The present paper describes the P wave modeling of two 6769

A carbon dioxide flood pilot is being conducted in a section of Chevron's McElroy field in Crane County, west Texas. Prior to CO 2 injection, two high-frequency crosswell seismic profiles were recorded to investigate the use of seismic profiling for high-resolution reservoir delineation and CO 2 monitoring. These preinjection profiles provide the baseline for timelapse monitoring. Profile #1 was recorded between an injector well and an offset observation well at a nominal well-to-well distance of 184 ft (56 m). Profile #2 was recorded between a producing well and the observation well at a nominal distance of 600 ft (183 m). The combination of traveltime tomography and stacked CDP reflection amplitudes demonstrates how highfrequency crosswell seismic data can be used to image both large and small scale heterogeneity between wells: Transmission traveltime tomography is used to image the large scale velocity variations; CDP reflection imaging is then used to image smaller scale impedance heterogeneities. The resolution capability of crosswell data is clearly illustrated by an image of the Grayburg-San Andres angular unconformity, seen in both the P-wave and S-wave velocity tomograms and the reflection images. In addition to the imaging study, cores from an observation well were analyzed to support interpretation of the crosswell images and assess the feasibility of monitoring changes in CO 2 saturation. The results of this integrated study demonstrate (1) the use of crosswell seismic profiling to produce a high-resolution reservoir delineation and (2) the possibility for successful monitoring of CO 2 in carbonate reservoirs. The crosswell data were acquired with a piezoelectric source and a multilevel hydrophone array. Both profiles, nearly 80 000 seismic traces, were recorded in approximately 80 hours using a new acquisition technique of shooting on-the-fly. This paper presents the overall project summary and interpretation of the results from the near-offset profile.

A knowledge of permafrost conditions is important for planning the foundation of buildings and engineering activities at high latitudes and for geological mapping of sediment thicknesses and architecture. The freezing of sediments is known to greatly affect their seismic velocities. In polar regions the actual velocities of the upper sediments may therefore potentially reveal water saturation and extent of freezing. We apply various strategies for modeling seismic velocities and reflectivity properties of unconsolidated granular materials as a function of water saturation and freezing conditions. The modeling results are used to interpret a set of high‐resolution seismic data collected from a glaciomarine delta at Spitsbergen, the Norwegian Arctic, where the upper subsurface sediments are assumed to be in transition from unfrozen to frozen along a transect landward from the delta front. To our knowledge, this is the first attempt to study pore‐fluid freezing from such data. Our study indicates that the P‐ and S‐wave velocities may increase as much as 80–90% when fully, or almost fully, water‐saturated unconsolidated sediments freeze. Since a small amount of frozen water in the voids of a porous rock can lead to large velocity increases, the freezing of sediments reduces seismic resolution; thus, the optimum resolution is obtained at locations where the sediments appear unfrozen. The reflectivity from boundaries separating sediments of slightly different porosity may depend more strongly on the actual saturation rather than changes in granular characteristics. For fully water‐saturated sediments, the P‐wave reflectivity decreases sharply with freezing, while the reflectivity becomes less affected as the water saturation is lowered. Thus, a combination of velocity and reflectivity information may reveal saturation and freezing conditions.

Reliable crosswell reflection imaging is a challenging task, even after the data have been wavefield‐separated in the time domain. Residual, strong coherent noise is still present in the data. Stacking is complicated by the wide range of reflection incidence angles available for imaging. With wavelengths of a few feet, small misalignments as a result of velocity or geometric errors produce destructive interference and degrade the quality of the stacked image. We present an imaging sequence that addressed these complications and allowed us to produce high‐quality stacked images for both P‐ and S‐waves from a large‐volume crosswell data set. A very good tie was achieved at both wells. Heterogeneities imaged from well to well included very thin beds [less than 5 ft (1.5 m) thick] within the reservoir, pinchouts, and a major angular unconformity—the Grayburg/San Andres—that could not be observed reliably with any other technique (log correlation, surface seismic imaging, or tomography). In fact, the produced crosswell reflection images exhibit dramatically higher resolution and continuity than the P‐wave traveltime tomogram.