You cannot take the log of a negative number or 0. The number must be positive and non-zero. This is not Surfer's rule, but a mathematical one. The log of (X) becomes asymptotic as X approaches 0.
If you are gridding data that contains negative or 0 values, then these data points are ignored if you are gridding with the Z Transform set to either Log, save as log or Log, save as linear.
Updated January 26, 2017