H23A-1002
Time-lapse crosshole radar profiling of infiltration process in a vadose zone
Ground-penetrating radar (GPR) is an effective tool for imaging spatial distribution of hydrogeologic parameters. An artificial groundwater recharge test was conducted in Nagaoka City in Japan, and time-lapse crosshole GPR data were collected to monitor the infiltration process in a vadose zone. In the experiment, zero-offset profiling (ZOP) was utilized to rapidly scan the inter-borehole region. The infiltration process is clearly observed as a variation of EM wave velocities, which can be transformed into dielectric constants and further converted to water contents. GPR responses in a vadose zone are largely controlled by variations in water saturation. An increase in traveltimes can be attributed to an increase in water saturation, and a wetting front moves downward with an average velocity of about 2.7 m/h in the test zone. Interpretation of experimental results was guided by a finite- difference time-domain (FDTD) method for two-dimensional cylindrical coordinates to simulate radargrams associated with the advance of a wetting front during the filtration process. To accurately determine the water content profile, correct velocity analysis requires identification of first-arriving critically refracted waves from the traveltime profile. The standard ZOP for which all first arrivals are assumed to be direct waves, results in 4.5 - 16 % underestimation of water content in a transition zone.
H23A-1003
Influence of different fertiliztion and cultivation on geophysical parameters
Applications of geophysical methods for agricultural use increased during the last years. Since there is mostly an indirect relationship between geophysical and soil parameters, geophysical measurements within soils lead generally to ambiguous results with respect to the desired information. Our investigations were conducted at the Static Experiment' Bad Lauchstaedt, which was laid out in 1902 with a major focus on examining the influence of organic and mineral fertilization on yield and quality of crops as well as on soil fertility. This site gives an excellent opportunity to study the correlations between geophysical measurements and soil parameters. The systematic design of the experiment consists of 18 fields with different management suited to investigate influence of mineral and organic fertiliztion, different developed carbon stocks and the impact of plants in terms of water availability on geophysical measurements. In our study we will present results from DC-geoelectrical measurements (profile length 80-160 m, electrode distance 0.5 m) at two different times, in August after harvest and in January. Additionally electromagnetical investigations with an EM38DD were conducted in January and extensive data sets of soil parameters (e.g., soil water content, particle sizes, carbon stocks) are available. Geophysical measurements show pattern that correlate with different fertilization. For investigation of parameter relationships a multicriterial evaluation of data sets is necessary. The analysis is performed with respect to the complex parameter dependencies; for example higher carbon content tends to a higher water storage capacity. Thus, at higher soil moisture contents a lower resistivity is measured. On the other side the surface plant growth is increased with a better supply level and the plants can take up more water. This water comes rather from the upper layers of the soil. The consequence is a higher resistivity despite higher carbon content or mineral fertiliztion. Without fertilization an increased root growth in lower soil horizons can be observed that leads to an increased water uptake in the area and consequently a higher resistivity. Hence, there is a need to look at the conditions, at the location and the management simultaneous.
H23A-1004
Geophysical mapping of buried valley aquifer systems: the project BurVal
Sub-glacial erosion during the ice ages carved deep valleys down to 400 m depth into the Tertiary subsurface of the Permian sedimentary basins in Northern and Central Europe. These valleys are several km wide and extend over distances of up to 100 km. Refilled with glacial material (sand and till), they can have complex internal structures which are furthermore difficult to image. Despite this, buried valleys can have an important influence on groundwater conditions: if sandy material dominates the valley fill, this may host important groundwater reservoirs which are increasingly used for water supply. Targets for geophysical exploration of buried valleys are: 1) localisation and lateral extent (the valley is not visible at the surface), 2) shape and depth extent, 3) composition of the valley fill in terms of aquifers and impermeable layers, 4) structure of the covering layers in order to quantify the aquifer vulnerability. Within the EU-project BurVal six buried valleys in Denmark, Northern Germany and the Netherlands were intensely investigated with geophysical and hydrogeological methods, including reflection seismics, gravity, ground-based resistivity and airborne electromagnetic methods (time- and frequency domain). Fieldwork and data interpretations were carried out by GEUS (Geological Survey of Denmark and Greenland, Copenhagen), University of Aarhus, University of Kiel, BGR, GGA-Institute of Applied Geosciences (both Hannover, Germany), TNO (Geological Survey of the Netherlands, Utrecht) and some private contractors. Combined with drilling results, the geophysical results were used to construct geological models of the valley which can be used for groundwater modelling. Results of the investigation program showing the combined use of geophysical techniques to characterize buried valleys are presented and discussed.
