In-Loop Transient Electromagnetic Responses With Induced Polarization Effects of Deep-Sea Hydrothermal Deposits

摘要

A transient electromagnetic (TEM) system using a small loop source is advantageous to the development of compact autonomous instruments, which are well suited to a submersible-based survey. Since the electrical conductivity of subseafloor materials can be frequency dependent, these induced polarization (IP) effects may affect the reliability of TEM data interpretation. In this paper, we investigate the IP effects on the TEM responses of deep-sea hydrothermal mineral deposits with a thin sediment cover. Time-domain target signals are larger and appear earlier in horizontal magnetic fields than in vertical magnetic fields. The IP effects cause transient magnetic fields to enhance initially, to decay rapidly, and then to reverse the polarity. The dc conductivity and chargeability in Cole-Cole parameters influence the time of sign reversals and the enhancement of target responses, simultaneously. A damped least squares inversion scheme is applied to reconstruct a polarizable hydrothermal deposit model using synthetic TEM sounding data sampled from a time interval before the sign reversal. The test example demonstrates that the Cole-Cole parameters of a highly conductive layer can be resolved more effectively from vertical magnetic fields than from horizontal magnetic fields.