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Cross-borehole tomography can be thought of as an extension of sonic logging to the reservoir cross section between two boreholes. The information obtained from cross-borehole tomography, when properly interpreted in the context of all available information, can often be invaluable for preparing accurate [[geological cross sections]] for reservoir development and planning.

Cross-borehole tomography can be thought of as an extension of sonic logging to the reservoir [[cross section]] between two boreholes. The information obtained from cross-borehole tomography, when properly interpreted in the context of all available information, can often be invaluable for preparing accurate [[geological cross sections]] for reservoir development and planning.

Important differences exist, however, between cross-borehole seismic velocity images and sonic logs. Sonic logs usually measure velocity of sound in rocks at very high frequencies (5 to 40 kHz), whereas seismic tomography measures the velocity of sound at seismic frequencies, usually in the frequency range of 20 Hz to as high as several kilohertz. Sonic logs represent the measurement of velocities at the wellbore and are usually plotted as a continuous curve in depth. Because seismic tomography measures the two-dimensional velocity field between the wellbores, it is usually represented by a color-coded map in which a color is assigned to the seismic velocity at each point. This map, or plot, is referred to as a ''tomogram''. Other displays such as [[contour]] plots can also be used, but this is not common.

Important differences exist, however, between cross-borehole seismic velocity images and sonic logs. Sonic logs usually measure velocity of sound in rocks at very high frequencies (5 to 40 kHz), whereas seismic tomography measures the velocity of sound at seismic frequencies, usually in the frequency range of 20 Hz to as high as several kilohertz. Sonic logs represent the measurement of velocities at the wellbore and are usually plotted as a continuous curve in depth. Because seismic tomography measures the two-dimensional velocity field between the wellbores, it is usually represented by a color-coded map in which a color is assigned to the seismic velocity at each point. This map, or plot, is referred to as a ''tomogram''. Other displays such as [[contour]] plots can also be used, but this is not common.

Numerous interpretation pitfalls, which can make part or all of an interpretation uncertain, are also important to evaluate. Since velocity is affected by many factors, incomplete geological characterizations or log data increase the uncertainty of tomography interpretations. Sufficient well data are therefore necessary to support interpretation adequately. In reservoirs where EOR processes cause rapid changes, correlation of tomograms with older logs may not be valid. Efforts should be made to run logs at the same time the tomography data are acquired.

Numerous interpretation pitfalls, which can make part or all of an interpretation uncertain, are also important to evaluate. Since velocity is affected by many factors, incomplete geological characterizations or log data increase the uncertainty of tomography interpretations. Sufficient well data are therefore necessary to support interpretation adequately. In reservoirs where EOR processes cause rapid changes, correlation of tomograms with older logs may not be valid. Efforts should be made to run logs at the same time the tomography data are acquired.

Tomography display parameters are one of the more important factors governing the ease of interpretation and acceptance of results. To facilitate correlations with cross-borehole geology, tomogram velocity fields should be scaled to match major stratigraphic and reservoir units ([[:file:cross-borehole-tomography-in-development-geology_fig4.jpg|Figure 4]]). The definition of smaller velocity fields may be appropriate to show details such as reservoir heterogeneity within selected major velocity fields. Stratigraphic units such as formation tops and sedimentary facies should also be delineated. In addition to defining velocity fields that match geological units, the use of standard colors and/or graphic symbols for rock units is important. Cold colors (blues) should be used for high velocity fields with a transition to hot colors (reds) for low velocities. Using these guidelines for display should result in tomograms that closely resemble geological cross sections and are more readily understood and utilized by geologists, engineers, and management. Analysis of tomograms with reference to reservoir models and any available surface seismic should provide the basis for interpretations that can be summarized with integrated data displays, as illustrated in [[:file:cross-borehole-tomography-in-development-geology_fig5.jpg|Figure 5]]. Most important are the well data to velocity field correlations and the log to tomography correlations documenting lithology, porosity, fluid saturation, and temperature. The resulting log and tomography display should provide the data needed to qualitatively document cross-borehole structure, especially dip and faults, reservoir heterogeneity and homogeneity, fluid contacts and any EOR flood fronts.

Tomography display parameters are one of the more important factors governing the ease of interpretation and acceptance of results. To facilitate correlations with cross-borehole geology, tomogram velocity fields should be scaled to match major stratigraphic and reservoir units ([[:file:cross-borehole-tomography-in-development-geology_fig4.jpg|Figure 4]]). The definition of smaller velocity fields may be appropriate to show details such as reservoir heterogeneity within selected major velocity fields. Stratigraphic units such as formation tops and sedimentary facies should also be delineated. In addition to defining velocity fields that match geological units, the use of standard colors and/or graphic symbols for rock units is important. Cold colors (blues) should be used for high velocity fields with a transition to hot colors (reds) for low velocities. Using these guidelines for display should result in tomograms that closely resemble [[geological cross sections]] and are more readily understood and utilized by geologists, engineers, and management. Analysis of tomograms with reference to reservoir models and any available surface seismic should provide the basis for interpretations that can be summarized with integrated data displays, as illustrated in [[:file:cross-borehole-tomography-in-development-geology_fig5.jpg|Figure 5]]. Most important are the well data to velocity field correlations and the log to tomography correlations documenting lithology, porosity, fluid saturation, and temperature. The resulting log and tomography display should provide the data needed to qualitatively document cross-borehole structure, especially dip and faults, reservoir heterogeneity and homogeneity, fluid contacts and any EOR flood fronts.

==See also==

==See also==