Schlumberger

Presurvey Design and Acquisition

A typical geometry when borehole seismic data is acquired in conjunction with surface seismic data is to have an array tool clamped downhole and have the vessel stream in a series of parallel lines over a grid. However, in the case of a stand-alone 3D VSP survey, spiral patterns are the preferred sailing geometry. This technique has the advantage that the source can shoot continuously and there is no acquisition downtime while the vessel turns at the line ends. On land, a source arrangement on the surface is sometimes preferred.

Building a 3D velocity model

Presurvey modeling and survey design involves building a 3D velocity model using all information available, including surface seismic velocities, well logs, checkshot/VSP surveys, and interpreted horizons. Later, this initial 3D model is used in the processing flow. Presurvey acquisition parameters needed for a 3D VSP survey include tool depth, shotpoint separation, distance between spiral arcs, and maximum spiral radius.

Enhanced tool efficiency

To minimize rig time and maximize 3D coverage, long toolstrings are used downhole, typically the VSI versatile seismic imager tool with up to 40 shuttles. Operating crews can achieve rig times of around 4 hours for the VSI tool, and once in place downhole the toolstring remains in the same position for the 3D VSP survey and for any additional walkaway VSP surveys.

Data acquisition planning

Typically, the shooting vessel navigates into position at the center of the spiral and starts acquisition of the 3D survey. Acquisition can use a flip-flop source configuration in which airguns alternate firing between two arrays. By starting at the center of the spiral, the most important data is acquired first. It is critical that the acquisition geometry follows the plan to a high degree of accuracy.

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