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.

Approximate velocities can be calculated using the stacking velocities that were picked during processing<ref name=pt07r9>Dix, C. H., 1955, Seismic velocities from surface measurements: Geophysics, v. 20, p. 68–86., 10., 1190/1., 1438126</ref> ([[:file:mapping-with-two-dimensional-seismic-data_fig4.png|Figure 4]]). This is the poorest source of velocity information, but it may be the only source in areas where no wells have been drilled. Stacking velocities are usually printed at the top of each seismic line. Use the nearest shotpoint printed under the stacking velocities as the “ground position” for your calculated average velocities. Keep in mind that stacking velocities are not true velocities; they are just the velocities that the processor interpreted as the best at tuning events during processing. Occasionally, these velocities can vary from true velocities by more than 20%. However, they generally approximate the root mean square velocities from which average velocities can be calculated.

Approximate velocities can be calculated using the stacking velocities that were picked during processing<ref name=pt07r9>Dix, C. H., 1955, Seismic velocities from surface measurements: Geophysics, v. 20, p. 68–86., 10., 1190/1., 1438126</ref> ([[:file:mapping-with-two-dimensional-seismic-data_fig4.png|Figure 4]]). This is the poorest source of velocity information, but it may be the only source in areas where no wells have been drilled. Stacking velocities are usually printed at the top of each seismic line. Use the nearest shotpoint printed under the stacking velocities as the “ground position” for your calculated average velocities. Keep in mind that stacking velocities are not true velocities; they are just the velocities that the processor interpreted as the best at tuning events during processing. Occasionally, these velocities can vary from true velocities by more than 20%. However, they generally approximate the root mean square velocities from which average velocities can be calculated.

===Depth maps===

===Depth maps===

Basically, depth maps are constructed by multiplying a one-way time map by a velocity gradient map. Hence, the times on a two-way map must be halved before this calculation takes place. A basic map can be made by simply multiplying one-way time by velocity at every point where velocity data exist and then contouring the products. A much better result is obtained by gridding both the time and velocity maps ([[:file:mapping-with-two-dimensional-seismic-data_fig5.png|Figure 5]]) and then multiplying the time by the velocity at each grid point. Both grids can be constructed by interpolating values between points where data are available. After multiplying time by velocity at each grid point, contour the products. The gridding method is easily done with a computer and mapping software.

Basically, depth maps are constructed by multiplying a one-way time map by a velocity gradient map. Hence, the times on a two-way map must be halved before this calculation takes place. A basic map can be made by simply multiplying one-way time by velocity at every point where velocity data exist and then contouring the products. A much better result is obtained by gridding both the time and velocity maps ([[:file:mapping-with-two-dimensional-seismic-data_fig5.png|Figure 5]]) and then multiplying the time by the velocity at each grid point. Both grids can be constructed by interpolating values between points where data are available. After multiplying time by velocity at each grid point, contour the products. The gridding method is easily done with a computer and mapping software.

===Time interval maps===

===Time interval maps===