the amplitudes V1 and V2 of the first and second peaks of the waveform after the switch S1 is opened are of great interest as is also the ratio of these two magnitudes. The embodiment of Figure 2 can be used for exact quantitative testing of the geophone or string G or for rather more straightforward qualitative testing. The quantitative testing will provide detailed information concerning the parameters of the geophone or string G which can be useful for interpreting data acquired, in use, by the geophone or string G while the qualitative testing is useful to determine rapidly whether the geophone or string used is working correctly i.e. has electrical parameters falling within a desired range of values or whether, for example, it has an open circuit coil or a. "stuck" armature. For such qualitative testing, to provide a "go no-go" indication of whether the geophone or string is working it is in general sufficient to apply a known offset current when the switch S1 is closed and to determine whether the magnitude of the second peak V2 is within a predetermined range. It should be noted that with prior testing apparatus it has not been practical to measure the second peak V2 satisfactorily without physically disconnecting the damping resistor or resistors. The use of the feedback circuitry in cancelling the effect of the damping resistors, results in the second peak having a higher and thus more easily measurable amplitude. It is desirable to use the amplitude of the second peak for testing purposes since if a string includes a geophone with a stuck armature the effect of the stuck armature is much greater, and hence more easily noticeable on the second peak than it is on the first peak Respective inverting peak detectors 105 and 107 are provided for detecting the first and second peaks respectively. Each peak detector may comprise a diode-pump arrangement with an operational amplifier monitoring the voltage across the capacitor and providing a buffered output. The forward voltage drop across the diode can, of course, be eliminated by conventional circuit techniques. The output from the respective peak detectors 105 and 107 are applied to an analogue to digital converter 109. The output from the first peak detector 105 is used as the reference voltage in the A/D converter with which the magnitude of the output of the second peak detector 107 is compared to produce the digital output. By this arrangement the output of the A/D converter 109 is representative of the ratio between the first and second peaks V1 and V2 respectively. A combined oscillator and timing control block 111 serves to control the operation of the peak detectors 105, 107 and the switch S1 and can also be used to produce the necessary clock signals for the A/D converter 109. Discharge logic 113 is provided to ensure that the storage capacitors in the respective peak detectors 105 and 107 are discharged at the start of a test while an inhibiting block 115 serves to inhibit operation of the second peak detector 107 until after the switch S1 has opened. This is simply because this can clearly be seen from Figure 3 the magnitude of the second peak V2 is less than the magnitude VP of the peak which occurs when the switch S1 is initially closed. The oscillator and timing block 111 can, of course, also by interconnection with suitable zero crossing detectors produce an indication of the parameter Tx shown in Figure 3.

The output from analogue to digital converter 109 can be used in any one of the number of ways, for example, to operate a digital display indicating the ratio v2 and/or a recording device for recording this ratio for each geophone or string under test.

A circuit 111 can be connected to the second peak detector 107 to provide an indication if the magnitude V2 of the second peak is outside the range of values necessary to give desired results from the geophone or string when actually in use. Thus the circuit 117 receives the output of the second peak detector 107 and has respective maximum and minimum level detectors 119 and 121 respectively. If the output of detector 107 is outside the detection "window" defined by the detectors 119 and 121 an alarm 123 which may, for example, give an audible or visual warning, is operated. As shown, the outputs of the level detectors 119 and 121 may be in a wired-or configuration.

A meter M maybe provided to monitor the output of the peak detectors 105 and 107 as itis sometimes more convenient to have the values displayed on an analogue meter than on a digital display. In order that a single meter may be used to monitor both magnitudes V1 and V2 a switch S2 is provided to enable the operator to select which output is monitored.

The embodiments shown in Figure 2 can be modified in a number of ways according to its intended application. Thus where the embodiment is used in an application where it is only required to indicate whether the magnitude V2 of the second peak associated with the network under test is within a predetermined range of values, the first peak detector 105 meter M and analogue to digital converter 109 may be omitted. Equally in other applications it may be convenient to omit the m

