Resistivity & IP
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Electrical resistivity and induced polarization (IP) are techniques which have blossomed in recent years because of advances in equipment. Computers have allowed for rapid automated data collection and analysis making this a very cost-effective detailed technique.

The electrical resistivity method uses a series of electrodes nailed into the ground about six inches deep along a selected straight profile or 3D grid. The distance between the electrodes depends on the desired depth of investigation and the target being imaged.  The farther apart the electrodes, the “deeper” the electricity goes into the earth, at the expense of resolution.  The most commonly used configurations activate four electrodes at a time, two for passing current into the ground and two for measuring the potential difference.


 

2D profiling and 3D imaging produce modeled cross-sections of the earth's resistivity structure below the survey. The depth of investigation depends on many factors but typically ranges from a few meters to over a hundred meters.

This example from a groundwater mapping project in southwest Montana shows three different electrode configurations collected at the same location.  The Schlumberger, Dipole-Dipole, and Pole-Dipole all show similar results at this uniform site though each configuration has advantages and disadvantages depending on the physical properties of the host environment and the desired target.  Cultural interference like power lines and fences, or project requirements like a required depth of investigation, can influence which configuration will work best.

The measured apparent resistivity values plotted in the top panel represent the actual Schlumberger data from this project.  The total length of this 44-electrode profile was 215 meters and the total depth of investigation ranged from 40 to 70 meters depending on the electrode configuration.

Measured ranges of resistivity for some typical materials are:

Igneous
 Altered granite
 Limestone
 Sandstone
 Dry Gravel
 Alluvium and Sand
 Clays
 Soil
 Fresh Water
 Copper (native)
100 – 1,000,000 ohm-m
1-100 ohm-m
10 – 10,000 ohm-m
10 – 1,000 ohm-m
600 – 10,000 ohm-m
10 – 800 ohm-m
10 – 100 ohm-m
1 – 10 ohm-m
3 – 100 ohm-m
0.0000002 ohm-m

Induced polarization data is collected by measuring the voltage at specified time intervals after the current is turned off.  Some materials, clay and metals for example, take time to dissipate the electric charge induced by the applied current.  If the ground is “chargable” then there will be a decay in the signal which is measured after the current is turned off.  This can be very useful for telling the difference between a localized perched aquifer (not chargable) and a buried refrigerator graveyard (chargable).

With the proper instrumentation and techniques, resistivity and IP data can be collected at the same time.  The target of interest would dictate if this useful addition to the electrical survey is worth the added expense.