A HIGH RESOLUTION QUASI-3D GROUND PENETRATION RADAR SURVEY TO DETECT BURIED CAVITIES WITHIN THE KARST MIDDLE TERTIARY FORMATIONS OF NORTHERN PUERTO RICO

Near-surface cavities can pose serious hazards to human safety, especially in highly urbanized town centers. The location of subsurface voids, the estimation of their size and the evaluation of the overburden thickness are necessary to assess the risk of collapse. In this study, a previously acquired Quasi-3D ground penetration radar survey was processed for cavity location in the Karst Middle Tertiary Formations of Northern Puerto Rico. The urban study site is characterized by numerous cavities and is overlaid by a deformed concrete-slab of an abandoned basketball court. The geology of the site is characterized by broad areas covered by surface deposits of Quaternary age, locally called “Blanket Sands”, which unconformably overlie the Aymamón Limestone. The basketball court forms an elongated sinkhole with numerous cracks. Three main subsurface features were imaged by the GPR data at the study site: (a) vertical shafts; (b) collapsed voids, and (c) horizontal fissures or cavities. The vertical shafts are characterized by multiple high amplitude reflections and diffractions with alternating polarity. One collapsed void was imaged within the GPR data set beneath the concrete-slab sinkhole. The collapsed zone is market by numerous normal faults and tilted reflectors. The collapsed void supports Briggs(1962) Sink-hole Development Hypothesis, that is, as limestone dissolve beneath the sands, the sands subsides gradually into depressions. In addition, numerous shallow horizontal high amplitude reflectors were observed within the study site. These were interpreted as horizontal fissures or elongated cavities, probably caused as limestone dissolution beneath the “Blanket Sands”, and gradual subsidence and fracturing along “Blanket sands” bedding planes.