The soil is probably acidic and reacting with your probes, and generating a small E.M.F. You could remeasure, using probes made of a less reactive metal. {I have some spare gold probes I can give away to the first requester}. (not really!).

 Thanks Dr. Kennedy for your answer.I appreciate it. 

You could use a porous ceramic pot with an internal copper electrode in a copper sulfate solution in it for electrodes. This can avoid the metallic probes reacting with acidic soils. Google copper-copper sulfate electrode for sources to purchase these.

Thank you Dr. Campbell for answering , it looks like a reasonable method to avoid soil skin effect , have you tried this during survey? i would like to know if you don't mind.

I appreciate it.

Wadhah

Yes, we used to use these for IP and AMT surveys in mineral exploration, with great success.