The Sub-bottom Depth Recorder (SDR) has been developed to study the underlying geologic structure of water-covered areas. The SDR is essentially a broad-band, high-powered echo sounder which utilizes 1 of 2 sound sources. The first, an electrical spark discharge (Sparker), has produced penetrations in excess of 600 ft. in 50 ft. of water. The second, a combustion chamber using a mixture of propane and oxygen (RASS), has produced penetrations in excess of 1,400 ft. in 80 ft. of water. Results obtained from the SDR have been found to be in excellent agreement with test boring and seismic refraction data.