We describe a new application of the Virtual Source Method, the Virtual Checkshot, for accurately estimating sub-salt interval velocities of P- and S-waves from walk-away or 3D VSP data. Complex overburden may lead to erroneous velocity profiles when conventional check-shots and zerooffset VSP-s are used. The new technique overcomes the challenge by creating a Virtual Source at each downhole geophone. When both Virtual Source and receivers are placed below the most complex part of the section, the obtained velocity profiles are not distorted by the overburden. The Virtual Checkshot can correct for overburden of any complexity since no velocity information between the surface and the geophones is required. We apply the Virtual Checkshot technique to a sub-salt deep-water prospect in the Gulf of Mexico. We obtain P- and S-wave velocity profiles that are in excellent agreement with sonic logs in salt and below salt.
Extracting velocity profiles from checkshots under complicated overburden is challenging because:
first arrival waveforms may be distorted and difficult to pick;
the fastest arrival may not come along the shortest path between the source and the receiver (Fig. 1a).
Both of these problems could be alleviated by placing the source in the well, so that the travel-path of the first arrival is short and close to a straight line between the source and the receiver (Fig. 1b). In practice, it is difficult to place a physical source downhole. However, it is easy to create a Virtual Source in the well from walk-away or 3D VSP data (, , ). Measuring the first arrival traveltime from a Virtual Source to a number of receivers below it, we can construct a Virtual Checkshot' that is insensitive to overburden complexity. Figure 1: Checkshot in complex medium (available in full paper) An extension of this idea would be to create a Virtual Shear Source in the borehole and construct a Shear Virtual Checkshot. Conventional checkshots measure only P-wave velocity from first arrivals. Shear waves, even if emitted from the surface source or generated by P-S conversions in the overburden, arrive later and are difficult to unravel and pick (for a successful attempt see ). Bakulin and Calvert  showed that we could harvest P-S conversions in the overburden to create a pure Shear Virtual Source in the borehole. The first arrival from such a source (easy to pick) would be an S-wave, yielding a shear-velocity profile.
We have tested these ideas on a data set from a sub-salt prospect in the deepwater Gulf of Mexico (GOM). We obtain profiles of P- and S-wave velocities that are in good agreement with sonic logs under salt at more than 7 km depth.
Virtual Checkshots are generated from conventional walk-away walkaway (WAW) VSP data. Our example dataset was acquired in a vertical well through a massive salt body in the deepwater GOM.