Seismic Refraction (M-1)
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Q: How does it work?
A: A hammer blow or explosive charge (the shot) 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.
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Q: What is the geologic model?
A: Geologic materials which increase in velocity with depth. Layered and non-planar boundaries between strata of differing velocities are sensed.
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Q: What are the requirements?
A: A reasonable source:
sledge hammer and metal plate OR
accelerated weight drop OR
explosives OR
vehicle-weighted plank and sledge hammer
Low ambient noise: Stay Away From:
all-night gravel processors
interstate highways
busy airports
Surface access:
offset shots beyond ends of lines
geophone emplacements (pavement, snow, etc.)
Ground truth:
drill holes
well logs
outcrops
water table location
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Q: What are the pitfalls?
A: Low velocity layers within higher velocity milieu are not detected.
Water table location may not be detected.
High speed stringers may be mistaken for bedrock.
Ambiguity: velocity vs. structure tradeoff.
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Q: What logistics are needed?
A: Crew size usually 2 or 3 people.
Portable(no vehicles) if source can be carried.
Line location and actual elevations by surveying or GPS.
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Q: What are the deliverables?
A: Plan maps, travel-time curves, seismic cross-sections, geologic interpretation of seismic cross-sections and narrative description of work done.
