Energy Diversification Sub Committee

Near Surface Geophysical Mapping to Support Solar Power Development

Kate McKinley, PG, PGp

Kate McKinley, PG, PGp
THG Geophysics, Ltd.

Thursday, April 21st, 2022 – 12:00 PM MST


With the recent increase in utility-scale solar projects, engineers are faced with the challenge of determining the appropriate foundation design in geographic areas that are dominated by varying shallow subsurface conditions.  The shallow problems encountered are similar to conventional construction sites: the challenge of mapping subsurface utilities, the presence of inconsistent depth to bedrock surfaces or pinnacled rock, and the determination of cut and fill volumes.  Traditional geotechnical testing along with geophysics can adequately characterize all of these concerns on a project site, but the size of these projects make point measurements and traditional geophysical mapping insufficient to characterize the variability encountered across these vast footprints.  As a result, THG Geophysics applies unconventional approaches to help characterize these sites.

Construction of solar projects in areas with very shallow bedrock poses challenges to design engineers.  Unpredictable karst conditions may be missed by limited geotechnical investigations.  Additionally, there is a significant financial benefit in determining the depth to bedrock prior to finalizing foundation and pile installation designs.  Terrain conductivity mapping (TCM) with multiple-dipole tools provides site-wide views of the shallow subsurface.  Alone, these data provide limited or regional interpretations.  However, when inverted and paired with geotechnical boring data, this method results in depth to bedrock maps and karst hazard delineation.

The solar projects are designed to have minimal impact and can be designed around existing infrastructure such as utilities and drain tiles, but mapping these un-inventoried structures in agricultural settings can be extremely challenging.  These types of utilities are often nonconductive and ground penetrating radar (GPR) is required to locate them.  Unfortunately, conventional 2-D GPR is not an efficient method to map large sites.  Even though GPR can be towed and interpreted in real time, discrimination of point targets from linear targets is not possible in 2-D data paths.  Post-processing data from a towed 3-D GPR array results in plan-view visualizations that allow for discrimination between linear targets and point features without sacrificing acquisition speed.

TCM and 3-D GPR are  fast and efficient methods of assessing large areas, but they require data processing and quantification of interpretation errors.  However, the results are valued for efficiency,  land coverage, and data resolution.


Kate McKinley is a Senior Geophysicist and Vice President at THG Geophysics, a full service geophysical firm dedicated to mapping the shallow subsurface and to providing utility location services.  With 20 years of experience in geophysics and business development, Kate still enjoys the everyday adventure of applying the principles of physics to imaging what lies below the surface. She has, gratefully, had the opportunity to provide these services to diverse projects in engineering, environmental, and archaeological disciplines. 

Kate enjoys sharing her experiences through teaching, helping to develop her trade through her activity in several professional organizations and helping her community by participating in several local, civic organizations.

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