Sound propagation using impulsive sources on the New Jersey Continental Shelf

Abstract

An acoustic experiment was performed on the New Jersey Continental Shelf in a well surveyed area with known bottom and sub-bottom properties. The measurement system was a calibrated vertical array 14–18 km from a known geological feature. Calibrated airgun shots (AG) and special small omni-directional explosives shots (SO) were used to generate impulsive sounds monitored by a calibrated source hydrophone. The repeatability of the SO and AG shots was excellent. The sea state during this experiment was between 4 and 6 on the Beaufort scale and the ship was held in position on either side of the geological feature. The sound velocity profiles were stable over the measurement period due to the fully developed seas. A mixed layer depth of approximately 20–30 m was followed by a steep thermocline to a depth of 48 m with a gradient of approximately 0.5 sec and a relatively isothermal layer to the bottom which ranges between 70 m at the source location and 80 m at the receiving array. Measurements received on the vertical array were variable but over short periods of time remarkably consistent. Time of flight enabled the separation of the first three modes whose shape and arrival times were to agree with calculations. Comparisons of the arrival structure changes due to the feature and estimates of the backscatter from this feature are presented in relation to the mode arrival statistics for different groups of shots.