Summary: FORCE
Deep Water Seismic Challenges Seminar
By John Hughes, FORCE
Seismic Methods Work Group Leader
The FORCE Seismic Methods Work Group organised a “Deep Water
Seismic Challenges” Seminar hosted by the NPD on
Three principal themes were evident:
This included a variety of technologies, but for the
It is convenient to break “improving the seismic image” into three sub-categories: (a) acquisitions, (b) processing, and (c) Analysis of DHI’s and Lithological Responses:
Seismic acquisition techniques described were largely geared to improve signal to noise (mostly through appropriate data acquisition for multiple attenuation) and improving resolution. Methods described included high density 3D data acquisitions for higher fold and reduced cross line aliasing, wide towed streamers appropriate for 3D SRME de-multiple, multi-azimuth acquisition for improved illumination, and OBC techniques notably for Pz summation multiple attenuation. Mark Thompson of Statoil laid down an intriguing challenge for research and development groups to develop methods to measure vertical pressure gradients or particle motion with towed streamers to enable de-multiple through Pz summation rather than more costly OBC deployment.
A key point in the data processing world is the step change impact enabled by modern clustered parallel computing facilities which have enabled 3D de-multiple and depth migration codes to be affordable. Key advances in demultiple techniques included:
· 2D and notably 3D SRME (as demonstrated by the successful Magic3D application by Shell at Ormen Lange).
· 3D Wavefield extrapolation de-multiple (a complementary and intriguing alternative to 3D SRME applied by BP at Ormen Lange)
· Data “Friendly” demultiple methods such as designature de-multiple.
·
SRME
for OBC data
There was also interesting discussion on the appropriate use of SRME, radon and DIMAT from CGG.
Fred Herkenhoff of ChevronTexaco presented an optimum amplitude preserving processing sequence and a
very enlightening view of the lack of repeatability in amplitude preservation
between of 6 contractors processing of a seismic line from offshore
Pre-stack depth migration is a key deep-water seismic
challenge in many parts of the world notably in sub-salt environments including
as seen in
It should not be overlooked that pre-stack depth migration
is also an important step in areas of mid
Once the seismic data has passed through the optimum data acquisition and processing steps, as described above, the next step is interpretation and analysis. The analysis of seismic amplitudes for fluid and lithological discrimination was also a significant theme at the seminar. Topics discussed included:
a) Pz summation of OBC data to remove flat multiple events and reduce the risk of improper identification of fluid contact related flat spots.
b) Use of long offset data was advocated for lithology discrimination and inversion techniques.
c) EM methods were described by EMGS, which provide resistivity based information for fluid type discrimination independent of seismic amplitudes and thus add an extra parameter to reduce uncertainty in this non-unique process.
3. Cost
Cost is a major hurdle for application of many of the deep-water seismic technologies as the oil and gas industry continues to reduce expenditures. However we should be encouraged by successful case examples presented at the seminar which clearly demonstrate considerable added value through techniques such as 3D de-multiple, 3D pre-stack depth migration and recent advances in seismic acquisition such as OBC methods, wide towed streamers, long offset data and high density 3D. Success has also been shown with non-seismic methods such as the recent developments in EM seabed logging. All of these require expenditure over and above what is generally accepted as “standard”. It is therefore a critical challenge for all of us involved in geophysical work in deep water areas to demonstrate the value of these technologies to the decision makers within the oil and gas industry.