High-Resolution Seabed Mapping: New Developments

Abstract

ABSTRACT This paper describes several new developments in the evolution of side scan sonar sea bed mapping systems. A new advanced microprocessor-controlled tuning improves record quality. New deep tow vehicles allow for relatively high speed medium depth towing or low speed full ocean depth towing. Detailed bottom topography can be overlaid on the side scan sonar/sub-bottom profiler data to enhance record interpretation. INTRODUCTION In the past few years, side scan sonar systems have evolved into sophisticated sea bed mapping systems. Microprocessor technology has been used to provide two-axis scale correction of the sonar data in real time. Low frequency systems are available which can cover swaths of many kilometers with low resolution, and medium and high frequency systems are available which cover smaller areas in more detail. Corrected strips of sonar data are placed side by side to produce a fully corrected sonar map of the sea bed. Figure 1 shows a sea-bed mosaic made with a 50 kHz side scan sonar. Because these sonar maps resemble aerial photographs, they are far easier to interpret and utilize than the older, uncorrected sonar data. As the sonar mapping technique gains increased acceptance, new developments are continuing to improve the quality of the maps and the efficiency of data gathering. Figure 2 shows the shipboard portion of a typical sea bed mapping system. The system allows the sonar records to be annotated with data such as event mark numbers, navigation coordinates, time, date, etc. in order to facilitate assembly of sonar mosaics. A keyboard allows manual entry of additional data. Parallel or serial (RS-232) ports are available to accept navigational and other data from external sources. ADVANCED HANDS-OFF-TUNING® Side scan sonar system designers have always been faced with a difficult situation regarding the dynamic range of the sonar echo signals. During the course of a sonar sweep, the signals may vary in level over a wide range, on the order of 120 db. The hard copy graphic recording media however, typically only have a dynamic range on the order of 20 to 40 db. A variety of automatic gain control systems and time variable gain systems have been developed to deal with the problem. Manually tuned time variable gain controls allow the operator adjust the gain curves to produce reasonably uniform sonar records under most conditions. Unfortunately, these systems tend to need relatively skilled operators to achieve optimum results. Other systems have been developed which set up a fixed time variable gain curve based on theoretical calculations of the various factors which affect the sonar signal. These are called predictive systems. The factors taken into considerations include the sonar spreading losses, attenuation losses and the bottom backscatter functions. Some systems also include a calculation for the vertical beam angle of the sonar transducers.