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Line 127: − + − [[file:using-and-improving-surface-models-built-by-computer_fig13.png|left|thumb|{{figure number|13}}Separate surface models are built for each fault block. (a, b, and c) The surface for each fault block is allowed to extend past faults defining the block edge. (d) When displayed, contours are constrained to inside the fault block polygon and all models are displayed on the same map. (After <ref name=pt08r11 />.)]]+ + + + + + Line 139:
Line 144: − − [[file:using-and-improving-surface-models-built-by-computer_fig14.png|thumb|{{figure number|14}}Surface models are constructed for the faults and for each surface on each side of each fault. Operations between surface models prevent them from projecting past one another.]] Line 147:
Line 150: − − [[file:using-and-improving-surface-models-built-by-computer_fig15.png|left|thumb|{{figure number|15}}Faults act as barriers beyond which data cannot be seen from the location for which a surface value is being calculated. (a) A grid node (indicated by +) to the west of fault A can only see data in the hatchured area. (b) A grid node farther to the south of fault A can see more data, thus the surface smoothly changes form around the fault ends.<ref name=pt08r11 />]] Line 157:
Line 158: − − [[file:using-and-improving-surface-models-built-by-computer_fig16.png|thumb|{{figure number|16}}(a) Surface model built with no fault constraints. (b) Model of vertical separation. (c) Unfaulted structure model built after removing vertical separation from data. (d) Faulted structure model built by subtracting separation model from unfaulted structure model. (After <ref name=pt08r11 />.)]] Line 165:
Line 164: − − [[file:using-and-improving-surface-models-built-by-computer_fig17.png|thumb|left|{{figure number|17}}The data value is adjusted by the separation of faults crossed by the line connecting the data point and the location for which an estimate is being made.]]
Using and improving surface models built by computer (view source)
Revision as of 13:38, 8 July 2014
, 13:38, 8 July 2014→Faults
==Faults ==
==Faults ==
<gallery mode=packed heights=200px widths=200px>
using-and-improving-surface-models-built-by-computer_fig13.png|{{figure number|13}}Separate surface models are built for each fault block. (a, b, and c) The surface for each fault block is allowed to extend past faults defining the block edge. (d) When displayed, contours are constrained to inside the fault block polygon and all models are displayed on the same map. (After <ref name=pt08r11 />.)
using-and-improving-surface-models-built-by-computer_fig14.png|{{figure number|14}}Surface models are constructed for the faults and for each surface on each side of each fault. Operations between surface models prevent them from projecting past one another.
using-and-improving-surface-models-built-by-computer_fig15.png|{{figure number|15}}Faults act as barriers beyond which data cannot be seen from the location for which a surface value is being calculated. (a) A grid node (indicated by +) to the west of fault A can only see data in the hatchured area. (b) A grid node farther to the south of fault A can see more data, thus the surface smoothly changes form around the fault ends.<ref name=pt08r11 />
using-and-improving-surface-models-built-by-computer_fig16.png|{{figure number|16}}(a) Surface model built with no fault constraints. (b) Model of vertical separation. (c) Unfaulted structure model built after removing vertical separation from data. (d) Faulted structure model built by subtracting separation model from unfaulted structure model.<ref name=pt08r11 />
using-and-improving-surface-models-built-by-computer_fig17.png|{{figure number|17}}The data value is adjusted by the separation of faults crossed by the line connecting the data point and the location for which an estimate is being made.
</gallery>
Few programs automatically identify faults based on input data; therefore, an interpretation of the presence and position of faults must be made prior to computer mapping. Once the fault interpretation has been made, several techniques exist for incorporating them into a surface model. Some commonly used methods are described here.
Few programs automatically identify faults based on input data; therefore, an interpretation of the presence and position of faults must be made prior to computer mapping. Once the fault interpretation has been made, several techniques exist for incorporating them into a surface model. Some commonly used methods are described here.
Fault gaps imply nonvertical faults, and traces with gaps would be expected to shift position from one surface to the next. Since digitizing polygons requires significant effort, the same set of polygons is often used for all surfaces (vertical faults). It is important to understand how a vertical assumption affects unit volumes.
Fault gaps imply nonvertical faults, and traces with gaps would be expected to shift position from one surface to the next. Since digitizing polygons requires significant effort, the same set of polygons is often used for all surfaces (vertical faults). It is important to understand how a vertical assumption affects unit volumes.
===Fault plane===
===Fault plane===
If this technique is used, then creating displays and volumes for a surface or zone requires careful manipulation of a large number of surface models. This is because each surface is represented by a suite of surface models, one for each fault block. Also, much care is required to model surfaces cut by faults that fade out in the map area.
If this technique is used, then creating displays and volumes for a surface or zone requires careful manipulation of a large number of surface models. This is because each surface is represented by a suite of surface models, one for each fault block. Also, much care is required to model surfaces cut by faults that fade out in the map area.
===Fault trace===
===Fault trace===
Contouring, cross section, volumetrics, and other surface display and manipulation algorithms must be modified to use fault traces. When modified, these algorithms do not use surface model values from one side of a fault for contouring and volume calculations on the other side of the fault.
Contouring, cross section, volumetrics, and other surface display and manipulation algorithms must be modified to use fault traces. When modified, these algorithms do not use surface model values from one side of a fault for contouring and volume calculations on the other side of the fault.
===Vertical separation===
===Vertical separation===
Vertical separation modeling is usually handled either by (1) building a separation model for each fault and adjusting all the data at once or (2) adjusting each data point for vertical separation at the time it is used for surface modeling. In the first approach, a vertical separation model is built for each fault. All of the separation models for faults that affect a particular surface are added together. Data for that surface are shifted by the total separation at each location, moving them to their prefault position. A surface model is constructed using the adjusted data, and the total vertical separation model is then subtracted from the unfaulted surface model, creating the final faulted surface model ([[:file:using-and-improving-surface-models-built-by-computer_fig16.png|Figure 16]]).
Vertical separation modeling is usually handled either by (1) building a separation model for each fault and adjusting all the data at once or (2) adjusting each data point for vertical separation at the time it is used for surface modeling. In the first approach, a vertical separation model is built for each fault. All of the separation models for faults that affect a particular surface are added together. Data for that surface are shifted by the total separation at each location, moving them to their prefault position. A surface model is constructed using the adjusted data, and the total vertical separation model is then subtracted from the unfaulted surface model, creating the final faulted surface model ([[:file:using-and-improving-surface-models-built-by-computer_fig16.png|Figure 16]]).
The second approach is similar to the fault trace method in that it alters the use of a data point on the opposite side of a fault. However, instead of not using the point, it adjusts the point's z value by the vertical separation of the faults that lie between the point and the node being calculated ([[:file:using-and-improving-surface-models-built-by-computer_fig17.png|Figure 17]]).
The second approach is similar to the fault trace method in that it alters the use of a data point on the opposite side of a fault. However, instead of not using the point, it adjusts the point's z value by the vertical separation of the faults that lie between the point and the node being calculated ([[:file:using-and-improving-surface-models-built-by-computer_fig17.png|Figure 17]]).