Application Of Surface Geophysical Methods To Delineate Fracture Zones Associated With Photolinear Features In West-Central Florida

This study was designed to investigate the geophysical characteristics of fracture zones associated with<br>photolinear features in West-central Florida and to determine the most effective geophysical profiling methods for<br>rapid field characterization. Multiple surface-geophysical techniques were employed to verify the geologic<br>significance of two photolinear features at the study site. Interpretation of the geophysical data was constrained by<br>using multiple geophysical methods, site lithologic data and information obtained in previous studies of photolinear<br>features in West-central Florida.<br>Closely spaced horizontal-loop electromagnetic (HLEM) and very low frequency electromagnetic (VLF)<br>tilt-angle data were used to locate geophysical anomalies associated with the fracture zone. Resistivity profiling<br>data correlate well with the results of the HLEM and VLF methods. The optimum-offset, seismic-reflection<br>technique accurately delineates the large-scale fracture zone; however resolution is insufficient for detailed<br>stratigraphic interpretation. Self-potential data show a negative-streaming potential associated with the center of the<br>fracture zone. Microgravity data along a section of the fracture zone indicate lateral variations in density that<br>correlate well with anomalies identified with the other geophysical methods. Future studies of photolinears in<br>West-central Florida should use high-density VLF and HLEM surveys as rapid reconnaissance techniques to locate<br>anomalies associated with photolinears. High-resolution, shallow, seismic reflection, resistivity, SP and gravity<br>methods can be used to provide additional correlation.<br>Geophysical and lithologic data from the study site are incorporated into a geologic model of the fracture<br>zone. Data suggest that the two photolinears are related to a large-scale fracture zone in the Floridan Aquifer. The<br>fracture zone appears to be greater than 700 m wide with a 100 m wide sand-filled bedrock low. The bedrock low is<br>flanked by zones of higher bulk density approximately 200-400 m wide. The zones of higher bulk density are<br>interpreted as zones of recrystallized limestone caused by the precipitation of calcite along the fracture zone. The<br>recrystallized limestone zones appear to host multiple limestone pinnacles and clay-tilled fractures. The observed<br>photolinears correspond to a sand-filled bedrock low and a zone of dense, recrystallized limestone adjacent to the<br>bedrock low.