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Line 48: − + − [[file:cross-borehole-tomography-in-development-geology_fig3.png|left|thumb|{{figure number|3}}Time lapse tomograms acquired at 6 month intervals across a thermal EOR project. (After <ref name=pt07r25 />.)]]+ + + − − [[file:cross-borehole-tomography-in-development-geology_fig4.jpg|thumb|{{figure number|4}}Single image tomograms documenting stratigraphy and reservoir zones. Heterogeneity of the lower reservoir is documented by the contrast in log curves and the corresponding change in the tomographic velocity fields. (After Justice et al., 1990.)]]
→Survey types
==Survey types==
==Survey types==
<gallery mode=packed heights=300px widths=300px>
file:cross-borehole-tomography-in-development-geology_fig3.png|{{figure number|3}}Time lapse tomograms acquired at 6 month intervals across a thermal EOR project. (After <ref name=pt07r25 />.)
file:cross-borehole-tomography-in-development-geology_fig4.jpg|{{figure number|4}}Single image tomograms documenting stratigraphy and reservoir zones. Heterogeneity of the lower reservoir is documented by the contrast in log curves and the corresponding change in the tomographic velocity fields. (After Justice et al., 1990.)
</gallery>
Two major types of tomographic surveys are in use today. These consist of single tomograms for imaging reservoir characteristics, and time-lapse tomography for imaging time evolution processes in reservoirs such as those associated with [[enhanced oil recovery]] (EOR).
Two major types of tomographic surveys are in use today. These consist of single tomograms for imaging reservoir characteristics, and time-lapse tomography for imaging time evolution processes in reservoirs such as those associated with [[enhanced oil recovery]] (EOR).
In time-lapse tomography, a baseline survey is taken, ideally before an EOR (or similar) project begins. The resulting tomograms may or may not be interpreted at this time. After an appropriate time interval, during which the EOR program is operating and inducing changes in the reservoir, a second survey is taken. The resulting tomograms will be of little additional value unless the changes in the reservoir have also induced changes in the seismic velocity field. Fortunately, there are some well-documented situations in which this does occur.<ref name=Nur_1984>Nur, A. M., 1984, Seismic monitoring of thermal enhanced oil processes: Society of Exploration Geophysicists Expanded Abstracts, 54th Annual Meeting, p.337-340.</ref><ref name=pt07r25>Justice, J. H., Vassiliou, A. A., Singh, S., Logel, J. D., Hansen, P. A., Hall, B. R., Hurt, P. R., Solanki, J. J., 1989, Acoustic tomography for monitoring enhanced oil recovery: Leading Edge, v. 8, p. 12–19., 10., 1190/1., 1439605</ref><ref name=pt07r26>Justice, J. H., Vassiliou, A. A., Mathisen, M. E., Singh, S., Cunningham, P. S., Hutt, P. R., 1992, Acoustic tomography in reservoir surveillance: Society of Exploration Geophysicists, Reservoir Geophysics, p. 321–334.</ref><ref name=pt07r27>Justice, J. H., Mathisen, M. E., Vassiliou, A. A., Shiao, I., Alameddine, B. R., Guinzy, N. J., 1993, Crosswell seismic tomography in improved oil recovery: First Break (in press).</ref> For example, heating of heavy oils by steam injection can produce large reductions in the seismic velocity field in the affected part of the reservoir.
In time-lapse tomography, a baseline survey is taken, ideally before an EOR (or similar) project begins. The resulting tomograms may or may not be interpreted at this time. After an appropriate time interval, during which the EOR program is operating and inducing changes in the reservoir, a second survey is taken. The resulting tomograms will be of little additional value unless the changes in the reservoir have also induced changes in the seismic velocity field. Fortunately, there are some well-documented situations in which this does occur.<ref name=Nur_1984>Nur, A. M., 1984, Seismic monitoring of thermal enhanced oil processes: Society of Exploration Geophysicists Expanded Abstracts, 54th Annual Meeting, p.337-340.</ref><ref name=pt07r25>Justice, J. H., Vassiliou, A. A., Singh, S., Logel, J. D., Hansen, P. A., Hall, B. R., Hurt, P. R., Solanki, J. J., 1989, Acoustic tomography for monitoring enhanced oil recovery: Leading Edge, v. 8, p. 12–19., 10., 1190/1., 1439605</ref><ref name=pt07r26>Justice, J. H., Vassiliou, A. A., Mathisen, M. E., Singh, S., Cunningham, P. S., Hutt, P. R., 1992, Acoustic tomography in reservoir surveillance: Society of Exploration Geophysicists, Reservoir Geophysics, p. 321–334.</ref><ref name=pt07r27>Justice, J. H., Mathisen, M. E., Vassiliou, A. A., Shiao, I., Alameddine, B. R., Guinzy, N. J., 1993, Crosswell seismic tomography in improved oil recovery: First Break (in press).</ref> For example, heating of heavy oils by steam injection can produce large reductions in the seismic velocity field in the affected part of the reservoir.