Limitations in sonic and density logs and marine COP data produce ambiguities in analysis and interpretation of this information for exploration prospect evaluation and production reservoir delineation. Marine vertical seismic profiling (VSP) provides an added tool to help unravel many of these ambiguities by measuring seismic wave reflections and propagation parameters (e.g., travel time, velocity, and attenuation) in the vicinity of the borehole at seismic frequencies.
In the marine environment, special restrictions are imposed on VSP acquisition by the drill rig, deviated boreholes, and high offshore operating expenses. Moreover, marine VSP signal processing must contend with the effects of these problems, such as source reverberation and limited frequency bandwidth. These processed VSP resu1ts are used to correlate seismic data to the borehole for seismic interpretation. The basic applications for a marine VSP are:
Accurate time-to-depth relationship
Interval velocities at seismic frequencies
Correlation of the seismic data near and below the borehole
Correlation of the seismic data to both logs and drilling information measured in depth.
Seismic wave attenuation estimates
Improving modeling of synthetic seismograms
Improving processing of surface COP data and
Improving structural and stratigraphic interpretation of surface COP data.
This overview of marine Vertical seismic profiling (VSP) consists of discussions on:
acquisition and processing of VSP data,
VSP interpretation and integration (with logs and surface COP data),
VSP in deviated boreholes, and
VSP applications. Emphasis is placed on those unique applications and problems of VSP for the offshore environment.
Major limitations arise in the analysis and application of surface seismic data and borehole logs in exploration and production. For surface seismic data, the seismic profiles provide a continuous view of the reflecting horizons in the subsurface and are interpreted in terms of their geological significance. The interpretation of these data is limited by the resolution (25 to 100 ft) and the discrete subsurface sampling along survey lines. The data are recorded in time and must be converted to depth and mi grated to the proper spatial position. On the other hand, typical logs have excellent vertical resolution in depth and provide the needed detailed information about the reservoir and its geology. However, the log investigates only the subsurface penetrated by the bit and looks only at a few feet of the formation surrounding the borehole. The correlation and mapping of this log information between wells is especially difficult in complex geology (e.g., faulting). The trend in interpretation is to integrate as much log and drilling information as available with the surface seismic data to fill the information gap between these data sets. VSP, as a seismic log, can aid in the integration while providing critical additional information.