Chirp subbottom profiler for quantitative sediment analysis

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.