Variability of Karstic Limestone Quantified Through Compressional Wave Velocity Measurements

Within the geotechnical and geological communities it is common knowledge that karstic limestone is highly variable however this variability is often not well defined or quantified. To assess the variability in karstic Ocala Limestone, an investigation was conducted at the shared University of North Florida and University of Florida karstic limestone geophysical/ground truthing test site in central Florida. The variability was assessed via delineating the limestone bedrock surface and through laboratory measurements made on core specimens taken from the site. The bedrock surface was identified using surface geological mapping, intrusive and non-intrusive (seismic refraction) techniques. The core specimens were characterized using compression wave (p-wave) velocity measurements. Analyses of the field data revealed a highly erratic limestone bedrock surface; the bedrock surface decreased in elevation from ground level to nine meters in depth within a horizontal distance of 24 meters. Additionally, seismic refraction tomography data revealed marked differences in the compression wave velocities at the top of the bedrock surface at various locations along one of the survey lines; wave velocities were highest within slots or valleys and lowest at the tops of blocks or pinnacles. Laboratory results also showed high variability; there were no clear velocity trends with depth within a single bore hole or at constant depths across the survey line. However, the geological history of the limestone provides explanations for the variability for both the limestone bedrock surface and compression wave velocity measurements.