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- On the performance of geo-acoustic estimation for a distributed sensor arrayPublication . Jesus, Sergio; Mantouka, A.; Felisberto, Paulo; Soares, CristianoThe vision underlying the Widely scalable Mobile Underwater Sonar Technology (WiMUST) project is that of developing advanced cooperative and networked control / navigation systems to enable a large number (tenths) of marine robots towing small acoustic arrays to act as a coordinated team for seismic sub-bottom imaging. The space-time coherent processing of bottom returns requires the ensemble of short acoustic arrays to be seen as a single spatially distributed sensor array. Since the vehicles are free to move along range, cross-range and depth the resulting distributed sensor array may take, at least conceptually, any spatial shape. With array shape freedom comes the question of which is the most suitable (or optimal) array geometry for sub-bottom imaging and inversion. The answer to this question hinges, among others, on the definition of performance of a seismic sub-bottom profiling system. Determining the optimal sensor array geometry is clearly a ill-posed problem, since the optimal geometry is itself bottom dependent, and there is no such environment as "one size fits all". This work addresses several criteria for sub-bottom profiling system performance including gain, resolution and probability of detection. Two physical models will be tested: one based on acoustic wave reflection used in traditional seismic imaging, and another normally used in matched-field bottom properties estimation, that includes propagation and refraction. Simulations to support the theoretical developments and algorithms were obtained on a scenario inspired in a real environment off the coast of Peljesac (Croatia).
- Seismoacoustic bottom inversion with AUV towed streamers: a multi-stage approachPublication . Soares, Cristiano; Jesus, Sergio; Mantouka, Agni; Felisberto, PauloThe WiMUST (Widely scalable Mobile Underwater Sonar Technology) Project envisions using a team of autonomous underwater vehicles towing short acoustic arrays for seismic surveying of seabottom geoacoustic properties. One of the objectives in the project is to tackle the inversion of acoustic data collected with short towed horizontal arrays by means of a Matched-Field Inversion (MFI) technique. While there is great deal of experience in MFI and the so-called focalization applied to horizontal propagation scenarios, in near vertical propagation scenarios, with a source receiver horizontal distance limited to a few tens of meter or less, there is little understanding in terms of feasibility of the acoustic inversion of bottom properties. In particular, the simultaneous inversion of bottom properties (soundspeeds, densities, attenuations) of multiple bottom layers needs to be tackled, since the experimenter has to account for the admissible mismatch of other environmental properties such as water soundspeed and depth, and the potential solution ambiguity inherent to an optimization problem with ten or more unknown parameters. The actual simulation study, carried out with an environmental scenario and geometric set up based on the Peljesac data set, considers a shallow water acoustic propagation scenario with a short array. A sensitivity analysis in MFI provides understanding on the observability of the unknown parameters of interest. A mismatch analysis indicates that watercolumn mismatch (soundspeed and depth) may cause the MFI procedure to break down. Based on the conclusions taken from the sensitive and mismatch analysis, an iterative acoustic inversion concept with feedback of intermediate parameter estimates is developed and tested with simulated data.