What is Conductivity?
Conductivity is the measurement of how well a
substance or formatin conducts an electric current. The reciprocal
of conductivity is resistivity, which is the measurement of resistance
to a current. Hydrocarbons, the rock, and freshwater all act as
insulators and are, therefore, non-conductive and highly resistive to
electric flow. Saltwater, however, is a conductor and has a low
resistivity.
Why Use the Electrical Conductivity Tool?
Soil conductivity and resistivity 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 electrical conductivities. A
few site specific core samples, either from discrete depths or a
continuous core, can be used to verify the lithology represented by
electrical conductivity 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.
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
VOC's, or metals) in the subsurface. The lithologic information
gathered with the conductivity tool 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.
