ISSN:
1365-246X
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
Notes:
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-246X.1990.tb01782.x
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