Electrical Conductance Probe Profiling

USGS scientists utilized electrical conductance (EC) probe profiling on August 22-23, 2005, in the sediments underlying Salt Pond and adjacent areas at CCNS (Figures 1, 3, and 6). Use of the EC probe was specifically intended to provide ground-truth for the results of 2004 CRP surveys in the area and to guide subsequent submarine ground-water sampling beneath Salt Pond and Nauset Marsh.

Figure 6: Oblique aerial view of Salt Pond and Nauset Marsh (Salt Pond Bay) facing toward the southeast.

Saline and fresh waters were detected at varying depths at multiple locations beneath the saltwater body by using a modified version of Geoprobe Systems' Direct Image Electrical Conductivity tool (Geoprobe Systems, Salina, Kansas). The EC system is typically used to measure soil conductivity (or inversely, resistivity) to classify soils and geologic formations. The system measures the bulk electrical conductance of the formation (soil and pore water) surrounding the probe. Pore-fluid conductivity, lithology, and porosity affect the electrical conductance of the sediments (Archie, 1942). In general, fine-grained sediments, such as clays, have higher conductivity signals than silty sediments; in turn, silty sediments have higher conductivity signals than coarse sediments (sand and gravel). This pattern is similar to that in borehole resistivity logs.

During typical terrestrial logging, the electrical conductance of the fresh ground water is very low (less than 10 mS/m), and the soil characteristics primarily determine bulk conductance. In settings where brackish or saline pore fluids are present, however, the relative changes in conductance are largely determined by the salinity of the fluids rather than by changes in lithology (Manheim et al., 2004). The EC tool is therefore useful in the estuarine environment for determining the presence of fresh or brackish ground water, because the difference in bulk electrical conductance between fresh pore water (2-150 mS/m) and saline pore water (500-3500 mS/m) is so great (McCobb and LeBlanc, 2002).

The EC method involves attaching a Wenner-array probe (Geoprobe SC200) to 25-mm-diameter probe rods. The probe is connected to the surface (a floating barge in this case, Figure 7) by a water-tight, shielded cable strung inside of the complete length of probe rods (Figure 8). The Wenner-array probe consists of four contact rings; one pair of rings carries a known current, and another pair measures the resulting voltage through the formation. The conductivity is the ratio of current to voltage multiplied by a constant.

Figure 7: The USGS ATV-mounted drilling rig.

Figure 8: Geoprobe System's Direct Image Electrical Conductivity equipment.

At five locations (sites 1-5) in Salt Pond and adjacent areas (Figure 1 and Figure 3), the probe was placed on the bottom (typically, the top of the soft sediment), and an initial reading was recorded (surface water ~4000 mS/m). The probe and probe rod were advanced by hand (when possible) or by automatic vibratory hammer at 30-cm intervals. After advancing the probe to each successive depth, a measurement was made by moving the attached stringpot cable. When the cable was moved, the measurement was triggered and recorded on the field computer (Geoprobe FC4000, Figure 9). After multiple measurements had been made at each interval, an average EC (electrical conductance) value (in mS/m) was recorded. Changes in the ease of probe advancement were also noted by interval. Depth of water, start and end time, and sediment/drive descriptions were also noted. The profile location was recorded by using a global positioning system (GPS) device. Profiles of EC probe data are shown in Figure 10, and data are compiled in Table 5. The complete Excel spreadsheet, including the summary data presented in Table 5 as well as the individual logs from each geoprobe, is available from the Data Catalog page. Individual probe logs and results are also available from the following PDF files: SP2005-1, SP2005-2, SP2005-3, SP2005-4, SP2005-5

Results of the EC probe profiling are generally consistent with surface-towed CRP survey results and submarine ground-water sampling and pore-water extraction results, which showed shallow fresher ground water beneath the tidal channel and northeastern Nauset Marsh (Salt Pond Bay) and shallow brackish groundwater beneath Salt Pond itself. The technique was especially useful for quickly constraining relative salinities of pore water in fine-grained units that were difficult to sample by using piezometer techniques because of low fluid yields.

Figure 9: Operating the EC field computer at Salt Pond, Cape Cod, MA.

Figure 10: Electrical conductance profiles from each of the probe locations indicated in Figure 3.