H23A-1005
More Accurate Estimation of Water Table Elevation in the Unconsolidated Sediment Aquifer Using Ground Penetrating Radar
More than two decades, Ground Penetrating Radar (GPR) as a non-invasive sounding technique has been used to detect and map the water table in the unconfined sediment aquifer under the favorable conditions such as in the coarse grained sediment aquifer. Therefore, more accurate estimation of the water table elevation (or depth) using GPR has become a great environmental concern to geophysicists. In general, the vadose zone can be conceptualized as three zones including: (1) a pendular zone with a residual saturation, (2) a transition (or funicular) zone, and (3) capillary fringe (tension-saturated zone) from the ground surface to the water table. In the transition zone, water content increases gradually until it reaches to the tension-saturation (close to 100 percent) at the height of air-entry head. The thicknesses of the transition zone and capillary fringe are greatly controlled by the soil texture in the subsurface. It is known that the GPR reflection associated with water table is from the top of the capillary fringe above the water table. In this study, both numerical and physical model experiments with GPR frequencies of 500 and 1,000 MHz antenna were conducted to more accurately estimate the water table elevation by simulating the vadose zone with various thickness of the transition zone above the capillary fringe in the different sediment aquifers. In addition, the heights of the capillary fringe and transition zone in the different sediments were measured in the laboratory. The results of the experiments demonstrate that the GPR reflection associated with the water table actually occurs at the uppermost part of the transition zone, not the top of the capillary fringe above the water table. From the study results, the water table elevation can be obtained by subtracting the estimates of the heights of both capillary fringe and transition zone from the GPR-detected water table elevation. Therefore, the main sources affecting the estimation error in the water table elevation can be reduced by considering the presence of the transition zone and capillary fringe above the water table in the sediment aquifer.
H23A-1006
Use of high-resolution geophysical data to characterize the porosity distribution in heterogeneous aquifers: Influence of inversion and data integration method on hydrological predictions
Knowledge of the detailed distribution of hydrological parameters in heterogeneous aquifers is a key prerequisite for accurate simulation of groundwater flow and contaminant transport. The integration of high-resolution geophysical data into the subsurface characterization problem has been shown in many cases to significantly improve our knowledge of hydrological parameters by providing information at spatial scales that are unattainable using conventional measurement techniques. However, the hydrological significance of many of the choices made during the complex process of geophysical data integration (e.g., data processing and inversion techniques, integration/simulation methodology) has not been fully evaluated. In addition, evaluation of how much benefit is brought by the geophysical data for various types of hydrological models has not been thoroughly investigated. Understanding these issues is critical to making wise decisions about how to use the geophysical data. Here, we examine some of these issues for the case of porosity characterization in saturated heterogeneous aquifers using crosshole ground-penetrating radar (GPR) and borehole porosity log data. To begin, we generate a number of different porosity fields that exhibit varying degrees of continuity and structural complexity. Next, we simulate the collection of crosshole GPR data between several boreholes in these fields, and the collection of porosity log data at the borehole locations. The synthetic GPR data are then tomographically inverted for the spatial distribution of electromagnetic wave velocity using a variety of inversion techniques. Together with the synthetic porosity logs, the resulting tomographic images are used to reconstruct the porosity field through Monte- Carlo conditional simulations. To accomplish this, we again use a variety of methods, including sequential simulation and simulated annealing. The resulting realizations of porosity, obtained using the various combinations of inversion and data integration techniques, are then used as basis information to infer the hydraulic conductivity field and perform groundwater flow and contaminant transport simulations. This allows us to assess the hydrological significance of our choice of geophysical inversion and data integration methods.
