Seismic Refraction Seismic refraction methods use seismic energy sourcesgenerated from the ground surface to transmitgenerated from the ground surface to transmit“refracted waves” along higher velocity subsurfaceinterfaces. According to Snell’s Law these refractionevents occur at higher velocity interfaces when theangle of incidence of incoming seismic raypaths reaches“critical incidence” (shown on the left) As the refractedcritical incidence (shown on the left). As the refracted“headwave” moves along this interface its vibrationssend back seismic waves through the overlying layer.These returning refracted waves are recorded on theground surface by numerous geophones. The refractionsurvey initiates the energy source at several pointssurvey initiates the energy source at several pointsalong a line of geophones to record a set of “fieldrecords” showing vibration‐amplitude versus time ateach geophone position. The direct and refracted wavesare the “first arrivals” on these field records. Their timeis picked and used to construct “travel time curves” foris picked and used to construct travel time curves forthe refraction survey. These curves are used incomputer programs (such as RAYFRACT or IXRefraX) togenerate a two‐dimensional model of subsurfacevelocity structure (refraction velocity profile).

The refraction velocity profiles can be used to imagevelocity variations due to shallow bedrock structure andfaulting, and provide velocity information for theevaluation of rippability and conversion of reflectionti fil t d th fil S i h

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time profiles to depth profiles. Surveys using a shear‐wave energy sources can also be used to generateseismic shear‐wave velocity profiles.

Seismic Refraction Velocity Profile Generated Using the “Generalized Reciprocal Method” (GRM) for 3‐Layer Earth Model with Computer Program GREMIX (IXRefracX)

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Seismic Refraction Tomography Velocity ProfileGenerated Using Computer Program RAYFRACT

Shows Estimated Range of Bedrock Rippability for Caterpiller EquipmentShows Estimated Range of Bedrock Rippability for Caterpiller Equipment

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