Zoom on

Bathymetry: mapping the seabed

Publié le 30 November 2022
par CADDEN

What is bathymetry?

Bathymetry consists of defining the relief of the seabed by determining the height of water georeferenced by means of underwater acoustic sensors (single-beam or multibeam echosounders, sonars, probes, etc.), a surface GNSS receiver and analysis of the acquired data.

The term bathymetry comes from ancient Greek: “bathys” (deep) and “metron” (measurement, instrument for measurement).

Why the bathymetry?

In comparison with the surveyors who carry out the topography of the earth’s surface, the bathymetry aims to map the relief of the seabed.

By conducting bathymetric surveys, the analysis of these reliefs aims to improve:

Topographical knowledge under the water surface;
Monitoring the marine environment;
Marine and underwater navigation safety;
The monitoring of works of art;
The evaluation of volumes in the context of dredging;
Measuring water levels of rivers, lakes, quarries…

See our hydrographic equipment

What is a bathymetric survey?

A bathymetric survey is a measure of water height and distance made with a sensor (single beam or multibeam echosounder, probe, sonar, etc.) combined with a professional and precise GNSS system to determine the geographical positioning.

How to do a bathymetric survey?

The steps for conducting a bathymetric survey vary depending on the type of boat and the environment to be surveyed; here is a non-exhaustive list:

Mission preparation and definition of desired line spacing, choice of echosounder type…;
Installation of equipment according to boat constraints (hydrographic vessel, bathymetric drone (USV = “Unmanned Surface Vehicle”), etc.);
Monitoring of previously planned lines, data collection and quality control in real time, using a software dedicated to bathymetry to combine the position and depth information collected;
Analysis and processing of data using software.

How long does it take to complete a bathymetric survey?

There is no defined time since each bathymetric survey is different and depends on several criteria: the area to be raised, the weather (sometimes tides) and the density of the survey.

What are the data obtained after a bathymetric survey?

Once the survey is carried out with the bathymetric probes, the raw data can be used in several forms after analysis and processing.

During the processing process, inconsistent data is discarded when they do not correspond to the reality of the field, this is done automatically (using filters) or manually. This step is necessary to achieve accurate and reliable results.

Once processed, these results are available in several formats:

A textured 3D model (also called a digital terrain model) calculated using bathymetry measurements;
A cloud of X/Y/Z points mapped following a bathymetric survey conducted exclusively with a multibeam sounder. These results are sometimes combined with data obtained by topographical survey with an aerial drone (UAV = “Unmanned Aerial Vehicle”);
An acoustic image mosaic created from the acoustic signal reflectivity on the bottom (only for multi-beam echosounders and side sonars).

In what type of applications should bathymetry be used?

Bathymetry has many different applications:

Marine geophysical work;
Detection of wrecks, obstacles;
Securing navigation (dredging monitoring, etc.);
Hydraulic installations (installation and maintenance of dams, canals, dikes, etc.);
Flood prevention;
River management;
Monitoring of sediment deposits;
Inspection of structures, etc.

Where to conduct a bathymetry survey?

All the different water zones, natural or artificial, and the different depths are conducive to the realization of bathymetry surveys: sea, ocean, lake, lagoon, river, canal, stretch, pond, bridge, port area, dam, dike, quarry, etc.

What equipment is required for bathymetry?

A complete bathymetry system consists of a single-beam or multibeam sounder, usually associated with other sensors. In the case of a boat with the integration of a single beam sounder, most situations require only a GNSS receiver in addition.

Equipment with a multibeam sounder:

One or more professional and accurate GNSS receivers (accurately determine a geographical position);
An inertial navigation system (indicates the compensation of the bearer’s movements, star, ROV (ROV = “Remotely Operated Vehicle”) or AUV (AUV = “Autonomous Underwater Vehicle”));
A speed profiler (measures the speed of sound according to several criteria: pressure, temperature, salinity, etc.);
A Lidar (mapping in a single passage the land surface: bridge, bank, rocks, islets, etc.) for coastal applications.

A single-beam or multibeam system is usually directly integrated on a hydrographic vessel, a marine drone or an underwater drone (ROV/AUV). A software suite completes the installation to perform the acquisition, analysis, processing and interpretation of bathymetric data.

What is a bathymetric echosounder?

