The CTDI group is mainly involved in acoustic data acquisition and processing to map the seabed, in terms of morphology and nature. The acoustic technology is the best and only way for measuring the seabed, whatever the depth.
Multibeam echosounders and side scan sonar are used for mapping surveys. This equipment delivers three types of data:
Measures water depth and consists of measuring the travel time of acoustic signals. Unlike single beam echosounders, multibeam echocounders have a much denser coverage of the seabed during the mapping survey.
Measures the seabed reflection level. This data contributes to the study and characterisation of the nature of the seabed.
As for the seabed, the intensity of echoes in the water-column allows to visualise fluid emission (gas, other liquids…)
Computing systems and software on-board the research vessels ensure the acquisition and back-up of a full range of data related to the ship itself (navigation, heading, pitch, roll, heave, weather…) and to the scientific sensors on-board (multibeam echosounders, gravimeter, magnetometer…).
The software provide the scientists and technicians with bathymetric coverage in real time as the survey advances. The team can therefore check the data acquired and their quality, and validate in real time the ship’s workflow. A survey consists of a succession of acquisition profiles obtained over a given area for complete coverage.
The acquisition of acoustic data is subject to a number of hazards and parameters that may affect the quality of the measured data: weather and sea state, vessel speed, positioning quality, current, celerity, tide, etc.
- Some of the recorded data may be noisy, or even erroneous, due to adverse sea conditions affecting emission and reception of the acoustic signal.
- Spatial or temporal variations of sea temperature and salinity may also modify signal propagation in the water, and hence influence the ping depth and positioning on the bottom.
- Several other parameters may influence data quality and precision, such as the ship’s position or movement. These parameters must be checked and if possible corrected to ensure the best quality of bathymetric data
The aim of data processing is to transform measured acoustic data into validated digital terrain models for analysis and interpretations in terms of seabed morphology and nature. Specific processing software, such as Globe, validate, correct and refine data, using automatic algorithms or manual processing by in-house experts.