Acoustic Surveying of Ultra-Shallow Water Bottoms (&lt; 2.0 m) for Both Engineering, and Environmental Applications
Authors: Roberts H.H. , Wilson C. , Supan J. 
Published: October 2008

This paper presents the methodologies, instrumentation platform constraints, and post-acquision processing techniques for producing high quality acoustic data from the surface and shallow subsurface of very shallow water settings (&lt;2.0 m deep) typical of the bays and lakes common to the coastal areas of the northern and western Gulf of Mexico coastal zone. The instrument suites, deployment strategies, and acquision-processing software programs have recently been tested in central Barataria Bay (Louisiana) where water depths average &lt;1.5m.

Results of the first major data collection project have demonstrated that excellent acoustic images of the bay bottom and shallow subsurface can be acquired, calibrated to sediment properties and surface reflectance characteristics, and quantified using image-processing software. The study was conducted in two phases:the digital acoustic data acquisition phase using side-scan sonar and chrip sonar as the acoustic sources anda field verification phase that included surface sediment sampling, quadrate studies, vibracoring, and penetrometer profiling.

Results of this project support the use of this technology for ultra-shallow water applications. For oyster bed assessments, the objective of the initial project results indicate an extremely close tie between surface reflectance strengths and scattered surface shell, oyster reef, burrowed mud bottom, and other bottom types. Enhanced differential GPS location data provided real-time submeter accuracy to all survey and ground truth data. Image processing of acoustic data proved useful for quantifying reflectance patterns and relating these data to various bottom types ranging between oyster reefs to smooth mud bottoms. Disturbances of the bottom such as drag marks, pipeline canals, dredge spoil, and man-made objects were clearly visible in our data sets.

These newly developed procedures for acquiring high quality acoustic data in ultra-shallow water can be used for planning industrial operations to minimize environmental damage and choosing installation sites for industrial infrastructure that provide the best geologic and biologic conditions. These data are also valuable for providing accurate and objective damage assessments associated with environmental litigation.

Introduction

The continued success of offshore hydrocarbon exploration and production around the world has created a complex infrastructure of production facilities and especially pipelines that, in many cases, directly impact shallow water and low relief coastal areas. In the northern Gulf of Mexico alone there are over 4000 offshore oil and gas-related structures and their associated pipelines. Many of these pipelines come ashore across coastal bays and marshlands where they join other pipelines and production facilities associated with onshore fields found in the coastal zone. With renewed production in coastal and onshore areas stimulated by newly acquired 3D-seismic data, the need for high resolution seismic and side-scan sonar surveys of ultra-shallow water bottoms has increased. This need arises from the engineering sector for planning pipeline routes and production facility sites as well as for inventorying existing infrastructure in these shallow water areas where instrumentation may not work well and standard survey vessels cannot travel. A need for data collection in this setting also arises from legal pressures to develop better environmental assessment methodologies.