INTRODUCTION

Single-beam echo-sounder sidescan sonar is currently the main sensor type being used for seabed mapping and surveying. In the future, new developments within high-resolution wide-swathe covering sensors will be available, and will to a large extent replace existing instrumentation.

A wide-swathe bathymetric system is described, viz. the Multibeam Echo-sounder EM 100. The first part of this paper presents ideas about how new developments within high-resolution swathe covering systems will influence instrumentation for seabed mapping and surveying in the future. The second part described the Multibeam Echo-sounder.

Seabed Mapping And Surveying With Existing Instrumentation

The current seabed mapping technique is based on the use of the single-beam echo-sounder with sidescan sonar for visual inspection between the sounding lines.

The echo-sounder provides a high-density sampling of the topography along survey lines and no information of the seabed between the lines. For detailed mapping a line density down to 25 m is required, which is a time-consuming and costly investment. Production of maps from these data required highly skilled staff and advanced interpolation techniques.

Sidescan sonar provides a means for visual inspection of the seabed between the sounding lines. Despite the many favourable quanlitites of sidescan sonar, the use of this technique for bathymetric mapping has been limited by the inability to provide quantitative depth information.

The sidescan sonograph shows echo level as a function of slant range from the towfish, giving a distorted picture of the seabed. This makes the interpretation of the sonograph time-consuming and costly.

Sidescan sonar provides useful results in areas with homogeneous seabed materials, while the results from areas with non-homogeneous materials may be of very little use. In areas with rugged topography, iceberg plough marks or pockmarks, the sonograph from sidescan sonar gives a very distorted picture of the seabed. In such areas it is very difficult to distinguish between sidescan effects and material effects.

Figure 1 illustrates current instrumentation for seabed mapping and surveying.

Fig 1. Exiting instrumentation for mapping and surveying. Vessel: navigation, echo-sounder, towfish tracking. Towfish : sidescanning sonar, depth sensor, attitude sensor Scale 1 to 10 000–50 000. Contours 2–20m (available in full paper)

Future Seabed Mapping And Surveying With Wide-Swathe Covering Systems

Two main factors will push forward a change from conventional echo-sounder and sidescan sonar to a new generation of wide swathe covering systems.

  1. The demands for reliable and accurate seabed mapping is increasing. This requires a shoter distance between the sounding lines using a conventional echo-sounder. Wide-swathe systems have be used in order to keep surveying costs within reasonable limits, and also to meet the required level of detail and reliability of the final maps.

  2. Current technology makes it possible to bring forward wide-swathe systems within acceptable dimensions and cost limits.

The Multibeam Echo-sounder (EM 100) will to a large extent replace single-beam echo-sounder and sidescan sonar techniques. The Multibeam Echo-sounder will have accuracy Comparable with the single-beam echo-sounder, and the need for visual inspection between the sounding lines is no longer present.

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