Soil conductivity
and resistivity (the inverse of conductivity) have long been used
as tools to classify soils. The power of this tool stems from the
fact that higher electrical conductivities are representative of
finer grained sediments, such as silts or clays, while sands and
gravels are characterized by distinctly lower EC’s. A few
site specific core samples, either from discrete depths or a continuous
core, can be used to verify the lithology represented by EC values
at a site. The electrical logs are then correlated across the site
to show changes in thickness or elevation of lithologic units of
interest. Soil conductivity logging continues to increase in usage
because it can be efficiently performed with the highly mobile and
cost-effective percussion probing equipment.
EC logs are used to define zones of lower conductivity, equivalent
to coarser grained, more permeable sediments, which will allow the
movement of contaminants (hydrocarbons, chlorinated VOCs, or metals)
in the subsurface. The lithologic information gathered with the
Conductivity System can be used to aid the investigator in understanding
the movement and location of contaminants in the subsurface. This
information will also assist in the proper placement of monitoring
or extraction wells.
The EC theory of operation
A current is sent through the formation between two probe
contacts. This current is measured along with the voltage
that results. The conductivity is a ratio of current to voltage
times a constant. The resulting reading is in milli-Siemens
per meter (mS/m). The conductivity of soils is different for
each type of media. Finer grained sediments, such as silts
or clays, will have a higher EC signal. While coarser grained
sediments, sands and gravel, will have a lower EC signal.
The coarser grained sediments will allow the migration of
contaminants and the finer grained sediments will trap the
contaminant. The EC system is a powerful tool when used properly.
The EC gives the investigator real-time, on-screen logs allowing
onsite decisions. |
The features and benefits of the EC
logging system are:
- Measures soil conductivity and probing speed simultaneously
- Greatly reduces soil core required to characterize
lithology
- Real-time, on-screen log during probing
- Cost-effective alternative to borehole logging or cone
penetrometers
- Expendable dipole allows retraction grouting to seal
logging holes
- Designed for percussion probing
- Spreadsheet compatible data storage
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