Propagation and dissipation of VHF Rayleigh waves in Scotland
This work is an analysis of Rayleigh waves with frequencies of
around 1 to 3 Hz, observed from four explosions fired in Scotland
during the large scale refraction project LISPB.
Time domain measurements are made from reduced tra.vel time
sections showing recordings from the linear LISPB array in northern
Scotland and for isolated paths around the LOWNET network in the
Midland Valley and to the EKA array in the Southern Uplands. These
show a division of the country into seven separate provinces with
boundaries marked by sharp velocity changes and sudden attenuation,
co-incident with features of the mapped surface geology.
In the frequency domain, group and phase arrival times and
amplitude are obtained for each Rayleigh wave recorded. After least
squares analysis, phase and group slowness (s (f), s (f)) are
obtained for each of the seven provinces and the specific dissipation
factor (Q~1(f)) for five.
s (f) ranges from 0.329 to 0.610 s/km and generally increases
with frequency, consistent with s (f) which is always larger.
ranges from 0.015 to 0.050.
These data are inverted using both linear and Hedgehog methods
to obtain shear wave velocity (~) and the shear wave dissipation
factor (Qp1) in layered models for the upper 2 km of the crust in
generally increases with depth, due to compaction and near
surface weathering. It also increases with geological age, which
ranges from the Carboniferous of the Midland Valley to the Moine of
northern Scotland. A low velocity zone beneath the Ochil Hills
indicates that Devonian lavas overlie Old Red Sandstone whichoutcrops further north. Q~ generally increases with depth also.
Introducing a Q~1(f) which varies as f-O.5 makes no marked change
to the models, with a tendency for larger values at shallow depth
and smaller values deeper down