Why am I getting negative resistivity values?
There are many possible causes of negative readings, the majority of which are not a result of instrument malfunction. The most common issues are poor electrode contact, inappropriate power settings, battery capacity or dirty connections and/or damaged cables.
Poor electrode contact
If the contact resistance at the electrodes is too high, it can cause instability in the measurement. Use the electrode test to identify electrodes with poor contact resistances compared with the others. This problem can be reduced by:
- Hammering electrodes deeper (using longer electrodes if necessary).
- Using more than one electrode at each cable take-out and connecting them together ‘in parallel’ (with cable jumpers).
- Using some kind of conductive material to improve the contact between the electrode and the ground. This can be pouring water/salt water/electrolyte gel around the probes.
- Use conductive clay such as bentonite around the electrodes to increase the effective surface area.
- Pushing the electrodes through water-soaked sponges to slowly release water into the ground during measurements.
- Use an alternative electrode design such as plate electrodes.
Sometimes, if the maximum transmitter settings on the Terrameter are too low, the system will struggle to get a reliable ‘signal’ into the ground: poor signal in equates to poor signal out. Try to increase the maximum output current, voltage or power setting. Conversely, in conductive ground, using settings that are too high will rapidly drain the external battery leading to similar problems as seen when using a poor-quality battery (below). Monitoring the quality of the results in real time, by watching the results page, will help get the right balance of settings.
A very common problem is that the battery does not have enough power to maintain a steady measurement, either due to its design or capacity depletion through age. If you are using an external 12V battery, ensure that it is well charged and in good condition. If possible, use the instrument status or a multimeter to test how the battery voltage changes during a measurement. A poor condition battery can show over 12V when there is no load on it, but as soon as you try to take a measurement the voltage in the battery collapses. If the battery voltage drops below 12V it is possible to start seeing lower quality readings and, with very low battery voltages, negative values can begin to appear.
Dirty connectors and/or damaged cables
Ensure that there is no dirt in any of the connectors on either the cables or the Terrameter. If so, clean with a toothbrush and a residue-free contact cleaner or compressed air. Check that the cables are all in good condition with no bad grazes, compressions or kinks in them. Utilize the onboard cable tests to ensure the integrity of all connectors and cables.
A final source of bad readings is “environmental” and out of your control – if there are metal fences in contact with the ground very close to the survey line, or metal pipes/cables in the ground where you are measuring, these can also return negative values.
More to read