Geophysical Techniques for Offshore Site Investigation

Abstract

Abstract Geophysics is an essential tool for the detection and understanding of variations in the properties of the Earth, geological structure, and for the identification of potential geohazards. The understanding of the geology and geological processes active within an area can have significant implications for locating infrastructure, for safety in its design and design optimization, and for potential time and cost savings associated with its installation. In‐depth knowledge of the active geological processes will also provide added confidence in the success of engineering projects over their operational lives. The advantage of using geophysical techniques in the marine environment is that large areas of the seafloor can be mapped relatively easily and in a relatively short time. Geotechnical boreholes are an effective means of investigating subsurface shallow geology but are expensive to conduct and only provide information at discrete locations. Nevertheless, boreholes (and wells) are essential to provide calibration and ground truthing of geophysical data. The applications of marine geophysics are as varied as the techniques and instruments that have been developed for surveying the sea floor. In the context of engineering, marine geophysics is an essential component of data integration and the development of a detailed geological model. The platforms from which data are acquired have evolved rapidly. Survey vessels are still the mainstay of site investigations but there is now much greater utilization of vehicles such as autonomous underwater vehicles (AUVs) to acquire data with higher resolution. The properties required to be measured, accurate positioning and coverage, adequate penetration, and sufficient resolution of geophysical data are of primary concern when planning and executing a geophysical survey. Seafloor variability is typically mapped using acoustic techniques such as echo sounding, side scan sonar, and seismic reflection. Subseafloor variability is most commonly mapped using seismic reflection methods, and to a lesser degree with magnetic methods, seismic refraction, seismic converted waves and shear waves, electrical methods, and electromagnetic methods. The various geophysical techniques yield complementary information, and as such there is a trend toward multidisciplinary approaches for obtaining improved seafloor and seabed characterization. The subsequent processes of integration and interpretation of these datasets is essential to the development and refinement of a geological model, which can be applied to engineering requirements.