Seismic Reflection
Overview
A hammer blow or explosive charge (the shot) generates a shock wave which travels through the earth and is reflected by disconti-nuities in earth materials. Energy received at the surface (by an array of sensors or geophones) is analyzed for the location of the reflecting interfaces and the velocity of the materials between them.
Geologic Model
Layered geologic strata with abrupt velocity and/or density changes at the boundaries. Non-planar boundaries between homogeneous layers.
Requirements
A. A reasonable source:
sledge hammer and metal plate
accelerated weight drop
controlled vibration source
explosives
B. Low ambient noise: stay away from:
all-night gravel processors
interstate highways
busy airports
surface access offset shots at ends of lines
geophone placements
homogeneous surface
C. Ground truth:
drill holes
well logs
outcrops
water table location
D. Favorable geometry:
isolated interface of interest
sufficient depth
sufficient frequency content
E. Experienced processor:
experience in processing and interpreting shallow seismic as opposed to deep seismic for petroleum exploration
Considerations
For near surface (<100m) it is difficult (not impossible, difficult) to generate high enough frequencies to discriminate useful targets. The velocities are poorly determined, though the relative placement of the interfaces is superb. Multiples and refractions can be mistaken for real interfaces.
Logistics
Crew size is generally 3-5 persons. Larger sources and multiple sets of equipment are usually employed to stack the data and improve quality. Driving the line is the rule rather than the exception. Line location and actual site elevations are done by surveying or GPS.
Deliverables
Plan maps of line locations, a flow chart of the processing scheme, seismic cross-sections in time and depth, a geologic interpretation of the work done.