Abstract

A wide-band, frequency-modulated, subbottom profiling system (the chirp sonar) can remotely determine the acoustic attenuation of ocean sediments and produce artifact-free sediment profiles in real time. The chirp sonar is controlled by a minicomputer which performs analog-to-digital and digital-to-analog conversion, correlation processing, and attenuation estimation in real time. The minicomputer generates an FM pulse that is phase- and amplitude-compensated to correct for the sonar system response. Such precise waveform control helps suppress correlation noise and source ringing. The chirp sonar, which has an effective bandwidth of 5 kHz, can generate chirp (Klauder) wavelets with a tuning thickness (Rayleigh's criterion for resolution) of approximately 0.1 ms. After each return is correlated, a computationally fast algorithm estimates the attenuation of subbottom reflections by waveform matching with a theoretically attenuated waveform. This algorithm obtains an attenuation estimate by minimizing the mean-square error between the autocorrelation function of the theoretically attenuated wavelet and the autocorrelation function of the subbottom reflection.The chirp sonar was tested in Narragansett Bay, R.I. along a line that had been previously cored. Experimental results show that correlation noise from the seafloor reflection was below -60 dB, the quantization noise level, thereby allowing detection of small subbottom impedance contrasts and accurate estimation of attenuation. Attenuation coefficient estimates from this sandy region agree with in-situ measurements made by other investigators.

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