EurOCEAN 2000 - Vlaams Instituut voor de Zee

2. OBJECTIVES
The first objective of ISO-3D has been the development of the computer modelling code. Such
a code did not previously exist, so it will have very wide applications in both physical
oceanography and geophysics. Modelling studies for natural sources are investigating (i) the
effects of induction on electromagnetic observations of basin-scale movements of water masses, at
time scales up to and including long-term observations of climate-related oceanographic processes;
and (ii) tidally generated electromagnetic signals observed both within the oceans and on land, and
their applications in both physical oceanography and geophysics. The CSEM demonstration
survey at the Mid-Atlantic Ridge is for the first time providing 3-Dimensional constraints on
crustal electrical resistivity structure to a depth of at least 2 km beneath the seafloor.
The specific objectives of ISO-3D are:
1. To develop a numerical computer code for modelling the propagation of electromagnetic
fields through a 3-Dimensional earth and ocean structure, with excitation by an arbitrary 3-
Dimensional source in the ocean.
2. To apply the code to model studies of the use of basin-scale submarine cables for monitoring
climate-related transport of water masses.
3. To apply the code to model studies of controlled source electromagnetic sounding (CSEM)
experiments for investigating environmentally and/or economically significant structures and
processes at and beneath the ocean floor.
4. To apply the code to model studies of the use of tidally generated electromagnetic (EM) field
measurements for :
(i) assessment of the possibility of using seafloor EM measurements for the improvement of tidal
corrections in satellite altimetry,
(ii) deep induction studies of the earth, using seafloor electric field or submarine cable voltage
measurements,
(iii) structural studies of the continental margin, using fields measured on land,
(iv) possible monitoring of oceanic velocity fields by electromagnetic observing stations on land.
5. To carry out a demonstration CSEM experiment at sea to establish the viability of the technique
for imaging 3-Dimensional sub-sea-bottom targets associated with structures at the Mid-
Atlantic Ridge south of the Azores, within European waters.
6. To analyse and interpret the results of the demonstration experiment and hence to provide
(i) confirmation that the CSEM method can provide constraints on structure in complex 3-D
environments,
(ii) an improved understanding of the physical environment of the upper oceanic crust within
which hydrothermal circulation takes place at zero age on the Mid-Atlantic Ridge.
3. NUMERICAL CODE DEVELOPMENT AND TESTING.
The numerical model we have developed was motivated by the lack of a general-purpose
numerical model code capable of predicting the electromagnetic field generated by arbitrary
induction sources within the ocean in models of the Earth where the electrical resistivity varies
in all directions. In particular, our aim has been to provide a controlled-source code capable of
handling a three-dimensional Earth, and a motional-induction code capable of simulating the
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