A high-frequency sparker source for the borehole environment

For tomographic investigations of shallow subsurface features of limited lateral extent, a high-frequency, low-cost borehole seismic source would be highly desirable, particularly for investigators with limited budgets. We constructed a simple, arc-discharge seismic source from off-the-shelf items. This source consists of a salt water filled bottle containing exposed conductors of a coaxial cable, across which 100 to 300 joules of electrical power were discharged. This source produced a seismic pulse with a dominant frequency in the neighborhood of 1.5 kHz and a half-power bandwidth in excess of 1 kHz. Repeatability of seismic signatures in a variety of environmental settings was excellent. Sufficient power was generated to observe seismic signals with at least a 35 dB signal-to-noise ratio at horizontal borehole separations of 100 m. For a borehole separation of 33.2 m, signals with at least a 35 dB signal-to-noise ratio were observed at angular ranges in the vertical plane to 68 degrees. The hydrostatic head limit for this source was determined to be approximately 430 m.