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 have 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 grid nodes. 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. Note that 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.
Image examples of the effects of gridding with breaklines and faults are below. The Surfer SRF project file and the BLN file used for the faults and breaklines are attached to this issue. The BLN file can be used as either a fault file or a breakline file. The 0's in the third column (the Z value per vertex) are only used when the BLN is used as a breakline file. They are ignored when the file is not used as a breakline file.
Original contour map without faults or breaklines.
The same data set gridded with two
faults and displayed as a contour map.
The same data set gridded with
two breaklines and displayed
as a contour map
Updated October 16, 2018