Spatial and temporal distributions of accumulation rates on the catchment of Thwaites Glacier, West Antarctica

textWe make a first-order calculation of accumulation rates in the catchment of Thwaites Glacier (TG), West Antarctica using the Nye and Daansgard-Johnson methodologies. Both formulations compute accumulations as a function of the age-depth relationship, including a thinning correction due to ice flow. For this purpose, I track and firn-correct two continuous, shallow ice layers obtained from radio echo soundings surveyed during the 2004-05 AGASEA expedition. The layers range from 60 to 700 meters depth between the ice divide and the coast. Dating of layers come from the ice core WDC06A, located on the West Antarctic Ice Sheet (WAIS) ice divide, which have ages 548 and 725 years, respectively. We compare our accumulation results with four independent datasets: 1)IceBridge snow radar (2009-2010), optimized for tracking near-surface layers; 2) a contemporary model of snowfall precipitation, 3) an interpolation of ice core data using satellite passive microwave; 4) ice cores data. We test the hypothesis that accumulation rates have increased since the beginning of the industrial era, a change that has not been observed. Indeed, I find that observations indicate that accumulation rates in the TG catchment have not changed during the past ~700 years. From here I assess the mass balance of the system and analyze what it tells about the history of the glacier.Geological Science

Modelo gravimétrico del espesor de la corteza terrestre en colombia

La determinación del espesor de la corteza terrestre en Colombia, a partir de datos gravimétricos, se hizo utilizando datos de Anomalia Total de Bouguer y altura topográfica suministrados por el Instituto Geográfico Agustin Codazzi. Estos fueron representados mediante series bidimensionales de Fourier, que al ser combinadas con expresiones matemáticas del potencial gravitacional terrestre, permitieron plantear el grosor cortical en términos observables (gravedad y altura) sobre la superficie terrestre. Los resultados han sido dispuestos en una malla de puntos que se interpolan a una superficie utilizando el algoritmo de mínima curvatura. Para la estimación de la confiabilidad de los resultados se ha realizado una confrontación con las deducciones geológicas obtenidas de algunas interpretaciones sismicas realizadas en el pais.The gravimetric model of the crustal thickness of the earth in Colombia was made from Total Bouguer Anomaly and topographic height given by the Instituto Geográfico Agustín Codazzi. They were transformed to bidimensional series of Fourier, that combined with mathematics expressions of the gravitational potential, both, they made possible to find the crustal thickness starting from observable values across the earth's surface (gravity and topography). The results of several points were converted to surface through Minime Curvature algorithm. In order to obtain reliability of the results, seismic control was applied from another works

Radar interferometry measurement of land subsidence in El Paso, Texas

This work presents the application of radar interferometry to detect land subsidence associated with water pumping in El Paso, Texas and adjacent areas. Geological and hydrological information are compared with the radar information to validate the results. An error treatment of the measurements is performed using the singular value decomposition technique. Synthetic aperture radar interferometry is a proven remote sensing technique to detect ground deformation in a three-dimensional scale with millimetric precision. It has been applied successfully in earthquake monitoring, volcano deformation, glacier movement and aquifer compaction. El Paso and Ciudad Juarez are located over the Hueco Bolson aquifer, an unconsolidated alluvial aquifer that consists of gravel, sand, silt and clay. Because of increased water pumping since the early 20th century, the water table has changed and subsidence has occurred. Measurements of land subsidence are reported from the 1950s, 1970s, and 1980s. This work considers subsidence in the 1990sGeological Science

Parametric characterization of penumbra reduction for aperture-collimated pencil beam scanning (PBS) proton therapy

Recently, a commercial treatment planning system (TPS) has implemented aperture collimators for PBS dose calculations which can serve to reduce lateral penumbra. This study characterized the variation in magnitude of lateral penumbra for collimated and un-collimated PBS fields versus the parameters of air gap, depth, and range shifter thickness. Comparisons were performed in a homogenous geometry between measured data and calculations made by a commercial TPS. Beam-specific target volumes were generated for collimated and un-collimated PBS fields and optimized for various range shifter thicknesses and air gaps. Lateral penumbra (80%-20% distance) was measured across each target volume to characterize penumbra variation with depth and air gap. An analytic equation was introduced to predict the reduction in lateral penumbra between un-collimated and collimated PBS treatments. Calculated penumbra values increased with depth across all combinations of range shifters for a constant air gap. At 2 cm depth, the reductions in penumbra due to the aperture were 2.7 mm, 3.7 mm and 4.2 mm when using range shifter thicknesses of 0 cm, 4.0 cm and 7.5 cm, respectively. At a depth of approximately 20 cm and air gap of 5 cm, differences between penumbras of collimated and un-collimated beams were less than 1 mm. Penumbra reductions for the collimated beams were largest at small air gaps. All TPS-calculated penumbra values derived in this study were within 1 mm of film measurement values. Finally, the analytic equation was tested using a clinical CT scan, and we found good dosimetric agreement between the model predictions and the result calculated by the TPS. In conclusion, application of collimators to PBS fields can sharpen penumbra by several mm and are most beneficial for shallow targets. Furthermore, measurements indicate that the dose calculation accuracy in the penumbra region of PBS-collimated fields is adequate for clinical use