 Interpretation of EC logs comes with field experience and an initial core sample to confirm lithologic changes. Each EC log is unique per site. As a generalization a high conductivity reading indicates a smaller particle size and a low conductivity reading indicates a larger particle size.   The two direct push electric logs to the right were obtained with an SC400 Wenner array probe in the Midwestern United States on the same site. In these two examples we observed a high conductivity to a depth of 8 or 9 feet indicating a silt-clay soil is present. Below the soil horizons the conductivity drops abruptly to a range of 5 to 10 milliSiemens per meter (mS/m) indicating a sharp contact between the relatively fine grained soil and dry sands. On both logs at a depth of about 15 feet the conductivity increases to a range between 20 to 40 mS/m. This increase correlates with the static water level observed in local wells and indeed indicates that the sandy formation becomes saturated at this depth. Having a coarser grained/relatively clean sandy formation allows us the detect the abrupt transition from the vadose to saturated zone in these logs. In finer grained materials the water level would be masked because the transition from the unsaturated to the saturated conditions is brad due to the capillary effect, and not abrupt as in these sandy materials shown. Also observed on these logs are the various abrupt increases and decreases (spikes) between 35 and 70 feet. These spikes indicate the presence of fine grained lenses or layers which are separated by zones of lower conductivity, sand/gravel. Between these two logs most of the clay layers do not correlate. That is the clay layers are limited in lateral extent. Additional logs would be required to determine if any of these lenses would be consistent throughout the site to form an extensive aquitard. It is also interesting to note that below this clay layer (45ft) the average speed of probe advancement drops from about 20+ft/min to between 5 to 10 ft/min on both logs. This indicates that the sediments below this depth (45ft) have a higher density. Also on both speed graphs at about 70 feet the speed of advancement decreases to near zero. This indicates refusal. At the same time of obtaining refusal on the speed graph the conductivity abruptly increases. This decrease in speed and increase in conductivity matches the depth where samples show contact with the shale bedrock. So, both the speed graph and the conductivity graph work to verify contact with bedrock at this site. |