Electric potential arising from a point-source near a cylinder in layered earth structures: design criteria for a modified marine heat-flow probe

Abstract

A modified marine heat-flow probe has been proposed to conduct simultaneous measurements of thermal and electric properties of seafloor sediments. A major concern in the electric aspect is the shielding effect caused by an extremely conductive solid steel strength-member of the probe. To appraise the feasibility of the proposed probe in electric measurements, we develop a novel numerical algorithm to calculate potential and hence apparent resistivities arising from a point-source of current near a cylinder situated in layered earth structures. The scheme is validated against an analytical solution with respect to a uniform whole-space in the presence of the cylinder. Image theory is employed to transform the layered earth structures into the whole-space models. Model parameters are taken from the design blueprint. Numerical computations for varieties of electrode configurations reveal that the dipole?dipole array suffers the smallest disturbance, and can be employed to directly measure the true electric properties of seafloor sediments as long as its electrode spacing is less than 10 cm. The electric properties are also measurable when the pole?dipole array or the Wenner array is used with the electrode spacing less than 5 cm. The pole?pole array is however not applicable.