A depth echosounder is used to measure the depth below the surface of the water. There are two types of bathymetric echosounders: the single-beam sounder and the multibeam echosounder.

How does a bathymetric echosounder work?

The bathymetric echosounder sends an acoustic wave reflected from the seabed, the echo emitted is received by the same transducer. The travel time of the sound gives information on the distance from the background.

The speed of sound varying below the surface of the water (about 1450 m/s) as a function of salinity, temperature and pressure, the depth sounder provides a precise distance between the sensor and the ground in real time.

Depending on the case, these many parameters require real-time or delayed corrections to best adjust the raw water height measurement.

What is a single beam echosounder?

The single-beam sounder has a single emission cone. Depending on configurations and vendors, it can be single and/or bi-frequency:

Low frequencies (less than or equal to 30 kHz) are capable of reaching very great depths beyond 200m. These frequencies obtain better penetration into soft-bottomed soils (e.g., muddy soils);
Higher frequencies (200 kHz or higher) are preferred for lower water heights up to 100/200m, with higher resolution measurements (ability to distinguish two objects of identical size).

What is a multibeam echosounder?

Unlike the single beam echosounder, the multibeam echosounder emits a pulse several times per second, in several hundred emission cones. This method offers many advantages; the main one being to obtain a larger coverage in a single passage.

The latest equipment, such as the R2Sonic series of depth echosounders, offer several additional features related to the georeferenced water height measurement:

Inspection of structures (dock edges, bridge piers);
Monitoring of pipelines;
Classification of funds, etc.

What is the difference between mono-frequency and bi-frequency?

Bathymetric probes are generally available in single-frequency mode, with some versions in bi-frequency mode.

Unlike the mono-frequency echosounder, the bi-frequency echosounder is able to provide a first level of characterization of the seabed, by detecting the layer of loose sediment: the highest frequency (200 kHz) stops on the top of the sediment, the lowest frequency (about 30 kHz) penetrates the loose sediment.

Other single beam echosounders

What is a sediment echo-sounder?

The sub bottom profilers uses very basic frequencies (from 1 to 15 kHz) and aims to detect the different strata of the marine subsurface. This requires a sufficient height of water for the strong acoustic signal to form properly in the water column.

What is a scientific echosounder?

The scientific echosounder favours multi-frequency transducers (high and low frequencies) and processes the feedback of acoustic signals in order to detect and measure biomass:

Detection of schools of fish;
Fish characterization and counting;
Detection of underwater flora;
Measurement of underwater vegetation height, etc.

Bathymetric systems

Hydrographic vessels

Integrated with single beam or multibeam echosounders, a hydrographic boat is a boat generally used in maritime areas with great depths and over long distances: oceans, lakes, rivers, channels…

Its objective is the search for wrecks, rocks and obstructions, the monitoring of cables or the environmental analysis for work at sea.

Discover the Port Atlantique de La Rochelle case study

USV for bathymetry

Unlike the hydrographic boat, the USV for bathymetry is a compact and easy to operate system. It also incorporates a single-beam or multibeam echosounder as well as numerous sensors to collect data on the seabed terrain.

The USV carries out bathymetric surveys in contexts where access is restricted for a conventional boat because of a low draught, for safety reasons, such as at the foot of a dam or in areas with strong eddies. Operators stay ashore dry and safe.

What’s the point of the USV for bathymetry?

Compact, manoeuvrable and responsive, a USV for bathymetry maps the seabed precisely with several speeds (up to 5 m/s) depending on the need and conditions of navigation.

With a manual and autonomous mode, the surface drones dedicated to bathymetry are designed to follow a programmed path at the beginning of the survey using the integrated precision GNSS receiver.

Compared to a traditional boat, with at least one operator and one navigator in charge of safety, launching operations are much faster since a USV requires only one operator.

Several options are provided to get further information.

The USV100 options:

EXO2 multiparameter probe;
ADCP;
Side-scan sonar;
Oceanographic multiparameter probe;
Orphie camera, etc.

The USV200 options:

Lidar: laser sensor for land mapping (bridge, banks, buildings, etc.);
Launching trolley;
Electric winch;
Oceanographic multiparameter probe;
Orphie camera, etc.

Did you like this article ?
Share it with your colleagues or friends