Survey Planning
Pre-survey planning technology at PGS
PGS has the leading pre-survey planning tools and capabilities in the industry.
The NucleusTM package of modeling and processing routines is comprehensive and complete, able to model any acquisition configuration, parameter, or scenario:
- Any streamer, seafloor ocean bottom cable (OBC), land, vertical seismic profile (VSP), or cross-well acquisition geometry: For any marine, transition zone (TZ), or land environment
- Any acquisition or hardware system, including all major manufacturers of recording instruments, streamers, hydrophones, geophones, and air guns
- At all times, all modeling is frequency dependent, elastic (both compressional and converted-wave propagation is included), visco-elastic (attenuation effects in the Earth are included in the recorded frequency response), and anisotropic (the physical properties of the Earth are allowed to vary in different directions)
- Modeling algorithms include Recursive Reflectivity (i.e. modified Kennett), ray tracing, and finite difference: Each for 1D, 2D, and 3D models
No other seismic contractor owns such capabilities. Combined with over 200 man years of international pre-survey planning experience, PGS can offer the most professional, innovative, and comprehensive pre-survey planning services anywhere, any time, and for any challenge. Coupled with a seamless integration with the PGS operations, engineering, processing, and business units, PGS provides tailor-made solutions with immediate and practical applications.
Each of our international locations collaborate on projects on a daily basis, providing “virtual” teams and competence to important projects, continually ensuring that the best effort is made, and the best resources are applied. Finally, by having pre-survey planning teams in each major international location, PGS will provide personalized service and feedback, on time, every time.
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A few details on Pre-survey Planning and Modeling
Modern seismic pre-survey planning must utilize a suite of modeling and processing tools, each incorporating the full acquisition system response for any given survey. Every facet of the seismic method must be replicated and understood. Therefore, the user can discriminate between acquisition and Earth effects upon the data. Pre-survey planning must incorporate all relevant knowledge of the geology, acquisition systems, processing requirements and interpretation objectives – all accounted for whilst honouring geophysical integrity and accuracy. Then the data implications of any given acquisition approach (streamer, land, ocean bottom sensor, vertical cable) can be understood and treated throughout the entire processing and interpretation workflow. In particular, the ability to accurately model the Earth reflectivity sequence is critical for AVO, reservoir characterization and time-lapse studies.
Modeled synthetic shot gathers from a simple 1D model. Different modeling algorithms yield different results: Recursive Reflectivity (left), Ray Tracing (middle), and Finite-Difference (right).
PGS uses the “source-system-model” approach during any pre-survey planning exercise. PGS will attempt to replicate the exact frequency-dependent, visco-elastic 3D behaviour of the seismic source and acquisition equipment for any given survey parameterization. Then, the only unknown is the model, which is of course specific to the survey location and geology.
Using the source-system-model approach at all times, a typical survey planning exercise begins with the assimilation of all available geological and geophysical data. Ideally, a full suite of well logs will be provided, enabling an accurate reflectivity analysis of the seismic response and resolution of the target lithology and fluid characteristics. Such studies are of particular significance for converted-wave ocean bottom cable (4C OBC) surveys, where we seek to understand the difference between P-P and P-S reflection events. Any existing seismic data of relevance will be incorporated into the analyses at this stage, as there is no substitute for real data that incorporates the full wavefield response of the target Earth model. However, the use of real data in the overall survey planning scheme is typically limited, as we are strongly constrained by the acquisition parameters used, which will likely be quite different to those for any new survey. 4C OBS survey planning will almost always have no precedent, which is why the availability of full wavefield logs (including S-wave sonics) is critical. For all survey planning scenarios, the unavailability of full log data will demand some kind of prediction of the elastic model parameters, and consequently, the integrity of all (P-P and P-S) reflectivity and AVO results will be at best approximate.
