During an investigation of alluvium [in Wyoming] using a shallow refraction seismograph, time-distance curves obtained indicated two refraction interfaces. The shallower of the interfaces is the ground-water table, whose depth was obtained with an accuracy of + or -10 percent. The deeper interface is either the surface of the bedrock or the base of the weathered zone on the bedrock. If this lower interface is the surface of the bedrock, the velocity of seismic waves in the weathered material will be considerably lower than if the interface is the base of the weathered zone. Using this criteria makes it possible to determine whether the seismic wave is refracted from the surface of the bedrock or the base of the weathered zone.