High Resolution Near Sub-Bottom Profiling
Authors: Chramiec M.A. , Morton R.W. 
Published: October 2008

The wide dynamic range of acoustic signals reflected from the various ocean bottoms, coupled with reverberations and a noisy environment, make the problem of receiving, processing, and displaying these signals difficult. Application of real time correlation processing, where a large bandwidth/long duration signal is transmitted, stored, and used as a reference replica for returning echoes, improves results in several ways. It makes maximum use of the high resolution inherently available in the large signal bandwidth and the high total energy contained in the long duration signal. It also provides a display which is relatively noise free since the particular characteristics of the transmitted signal are normally not present in the interfering noise, and therefore do not correlate with the stored replica. Furthermore, reverberations a used by various sub-bottoms do not correlate with the stored replica, consequently closely spaced layers are not obscured by reverberations from those adjacent. The development of a correlation processor for use with medium and low frequency survey sonars, the experimental verification of design parameters, and some sub-bottom profiles obtained at 3.5 KHz are described in this paper. Results indicate that the correlation technique should prove valuable in the processing of near subbottom signals, and should also improve the ability of survey sonars to provide depth data which can be readily digitized.

INTRODUCTIQN

Standard near sub-bottom profiling sonars require higher energies for deeper penetration and shorter pulses for better resolution. Since most systems are limited in peak power by either transducer cavitation or transmitter output, higher energies (power -x time) can only be achieved through increasing pulse length, which consequently degrades resolution. Furthermore, higher transmitter powers cause increased bottom reverberations, which tend to obscure closely spaced bottom layers. A conflict therefore exists in simultaneously attempting to improve the sub-bottom penetration and the resolution characteristics of standard systems.

A signal processing technique known as replica correlation resolves the apparent conflict between higher energies at shorter pulses desirable in sub-bottom profiling sonars. In addition, it provides improved immunity to ever present environmental noise. Finally, it delivers an output signal which is well matched to the standard output display, a line scan paper recorder.

A correlation echo sounder processor, referred to as CESP, has been develope dat Raytheon's Submarine Signal Division. Experiments have been conducted at 3.5KHz and 12 KHz in shallow New England Continental Shelf waters and the deeper waters of the southwest Caribbean. The development, experimental verifications, and some near sub-bottom profiling results taken over a wide range of conditions are presented in the following paper.

PRINCIPLES OF OPERATION

Correlation signal processing is based on the principle that time and frequency are equivalent, and that operations carried on in the time domain can as well be carried on in the frequency domain. Figure 1 shows two examples of this equivalence, with time functions on the left and their corresponding frequency functions on the right. In the upper left, a one millisecond long rectangular pulse is shown. To the right, the spectrum of frequencies representing the same pulse are presented. In the lower part of the figure we start with the rectangular frequency spectrum on the right, and transform it to the time function on the lower left.