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: |
