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Long range shock waves from a large explosion underwater: experiment and data
6.2.2.1 BGS si te model: The BGS site model (Table D2b) has a sea
thickness of 60 m, thinner than that of the shot-point Central Model. The
parameters of the two sea-bed layers are identical in the two models. The
significant differences are confined to the shallow upper crust (layers
4-5) and the upper crust (layers 6-8), inspection of Fig. 6.1 underlines
the reasons for this. Parameters for the mid crust, starting at 9 km
depth, are identical in the two models. Indeed, the seismic parameters of
the deepest upper crustal layer (8) are also identical, layer 8 in the BGS
site Model simply being thinned to extend from 5.3 to 9.0 km. Differences
in seismic properties between the two models are therefore constrained
within depth above 4 or 5 km and mainly reflect the thicker Triassic and
the identification of Devonian layers.
6.2.2.2 Tentsmuir hard rock site model: This model (Table D2c)
obviously excludes a sea layer. The two sea-bed layers of the previous
models are also excluded and no matching to the unconsolidated sediment
layer in the Firth of Forth model FF1 (MacBeth & Burton 1988) is
considered, although a model representative of the three-component
seismometer site at THA on soft rock would need this. The general
interpretation of the geological strata at Tentsmuir is shown in Fig. 6.1
for the shallow uppr crust down to 2 km depth (layers 1-3).
The 300 m thickness of the Lower Devonian Old Red Sandstone upper layer
is inferred from the BGS Arbroath Solid Geology Map (1:250000); similarly
inferred are the Lower Devonian volcanics, basalt and sandstone extending
down to 2 km and devided into layers 2-3 for seismic parameterisation.
Shear wave velocity is drawn from Evans' (1981) inversion of Rayleigh wave
velocities in the Old Red Sandstone and in Devonian Lavas. Footnotes to
Table D2c explain the derivation of the seismic parameters in the shallow
and upper crust. Parameters of the mid and lower crust, and upper mantle
down to over 90 km (layers 5-10) are identical to the bottom six layers of
the two offshore models previously constructed. Four more layers are
added to the Tentsmuir hard rock model extending it to below 12000 km into
the upper mantle. The parameters of this extension derive largely from
the Jeffreys-Bullen 'A' model and are incorporated simply with an eye to
future modelling of the seismic waveforms up to 1 Hz energy at Tentsmuir,
a range of 148 km from the shot-point.
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