Changes

==Intersecting surface techniques==

==Intersecting surface techniques==

===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===

For computer mapping, the term ''baselap'' can be defined as the abrupt termination of a higher surface (usually depositional) against a lower surface. A similar definition could be used for ''truncation''—the abrupt termination of a lower surface against a higher surface (usually an unconformity).

For computer mapping, the term ''baselap'' can be defined as the abrupt termination of a higher surface (usually depositional) against a lower surface. A similar definition could be used for ''truncation''—the abrupt termination of a lower surface against a higher surface (usually an unconformity).

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 (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.

[[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.]]

[[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).]]

====Baselap====

====Baselap====

Baselap can be achieved in several ways. To baselap one grid onto another for the purpose of cross section display and volumetrics work, the elevation ''z'' values at each grid node are compared and the maximum value is retained in a new grid. This makes the two grids coincident in the area where the upper grid is missing due to baselap (Figure 9).

Baselap can be achieved in several ways. To baselap one grid onto another for the purpose of cross section display and volumetrics work, the elevation ''z'' values at each grid node are compared and the maximum value is retained in a new grid. This makes the two grids coincident in the area where the upper grid is missing due to base lap ([[:file:using-and-improving-surface-models-built-by-computer_fig9.png|Figure 9]]).

[[file:using-and-improving-surface-models-built-by-computer_fig9.png|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).]]

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 (Figure 9).

[[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.]]

 

The ''subcrop line'' is the line of contact between a baselapping surface and the surface upon which it baselaps (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.

[[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.]]

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 (Figure 11).

[[file:using-and-improving-surface-models-built-by-computer_fig11.png|thumb|left|{{figure number|11}}Map showing contours and subcrop lines.]]

[[file:using-and-improving-surface-models-built-by-computer_fig11.png|thumb|{{figure number|11}}Map showing contours and subcrop lines.]]

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]]).

====Truncation====

====Truncation====

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===

In projects involving more than two surfaces, the techniques for baselap, truncation, and conformity are often used in combination (Figure 12). The following rules are used to order the techniques:

In projects involving more than two surfaces, the techniques for baselap, truncation, and conformity are often used in combination ([[:file:using-and-improving-surface-models-built-by-computer_fig12.png|Figure 12]]). The following rules are used to order the techniques:

* On a hand-drawn cross section showing all surface relationships, identify the unconformities.

* On a hand-drawn cross section showing all surface relationships, identify the unconformities.

* For each sequence, use the conformable technique to build all of the noncontrol grids.

* For each sequence, use the conformable technique to build all of the noncontrol grids.

* Starting with the lowermost surface, perform truncation and baselap operations, working up from the bottom.

* Starting with the lowermost surface, perform truncation and baselap operations, working up from the bottom.

Unique aspects of a project often require these procedures to be modified. However, the procedures provide a useful outline for getting started and guiding you through a project.

Unique aspects of a project often require these procedures to be modified. However, the procedures provide a useful outline for getting started and guiding you through a project.