Changes

==Tomographic data acquisition==

==Tomographic data acquisition==

To acquire seismic tomography data, a source of seismic energy is lowered to the survey depth in the source borehole, and receivers are lowered to an appropriate depth in one or more boreholes that are used to record the seismic data (Figure 1). The source and receivers each occupy a number of stations, usually regularly spaced over a depth range that includes the zone of interest in the reservoir. The spacing of these stations and the vertical interval they cover (aperture) play a role in determining the final spatial resolution of the tomogram. With receivers operating in additional wells, data for several tomograms can be acquired simultaneously.

[[file:cross-borehole-tomography-in-development-geology_fig1.png|left|thumb|{{figure number|1}}Schematic illustration of tomographic data acquisition.]]

[[file:cross-borehole-tomography-in-development-geology_fig1.png|thumb|{{figure number|1}}Schematic illustration of tomographic data acquisition.]]

[[file:cross-borehole-tomography-in-development-geology_fig2.png|thumb|{{figure number|2}}Ray path diagram documenting the distribution of ray paths across a thermal EOR project tomogram.]]

With source and receivers in place, the source is fired and a recording is made. In traveltime tomography, this recording is later used to compute the time required for the seismic energy to travel from source to receiver. Data are acquired for every combination of source and receiver stations. Ray paths connecting sources and receivers for a typical survey are shown in Figure 2. These traveltime data are then used to infer the seismic velocity field between the wellbores. If attenuation measurements can be made on the data, then the seismic Q-factor can also be imaged in the reservoir. Since the number of source/receiver pairs is usually in the thousands, data acquisition can be a slow process. Wirelines with multiple receivers appropriately placed can reduce the survey time.

To acquire seismic tomography data, a source of seismic energy is lowered to the survey depth in the source borehole, and receivers are lowered to an appropriate depth in one or more boreholes that are used to record the seismic data ([[:file:cross-borehole-tomography-in-development-geology_fig1.png|Figure 1]]). The source and receivers each occupy a number of stations, usually regularly spaced over a depth range that includes the zone of interest in the reservoir. The spacing of these stations and the vertical interval they cover (aperture) play a role in determining the final spatial resolution of the tomogram. With receivers operating in additional wells, data for several tomograms can be acquired simultaneously.

[[file:cross-borehole-tomography-in-development-geology_fig2.png|thumb|{{figure number|2}}Ray path diagram documenting the distribution of ray paths across a thermal EOR project tomogram.]]

With source and receivers in place, the source is fired and a recording is made. In traveltime tomography, this recording is later used to compute the time required for the seismic energy to travel from source to receiver. Data are acquired for every combination of source and receiver stations. Ray paths connecting sources and receivers for a typical survey are shown in [[:file:cross-borehole-tomography-in-development-geology_fig2.png|Figure 2]]. These traveltime data are then used to infer the seismic velocity field between the wellbores. If attenuation measurements can be made on the data, then the seismic Q-factor can also be imaged in the reservoir. Since the number of source/receiver pairs is usually in the thousands, data acquisition can be a slow process. Wirelines with multiple receivers appropriately placed can reduce the survey time.

Because usable seismic frequencies are an important factor in resolution, sources with the widest possible bandwidth are desirable, and data sampling rates of the recording instruments must be correspondingly high (usually on the order of 10,000 samples per second for each channel). Usually multicomponent receivers, which record data using three orthogonally mounted geophones, are used, so that both shear and compressional waves can be identified. If it is possible to reconstruct both compressional and shear wave tomograms, then ''V''_p/''V''_s and Poisson's ratio can be computed in the interwell volume.

Because usable seismic frequencies are an important factor in resolution, sources with the widest possible bandwidth are desirable, and data sampling rates of the recording instruments must be correspondingly high (usually on the order of 10,000 samples per second for each channel). Usually multicomponent receivers, which record data using three orthogonally mounted geophones, are used, so that both shear and compressional waves can be identified. If it is possible to reconstruct both compressional and shear wave tomograms, then ''V''_p/''V''_s and Poisson's ratio can be computed in the interwell volume.