Overthrust velocity models from refraction arrivals

Abstract

Accurate near surface velocity models can be derived from refraction arrivals on conventional field records (shot for reflection data) in complex overthrust areas. This is true even when the reflection image is poor and reflection based interval velocities are not practical. Refractions are easiest to interpret when the refracting surface is a buried erosional unconformity with overthrust layers subcropping along the unconformity. Velocity accuracy is about 10%. Velocity resolution is about 2%. Spacial resolution for locating the boundaries between velocity units is about 3 group locations. Velocities can be measured when the lateral extent of the formation along the refracting surface is as short as 500 m. Refractions are more difficult to interpret where thrusted units outcrop at the surface. In this case, the refracting surfaces are often the transitions between weathered, less weathered and non-weathered rock of the same formation. Velocity resolution is still about 5% but the spacial resolution is less than above. This paper presents refraction velocity models from both types of terrain, confirmed by well data obtained after the refraction analysis was completed.