An EC log showing EC soil log (left) with Rate of Push log (right). The first 30 ft. is primarily fine grained soils with some slightly coarser seams that transition into coarse grained material below 30 ft.
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, in general, silts and clays exhibit higher electrical conductivity readings than sands and gravels (Fig. 1). As with any of the Direct Image® logging tools, the collection of a few confirmation soil samples, either from discrete depths or a continuous core, should 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.
Soil conductivity logging continues to increase in usage because conductivity logging can be efficiently performed with highly mobile and cost-effective percussion probing equipment.
The EC probes come in two different configurations, Dipole Array and Wenner Array, with the same theory of operation. A current is sent through the formation between two probe contacts. This current is measured along with the voltage that results (Figure 2). The conductivity is a ratio of current to voltage times a constant. The resulting reading is in milli-Siemens per meter (mS/m).
Soil conductivity, in general, varies with grain size. Finer grained soils, such as silts or clays, tend to produce higher EC signals than coarser grained sand and gravels. Figure 3 shows that while specific values cannot be assigned to each soil type, each soil type should provide a different response on a specific site. The coarser grained sediments will allow the migration of contaminants and the finer grained sediments will trap and store contaminants.
Fig. 3 : Generalized graph of potential soil EC values. Soil EC values are also influenced by the ionic strength of the soil pore water. Brine contaminated soils may exhibit EC values higher than normal soil ranges.
The EC gives the investigator real-time, on-screen logs allowing onsite decisions (Fig. 4).
Fig. 4 : The EC Log shows changes in subsurface lithology as the EC probe is driven into the soil with the Geoprobe® direct push rig. What the probe encounters is seen in real time on the field laptop at the surface.
Fig. 5 : This graph shows a series of EC logs from across a site created in the DI Viewer software. Logs are expected to be different as lithology features transition as the investigator moves across a site.
Observing general log reproducibility is a common way to quality control (QC) the log data.
EC is frequently used to map the contaminant pathways of a site. Zones of lower conductivity indicate coarser grained, more permeable materials.
It is these permeable zones which allow contaminants (hydrocarbons, chlorinated VOCs, or metals) to be transported in the subsurface.
Using a cross sectional view of a series of logs is very helpful in visualizing how these preferential pathways connect across a site.
The 4 EC logs shown in Figure 5 indicate primarily finer grained soils near the surface with coarser grained zones at 10’-11’ and 16’- 24’. Finer grained soils transition into coarse grained soils at depths ranging from 33’-45’ across the site. 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.
EC Combined With Other Logging Techniques :
Electrical conductivity has been combined with the following logging tools:
- MIP : The MIP maps the position and relative concentration of VOC contaminants in the subsurface while EC maps the lithology. Knowing information about the lithology where contaminants are present is helpful.
- HPT : The HPT provides a measurement of the injection pressure of a set flow of water into the formation. EC and HPT are very useful companion measurements which often mirror one another such as: courser grained materials – results in low injection pressure – means zone of higher permeability. Often they will diverge from one another which helps provide us with more information to better understand if there are ionic influences or some other factor influencing permeability.
- MiHpt : The MiHpt is a combined MIP-HPT system with an integrated EC dipole.
- CPT : This combined tool string incorporates a CPT onto a MIP-EC sub.
- HPT-GWS : This unique tool allows us to perform HPT and EC logging and then stop and collect groundwater samples at appropriate depths in the same push.