A high‐frequency sparker source for the borehole environment

Abstract

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.