Changes

[[file:using-and-improving-surface-models-built-by-computer_fig2.png|thumb|300px|{{figure number|2}}Cross section showing the output from filtering being constrained between models built by shifting the initial surface model up and down slightly.]]

[[file:using-and-improving-surface-models-built-by-computer_fig2.png|thumb|300px|{{figure number|2}}Cross section showing the output from filtering being constrained between models built by shifting the initial surface model up and down slightly.]]

A common problem with computer-generated surface models are surface structures that are not supported by data. That is, the structures are by-products of the surface-modeling algorithm. Filters such as least squares, biharmonic, laplacian, and others can be applied to existing surface models to remove these unsupported structures. Data should be honored while filtering so that data values continue to fall on the correct side of contours.

A common problem with computer-generated surface models are surface structures that are not supported by data. That is, the structures are by-products of the surface-modeling algorithm. Filters such as least squares, biharmonic, laplacian, and others can be applied to existing surface models to remove these unsupported structures. Data should be honored while filtering so that data values continue to fall on the correct side of [[contour]]s.

Sometimes undesired contour wobbles are caused by data. For example, shot point values for a seismic line that parallels strike and whose shot point z values fluctuate about a contour value will cause contours to wobble through the data. This wobble is due to noise in the seismic data and is usually undesirable. Many filter programs allow surface models other than the one being filtered to act as upper and lower constraints within which the resultant surface model must stay. By shifting the original surface model up and down by small amounts (magnitude of the wobbles) and using these new surfaces to constrain filtering, a smoother model that still honors surface form can be created ([[:file:using-and-improving-surface-models-built-by-computer_fig2.png|Figure 2]]).

Sometimes undesired contour wobbles are caused by data. For example, shot point values for a seismic line that parallels strike and whose shot point z values fluctuate about a contour value will cause contours to wobble through the data. This wobble is due to noise in the seismic data and is usually undesirable. Many filter programs allow surface models other than the one being filtered to act as upper and lower constraints within which the resultant surface model must stay. By shifting the original surface model up and down by small amounts (magnitude of the wobbles) and using these new surfaces to constrain filtering, a smoother model that still honors surface form can be created ([[:file:using-and-improving-surface-models-built-by-computer_fig2.png|Figure 2]]).

====Regional trend assist====

====Regional trend assist====

This technique is sometimes used when the surface is tilted, projection up and down dip is desired, and algorithms that project slope do not produce acceptable results ([[:file:using-and-improving-surface-models-built-by-computer_fig3.png|Figure 3]]). The general steps are (1) build a first- or second-order trend surface through the data, (2) back interpolate from the trend surface a ''z'' value at each data location, (3) subtract the back interpolated values from the original data creating difference values, (4) build a surface model of the difference, and (5) add the difference surface to the trend surface.

This technique is sometimes used when the surface is tilted, projection up and down [[dip]] is desired, and algorithms that project slope do not produce acceptable results ([[:file:using-and-improving-surface-models-built-by-computer_fig3.png|Figure 3]]). The general steps are (1) build a first- or second-order trend surface through the data, (2) back interpolate from the trend surface a ''z'' value at each data location, (3) subtract the back interpolated values from the original data creating difference values, (4) build a surface model of the difference, and (5) add the difference surface to the trend surface.

[[file:using-and-improving-surface-models-built-by-computer_fig4.png|300px|thumb|{{figure number|4}}Cross sections through the same data. (a) The surface model does not honor the data. (b) Surface is shifted to data by modeling the error between the data and the original surface and then adding the original and error models.]]

[[file:using-and-improving-surface-models-built-by-computer_fig4.png|300px|thumb|{{figure number|4}}Cross sections through the same data. (a) The surface model does not honor the data. (b) Surface is shifted to data by modeling the error between the data and the original surface and then adding the original and error models.]]

===Digitize and model hand-drawn contours===

===Digitize and model hand-drawn contours===

Often too few data are available to build an acceptable surface model or to support the detailed shape of a geologist's interpretation. If this problem exists over most of the map area, then editing the output model or using dummy points is not feasible. Instead, hand-drawn maps should be used. Contours from hand-drawn maps are digitized and used as input for the surface modeling algorithms. Some algorithms are specifically designed for digitized contours. If one of these is not available, there are usually specific parameter settings that make point-modeling algorithms effective for modeling digitized contours.

Often too few data are available to build an acceptable surface model or to support the detailed shape of a geologist's interpretation. If this problem exists over most of the map area, then editing the output model or using dummy points is not feasible. Instead, hand-drawn maps should be used. [[Contour]]s from hand-drawn maps are digitized and used as input for the surface modeling algorithms. Some algorithms are specifically designed for digitized contours. If one of these is not available, there are usually specific parameter settings that make point-modeling algorithms effective for modeling digitized contours.

==Intersecting surface techniques==

==Intersecting surface techniques==

====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===

[[Category:Integrated computer methods]]

[[Category:Integrated computer methods]]