GB2377496A

GB2377496A - Multi-frequency spark discharge seismic wave generator.

Info

Publication number: GB2377496A
Application number: GB0212674A
Authority: GB
Inventors: Stephen Lionel Shedlock
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-05-31
Filing date: 2002-05-31
Publication date: 2003-01-15
Anticipated expiration: 2022-05-31
Also published as: GB0113162D0; GB0212674D0; GB2377496B

Abstract

A seismic wave generator is disclosed comprising a probe 3 including a spark discharge chamber (Fig.3 42) having a resonant frequency and means 1 for generating a spark discharge within the chamber (Fig.3 42). There is further provided means for varying the resonant frequency of the spark discharge chamber (Fig.3 42) and means (25, Fig.2) for varying the repetition rate of sparks generated within the spark discharge chamber (Fig.3 42).

Description

Title - Multi-Frequency Seismic Source This invention relates to a seismic source, that it to say apparatus for the generation of seismic signals suitable for use in geophysical experiments.

Seismic signals are used in a variety of site investigations for civil engineering, environmental and mining applications. Such investigations rely on the transmission of a seismic signal through sub-surface strata, from the surface or from boreholes drilled to assess these ground conditions. A seismic wave is generated and, by means of appropriately positioned detectors, the manner in which the seismic wave is propagated through the ground is studied. The seismic wave is commonly generated by means of an electrical spark discharge in a probe or"sonde"which may be mounted on the surface of the terrain under investigation or inserted into a borehole.

A problem with known equipment for this purpose is a relative lack of flexibility.

Different types of study, or studies of different types of terrain, require rather different operating parameters and these cannot be provided by the same equipment. For example, the resolution of the equipment (ie the minimum size of underground geological features that it is possible to identify) depends upon the frequency (or wavelength) of the seismic wave. The resolution of smaller features requires the use of higher frequencies/shorter wavelengths. Hitherto, it has not been possible rapidly and easily to vary the frequency of the seismic wave.

Another parameter that it is sometimes desirable to vary is the repetition rate of the electrical spark discharge. Currently available equipment is limited to fixed recycling periods. Such periods are either rather slow (of the order of 10 seconds), which is suitable for so-called tomographic inversion studies, or more rapid (of the order of 1 second) for reflection/refraction studies in shallow water locations (lakes etc).

Versatile equipment that may readily adapted for the generation of seismic waves with variable frequency and at variable repetition rates has hitherto not been available.

There have now been devised improvements to seismic sources suitable for geophysical experiments that overcome or substantially mitigate the abovementioned and/or other disadvantages of the prior art.

According to the invention, there is provided a seismic wave generator comprising a probe including a spark discharge chamber having a resonant frequency and means for generating a spark discharge within the chamber, there being further provided means for varying the resonant frequency of the spark discharge chamber and means for varying the repetition rate of sparks generated within the spark discharge chamber.

The seismic wave generator according to the invention is advantageous primarily in that it is versatile. The resonant frequency (and hence the frequency of the generated seismic wave) can be adjusted to provide the desired resolution in the terrain under investigation. Likewise, the spark repetition rate can be adjusted to suit the type of investigation being performed. In preferred embodiments, the generator is very compact and portable and can be very easily transported to the site of use.

The means by which the resonant frequency of the spark discharge chamber is varied most conveniently comprises means for varying the physical dimensions of the spark discharge chamber. The generator may therefore be supplied or useable with any one of a range of interchangeable spark discharge chambers of differing dimensions. Conveniently, the spark discharge chambers are cylindrical and differ from each other in their diameters. It is particularly preferred for the

spark discharge chamber to have one of a range of diameters less than 100mm, so that they fit within a standard 100mm borehole.

In preferred embodiments, the spark discharge chambers are formed of reinforced elastomeric or rubber material and are filled with a salt solution Such embodiments are typically used whilst submerged in a liquid, typically water, with the water transmitting the generated seismic wave into its surroundings.

In alternative embodiments, the spark discharge chamber may be formed from a relatively pliant elastomeric or rubber material. In such embodiments, the probe preferably includes means for increasing the pressure of the salt solution within the spark discharge chamber, thereby extending the pliant elastomeric or rubber material and enlarging the spark discharge chamber. Such embodiments may therefore be used in air-filled boreholes because contact with the borehole wall allows the generated seismic wave to be transmitted effectively into the surroundings of the borehole.

