Photolinears, Fractures, and Fallacies: A Post Hoc Study of Photolineaments, Hillsborough County, Florida

There is a misconception by some in the geologic and non-geologic communities of Florida that photolineaments and vertical bedrock fractures are one and the same. The main objectives of this paper are (1) to document a case study where a comprehensive geophysical and geotechnical exploration program was undertaken to verify a high-quality photolinear analysis; and, based on the case study, (2) evaluate the validity of photolinears as indicators of vertical bedrock fractures in the covered karst terrain of west-central Florida. The case study, an investigation by Upchurch et al. (1999), was an analysis of photolineaments at a 445-ha site intended for construction of an above-grade reservoir in west-central Florida. The photolineaments were ground truthed using ground penetrating radar (GPR), refraction and reflection seismic profiling, standard penetration testing (SPT), and cone penetrometer testing (CPT; Dobecki and Upchurch 2010). The post hoc review, based on the comprehensive site geophysical and geotechnical testing and resulting data, determined that fifty-eight percent of the photolinears do not correspond to vertical fractures in the limestone bedrock. This review demonstrates the fallacies of assuming all photolinears represent vertical bedrock fractures in the covered karst terrain of west-central Florida. Based on this case study and the post hoc review, it is our belief that in the covered karst terrains of Florida, all photolinear evaluations should have some form of field verification in order to equate them to vertical bedrock fractures or karst features.