SiPLABoratory

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.).

Conclusion

The towed array system for geophysical exploration has been proved successful in operation at sea and in the analysis of the experimental data. A great wealth of recommendation and results are now available for the application of such a system in shallow surveys. These results can be considered as a feasibility test of the approach, and lead towards the development of easier-to-use and less expensive systems for seafloor exploration.