Changes

===Velocity gradient maps===

===Velocity gradient maps===

A velocity gradient map is constructed at an intermediate step between a time map and a depth map. During conversion from time to depth, a velocity gradient map compensates for lateral changes in velocity, which is preferable to using a single velocity function ([[:file:mapping-with-two-dimensional-seismic-data_fig3.png|Figure 3]]). Construction requires a base map and velocity data. The object is to contour the average velocity down to an event.

A velocity gradient map is constructed at an intermediate step between a time map and a depth map. During conversion from time to depth, a velocity gradient map compensates for [[lateral]] changes in velocity, which is preferable to using a single velocity function ([[:file:mapping-with-two-dimensional-seismic-data_fig3.png|Figure 3]]). Construction requires a base map and velocity data. The object is to contour the average velocity down to an event.

Velocity data generally come from five sources: vertical seismic profiles (VSPs), checkshot surveys, [[synthetic seismograms]], stacking velocities, and well depth to time correlations. The latter is an easy and reliable method of determining velocities. Simply match the time of a horizon on a seismic line with the depth of that horizon in an adjacent well and you can calculate a velocity. This method may not work in highly deformed rocks, in which one is unsure exactly what the two-dimensional seismic line is imaging. However, depth to time correlations generally work well.

Velocity data generally come from five sources: vertical seismic profiles (VSPs), checkshot surveys, [[synthetic seismograms]], stacking velocities, and well depth to time correlations. The latter is an easy and reliable method of determining velocities. Simply match the time of a horizon on a seismic line with the depth of that horizon in an adjacent well and you can calculate a velocity. This method may not work in highly deformed rocks, in which one is unsure exactly what the two-dimensional seismic line is imaging. However, depth to time correlations generally work well.

===Time interval maps===

===Time interval maps===

Time interval (or isotime or isochron) maps are commonly used for interpreting changes in thickness between interpreted horizons ([[:file:mapping-with-two-dimensional-seismic-data_fig6.png|Figure 6]]). To map time intervals, calculate the difference in time (normally two-way time) between two events at each shotpoint and contour the resultant values.

Time interval (or isotime or [[isochron]]) maps are commonly used for interpreting changes in thickness between interpreted horizons ([[:file:mapping-with-two-dimensional-seismic-data_fig6.png|Figure 6]]). To map time intervals, calculate the difference in time (normally two-way time) between two events at each shotpoint and contour the resultant values.

===Time slice maps===

===Time slice maps===

[[Category:Geophysical methods]]

[[Category:Geophysical methods]]