Incoherent Shallow-Water Seabed Logging Using Numerical-Model-Based Optimization for the Prediction of Unperturbed UEP Signatures

Abstract

Underwater electric potential (UEP) ship signatures are evaluated at a test site that provides only incoherent data from electric field sensors placed on the seabed. This paper shows how these data sets can be used to extract significant seabed parameters, which are evaluated by a numerical model combined with an optimization process in the frequency range from 1 Hz up to 100 kHz. This is in contrast to the controlled electromagnetic source technology used by geophysicists, which has to track both phase and amplitude, and, therefore, cannot simply be applied to data from per se stationary systems. Using the aforementioned frequency range within COMSOL Multiphysics (v6.0), the finite-element-based simulation software, a sensitivity analysis of seabed parameter behavior within a seabed depth range from 1 m to 20 m is evaluated for the highest sensitivity for each of the measurable field amplitudes of the locally available sensors. From this, the electrical conductivities of two seabed layers and the depth of the first layer can be extracted using a combination of a genetic algorithm and the Nelder–Mead method. In summary, phase information is not required to characterize the stratified seabed, providing a cost-effective approach to obtaining effective environmental conditions for free-water signature calculations.

Keywords

• seabed logging
• current dipole
• UEP signature
• frequency sweep
• optimization algorithm
• inverse modelling