Depth determination is a fundamental

 

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INTRODUCTION

Depth determination is a fundamental task for a hydrographer, which requires specific knowledge of the medium, of underwater acoustics, of the plethora of devices available for depth measurement, of complementary sensors for attitude and heave measurement and proper procedures to achieve and meet the internationally recommended standards for accuracy and coverage as articulated in IHO publication S-44 4th Edition. Lead line and sounding pole were the earliest methods used for directly measuring water depth. Their easy principles of operation ensured their continued use over many centuries. Single beam echo sounders, derived from military sonars, were a major development and have been used in hydrographic surveying since the mid 1900s. During the last decade, hydrographic surveying has experienced a conceptual change in depth measurement technology and methodology. Multibeam echo sounders (MBES) and airborne laser sounding systems (ALS) now provide almost total seafloor coverage and depth measurement. The high data density and high acquisition rates have led to huge bathymetric data sets and much ancillary data. The state of the art of the depth measurement equipment was evaluated by the working group of the S-44 [IHO, 1998] as follows: “Single beam echo sounders have reached a sub-decimetre accuracy in shallow water. The market offers a variety of equipment with different frequencies, pulse rates etc. and it is possible to satisfy most users' and, in particular, the hydrographers' needs. (…) Multibeam echo sounder technology is developing rapidly and offers great potential for accurate and total seafloor search if used with proper procedures and provided that the resolution of the system is adequate for proper detection of navigational hazards. Airborne laser sounding is a new technology which can offer substantial productivity gains for surveys in shallow, clear water. Airborne laser systems are capable of measuring depths to 50 m or more.” Despite these new technologies, single beam echo sounders (SBES) still remain, for the present, the traditional equipment used on hydrographic surveys worldwide. These echo sounders have also evolved from analogue to digital recording, with greater precisions and higher accuracies and with specific features which allow a wider variety of purposes to be met. The use of digital echo sounders along with motion sensors, satellite positioning systems (such as GPS) and software for data acquisition have combined to optimize productivity with corresponding reductions in personnel for survey operations. MBES have become a valuable tool for depth determination when full seafloor ensonification is required. An increasing number of National Hydrographic Offices (NHO) has adopted multibeam technology as the methodology of choice for the collection of bathymetric data for new chart production. The acceptance of 120 M-13 multibeam data for use in published nautical charts is a sign of growing confidence in the technology. Notwithstanding their impressive capabilities, it is vital that planners, operators and checkers have indepth knowledge of MBES operating principles, as well as practice in data interpretation and validation. Airborne laser sounding systems are being used by a few NHOs; these systems have, by far, the highest data acquisition rates and are particularly suited to near shore and shallow water areas. However, the high costs for the assets involved in data collection and their operation do not currently allow a more general use. In this Chapter, Section 2 covers the broad acoustic fundamentals necessary for the understanding of sea water acoustic waves and physical characteristics, acoustic wave propagation and acoustic parameters. Section 3 deals with motion sensors. Section 4 covers transducer characteristics, their classification with regard to beam pattern, principles of operation and their installation. Section 5 describes the acoustic systems of single beam echo sounders and swath systems, both multibeam and interferometric sonars, with regard to their characteristics, principles of operation, installation and operational use. Finally, Section 6 explains non-acoustic systems, such as airborne laser and electromagnetic induction systems, remote sensing systems and classic mechanical devices. The terminology used in this chapter follows, as far as possible, the Hydrographic Dictionary [IHO SP-32 5th Edition, 1994].

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