Effect of Suspended Sediment on Acoustic Detection Using Reverberation

The suspended sediment layer occupies the lower water column. Presence of the suspended sediments creates a volume scattering layer which affects acoustic detection. Understanding the acoustic effects of the suspended sediment layer leads to the development of acoustic sensors with capability to scan the seafloor and to detect ordnance such as sea mines. 2. Comprehensive Acoustic Simulation System Sonar equations provide guidelines for system design (Urick, 1983). The governing equation for beam patterns with dominating volume reverberation is given by SL – TL i – TL r + TS RL = SNR (1) where SL is the source level; TL i is the transmission loss of the incident wave; TL r is the transmission loss of the reflected echo; TS is target strength; RL is the reverberation level of sediments; SNR is the signal-to-noise ratio of the sonar data. The transmission losses of the incident and reflected waves, TL i and TL r , account for spherical loss (such as spherical spreading), acoustic attenuation, and boundary loss. Volume reverberation depends on the physical properties of the water column. With the presence of a suspended sediment layer, large quantities of sediment remain in the water column and significantly affect the acoustic transmission in the water. The denser the suspended sediment layer is, the harder it would be for sonar to penetrate through the water. Moreover, suspended bottom sediment layer increases the density of the lower water column and in turn changes the sound velocity profile and prevents acoustic energy from reaching a possible buried object. Reverberation can be used to represent the effects of a suspended sediment layer on acoustic detection. The differential form of the reverberation can be obtained from integration over the ensonified area (Keenan, 2000), d(RL) = SL + 10 log (SA) + SS – TL t TL f + BP t + BP r (2) where SA is the scattering area; SS is the scattering strength per unit area; TL t is the transmission loss to scatterer; TL f is the transmission loss from scatterer; BP t is the beam pattern to scatterer; and BP f is the beam pattern from scatterer. The most important design criterion for detecting mine-like objects is to maximize the SNR, the target echo to scattering noise ratio in decibels. I N T R O D U C T I O N coustic detection of undersea objects is difficult due to the uncertain environment (Chu et al., 2002, 2004) and even more difficult when the objects are buried in the seabed. First, sediments generate high backscattering noise due to heterogeneous scatters within the sediments clouding the object. Second, the acoustic wave attenuation in sediments is much higher than in water. Acoustic shadows make the buried targets absent in the sonar images due to diffraction around the target, transmission through the target and relatively high acoustic noise due to backscattering from sediments surrounding the target. Classification of buried targets is also more difficult since there are no shadows, and the images do not contain much information about target shape since scattering from oblique target surfaces is not detectable. Acoustic images of buried targets primarily consist of echoes from the target surfaces that are normal to the incident acoustic ray path. Target surfaces with an oblique aspect to the incident ray path will backscatter much less energy at the lower operating frequencies of sub-bottom profilers since the acoustic wavelength is much longer than the surface roughness of most targets of interest.