Moving towards full-sampling in land 3D acquisition

Marty Williams and Scott Hoenmans, Input/Output, explain the requirements for modern 3D seismic on land and anticipate the introduction of new operating technologies and methods to obtain fully sampled reflected seismic energy. Geophysicists need seismic data to convey a reliable, geologically meaningful picture of the subsurface both at the prospect and geologic system levels. This requires that spatial sampling be sufficient to provide adequate resolution not only for identifying drillable targets, but also for characterizing depositional environments, intra- and inter-reservoir discontinuities, and other subtle features associated with finding and developing hydrocarbon reservoirs. To address these imaging needs onshore, sampling density needs to improve significantly by deploying more recording stations than is the current convention. To date, high station count surveys that fully sample reflected seismic energy have been cost prohibitive and difficult to implement. Fortunately, a new generation of cableless, single-station seismic recording systems will soon be commercialized which should provide geophysicists with a viable platform for acquiring high station count surveys and obtaining fully sampled data. Spatial sampling Spatial resolution is not just a matter of bin spacing. In the era of pre-stack migration and anisotropy analysis, all pre-stack domains need to be sampled. These criteria are rarely met in contemporary seismic acquisition since cost and recording system constraints force interpreters to cope with inferior resolution and unnecessarily noisy data that result from footprint, migration artifacts, AVO uncertainties, or unresolved anisotropy. The solution for overcoming these modern interpretation limitations is to spatially sample the reflected seismic energy with increased station densities such that: ■ Source and receiver intervals are equivalent in both the in-line and cross-line directions (an orthogonal geometry) ■ Group intervals are small enough to avoid aliasing signal and organized noise ■ The source effort is balanced with the receiver effort ■ Sources and receivers are symmetric in response (Vermeer, 1990) ■ Data are sampled in the offset, azimuth, and CMP domains so as to not introduce either gaps or aliasing of the reflections within these domains Acquiring this ideal, fully sampled survey is a tall order. In one region we evaluated, it would have taken nearly 100,000 live stations. This would have exceeded any previous station count deployment for this development trend by more than an order of magnitude and satisfied only a portion of the ideal sampling criteria noted above. Conventional land acquisition systems would also have been technically challenged to acquire this quantity of data. Even if technically feasible, the acquisition cost would have been exorbitant. The time is fast approaching, however, when this level of effort will become an economic necessity. Competition for resource access continues to increase worldwide. E&P operators are compelled to showcase environmentally-friendly seismic acquisition solutions with agencies like the Bureau of Land Management that control resource development activities on government lands in the US. They also must prove their sources of technological advantage when vying for production sharing agreements with sovereign host governments. As the pressures to secure and efficiently monetize resources mount, the winners will be those companies that have an edge in finding and developing subtle and more complex hydrocarbon reservoirs in increasingly difficult imaging environments. Fortunately, the costs of acquiring high station count surveys should continue to decrease. On a per station basis, the up-front capital cost of equipment and the ongoing cost of field acquisition should fall as commercial deployments of cableless, single-station recording systems become more widespread. Supporting technologies in areas such as data storage, power systems, and LiDAR (light detection and ranging), as well as tighter, software-enabled integration among planning, surveying, and field acquisition workflows will drive further cycles of efficiency, especially as acquisition crews become more proficient in high station count operations. We are rapidly approaching the point where acquisition technology and operational methods will support a step-change improvement in land imaging. By removing the constraints imposed by cable-based recording systems, geophysicists will be able to focus their land imaging programmes on full sampling of the entire seismic wavefield and to de-emphasize noise rejection sampling as the primary objective in land survey design.