Shear Wave Seismic Refraction (M-8)

1. Q: How does it work?
   A: This method is a variation of the seismic refraction method (above) and that discussion is repeated here. 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.
   

2. Q: What is the geologic model?
   A: Geologic materials which increase in velocity with depth. Layered, nonplanar boundaries between strata of differing velocities are sensed.
   

3. Q: What are the requirements?
   A: A reversible source:
   sledge hammer and vehicle-weighted plank
   person-weighted mechanical device
   
   Low ambient noise: Stay Away From:
   all-night gravel processors
   interstate highways
   busy airports
   
   Surface access:
   offset shot away from ends of lines
   geophone plants (can be done on snow or pavement)
   
   Ground truth:
   drill holes
   well logs
   outcrops
   water table location
   
4. Q: What are the pitfalls?
   A: 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
   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.
   
5. 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
   
6. 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
   
7. More detailed information.