Topic:
Why doesn't convergent gridding honor fault boundaries correctly?

Description:
It appears that incorrect results are generated. This results in grid nodes and
contours generated inside closed fault boundaries with no data points present
inside. It also may use data points from the other side of the fault boundary
even though those should not be used during gridding based on documented
criteria. (This is mostly in CPS documentation.)

Basemap Plus uses CPS-3 functions for gridding. There are several grid
interfaces in CPS for gridding. One is called "quick map", another is called
"single surface gridding" (CPS Main module > Modeling > Single surface). Using
"single surface" convergent gridding, a user can make a choice regarding the use
of fault boundaries. There is a parameter that can be set which is called
"percent of iterations to use faults", by default it is set to "All" (use in all
iterations). Convergent (CPS 4.2 style) uses a similar option which is worded a
bit differently - "Fault handling in convergent gridding passes", the choices
are "use faults in all passes" (default) and "use faults only after coarse
gridding passes". However there is no similar parameter that can be set by the
user in Basemap Plus. Gridding in Basemap Plus is similar in this regard to how
CPS "quick map" works - there is no similar parameter there either. Basemap Plus
default regarding usage of fault boundaries is not the same as CPS default - it
honors fault boundaries but not 100%. 

Solution:
Several solutions/workarounds are available depending on circumstances. If the
used algorithm and produced results are completely unacceptable, one can use
least squares gridding instead of convergent in Basemap Plus - least squares
gridding works with fault boundaries correctly and honors them 100% of the time
(starting in GeoFrame 3.5), however, it is also slower because of this. One can also move the data to CPS via IESX-CPS Link or GF Link and perform gridding there.Grids generated in CPS can be saved directly to GeoFrame database starting in GeoFrame 3.5.

If the grid and contours generated inside fault boundaries look wrong but
results are acceptable in other respects, one can use "convergent gridding with
fault trace blanking" to solve the problem.

One can also change other parameters while using convergent gridding in Basemap
Plus to make it honor fault boundaries to a greater extent. By changing "grid
coverage" from "high" to "medium" or "low", we reduce the initial grid
increments for convergent gridding and make it pick less points across the fault
boundary. Initial intervals for "High coverage" are 7 times final, "Medium
coverage" : 4 times final, and for "Low coverage" initial = final.

Gridding in Basemap Plus is going to be substantially enhanced for GeoFrame 4.0
and users will have more control over use of fault boundaries as well as other
parameters.




Last Modified on: 09-JUN-00