A suite of 2D and 3D elastic models are then constructed, the complexity and accuracy of which are dictated by the amount of available data for model building. It is often the case that a new 3D survey will occur in a relatively virgin area, so the models built will be by necessity simplistic, and the experience and technical skill of the survey planning geophysicists will be of particular importance. Fundamental 3D issues like subsurface fold and illumination, analyses of acquisition footprints, shooting direction etc., are all typically addressed by dynamic 3D ray tracing and processing. General offset and spatial sampling requirements can be addressed by all the modeling methods, depending upon the complexity of primary and noise interference at larger offsets, and upon the detail of resolution and AVO criteria specified for the survey. Each acquisition parameter is addressed individually by a variety of real data and modeling tests, and an understanding of the overall wavefield phenomena for the target area will develop. In areas of existing seismic data, incorporation of the real navigation data will increase the relevance of any modeling results.
High-end studies involving reservoir characterization, fracture analysis and reservoir monitoring feasibility studies all require the full wavefield modeling power of 2D and 3D visco-elastic finite difference algorithms, and will involve comprehensive log analysis, rock physics investigation, and seismic modeling, greatly expanding upon the scope of exploration-scale survey planning.
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How accurate is the PGS pre-survey planning?
It is reasonably straightforward to constrain and optimize the basic streamer spread or seafloor cable dimensions: 1. Maximum acceptable streamer spread width, 2. Minimum required streamer length, and 3. Optimum CMP bin dimensions (incorporates source and frequency bandwidth modeling). These are the key issues affecting survey cost and efficiency. The next level of planning effort should address different shooting strategies (such as shooting direction, degree of sail line overlap, multi-azimuth, etc.).
In cases where modeling results are an explicit function of a priori information, such as kinematic analyses of illumination and offset statistics, then a rather predictable link exists between the accuracy of the result and the accuracy of the input model. It is demonstrable that dynamic (elastic) analyses of the acquisition footprint and the frequency bandwidth can also be quantitatively reliable even when the input model is reasonably simplistic and unconstrained by a detailed knowledge of the Earth. Provided that appropriate modeling tools are available, that sufficient diligence is given to building realistic elastic Earth models, and that the principles of elastic wave propagation and the seismic method are understood, then it is repeatedly demonstrated by PGS that pre-survey simulations of any 3D seismic experiment are both quantitatively accurate and robust.
The synthetic cross-line DMO stack event at the upper-right represents the result of 3D ray tracing and processing with a 3D dipping plane-layer model based upon a survey location in offshore Indonesia. An actual cross-line from the production DMO stack cube (left) was then used to quantitatively interrogate the footprint amplitudes. An event at ~ 0.7 s TWT was auto-tracked, and compared at the bottom right with the predicted footprint amplitude (80 cross-line stack traces equates to five nominal sail lines). As expected, the modeled footprint (red) slightly overestimates the actual footprint (blue), but the match is good. The modeled amplitudes are asymmetric because 2°feathering is used in modeling. The real amplitudes are irregularly periodic because vessel steering varies from line-to-line.
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What is PGS doing to improve its pre-survey planning technology?
The Technology division of PGS pursues a holistic approach to survey planning technology. Just as the implementation of pre-survey planning should acknowledge operations, engineering, processing, and business considerations, so should the development of new technologies make such acknowledgements.
New developments of the NucleusTM modelling and processing package, the gAS-VIPER onboard recording and QC systems, the holoSeisTM visualization technology, and the suite of technologies contributing to the HD3D data products, are all done in concert. NucleusTM and holoSeisTM are becoming standard installations on the PGS vessels and crews, thereby providing invaluable feedback to ongoing R&D efforts. Most notably, all PGS technologies are being developed to provide new complements to the PGS HD3D value chain of products.
Time slice from the East Java HD3D survey at a very shallow 0.15 seconds two-way time (TWT). Resolution and quality of the complex meandering channel system is excellent. Horizontal scale ≈
15 km. This outstanding data quality was the result of a comprehensive pre-survey planning exercise by PGS, which identified the solution to a location historically affected by poor resolution and extremely noisy data.
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