Changes

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.

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.

When there is sufficient velocity and dip information and adequate presurvey preparation time to allow ray trace modeling of the source—receiver travel path, it is helpful to calculate and display the anticipated ray paths for several possible source and receiver locations to determine which source position produces the best approximation of a straight line travel path to each desired receiver location.

When there is sufficient velocity and dip information and adequate presurvey preparation time to allow ray trace modeling of the source—receiver travel path, it is helpful to calculate and display the anticipated ray paths for several possible source and receiver locations to determine which source position produces the best approximation of a straight line travel path to each desired receiver location.