H23A-1007
Hydromechanical response characterization by integration of geophysical and hydrological data, San Lorenzo, California
A deep (317 m) borehole constructed with 6 piezometers was drilled adjacent to a dual-stage extensometer (under construction) near the modern San Francisco Bay shore in San Lorenzo, California for the purpose of monitoring pore-fluid pressure changes that may result from a proposed aquifer storage and recovery (ASR) program. Continuous lithological and geophysical logging of the borehole along with discrete measurements of vertical hydraulic conductivity and of consolidation from selected core samples characterizes the distribution of properties controlling the aquifer-system response to stress. Characterizing the distribution of these properties will contribute details to the analysis of the bulk deformation measured by the extensometers. The lithologic and geophysical logs collected at the site generally indicate unconsolidated to partly consolidated continental and marine alluvial deposits consisting mostly of silt and clay, but include three coarse-sand and gravel layers totaling nearly 30 m between 155 and 198 m below land surface. A shear- and compressional-wave suspension log of the uppermost 30 m indicated the shear-wave velocity is 209 m/s, classifying it as a National Earthquake Hazards Reduction Program Class D site that can be expected to amplify strong ground motions by a factor of 2.4 for low (0.1 g) spectral accelerations. Pore-water chemistry (representing the water released from compaction if clay beds compact in response to ASR activities) was characterized and subsequently synthesized for use in saturated vertical hydraulic conductivity measurements; in general, the resulting vertical conductivity values decrease with depth and range from 0.0004 to 24 cm/d (geometric mean of 0.04 cm/d). Low overconsolidation ratios measured from consolidation tests reveal the sediments are normally consolidated; the compression indices indicate that the elastic and inelastic specific storage values generally decrease with depth and range from 1.8x10-5 to 1.9x10-4 m-1 and from 2.5x10-4 to 2.8x10-3 m-1, respectively. The property-distribution information derived from geophysical and hydrological data is valuable for characterization of the hydromechanical response from both natural and ASR-induced stresses, which will aid in understanding the composite depth-integrated measurements from nearby extensometers.
H23A-1008
Contribution of a combined TDEM (Time-Domain electromagnetism) and geoelectrical survey to the investigation of the coastal aquifer of Puerto Morelos, Quintana Roo, Mexico
This study reports initial results of combined Time Domain Electromagnetic (TDEM) and vertical electrical sounding (VES), geophysical characterization of the Quintana Roo coastal aquifer, with the aim of establishing effective protocols for subsequent surveys in the area, through the association of TDEM and VES. The high resistivity of the carbonate terrain, combined with the very low resistivity range of fresh-water and sea-water, are ideal to use both tools in combination. The results show that both methods used in a combination may provide a useful tool for hydrogeologial studies. In this survey we were able to identifiy a fracture 100 m x 40 m, that was correlated to fresh-water discharges in to the Puerto Morelos Reef lagoon.
H23A-1009
Repeated Resistivity and GPR Surveys for the Monitoring of Water Content and Temperature in the Unsaturated Zone
Repeated resistivity and GPR surveys were carried out in the KR-1 groundwater observation site, North-Kanto, Japan, where the measurements of near-surface water content and temperature have been conducted with the meteorological observation. The purposes of these repeated surveys were to detect the changes of hydrogeological environment in the unsaturated zone. Twenty-five electrodes were placed at 1 m intervals along a 24-m long line that crossed immediately near the soil moisture and temperature meters. The volumetric water content was measured every 10 minutes at ten depths from 0.6 to 10 m. The soil temperature was measured at depths of 0.6, 3, 10 m. Using dipole-dipole and Wenner electrode arrays, resistivity data were collected for every month from August 2006 to August 2007. A 2-D resistivity section was obtained for each resistivity data set and the seasonal resistivity changes were recognized. GPR surveys were carried out in December 2006, January 2007, February 2007 and July 2007. In these surveys, 3-D GPR data were acquired with a 350-MHz GPR system in the area of 16 m ~ 17 m around the soil moisture and temperature meters. 2-D GPR profile data for deep depth were collected with a continuous wave (5 to 150-MHz) GPR system along 53-m long and 55-m long lines. At several points, wide angle measurements were done to estimate the EM wave velocity. We compared the changes of resistivity and GPR profiles with those of precipitation, volumetric water content, and soil temperature. The resistivity near the surface increased from summer season to winter season and then decreased. However the changes of EM wave velocity and water content were relatively small. It seems that the change of resistivity was most influenced by the change of soil temperature.
