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Durham University

Department of Earth Sciences

Profile

Publication details for Professor Richard Hobbs

Hobbs, R.W, Drummond, B.J & Goleby, B.R (2006). The effects of three-dimensional structure on two-dimensional images of crustal seismic sections and on the interpretation of shear zone morphology. Geophysical journal international 164(3): 490-500.

Author(s) from Durham

Abstract

Crustal scale seismic images provide information on the geometry of subsurface structure.
In this paper we examine shear zones as they provide geometrical constraints on the evolution
of the crust and as they provide pathways for the migration of mineral-rich fluids from
the lower crust. However, they typically appear in seismic images of the deep crust as laterally
continuous bands of discontinuous reflections with individual reflections often having
high amplitudes. Geological mapping of exposed shear zones show them to have a complex
3-D structure yet crustal-scale seismic reflection surveys use single or at the most only a few
profiles, and therefore only create 2-D images of these structures. The processing and imaging
of the multifold common midpoint (CMP) data assumes that the seismic energy comes entirely
from within the plane of the section. In this paper, we use full-waveform 3-D synthetic data
to consider the effects that 3-D topography on a reflector has on reflection character on a 2-D
profile. We base our synthetics on an observed shear zone and test models with both a single
layer and anastomizing layers. We show that topography on the reflector out of the plane of
the section may cause spurious events both above and below the expected target depth. We
derive the basic understanding using a simple isotropic homogeneous model, however, we then
demonstrate that this is a robust phenomenon and is endemic on all 2-D sections even if the
overburden is not homogeneous.We demonstrate that we obtain similar results with a velocity
gradient or, for a more extreme case, with a 2-km-deep basin filled with low-velocity sediment.
For crustal scale seismic profiles, in particular, the effect is pervasive as neither stacking nor
migration can discriminate against out-of-plane energy and the 2-D stack represents the 3-D
seismic response of a broad swath centred on the profile. However, we conclude that using the
modelled data it is possible to identify qualitatively where there is significant contamination
from out-of-plane topography and show examples from a shear zone in the Archaean Yilgarn
Block in Western Australia.

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