Changes

==Mapping two-way time==

==Mapping two-way time==

Most two-dimensional seismic reflection lines are presented in the format of horizontal distance versus two-way traveltime (time sections). Using interpreted time sections and a geographic base map, one can draft structure contour maps.

Most two-dimensional seismic reflection lines are presented in the format of horizontal distance versus two-way traveltime (time sections). Using interpreted time sections and a geographic base map, one can draft structure [[contour]] maps.

===Preparation for mapping===

===Preparation for mapping===

===Misties===

===Misties===

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mapping-with-two-dimensional-seismic-data_fig1.png|{{figure number|1}}False structure (dashed lines) created when misties are averaged. Dots show times of events on seismic lines A, X, and N where those lines intersect line Q. Solid lines show true attitude of beds. If dashed events were mapped, false structure would appear.

mapping-with-two-dimensional-seismic-data_fig1.png|{{figure number|1}}False structure (dashed lines) created when misties are averaged. Dots show times of events on seismic lines A, X, and N where those lines intersect line Q. Solid lines show true attitude of beds. If dashed events were mapped, false structure would appear.

mapping-with-two-dimensional-seismic-data_fig2.png|{{figure number|2}}(a) Crude base map illustrating seismic line intersections. (b) Table showing misties at seismic line intersections (times in milliseconds). Circled lines constitute a group having small misties. A group can be used as a base to which times on all other lines are adjusted. For example, times on line A561–80 could be shifted down about 52 msec.

mapping-with-two-dimensional-seismic-data_fig2.png|{{figure number|2}}(a) Crude base map illustrating seismic line intersections. (b) Table showing misties at seismic line intersections (times in milliseconds). Circled lines constitute a group having small misties. A group can be used as a base to which times on all other lines are adjusted. For example, times on line A561–80 could be shifted down about 52 msec.

===Contouring===

===Contouring===

After the misties are adjusted, the revised two-way times can be plotted on the final base map. Finally, the points can be contoured at an interval that you deem appropriate.

After the misties are adjusted, the revised two-way times can be plotted on the final base map. Finally, the points can be [[contour]]ed at an interval that you deem appropriate.

==Other types of maps==

==Other types of maps==

 

[[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.]]

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.

===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.

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===

===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===

A time slice map shows geology in the horizontal plane at a given time (normally two-way time) ([[:file:mapping-with-two-dimensional-seismic-data_fig6.png|Figure 6]]). In essence, making a time slice map is analogous to making geological maps from cross sections. As a first step toward construction, imagine that all the data above your chosen time disappear and that your base map lies directly on top of the chosen time. Next, locate each intersection between an interpreted event and the chosen time and plot that intersection at the ground position immediately above. Finally, link the points for each respective event in a manner that is consistent with your knowledge of the structure on the lines.

A time slice map shows geology in the horizontal plane at a given time (normally two-way time) ([[:file:mapping-with-two-dimensional-seismic-data_fig6.png|Figure 6]]). In essence, making a time slice map is analogous to making geological maps from [[cross section]]s. As a first step toward construction, imagine that all the data above your chosen time disappear and that your base map lies directly on top of the chosen time. Next, locate each intersection between an interpreted event and the chosen time and plot that intersection at the ground position immediately above. Finally, link the points for each respective event in a manner that is consistent with your knowledge of the structure on the lines.

==Computer-aided mapping==

==Computer-aided mapping==

Interpretations derived from seismic lines can be mapped efficiently with the help of a computer workstation, which performs repetitive calculations very quickly. First, the interpretations must be entered into a workstation, either by interactive (on-screen) interpretation or by digitizing interpretations that exist on printed lines. Mistie corrections are then performed by the computer. Two-way times and time intervals can then be posted on a base map. Contouring time and time interval maps can be done by the workstation, but the result usually requires some hand editing (see [[A development geology workstation]]).

Interpretations derived from seismic lines can be mapped efficiently with the help of a computer workstation, which performs repetitive calculations very quickly. First, the interpretations must be entered into a workstation, either by interactive (on-screen) interpretation or by digitizing interpretations that exist on printed lines. Mistie corrections are then performed by the computer. Two-way times and time intervals can then be posted on a base map. [[Contour]]ing time and time interval maps can be done by the workstation, but the result usually requires some hand editing (see [[A development geology workstation]]).

Constructing depth maps is possible once hand-drawn time and velocity maps are digitized into the workstation. The computer grids and multiplies the time and velocity maps, and the resultant values can then be contoured.

Constructing depth maps is possible once hand-drawn time and velocity maps are digitized into the workstation. The computer grids and multiplies the time and velocity maps, and the resultant values can then be contoured.

[[Category:Geophysical methods]]

[[Category:Geophysical methods]]