Question
Is it possible to model the frequency dependence of the soil parameters when doing a transient study with HIFREQ and FFTSES?

HIFREQ does not directly take the frequency-dependence of the soil parameters (resistivity and permittivity) into account in its computations. The model for a layer of soil in HIFREQ allows you to specify only a real part for the resistivity and the permittivity.

This is not a big limitation in itself, since an imaginary part for the permittivity is equivalent to a linear frequency dependence of the resistivity. The problem is really how to specify a frequency dependence for the resistivity and the permittivity of the soil.

You can do this by creating one HIFREQ file per frequency, defining the characteristics of the soil based on the real and imaginary parts of the complex conductivity for that frequency. You can use any model to compute the complex conductivity, for instance the models discussed by C. M. Portela et al.

The AutoTransient program does that automatically for you. It allows you to specify the resistivity and permittivity as a function of frequency by filling a table (use Project | Edit Soil Model). Then, the program interpolates the resistivity and permittivity data, and generates all necessary HIFREQ files to carry out the transient study.