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MAST 2 project, achievements
The project "Geophysical seafloor exploration with a towed array in shallow water"
involved the interaction of expertise coming from system engineering, signal processing, functional optimization, acoustic modelling and geophysics. It comprised a theoretical study with simulated data, the design and assembly of a non-acoustic positioning system on an existing acoustic instrumentation, a major sea trial for data collection, and a validation study on field data. The main result of the project is the conclusion that the proposed methodology is indeed feasible to obtain a detailed and spatially localized estimate of the sea bottom structure with a considerable saving in ship time and cost of the operations required. The more detailed findings of the project can be resumed as follows:- a towed array of 156m length, with 40 receivers spaced at 4 meters from each other is able to collect data sufficient for the estimation of sea bottom geophysical properties; the length of the array is such to allow its operation in shallow water with minimum navigation risk; source and receivers should be towed at about mid water depth, and with about 10 meters difference in depth between them; in any case, the towed instrumentation should stay below the thermocline; in the sea trial the experimental configuration has been of 40m depth for the source, 50-60m depth for the receiver array, a total distance of 685m between the towing ship and the tail of the array, all this in 120m deep waters;
- cw signals in the frequency range from 100 to 200 Hz were experimentally shown able to resolve structures with thickness of 3 meters, and with penetration up to 20 meters; use of lower frequencies can increase the penetration ability of the method, while higher frequencies will enhance resolution; incoherent broadband summation of different signals did not enhance the performance;
- position of the receiving sensors has to be known with an accuracy of lambda/2 in range and lambda/5 in depth (lambda being the acoustic wavelength); a system measuring the deformation of the array shape under tow has to be included in the instrumentation, and it is critical for the success of the estimate; the system developed and tested in the project, composed by non-acoustic sensors as tiltmeters, compasses and pressure gauges can measure in real time the array deformation with the required accuracy and without interfering with the acoustic measurement;
- data inversion and bottom parameter estimation require the use of global search methods; genetic algorithms and neural networks were tested and proved able to come with correct estimates in a variety of environments with simulated data; with field data, both methods produced estimates in agreement with an independent bottom model of the same area obtained from standard instrumentation (coring, seismic surveys, geophone stations); neural network inversion allows for a great reduction in the number of computation required; genetic algorithms allow for a more robust inversion, and can take into account in a much easier way the changes in the environment (like sloping bottoms, etc.).