bathymetry

Bathymetry is the measurement and representation of the underwater topography of oceans, lakes and rivers. It is the underwater equivalent of topographic surveying and provides information about water depth as well as the shape and structure of the seabed or riverbed.

Bathymetric data is used to create nautical charts, identify navigational hazards, monitor coastal and river environments, investigate erosion and sediment transport, model flooding, map aquatic habitats and plan marine infrastructure.

Bathymetric measurement methods

Modern bathymetric surveys use several measurement techniques. Single-beam and multibeam echo sounders determine depth from the travel time of acoustic pulses reflected by the seabed. Multibeam sonar is particularly suitable for mapping large areas and deep water.

Bathymetric LiDAR is an optical alternative primarily used for shallow coastal waters, rivers and lakes. It can efficiently survey areas that are difficult or unsafe to access by boat and can produce a continuous elevation model covering both land and underwater terrain.

How bathymetric LiDAR works

A bathymetric LiDAR system emits short laser pulses towards the water and measures the time at which the reflected light returns. Green laser light, commonly with a wavelength of approximately 532 nm, is frequently used because it penetrates clear water more effectively than the near-infrared wavelengths typically used for topographic LiDAR.

A measurement may contain several return components. An initial reflection is produced at the water surface, while a later return originates from the bed of the water body. The difference between their arrival times is used to determine the optical path length through the water and, after correcting for the refractive index and measurement geometry, the water depth.

The reflected signal may also contain contributions from particles, vegetation and other objects within the water column.

Limitations of LiDAR bathymetry

The maximum measurable depth depends strongly on water clarity, absorption, scattering, seabed reflectivity, surface waves, ambient light and the sensitivity of the receiver. Bathymetric LiDAR is therefore mainly suitable for clear, shallow or moderately deep water and does not replace sonar for surveying the deep ocean.

Accurate measurements also require corrections for the refraction of the laser beam at the water surface, aircraft or drone movement, the changing orientation of waves and the position and orientation of the sensor platform.

Time-resolved signal acquisition

Bathymetric LiDAR relies on precise time-of-flight measurements. In photon-counting systems, detectors such as SPADs or photomultiplier tubes generate electrical pulses for individual detected photons. A time-to-digital converter records their arrival times relative to the transmitted laser pulse. Repeated measurements can then be combined into time-of-flight histograms that reveal the water-surface and bottom returns.

Full-waveform LiDAR systems instead digitize the complete analog detector signal with a high-speed analog-to-digital converter. This preserves information about the shape, amplitude and temporal width of the return waveform and enables detailed analysis of overlapping reflections and scattering within the water column.

cronologic TDCs and TimeTaggers provide precise multichannel event timing for photon-counting LiDAR systems. cronologic high-speed ADCs enable continuous full-waveform acquisition, while integrated zero suppression can reduce the amount of transferred data by selecting the waveform regions that contain relevant return signals.

‍