A high-frequency downhole sparker sound source for crosswell seismic surveying

Abstract

Traditionally seismic exploration techniques have not been widely used in hard rock mining and exploration environments. To facilitate faster and more accurate shallow high-resolution seismic surveys a high-frequency electric discharge (sparker) seismic source was developed. The main design requirements of this source were that it (1) achieve rapid downhole firing, (2) fit down surface diamond drill holes (70 mm diameter), (3) provide similar energy level and frequency content to a seismic detonator so as to produce useful sharp signals over a distance up to 200 meters, and (4) it should he highly repeatable and reliable. The sparker system consists of a surface control and' power source, winch cable and depth encoder, and a downhole probe. The 22 kg downhole probe, which has a diameter of 60 mm and leneth of approximately 3m, produces a discharge energy output of 480 J. The innovation with this sparker is that the 60 mF 5000 V discharge capacitor is contained within the downhole probe, thus overcoming the problem of significant power loss through the high resistance of the cable and the effects of inductance experienced by other surface capacitor arc discharge sources. The sparker described does not include a discharge chamber, hut instead relies on the saline conditions of a fluid filled borehole to enable explosive plasma bubble formation to occur. Initial field tests in the hard rock environment of Kambalda Western Australia provided some very encouraging results. The ability of the sparker to operate in a borehole under hydrostatic heads in excess of 250m and to produce clearly received signal at distances in excess of l00 m was demonstrated. The output signal of a single shot of the sparker compared very favourahly with that produced by a seismic detonator under the same conditions. The firing rate in the field was found to be approximately 5 shots per minute. Repeatability of the signal was excellent.