H51E-01
A Modeling Study Using Oscillatory Pressure Waves for Hydraulic Tomography
We have been doing experimental field work using oscillatory pressure waves to investigate the hydraulic conductivity distribution between source and receiver wells. Theory for a homogeneous aquifer predicts that the amplitude should decay exponentially with radial distance and the phase shift should increase linearly with radial distance. Both the exponential decay coefficient and the phase shift should vary inversely with the square root of the hydraulic conductivity. Extension of this simple theory to heterogeneous aquifer systems suggests the hydraulic conductivity should be replaced with the spatially averaged value. Most of our presented experimental work to date has used horizontal ray paths. Recently, diagonal ray path data has been collected for use in a tomographic reconstruction of the hydraulic conductivity distribution. We are faced with how to process this data effectively. This paper presents the results of modeling studies to evaluate the potential of this method. First, we look at the capability of extending the simple homogeneous analytical solutions to heterogeneous situations. Modeling studies are necessary because we do not have analytical solutions for general heterogeneous situations. The modeling work indicates a useful extension of the homogeneous formulas is possible, making interpretation of the heterogeneous data more efficient. We also deal with resolution issues by considering: what is the minimum detectable zone of heterogeneity? The effect of an upper or lower barrier boundary on the interpretation of the data is also presented. In summary, the use of oscillatory pressure waves for hydraulic tomographic reconstruction of hydraulic conductivity distributions looks promising. This work funded by the Strategic Environmental Research and Development Program (SERDP) as project ER-1367.
H51E-02
A Comparison Between Solute Transport in a Discrete Fracture and in a Fracture Network Using a Novel Method for Tracer Detection
Field scale characterization and modeling of transport in bedrock aquifers has become more widespread due to increasing occurrences of groundwater contamination in these settings. As a result of the difficulty in discrete sampling, the processes of large-scale transport in fractured rock aquifers are poorly understood. The objective of this study is to explore the transport of a tracer solute at large scale using in-situ technology which can identify breakthrough of tracer at discrete fracture locations within a borehole. The tracer experiment was conducted at a well characterized field site in Smithville, Ontario. The site is underlain by a flat-lying dolomite which is dominated by several large-scale bedding-plane fractures. A total of seven boreholes were used for this experiment, aligned closely to the direction of groundwater flow over a distance of approximately 200 m. For this experiment, water was injected to establish a flow field over the thickness of the upper aquifer (~10 m). The tracer was then introduced over a short period of time and the water injection was continued for an additional 69 hours. Lissamine FF, a conservative fluorescent dye, was used as a tracer. A submersible fluorometer was used to detect the tracer arrival in-situ and obtain vertical concentration profiles in each of the boreholes. Using the concentration profiles obtained with the fluorometer and based on previous hydraulic testing data, the dominate transport pathways were identified. A finite element model which solves for transport and flow in discrete fracture features (FRAC3DVS) was then used to simulate the tracer experiment and recreate the breakthrough curves measured in each borehole. The results of this modeling exercise were compared to those from a model study conducted on the results of a tracer experiment conducted previously in a single fracture at the same location. Assessment of the modeling studies at different scales provides insight into the difficulties of modeling transport at the network scale and into the importance of considering matrix interactions, and tortuous flow paths at either scale.
