Manufacturers Data Sheet
Safety Information Sheet 2019 08 31.pdf
Information about usage and storage replacing the electrodes
Steel rods can be used for short recordings around 1-2 hours.
The deliver data good down to 10 Hz. Maybe more.
In dry areas you notify that by hammering the rods into the ground, the hole widens and the rod looses contact to the ground - and your measurment is useless!
Use the EFP-06. Use bentonite in case of dry ground. Never use salt! That destroys the chemistry of the electrodes (despite many people say “use salt” … don’t use salt!).
Hence that above 100 Hz you’ll see some data - but that is data from the E-filed line and not from the electrodes.
In case you get 100 mV offset: check the electrodes. replace electrodes