Recent Developments in Side Scan Sonar and Applications in the Offshore Oil Field
Authors: Henderson Robert F. 
Published: October 2008

The side scan is used for site surveys, inspection of pipelines, antiquities surveys and wellhead relocation. To improve its usefulness, the following have been developed recently:new amplifier circuit to improve data presentation and reduce need for operator adjustments of controls,new tow fish design to improve recovery possibility when it snags the bottom or a submerged object, andcombination of low cost fairing and lightweight efficient depressor to significantly improve tow depth for a given length of cable.

INTRODUCTION

Prior to 1967, side scan sonar use was limited mostly to scientific investigation and recovery of lost aircraft primarily because there were no commercially available systems costing less than $ 100,000, except for ineffective converted echo sounders. In that year, EG&G introduced its Mark I System at a price of $ 20,000, and commercial use developed rapidly thereafter. Today, side scan sonar is considered a practical useful tool for seafloor engineering, and its use is world wide. Specific applications in the oil patch include site surveys, pipeline inspections, antiquities surveys and wellhead relocation. Whenever a structure is to be placed on the ocean floor, it is imperative to know the nature of the bottom in the chosen site. For example, before installing a pipeline from an offshore production area to land, a survey must be made to select the best route. Similar to laying out a proposed highway on land, the best route avoids steep slopes and rocky ledges. To map the area, a number of sounding lines can be run criss-crossing it. These lines must be run very close together if much detail is to be achieved. A more cost-effective method to obtain this information is to use side scan sonar. Again, after the pipeline has been installed, its exact location can be readily determined by side scan. How does the side scan sonar provide this picture of the ocean bottom? Think of it as a camera system which uses sound instead of light rays to create a picture line by line on a continuous strip of paper. The sound emanates from a towed body ? known as the fish ? in a fan shaped beam which is very narrow in the horizontal plane and sufficiently broad in the vertical plane to ensure that the energy reaches the bottom from directly under the fish to the full range of the system (Figure 1). The sound is reflected by the bottom and any objects on the bottom, and some portion is returned to sound receivers in the fish. The returned energy is amplified and presented on a graphic recorder which draws a single line for every transmission of sound energy. Variations in strength of the returned signals are represented by making this line darker for strong signals and lighter for weak signals. The sweep is linear with time, hence the distance to the point from which a particular echo is received is indicated by its relative position along the sweep line. As the fish is towed through the water, it transmits and receives sound for a series of lines representing the bottom over which it is passing (Figure 2).