Changes

==Checkshot source-receiver geometry==

==Checkshot source-receiver geometry==

The purpose of a velocity survey is to produce a down-going seismic wavelet at the surface near a well and then to measure the time required for that wavelet to travel to a known depth where a seismic receiver is positioned in the well. This borehole receiver is locked successively at several different depth levels, and the vertical traveltime to each level is measured<ref name=pt07r1>Anstey, N. A., Geyer, R. L., 1987, Borehole velocity measurements and the synthetic seismogram: Boston, MA, IHRDC, 355 p.</ref>. Each measurement of the source-receiver traveltime is a ''checkshot'', and the compilation of all of the traveltime measurements into a time-depth calibration function is referred to as a ''checkshot survey''.

[[file:checkshots-and-vertical-seismic-profiles_fig1.png|thumb|left|{{figure number|1.}}The source-receiver geometry commonly used in onshore checkshot surveys.]]

The source—receiver geometry used in onshore velocity checkshots is shown in Figure 1. If possible, the energy source should be the same as that used to record the surface seismic data near the well. A buried explosive charge is shown in this diagram, but other common onshore energy sources include Vibroseis or air guns operated in a water-filled pit near a well. Offshore, essentially all checkshot surveys involve air guns as the seismic energy source.

The purpose of a velocity survey is to produce a down-going seismic wavelet at the surface near a well and then to measure the time required for that wavelet to travel to a known depth where a seismic receiver is positioned in the well. This borehole receiver is locked successively at several different depth levels, and the vertical traveltime to each level is measured.<ref name=pt07r1>Anstey, N. A., Geyer, R. L., 1987, Borehole velocity measurements and the synthetic seismogram: Boston, MA, IHRDC, 355 p.</ref> Each measurement of the source-receiver traveltime is a ''checkshot'', and the compilation of all of the traveltime measurements into a time-depth calibration function is referred to as a ''checkshot survey''.

[[file:checkshots-and-vertical-seismic-profiles_fig1.png|thumb|{{figure number|1}}The source-receiver geometry commonly used in onshore checkshot surveys.]]

The source—receiver geometry used in onshore velocity checkshots is shown in [[:file:checkshots-and-vertical-seismic-profiles_fig1.png|Figure 1]]. If possible, the energy source should be the same as that used to record the surface seismic data near the well. A buried explosive charge is shown in this diagram, but other common onshore energy sources include Vibroseis or air guns operated in a water-filled pit near a well. Offshore, essentially all checkshot surveys involve air guns as the seismic energy source.

Ordinarily, the borehole receiver is first lowered to the deepest checkshot level, and the traveltime to this deepest receiver position is measured for one or more surface shots. The receiver is then moved upward a distance of 200, 500, or [[depth::1000 ft]] (61, 152, or 305 m) to record the checkshot, or vertical traveltime, at successively shallower levels.

Ordinarily, the borehole receiver is first lowered to the deepest checkshot level, and the traveltime to this deepest receiver position is measured for one or more surface shots. The receiver is then moved upward a distance of [[depth::200 ft|200]], 500, or [[depth::1000 ft]] (61, 152, or 305 m) to record the checkshot, or vertical traveltime, at successively shallower levels.

The time-depth calibration function and velocity analyses that can be calculated from checkshot measurements are more reliable if each source-receiver travel path is a vertical straight line rather than an oblique, refracted path<ref name=pt07r15>Goetz, J. F., Dupal, L., Bowles, J., 1979, An investigation into discrepancies between sonic log and seismic check-shot velocities: Australian Exploration Association Journal, v. 19, pt. 1, p. 131–141.</ref>. Consequently, if a well is deviated, then the surface position of the source should be readjusted each time the downhole receiver is moved to a new depth level, as shown in Figure 2, so that the travel path always remains as vertical as possible.

The time-depth calibration function and velocity analyses that can be calculated from checkshot measurements are more reliable if each source-receiver travel path is a vertical straight line rather than an oblique, refracted path.<ref name=pt07r15>Goetz, J. F., Dupal, L., Bowles, J., 1979, An investigation into discrepancies between sonic log and seismic check-shot velocities: Australian Exploration Association Journal, v. 19, pt. 1, p. 131–141.</ref> Consequently, if a well is deviated, then the surface position of the source should be readjusted each time the downhole receiver is moved to a new depth level, as shown in Figure 2, so that the travel path always remains as vertical as possible.

[[file:checkshots-and-vertical-seismic-profiles_fig2.png|thumb|{{figure number|2}}The source-receiver geometry commonly used to record checkshots in deviated wells.]]

[[file:checkshots-and-vertical-seismic-profiles_fig2.png|thumb|{{figure number|2}}The source-receiver geometry commonly used to record checkshots in deviated wells.]]