Subseafloor geoacoustic characterization in the kilohertz regime with a broadband source and a 4-element receiver array

Maritime Rapid Environmental Assessment experiments (MREA/BP´07) were carried out southeast of the island of Elba in the Mediterranean Sea in the spring of 2007 with the aim to produce an integrated 4D (3-dimensional space and time) picture of that shallow water environment. Several standard and advanced techniques of environmental characterization covering the fields of underwater acoustics, physical oceanography and geophysics were combined within a coherent scheme of data acquisition, processing and assimilation. This paper focuses on a technique developed for the rapid 3D geoacoustic characterization of the subseafloor via ldquorange-independent inversionrdquo runs in subareas predetermined on the basis of geological ground truth and an initial hydrographic and geophysical survey. The technique uses a broadband source with 1.2-kHz center frequency, a drifting vertical array of four 5-m spaced, model-based matched filter (MBMF) processing and Bayesian inversion. Experimental results are in excellent agreement with earlier Yellow Shark sparse-array results in the same area demonstrating the feasibility of using the fully-coherent MBMF processing at higher frequencies (0.85-1.55 kHz) and shorter ranges (1-2 km), and therefore lighter instrumentation that is easily deployed by hand from a small high-speed boat or mounted on a remotely operated or autonomous underwater vehicles.