First go over Appendix F, Periodic Tests


Moisture or dirt between terminals will degrade the accuracy of the reading.


If there is a rattle inside the case: remove the instrument from the case and determine what is rattling. Any metal object that is loose within the case poses a potential for electrical damage to the instrument.


If there is moisture inside the case: water left inside the case will corrode and destroy the instrument. Remove the instrument from the case and dry thoroughly.


There is a fuse inside the instrument. To date, one has never blown. If the instrument is “dead”, remove the fuse and examine for a broken or missing filament.


If there is poor contact between the potential electrodes and the earth, the potential cables tend to act as antennas and the readings can be noisy. Appendix Q gives an insight into problems with electrodes in dry ground.


If reels are used for the transmitter cables, see Appendix E if:

·        There are wandering readings

·        There are unusually low readings

·        Readings that seem stable for a time then change erratically

·        Readings that change drastically depending on battery voltage

·        Readings that change drastically when cable on the transmitter spool is retracted or extended



Anytime that negative resistance readings appear every bell and whistle should go off. Although negative resistance readings are theoretically possible in geology of extreme resistivity contrasts and extremely irregular geological geometry, they are very RARE.

More than likely there is an error in measurement due to:

1) Incorrect polarity of transmitter or receiver cables. This is the most likely cause.

2)  Using cables which allow excessive cross-talk or cross-leakage between the transmitter and receiver cables.

3)  A malfunction in the instrument. In the strange case of negative resistance this should be examined by:


a)     Assuring that the measured value of the calibration resistor is proper. The standard LRI calibration resistor harness that comes with each new MiniRes has a value of 19.00 ohms. It is marked on the resistor cover. 


b)     Assuring a good zero value with shorted receiver inputs. The shorted inputs test is described in Appendix F, Periodic Tests. Clamping a simple paper clip between the receiver terminals and pressing the red reading button should give a resistivity of zero.

c)     Exchanging receiver and transmitter cable/electrodes (using the reciprocity theorem) to see that the same value of resistance is obtained.


4)     An incomplete  or open circuit (may be intermittently open)  in  one or  both of  the  receiver wires or receiver  wire-to-electrode connections. This error  will clearly show up when doing the  reciprocity  check  (in 3c above)  since  the  receiver  open  circuit  will  be put  into  the transmitter side and then the LINE OPEN LED will illuminate brightly. The LINE OPEN LED should always be OFF during normal or reciprocal array arrangements.

5)     Using  transmitter cable spools with  many  turns of wire on  them  can also cause  negative resistance readings. See the last section of Appendix E in the manual  for a description and

solutions to this problem.

If any negative resistance readings show up again OR if the operator does not see a monotonic decrease in resistance readings with increasing "a" spacing THEN the operator should go through all of the checks listed above in order to locate and define the problem(s).
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