Seismic imaging is an indirect measuring technique that involves recording surface echoes from the propagation of a induced seismic wave in the seabed.
High Resolution ocean bottom seismic methods implemented for shallow seabed surveys use frequencies up to kHz. This provides high resolution quality and penetration down to a few hundred meters depending on the nature of the soil. This technology is commonly used for seabed surveys in shallow waters. Its accuracy in deep seas decreases rapidly due to the increased distance between the system and the terrain to be imaged.
To counter this, Ifremer initiated the Sysif programme in 2006 for the development of a High Resolution seismic reconnaissance system for work in deep waters.
Until now, Sysif, which carried out its first operational dive in 2008 as part of a collaborative project with the French company TOTAL off the coast of Nigeria, was equipped with a single receiver or seismic trace (a so-called monotrace system) consisting of a small group of analog hydrophones. This system did not allow to determine propagation speeds in the sediments thus limiting the image obtained from the seabed to an image in travel time compared to an image in depth.
The analog technology limited the potential number of seismic tracks but an innovative technical solution was found through the development of digital hydrophones using the Ethernet transfer protocol. The data-acquisition electronic boards are housed inside the hydrophones and have, in addition, an attitude sensor Cap, Roll and Pitch, allowing to reconstruct the deformed system in operation. A patent application is currently under evaluation for this technology.
The 52 hydrophones have been integrated by the French manufacturer Sercel into a conventional oil-filled towed array, 117 meters long, successfully implemented for the first time in March 2013, on board the research vessel Pourquoi Pas? during the ESSSYPEN expedition.
The raw seismic data are not in themselves useable due to the movement of equipment during the dive. The data need to be processed prior to providing the users with interpretable images.
- The first step in processing is to realign the equipment with the sensors. Acoustic positioning, depth meter, altimeter, accelerometer, attitude sensors: all these data must be processed with great precision in order to elaborate a final seismic image.
- The work of the geophysicist will now be to use positioning data through seismic processing algorithms to correct any artifacts. The image obtained from the newly developed tool is an in-depth image of unparalleled accuracy.
This multitrace High Resolution seismic system towed on the seabed is a first on an international level, providing an exceptional tool for researchers and engineers in charge of the understanding of the phenomena taking place on the seabed.