The project describes detailed 3D reconnaissance of seabed aimed at understanding sediment properties for geological, geotechnical and environmental site investigation purposes, based on Very High Resolution 3D seismic. The term Very High Resolution (VHR) seismic stands for mean seismic source frequencies between 200 Hz and 1.2 kHz with an expected vertical resolution in the order of decimeters and a penetration down to about 100 m. The realisation of the objectives was twofold with the design of a flexible 3D seismic method for optimal coverage of targets of different size and the development of a processing sequence tailored for VHR data with regards to its frequency content This contribution focuses on the proposed approach as well as on different geological and geotechnical targets successfully surveyed using this methodology:
A site in the Dover Strait (1999) - located off Cape Gris Nez - was chosen in a tectonised environment where folds and faults affect Jurassic limestones
A site off the south coast of Cornwall (2000) - located in St Austell Bay - was chosen on the basis of access to existing geotechnical and geophysical data. The location provides an unconsolidated sequence of sands and clays in a fluvial environment, including peat beds and cobbles overlying a consolidated shale (Devonian) substrate
A site in the Adriatic Sea (2001), where a variety of deformation features affects the late Holocene high-stand deposits on the south-western portion of the shelf
The term Very High Resolution (VHR) seismic stands for a resolution of submetric scale in the medium. The seismic source which is the most suitable answer to the needs of the project and which provides adequate frequency content vas sparker technology with a frequency bandwidth centred on 700 Hz.
From the imagery point of view, the main key constraints for a survey design are the following: maximum depth and dip of layers, horizontal and vertical resolution. These constraints may, in turn, be related to the acquisition geometry addressing the following parameters signal frequency bandwidth, trace length, trace interval, streamer length, streamer spacing, line spacing and shot interval The proposed 3D acquisition system is capable of being dimensioned m a dynamic way for optimal coverage of geotechnical and geological sites of small extent (2 km * 1 km) in water depths up to 100 metres and penetration up to 100 metres and a target vertical resolution of decimetric scale. Investigation areas might include large geotechnical sites (dams, artificial island sites, pipelines routes on the continental shelf …) or sedimentary bodies of key geological importance.
Due to its frequency content, and thus its ability to correctly image dipping reflectors, the sparker source is used together with the flexible 3D acquisition system Paying close attention to the expected geology of the surveyed area allows to complete the survey using different cross-line receiver spacing, therefore decreasing the overall duration of the survey: