Rapid and robust single receiver geoacoustic inversion in shallow water

Estimating geoacoustic parameters is important for many applications in underwater acoustics. Conventional techniques generally employ vertical or horizontal receiver arrays whose output is matched field processed to invert for bottom parameters. Recent work has investigated whether the receiver array could be replaced with a single receiver with a moving source creating a virtual source array. This paper presents a new technique applicable to shallow-water geoacoustic inversion based on a moving source and a single stationary receiver. In contrast to other single receiver geoacoustic inversion methods, we use the multi-path structure of the signal to create a virtual vertical array to estimate the source-receiver geometry. Since the source is moving we effectively create both an incoherent synthetic horizontal source aperture and a coherent synthetic vertical aperture by means of the multipath. This method can be used to obtain the sound speed and density of the superficial seabed both quickly and inexpensively. To provide multipath resolution, we use a linear frequency modulated signal varying between 1.5-5.5 kHz. Applying a Wigner-Ville transform to the received pulse train and stacking the result gives estimates of the time lags between the arrivals, permitting the multi-path structure to be determined. An inverse Wigner-Ville transform of the isolated direct arrival provides an estimate of the matched filter required in the time domain to estimate transmission losses for each path. Once the source-receiver geometry is estimated from the multipath structure then the angles of arrival for the bottom interacting paths can be computed. The inversion of bottom parameters (density and compressional sound speed) is performed by optimising an objective function that makes use of the angular-dependent reflection coefficient derived from the amplitudes of bottom-interacting paths, given the direct arrival amplitudes and path lengths. Data from experiments conducted in local Singapore waters and the geoacoustic inversion results will be presented.