A Short Baseline Transient Electromagnetic Method for Use on the Sea Floor

Abstract

A towed electromagnetic system capable of mapping the electrical conductivity of the sea floor over a large area has many possible applications, including mapping Quaternary geology, and understanding the physical properties of midocean ridge hydrothermal systems in association with massive sulphide deposits. However, the electrical conductivity of seawater is usually much greater than that of the sea floor, rendering the majority of electromagnetic systems presently employed on land unsuitable for marine use. A theoretical study of the transient step-on responses of some common controlled source, electromagnetic systems to adjoining conductive half-spaces shows that for two systems, the horizontal, in-line, electric dipole-dipole (ERER) and horizontal, coaxial, magnetic dipole-dipole (HRHR), the position in time of the initial portion of the transient indicates the conductivity of the sea floor, while at distinctly later time, a second characteristic of the transient is a measure of the larger seawater conductivity. The diagnostic separation between the two parts of the transient response does not occur for many other systems, including several commonly used for exploration on land. The concept of apparent conductivity is defined for the transient system in terms of the time of the first arrival of the transient signal. This apparent conductivity is used to produce characteristic curves of the HRHR system response for different layered earth models. A prototype HRHR system operating on a 100 m scale has been designed and constructed. A successful test of the system in shallow water was conducted in the coastal waters of Vancouver Island. The survey yielded 37 conductivity measurements along three lines. The instruments were towed by a ship along the sea floor. The tow cable carried both current to the transmitter coil and the received signal back to the ship for processing. Both the shape and amplitude of the received signal are indicative of the conductivity of the bottom sediments. Inversion of the data suggest that a varying thickness of 1.2 S·m-1mud overlies rock or sediment with a conductivity of about .1 S·m-1 . Improvements in equipment design might allow the production in real time of a continuous map of the conductivity structure of the sea floor.