POWERFUL LOW-FREQUENCY VIBRATORS FOR ACTIVE SEISMOLOGY

Abstract

In the past two decades, active seismology studies in Russia have made use of powerful (40- and 100-ton) low-frequency vibrators. These sources create a force amplitude of up to 100 tons and function in the 1.5-3, 3-6, and 5-10 Hz frequency bands. The mobile versions of the vibrator have a force amplitude of 40 tons and a 6-12 Hz frequency band. Recording distances for the 100-ton vibrator are as large as 350 km, enabling the refracted waves to penetrate down to 50 km depths. Vibrator operation sessions are highly repeatable, having distinct ''summer'' or ''winter'' spectral patterns. A long profile of seismic records allows estimation of fault zone depths using changes in recorded spectra. Other applications include deep seismic profiling, seismic hazard mapping, structural testing, stress-induced anisotropy studies, seismic station calibration, and large-structure integrity testing. The theoretical description of the low-frequency vibrator is given in the appendices, which contain numerical examples.