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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