H23A-1010
Microgravity Monitoring of Artificial Reservoir Recharge at Little Cottonwood Canyon, Wasatch Front, Utah
A series of repeated high-precision (±5 μGal) gravity surveys are being used to monitor an artificial recharge project at the mouth of Little Cottonwood Canyon, Utah. This recharge location allows direct infiltration into the main municipal supply aquifer for the Salt Lake valley which provides water to >33% of Utah's population. The pilot project under study has three infiltration sites within 100 m of each other to test differing infiltration techniques: open shallow infiltration pond, medium-depth (20 m) cased well, and near-surface- (1 m) buried drain field. To capture the expected horizontal migration (>500 m/yr) of the infiltrated water, the survey network includes 12 stations within 500 m of the three closely-spaced infiltration points, ll stations within 500 and 1000 m from the site, and 4 stations over 1500 m distant from the infiltration site for regional and environmental background control. Prior to infiltration, a set of five background campaigns have been acquired on the survey network between spring 2006 and summer 2007. Background (natural/environmental) variability is thus robustly estimated at 10 μGal. Infiltration commenced in mid- September 2007 with bimonthly gravity surveys over the entire network during the last quarter of 2007. Based on a previous artificial recharge experiment approximately 60 km north of the current project, we expect a peak signal of >40 μGal from the approximately 300 acre-ft to be infiltrated. Detailed study of the gravity changes at each infiltration site will allow a comparison of the relative recharge and migration rates from each technique, and will inform future large-scale recharge projects for the southern portion of the Salt Lake valley.
H23A-1011
Spatial delineation of groundwater salinity using deep TDEM geophysical measurements: a feasibility study
The objective of this study was to delineate the spatial distribution of fresh, brackish and saline groundwater bodies, as well as brine, beneath the Judea Desert, Israel. The deep time domain electromagnetic (TDEM) geophysical system, named Cycle-5M, providing the maximum exploration depth, down to ~1.5-2 km below land surface, was employed. The study was conducted at 21 locations at the desert plateau, where fresh groundwater flows through the upper aquifers from the replenishment area at the Judea Mountains toward the Dead Sea springs. At deeper aquifers, brackish and saline groundwater bodies exist, which originated from lakes and lagoons, existed at the Dead Sea Rift valley during the Pleistocene and Pliocene, respectively. The geophysical calibration results showed that, fresh groundwater (C<10e3 ppm TDS salinity) is characterized by resistivity of more than 15 ohm-m; brackish groundwater (10e3<C<10e4 ppm salinity) by 5- 15 ohm-m; saline groundwater (10e4<C<10e5 ppm salinity) by 0.5-5 ohm-m; and brine (10e5<C ppm salinity) by less than 0.5 ohm-m. The hydrological results of the survey show that the upper Judea Group aquifer is usually saturated with fresh groundwater, while the interface between fresh and brackish water exists within the lower Judea Group aquifer and in the underlying Kurnub Group aquifer. According to the scarce calibration data available, the salinity of the brackish groundwater in most cases does not exceed a half of the normal seawater salinity.
