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Long range shock waves from a large explosion underwater: experiment and data
6.2. /.5 Density: Derivation of density from velocity values follows
Stuart's (1978) prescription for the North Sea, specific details are
appended as footnotes to tabulated values in the shot-point central model.
6.2./.6 Specific attenuation factors: Regional measurements of
attenuation appropriate to Scotland, the Midland Valley in particular, and
to the North Sea are in short supply. Allen's (1988) review of physical
properties of marine sediments was used to guide selection of attenuation
values in different sand types (Hamilton 1976c) in the sea bottom layers.
1/Q a in the Carboniferous of the Midland Valley has been measured by
Evans (1981) down to about 2 km using Rayleigh waves and their values are
reasonably consistent with MacBeth & Burton's (1987) general results for
attenuation in the upper shallow crust in Scotland. No regionally
specific attenuation values for the upper crust through to the upper
mantle are available and so values are selected from those generally
representative of the continental crust and lithosphere (e.g. Burton
1976).
6.2.2 Geophysical constraints on seismic models and range of applicability
The Shot-point Central Model, the derivation of which is mostly described
above, is given in Table 02a.
The range of applicability of this model depends on the relative
consistency and simplicity of the structure parallel to the shot-point to
Tentsmuir great circle. There is considerable other geophysical evidence
for simplicity of structure, being valley-like, parallel to the shot-line
axis out to the shot-point. Secondly, there is evidence that the AMOCO
VeIl is on the flanks of this structure - this is why deeper layers are
used in the shot-point velocity depth model for the shallow upper crust
(i.e. the Triassic and Zechstein), as the final adjustment of parameters
with respect to those observed at the AMOCO VeIl.
Examination of the contours on the Aeromagnetic Map of Great Britain
(Sheet 1, England, Scotland & Northern Ireland) shows them to be
consistent with a synclinal valley with axis parallel to the shot-line.
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