TRENDSMOOTH.GX – Author: Aaron Balasch.
The TRENDSMOOTH.GX partitions the input grid into ‘subset grids’ and applies a trend surface(0-3rd degree)
to each of the ‘subset grids’. It then recombines all the trended ‘subset grids’ through a Boolean AND operation
to end up with an output grid the same size as the input grid.
The input parameters are;
Input Grid file
Output Trend Surface Grid file
Output Residual Grid (Input grid with trend surface removed)
Window size in X direction in kilometres (i.e. width of window for subset grids)
Window size in Y direction in kilometres (i.e. height of window). Default=same as window width
Window overlap in kilometers(Defualt = 15% of window width))
Trend order (0-3) to apply to each subset grid
Lowpass filter to apply to the residual grid In KM (Default = 6 times the cell size)
Smooth the output residual surface grid
Number of pass of 3x3 Hanning filter(if Smooth=yes, Default = 3)
Low end percentile for clipping(Default = 5th%ile)
High end percentile for clipping(Default = 99.9 %ile)
Keep residual subset grids
In addition to the two output grids, a group called RECTANGLE is created. The RECTANGLE group contains
a series of horizontal and vertical lines(similar to graph paper) that show where the input grid has been
partitioned into ‘subset grids’.
Once the input grid has been partitioned into a subset grids, the trend surface is then applied to that subset
grid to create a residual surface for the subset grid. Each residual subset grid is now ‘pasted’ to the previous
residual subset grid by means of a Boolean AND operation to obtain a residual surface for the entire input grid.
A lowpass filter can now be applied in orthogonal directions(First in the X direction and then again in the Y
direction). Smoothing with a 3x3 Hanning filter can also be applied to obtain the final residual surface. A trend
surface is then created by simply subtracting the residual surface from the input grid.
With low end percentile clipping set to zero(i.e. no low end clipping), the residual surface is offset such that the
residual surface contains no negative numbers. The low end percentile for clipping is intended to compensate
for low end outliers. The high end percentile for clipping is intended to compensate for outliers on the high end.
The last option provides the user with a choice to keep the residual subset grids or not. Residual subset grids
can be kept for data verification purposes or additional analysis. The subset grids are named according to
ROW-COLUMN, where ROW and COLUMN correspond to the origin of the subset grid. For example, the first
subset grid is always named 0-0.grd. For an input grid cell size of 100, a window size of 2 km, and an overlap
of 15% the second subset grid will be named 0-17.grd.(row=0, column = 2km window – 300 metre overlap =
17 cells @ 100metre cell size)
I
A small example is presented below that illustrates TRENDSMOOTH.GX. The example illustrates how
trendsmooth can be used to perform trend analysis for oil and gas exploration
The top surface of the Keg River formation (an oil bearing formation in northern Alberta) was mapped for a
small area in northern Alberta. The picks for the top surface of the Keg River formation were taken from a
database of well log picks. Figure 1 on the next page shows the Keg River surface. The Keg River surface is
not mapped where there are large gaps in the well control.
Trendsmooth was applied to the surface of the Keg
River formation. The parametes for were,
Window size = 5 x 5 km
Overlap = default = 15%
Trend order = 2
Low %ile for clipping = default = 5
High %ile for clipping = default = 99.9
Figure 1: Keg River Surface
The map below shows the residual surface
Figure 2: Residual Surface
The figure below shows profiles of for the Keg River surface(Figure 1), the residual surface (figure 2), and the
trend surface along the line of section. The residual surface and the trend surface are the two surfaces created
by TRENDSMOOTH
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· For any of your GX programming or Oasis Montaj needs, please contact me at the above address
· Over 10 years as a GX programmer and 15 years working with Oasis Montaj.
· A library of over 40 custom GX’s
· For a list of other all GX's or any questions or enquiries, please email abalasch@telus.net or
aaron.balasch@skyhunter.ca
Questions/Problems/Requests or for any other custom GX programming needs contact abaslasch@telus.net
or aaron.balasch@skyhunter.ca