Measurements of seabed damping coefficients from head waves in shallow water

Abstract

Acoustic signals from small explosive charges have been measured with sonobuoys on several tracks in shallow water. Using the frequency band from 14 to 70 Hz, travel times of head waves were monitored, and the sound speeds and depths of corresponding interfaces in the seabed were derived. Head waves from interfaces indistinguishable from the seafloor (the water/seabed interface) were detected on only two of the tracks, with sound speeds around 2300 m/s. The amount of data obtained for the seafloor was limited by incoherence of the signals and, for some tracks, by inappropriate experimental method. Range dependencies of head wave energy in the 25-Hz third-octave band were measured on four tracks whose seafloor depths ranged from 60 to 100 m. These interfaces were between 0 and 220 m beneath the seafloor, and their sound speeds ranged between 2000 and 2400 m/s. Results for damping coefficient range from 0.4 to 0.8 dB/cycle, with standard errors of around 0.1 dB/cycle. Damping in the deepest interface (whose sound speed was 2200 m/s) was measured at six frequencies from 16 to 63 Hz, and over this band there is a weak tendency for the damping per cycle to decrease with frequency.