H51E-03
Characterization of radar (GPR) signatures and physical properties of a near-surface granular stratigraphy in the Valley of Queretaro, Mexico
Coupled use of GPR surveying with three different frequencies (300, 900, and 2000 MHz) and physical attributes determinations in a granular near-surface sequence gives insights into the effects of water content and type to the velocity of EM wave propagation (permittivity). Using known depth reflectors in the estimation of velocities for each frequency allowed obtaining vertical differences in velocity. Characteristic radar signatures were obtained for each of the 5 layers composing the horizontally stratified near-surface sequence studied. The sequence is composed of a cover of organic (0 - 0.5 m) and carbonated (0.5 - 1.0 m) soils over a pyroclastic (1 - 4.5 m), sandy gravel (4.5 - 5.6 m), silty sand (5.6 - 7.5 m), and welded tuff (7.5 - ? m). Selected physical attributes measured in detail each 15 cm of the sequence included grain size, relative strength, bulk dry density, water content (gravimetric and volumetric), specific density, and electrical conductivity. The correlation of physical properties with radargrams show the potential of GPR for characterizing layers with a similar set of interrelated physical attributes. However, the analysis of radar signatures assumes a certain degree of homogeneity in the attributes of a layer and the record of certain attributes depends, at least partially, of the frequency. For instance, small voids at the base of the carbonate soil were recorded as air wave hyperboles only with the 900 MHz frequency. Since no significant water content contrasts are observed in the sequence, the difference in the estimation of velocities of propagation from water content measurements using the Topp equation and the velocities obtained with the known reflector method can be associated to the contribution of dielectric losses. Thus, any model representing the bulk permittivity of clay bearing sequences should consider the effects of conductive losses. In our case, these effects can be related to the presence of different clay materials that have an effect in the type of water. Furthermore, bulk velocities of propagation in a simple layered near-surface sequence may be strongly frequency dependent. Valuable information can be obtained if the depositional history of the sequence and its physical attributes are determined experimentally and analyzed in detail.
H51E-04
Non-Parametric Statistical Methods for Evaluating Heavily-Censored Hydraulic Conductivity Data
A statistical analysis was performed to evaluate the relationship between hydraulic conductivity and rock classification in seven boreholes at the Underground Testing Area near Las Vegas, NV. High-resolution flowmeter data were collected at 6 cm intervals and related to the rock classifications: hydrostratigraphic unit, hyrogeologic unit, lithology, stratigraphic unit, and alteration. The combined length of borehole measurements is greater than 1750 m, providing hundreds of hydraulic conductivity values. The complicating factor in this analysis is that over 70% of the data are censored, or less-than some minimum detection limit. Non-parametric censored data techniques described in Helsel (2005) were used to describe the data and determine the best classification method for describing hydraulic conductivity. Results show that 24% of the rock classifications exhibit a significant decrease in K with depth. 90% of the rock classifications are heterogeneous, with hydrostratigraphic unit being the most heterogeneous. The greatest variability in hydraulic conductivity among rock classifications was found in stratigraphic units, while the lowest variability was found in lithology.
H51E-05
Artificial Recharge Sites Identification In A Typical Semi-Arid Terrain Of Progressively Lowering Groundwater Levels
Shamshabad basin of Ranga Reddy district, Andhra Pradesh (A.P) is one of the severe drought prone areas in the Telangana region of A.P. India. The basin is about 159 km2. The average annual rainfall is about 740mm. At present the basin is experiencing severe drought conditions due to over exploitation of groundwater, in spite of normal rainfall for several years. Integrated investigations such as Hydrogeomorphological investigations and hydrogeophysical investigations were carried out in the basin to identify artificial recharge sites, water and soil conservation sites to improve the groundwater levels. Traditionally artificial recharge sites are identified by remote sensing method by preparing various thematic maps such as hydrogeomorphological map, slope map, drainage map etc. However this method may not throw much light on the depth to basement, which is very much essential in identifying the artificial recharge sites. Therefore in addition to the thematic mapping by using IRS LISS III data, 32 vertical electrical soundings were carried out in the basin delineating the aquifer resistivity and depth to basement. By integrating all these studies artificial recharge sites were identified in such a way that, the drainage map is overlaid on the slope map, the drainage pattern and slope of watershed is carefully studied and then Check dams are located where the ground is fairly level after steeper slopes on the upstream side and where the drainage path is nearly straight on a moderate Pediplain. At this location it is verified that whether the basement depth is sufficient or not and also verified whether the aquifer formation is amenable to recharge or not by studying the aquifer resistivity and groundwater levels. All together three check dams sites were identified in addition to afforestration and vegetative barriers and field bunding at some places
