Low frequency underwater sound propagation measurements west of the NOSC oceanographic tower

Abstract

Acoustic signals from 50 SUS charges detonated at ranges from 1 to 50 mi west of the NOSC tower were recorded in December, 1978 on both hydrophones and three-component geophones buried about 1 ft in the sediment and cable connected to the tower. An additional hydrophone was placed on the bottom. Both ground and water arrivals were observed for shots within the 400 fa curve (at ranges up to 10 miles) with only water arrivals noted for shots in deeper water of the San Diego trough. Propagation paths in the water were SR/BR (Surface Reflected/Bottom Reflected) throughout, with pronounced up-slope propagation near the receivers. Spectrum analysis of the various arrivals shows that the S/N ratios of the ground and water arrivals are similar below 20 Hz for shots within the 400 fa curve. The ground arrivals, however, have a longer duration (several seconds) than the water arrival (<1 s) because recordings were obtained of both compressional and shear waves refracted in the sediment and rock layers below the sea floor. [Work supported by ONR.]