Deep Downhole Seismic Testing Using a Hydraulically-Operated, Controlled-Waveform Vibroseis

Abstract eng:
Downhole seismic testing is one field test that is commonly used to determine constrained compression-wave (P) and shearwave (S) velocity profiles in geotechnical earthquake engineering investigations. Traditional downhole testing has generally involved profiling in the 30- to 200-m depth range using hand-operated or small, mechanically-assisted sources. As the number of field investigations at locations with critical facilities has increased, profiling depths have also increased. An improved downhole test for P- and S-wave velocity profiling to depths exceeding 400 m is presented. The improvements include: (1) a more powerful source, (2) generation of simple sinusoidal waveforms, (3) the ability to “tune” the sinusoidal waveform to site conditions and (4) high-fidelity, post-processing of the time-domain records to increase signal-to-noise ratios at deeper depths. The seismic source is a large, hydraulically-operated, triaxial vibroseis named T-Rex that generates both P (vertically shaking) and S (horizontal shaking) waves. The test procedure and signal processing are discussed. Examples of raw and processed time-domain records, P- and S-wave travel-time plots, and interpreted wave velocity profiles measured to a depth of 415 m in one borehole are shown. Comparisons are also made with traditional downhole testing performed to a depth of 183 m in the same borehole.

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