H51E-06
Geostatistical Characterization of the Unsaturated Zone Using Cross-Borehole Ground Penetrating Radar
The water infiltration through the unsaturated zone is a determining factor for the quantity and quality of the underlying groundwater. An accurate understanding and description of the important processes in the unsaturated zone is needed in order to produce reliable decision tools for groundwater exploitation and protection. High resolution tomographic images obtained using cross-borehole ground penetrating radar may provide valuable information regarding the characteristics of the shallow subsurface. More specifically, geostatistical properties of hydrological state variables, such as moisture content, may be estimated from derived velocity distributions. Unfortunately, commonly used least-squares inversion techniques result in smooth, minimum variance estimates of the subsurface radar wave velocity structure, which may diminish the utility of these images for geostatistical inference. To address this limitation, we present here a recently developed stochastic inversion technique to infer the subsurface geostatistical properties using cross-borehole ground penetrating radar data alone. For a specific choice of prior covariance model, we evaluate how likely it is that samples of the posterior Gaussian probability density function (PDF) are samples of the prior Gaussian PDF. The properties are inferred without using the inversion images directly and are not affected by the excessive smoothing/damping often observed in previous literature. The velocity distributions and obtained correlation structures are compared with similar estimates found using traditional least-squares inversion using both synthetic studies as well as travel time data collected at a field site in Denmark. In comparison to the traditional inversion algorithm the stochastic inversion technique produces images containing a higher degree of spatial variability, in addition the subsurface structures are less connected and more contained. Small-scale changes in the velocity distribution may result from local moisture content changes, and these small-scale features may potentially be extremely important if small-scale flow patterns like fingering and preferential flow occur.
H51E-07
An Extension of a Nonstationary Inversion Method with Approximation Error Analysis Applied to Hydrological Process Monitoring
We extend the previously presented methodology for imaging the evolution of electrically conductive fluids in porous media. In that method, the nonstationary inversion problem was solved using Bayesian filtering. The method was demonstrated using a synthetically generated test case where the monitored target is a time-varying water plume in an unsaturated porous medium, and the imaging modality was electrical resistance tomography (ERT). The inverse problem was formulated as a state estimation problem, which is based on observation- evolution models. As an observation model for ERT, the complete electrode model was used, and for time- varying unsaturated flow, the Richards equation was used as an evolution model. Although the "true" evolution of water flow was simulated using a heterogeneous permeability field, in the inversion step the permeability was assumed to be homogeneous. This assumption leads to approximation errors that have been taken into account by constructing a statistical model between the different realizations of the accurate and the approximate fluid flow models. This statistical model was constructed using an ensemble of samples from the evolution model in a way that the construction can be carried out prior to taking observations. However, the statistics of approximation errors actually depends on observations (through the state). In this work we extend the previously presented method so that the statistics of the approximation error are adjusted based on the observations. The basic idea of the extension is to gather those samples from the ensemble which at the current time best represents the observed state. We then determine the statistics of the approximation error based on these collated samples. The extension of the methodology provides improved estimates of water saturation distributions compared to the previously presented approaches. The proposed methodology may be extended for imaging and estimating parameters of dynamical processes using a variety of geophysical methods. This work was supported, in part, by the Finnish Funding Agency for Technology and Innovation (TEKES), projects 40285/05 and 40347/05, and by the U.S. Dept. of Energy under Contract No. DE-AC02-05CH11231.
