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