Fault application functions
Faults represented as dtm surfaces or traces (in the case of vertical faults) may be used to control the generation of a discontinuous surface (that is, partitioned into hanging and foot walls of the fault) from a set of points representing that surface. The algorithms are described below as complex geometries of faults and surfaces may not give satisfactory results and it may be necessary to model the faults manually.
This function applies an oblique fault represented by a dtm to a set of points representing a surface to produce two trisolations representing the hanging and footwall components of that surface.
To run this function: Choose Surfaces > Fault modelling > Create faulted surface - oblique, or...
First, the points are partioned into those on either side of the fault. The definition of being on one side or the other of the fault relates to the plane of best fit of the fault and to the deviation of the fault surface from that plane. If we consider the plane to be rotated so that it is horizontal and the surface points and the fault are rotated in sympathy, then all points above the maximum upward deviation of the fault from the plane are on one side of the fault while points below the maximum downward deviation of the fault from the plane are on the other side. Points that fall within these maximum deviations are tested as to whether they are above or below the unrotated fault surface and assigned to the appropriate partition. Any points that could not be so partitioned are left undefined and are ignored in subsequent operations.
Once the two partitions have been determined, a dtm surface of each is created and expanded to a boundary that extends beyond the fault. The line of intersection between the fault and the surface is determined and a boundary comprising the original boundary of the partition and the line of intersection is derived and the expanded surface is clipped to that boundary.
Select a triangle on the fault surface
Select a triangle and the trisolation containing that triangle will be used as the fault surface. The prompt:
Select a point on the surface to be faulted
will then be displayed and you should select a point. All points in the layer containing the selected point are considered to be on the surface to be faulted.
Note: The layer containing the dtm representing the fault must be different to that containing the points representing the surface to be faulted.
Apply fault to surface form
Surface expansion method
Nominate the method to be used for expanding the surface.
Boundary dip. The dips at the points on the old DTM boundary are estimated by using a weighted average of the dips of the triangles attached to the point. If there is a lot of local fluctuation of dip at the old boundary, you might find that this creates a "saw-toothed" effect at the new boundary. If you choose Boundary dip, you also enter a Smoothing factor.
The Plane Fit method extends the DTM to the new boundary using the plane of best fit of the data in the DTM. You will find that the Plane Fit method is the most appropriate for expanding data that lies approximately in a plane.
Smoothing factor
If the BOUNDARY DIP method is to be used to expand the surface, specify the smoothing factor to be used to average the dips. The Smoothing Factor is an integer between 0 and 50. The Smoothing Factor identifies the number of points on each side of the point being expanded. The points are then averaged to determine the dip for the expanded point. For example, if the Smoothing Factor is 5, five points on either side of the point on the old boundary are used in the averaging, as well as the point itself.
Output layer
Enter or select the graphics layer to which the fault DTM is to be drawn. If the nominated layer does not exist then it will be created.
Object ID, Object name
Define the ID and name that will assigned to the DTM. The ID must be a positive integer. The name can be blank or any character value. The name is only used in DTM reports.
Results
The components of the resultant surface are drawn in the nominated layer.
Errors
The fault and the surface to be faulted must be in different layers
Ensure that the fault dtm and the surface points are in different graphics layers.
Cannot partition points in String 999 using String/Object 999, Segment/Trisolation 999
The points on the surface could not be partitioned using the dtm surface nominated.
Cannot expand the dtm surface for partition
An error occurred trying to expand the partition surface.
Cannot generate the dtm surface for partition
An error occurred trying to create a surface from the points in a partition.
Vertical Fault
This function applies a vertical fault represented by a segment to a set of points representing a surface to produce two trisolations representing the hanging and footwall components of that surface.
To run this function: Choose Surfaces > Fault modelling > Create faulted surface - vertical, or...
First the points are partitioned into those on either side of the fault. The definition of being on one side or the other of the fault relates to the line of best fit of the fault and to the deviation of the fault trace from that line. If we consider the line to be rotated so that it is horizontal and the surface points and the fault trace are rotated in sympathy, then all points above the maximum upward deviation of the fault trace from the line are on one side of the fault while points below the maximum downward deviation of the fault trace from the line are on the other side. Points that fall within these maximum deviations are tested as to whether they lie in one or the other of the two polygons derived from the fault trace and the maximum deviations and assigned to the appropriate partition. Any points that could not be so partitioned are left undefined, as are points that lie beyond rays cast orthogonally to the end points of the line of best fit, and are ignored in subsequent operations.
