It is self potential measurements (a very old school geophysical method):
2 electrodes in the soil with a certain spacing can mesure a potential in millivolts created by the soil itself.
It is often a very small voltage from tenth of mV to hundreds of mV rarely reaching 1 volt.
Measuring the resistance between these 2 electrodes gives also at least an acurate information about your contact between soil/electrode.
On a volcanic medium it can be kOhms to hundreds of kOhms.
This processing is often done with a standard multimeter with no problem (even the ones very cheap).
I tested 2 configurations :
– DC measurement with the main V channel, and then jumped to the Ohm channel with a switch
– DC measurement with the Ohm channel on auxiliary mode, then resistance measurement with the same channel.
You have to notice that once we have choosen the place of the negative electrode, the measured values in mV can be positive or negative according to ions migrations in the soil, this is normal and in fact this is one of the targets to get these kind of variations.

But why does the Ohmmeter give negative values, that is the problem…
and these negative resistance values always occur when we have some negative mV readings, but they are not all the time negative.
With negative mV readings we can have positive resistance readings but they are non sense according to comparative measurements with standard voltmeter.
So to summarize :
– negative millivolts can give negative ohms, but not at 100%
– in some cases of negatives mV, some resistance values look bullshit even when these resistances are positive
– in some other cases the “absolute value” of negative resistance look good!

thanks for helping.