An Investigation of Cross-Borehole Ground Penetrating Radar Measurements for Characterizing the 2D Moisture Content Distribution in the Vadose Zone

The use of cross-borehole ground penetrating radar (GPR) imaging for determining g the two dimensional (2D) in situ moisture content distribution within the vadose zone is being investigated. The ultimate goal is to use the GPR images as input to a 2D hydrologic inversion scheme for recovering the van Genuchten parameters governing unsaturated ,hydraulic flow. Initial experiments conducted on synthetic data have shown that at least in theory, cross-borehole GPR measurements can provide realistic estimates of the spatial variation in moisture content that are needed for this type of hydrologic inversion scheme. However, the method can not recover exact values of moisture content due to the break down of the empirical expression often employed to convert GPR velocity images to moisture content, and to the smearing nature of the imaging algorithm. To test the applicability of this method in a real world environment cross- borehole GPR measurements were made at a hydrologic/geophysical vadose zone test site in Socorro, New Mexico. Results show that the GPR images compare well with the uncalibrated borehole neutron log data. GPR data acquisition will continue once an infiltration test has started, and the results from these measurements will be employed in a 2D hydrologic inverse scheme

A Resolution Analysis of Two Geophysical Imaging Methods For Characterizing and Monitoring Hydrologic Conditions in the Vadose Zone

The objective of this research project was to analyze the resolution of two geophysical imaging techniques: electrical resistivity tomography (ERT) and cross-borehole ground penetrating radar (XBGPR) for monitoring subsurface flow and transport processes within the vadose zone. This was accomplished through a coupled approach involving very fine-scale unsaturated flow forward modeling, conversion of the resultant flow and solute fields to geophysical property models, forward geophysical modeling using the property model obtained from the last step to obtain synthetic geophysical data, and finally inversion of this synthetic data. These geophysical property models were then compared to those derived from the conversion of the hydrologic forward modeling to provide an understanding of the resolution and limitations of the geophysical techniques

Teasing Apart Impacts of Human Activity and Regional Drought on Madagascar's Large Vertebrate Fauna: Insights From New Excavations at Tsimanampesotse and Antsirafaly

Madagascar experienced a major faunal turnover near the end of the first millenium CE that particularly affected terrestrial, large-bodied vertebrate species. Teasing apart the relative impacts of people and climate on this event requires a focus on regional records with good chronological control. These records may document coeval changes in rainfall, faunal composition, and human activities. Here we present new paleontological and paleoclimatological data from southwestern Madagascar, the driest part of the island today. We collected over 1500 subfossil bones from deposits at a coastal site called Antsirafaly and from both flooded and dry cave deposits at Tsimanampesotse National Park. We built a chronology of Late Holocene changes in faunal assemblages based on 65 radiocarbon-dated specimens and subfossil associations. We collected stalagmites primarily within Tsimanampesotse but also at two additional locations in southern Madagascar. These provided information regarding hydroclimate variability over the past 120,000 years. Prior research has supported a primary role for drought (rather than humans) in triggering faunal turnover at Tsimanampesotse. This is based on evidence of: (1) a large freshwater ecosystem west of what is now the hypersaline Lake Tsimanampesotse, which supported freshwater mollusks and waterfowl (including animals that could not survive on resources offered by the hypersaline lake today); (2) abundant now-extinct terrestrial vertebrates; (3) regional decline or disappearance of certain tree species; and (4) scant local human presence. Our new data allow us to document the hydroclimate of the subarid southwest during the Holocene, as well as shifts in faunal composition (including local extirpations, large-vertebrate population collapse, and the appearance of introduced species). These records affirm that climate alone cannot have produced the observed vertebrate turnover in the southwest. Human activity, including the introduction of cattle, as well as associated changes in habitat exploitation, also played an important role

Iterative electromagnetic Born inversion applied to earth conductivity imaging

This thesis investigates the use of a fast imaging technique to deduce the spatial conductivity distribution in the earth from low frequency (< 1 MHz), cross well electromagnetic (EM) measurements. The theory embodied in this work is the extension of previous strategies and is based on the Born series approximation to solve both the forward and inverse problem. Nonlinear integral equations are employed to derive the series expansion which accounts for the scattered magnetic fields that are generated by inhomogeneities embedded in either a homogenous or a layered earth. A sinusoidally oscillating, vertically oriented magnetic dipole is employed as a source, and it is assumed that the scattering bodies are azimuthally symmetric about the source dipole axis. The use of this model geometry reduces the 3-D vector problem to a more manageable 2-D scalar form. The validity of the cross well EM method is tested by applying the imaging scheme to two sets of field data. Images of the data collected at the Devine, Texas test site show excellent correlation with the well logs. Unfortunately there is a drift error present in the data that limits the accuracy of the results. A more complete set of data collected at the Richmond field station in Richmond, California demonstrates that cross well EM can be successfully employed to monitor the position of an injected mass of salt water. Both the data and the resulting images clearly indicate the plume migrates toward the north-northwest. The plausibility of these conclusions is verified by applying the imaging code to synthetic data generated by a 3-D sheet model