Refraction velocity sections: An aid in shallow reflection interpretation

Refraction velocity sections, made by plotting large numbers of depth and velocity solutions from refraction analysis of shallow reflection data and then contouring them, have proven to be useful aids in interpreting shallow reflection data. Two examples are given. One is a reflection line recorded across a modern earth fissure in south-central New Mexico. The second is a reflection line recorded across a major step fault on the eastern margin of the Rio Grande Rift in central New Mexico. The first line shows that the fissure is located over a shallow graben, but does not show clearly whether the graben reaches to the surface. The refraction velocity section, however, shows that the graben does extend nearly to the surface. The second reflection line reveals several possible faults, but does not indicate which are the faults which bring the Paleozoic strata to shallow depths. The refraction velocity section does show which of these faults bring bedrock near the surface, and thus are the major faults of the fault zone.