kriging

Hi all,
I have a data set that I would like to krige to interpolate between 
transects. There is a non-linear trend between two of the variables...my 
impression from reading the gstat help file is that there must be a 
linear relationship between the data to use universal kriging?
Second, would a method of non-linear regression followed by modelling of 
the residuals with a semivariogram be an appropriate solution?

Thanks,

Dave