EurOCEAN 2000 - Vlaams Instituut voor de Zee

electromagnetic fields generated by large scale, relatively high frequency seawater motions
such as the open-ocean tide, where self- and mutual induction must be considered in realistic
models of the oceans and solid Earth on ocean basin or global scales.
Our approach to this problem has been to adapt a widely used and well tested numerical code
for the related magnetotelluric (MT) problem to compute the electric and magnetic fields
induced in the ocean and the underlying earth by general electromagnetic sources within the
water column. Our new modelling code has therefore been based on the Mackie and Madden 3-
D magnetotelluric code. The code uses conjugate gradients with incomplete Cholesky
decomposition to solve for the electromagnetic field on staggered finite-difference grids. It first
solves for the magnetic field, then computes electric field values as required. Including
Mackie's incorporation of the correction first introduced by Smith for the divergence of the
magnetic field, a number of the recent modifications by Booker and Mackie, and the addition
of more general geometries based on earlier work of Flosadottir et al. with the Smith code, the
new code can still be used to model the original MT field, but the parts of the code that set up
the equation to be solved and afterwards calculate the electric field from the circulation of the
magnetic field and the electric source currents have been extensively rewritten. The
modifications have been designed so that the model equations are based strictly on the integral
forms of the Maxwell equations. The code can now be used to model oceanic or man-made
sources in Cartesian, spherical-Earth, or terrain-following coordinates for spatial scales ranging
from crustal-scale controlled source sounding to basin-scale models of induction by the ocean
general circulation and tides.
The most important added capability of the new code is the possibility of turning off the MT
forcing and boundary conditions, instead specifying a prescribed electric current at one or more
of the points where electric fields are defined on the model grids. Point sources, such as a
controlled-source horizontal electric dipole, may be specified by input of a non-zero source
current at one or a few grid points, while oceanic sources are modelled by source currents
specified at all points within the model ocean.
Another new capability is made possible by the inclusion of metric factors that can be invoked
to scale distances between points on the model grid so as to simulate geometric effects such as
convergence of the meridians (and the shrinking of horizontal distances with depth) on a
spherical Earth. In addition to large-scale models in which the Earth's curvature needs to be
taken into account, this will also be used for terrain-following co-ordinates based on idealized
topography or bathymetric data.
4. DEMONSTRATION 3-D CSEM SURVEY: THE ‘MADRIGALS’
CRUISE, CD120
RRS Charles Darwin Cruise 120 – designated ‘Madrigals’ (Mid-Atlantic Deep-towed
Resistivity and Induction Geophysics), investigated the ‘Lucky Strike’ segment of the Mid-
Atlantic Ridge - an area which lies to the SW of the Azores archipelago, and within the EEZ of
Portugal. During the cruise we carried out a 3-D controlled-source electromagnetic (CSEM)
sounding study of the upper and middle crust beneath the central volcano of the segment, in
order to determine crustal electrical resistivity structure and hence to constrain the physical
properties of the crust beneath a region of very active recent volcanism and current high- and
low-temperature hydrothermal venting. The technological and methodological objective was
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