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Line 48: − + − [[file:mapping-with-two-dimensional-seismic-data_fig3.png|left|thumb|{{figure number|3}}Illustration showing effect of lateral differences in velocity on conversion of time and depth. If a single velocity function of 8300 ft/sec were used, errors of +6 ft at A and -440 ft at B would appear on the depth map.]]+ + + + + − − [[file:mapping-with-two-dimensional-seismic-data_fig4.png|thumb|{{figure number|4}}(a) Illustration of ray paths, intervals, and interfaces used to help explain the Dix formula. (b) The Dix formula for calculating interval velocities, which assumes that interfaces are flat and smooth.]] Line 60:
Line 62: − − [[file:mapping-with-two-dimensional-seismic-data_fig5.png|left|thumb|{{figure number|5}}Velocity map with velocities marked at grid intersections.]] − [[file:mapping-with-two-dimensional-seismic-data_fig6.png|thumb|{{figure number|6}}(a) Block diagram showing the time that Is mapped for a time slice map. (b) Interval that is mapped on time interval map. (c) Time interval map.]]
Seismic data - mapping with two-dimensional data (view source)
Revision as of 20:14, 24 June 2014
, 20:14, 24 June 2014→Other types of maps
==Other types of maps==
==Other types of maps==
<gallery mode=packed heights=200px widths=200px>
mapping-with-two-dimensional-seismic-data_fig3.png|{{figure number|3}}Illustration showing effect of lateral differences in velocity on conversion of time and depth. If a single velocity function of 8300 ft/sec were used, errors of +6 ft at A and -440 ft at B would appear on the depth map.
mapping-with-two-dimensional-seismic-data_fig4.png|{{figure number|4}}(a) Illustration of ray paths, intervals, and interfaces used to help explain the Dix formula. (b) The Dix formula for calculating interval velocities, which assumes that interfaces are flat and smooth.
mapping-with-two-dimensional-seismic-data_fig5.png|{{figure number|5}}Velocity map with velocities marked at grid intersections.
mapping-with-two-dimensional-seismic-data_fig6.png|{{figure number|6}}(a) Block diagram showing the time that Is mapped for a time slice map. (b) Interval that is mapped on time interval map. (c) Time interval map.
</gallery>
===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.
Vertical seismic profiles and checkshot surveys are also excellent sources because they show the actual traveltime of sound through material over a known distance, thereby yielding true velocities. Good estimates of velocities are provided by synthetic seismograms. Synthetics are made from sonic logs and show the cumulative travel time through the rocks where a sonic log was run. Knowing the cumulative travel time for a given depth, one can calculate a velocity (distance divided by time).
Vertical seismic profiles and checkshot surveys are also excellent sources because they show the actual traveltime of sound through material over a known distance, thereby yielding true velocities. Good estimates of velocities are provided by synthetic seismograms. Synthetics are made from sonic logs and show the cumulative travel time through the rocks where a sonic log was run. Knowing the cumulative travel time for a given depth, one can calculate a velocity (distance divided by time).
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, DOI: [http://library.seg.org/doi/abs/10.1190/1.1438126 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, DOI: [http://library.seg.org/doi/abs/10.1190/1.1438126 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===