GB2219087A - Seismic wave generating apparatus - Google Patents

Abstract

Seismic wave generating apparatus comprises a probe (10) adapted to be received in the ground (18) and including spark discharge electrodes (20, 22) connected to an electrical discharge apparatus including a voltage converter (76) to convert a low (24V D.C.) voltage to a high output voltage (4000V D.C.). A storage capacitor (84) is charged by the converter (76). Switch means (90) switches to a conducting condition when the potential difference across the capacitor (84) reaches a preselected level and connects the storage capacitor (84) to discharge electrodes (20, 22). Feed back means (86, 60) transmits a signal to reset means (68) on discharge to reset the converter (76) to an operative condition. The apparatus is relatively portable and can produce seismic waves at regular intervals of seven to fifteen seconds over a considerable period of time.

Description

SEISMIC WAVE GENERATING APPARATUS This invention is concerned with seismic wave generating apparatus which is suitable for use in geophysical experiments.

In carrying out geophysical experiments one known practice is to create a seismic wave (e.g. a compression wave or a shear wave) and, by means of appropriately positioned detectors, study the manner in which the seismic wave is propagated through the ground. Various ways of generating a suitable seismic wave have been proposed, for example small explosions, or powerful sparks.

Apparatuses for creating shock waves by means of powerful electrical discharges or 'sparks have been proposed but have been expensive equipment and have been extremely cumbersome. In order to create a sufficiently strong spark a high voltage must be discharged and in the known apparatuses, the power for generating a spark has been supplied by a generator of considerable size such that it must be mounted on a vehicle for transport to and from the site at which the experiments are to be carried out. This is expensive and inconvenient, especially where the experiments are to be carried out in a confined space into which a generator of sufficient power cannot readily be transported. In the existing apparatuses, when the spark is struck a large current, for example 600 to 700 amps, is drawn for a short period of time.The known apparatuses utilise a rectifier and a voltage multiplier associated with the A.C. generator to step up the voltage to the required level: about 4000 volts has been found to be satisfactory. Not only is the known apparatus expensive and cumbersome but also difficult to operate, requiring careful control of the apparatus and manual activation of the spark.

One of the various objects of the present invention is to provide an improved seismic wave generating apparatus.

In one aspect the invention may be considered to provide a seismic wave generating apparatus suitable for use in geophysical experiments comprising a seismic source including a housing and a spark discharge means within the housing, and an electrical discharge apparatus connected to the spark discharge means comprising input means by which a low voltage may be supplied in the operation of the apparatus, a voltage converter for converting low voltage applied to the input means to a high output voltage, a storage capacitor connected to the converter to be charged by the high output voltage, switch means adapted to switch from a non-conducting to a conducting condition when the potential difference across the capacitor reaches a pre-selected level and to connect the storage capacitor to discharge the storage capacitor through the spark discharge means, and feed back means adapted to transmit a signal to reset means when the storage capacitor discharges, the reset means being adapted to reset the voltage converter.

A preferred input means comprises input lines connected to a low voltage D.C. source e.g. a battery source, suitably two 12 Volt car batteries connected in series to provide a 24V D.C. supply.

The voltage converter may be any converter capable of converting the low voltage input to a suitable high voltage. One preferred voltage converter may comprise a BRANDENBURG P.S.U. which will convert a low D.C. voltage to a high output D.C. voltage e.g. a 24V D.C. input to a 4000V high output voltage, providing a current of up to about 5 mA; this type of converter may be adjusted to deliver from 0 to 5 mA. Where a higher charging rate is required two or more such converters may be connected in parallel and appropriately adjusted.

Commonly voltage converters include circuitry to protect them against surges when the storage capacitor discharges, the protective circuitry is actuated when the storage capacitor discharges and before the converter can again become operational it must be reset. In a preferred apparatus in accordance with the invention the converter is reset by connecting the input of the converter to earth and the reset means is arranged to do this, comprising a circuit including a switch adapted to be operated to switch an input connection of the converter from connection to the input means to earth for a period sufficient to reset the converter, suitably about one second, and then to switch back to connect the input connection back to connect tc the input means, restarting the convertor.

