Simulation and inversion of deep-towed marine electrical resistivity survey data

Electrical resistivity surveys in marine environments are often subject to a strong screening effect because conductive seawater can short-circuit most current and prevent the currents from entering the seafloor. We study the deep-towed electrical survey that deploys electrodes as close to the seafloor as possible and still benefits from high mobility and efficiency. In particular, we propose its application in the nearshore and shallow offshore environmental and engineering problems, for which a fault model and a boulder model are tested with a water depth of 50 meters. Most towed marine electrical surveys use the dipole-dipole configuration for convenience, but the depth of penetration can be significantly limited when the thickness of water is much greater than the source electrode spacing. Therefore, we also investigate a deep-towed pole-dipole configuration, in which the remote electrode floats on the water surface sufficiently far away to generate a point-source field near the seafloor. Our simulations have shown that the measured potential difference at long source-receiver offsets decays much more slowly in the pole-dipole than in the dipole-dipole configuration. This advantage enables the pole-dipole configuration to enhance detectability for deep structures. Synthetic inversion experiments also confirm that our deep-towed pole-dipole array can more reliably recover the deep part of targets below the seafloor, whereas the imaging results of the dipole-dipole array may be easily distorted by motion-induced and ambient noises.