BATHYMETRY & BACKSCATTER

Bathymetric data has been collected for centuries, primarily with the aim of improving the safety of navigation. One of the first tools used to estimate depths, the lead line, is still being used today. A lead line is basically a rope with a weight attached to the end; the weight is dropped to the bottom of the ocean and the length of the rope is measured. In some cases, the weight has a hole at the bottom that can be filled with grease or tallow that allows collecting a sediment sample. To this day, many nautical charts in use contain some of the early measurements made from lead lines. However, measurements from lead lines are not very accurate; they are impacted by variable current layers causing the line to drift, can sink into sediments, and can miss shoals and other elements dangerous for navigation. Other mechanical methods still in use in shallow waters include the sounding pole, or sounding rod. Sounding poles are less impacted by drift and can be mounted on a foot that prevents penetration into the upper sediment layers. A GPS receiver can be attached to the top of the pole to derive a more accurate position. Wire sweeps and bar sweeps are two methods that are used to ensure clearance over obstacles, once again for navigation purposes. A float wire or bar is lowered at a set depth, and the tension in the wire or the movement of the buoys indicates whether an obstacle is found at that given depth. Despite technological developments, these mechanical methods are still in use and can serve to calibrate more advanced acoustic sounding equipment and to verify measurements when necessary.

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An increase in the understanding of underwater acoustics led to the development of the first echosounders in the early 1900s. Echosounders transmit sound waves in the ocean and an estimate of depth can be made by measuring the time between the transmitted and the returned echo when the speed of sound in the water is known. Most echosounders can also provide backscatter data, which correspond to the intensity of the acoustic return. Backscatter data have been found to be correlated with sediment types; hard bottoms may reflect more sound than soft bottoms that will absorb some of it. Different types of echosounders are used depending on the water depth and the need for accuracy and bottom object detection criteria. Acoustic echosounders are precise and versatile instruments that are less affected by water clarity and are hull-mounted, pole-mounted, towed by a ship, or outfitted on an autonomous vehicle. Singlebeam echosounders are the simplest. They measure the shallowest depth in a circular area under the ship called the footprint by calculating the travel time through the water column of a single sonar pulse. The footprint size, which drives the resolution of the resulting data, depends on water depth and sound frequency. The shape and frequency of the acoustic backscatter return can be measured to inform on seafloor hardness or, in some cases, the type of benthic community. Although providing a wealth of information, singlebeam echosounders are very time-consuming; because the acoustic footprints do not cover a wide swath, they require many passes over the seafloor to gain full coverage, and often a lot of interpolation is needed to estimate the depth of the seafloor over greater extents. To increase coverage, multiple singlebeam echosounders can be mounted on a horizontal support to create a sweep system. Multibeam echosounders are preferred over singlebeam echosounders because they provide a much more complete coverage in a continuous swath by using multiple pulses angled outward to both sides of the platform. The backscatter returns can be used to determine some seafloor properties, but data reliability depends on the return angle, with the beams furthest away from the nadir being less accurate. Multibeam data resolution also depends on the distance of the sensor from the seafloor. Equipping an underwater vehicle with a multibeam sensor allows for very high-resolution dataset by reducing the sensor-to-seafloor distance. However, the closer the sensor is to the bottom, the smaller the swath width, which requires more passes to get full coverage. Finally, sidescan sonars are typically towed instruments that use acoustic pulses to image both sides of the platform track and provide imagery. Their primary use is for detecting objects on the seafloor, whether natural features such as rocks and sand waves or human-made features such as shipwrecks and pipelines. Traditional sidescan sonars most often do not collect bathymetry, except for newer interferometric systems that use multiple antennas and synthetic aperture sonars. Sidescan sonars are deployed when investigating bottom contacts that may pose a danger to navigation to collect information on navigational hazards that a downward-looking hull-mounted echosounder may not measure precisely. Sidescan sonars provide very high-resolution imagery.

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Finally, optical remote sensing also enables estimating the depths of the ocean. This can be done using principles behind radar altimetry, satellite-derived bathymetry from multispectral data, or using lidar systems. A precise and efficient way to map shallow, relatively clear waters down to 40 m depth is with topo-bathymetric or bathymetric lidar systems mounted on an aircraft. These surveys are relatively fast and provide loads of data to build high-resolution digital bathymetric models based on the return of laser signals bouncing off the objects below. However, these systems are limited to shallow depths because of light attenuation in water, and data processing can be laborious and crucial to the surveys’ outcomes. Most often, bathymetric lidar systems are mounted on aircraft, and aerial photography is collected in concert with lidar for reference during data processing. However, more recent systems are light enough to be mounted on drones, and space-based lidar systems like ICESat-2 can also provide bathymetric data. Backscatter from lidar is also collected, but challenges in interpreting those are plentiful, currently making them of limited use. Lidar data can help reduce seafloor mapping costs and increase resolution in shallow waters (less than 40 meters) where multibeam systems become less efficient to collect; therefore, many seafloor mapping programs use lidar in shallow water and multibeam in deeper waters. Of the various bathymetric data types and collection platforms, satellite data are typically used to capture broader-scale seascape patterns and are most valuable for applications evaluating frequent changes. In seafloor mapping, these are primarily limited to the clear shallow waters of the tropics. Imagery from airplanes or drones can be used to increase the spatial resolution of data, but will require more time and effort to cover an area comparable to one satellite image. When deriving bathymetry from light, the measurements are not direct but instead are estimated from imagery collected at different wavelengths. Physics-based methods are grounded in the radiative transfer equation inversion that describes the path sunlight follows through the atmosphere and the water column; the interactions of light with the atmosphere, air-water boundary, water column, and the seafloor are first modeled. Then, depth can be deduced based on the amount of reflected energy. Empirical methods for deriving bathymetry from multispectral sensors rely on linking ground-truthing data to brightness values based on principles of light absorption through the water column. Other physical methods that do not rely on ground-truthing methods have also been applied based on bottom reflectance. More recent methods have also tested the potential of through-water photogrammetry to derive bathymetry, which requires correcting the air-water interface. On the other hand, underwater photogrammetry is commonly applied to produce bathymetry over small areas. Another approach for satellite-derived bathymetry uses radar altimeters, which can estimate the height of Earth’s geoïd. Anomalies in the geoïd are known to be correlated to coarse-scale bathymetry, enabling its derivation. Finally, theoretical concepts of wave kinematics applied to multispectral or radar imagery can be used to estimate bathymetry in coastal waters. It is mainly based on the linear wave dispersion relation, which states that the length and period of ocean waves change when they enter shallow waters. Water depth can be derived using the linear wave dispersion relation by deriving wave speed, period, and gravity acceleration from satellite data.