Geologic Mapping of Sand Unit Thickness
Pyramid was contracted to perform an electrical resistivity survey across 230 acres of undeveloped land in North Carolina. The purpose of the survey was to quantify the lateral extents and approximate depth/thicknesses of sandy soils near the ground surface. The client was interested in the potential of the site to be utilized as a sand mine, and wanted to understand the potential yield of sandy soils at the property.
Prior to the geophysical survey, shallow soil borings were performed to obtain physical soil samples across the property that would provide groundtruth data for the geophysics. The soil borings indicated that sandy soils ( a mix of sand, silty sands and clayey sands) that could potentially be used for mining purposes started at the ground surface, and were present at thicknesses ranging from approximately 7 to 30 feet. Having physical soil samples with which to groundtruth geophysical data is extremely important, and these recent soil borings provided a good baseline for geophysical analyses.
Pyramid performed a total of 12 electrical resistivity transects across the property. Although this was a relatively small number of transects for such a large area, the data provided a good generalized understanding of the geologic conditions across the 230 acres. The terrain was typically medium to dense forested areas. The benefit of resistivity surveys relative to other methods is that the test can be set up in areas that are largely inaccessible to other instruments that need to be dragged or pushed continuously, such as electromagnetics or ground penetrating radar.
The resistivity testing provided geologic data down to depths of approximately 110 feet. The results of the geophysical testing indicated four main geologic strata that characterized the subsurface across the property:
- Stratum 1 – High resistivity (2000+ Ohm-m) dry sandy soils with varying percentages of fines in the upper ~2 to ~50 feet.
- Stratum 2 – Mid-resistivity (500-1000 Ohm-m) dry clayey soils with variable sand content directly underlying the sandy soils
- Stratum 3 – Low resistivity (Stratum 4 – High resistivity materials (sand or rock) below the water table in the lower portion of most profiles.
Stratum 1 was considered to be the soils that could potentially be used for mining purposes. The resistivity data were analyzed, and depths/thicknesses of viable sandy soils were extracted from each transect to generate contour maps of the sandy soils across the property. The contour maps helped to show both the average thickness of the sandy soil unit, as well as specific locations that may provide increased or decreased volumes of sand.
Contour Map of Sand Thickness
The application of electrical resistivity to this project allowed for a relatively detailed understanding of the geologic conditions at the property that was obtained in a short amount of time. The alternative would be to perform a significant number of additional soil borings, which would have resulted in exponentially higher costs to the client and additional time to realize the results. Furthermore, the continuous profiles of geology that are obtained using the geophysical methods eliminate gaps in the data that would otherwise be present if the boring program was the only approach taken. The project shows the benefit of using geophysics, in conjunction with physical soil samples (groundtruth data) to effectively characterize a project site in a short amount if time.