Characterization of the texture, composition and porosity of the Aymamon Limestone Formation and laboratory derivation of elastic properties and seismic wave velocities

The study area of this project lies in the central portion of northern coastal plain of Puerto Rico, which is characterized by mogote karst terrains that form on the Tertiary Aymamon Limestone Formation. Lowlands between the mogote hills and ridges are covered by Quaternary blanket deposits that obscure and fill underlying sinkholes in the Aymamon Limestone. Subsurface investigations of these features and their potential for collapse rely heavily on seismic surveys. The purpose of this thesis was to investigate the physical properties of the Aymamon Limestone Formation that control its seismic wave velocity to enhance interpretation of subsurface seismic analyses. A test boring (B-1) was drilled at the Monte Verde Urbanization's basketball court at Manati, in order to correlate the derived MASW seismic velocity profile with the subsurface geology of the site and collect core samples for testing. The boring encountered cavernous porosity at depths between 21.5-27.4 m that precluded core sampling at the target depths. Only two indurated recrystallized limestone fragments with densities of 2,084 kg/m3 and 1,602 kg/m3, respectively, were recovered from a depth of 25 meters.;As an alternative Aymamon Limestone core samples from four (4) different core holes (NC-4, NC-5, NC-9, and NC-10) drilled by the USGS were collected in order to characterize their composition, texture and porosity and select sample for testing. Much of the Aymamon Limestone cores from this mogote karst zone are comprised of well-indurated, recrystallized and well-cemented limestones. Samples were mostly packstone and less commonly wackestone and grainstone textures. Most of the porosity is vuggy and mouldic with much less intra-particle porosity. The fossils in the core samples included calcareous algae, benthic foraminifers, and echinoderms that have a calcite skeletal composition and corals, mollusks, and other fragments that have an aragonite skeletal composition. The micro-porosity percentages for cores NC-4, NC-5, NC-9, and NC-10 were 1.49%, 1.72%, 3.31%, and 2.54%, respectively. The mineralogy of the carbonate samples is dominantly composed of calcite with some samples consisting of high Mg calcite and dolomite.;Four core samples from the original target depths where chosen from the closest NC-5 and subjected to unconfined compression tests to determine their elastic properties and derivative seismic velocities. The samples from 20m and 27.5m consisted of wackestones and those from 21.5m and 23m were packstones. All four samples were composed of calcite or high Mg calcite. Vug porosity dominated the four samples with some inter-particle porosity in the 21.5m sample and both intra-particle and fracture porosity in the 23m sample. The estimated macro-porosity percentages for samples from 20, 21.5, 23, and 27.5 meters deep were 19.33%, 1%, 3.5%, and 39.67%; and the micro-porosity values were 1.69%, 0.88%, 2.15%, and 0.39%, respectively. The density values of the samples from core NC-5 range from 2,295 to 2,510 kg/m3. The results of the tested samples showed that Young's modulus ranged from 1.02-3.04 GPa, Poisson's ratio from 0.27-0.37, the shear modulus from 0.5-1.3 GPa, and the bulk modulus from 1.06-2.6 GPa. The Young's modulus and the shear modulus consistently increased with sample depth. The resultant calculated P-wave velocities ranged from 859-1,360 m/sec and S-wave velocity ranged from 446-740 m/sec. Calculated S-wave velocities increased consistently with depth of sample and reached a maximum at the sample with the highest mega-porosity and least density. The test results are consistent with field observations at Monte Verde where cavernous porosity was encountered at depths where seismic profiles increasingly higher shear wave velocities. The association of higher calculated P-wave velocities with lower micro-porosity suggests that recrystallization of the limestone between mega-pores to a denser mass may be controlling seismic wave velocity in limestones in advanced stages of dissolution.