It will be appreciated that by selecting an appropriately sized storage capacitor and other components a high voltage/high current electrical discharge of short duration may be produced at regular intervals, thus to create a spark at the spark means and thereby a seismic wave, at regular intervals without the presence of an operator.

The reset means may conveniently be adapted to also produce a trigger output signal each time the storage capacitor is discharged: this is preferably connected to a suitable recorder to provide a datum signal indicating the time at which the seismic wave is generated.

Any suitable switch means may be used in apparatus in accordance with the invention. One convenient switch means comprises a so-called "spark gap" which may be arranged to switch from a non-conductive to a conductive, very low resistance, condition at a pre-selected voltage.

A preferred spark gap is a gas tube spark gap, which suitably comprises a hydrogen-filled tube (of known construction and currently used in, for example, lightening conductors) having input and output electrodes, which has a resistance of about 1014 ohms in its non-conductive state but switches to a resistance of about 1 ohm in its conductive state, conducting the discharge to the spark discharge means. Other spark gaps are, however, also possible - for example a two inch air gap together with a "starter" electrode which, when the charge on the capacitor reaches the pre-selected level is actuated to ionise the air in the spark gap thereby rendering it conductive.

The feed back means conveniently comprises a high resistance potential divider connected across the capacitor terminals with an output arranged so that a suitable voltage pulse e.g. of about 5 volts is transmitted to the reset means when discharge occurs.

The housing is adapted to be mounted in firm contact with the ground, for example in a borehole, whereby to permit transmission of a seismic wave generated within the housing to the surrounding ground. The spark generating means within the housing is operable to provide a discharge in the housing thus to generate a seismic wave. In a preferred apparatus a liquid electrolyte fills the housing and the spark generating means comprises a pair of suitably spaced electrodes in the electrolyte; suitably the electrolyte is aqueous sodium chloride solution of appropriate concentration.

The electrolyte aids in transmitting a seismic wave generated by the spark discharge to the surrounding ground.

There now follows a detailed description, to be read with reference to the accompanying drawings, of a seismic wave generating apparatus embodying the invention. It will be realised that this apparatus has been selected for description to illustrate the invention by way of example.

In the accompanying drawings- Figure 1 is a circuit diagram of an electrical discharge apparatus suitable for use in a seismic wave generating apparatus embodying the invention; Figure 2 is a circuit diagram of an auto reset circuit of the electrical discharge apparatus; and Figure 3 is a diagrammatic view of a probe of the seismic wave generating apparatus embodying the invention.

A seismic source of the illustrative apparatus comprises a probe 10 including a housing having a body portion 12 and a lower cover portion 14 secured to the body portion 12. The probe is adapted to be received in a borehole 16 in the ground 18. Electrodes 20, 22 project from the body 12 into the space bounded by the cover 14.

The electrodes 20, 22 comprise a central high voltage power electrode 20 and an earthed outer electrode 22. The space defined by the cover 14 and body 12 is filled with an electolyte, conveniently sodium chloride solution 24 of a predetermined concentration. A cable 26 is connected to output lines 28, 30 of an electrical discharge apparatus. The outer electrode 22 is connected to the earth line 28, whilst the central electrode 20 is connected to the high voltage output line 30 (see Figure 1).

The electrical discharge apparatus shown in Figure 1 is intended to be driven by input means including a battery source (not shown) or other source supplying 24 volts D.C. between the earth line 28 and a power input line 32. Line 32 is connected through a diode 34 which provides a protection against reverse current flow to a two-way manually operable switch 36. The input line 32 is also connected by a line 38 to a converter 40 which gives an output of 5 volts on an output line 42 connected to a digital volt meter 44 having an input line 46 to receive a signal as described later.

