Electromagnetic Induction-Based Regional Subsurface Characterization of Mountainous Environments: Examples from Puerto Rico

The physical and mechanical characterization of the subsurface under hillslopes is critical for understanding the distribution of mass wasting by landslides and debris flows, which are two significant processes that drive topographic change in steep environments. Hillslope environments are notoriously spatially complex. However, subsurface characterization is rarely performed on regional scales due to challenges in characterizing highly localized variations. Common methods for subsurface characterization involve costly site-specific sampling and testing leading to point measurements or regional remote sensing techniques. Currently, a gap in scale compatibility exists between site-specific analysis where characterization of the subsurface is robust but spatially limited and regional hazard analyses where characterization is limited to regionally broad categories based on lithology, slope, and/or vegetation. This research explores electromagnetic induction (EM) as a scalable approach to characterize relative differences in physico-mechanical properties of the subsurface in steep terrain where mass wasting is common. For this study, EM data was collected in different geologic units across southwest Puerto Rico (PR), then converted to apparent resistivity values. The apparent resistivity of lithologic groups and limestone subunits were statistically compared leveraging the large datasets collected. Two study areas are selected to examine spatial trends and the effects of topographic factors. The results indicate that EM can detect differences between and within geologic units/subunits and help estimate subsurface heterogeneity where detailed mapping is unavailable.