The buffered cables have an integrated amplifier (“line driver”).
At higher frequencies (e.g. 10 kHz and above) and higher resistivities (1 kOhm and above) you get a phase shift from the electrode-cable input.
In order to avoid that you will use buffer cables (length 25m (@64kHz target f) or 5m (@524 target f) ). For the SHFT sensor this is a must. For the MFS-07e you also can reach this necessity.
As electrodes you should use a steel rod.