The switch 36 is arranged in a first position (in which it is shown in Figure 1) to connect the input 32 to a line 46 and a line 48 each of which are provided with fuses. In the other position of the switch 36, the lines 46, 48 are connected to a line 50 which itself connects to the earth line 28, for a purpose to be described hereinafter. The line 46 leads to poles 52, 54 of a twoway double gang switch 56. The line 48 provides a power input to an auto reset circuit 58 shown in more detail in Figure 2 and which will be described in greater detail hereinafter.The auto reset circuit 58 is provided with a second input line 60 on which is transmitted a reset signal, as will be described hereinafter, and has three output lines a first output line 62 providing a trigger output to further apparatus, for example an event recorder (not shown) and the remaining two output lines 64, 66 being connected to the other poles 68, 70 of the switch 56.

By operation of the switch 56 the poles 52 and 54, or alternatively 68 and 70 may be connected to power input connections 72, 74 to two voltage converters 76 which have common earth line 78 and chassis earth line 80. The voltage converters are suitably of the type known as BRANDENBURG Model 732 positive voltage converters and are adapted to provide, from a 24 volt D.C. input, a 4000 volt D.C. output on a common output line 82. The output current of each converter 76 is a maximum of 5 milliamperes. A storage capacitor 84 is connected between the earth line 28 and the output line 82 from the converters 76.The capacity of the capacitor 84 is chosen according to the characteristics required of the illustrative seismic wave generating apparatus and, for usual geophysical purposes, utilising the other components shown in the circuit is suitably a capacitor of 15 microfarads capacity rated at 4000 volts.

A series of bleed resistors 86 is also connected between the earth line 28 and the output line 82 from the converters 76. The reset signal line 60 is connected at a suitable point in the bank of resistors 86 (e.g. 10 resistors each 2.7 M ohms and one 87 of 33 k.ohm) to carry 5 volts when the potential difference between the lines 28, 82 is 4000 volts. Further bank of resistors 88 (e.g. 10 resistors each 10 M ohm and one 89 of 10 k.ohm) provides a potential divider to which the line 46 is connected at a suitable position to give a voltage of 0.5 volts on the line 46 when the potential difference between the lines 28, 82 is 5000 volts: the volt meter 44 can therefore be calibrated to give an indication of the potential difference between the first line 28 and the output line 82 from the converters 76.

Finally the output line 82 is connected to one terminal of a spark gap 90, the other terminal of which is connected to the output lead 30 from the electrical discharge apparatus. Any suitable spark gap apparatus may be provided as the spark gap 90 which is effectively acting as a switch in the operation of the illustrative apparatus. One suitable gas tube spark gap is made by the English Electric Valve Co. Ltd., and can be arranged to become conductive when the voltage applied across its terminals reaches 4000 volts.

A coil 92 around the output line 30 provides an inductive trigger output 93 which will provide an event trigger for appropriately sited recorders; however, the digital trigger output provided on the line 62 by the auto reset circuit 58 is preferred as this is more controlled. The discharge of the storage capacitor 84 through the output line 30 induces a current in the coil 92 thus providing a signal when the storage capacitor is discharged.

The auto reset circuit 58 is shown in more detail in Figure 2. The input line 48 to the auto reset circuit 58 is connected to a two-way switch 94, to a relay circuit 96 and to a voltage conversion circuit 98 which provides a 5 volt output on a line 100. The line 100 is connected to various terminals of an integrated circuit chip 102.

One suitable integrated circuit is a timer circuit supplied under the designation NE555V by Mullard/ Signetics and obtainable from Radio Spares. The TTL version is preferred. The input reset signal line 60 is connected to another chip terminal of the chip 102 and an output line 104 leads to two transistor switches 106, 108. The transistor switch 106 is connected in circuit with the relay 96 so that actuation of the transistor switch 106 to a conductive condition operates the relay 96. Operation of the relay 96 operates the two-way switch 94 to cause it to move from the charge condition in which it is shown in Figure 2, in which the input line 48 is connected through the switch 94 to the lines 64, 66 to the converters 76, to a reset condition in which the lines 64, 66 are connected by a line 110 to earth. The transistor switch 108 is connected to provide a signal on the digital trigger output line 62 when in a conductive condition. The circuit connections to the chip 102 are such that, on receipt of a signal on the reset line 60 the transistor switch 106 is switched to a conductive state for about one second, thus holding the relay in an activated condition for one second and retaining the switch 94 with the lines 64, 66 connected to the line 110 and thus to earth for one second; after the switch 106 ceases to be conducting, the relay becomes inactivated and the switch 94 is returned to the condition in which it is shown in Figure 2 so that a charging supply is connected from the line 48 to the lines 64, 66.Likewise, upon receipt of the signal on the line 60, the switch 108 is put into an operative condition for one microsecond thus providing a short duration trigger pulse on the line 62.