Claims

**WARNING** start of CLMS field may overlap end of DESC **. also be used to produce the necessary clock signals for the A/D converter 109. Discharge logic 113 is provided to ensure that the storage capacitors in the respective peak detectors 105 and 107 are discharged at the start of a test while an inhibiting block 115 serves to inhibit operation of the second peak detector 107 until after the switch S1 has opened. This is simply because this can clearly be seen from Figure 3 the magnitude of the second peak V2 is less than the magnitude VP of the peak which occurs when the switch S1 is initially closed. The oscillator and timing block 111 can, of course, also by interconnection with suitable zero crossing detectors produce an indication of the parameter Tx shown in Figure 3. The output from analogue to digital converter 109 can be used in any one of the number of ways, for example, to operate a digital display indicating the ratio v2 and/or a recording device for recording this ratio for each geophone or string under test. A circuit 111 can be connected to the second peak detector 107 to provide an indication if the magnitude V2 of the second peak is outside the range of values necessary to give desired results from the geophone or string when actually in use. Thus the circuit 117 receives the output of the second peak detector 107 and has respective maximum and minimum level detectors 119 and 121 respectively. If the output of detector 107 is outside the detection "window" defined by the detectors 119 and 121 an alarm 123 which may, for example, give an audible or visual warning, is operated. As shown, the outputs of the level detectors 119 and 121 may be in a wired-or configuration. A meter M maybe provided to monitor the output of the peak detectors 105 and 107 as itis sometimes more convenient to have the values displayed on an analogue meter than on a digital display. In order that a single meter may be used to monitor both magnitudes V1 and V2 a switch S2 is provided to enable the operator to select which output is monitored. The embodiments shown in Figure 2 can be modified in a number of ways according to its intended application. Thus where the embodiment is used in an application where it is only required to indicate whether the magnitude V2 of the second peak associated with the network under test is within a predetermined range of values, the first peak detector 105 meter M and analogue to digital converter 109 may be omitted. Equally in other applications it may be convenient to omit the meter M and/or the alarm circuit 117. Each of the above described apparatuses may, of course, be used to test networks other than geophones. WHAT WE CLAIM IS:

1. A method of testing an electrical network having an electrical output port and a component which will damp the electrical response, at said output port, of the network, in which the time dependent electric response of the network to the application of a predetermined electrical stimulus is measured, comprising the steps of applying said electrical stimulus and simultaneously causing an electric current to flow, via the port, through the network, which current has a magnitude and polarity appropriate to reduce by a substantial amount, or to cancel, the effect, on the measured time dependent response, of said damping component and measuring said response via said port.

2. A method according to claim 1, wherein said stimulus applying step comprises applying to the network an alternating current having a frequency substantially equal to the resonant frequency of the network and a direct current having a magnitude substantially equal to one half the peak to peak amplitude of said alternating current.

3. A method according to claim 2, wherein the measuring step comprises sampling peaks of the voltage waveform appearing across the network in response to the application of the alternating and direct currents thereto.

4. A method according to claim 1, wherein said network is, or approximates to, a network having a damped second order time dependent response to the application of a step change in said predetermined stimulus.

5. A method according to claim 4 wherein said stimulus applying step comprises connecting the network to a circuit which provides a steady direct current and causing a step change in said direct current.

6. A method according to claim 5 wherein the measuring step comprises sampling the peaks, subsequent to said step change, in the response of the electrical network.

7. A method according to claim 5 or 6 and including the step of measuring the time elapsed between predetermined portions of the response of the network subsequent to said step change.

8. A method according to any one of the preceding claims wherein the network comprises at least one electrical transducer.

9. A method according to any one of the preceding claims wherein said component comprises at least one electrical resistor.

10. A method according to any one of the preceding claims wherein said electrical network is at least one geophone in combination with at least one damping resistor.

11. Apparatus for testing an electrical network, the network having an electrical output

port and a component which will damp the electrical response at said output port to an electrical stimulus applied to the network, the apparatus comprising means for applying such electrical stimulus to the network means, having a pair of input terminals for connection across said ports for measuring the time dependent electrical response at said output port to said electrical stimulus and means for monitoring said electrical response at said output port to such electrical stimulus and for applying a further electrical stimulus to said network via said input terminals, the arrangement being such that, in use, the magnitude and polarity of said further electrical stimulus are appropriate to reduce by a substantial amount, or to cancel, the effect of said damping component on said response.

12. Apparatus according to claim 11, wherein the stimulus applying means comprises an operational amplifier circuit having a virtual earth input and an output to which input and output the input terminals are connected and the monitoring and further stimulus applying means comprises an inverting amplifier having an input connected to receive the output of said operational amplifier circuit and an output connected by a resistor to said operational amplifier circuit input.

13. Apparatus according to claim 12 and comprising an oscillator adapted to provide, in use, an alternating current output of a frequency substantially equal to the resonant frequency of said network and means for applying said alternating current output to said operational amplifier circuit input.

14. Apparatus according to Claim 13 and comprising a circuit for applying to said operational amplifier circuit input a direct current having an amplitude substantially equal to one half the peak to peak value, at the operational amplifier circuit input, of said alternating current.

