Side-scan sonar
Side-scan sonar is a device that transmits and receives ultrasonic waves towed by a ship. It allows objects on the seabed to be located and positioned.
This tool allows for a continuous representation of the morphology and nature of the seabed, comparable to a view of the earth from the sky.
The acoustic imaging obtained allows a representation of the reflectivity of the bottom along the swath and especially of the presence of irregularities or small obstacles that are "seen" by the signal.
Principle
A transducer, commonly called the fish, attached to a cable connected to the boat, sends out sound waves consisting of an emitted beam
perpendicular to the direction of the boat.
This beam is reflected off the seabed and then received by the fish. The time elapsed between the emission and reception of the sound signals allows us to determine the depth (the first echo informs us of the sonar's distance from the seabed).
Depending on the intensity of the return signal, which depends on the nature of the seabed, the recorded waves will be different.
Equipment
CERES operates an EdgeTech 4200 side-scan sonar, a recognised standard in geophysical surveying, with an EdgeTech 4100 available as a backup system.
EDGETECH 4200
Frequency: 300/600 kHz Range: 230/120 m Depth rating: 2,000 m Resolution: 0.03 m at 300 kHz and 0.015 m at 600 kHz Vertical beamwidth: 50°
EDGETECH 4125
Frequency: 300/600 kHz
Range: 230 / 120
Depth Range: 2,000 m
Resolution: 0.03 at 300 kHz and 0.015 at 600 kHz
Lobe opening: 50°
KONSBERG PULSAR 6100
Frequency: 500kHz-1MHz
range: 100m at 550 kHz
Depth range: 1000m
Resolution: 0.07m at 10m range, 0.35m at 50m range, 0.70m at 100m range
lobe opening: 50°
Methodology
Data Acquisition and Processing
Mapping
Acquisition profiles must be adapted to the depth in order to obtain a complete mosaic of the area.
The CERES team provides this information during the technical proposal phase, often accompanied by a site plan with the
forecast lines.
Navigation, acquisition and processing are carried out within the Isis software (for acquisition and processing).
The vessel's position is accurate to the centimeter using our own RTK or, where applicable, the Orpheon system. The relative positioning of the fish in relation to the boat is determined by two methods: acoustic (USBL) or the so-called "Layback" method.
Bathymetric acquisition is carried out on average at 3 knots.
On completion of the work, the acoustic mosaic and all raw and processed data are delivered in GeoTIFF format.