Once the two partitions have been determined, a dtm surface of each is created and expanded to a boundary that extends beyond the fault trace. A boundary comprising the original boundary of the partition and the fault trace is derived and the expanded surface is clipped to that boundary.
Select a point in the fault string
Select a point and the segment containing that point will be used as the fault trace. The prompt:
Select a point on the surface to be faulted
will then be displayed and you should select a point. All points in the layer containing the selected point are considered to be on the surface to be faulted.
Note: The layer containing the segment representing the fault trace must be different to that containing the points representing the surface to be faulted.
The APPLY FAULT TO SURFACE is then presented.
Surface expansion method
Nominate the method to be used for expanding the surface. This may be BOUNDARY DIP or PLANE FIT.
Smoothing factor
If the BOUNDARY DIP method is to be used to expand the surface, specify the smoothing factor to be used to average the dips.
Output layer
Enter or select the graphics layer to which the fault dtm is to be drawn. If the nominated layer does not exist then it will be created.
Results
The components of the resultant surface are drawn in the nominated layer.
Errors
The fault and the surface to be faulted must be in different layers
Ensure that the fault dtm and the surface points are in different graphics layers.
Cannot partition points in String 999 using String/Object 999, Segment/Trisolation 999
The points on the surface could not be partitioned using the segment nominated.
Cannot expand the dtm surface for partition
An error occurred trying to expand the partition surface.
Cannot generate the dtm surface for partition
An error occurred trying to create a surface from the points in a partition.
Multi Surfaces
Fault Multiple Surfaces
This function applies a vertical or oblique fault to multiple surfaces as contained in a range of string files. The algorithms employed are those for the application of oblique and vertical faults described elsewhere. The results of the function may be written to dtm files and/or drawn in graphics layers (if invoked while in graphics mode).
To run this function: Choose Surfaces > Fault modelling > Create multiple faulted surfaces, or...
Apply fault to multiple surfaces form
FAULT
This specifies the object to act as the fault. If oblique, it must be a DTM and if vertical, a segment.
Location, Object ID and Trisolation ID
Specify the location and ID of the string or DTM file containing the fault surface or trace.
String/Triobject
Specify the string (vertical fault) or triobject (oblique fault) number of the fault object.
Segment/Trisolation
Specify the segment (vertical fault) or trisolation (oblique fault) number of the fault object.
SURFACES
Location, ID Range
Specify the location and ID range of the string files containing the points representing the surface to be faulted. All points in a given string file are deemed to represent the surface.
Output
The name specified here will act as the location of the dtm files and the root name of the graphics layers to which the results of faulting each surface are written or drawn. The IDs will be those of the input string files. The names of the graphics layers will be the same as those of the resultant dtm files (without, of course, the ".dtm" extension). So, if a string file "input1.str" is processed and "output" is specified as the output name, the results may be written to "output1.dtm" and drawn in a graphics layer called "output1". If this field is left blank then the location of the input surface files is used.
Object ID, Object name
Define the ID and name that will assigned to the DTM. The ID must be a positive integer. The name can be blank or any character value. The name is only used in DTM reports.
Surface expansion method
Nominate the method to be used for expanding the surface. This may be BOUNDARY DIP or PLANE FIT.
Smoothing factor
If the BOUNDARY DIP method is to be used to expand the surface, specify the smoothing factor to be used to average the dips. An example value is 10.
Save to file?
Select "Y" if the results are to be written to dtm files. This option is fixed to "Y" if the function is not being invoked from graphics.
Save to layer?
Select "Y" if the results are to be drawn in graphics layers. This option is fixed to "N" if the function is not being invoked from graphics.
Fault type
Select OBLIQUE if an oblique fault is to be applied in which case a dtm having the specifications given for the fault is expected.
Select VERTICAL if a vertical fault is to be applied in which case a segment having the specifications given for the fault is expected.
Results
Each string file is processed in turn and the results written out to a dtm file or drawn in a graphics layer as nominated.
Errors
Must nominate to save results to files or layers
Ensure that at least one of "Save to file?" or "Save to layer?" is "Y".
Error loading file xxx
The specified surface file could not be loaded.
File xxx not found
The specified file could not be found.
Fault object String/Triobject 99, Segment/Trisolation 99 not found
The specified object could not be found in the file.
Error loading file xxx
If a warning then the specified surface file could not be loaded but processing continues. If an error then the file containing the fault object could not be loaded. Ensure that the dtm file exists if an OBLIQUE fault has been nominated or that a string file exists if a VERTICAL fault has been nominated.
Cannot expand the dtm surface for partition
An error occurred trying to expand the partition surface.
Cannot generate the dtm surface for partition
An error occurred trying to create a surface from the points in a partition.