Investigating fault shadows in seismic imaging of data from the Upper-Rhine Graben

Complex fault areas with rapid lateral velocity changes are challenging when consid-
ering a seismic interpretation. In addition, the fracture zones of such regions are also a
primary focus of study for their reservoir opportunities. In some cases, seismic profile
pitfalls can resemble false structures caused by velocity anomalies affected by fault
structures. They identify as fault shadows.
The fault shadow problem has been discussed for many years, starting with (Fagin,
1996) and (Trinchero, 2000). There, the idea of this imaging problem in fault was
suggested due to the lateral velocity variations. Further studies pointed out that a
Prestack Depth Migration (PSDM) with an addition of a well precise velocity model
can give successful results.
This thesis focuses on identifying fault shadow problem cases in seismic imaging for
complex faulting areas, understanding their origin and studying the imaging strate-
gies to solve the problem. For that, synthetic seismic data is used a synthetic study
based on real 3D data is done.
Based on the field data provided, two blocky velocity models are built. Each block
contains the average slowness of the real velocity model, with the idea of representing
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velocity contrast between each block. Then, synthetic seismic data is obtained with
a 2D finite difference using the velocity models created. Followed by a Kirchoff mi-
gration performance in time and depth for each model to create a final image of the
subsurface. The approach here is to reproduce the imaging effect of fault shadows.
The fault shadow effect after a depth migration was only reproduced after using a non
accurate velocity model. Therefore, the velocity model used to migrate the field data
could be missing some feature or high contrast layer.
Additionally, an energy flow for each velocity model is built and correlated with a
seismic ray tracing to study whether the fault shadow effect, in this case, is produced
due to an acquisition problem and a poor illumination of the subsurface. The energy
flow and ray-tracing results showed that receivers in the subsurface could record the
energy that reached the deeper reflectors.
Therefore the fault shadow effect for this study is not an acquisition problem but rather
an imaging problem, and a more precise velocity model would be recommended to
overcome these imaging problems.