It is particularly preferred that the spark discharge chamber be constructed in such a way as to generate a substantially uniform and omni-directional energy field. This is preferably achieved by creating the spark discharge between a central electrode and a plurality of outer electrodes equiangularly spaced around the central electrode.

Typically, the wavelength of the seismic wave can be set within a range of 0.2 to 5. 0m.

The spark discharge is preferably created by charging and then discharging a capacitor. In such a case the variation in pulse repetition rate is preferably achieved by controlling the magnitude of the current available to charge that capacitor, a higher current leading to a shorter charging time, and hence a more

rapid spark repetition rate. The interval between pulses is typically between 2 5 and 30s.

The control unit for the seismic source generator is preferably housed remotely from the probe containing the spark discharge chamber, the probe and the control unit being connected by a high voltage cable. For borehole applications, such a cable may be typically 100-200m or more in length.

Further details of the seismic source generator according to the invention will become evident from the following description of a preferred embodiment, which is described in greater detail, by way of illustration only, with reference to the accompanying drawings, in which Figure 1 is a schematic view of a seismic source according to the invention; Figure 2 is a block schematic diagram of the seismic source; Figure 3 is a side view of a downhole sonde forming part of the seismic source of Figure 1; Figure 4 is a cross-sectional view on the line IV-IV in Figure 3; Figure 5 illustrates the energy field generated in operation of the sonde of Figure 3; Figure 6 is a circuit diagram of a variable current and variable voltage circuit forming part of the control unit 1 of Figure 1; Figure 7 is a circuit diagram of an output voltage monitor forming part of the control unit 1 of Figure 1;

Figure 8 is a circuit diagram of a safety circuit forming part of the control unit 1 of Figure 1; Figure 9 is a circuit diagram of a remote operation circuit forming part of the control unit 1 of Figure 1; and Figure 10 is a view, partly in section, of a borehole containing a downhole sonde forming part of a further embodiment of a seismic source according to the invention.

Referring first to Figure 1, a seismic source according to the invention comprises essentially three components: - a control unit generally designated 1 - a connecting cable generally designated 2 - a downhole sonde generally designated 3 The control unit 1 has a housing 20 approximately 400mm in width and 200mm in both height and depth. The front panel of the housing 20 is fitted with - a power ON switch 21 - an earth connector 22 - a fault indication lamp 23 - an output voltage display (digital multimeter-DMM) 24 - a pulse repetition frequency (prf) selector dial 25 - a pulse trigger 26 - a remote operation connector 27 - a pulse repetition control 28 The connecting cable 2 may typically be 150-200m in length and is described more fully below.

The sonde 3 is also described more fully below, in relation to Figures 3 and 4.

As shown in Figure 2, the system comprises a high-voltage power supply (HVPS) 31 which is supplied by one or more 12V DC car batteries (not shown) and serves to generate an output voltage of typically up to 6000V. The power supply 31 charges a capacitor 32 which discharges across a spark gap 33. The resulting high output voltage is applied to the connecting cable 2 which is connected to the downhole sonde 3 described below.

One form of spark gap 33 that has been found to be suitable is the GXH series supplied by Marconi Applied Technologies Ltd, Waterhouse Lane, Chelmsford CM1 2QU, Unitd Kingdom.

As also indicated in Figure 2, the system incorporates a pulse repetition rate adjustment circuit 34 (which is connected to the selector dial 25), a reset circuit 35 and a safety cut-out circuit 36 connected to the fault indication lamp 23.

Power conversion from the incoming DC low voltage supply to that of the high voltage DC generated voltage for the charging of capacitors is important to the

operation of the unit. The characteristics of the HPVS 31 are preferably as follows : Input voltage : 12-24V DC Output voltage : 0-6000V DC (adjustable) 1out 0-40 mA (adjustable) Power rating: 250 W Suitable high voltage power supplies include those supplied by Ultra Volt Inc, CS9002, Ronkonkoma, NY 11779, USA, as the 6C series power supply.

