Sonar research conducted during the period 1 April - 30 June 1961

Abstract

This is a progress report of the Institution's research between April and 30 June 1961 supported under Contract NObsr-72521 with the Bureau of Ships, Navy Department. The program consists of studies of compressional wave transmission (sound and seismic waves) through sea water and the underlying earth's crust, the ambient noise of the ocean, and the scattering of sound in the sea and beneath the ocean floor. During the present reporting period sound transmission studies have included laboratory analysis of data taken previously in a continuing program directed toward describing near-surface transmission into the shadow zone, and toward accounting for the variability of near-surface . transmission under nominally "good" sonar conditions . Seismic studies consisted of analysis of seismic refraction data taken two years ago in: the Mediterranean, completion of a report of a seismic reflection study of the southern part of Narragansett Bay, extension of this latter study into Rhode Island Sound, and a series of new measurements of the apparent reflectivity of the bottom at 12 kcps. In the sound-scattering program we accepted delivery of a small towed fish containing sound gear for scattering experiments over near-horizontal paths. This instrument was taken to sea on CHAIN for initial testing and self-noise measurements. A series of tests of new sound sources, supported under Contract Nonr-1367, have demonstrated that seismic reflection observations can be made in deep ocean basins from a ship underway slowly using only such electrically-powered sources as Thumper or Sparker. Sub-bottom structures have been mapped in fine detail experimentally in water over 2600 fathoms deep. This work was done with a Thumper having a 5000-joule electrical input . Plans have been made for increasing the electrical input of Thumper to about 13, 000 joules and Sparker to 25, 000 joules. We plan to use these new sound sources in sound transmission experiments this summer and fall in the Mediterranean Sea. As an extra-curricular hobby Miss Broughton, a technical assistant at the Institution, made interesting magnetic tape recordings of the sounds of a small pilot whale stranded on a local beach. She held microphones on the whale's head near the blow hole. The recorded sounds are rather different from those heard through the water near pilot whales. These and other parts of our work are detailed below.