Bi-Directional is a technique for roughly parallel lines. It is 1D method, it cannot use tie lines. It creates grid lines (columns, rows) independent of each other, which may lead to discrepancies and visible artifacts, especially with nonlinear (Akima) interpolation. Its fitting in data nodes is not obvious.

Minimum curvature gridding method creates a minimum curvature surface to the random data points. It is 2D method, it uses all the input data. This surface is the smoothest surface that fits the given data values. The method is linear. Its fitting in data nodes can be regulated.

I would use minimum curvature method in any regular situation.

You can find much more information (for much more time) searching documents at http://www.geosoft.com.

Thank you for you reply Eugene.

I found your point valid. The use of all the input data in minimum curvature seems more logic than the bi-directional technique.

However, Geosoft indeed recommends the use of bi-directional for not only roughly parallel lines but for "true" parallel ones. ( p.3).

It was told that bigrid tends to strengthen trends perpendicular to the direction of the survey lines.

Maybe because of the poor frequency of sampling in gamma-ray spectrometry data minimum curvature should be applied in my example.

Do you think that bi-directional interpolation could lead to much more artefacts than minimum curvature?

I do think that bi-directional interpolation could lead to much more artefacts than minimum curvature: as a method to build grids it is 1D procedure, i.e. anisotropic, therefore some trends along or across lines are quite possible. Note, this anisotropy is a naive interpretaion of 2D geologic reasons to make surveys with parallel lines.

The minimum curvature also has some lacks, but not so obvious. If you have some idea about quality of the results then just play with methods and their parameters. E.g., in minimum curvature play with % of points, cell size, tension, power, etc.

This is a issue about bi-directional interpolation that I did not think about.

Also playing with the parameters is one thing that I should do in order to get a better result.

Thank you again for you reply. I am just a beginner student in the interpolation technique. Your point of view was very helpful indeed.

Regards,