Bathymetry by drone
Bathymetry: Measurement by sounding the seabed
Bathymetry is a science to study marine depths, lagoons or even streams. It allows to determine the underwater topography of a lake with a very high precision. It is used to make surveys intended to secure surface or underwater navigation, measurements of underwater stockpiles or level of siltation, river bed surveys, but also in the quarry world. Therefore, bathymetry can favour many applications. It is also very useful for exploring the seabed and establishing reconnaissance missions.
3D mapping of water depth
Thanks to bathymetric measurements, it is possible to establish digital models representing the configuration of a seabed in 3D. Through the processing of the collected data, we can identify underwater reliefs such as ocean or terrain pits. The use of a drone during bathymetric surveys generates many benefits. Above all, this greatly reduces the cost and the time devoted to the mission, but also the workforce on the ground.
In addition, the aquatic drone equipped with an echo sounder, an inertial unit and a RTK GPS can be followed in real time. Operators take less risk by acting from the bank.
How does bathymetry work?
Bathymetry technology works with an echo sounder also called SONAR (Sound, Navigation and Ranging). It is a distance measuring tool that can measure the travel time of an acoustic signal reflected by the bottom or by any suspended element (fish, rock, seaweed etc.)
This device is composed in two parts:
– First, there is the transducer or probe that is located below the surface. It is a device that converts electrical signals to sound waves and vice versa. Therefore, it can switch from receiver mode to transmitter mode at any time.
– The second part is a box called sounder or echo sounder that receives the electrical impulses from the transducer and transmits them in images to the operator.
Ultrasound will be emitted by the transducer to hit the bottom. Once the wave reaches an obstacle, it bounces back to the probe and turns into an electrical signal. Then, the information is sent to the sounder that will be able to calculate distance of the obstacle from the speed of the wave and the time it took to reach the bottom. Therefore, the transducer is one of the centerpieces of the device. There are several sonar types that differ on various criteria such as angle or frequency but also on composition or power.
The single-beam echo sounder
At first, there is the category of single-beam echo sounders including:
• The mono-frequency echo sounder: it allows to give a depth for a given x, y, and z position.
• The bi-frequency echo sounder: for a given x, y, and z position, it gives two depths (the sedimentary deposit and the bottom of the lake)
The multi-beam echo sounder
In a second step, there is the category of multi-beam echo sounders with the multifrequency echo sounder which gives with a more refined precision the depth of the water a multitude of points.
Therefore, it is important to choose the most suitable method to apply depending on the mission, the client and the characteristics of the survey area.
Photogrammetry via drones makes it possible to model an area in 3D, create plans, make measurements (lengths, areas, volumes) and therefore ensures precision when mapping.
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