Seismic
Modelling of Large-Scale Conical Intrusions
Vigorito, Mario1, Andrew Hurst1,
Anthony Stephen John Scott1, J. A. Cartwright2 (1)
Aberdeen University, Aberdeen, United Kingdom (2) Cardiff University, Cardiff,
United Kingdom
Seismic investigations documented that
sand injections commonly show a conical morphology and occur either as isolated
units or clusters of conical features. However despite of the great abundance
of subsurface examples until now no analogue with a similar scale and geometry
has been documented from outcrop. The Panoche Giant Injection Complex (CA, USA) is probably the
largest injection complex occurring at outcrop. It extends for more than 54 km
in length and it is up to 800m thick. This injection complex is characterised
by the occurrence of winged sills and of low-angle dykes and dyke swarms, which
are arranged to form injected units with a semi-elliptical to horse-shoe
geometry in plan view and V- or U-shaped cross section. These features are 500
to 1.5km wide and in some cases cut vertically through more than 250 m of host
rock. One of these injected units has been selected in order to perform seismic
simulation. The selected unit is a 1.5km wide V-shaped, low-angle, asymmetric
dyke. This latter is 15-18m thick and is characterised by two diverging inner
segments which cut the host-rock strata with angles of 5-20º (total width 1.2
km) and pass outwards into more inclined segments 30-60º steep. A first attempt
to model the acoustic response of this injected unit generated an asymmetric
V-shaped seismogram, 75ms thick which closely match the dimension and geometry
of conical features imaged from subsurface in the off-shore Shetland-Faroe and
in the North
Sea.