Changes

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.

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 [[dip]]s, 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.

Several types of VSPs can be recorded by altering the position of the energy source relative to the receiver. The term ''offset'' is used to describe the horizontal distance between the source and receiver. If the receiver is directly below the source, the recorded data are called a ''zero offset VSP''. If there is a significant horizontal distance between the source and receiver, the recorded data are referred to as an ''offset VSP''. Examples of offset and zero offset geometries are shown in [[:file:checkshots-and-vertical-seismic-profiles_fig4.png|Figure 4]]. A common misuse of the term ''offset'' is in describing the horizontal position of the energy source relative to the wellhead rather than the position of the source relative to the location of the subsurface receiver. For this reason, the geometry in [[:file:checkshots-and-vertical-seismic-profiles_fig4.png|Figure 4(d)]] is an offset VSP, not a zero offset VSP.

Several types of VSPs can be recorded by altering the position of the energy source relative to the receiver. The term ''offset'' is used to describe the horizontal distance between the source and receiver. If the receiver is directly below the source, the recorded data are called a ''zero offset VSP''. If there is a significant horizontal distance between the source and receiver, the recorded data are referred to as an ''offset VSP''. Examples of offset and zero offset geometries are shown in [[:file:checkshots-and-vertical-seismic-profiles_fig4.png|Figure 4]]. A common misuse of the term ''offset'' is in describing the horizontal position of the energy source relative to the wellhead rather than the position of the source relative to the location of the subsurface receiver. For this reason, the geometry in [[:file:checkshots-and-vertical-seismic-profiles_fig4.png|Figure 4(d)]] is an offset VSP, not a zero offset VSP.

In a flat-layered earth, the reflection points associated with a zero offset VSP occur close to the vertical line passing through the source and receiver coordinates. Thus, the image made from these data will illuminate the subsurface in only a narrow vertical corridor passing through the receiver location. However, if there is structural dip, the reflection points associated with a zero offset VSP can occur at significant horizontal distances from the vertical line passing through the source and receiver. When properly processed, such data can produce high resolution images extending from the receiver position to the farthest reflection point coordinate.

In a flat-layered earth, the reflection points associated with a zero offset VSP occur close to the vertical line passing through the source and receiver coordinates. Thus, the image made from these data will illuminate the subsurface in only a narrow vertical corridor passing through the receiver location. However, if there is structural [[dip]], the reflection points associated with a zero offset VSP can occur at significant horizontal distances from the vertical line passing through the source and receiver. When properly processed, such data can produce high resolution images extending from the receiver position to the farthest reflection point coordinate.

For offset VSPs, reflection points are always distributed over some horizontal distance, so offset VSP recording geomety is often used to produce seismic images that traverse portions of a reservoir near survey wells.

For offset VSPs, reflection points are always distributed over some horizontal distance, so offset VSP recording geomety is often used to produce seismic images that traverse portions of a reservoir near survey wells.