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Shear-Wave Seismic Refraction

Overview

This method is a variation of the seismic refraction method. A hammer blow generates a shock wave which travels through the earth by refraction along material boundaries. The energy received at the surface (by an array of sensors or geophones) is analyzed for structure and velocity. The energy source and the receivers are arranged to record seismic energy traveling as SH waves, that is, particle motion parallel to the surface.

Applications

Using geophysics for geotechnical evaluation early in the infrastructure planning and engineering design process — before the 50% design level — can pay large dividends in public safety and reduced risk. Surface-wave seismic techniques, in particular, can provide appreciable cost and time savings over borehole methods for UBC/IBC site classification. The combined use of active and passive surface-wave techniques in urban areas can reduce the need for costly, heavy equipment such as bulldozers, electromechanical shakers and large weight drops.

Geologic Model

Geologic materials which increase in velocity with depth. Layered, nonplanar boundaries between strata of differing velocities are sensed.

Requirements

A. A reversible source:
sledge hammer and vehicle-weighted plank
person-weighted mechanical device

B. Low ambient noise: stay away from:
all-night gravel processors
interstate highways
busy airports

C. Surface access:
offset shot away from ends of lines
geophone plants (can be done on snow or pavement)

D. Ground truth:
drill holes
well logs
outcrops
water table location

Considerations

Low velocity layers within higher velocity milieu are not detected. Water table location probably will not be detected. High-speed stringers may be mistaken for bedrock. Possible ambiguity: velocity vs. structure tradeoff. S-wave sources are not as powerful as P-wave sources and one does not have explosives to fall back on as the ultimate source.

Logistics

Crew size usually 2 or 3 people. Portable (no vehicles) if source can be carried. Line location and actual elevations by surveying or GPS.

Deliverables

Plan maps, travel-time curves, seismic cross-sections, geologic interpretation of seismic cross-sections and narrative description of work done.

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