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Line 77: − + − [[file:using-and-improving-surface-models-built-by-computer_fig6.png|thumb|{{figure number|6}}Cross section showing two conformable surfaces. Dashed line represents direct modeling of lower surface data. Solid lines represent direct modeling of upper surface data and conformable modeling of lower surface data.]]+ + + + + + + + − − [[file:using-and-improving-surface-models-built-by-computer_fig7.png|thumb|left|{{figure number|7}}Cross section showing four conformable surfaces. The second from the top is the control and is modeled using structure data. The other surfaces are built using the conformable method.]] − − [[file:using-and-improving-surface-models-built-by-computer_fig8.png|thumb|{{figure number|8}}Cross sections showing that surfaces that intersect due to (a) baselap or (b) truncation will incorrectly cross one another.]] Line 95:
Line 98: − − [[file:using-and-improving-surface-models-built-by-computer_fig9.png|left|thumb|{{figure number|9}}Cross sections showing a baselapping surface (a) as coincident with the lower surface in areas of baselap (for cross section display) and (b) as missing in areas of baselap (for map display).]] Line 103:
Line 104: − − [[file:using-and-improving-surface-models-built-by-computer_fig10.png|thumb|{{figure number|10}}Cross section showing surfaces before baselap operations. The zero contour of the model built by subtracting the two surfaces defines the subcrop line.]] − − [[file:using-and-improving-surface-models-built-by-computer_fig11.png|thumb|left|{{figure number|11}}Map showing contours and subcrop lines.]] Line 115:
Line 112: − − [[file:using-and-improving-surface-models-built-by-computer_fig12.png|thumb|{{figure number|12}}The middle surface baselaps onto the lower surface and is truncated by the higher surface. (a) Cross section showing proper relationships. (b) Map showing surface contours and lines of baselap and truncation.]]
Using and improving surface models built by computer (view source)
Revision as of 13:22, 8 July 2014
, 13:22, 8 July 2014→Intersecting surface techniques
==Intersecting surface techniques==
==Intersecting surface techniques==
<gallery mode=packed heights=200px widths=200px>
using-and-improving-surface-models-built-by-computer_fig6.png|{{figure number|6}}Cross section showing two conformable surfaces. Dashed line represents direct modeling of lower surface data. Solid lines represent direct modeling of upper surface data and conformable modeling of lower surface data.
using-and-improving-surface-models-built-by-computer_fig7.png|{{figure number|7}}Cross section showing four conformable surfaces. The second from the top is the control and is modeled using structure data. The other surfaces are built using the conformable method.
using-and-improving-surface-models-built-by-computer_fig8.png|{{figure number|8}}Cross sections showing that surfaces that intersect due to (a) baselap or (b) truncation will incorrectly cross one another.
using-and-improving-surface-models-built-by-computer_fig9.png|{{figure number|9}}Cross sections showing a baselapping surface (a) as coincident with the lower surface in areas of baselap (for cross section display) and (b) as missing in areas of baselap (for map display).
using-and-improving-surface-models-built-by-computer_fig10.png|{{figure number|10}}Cross section showing surfaces before baselap operations. The zero contour of the model built by subtracting the two surfaces defines the subcrop line.
using-and-improving-surface-models-built-by-computer_fig11.png|{{figure number|11}}Map showing contours and subcrop lines.
using-and-improving-surface-models-built-by-computer_fig12.png|{{figure number|12}}The middle surface baselaps onto the lower surface and is truncated by the higher surface. (a) Cross section showing proper relationships. (b) Map showing surface contours and lines of baselap and truncation.
</gallery>
===Conformity===
===Conformity===
A technique used to model conformable surfaces is thickness addition or subtraction. It is used because directly gridding each surface in a group of conformable surfaces may not produce the best results. Often variations in data distributions allow one surface to project past another or to have significantly more form definition than others of that sequence ([[:file:using-and-improving-surface-models-built-by-computer_fig6.png|Figure 6]]). The conformable technique builds a grid for the surface with the best data distribution (control surface) within a sequence of conformable surfaces and then adds or subtracts the adjacent interval's thickness to generate conformable surfaces above or below it. The newly constructed structural surface now becomes the surface to which thickness is added or subtracted to produce the next higher or lower surface. The process continues upward and downward until all surfaces within the sequence are constructed ([[:file:using-and-improving-surface-models-built-by-computer_fig7.png|Figure 7]]). This approach works well for complete data sets and vertical wells, although additional steps are required to handle deviated wells, partial penetrations, or missing data<ref name=pt08r11 />.
