Summary
The marine controlled-source electromagnetic (CSEM) method has been evolving
into a geophysical imaging tool for increasingly complex geological settings in
which multiple resistive bodies can be resolved. An advanced subsurface imaging
workflow for 3D CSEM surveys is presented, which reproduces the resistivity
distribution to within a spatial resolution determined by the frequencies
included. The performance of our advanced-processing workflow is demonstrated
using a case study from the Gulf of Mexico (GOM), where a dense 3D grid was
acquired over an area of high-quality seismic data and well log control.
At the core of our 3D workflow is an inversion methodology with approximate
Hessian-based optimization and a fast finite-difference time-domain forward
operator. The optimization matches the synthetic to the measured field within
100?200 iterations and is sufficiently robust in three dimensions to avoid
expensive regularization schemes. The sensitivity of the gradient-based
inversion to the starting model is addressed by investing considerable effort
in building 1D inversion-based starting models. At the same time, 3D inversion
algorithms for survey layouts, including azimuthal data, demand high-quality
data conditioning for which we present a processing sequence from time-domain
electromagnetic data acquired by seabed receivers to frequency-domain data and
weights for inversion.
Detection and delineation of reservoirs in the presence of salt is
recognized as a major challenge to CSEM methods. In order to interpret 3D data
accurately in such a complex environment, the true-resistivity cube is built
from a sequence of constrained inversion-based interpretation steps. Using a
data set acquired in 2008 in the GOM, we demonstrate the ability of our 3D
technology to resolve small (2×2 km), low-resistivity pay (Δρ<5Ω·m) targets
within the vicinity (< 1 km) of large salt bodies. In this case study, the
3D method converged within 1 week, running on 150 parallel nodes.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
10 November 2009
- Meeting paper published:
5 May 2009
- Revised manuscript received:
1 September 2011
- Manuscript approved:
17 January 2012
- Published online:
1 June 2012
- Version of record:
12 June 2012
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