- On 25th July 2019
- In General, Sediment Management, Water Management
- By William
Bathymetry is the study of the bed (floor) of a waterbody, involving mapping of features on charts to provide information on water depth. Bathymetric charts are typically produced to aid navigation and inform dredging activities, for example during maintenance of navigational channels.
Bathymetry traditionally uses an echosounder attached to a survey boat. As the boat drives across the survey area, the echosounder generates electrical signals that are then converted to sound waves by a transducer under water. The sound waves bounce off of the underwater features and this echo is picked up by the echosounder which then calculates the distance to the feature. The system uses high accuracy GNSS (GPS) system to then link each distance measurement to a specific depth on a map.
Digital Terrain Models (Digital Elevation Models) can be produced through combining bathymetry with photogrammetry or LIDAR data. These techniques are traditionally used on land to measure elevations but do not work under water. By combining the three techniques, underwater features, intertidal features and land-based features can be stitched together into one seamless model/map.
How to do a bathymetric survey?
Here is a simple step by step guide on how to conduct a bathymetric survey.
- Equipment checks and calibration
This is important to make sure equipment is in prime condition when arriving on site.
- Mobilise all equipment
Mobilisation of all equipment can include transporting the survey boat and launching it in the survey area. In some cases, this is only transport of equipment which can then be deployed on a local hire boat as that can be more cost efficient.
- Deployment and equipment setup
This includes the setting up of a bespoke frame which is used to attach all the equipment securely to the boat. This keeps it firmly in place when the boat is moving to ensure high quality data. The transducer is attached to the underwater section of this frame and the GNSS (GPS) antennas are attached at the top of the frame to have as good a view of the sky as possible. The “control hardware” – the echosounder and the GNSS controllers are then connected via marine cables to all other equipment and also to the control computer. The software is then set up with details about the specific survey noted in the survey logs.
- Sound casting
Measurement of the speed of sound at a specific site to calibrate the transducer. This is because the speed varies with salinity and temperature which can significantly influence the reading of depth.
- Start the survey
This involves driving the boat whilst logging the depth data over the entire area to be surveyed. This is done by following predetermined survey lines. These lines will be very close together in shallow waters and where the accuracy required is high.
- Check the data before demobilisation
This is a quick look over all data collected to see whether there are any obvious areas which need more data collection.
Power down all equipment, disconnect and wash with fresh water.
- Process the data
The data must be processed using specialised software with a powerful workstation computer. The hydrographer checks the raw data for any false signals such as returns from floating objects or bubbles in the water. Any inaccurate data is removed and the final map is produced.
Bathymetric surveys are an essential tool to ensure safe navigation and effective dredging and maintenance of navigable waterbodies. They provide accurate measurement of depths of any waterbody.