Site prep, permitting, and obstacles

A land 3D survey is a regular grid superimposed on an irregular landscape. Every shot-point and receiver-station in the pre-plan lands somewhere real — a road shoulder, a wheat field, a suburban backyard, a wetland margin, a tribal land boundary. Field managers spend months before a crew arrives on permitting, scouting, and obstacle planning. The imaging consequences are directly measurable in the final fold map.

The obstacle classes

• Hard exclusions (highways, built-up areas, refineries, airports). The crew may not shoot within a minimum standoff — typically 50–150 m for roads, 300–500 m for buildings. Either skip the shot or displace it.
• Soft exclusions (conservation zones, archaeological sites, wetlands). Often can be shot if the method is modified — smaller vibrator trucks, air-blast gun, or buried source only.
• Permit-required zones (private farmland, tribal lands). Shooting is allowed with paperwork and often payment. Lead-time is weeks to months; sometimes whole seasons shift around a stubborn landowner.
• Terrain refusals (rivers, cliffs, dense forest). Physical access denied to vehicle-mounted sources; alternatives include portable drill + dynamite, man-pack vibrators, or helicopter-deployed nodal receivers.

Skip vs displace

When a planned shot cannot be fired at its nominal position, the crew has two choices. Skip leaves a hole: the shot-line jumps over the obstacle. Fold in the image drops locally — a dark patch where the skipped shots would have contributed. Displace moves the shot to the nearest clear location. Fold is preserved but the binning geometry is perturbed: the displaced shot contributes to slightly different bins than the original, which can distort fold uniformity.

Modern practice favours displace plus careful re-design: if the pre-plan anticipates an obstacle, the whole local pattern can be shifted to maintain fold. The survey planner runs fold simulations with the obstacles in place and iterates until the worst-case bin fold loss is below about 15%.

Permitting timelines

For a straightforward open-country 3D: 6–8 weeks of permit work before the crew lands. For a mixed-land survey with farmland, forestry, and protected zones: 4–6 months. For urban 3D (refinery or municipal): 9–12 months of paperwork for a 3-week field crew. The permit cost for a large land 3D can be 20–30% of the total survey budget — more than the crew day rate combined.

The fold-uniformity test

Before the first shot, the planner produces a fold map of the entire survey with every obstacle in place and every displacement applied. The test: in the image area (the interior of the survey, excluding tapered edges), no bin should drop below 80% of nominal fold and no patch larger than 250 × 250 m should average below 90%. If a patch fails the test, the template itself is re-designed — add a few shots, offset a shot line, double up a row — before the crew mobilises.

References

• Cordsen, A., Galbraith, M., Peirce, J. (2000). Planning Land 3-D Seismic Surveys. SEG Geophysical Developments 9.
• Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.
• Stone, D. G. (1994). Designing Seismic Surveys in Two and Three Dimensions. SEG Geophysical References 5.