By varying the voltage on the Remote Adjust terminal between 0 and +5V, the output voltage can be adjusted over the full range of operation of the unit, ie 0 to 6000V DC. The maximum output voltage will typically lie in the range of 4600 to 4900V DC. This equates to an input voltage at the Remote Adjust terminal of 3 6 to 3.8V DC (3.566V at 4600V output and 3.800V at 4900V output).

The amount of current available to charge the capacitor determines the firing frequency of the control unit 1. Where this DC current is low, then the firing period will be extended, while high current gives a reduced discharge period. If it is assumed that the capacitor is of 20pF at a nominal working voltage of 5000V DC, then the amount of current for a specific charge period is determined by:

where T = charge period in milliseconds I = supply current in mA C = capacitance in uF V = supply voltage in volts For an eight-step change in supply current, this equation yields the following firing periods:

Firing period/s Supply current/mA 2. 5 40 3. 0 33. 3 5.0 20 6.0 18.33 10. 0 10 20. 0 5 30. 0 3. 33

The HVPS 31 is programmable as a constant voltage supply, encompassing output voltages from OV DC up to the maximum rating of the power pack. When the application requires a constant current output, the current monitor feature permits the HVPS 31 to operate in a regulated current mode. An additional circuit is added externally to convert the unit from current regulation to voltage regulation. The current regulation offers a buffered output (LM158 opamp) from the current monitor and by incorporating a series of preset potentiometers the necessary current range (and hence the desired pulse repetition rate) may be selected, via the selector dial 25, from the front panel of the control unit 1.

Where it is necessary to limit the output voltage from the HVPS 31 then a 5 kQ rheostat connected potentiometer is connected from the Remote Adjust pin 6 to the Signal Ground Pin 5. This preset is mounted on the additional circuit board and is adjustable to account for the variation in the avalanche voltage of the cascade valve. This variable current and variable voltage circuit is shown in Figure 6 with the connections to the HVPS 31 and the prf selector dial 25 indicated.

As an additional safety feature the output voltage supplied to the load capacitor 32 is monitored on the front panel-mounted digital multimeter (DMM) 24. The HVPS 31 incorporates a potential divider network providing a 100: 1 output for supply to the DMM 24. This results in an output of 10V per 1000V. However should a 1000 : 1 ratio be necessary then an additional resistor and capacitor (102kQ and 0.1 uF) are connected in parallel with the DMM 24, providing 1V per 1000V. The signal for this voltage monitor is derived from the Eout Monitor (Pin 9) on the HVPS 31.

The schematic set-out for this 1000: 1 voltage monitor is illustrated in Figure 7 with the connections to the HVPS 31 and the DMM 24 indicated. The DMM 24 is

an LCD panel meter programmed for a full-scale output of 10 volts, thus indicating the output voltage, in kV, to two significant places. The input impedance of such a meter is in excess of 10 MQ.

With a predetermined voltage, half the product of the square of the voltage and the capacitance (CV2/2) determines the power output of such a system.

Assuming a terminal voltage of 5000V into a capacitor of 20uF, the power output of the control unit 1 at its output terminal is 250Ws. Regardless of the current variation this power output will be constant.

Operation of the HVPS 31 may be enabled via an external remote operation unit.

This unit is connected to the control unit 1 at the remote operation connector 27 and includes a push button switch. When this switch is activated, the remote operation circuit, within the control unit 1, provides the safety cut-out circuit 36 with a TTL high signal, thereby enabling one cycle of the HVPS 31. The remote operation circuit and the circuit for the remote operation unit are both shown in Figure 9. The connection between the remote operation circuit of Figure 9 and the safety cut-out circuit 36 of Figure 8 is indicated by the word"remote".

The control unit 1 is connected via the high-voltage cable 2 to the down-hole sonde 3. To achieve this the control unit 1, the sonde 3 and the cable 2 itself are fitted with mating connectors. Such connectors need to be capable of handling the pulsed current and to be able to do so when submerged in water to a depth of 200m.

In order that safe operation of the system may be undertaken, it is essential that the equipment is safely shut down in the event of any fault condition being measured.

The system comprises essentially three components, viz the control unit 1, the sonde 3 and the interconnecting cable 2. The safety features of these are:

The control unit 1 generates the high voltage to charge the internal capacitor 32.

