An instrument is described for use in seismic investigation of subsurface structure to a maximum depth of about 100 feet. A hammer is used as source of seismic energy. A cathode-ray tube replaces the usual galvanometers as display device, and seismic arrivals are timed electronically by a modified radar technique. Only one seismometer is used, and travel times for different ranges are obtained by moving the hammer along the traverse line. Applications will lie mainly in the field of civil engineering. Field tests made at a dam site are compared with direct borehole tests of the subsurface structure.