Geophysics - Seismic Refraction and Reflection

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Seismic techniques are useful for determining velocity contrasts in the subsurface.  Seismic velocity is the speed at which a surface generated p-wave or s-wave travels through soil and rock.  Seismic velocity correlates well with the rock hardness and density, and these in turn correlate with changes in lithology, fracturing, faulting, degree of weathering etc.  Seismic techniques, when applied correctly, can image the subsurface where traditional geophysical techniques fall over and fail.
The Refraction seismic technique is ideally suited for depth to bedrock determination e.g. finding palaeochannels and shallow environmental and engineering applications.  It is best suited where slow velocity layers overlay faster velocity layers and depth of investigation is usually limited to 100-150m (or 4 x the Source-Receiver Offset distance). The method is based on the fact that when a wave reaches a boundary between two layers having different seismic velocities, that seismic wave will be refracted (or bent) either toward the normal to the interface or away from the normal to the interface, depending on whether the velocity increases or decreases at the boundary. In the special case where layer velocity increases with depth at the boundary, critical refraction occurs where seismic waves travel along the interface between the two materials. The angle at which the seismic waves are critically refracted (the critical angle) is uniquely determined by the ratio of the velocities of the two materials: θc= sin-1(V1/V2), and because the critical angle is uniquely determined, the depth at which the boundary between the layers occurs can be calculated using geometry and the measured first arrival travel times.
For more complex geology, Reflection is superior, giving the ability to image subsurface layers and complex structures to significant depths (+500m).   The seismic reflection method is based on the fact that when a wave reaches a boundary between two materials having different acoustic impedances (product of velocity and density) that wave will be reflected back to the surface. The angle at which the seismic waves are reflected is determined by the angle of incidence of the waves.
GEOEARTH uses a very low impact, skid steer operated accelerated drop hammer for a seismic source. the seismic refraction utilizes acoustic waves generated by an impact or small explosive source to measure depths to bedrock or overburden layers of sedimentary rock and to infer bedrock faults or fracture zones.
•Seismic response are plotted through distance along the geophone array to identify individual layers, and to compute layer thicknesses and seismic velocities.
•Specific geologic conditions, such as bedrock fractures or valleys, may be interpreted directly from these time-distance plots or by using several seismic modeling techniques.
•Low-velocity zones and thin strata may be undetected using older interpretive methods, such as the crossover distance technique.
Geophysical Applications can use SeisOpt2D velocity modeling to identify these "hidden layer" conditions. SeisOpt2D also provides a means of quantitatively evaluating lateral seismic velocity variations that can represent lithologic contacts or fracture zones. Geophysical Application's software can also perform forward modeling to design geophone and shotpoint spacings needed to achieve specific survey objectives. 


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