Imágenes con el radar de penetración terrestre en la tefroestratigrafía reciente de los volcanes Poás e Irazú, Costa Rica
Revista Geológica de América Central
Ground penetratinG radar imaGinG of tephra stratiGraphy on poás and irazú volcanoes, costa rica imáGenes con el radar de penetración terrestre en la tefroestratiGrafía reciente de los volcanes poás e irazú, costa rica abstract: Ground penetrating radar (GPR) has been shown to be a useful tool for mapping geometry and thicknesses for volcanic fall, surge, granular flow, and lahar deposits. However, the success of GPR surveys is highly dependent on soil properties and the nature of stratigraphic
... e of stratigraphic layering. The efficacy of the method at a given site can be difficult to predict. Small-scale (10s to 100s of meters) test surveys with ground penetrating radar show that geologic features of interest can be resolved to depths up to 20 m on both Poás and Irazú volcanoes in Costa Rica. The antenna frequencies used in this pilot study, 50 MHz, 100 MHz, and 200 MHz, produce wavelengths too long to resolve most individual layers (mm's to cm's thick) in the near-surface tephra fall and surge deposits. However, these GPR profiles clearly show the attitude of beds and resolve some distinct contacts at depth, particularly the base of the 1963-1965 intracrater deposits on Irazú. On Irazú GPR profiles also confirm field observations that 1963 surge deposits thin consistently with distance from the crater rim, while packages from 1723 and older show uniform thicknesses or increasing thickness with distance from crater rim, suggesting reworking or reverse flow of surges returning from the Playa Hermosa caldera wall. On Poás, bright reflectors are present at depths below 2 m on near-vent profiles, but the lack of nearby stratigraphic observations precludes geological interpretation. The test profiles on both volcanoes also clearly show diffractions produced by blocks on the order to 5 cm or more in diameter embedded in the surficial deposits, as well as evidence of sag beneath blocks. Resolution of blocks decreases with depth, presumably due to both the inherent loss in lateral resolution with wave travel distance and to clearly observed dispersion (loss of high frequencies). Future studies, particularly with higher frequency antennas on Poás, could be useful for tracking depositional units between exposures, and for resolving the distribution of blocks or bombs in deposits in the uppermost few meters.