Operating from a 24V DC battery pack (not shown) the control unit 1 houses the HVPS 31, which is capable of providing power to the capacitor 32 at voltages of up to 5000V DC. This power is stored within the high voltage capacitor 32 and when the capacitor 32 stores sufficient energy then the power can be transferred via a cascade valve to the remote load (the downhole sonde 3). Any transfer of this energy must be undertaken in a safe and controlled manner under the direction of the operator. Should any fault or omission in proper operating procedure be detected then the system must be capable of preventing any discharge of this high energy and automatically prevent any further improper procedures.

The safety cut-out circuit 36 is shown in Figure 8 with the connections to the power ON switch 21, the pulse repetition control 28, the remote operation circuit and the cable 2 all indicated.

Such"faults"can result from the following : 1. Cable 2 to downhole sonde 3 not connected 2. Improper load connected to output 3. Downhole sonde 3 not connected to cable 2 4. Improper battery power connection 5. Unit shut down with capacitor in"charged"state Fault 1 The system relies on sensing a specific load being connected to the control unit 1 and where the safety cut-out 36 does not see this connection then the safety cutout 36 diables operation of the control unit 1.

The Enable connection to the HVPS 31 is normally supplied with a TTL high signal. However, in "fault" conditions this signal is a TTL low and operation of the HVPS 31 is disabled. The fault indicator light 23 is illuminated and further operation of the control unit 1 is not allowed until the fault condition has been cleared.

Fault 2 Where an extraordinary load has been connected to the output, the safety cut-out 36 senses that the correct connection has not been made and, as above, operation of the HVPS 31 is disabled. The fault indicator 23 is again illuminated and no further operation can be undertaken until the fault condition is cleared.

Fault 3 The downhole sonde 3 contains a circuit that ensures the integrity of the top connection cable and sonde connection. Where incorrect connections or cable faults are recorded then the safety cut-out 36 is activated and prevents subsequent operation of the control unit 1. The fault indicator 23 is illuminated.

Any damage to the cable 2 resulting from a break within the conductors will again prevent the unsafe operation of the unit, illuminated the fault indicator 23.

Fault 4 Incorporated within the safety cut-out circuit 36, an electromechanical relay 37 (see Figure 2) disconnects the high voltage from the Output. This relay 37 is energised at power-up and enables operation of the control unit 1. Where the operation of the control unit 1 is interrupted, either by incorrect power connection or sudden inadvertent power disconnection, the relay 37 is deactivated, disconnecting the cascade valve from the output. Any high voltage generated

from the HVPS 31 is isolated to within the control unit 1. No further operation of the unit can be undertaken until the fault condition is rectified.

Further safety features to account for reversed polarity power connection, prevent operation of the control unit 1 and any consequential damage to the internal components of the unit 1.

Fault 5 If normal operation of the control unit 1 is disabled whilst within a charge cycle and the unit depowered, the isolation relay 37 is enabled, disconnecting the Output from the high voltage components within the control unit 1. If left in this state, a chain of "bleed" resistors slowly decreases any residual high voltage to safe levels. This operation takes about 15 minutes and will ensure that any subsequent work, by a qualified engineer, within the control unit 1 will be undertaken in accordance with safe working practices.

The safety cut-out 36 senses the safe operation of components external to the control unit 1, disabling further operation within fault conditions. The safety cutout 36 is activated between discharges of the spark and comprises part of the HVPS Reset logic circuit 35. Unless the safety cut-out circuit provides an enable signal to the Reset 35 then the unit will maintain"power down"status.

Any fault within the safety cut-out circuit 36 will result in a fail-safe condition preventing further operation of the control unit 1.

As a component part of the safety cut-out circuit 36, the cable 2 between the control unit 1 and the downhole seismic sonde 3 transmits the power from the capacitor 32 to the spark chamber of the sonde 3 (see below). Of co-axial construction, this cable 2 has close knit inner and outer conductive braids under the outer sheath of the cable that overlies the insulating core over the central

conductor. The inner conductive braid is connected to the outer case of the sonde 3 and the system earth of the Control unit 1. The outer conductive braid forms part of the safety circuit and is electrically isolated from the inner braid The outer braid is electrically connected to the inner braid within the downhole seismic sonde 3. Cuts and/or breaks in the outer sheath of this cable will only expose the outer braid which is at earth potential and hence pose no safety risk.

