SiPLABoratory

Geophysical seafloor exploration with a towed array in shallow water

The geophysical properties of marine sediments just beneath the seafloor are of great importance for modelling of coastal areas. Accurate knowledge of these properties allows for correct initialization and testing of environmental models and increase their capability for predicting changes in the morphology of the coastal line. Moreover, a geophysical description of shallow marine sediments is a requirement in a large number of applications, such as environmental monitoring, ecological studies, underwater acoustics, geotechnical exploration and soil stability testing. This project was funded under the European MAST2 programme, with 800 kecu for 3 years. Starting date: January 1st, 1993.Coordinator: S.M. Jesus (UALG). Participants: A. Caiti (Univ. Genova, Italy), H. Zambujo (Instituto Hidrográfico, Lisbon) and A. Kristensen (Saclantcen, Italy). People involved from SiPLAB: S.M. Jesus, O. Rodríguez and P. Felisberto.

This project aimed at the study and development of a towed remote-sensing acoustic instrumentation able to quantitatively measure in a survey fashion the geophysical/geoacoustic properties of the seafloor in shallow waters (coastal areas and continental platforms). The instrumentation consists in a low frequency acoustic source and an horizontal array of acoustic receivers. Both source and receivers are towed from the the same platform. The source is transmitting pure tones at selected frequencies in continuous wave (cw) mode. Both source and receivers are submerged at a certain depth below the sea surface. The measured acoustic field at the receiver positions is exploited to estimate the geophysical properties of the seafloor sediments by using appropriate inversion algorithms.

The objective of this MAST 2 project can be summarized as follows:

• to determine the optimal configuration and mode of operation of the above described instrumentation, with the objective of being able to resolve geophysical structures within the sediment to scales of 0.5 - 1 meter, down to at least 30 meters from the seafloor surface; the optimization had to keep into account the constraints imposed by the operation in shallow water of the instrumentation proposed;
• to design computationally efficient data inversion techniques that rely on the optimal geometry;
• to assemble and test an experimental towed array localization system;
• to conduct a feasibility test at sea of the proposed methodology/instrumentation;
• to assess the merits and drawbacks of the proposed system, and come with recommendation as for design parameters and mode of operation of a commercial survey system