In the operation of the illustrative apparatus, with the spark discharge means connected to the electrical discharge apparatus and with a power source, for example a battery, connected between the input lines 28, 32 the apparatus may be arranged to provide single discharges upon actuation by an operator or to provide a series of discharges at regular intervals; the latter mode of operation is preferred for many geophysical experiments.

To operate the illustrative apparatus manually the two-way double gang switch 56 should be in the position in which it is shown in Figure 1 (marked "single"), that is with the line 72 connected to the pole 52 and with the line 74 connected with the pole 54. In this condition, the voltage converters 76 will charge the storage capacitor 84 when the switch 36 is in its "on" position in which it is shown in Figure 1. The capacitor 84 will be charged until the potential difference between the lines 28, 82 reaches the potential difference at which the spark gap 90 becomes conductive, in the case of the illustrative apparatus 4000 volts. When this voltage is reached, the spark gap 90 becomes conductive (its resistance dropping from about 1014 ohms to about 1 ohm) and discharges the storage capacitor 84 on lines 28, 30 to the spark generating means of the probe 10.A spark passes between the electrodes 20, 22 creating a shock wave in the probe 10 which is transmitted to the surrounding ground 18 and this seismic wave travels through the ground to various recording apparatuses positioned at appropriate points. Indication that the spark discharge has occurred is given by the volt meter 44.When the spark discharge occurs, a signal is given on the reset signal line 60 which causes a substantially instantaneous signal to be given on the digital trigger output line 62 and on the inductive trigger output lines 93 either, or both, of which may be connected to appropriate recording apparatuses to provide a time zero signal indicating the moment at which the spark discharge occurs: this can be used to trigger the recording apparatus or apparatuses so that the time taken for the seismic wave to reach the various detectors can be determined and appropriate calculations made using known techniques to establish the nature of the ground under survey.

Firing of the spark discharge will cause the protective circuits of the converters 76 to come into operation and no further charging of the capacitor 84 by converters 76 can take place until the protect circuits have been reset. When the illustrative apparatus is in its manual mode, resetting of the protective circuits must be carried out manually by operating the switch 36 to its "off" reset position, thus connecting the line 46 with the line 50 which itself is connected with earth, the converters thus being earthed which in the case of the illustrative apparatus is sufficient to reset the converters. A further spark discharge may then be caused by returning the switch 36 to its on position.Because it takes a certain amount of time (in the case of the illustrative apparatus about seven to ten seconds) to recharge the storage capacitor 84 at the charging rate of the converters 76, a spark discharge will occur about seven to 10 seconds after the switch 36 has been returned to its "on" position.

When it is wished to operate the illustrative apparatus continuously, the switch 36 is left in its "on" position but the switch 56 is moved to the position in which the pole 68 is connected with the line 72 and the pole 70 is connected with the line 74. When in this position, the input line 32 is connected to the line 48 and thus through the lines 64, 66 to the input lines 72, 74 to the converters 76 and the converters are operative to charge the storage capacitor 84 in the way discussed above. When the storage capacitor reaches the appropriate voltage, a spark discharge occurs in the probe 10 as outlined above and a signal is received by the auto reset circuit 58 on the line 60.This signal provides a trigger output on the line 62 as previously discussed but also causes a one second pulse on the line 104 thus causing the transistor switches 106, 108 to switch to their conductive conditions thus operating the relay 96 and providing a trigger output on the line 62. In the continuous mode, when the switch 94 is operated by the relay 96 to connect the lines 64, 66 to the line 110 (and thus to earth) the input lines 72, 74 of the converter 76 are connected to earth and thus the protective circuits of the converters 76 are reset. As soon as the transistor switch 106 becomes non-conducting the relay 96 allows the switch 94 to be returned to the position in which it is shown in Figure 2 thus again connecting the input line 48 to the input lines 72, 74 to the converters 76 and thus recharging the storage capacitor 84.

