gprMax has been really helpful in modeling and simulating various structures for my application. To create a more realistic model, I need to add surface roughness. The commands have been provided in the manual, namely, fractal_box and add_surface_roughness, whose syntaxes are respectively:

#fractal_box: f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 i1 str1 str2 [i2] [c1]

#add_surface_roughness: f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 str1 [i1]

The parameters f10 and f11 provide the maximum and minimum limits of the roughness. However, I do not find any other parameters that control the roughness (such as mean, std.deviation of roughness).

What parameters from the command control the Mean and Std. deviation of surface roughness? And how are they controlled?