Acoustic detection of motor vessel signals in very shallow water

A number of security threats and interdiction problems in shallow waterways and the very shallow littorals involve small vessels, which offer difficult detection problems. In water depths on the order of a wavelength, sound propagates with considerable involvement of the bottom, whose bulk and shear moduli, as well as velocities and attenuation vary with depth into the sediment. This problem has been studied with a two dimensional finite-element code, to illustrate the consequences of computational assumptions pertinent to these parameters and their gradients with depth into the sediment. Both time harmonic (steady state) and transient (animated pulse) sequences are treated in two dimensional depictions.