Equipment for In Situ Measurements of Sound Velocity and Absorption in Sea Floor Sediments

Abstract

With the intent of making in situ measurements of sound velocity and absorption in deep sea sediments from the bathyscaph Trieste, equipment for this purpose, useful also in shallow water, has been developed and field tested. Three probes, each containing two transducers 0.5 m apart, are fastened to a rigid beam for simultaneous insertion into the sediment. The receiving probes lie 1 and 2 m distant from the source probe. Pulse-actuated magnetostrictive transducers are used as sound sources, and barium titanate cylinders as receivers. Measurements at two depths in the sediment are possible by using upper or lower transducers. The travel time interval between reception of the transmitted pulse at the near and far receivers is measured on a decade scaler counting a precision oscillator. Oscilloscope presentation of the signals arriving at the two receivers along with the time of gating assures the operator of satisfactory operation and aids in setting certain controls. A frequency range between 7 kc/sec and 30 kc/sec is used, with filtering provided in the receiving circuit to allow measurements in a number of frequency bands. Absorption measurements are made by matching superimposed oscilloscope traces of the signals from the near and far receivers by means of precision bridge T attenuators in two matched amplifier channels.