Changes

==Checkshot source-receiver geometry==

==Checkshot source-receiver geometry==

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

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

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''.

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''.

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.

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.

[[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.

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.