However, where significant damage is effected, such that the core of the cable 2 is exposed then a potentially hazardous/lethal risk is present. Such ruptures in the conducting properties of the outer braid will render the cable beyond repair and activate the safety cut-out circuit 36. Temporary repairs, eg with adhesive electrical insulating tape, will affect the proper operation of the unit and invoke operation of the safety cut-out 36.

The safety cut-out 36 is operational at the power-up stage of the control unit 1 and senses the output connection to the downhole sonde 3 prior to enabling of the cycling of the HVPS 31. This safety cut-out circuit 36 provides a logic high to the enable input of the power pack and is an integral part of the reset circuit 35.

Operation of the safety cut-out 36 relies on sensing a finite current flowing within the downhole circuit. Contained within the downhole sonde 3 is a high impedance resistor that enables a small present current to flow within the loop consisting of the high voltage cable and the downhole sparker. Operating through a current sense circuit whose limits are set between an upper and lower threshold, and where this current meets the criteria for this current range, then a TTL high signal is transmitted to the gate contained within the Reset circuit 35.

As the amount of current flowing within the loop consisting of the downhole components is constrained within certain limits then unless the specific load is emulated, operation of the control unit 1 is disabled.

The safety cut-out 36 disabling safety circuit is the primary safety line within the control unit 1 and operates in conjunction with the reset circuit 35 and the

enabling relay 37 on the output positive terminal. These safety features ensure that any dangerously high voltages are restricted to the control unit 1. The operator may not gain access to high voltage components.

The Reset circuit 35 senses the change in the output potential of the HVPS 31.

During the charging cycle the voltage being applied to the high voltage capacitor 32 is monitored. This results in an increasing voltage to that of the cascade voltage of the"spark gap"when a sudden decrease in the potential is noted, following a discharge of the unit. The trailing edge of this voltage decrease is utilised to provide a high-low trigger to that of the Reset circuit 35 and enables the Enable input to be sent a TTL high (in continuous mode operation) for further cycling of the HVPS 31. Before such a TTL signal can enable operation of the HVPS 31 the condition of the safety cut-out 36 must be assessed and where continuity of the downhole components is compromised then this enable signal is disabled.

Within single shot operation, unless enabled by the safety cut-out circuit 36, charging of the capacitor 32 via the HVPS 31 will not be authorised.

The safety cut-out circuit 36 senses the amount of current flowing within the circuit comprising the downhole components and the cable and where this current is within preset limits the system is enabled. The upper and lower current limits are set to enable a maximum choice of options with regard to the source and cable configurations and only disable operation when open circuit or shorts conditions are measured.

The downhole sonde 3, shown in Figure 3, comprises two principal componentsthe sonde body 41 and a removable spark housing 42.

The sonde body 41 is cylindrical and is fitted at its top with a connector 43 for connection to the interconnecting high-voltage cable 2. The sonde body 41 has a diameter of 40mm.

The spark housing 42 is also cyclindrical and has one of a number of diameters In the range 40-80mm. The spark housing 42 is formed from a reinforced rubber material. In one preferred embodiment, spark housings having diameters of 41, 51,60, 70 and 76mm are supplied and are used interchangeably to generate seismic waves of the desired wavelengths.

As can be seen from Figures 4 and 5, the spark housing 42, which is filled with a saturated sodium chloride solution, contains a central electrode 44 and an arrangement of three outer electrodes 45, disposed equiangularly about the central electrode 44. This arrangement leads to the generation of an energy field which is omni-directional, the field strength being substantially uniform in all directions. As shown in Figure 5, the field has three lobes, extending outwardly from each of the three outer electrodes 45, the resultant field strength profile being indicated by the broken line.

Such a three-point discharge arrangement ensures long-term stability of the power source, reduces the impedance and leads to higher levels of energy transmission.

In use, the spark discharge chamber 42 of appropriate size for the desired wavelength of seismic wave is selected and connected to the sonde body 41.

