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| ==Mapping two-way time== | | ==Mapping two-way time== |
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− | 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. |
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| ===Preparation for mapping=== | | ===Preparation for mapping=== |
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| ===Misties=== | | ===Misties=== |
− | <gallery mode=packed heights=200px widths=200px> | + | <gallery mode=packed heights=250px widths=250px> |
| 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. |
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| ===Contouring=== | | ===Contouring=== |
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− | 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. |
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| ==Other types of maps== | | ==Other types of maps== |
− | <gallery mode=packed heights=200px widths=200px> | + | <gallery mode=packed heights=250px widths=250px> |
| 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_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_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. |
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| ===Velocity gradient maps=== | | ===Velocity gradient maps=== |
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− | 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. |
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| 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. |
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| ===Time interval maps=== | | ===Time interval maps=== |
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− | 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. |
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| ===Time slice maps=== | | ===Time slice maps=== |
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| ==Computer-aided mapping== | | ==Computer-aided mapping== |
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− | 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]]). |
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| 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. |
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| [[Category:Geophysical methods]] | | [[Category:Geophysical methods]] |
| + | [[Category:Methods in Exploration 10]] |