Undershooting

Key Takeaways

• Platform undershooting surveys restore seismic coverage to the most commercially valuable part of a field — the area directly beneath and immediately around the production platform, which is typically the heart of the accumulation and the target of the densest well cluster — and allow operators to make infill drilling decisions based on 3D seismic data with comparable quality to the open-water areas rather than relying on interpolation across a coverage gap; in the North Sea, where fixed jacket platforms are common in fields with 30-50 years of production history, undershooting has been used at fields including Statfjord, Brent, and Gullfaks to image the remaining hydrocarbon distribution below and around the platform topsides and conductor bundle; the imaging quality from an undershooting geometry depends on the source-receiver offset range achievable around the platform's physical dimensions and the depth of the target reservoir — a shallow reservoir requires short offsets for adequate near-vertical reflection coverage, while a deep reservoir at 3,000-4,000 meters can be adequately illuminated from sources many kilometers away on either side of the platform, giving more flexibility in the acquisition geometry.
• Ocean bottom cable (OBC) and ocean bottom node (OBN) receiver deployment beneath and around platforms is the enabling technology for modern undershooting surveys because it separates the receiver deployment from the source vessel operations — OBC cables and OBN nodes placed by ROV on the seabed can be positioned directly beneath the platform conductor bundle and topsides (where a towed hydrophone streamer could never be positioned) and record the seismic signals from sources fired at any azimuth around the platform; the OBN configuration (individual autonomous recording nodes rather than connected cables) is particularly useful in the complex umbilical and riser jungle beneath a production platform, where the ROV can thread individual nodes between obstacles that would prevent a continuous cable from being laid; the four-component (4C) OBN record also provides converted-wave (PS-wave) data alongside the P-wave data from the source, extending the subsurface information content of the undershooting survey beyond what a conventional marine streamer geometry would provide; node retrieval after the survey must also navigate around platform infrastructure, and ROV recovery operations beneath a live production platform require careful coordination with the platform operations team to avoid inadvertent contact with production umbilicals, control lines, and subsea completion equipment.
• Land undershooting uses explosive or vibroseis sources on one side of a restricted or sensitive area (a town, a mine, an industrial facility, a nature reserve) while receivers are deployed on the far side to capture reflections that have traveled through the subsurface beneath the restricted zone — this geometry allows seismic data to be acquired beneath areas where surface access for source operations is prohibited, expanding the survey coverage to target geological structures that would otherwise be in the shadow zone of the surface restriction; land undershooting requires careful survey design to ensure adequate subsurface illumination (the source-to-receiver geometry determines which subsurface reflection points are sampled and at what incidence angles), and the data processing must account for the very different source-receiver azimuth distributions inherent in an undershooting geometry versus a conventional cross-spread or orthogonal geometry; in urban areas where entire city blocks may sit above economically significant tight gas or shale formations, undershooting or similar wide-aperture acquisition geometries are sometimes the only practical approach to acquiring seismic data without interrupting surface operations above the target.
• Seismic processing for undershooting data faces the challenge that the source-receiver geometry is fundamentally non-symmetric — in conventional marine acquisition, sources and receivers are both on the same vessel (or nearby vessels) and the geometry is approximately symmetric around the common midpoint; in undershooting, the sources are on one side and receivers on the other, so the common midpoint between each source-receiver pair falls directly beneath or near the platform obstruction; this geometry concentrates the subsurface sample points exactly where coverage is needed but requires specialized processing flows to handle the large range of source-receiver offsets and azimuths present in the undershooting dataset; common midpoint stacking, which is the standard processing approach for conventional marine data, still works for undershooting data but the velocity analysis and multiple attenuation must account for the asymmetric geometry; pre-stack depth migration (PSDM) using a detailed 3D velocity model is the preferred imaging approach for undershooting data in complex subsalt or sub-platform environments, because it can correctly position reflections regardless of the acquisition geometry asymmetry.
• Simultaneous shooting during undershooting surveys (using encoded sources that allow multiple vessels to shoot simultaneously without crosstalk contamination) has dramatically reduced undershooting survey acquisition time and cost by allowing sources from multiple vessels on different sides of the platform to be active at the same time while the OBN or OBC receivers record all of the encoded signals simultaneously — simultaneous source technology (using unique source signatures for each vessel that can be separated in data processing) converts what would otherwise be a sequential one-vessel-at-a-time acquisition into a parallel multi-source acquisition that can triple or quadruple the data acquisition rate; for an undershooting survey around a large production facility in the North Sea or Gulf of Mexico where platform safety exclusion zones may limit source vessel positioning to specific corridors, the ability to shoot from multiple corridors simultaneously without interference between sources reduces the survey duration from weeks to days and correspondingly reduces the operational risk of weather, equipment failure, and platform operational conflicts during the acquisition.