Marine electromagnetic induction studies - Marine EM Laboratory

314 S.C. CONSTABLE
Alvin before other experimental priorities halted the exercise. The data yield
half-space resistivities of 13 and 20 f~m; in excellent agreement with established
values for seafloor basalt. It is unfortunate that this experiment could not be
completed and data collected over the sulphide deposit.
In what may be regarded as a related experiment, Francis (1985) deployed a
short (50 m) Wenner resistivity array from a submersible over a sulphide deposit on
the East Pacific Rise. Resistivities of around 10 f~m for the pillow basalts and as
low as 0.17 f~m for the sulphide deposit were recorded. Although the experiment
was extremely simple, these represent the only in situ conductivity measurements of
seafloor sulphides, and indicate what a useful tool resistivity mapping will be if
these deposits are to be exploited.
Having suspended a transmitter from a ship and floated a second from within a
hydrothermal vent field, it was clear that the next exercise for the Toronto/PGC
group would be to suspend one from a hole drilled through an ice sheet. This was
done through ice covering 18 m of water in the Beaufort Sea, in order to map the
electrically resistive permafrost zone which lies beneath the unfrozen seafloor
sediments (Edwards et al., 1988). Transmitter-receiver separations of 20-200 m
were obtained. The data did indeed show evidence of a buried zone at a depth of
15 m or so with a resistivity an order of magnitude greater than the sediments. An
attempt was made to study anisotropy by operating at a second, higher (39 Hz)
frequency, but the researchers were thwarted by noise.
The Toronto school has been prolific in the generation of background theory for
various aspects of underwater controlled source EM sounding, including the
response to transient systems and extension to simple 2D models (Edwards and
Chave, 1986; Edwards and Cheesman, 1987; Cheesman et al., 1987; Edwards,
1988a, b; Everett and Edwards, 1989). Most of this theory is directed to a
completely new area of marine research; that of exploring the continental shelves
and mid-ocean ridges for natural resources using EM methods. Although early
attempts have been made using DC methods and a recent dipole-dipole IP system
was used in search of seafloor mineral sand deposits (Wynn, 1988) both Toronto/
PGC and Scripps have recently tested inductive systems, using different geometries.
The electric dipole-dipole system that works so well in deep water can be modified
to operate in shallow water in search of shallower targets by increasing the
frequency of operation to hundreds of hertz (Constable et al., 1986). A magnetic
dipole-dipole system working in the time domain was operated by Cheesman et al.
(1988).
5.1. INTRODUCTION
5. Oceanographic Studies
There has been an increased usage of electromagnetic instruments for oceano-
graphic purposes. Measurement of the seafloor horizontal electric field gives a
measure of (orthogonal) horizontal water motion, based on the simple principle