Difference between a fault and a breakline in Surfer

When gridding (aka. interpolating) XYZ data, users select the data to grid and can also import a file of polylines or polygons to be used as breaklines or faults during the gridding process. Not all gridding methods support breaklines, and not all support faults. Both breaklines and fault lines need to be described in a BLN file (a Golden Software Blanking file), which is a simple text file format.

To visualize how Surfer uses faults and breaklines, we'll be using Demogrid.grd (from the Surfer Samples folder) displayed as a contour map. 

The Surfer SRF project file and the BLN file used for the faults and breaklines are attached to this article.


Surfer contour map without faults or breaklines
Original contour map, without faults or breaklines.

Faults occur when there are differences in Z values on either side of the line. This could be because of a geologic fault or another reason, such as a landslide or a building. Faults only need X and Y values in the BLN file. When gridding, Surfer sees the fault as a stop. It cannot look through the fault to the other side for data.  It can go around the fault, but the distance will increase and the data on the other side of the fault will have less of an effect on the calculated grid node value. If the fault is a polygon, and all data points are on the inside of the fault polygon, then the area outside the polygon in the grid file will be assigned NoData values. There is not a way to specify direction of movement or amount of displacement along a fault in Surfer, as they aren't specifically geological faults.

Surfer contour map with two faults
The same data set gridded with two faults and displayed as a contour map. Notice the contours can go right into the fault lines. 

Breaklines occur when there are changes in slope on either side of the line. This might be at the top of a ridge or the bottom of a cliff, at the shoreline of a lake, or anywhere a constant surface needs to be delineated. Breaklines have X, Y, and Z values in the BLN file. You can use the same BLN file as a breakline file, and also as a fault file or as a polygon boundary file for assigning NoData. The Z value is ignored if the BLN file is used as a fault or to assign NoData values. When gridding with breaklines, Surfer can look through the breakline to the other side for data points to use when calculating a grid nodes' Z value. The XYZ points of the breakline are also included as data points when gridding the data.

 Surfer contour map with two breaklines
The same data set gridded with two breaklines and displayed
as a contour map


Updated March 5, 2024

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