H51E-08
Spatial and Temporal Analysis of Mexico City Subsidence by Means of Interferometric Techniques
In Mexico city, water over-consumption leads to subsidence. Before the Spanish conquest, the southern part of the Mexico Valley, an endoreic basin surrounded by mountains, was filled by a large lake. Flooding problems oblige conquerors to dry the lakes, which by now have almost completely disappeared and have been replaced by buildings. The simplified hydrogeologic structure of Mexico Valley includes a superficial 50 to 300 m thick lacustrine aquitard overlying a thicker aquifer made of alluvial deposits. The aquitard layer plays a crucial role in the subsidence process due to the very high compressibility of its clay deposits separated by a less compressible sand layer where the biggest buildings are anchored. The aquifer over-exploitation leads to a depression of its piezometric level, inducing water downwards flow in the clays, yielding compaction and subsidence (Rivera, 1990). In order to quantitatively link subsidence to water pumping, the Mexico city subsidence needs to be mapped and analyzed through space and time. It will help identify possible variations related with seasonal recharge, anchored and non anchored buildings, old and new pumping areas with varying clay compressibility through time due to consolidation (Rivera, 1990, Ortega-Guerrero et al., 1999). Radar interferometry (InSAR, Interferometric Synthetic Aperture Radar) has been successfully applied to map subsidence caused by water pumping (e. g., Amelung et al., 2000). It uses two repeated SAR acquisitions to obtain distance measurements. After geometrical corrections, the interferometric phase contains deformation information as well as residual orbital and topographic errors and atmospheric delays. A previous work using levelling, interferometry and GPS techniques over Mexico city showed that the location of the maximum subsidence rates (about 400 mm/yr) has changed and moved from the downtown area to the east of the city over a 50 years interval (Cabral-Cano et al., 2006). In this previous study, no displacement time series could be obtained from interferometry due to difficulties in the unwrapping process. We present a work based on interferometry to measure subsidence evolution spatially and temporally using the whole 54 ERS1&2/ENVISAT images set and covering 11 years (1995-2006) of ground motion. The two main obstacles are temporal decorrelation and phase unwrapping. We test a 'traditional' interferometric method (Cavalié et al., 2006) and a Persistent Scatterer method (Hooper, 2006). To maximize coherence and facilitate unwrapping, the first method uses high coherent interferograms issued from short time span image pairs. Interferograms are corrected from a layered atmospheric phase screen and from residual orbital and topographical errors. We also derive a method to unwrap 6 months to one year interferograms. Corrected interferograms are inverted to obtain deformation time series and mitigate atmospheric artifacts. The second method uses all available images to construct interferograms with respect to a common master. It performs a correction of geometrical effects and uses amplitude and phase to select pixels not affected by decorrelation, thus carrying reliable phase information. We test the 3-D (space-time) unwrapping algorithm of Hooper to recover radar propagation delays and finally separate subsidence from atmospheric artifacts. Time series obtained from the application of both methods are analyzed, compared and discussed.
H51E-09
Exploring Seasonal Variations in Near Surface Soil Moisture Using Differential Electrical Resistivity Tomography
The rates of hydrologic processes are strongly influenced by spatial and temporal characteristics of near surface soil moisture. While climate variables play a major role in determining the distribution of soil moisture over large areas, vegetation characteristics may introduce significant spatial heterogeneity in soil moisture and recharge at local scales. Results from a Michigan site equipped for long-term electrical resistivity tomography (ERT), provides quantitative evidence of land cover effects on the spatial and temporal characteristics of near surface soil moisture. Data acquired during the late growing season of 2006 shows that the soil zone up to 5 meters deep below mature deciduous trees have lower moisture contents than similar soils under open grassy areas. During the early winter months, the measured resistivity in the deciduous forest declined significantly as the moisture content increased. The resistivity response in the upper meter of soil to seasonal fluctuations in atmospheric temperature and solar radiation is also different between the two land use types at the site during the late fall and early winter months. Such differences however diminish with the onset of soil freezing at the site. Our findings highlight important correlations between vegetation and near surface soil moisture characteristics in space and time, and they also demonstrate the usefulness of fixed surface ERT arrays for imaging such changes.