Detection of high conductivity objects buried in seafloor sediments

The problem of underwater imaging of submerged objects with conductivity contrasts, that are located in layers of sediments under seawater, is both interesting and challenging. One approach to addressing this inverse scattering problem is to probe the region surrounding the object with a very low frequency field, e.g., a few Hz, for which the associated skin depth in seawater is on the order of several meters. The measured data is subsequently processed to locate the buried object from the knowledge of its scattered field. Probing with low frequencies is necessary in order for the electromagnetic wave of an electric dipole located just below the sea surface to reach the sea floor. This is because the average conductivity of sea water is 4 S/m and the skin depth of several meters can be realized only by using probing frequencies on the order of a few Hz. Although the technique used in the work is similar to that of Wang and Hohmann (see Geophysics, vol.58, no.6, p.797-809, 1993) for conductors buried in the Earth, the present case is considerably more challenging because it deals with dipole fields radiating in sea water, which is orders of magnitude higher in conductivity in comparison to that of soil. We discuss the issues pertaining to the absorbing boundary conditions and the displacement current term in Maxwell´s equations, in the application of Yee´s finite difference time domain (FDTD) method to the problem at hand.