In the field of seabed surveying for nautical charting and offshore engineering applications, information on the topography is traditionally acquired by line surveys. The instrumentation is normally an echosounder with the required positioning, storage and presentation systems. Subject to geological aspects and survey specifications, the echosounder may be supplemented by a sidescan sonar, perhaps with fish positioning to obtain bottom information in between lines. To overcome the many short-coming and uncertainties of these systems, the Benigraph seafloor surveying system has been specified and developed.
The Benigraph system was conceived as a pipeline inspection system, designed to be able to assess coverage and free spans. The system contains a multibeam sonar with very high resolution both angularly and in range. It is fish mounted so as to allow an economic operating speed. This required the development of a new high-accuracy fish-positioning system. To allow quality control, the bottom is shown in real-time on a graphical display in 3D form. All sonar data are given with position in a global reference frame and stored on computer tape both in processed and raw form as is all the positioning data.
The Benigraph project started four years ago, and the first prototype went through an intensive field trial period outside Tromso in northern Norway this summer. In this paper, an overview to the system is given, followed by description of the subsystem, with performance figures, and concluding with the results of field trials.
The objective in the original pipeline inspection idea was found to be equivalent to producing a high-resolution real-time topographic seafloor mapping system, suitable for a wider field of application. The high accuracy in X, Y, and Z determination and the complete bottom coverage that the Benigraph achieves is essential in hydrographic survey work. For preroute pipeline surveys the system gives the information needed for safe determination of the pipeline route. The systems was intended for work in the North Sea and is therefore designed to map the seafloor at water depths down to at least 300 m without loss of accuracy.
Figure 1 shows an overall block diagram of the Benigraph. The system is completely modular and is divided into the following subsystems:
surface positioning;
towfish positioning;
scanning sonar;
data storage and presentation;
towfish and handling equipment.
The system used for surface positioning may be any type of high-quality radio positioning system, for example Syledis or Hyperfix. The fish-positioning systems, FIPOS, consists of two sub-units:
the acoustical positioning reference system (APR);
the inertial positioning reference system (IPR).
The measuring parts of the Benigraph are all mounted on the towifish while the towing vessel carries the data-processing equipment, displays, handling equipment, and a beacon for ship-to-fish range measurement the measured data are transmitted in digital links up to the surface, except for the hydrophone signals in the APR which are only amplified before being transmitted up the tow cable.
