If you use the Kriging gridding method, specifying a nugget effect would probably smooth things out. The nugget effect is made up of two components: Nugget Effect = Error variance + Micro Variance. To add a nugget effect, click the Advanced Options button in the Grid Data dialog. On the General page, click the Add button. In the Variogram Components dialog, select Nugget Effect, enter the Error Variance and Micro Variance, and click OK and OK. When testing the nugget effect, set the micro variance to zero and try some different values for the error variance to judge the effect.
If that doesn't help, you could try:
- A different gridding method (ie. Minimum Curvature or Triangulation with Linear Interpolation).
- Modifying the search radius under Advanced Options.
- Including "dummy data" in your original data file to help Surfer determine contour locations.
- Filtering the grid file (Grids | Edit | Filter). You may want to use a larger "neighborhood" in the filter than the typical 3x3 used in most pre-defined filters. For example, you could try a Gaussian low-pass filter with a 9x9 neighborhood and run it for 3 passes. This may certainly smooth things out, but you may not like the results. You may need to experiment with some different settings to see what you like.
- Editing the grid in the Grid Editor by clicking File | Open, selecting the grid and clicking Open. Edit the Z values of the grid nodes themselves using the Select tool or use the Push up or Pull down tools until the bull's eye effect is removed. Then save the grid by clicking File | Save (or Save As to save it to a new name) and click the X on the view tab to close it. Then update the map with the File | Reload Map Data command.
Updated September 13, 2017