A technique used to model conformable surfaces is thickness addition or subtraction. It is used because directly gridding each surface in a group of conformable surfaces may not produce the best results. Often variations in data distributions allow one surface to project past another or to have significantly more form definition than others of that sequence ([[:file:using-and-improving-surface-models-built-by-computer_fig6.png|Figure 6]]). The conformable technique builds a grid for the surface with the best data distribution (control surface) within a sequence of conformable surfaces and then adds or subtracts the adjacent interval's thickness to generate conformable surfaces above or below it. The newly constructed structural surface now becomes the surface to which thickness is added or subtracted to produce the next higher or lower surface. The process continues upward and downward until all surfaces within the sequence are constructed ([[:file:using-and-improving-surface-models-built-by-computer_fig7.png|Figure 7]]). This approach works well for complete data sets and vertical wells, although additional steps are required to handle deviated wells, partial penetrations, or missing data<ref name=pt08r11 />.
The conformable technique is often used to transfer the shape of an existing surface to a new surface while honoring the data of that new surface. Common applications include stream channels and the top of draping rock units<ref name=pt08r9>Hamilton, D. E., Jones, T. A., eds., 1992, Computer Modeling of Geologic Surfaces and Volumes: AAPG Computer Applications in Geology, n. 1, 297 p.</ref>.
The conformable technique is often used to transfer the shape of an existing surface to a new surface while honoring the data of that new surface. Common applications include stream channels and the top of draping rock units<ref name=pt08r9>Hamilton, D. E., Jones, T. A., eds., 1992, Computer Modeling of Geologic Surfaces and Volumes: AAPG Computer Applications in Geology, n. 1, 297 p.</ref>.
===Baselap and truncation===
===Baselap and truncation===
Most computer mapping systems build a surface model using only data for that surface. When one surface laps onto or truncates another, the initial surface models will almost always cross ([[:file:using-and-improving-surface-models-built-by-computer_fig8.png|Figure 8]]). This is expected and must be corrected. The following discussion describes methods for handling baselap and truncation in a grid-based mapping system.
Most computer mapping systems build a surface model using only data for that surface. When one surface laps onto or truncates another, the initial surface models will almost always cross ([[:file:using-and-improving-surface-models-built-by-computer_fig8.png|Figure 8]]). This is expected and must be corrected. The following discussion describes methods for handling baselap and truncation in a grid-based mapping system.
====Baselap====
====Baselap====
Contour maps of baselapping surfaces should have no contours in areas of baselap because the surface does not exist there. To baselap one grid onto another for map display, the elevation z values at each grid node are compared and the value of the baselapping grid is set to missing if lower or kept if higher than the other grid ([[:file:using-and-improving-surface-models-built-by-computer_fig9.png|Figure 9]]).
Contour maps of baselapping surfaces should have no contours in areas of baselap because the surface does not exist there. To baselap one grid onto another for map display, the elevation z values at each grid node are compared and the value of the baselapping grid is set to missing if lower or kept if higher than the other grid ([[:file:using-and-improving-surface-models-built-by-computer_fig9.png|Figure 9]]).
The ''subcrop line'' is the line of contact between a baselapping surface and the surface upon which it base laps ([[:file:using-and-improving-surface-models-built-by-computer_fig10.png|Figure 10]]). To generate this line, the elevation z values of one grid are subtracted from the other. The input grids must cross one another (grids before baselap) or the intersection cannot be established. The resulting intersection grid will have positive and negative values, and its zero contour will be the line of baselap.
The ''subcrop line'' is the line of contact between a baselapping surface and the surface upon which it base laps ([[:file:using-and-improving-surface-models-built-by-computer_fig10.png|Figure 10]]). To generate this line, the elevation z values of one grid are subtracted from the other. The input grids must cross one another (grids before baselap) or the intersection cannot be established. The resulting intersection grid will have positive and negative values, and its zero contour will be the line of baselap.
To display contours and the subcrop line, contour the baselapped (blanked) surface grid and then on the same map draw only the zero contour from the intersection grid ([[:file:using-and-improving-surface-models-built-by-computer_fig11.png|Figure 11]]).
To display contours and the subcrop line, contour the baselapped (blanked) surface grid and then on the same map draw only the zero contour from the intersection grid ([[:file:using-and-improving-surface-models-built-by-computer_fig11.png|Figure 11]]).
With only a few modifications, the approach used for baselap can be applied to truncation. For cross section and volumetrics work, the two grids are compared and the minimum is kept as the new truncated grid. For contour display, the grids are compared and the values of the truncated grid are set to missing if higher or kept if lower than the other grid. The intersection grid for subcrop display is built just as it was for baselap.
With only a few modifications, the approach used for baselap can be applied to truncation. For cross section and volumetrics work, the two grids are compared and the minimum is kept as the new truncated grid. For contour display, the grids are compared and the values of the truncated grid are set to missing if higher or kept if lower than the other grid. The intersection grid for subcrop display is built just as it was for baselap.
===Combining baselap, truncation, and conformity===
===Combining baselap, truncation, and conformity===