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|Title: ||Seismic Modelling for the Sub-Basalt Imaging Problem Including an Analysis and Development of the Boundary Element Method|
|Authors: ||Dobson, Andrew|
|Supervisor(s): ||Ziolkowski, Anton|
|Issue Date: ||Jun-2005|
|Publisher: ||University of Edinburgh; College of Science and Engineering; School of GeoScience|
|Abstract: ||The north-east Atlantic margin (NEAM) is important for hydrocarbon exploration
because of the growing evidence of hydrocarbon reserves in the region.
However, seismic exploration of the sub-surface is hampered by large deposits of
flood basalts, which cover possible hydrocarbon-bearing reservoirs underneath.
There are several hypotheses as to why imaging beneath basalt is a problem.
These include: the high impedance contrast between the basalt and the layers
above; the thin-layering of the basalt due to the many flows which make up a
basalt succession; and the rough interfaces on the top-basalt interface caused by
weathering and emplacement mechanisms.
I perform forward modelling to assess the relative importance of these factors
for imaging of sub-basalt reflections. The boundary element method (BEM) is
used for the rough-interface modelling. The method was selected because only
the interfaces between layers need to be discretized, in contrast to grid methods
such as finite difference for which the whole model needs to be discretized, and so
should lead to fast generation of shot gathers for models which have only a few
I have had to develop criteria for accurate modelling with the boundary
element method and have considered the following: source near an interface,
two interfaces close together, removal of model edge effects and precise modelling
of a transparent interface. I have improved efficiency of my code by: resampling
the model so that fewer discretization elements are required at low frequencies,
and suppressing wrap-around so that the time window length can be reduced.
I introduce a new scheme which combines domain decomposition and a far-field
approximation to improve the efficiency of the boundary element code further. I
compare performance with a standard finite difference code. I show that the BEM
is well suited to seismic modelling in an exploration environment when there are only a few layers in the model and when a seismic profile containing many shot
gathers for one model is required. For many other cases the finite difference code
is still the best option.
The input models for the forward modelling are based on real seismic data
which were acquired in the Faeroe-Shetland Channel in 2001. The modelling
shows that roughness on the surface of the basalt has little effect on the imaging
in this particular area of the NEAM. The thin layers in the basalt act as a
low-pass filter to the seismic wave. For the real-data acquisition, even the topbasalt
reflection is a low frequency event. This is most likely to be due to high
attenuation in the layers above the basalt. I show that sea-surface multiple energy
is considerable and that it could mask possible sub-basalt events on a seismic
shot gather, but any shallow sub-basalt events should still be visible even with
the presence of multiple energy. This leaves the possibility that there is only
one major stratigraphic unit between the base of the basalt and the crystalline
The implication of the forward modelling and real data analysis for acquisition
is that the acquisition parameters must emphasize the low frequencies, since the
high frequencies are attenuated before they even reach the top-basalt interface.
The implication for processing is that multiple removal is of prime importance.|
|Appears in Collections:||Earth and Planetary Science Research Institute thesis and dissertation collection|
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