Since the 1970s, remote sensing has increasingly been used to conduct underwater surveys. Several methodologies are employed in hydrographic surveying, from direct techniques such as probing rods to more sophisticated techniques such as satellite-derived bathymetry and altimetric radar. The growing need for data that is increasingly accurate and available with a quick turnaround has recently led to a worldwide effort to develop sensors and alternative techniques for measuring depth.
Bathymetric survey platforms include surface vessels, submersible platforms, aircraft and satellites (see Figure 1). Vessels range from large ships, used in offshore surveys, to uncrewed vessels, either remotely controlled or autonomous. The most commonly used submersible platforms are autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs), both of which are used for high-resolution mapping in deep water and can be controlled from a surface vessel. Acoustic sensors are preferred on these platforms, although AUVs and ROVs now also carry light detection and ranging (Lidar) systems and high-resolution photographic cameras.
Figure 1: Platforms used in hydrographic surveys: (a) NOAA Ship Fairweather for offshore surveys, (b) EchoBoat-ASV from Seafloor Systems, (c) Hugin AUV from Kongsberg.
The Ship Fairweather of the National Oceanic and Atmospheric Administration (NOAA) is mainly used in offshore hydrographic surveys and port surveys and is equipped with a Reson Seabat 8160 echosounder, which surveys depths of up to three kilometres. EchoBoat is a small autonomous surface vehicle (ASV) used in bathymetric surveys of inland waters and sheltered areas that are performed using acoustic sensors. The Hugin AUV from Kongsberg has, in addition to the EM2040 multibeam echosounder, a laser profiler, a coupled photographic camera and a HISAS 1032 synthetic aperture sonar, among other systems and sensors.
Crewed and uncrewed aircraft are also used in bathymetric mapping. These platforms are equipped with passive sensors, which allow depth estimates to be made through the spectral response of the submerged bottom, and active sensors, such as bathymetric Lidar. Similarly, satellites also function as bathymetric survey platforms, either through the use of orbital images (bathymetry by spectral response) or through the use of altimetric radars (active sensors), as in the Seabed 2030 project. There are also hybrid solutions that enable the acquisition of bathymetry using an echosounder towed by a low-flying unmanned aerial vehicle (UAV; Figure 2) or an integrated ground penetrating radar (GPR). Unmanned surface vehicle (USV)-based platforms are also increasingly used.
Bathymetric Survey Methods
Modern bathymetric surveys are carried out in various media, depending on the method used. Sound waves are used in the aquatic environment, visible light is used in the air and in the aquatic environment, and bathymetry derived from information from altimetric radars is used in the air.
Acoustic systems are used both in shallow water of around one metre deep and in deep water of many kilometres of depth. Such systems are preferred as they provide more accurate data than other methods. The high attenuation of visible light in the aquatic environment means that depth measurement by optical remote sensing (active and passive) is limited to shallow depths. While aerophotogrammetric (short distance) and orbital images are employed for bathymetry at depths of up to ten metres, Lidar systems operating in the green wavelength can reach up to 50 metres in clear waters. Finally, altimetric radars can be used to obtain depths in deep waters, especially where bathymetric information is scarce or non-existent.
Acoustic Sensors
The first record of the use of an echosounder, or sonar as it is popularly called, is when Leonardo da Vinci placed a tube in water to detect big ships, by positioning his ear in the tube. There are both passive and active sonar systems, but active sonar is used in depth measurement.
Between 1920 and 1930, the world saw the development and implementation of single beam echosounders (SBESs), which use sound to measure the depth directly below the sounding platform. By running a series of lines at a specified spacing, SBES greatly increased the speed of the survey process, allowing more data to be collected compared to direct methods. However, this method still left gaps in quantitative depth information between the survey lines. Between the 1950s and the 1980s, technological developments resulted in the emergence of sidescan sonars (SSSs) and multibeam echosounder (MBES) systems (beam-formers). Lateral scanning sonar technology offered a qualitative means of obtaining the sonic equivalent of an aerial photograph and improved the ability to identify submerged shipwrecks and obstructions. This proved to be an excellent aid for SBES surveys, since it enabled the search for submerged objects between the navigated lines. Beam-forming MBES made it possible to obtain quantitative depth information for almost 100% of the submerged bottom.
SBES is ideal for shallow water surveys and is very cost-effective. The planning, operation, processing and analysis are simple, and there is a huge range of equipment operating at low frequencies (12kHz–50kHz), high frequencies (100kHz–700kHz) and even dual frequencies (24kHz/200kHz, 33kHz/200kHz, 50kHz/200kHz, etc.).