A TEST OF A SHORT-BASE-LINE SEA-FLOOR TRANSIENT ELECTROMAGNETIC SYSTEM

We describe the conception, design, construction and testing of a towed electromagnetic system capable of mapping the near‐surface electrical conductivity of the sea‐floor. The transmitter and receiver coils are arranged coaxially, and dragged along the sea‐floor. The transmitter coil is 2 m in length and 1 m in diameter, and contains 100 turns of wire. It is energized from the surface by a constant voltage, whose polarity is reversed every 5 ms. The resulting transient magnetic field is detected in the receiver coil. Received signals are amplified and sent back to the surface for processing and analysis. Following a transition in the transmitter current, two distinct transients are observed at the receiver. These events correspond to electromagnetic energy which has diffused through the sea‐water and less conductive sea‐floor, respectively. The onset, amplitude and decay of the first transient are primarily a function of the conductivity structure of the sea‐floor. A successful survey with the system was carried out in shallow coastal waters east of Vancouver Island. The survey yielded 20 conductivity measurements along three lines. The data are stacked 512‐fold, and the shape and amplitude of the resulting noise‐reduced signal are compared with theoretical signals using a generalized linear inversion process. The shape, amplitude, and delay time of the received signal are indicative of the conductivity of the bottom sediments. The resulting model is a layer of mud of conductivity 1.2 S m‐1 and variable thickness overlying rock or sediment with a conductivity of about 0.1 S m‐1. The model is consistent with seismic log profiles obtained during the survey, and with conductivity values expected for surficial, marine sediments. Copyright © 1990, Wiley Blackwell. All rights reserved