Modified sonar transducers have been successfully used in water-covered areas to determine depths to the water bottom, layers within the bottom sediments, and depth to bedrock (Smith and Nichols, 1953). The possibility of detecting shallow horizontal beds by placing a sonar transducer directly on the ground is particularly attractive. Velocity discontinuities at depths of several hundred or more feet have long been successfully mapped by means of conventional seismic reflection methods. Recently, reflections from much shallower, closely spaced horizons have been recorded by increasing the filtered frequency range of the amplifiers to an upper limit of 300 to 500 cycles per second, and by other instrumental modifications (Pakiser, Mabey, and Warrick, 1954; Olson, 1955). Evison has recorded still shallower reflections by using an electromechanical vibrator that generates a square-enveloped pulse of sine waves at frequencies of 100 to 800 cycles per second and pulse widths of from 5 to 100 milliseconds (Evison, 1954).

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