Imaging axi-symmetric TAG-like structures by transient electric dipole seafloor electromagnetics

We investigate the response of a conductive axi-symmetric TAG-like structure to a transient in-line electric dipole-dipole seafloor system. Coupled differential equations for the azimuthal electric and magnetic fields are solved by finite differences iteratively for an off-axis source. We introduce an electrical model of TAG and investigate the sensitivity of first arrival data to the conductivities of the sulfide layer and the deeper root structure. The transmitter dipole is located on one side of the mound, 200 m from its center, while receiving locations are on the mound itself, or on an offset profile 200 m from the center of the mound, on the opposite side of it from the transmitter. We show that the time taken for a first arrival to be seen at the receiver following a transition in the transmitter is a robust estimator of seafloor conductivity because it is insensitive to the effects of topography. Times of first arrivals observed on the mound are sensitive to the conductivity of both the root structure and the sulfide. Times of first arrivals at locations on the far side of the mound are sensitive to the properties of the root structure only. Travel time effects are more pronounced on transmitter-receiver paths which pass through the TAG than on paths which graze the structure. Given sufficient data, the interior of TAG may be imaged using tomographic algorithms.