Zebra Geosciences does NOT offer a VSP acquisition service so has no obligation to promote in-house acquisition equipment / techniques and are free to offer impartial advice on “best for well” technology and survey parameters. Our expertise allows us to offer practical advice on the most efficient way to utilise the available equipment, the impact of compromises that may have to be made for practical reasons and whether a recorded data set could genuinely be processed to yield the desired results. If we feel a VSP does not have a reasonable chance of meeting the survey objective we will always say this as we feel it is in nobodies interest to record a VSP that fails to deliver.

Often a quick conversation will lead to an immediate conclusion as to the practicality of a proposed VSP survey although care must be taken not to overlook key factors. For example, lateral changes above a deviated wellbore can cause discontinuities in a Vertical Incidence (moving-source, moving-receiver) VSP making a Rig Source survey a better option. Similarly, when a walkaway survey is planned to give a sub-surface structural image, the assumption is often made that the same data set can be used to extract anisotropy parameters and/or AVO information at the horizon of interest. This is almost always NOT the case; these are specialist surveys and require specific design to optimise the results.

Of course, there will be occasions where a more quantitative approach is required. In such cases, the best available geological interpretation is used to build a local model. Details of the borehole design, principally deviation and casing/liner information, are used to estimate depth range of high fidelity recorded data and proposed VSP survey(s) simulated.

The predicted energy travel paths can be ray-traced to formations of interest, a simple example model with ray paths is shown here. The results of raytracing can then be used to create synthetic VSP data plots which will show how different incident energy arrivals may appear and interact, though of course in an idealised noise free environment. A more truly representative, although still noise-free, synthetic VSP can be generated through a wave equation simulation.

The combination of synthetic data generated by both conventional ray tracing and using a wave equation simulation is often crucial to a complete understanding of arrivals in more complex surveys during post-survey analysis.

Feel free to contact us if you need more information.