Low-frequency reverberation measurements with an activated towed array: Scattering strengths and statistics

Abstract

Results of reverberation measurements made with a low-frequency activated towed array sonar system in two deep-water locations of the Mediterranean Sea are presented. Measurements took

place under a variety of environmental conditions; the frequencies used, in the sub-kilohertz regions,

were in two bands around 370 and 740 Hz. Monostatic reverberation data were collected with a variety of waveforms (pulsed continuous wave and linear frequency modulated pulses) in order to obtain an adequate sample of the scattering function. The build-up of the data base has stressed data collection under possibly typical operational conditions for further performance analysis studies. Preliminary findings of the data analysis indicate that volume reverberation does not seem to be significant for this experimental set-up and that surface reverberation appears almost always overshadowed by seafloor scattering. Bottom scattering dominates returns through both the so-called fathometer effect and direct backscattering from the range of grazing angles. Scattering strengths appear independent of frequency and spectral histories show no significant doppler shift. Long-range returns are dominated by bottom highlights. Reverberation time series, which are highly nonstationary, can however be assumed to be locally covariance stationary, and logarithmic normalization techniques could usefully be applied.