Changes

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.

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 [[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.

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]], [[depth::500 ft|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.

[[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.]]

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 [[:file:checkshots-and-vertical-seismic-profiles_fig2.png|Figure 2]], so that the travel path always remains as vertical as possible.

Offshore, the vertical traveltime to a receiver is defined relative to sea level. Since the energy source is below sea level when it produces the down-going wavelet, an amount of time equal to the air gun depth divided by the sound velocity in water is added to the measured time to adjust it to a sea level origin.

Offshore, the vertical traveltime to a receiver is defined relative to sea level. Since the energy source is below sea level when it produces the down-going wavelet, an amount of time equal to the air gun depth divided by the sound velocity in water is added to the measured time to adjust it to a sea level origin.

Onshore, an arbitrary depth coordinate is chosen as the time datum. In Figure 1, the datum is above the shot depth, and in such a case, the vertical distance between the shot depth and the datum depth is divided by the velocity in that interval. That time adjustment is then added to the measured traveltime to each receiver. If the depth datum is below the shot depth, as in Figure 2, this adjustment time is subtracted from the measured traveltime.

Onshore, an arbitrary depth coordinate is chosen as the time datum. In [[:file:checkshots-and-vertical-seismic-profiles_fig1.png|Figure 1]], the datum is above the shot depth, and in such a case, the vertical distance between the shot depth and the datum depth is divided by the velocity in that interval. That time adjustment is then added to the measured traveltime to each receiver. If the depth datum is below the shot depth, as in [[:file:checkshots-and-vertical-seismic-profiles_fig2.png|Figure 2]], this adjustment time is subtracted from the measured traveltime.

When a checkshot survey well penetrates formations that exhibit complicated structural dips, it is advisable to position an energy source on both the updip and downdip sides of the well so that two different traveltime measurements are acquired at each receiver depth. One of the travel paths is usually a better approximation of a straight line than the other. For example, in Figure 3, source position A is preferred when the receiver is at depth ''Z''₁ but source position B is the better choice for a receiver at depth ''Z''₂. Usually, the traveltimes measured for sources A and B are simply averaged at each receiver depth because the structural dips and formation velocities are rarely known with enough precision to predetermine which travel path is the better approximation of a straight line.

[[file:checkshots-and-vertical-seismic-profiles_fig3.png|thumb|left|{{figure number|3}}The source position (A or B) should be chosen so that the travel path to each receiver is as nearly vertical as possible.]]

[[file:checkshots-and-vertical-seismic-profiles_fig3.png|thumb|{{figure number|3}}The source position (A or B) should be chosen so that the travel path to each receiver is as nearly vertical as possible.]]

When a checkshot survey well penetrates formations that exhibit complicated structural dips, it is advisable to position an energy source on both the updip and downdip sides of the well so that two different traveltime measurements are acquired at each receiver depth. One of the travel paths is usually a better approximation of a straight line than the other. For example, in [[:file:checkshots-and-vertical-seismic-profiles_fig3.png|Figure 3]], source position A is preferred when the receiver is at depth ''Z''₁ but source position B is the better choice for a receiver at depth ''Z''₂. Usually, the traveltimes measured for sources A and B are simply averaged at each receiver depth because the structural dips and formation velocities are rarely known with enough precision to predetermine which travel path is the better approximation of a straight line.

In surveys where the structure is simple horizontal layering but where significant lateral velocity variation occurs, it is also advisable to record traveltimes from shots on opposite sides of the well and average the times so that the checkshot values are not biased with a velocity that is unrepresentative of the prospect area.

In surveys where the structure is simple horizontal layering but where significant lateral velocity variation occurs, it is also advisable to record traveltimes from shots on opposite sides of the well and average the times so that the checkshot values are not biased with a velocity that is unrepresentative of the prospect area.