15. Apparatus according to claim 14, wherein said measuring means comprises a sampling circuit connected to receive said operational amplifier circuit output, the sampling circuit being synchronised with said alternating current to sample those peaks of the output of the operational amplifier circuit which correspond to those peaks of the alternating current which are opposite in polarity to the direct current.

16. Apparatus according to claim 14 or 15, in which said direct current applying circuit comprises a sample and hold circuit having an input receiving said alternating current and an output connected to said operational amplifier circuit input.

17. Apparatus according to any one of claims 13 to 16 wherein said oscillator is a voltage controlled oscillator in a phase locked loop configuration, the output of the operational amplifier circuit being connected to the phase comparator of said phase locked loop.

18. Apparatus according to claim 17 and comprising means for indicating the frequency of said alternating current.

19. Apparatus according to claim 18 wherein said indicating means comprises a further phase locked loop, having a frequency divider circuit.

20. Apparatus according to claim 17, 18 or 19 when appendant to claim 16 and comprising a first differential amplifier for providing an output proportional to the difference in the magnitudes of the signals at the input and output of said sample and hold circuit and a second differential amplifier for providing an output signal proportional to the difference between the signals appearing at an operational amplifier circuit and first differential amplifier outputs, the second differential amplifier having its output connected to said phase comparator.

21. Apparatus according to claim 12 and comprising a circuit for applying to said operational amplifier circuit input, a steady direct current and a switch operable to change the magnitude of said direct current between two step values.

22. Apparatus according to claim 21 and comprising at least one peak detecting circuit for producing an output signal proportional to one, or a respective one, of the peaks of the output signal occurring at said operational amplifier circuit output after the switch is operated.

23. Apparatus according to claim 22, wherein one such peak detecting circuit is arranged to produce an output proportional to only the second one of such peaks.

24. Apparatus according to claim 23, wherein said one such peak detecting circuit has circuitry connected thereto to inhibit the output thereof until after the circuit has been operated to change said value in a predetermined direction.

25. Apparatus according to claim 22, 23 or 24, wherein one, or a further one, such peak detecting circuit is arranged to produce an output signal proportional to only the first one of such peaks.

26. Apparatus according to any one of claims 22 to 25 and including an analogue to digital converter connected to the, or at least one of the, peak detecting circuits to produce a digital output.

27. Apparatus according to claims 13,15 and 16 or claims 13,14,15 and 16, wherein the analogue to digital converter is operative to compare the magnitude of that one of the output signals from the peak detecting circuits which represents the first such peak with that one of the output signals from the peak detecting circuits which represents the second such peak.

28. Apparatus according to any one of claims 22 to 27 and comprising means for receiving a signal representing one of said peaks and for indicating whether the magnitude of the peak is within a predetermined range of values.

29. Apparatus according to claim 28, wherein the arrangement is such that the said one of said peaks is the second such peak occurring after the switch is operated to change the value of said direct current in a predetermined direction.

30. A method of testing a geophone or string of geophones which comprises the steps of: taking a geophone or string of geophones; causing an electric current to flow through the geophone or string of geophones from one terminal to another terminal connected to the geophone or string of geophones, which current comprises a first alternating current component having a frequency substantially equal to the resonant frequency of the geophone or string of geophones., a second direct current component and a third current component which is arranged to cancel the effect of the part of the first and second components which flows through the effective damping resistance of the geophone or string of geophones whereby a substantially constant alternating current having a direct current offset is caused to flow through the geophone or geophones; and measuring the voltage produced across the terminals.

31. Apparatus for testing a geophone or string of geophones including a pair of terminals means for applying to the geophone or string of geophones via said terminals an electric current, which current comprises a first alternating current component having a frequency substantially equal to the resonant frequency of the geophone or string of geophones, a second direct current component and a third current.component which is arranged to cancel the effect of that part of the first and second components which flows through the effective damping resistance of the geophone or string of geophones whereby a substantially constant alternating current having a direct current offset is, in use, caused to flow through the geophone or geophones and means for measuring the voltage thus produced across the terminals.

32. Apparatus for testing an electrical network, such apparatus being constructed and arranged to operate substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

33. Apparatus for testing an electrical network, such apparatus being constructed and arranged to operate substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

34. A method of testing a geophone or string of geophones, such method being substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

35. A method of testing a geophone or string of geophones, such method being substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

36. A method of testing an electrical network such method being substantially as hereinbefore described with reference to and as illustrated in Figure 1 of the accompanying drawings.

37. A method of testing an electrical network such method being substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 3 of the accompanying drawings.