A New Technique for Acoustic Data Analysis

Abstract

INTRODUCTION A new technique for the spectral energy analysis of acoustic data taken during bottom and subbottom profiling surveys will be described. This technique is intended to be an additional tool for the evaluation and analysis of subbottom data. Single frequency acoustic systems [sonars] are widely used for bottom and subbottom profiling in shallow and deep waters. The system consists of a pulsed transmitter, a transducer or transducers for coupling acoustic energy into and out of the water, a receiver and a display such as a graphic recorder. A single transducer may be used as both the transmitter and receiver, or separate units can be employed. Transducers may be mounted to a ship's hull, or in towed vehicles [fish]. The echoes from the bottom and subbottom features are displayed on the recorder for immediate evaluation, and can also be recorded on magnetic tape for further processing. It is the purpose of the new analysis technique to extract additional information from these echo signals to allow more accurate identification of features and properties of the bottom and subbottom. This method, when used in conjunction with bottom sampling and coring operations, can establish the basic reference datums for a computer-aided automatic analysis system. When fully operational, this system with automatically identify subbottom materials and geologic features. The data display of conventional profiling recorders will be reviewed and then the basis of the new technique will be presented. The new display resulting from the energy analysis will be examined using acoustic signals from several sources, and the basic computer aided analysis system will be described. ACOUSTIC PROFILING Basic System Charts Classically, the data gathered from subbottom surveys is presented as a profile chart shown in Fig. 1. These records are interpreted and analyzed relying on prior experience in reading the records) and correlation with core borings is used to identify specific materials. The abscissa [horizontal axis] is the linear distance along the survey track and is a function of the speed of the transducer over the bottom. The ordinate is the time for the echo to return to the receiving transducer and is calibrated in depth [feet or fathoms] of the reflection below the transducer. The depth calibration is based on a sound velocity in water. The depth calibration is corrected for the two-way travel of the sound energy from transducer to bottom to transducer. The intensity of the echo is a function of both the reflecting material and the acoustic properties of adjacent strata. Therefore) in order to completely identify a particular subbottom layer) it is necessary to evaluate the echo from that layer of interest in conjunction with the echoes from overlying material. These echoes are qualitative indications of the acoustic parameters of the strata. The analysis and interpretation of these parameter must start from scratch, due to the lack of any established standards against which new data can be evaluated.