By appropriately choosing the various components in the circuit, it is possible to cause the illustrative apparatus to continuously provide spark discharges at appropriate regular intervals. In the illustrative apparatus, by adjustment of the converters 76 the intervals may be adjusted between about seven and fifteen seconds. It has been found that two fully charged car batteries will cycle the illustrative apparatus continuously for a considerable period of time, for example from six hours to forty eight hours depending upon the cycling rate chosen, when in the continuous mode.

The illustrative apparatus, including the battery power source can easily be carried on a single vehicle to a point for use and move into relatively small spaces for various geophysical surveys, which has been impossible with the previously known apparatus. It is, of course necessary, to ensure that the spark discharge cannot take place when the output terminals 28, 30 are accessible to an operator and for this purpose a protective cover is provided preventing access to the live terminal whilst the machine is operative. The illustrative apparatus is especially useful as a compression wave source.

Claims (13)

1. A seismic wave generating apparatus suitable for use in geophysical experiments comprising a housing, a spark discharge means within the housing and an electrical discharge apparatus connected to the spark discharge means comprising input means by which a low D.C. voltage may be supplied in the operation of the apparatus, a voltage converter for converting low voltage applied to the input means to a high output voltage, a storage capacitor connected to the converter to be charged by the high output voltage, switch means adapted to switch from a non-conducting to a conducting condition when the potential difference across the capacitor reaches a preselected level and connect the storage capacitor to discharge the storage capacitor through the spark means, and feed back means adapted to transmit a signal to reset means when the storage capacitor discharges, the reset means being adapted to reset the voltage converter.

2. Apparatus according to Claim 1 wherein the voltage converter is arranged to convert a 24 volt D.C. input to a 4000 volt output.

3. Apparatus according to either one of Claims 1 and 2 wherein the voltage converter is arranged to be reset by connecting the charging input of the converter to earth.

4. Apparatus according to any one of the preceding claims arranged so that seismic waves may be produced at regular intervals or at time intervals controlled by an operator.

D. Apparatus according to any one of the preceding claims wherein the apparatus is arranged to produce seismic waves at intervals of between seven and fifteen seconds.

6. Apparatus according to any one of the preceding claims wherein the switch means comprises apparatus arranged to switch from a non-conductive to a conductive state when a pre-selected voltage is reached.

7. Apparatus according to any one of the preceding claims wherein the feed back means comprises a potential divider connected across the capacitor terminals with an output arranged to provide a suitable voltage pulse to the reset means when discharge occurs.

8. Apparatus according to any one of the preceding claims wherein the reset means comprises a circuit arranged to operate a relay to move a switch from a charging position to a discharge position in which the input lines to the converter may be connected to earth.

9. Apparatus according to any one of the preceding claims comprising an output from the reset means to provide a trigger signal for recording apparatus.

10. Apparatus according to any one- of the preceding claims comprising an induction coil around the output lines from the electrical discharge apparatus to provide a trigger signal for recording apparatus.

11. Apparatus according to any one of the preceding claims wherein the input means comprises a battery low voltage source.

12. Apparatus for generating a seismic wave constructed arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.

13. Electrical discharge apparatus comprising input means by which a low voltage may be supplied in the operation of the apparatus, a voltage converter for converting low voltage applied to the input means to a high output voltage, a storage capacitor connected to the converter to be charged by ' the high output voltage, switch means adapted to switch from a non-conducting to a conducting condition when the potential difference across the capacitor reaches a pre-selected level and connect the storage capacitor to discharge the storage capacitor through the spark means, and feed back means adapted to transmit a signal to reset means when the storage capacitor discharges, the reset means being adapted to reset the voltage converter.