Abstract:

An electric arc source was designed to study high-frequency seismic surface wave propagation. The noncontact nature of this source made it feasible for use in synthetic aperture transmit arrays. The transmit signal, which was not linearly controllable, was found to be predominantly in the 1- to 4-kHz band with a Gaussian spectrum. This is an octave below the simultaneously generated air acoustic pulse. The source was used to create a synthetic line array in conjunction with a stationary receiver. The experiment was conducted in a sand-filled tank. Surface wave speeds in the range of 80 m/s were measured on the resulting seismograms. Significant dispersion occurred in the propagating waveform at distances both near and far from the source. Surface wave arrivals were discernable over 1 m from the source and compressional head waves could be observed within 30 cm. The data were in good agreement with lower frequency measurements made by other techniques. The surface wave generation was studied and found to be a combination of the surface interaction of the arc and the air acoustic interaction. Strong hysteresis was observed in the first arcing event. Later, the signal was smaller but sufficiently stable for averaging. [Work supported by ARO.]