The assembled sonde 3 is then introduced into a borehole and lowered to the desired depth. By triggering spark discharge within the spark discharge chamber 42 seismic waves are generated which propagate through the surroundings of the borehole. In practice, measurements might be taken at a series of depths, the sonde being incrementally lowered from one depth to another between measurements. In a typical set-up, one or more detectors might

be positioned at similar depth In a second borehole, the separation of the two boreholes typically being 50m. The propagation of seismic waves from the sonde 3 to the detectors is monitored and the information obtained may be used to establish, for instance, an image of the geological structures between the two boreholes.

In use, the sonde 3 of the above described embodiment is typically submerged within a liquid, typically water, within the borehole. This is because the water transmits the generated seismic wave very effectively into the surroundings of the borehole. If the sonde 3 is not submerged within a liquid, for example where the water table is too low or the borehole does not extend downwards, the seismic wave may be poorly transmitted by the surrounding air and hence only a very weak seismic wave may be transmitted into the surroundings of the borehole.

In an alternative embodiment (shown in Figure 10), the sonde 3 has a spark housing 42 formed from a relatively pliant rubber material. In use, the sonde 3 may be positioned within an air filled bore hole and the pressure of the saturated sodium chloride solution within the spark discharge chamber 42 is increased.

This causes the pliant rubber material to expand, thereby enlarging the central diameter of the spark discharge chamber 42, as shown in Figure 10. The spark discharge chamber 42 is preferably enlarged to such a size that there is contact between the spark discharge chamber 42 and the wall of the borehole. This contact enables the effective transmission of the seismic wave into the surroundings of the borehole.

Claims

Claims 1. A seismic wave generator comprising a probe including a spark discharge chamber having a resonant frequency and means for generating a spark discharge within the chamber, there being further provided means for varying the resonant frequency of the spark discharge chamber and means for varying the repetition rate of sparks generated within the spark discharge chamber.
2. A seismic wave generator as claimed in Claim 1, wherein the means for varying the resonant frequency of the spark discharge chamber comprises means for varying the physical dimensions of the spark discharge chamber.
3. A seismic wave generator as claimed in Claim 2, wherein the generator is supplied or useable with any one of a range of interchangeable spark discharge chambers of differing dimensions.
4. A seismic wave generator as claimed in Claim 3, wherein the spark discharge chambers are cylindrical and differ from each other in their diameters.
5. A seismic wave generator as claimed in any preceding claim, wherein the spark discharge chamber has a diameter less than 100mm.
6. A seismic wave generator as claimed in any preceding claim, wherein the spark discharge chamber is formed of reinforced elastomeric or rubber material and is filled with a salt solution.
7. A seismic wave generator as claimed in Claim 2, wherein the spark discharge chamber is formed from pliant elastomeric material and the probe includes means for increasing the pressure of fluid within the spark discharge chamber, thereby extending the pliant elastomeric material and enlarging the spark discharge material.

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8 A seismic wave generator as claimed in any preceding claim, wherein the spark discharge chamber is constructed in such a way as to generate a substantially uniform and omni-directional energy field.
9 A seismic wave generator as claimed in Claim 8, wherein the spark discharge is generated between a central electrode and a plurality of outer electrodes equiangularly spaced around the central electrode.
10. A seismic wave generator as claimed in any preceding claim, wherein the wavelength of the seismic wave can be set within a range of 0.2 to 5.0m.
11. A seismic wave generator as claimed in any preceding claim, wherein the spark discharge is generated by charging and then discharging a capacitor.
12. A seismic wave generator as claimed in Claim 11, wherein the variation in repetition rate is achieved by controlling the magnitude of the current available to charge that capacitor.
13. A seismic wave generator as claimed in any preceding claim, wherein the interval between sparks is between 2.5 and 30s.
14. A seismic wave generator as claimed in any preceding claim, wherein the control unit for the seismic source generator is housed remotely from the probe containing the spark discharge chamber, the probe and the control unit being connected by a high voltage cable.
15. A seismic wave generator as claimed in Claim 14, wherein the cable is greater than 100m in length.

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16. A seismic wave generator substantially as hereinbefore described and illustrated in accompanying Figures 1 to 9.