Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet

that the upper mantle beneath oceans is hotter than the mantle
beneath continents down to a depth of a few hundred kilometers.
If this is so, a corresphndingly higher conductivity should exist
beneath oceans which could be recognised from a reduced depth
of penetration in comparison to continents. Once a characteristic
conductivity difference between an oceanic and a. continental
substructure has been established, this could be used tb recog-
nize former oceanic mantle material beneath present-day con-
tinents and vice versa.
First houndings with recording instruments at the bottom of the
sea have been carrred out. They confirmed to some extent the
expectation of h'igh conductivities at extremely shallow depth,
but these punctual soundings may not be representative for the
oceans as a whole. Here the development of new experimental
techniques for expedient seafloor operations of magnetic and
geolectric instruments has. to be awaited.
~~sarvatioAs,on. . mid-oceanic islands provide a less expensive
way to study the induction in the oceans which in he case of
substorms and Sq is strongly coupled to the crustal and subthe
ocea s
crustal conductivities beneath . Buf again oceanic islands
are usually volcanic and their substructure may differ from that
of ordinary parts of ocean basins.
The island-effect itself is no obstacle fo$ soundings into the
deep structure. In fact, this effect represents a powerful tool
to investigate the inductive response in the surrounding open
'ocean, since the.theoretica1 distortion of the varia-tion fields
due to the islands can be regarded at sufficiently Low frequency
as a direct current problem for a given pattern of oceanic in-
duction currents at some distance from the island. Setting the
. ..
.density of these currents in relation to the observed magnetic
field on the island gives the impedance of the variation field
, for the surrounding oceari of known integrated conductivity. To a
first approximation induced currents in the ocean do not contribute
to the horizontal magnetic field on the island. Hence, by
knowing their density the horizontal magnetic field on the sea
s
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floor can be calculated from which the inductive response function
for the oceanic substructure follows.
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