Changes

When a seismic wave field is recorded with this small spatial sampling interval, several processing techniques can be used to separate the down-going and up-going wave fields. Once the up-going wave field is isolated from the more dominant down-going wave field, the up-going reflection events can be properly analyzed and interpreted and used to produce improved imagery of the subsurface.

When a seismic wave field is recorded with this small spatial sampling interval, several processing techniques can be used to separate the down-going and up-going wave fields. Once the up-going wave field is isolated from the more dominant down-going wave field, the up-going reflection events can be properly analyzed and interpreted and used to produce improved imagery of the subsurface.

==Vsp source-receiver geometries==

==VSP source-receiver geometries==

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 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 Figure 4(d) is an offset VSP, not a zero offset VSP.

[[file:checkshots-and-vertical-seismic-profiles_fig4.png|thumb|{{figure number|4}}Examples of the source-receiver positions involved in (a and b) zero offset and (c and d) offset VSP recording geometries.]]

[[file:checkshots-and-vertical-seismic-profiles_fig4.png|thumb|{{figure number|4}}Examples of the source-receiver positions involved in (a and b) zero offset and (c and d) offset VSP recording geometries.]]

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.