Seismic source

Details Seismic source Seismic source

1991-11-15

1994-04-05

Eldred, J. Woodrow

Communications, electrical: acoustic wave systems and devices

Signal transducers

Underwater type

181106, 181110, G01V 140

Patent

active

053011694

DESCRIPTION:

BRIEF SUMMARY
This invention relates to an apparatus for generating seismic waves, operable within a borehole.
Information about geological strata and structures can be obtained by causing seismic waves to propagate through the region of interest and sensing the times of arrival of those waves at different locations. Such a procedure enables the position and shape of seismic reflectors, which often are boundaries between different rocks, to be discovered. The amplitudes of the received waves can also provide information on the degree of attenuation caused by the rocks, which is dependent on the type of rock, and on the frequency of the seismic waves.
According to the present invention there is provided an apparatus for generating seismic waves operable in a borehole, the apparatus comprising a module and a cable to support the module within the borehole, the cable incorporating a conductor to provide electric current to the module, the module incorporating capacitor means, means for charging the capacitor means to a high voltage, a spark gap, and means for connecting the spark gap to the capacitor means, the spark gap comprising two electrodes separated by a gap, which gap in use is occupied by liquid acoustically coupled to the wall of the borehole, such that during operation sparks are created which extend from one electrode to the other, and pressure waves are thereby generated, characterized in that the capacitance and the voltage are such that the stored electrical energy is more than 1 kJ, the connecting means comprises a triggered arc gap, and the electrodes of the spark gap are of such a shape that the gap across which the sparks are to be created is of substantially uniform width, the width having a value between 5 and 20 mm, and such that the spark-creating current is concentrated in said gap, and the surfaces of both the electrodes are covered by an insulating layer except at those positions where the sparks are to be created.
Preferably the spark gap is enclosed by a rubber sleeve, the sleeve being filled with clean saline water, there being a pressure equalising duct connecting the water within the sleeve to the liquid in the borehole; the rubber sleeve is substantially transparent to the acoustic waves so that the saline water within the sleeve and the liquid in the borehole are acoustically continuous. The preferred arc gap is a tritium-filled triggered arc gap; when a 20 kV trigger pulse is provided to the arc gap an avalanche is initiated so the arc gap conducts electricity. This may be triggered whenever the capacitor means are almost fully charged.
The apparatus can operate at considerable depth in a borehole, for example at hydrostatic pressure up to 30 MPa and at a temperature up to 125.degree. C. (if the borehole is filled with pure water this is equivalent to a depth of up to 3 km). When the charged capacitor means is electrically connected to the spark gap, a spark is created in the gap which heats the liquid in the gap to form a vapour bubble; this occurs rapidly, creating a pressure wavefront which propagates through the liquid around the spark gap and the borehole liquid to the borehole wall. There seismic waves are generated in the surrounding rock. The seismic waves have a wide frequency range between about 200 Hz and 10 kHz; the predominant frequencies may be between about 5 and 6 kHz, or between 1 and 3 kHz, or between 200 Hz and 1.8 kHz, depending on the design of the spark gap.
One factor affecting the frequency range is the width of the gap between the two electrodes, the frequencies being lower for a larger gap. The electrode insulation lowers the capacitance of the electrodes, so lowering the frequency range, and also prevents the occurrence of multiple current paths which could suppress sparking altogether.
In one embodiment the spark gap is a cage structure comprising a central rod electrode and three inter-connected side electrodes extending parallel to the rod electrode and equally spaced around it, each side electrode being shaped so that over a portion of its length it is separa

REFERENCES:
patent: 3286226 (1966-11-01), Kearsley et al.
patent: 3700169 (1972-10-01), Naydan
patent: 4039042 (1977-08-01), Edwards et al.
patent: 4076980 (1978-02-01), Arnold et al.
patent: 4651311 (1987-03-01), Owen et al.
patent: 4715376 (1987-12-01), Nowacki et al.
"Electrohydraulics," Science Journal, Mar., 1968, pp. 61-66.

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