H23A-1012
How Streambed Temperatures can Contribute to the Determination of Aquifer Heterogeneity
The groundwater discharge to a stream may show small-scale heterogeneities caused by the structure of the connected aquifer. Traditional subsurface investigation techniques are often not capable of providing data in sufficient resolution to capture these small-scale variations in aquifer properties. In the streambed, the spatial pattern of the groundwater discharge can be investigated by temperature measurements. We hypothesize that the heterogeneity of the hydraulic conductivity (K) of an aquifer can be inferred from measured streambed temperatures. At a 220 m long section of a small stream in Germany, streambed temperatures were mapped with high resolution. A groundwater flow and heat transport model of the stream-aquifer system was set up including stochastically generated K fields. Direct-push injection logs and slug tests were performed in the connected aquifer to obtain the mean, variance and correlation lengths of K. Yet, the model results showed that the simulated streambed temperatures did not cover the range of measured temperatures. We concluded that the calculated variance of K was too low to cause the observed heterogeneities of the streambed temperatures and therefore generated new K fields with varying variances. From the model results we analyzed the relation between the variances of K and the simulated distribution of streambed temperatures. The required variance of K to reproduce the range of measured streambed temperatures could then be determined from this relation. From realizations of K fields generated with the adjusted variance, several could be selected that induced a range of groundwater fluxes and streambed temperatures in the model similar to the measured range. With the selected realizations the spatial distribution of groundwater fluxes in relation to the streambed area could also be well reproduced. Accordingly, these K fields are reliable input data for further modeling applications. This study showed that subsurface characterization methods based on heat constitute a valuable supplement to traditional exploration techniques.
H23A-1013
Groundwater Exploration in Baja California, Mexico, Using Audiomagnetotellurics
Guadalupe Valley, in Ensenada B. C. Mexico, basis of the winery industry of the region, is known by their climatic attributes for vineyard cultivation. In this place, the crop growing depends totally on underground water extracted of an aquifer contained in two separate small basins. In order to estimate the depth to water level, as well as the thickness of one of these basins, we carry out a geophysical survey using audio-magnetotellurics (AMT). We carried out five profiles in a frequency range between 1 Hz and 750 kHz to estimate the electrical conductivity of the ground. We know that this physical property is enhanced by the permeability as well as by the salinity of fluids in the aquifer. In contrast, the crystalline rocks forming the basement of the basin are very bad conductors of electricity. Based on the AMT observations we construct 2-D models of the ground resistivity distribution. Our results show a clear resistivity contrast between sediments and bedrock. The sediments have resistivity values that oscillate from 40 to 100 Ohm-m, associated with lithology and/or permeability changes. Some conductive bodies (5 to 15 Ohms-m) are observed at depths shallower that 150 m, which might be caused by the presence of water with higher salinity, or alternatively, by clay lens. The bottom of the basin has resistivity values larger than 300 Ohm-m typical of the granitic rocks composing the bedrock. The spatial variation of the basement depth suggests the presence a normal fault, in agreement with a graben structure proposed in former studies.
H23A-1014
Direct estimation of hydraulic conductivity using integrated inversion of resistivity data during a tracer test
We present an innovative integrated inversion technique that directly uses surface resistivity measurements to estimate hydrologic parameters during a tracer test. Determination of hydraulic conductivity (K) and dispersivity using only measured surface voltage potentials is a noninvasive and cost effective method for subsurface characterization. Unlike typical hydrogeophysical estimation problems, the need to invert the voltage data to produce an intermediate resistivity image is eliminated in our approach, thus our problem to be better constrained than typical hydrogeophysical estimation problems since we use many resistivity data points to estimate only a few key parameters (compared to the hundreds or thousands of parameters contained in a geophysical image). In addition, by optimizing for a minimal set of parameters we can eliminate dependence on prior knowledge of the subsurface that is required to regularize typical inverse problems. Another significant advantage of our approach is that very few current and potential electrodes are required, allowing for dynamic characterization of the subsurface during transient processes. Here we present a synthetic study of a tracer test performed in a homogeneous aquifer under a natural gradient. During the test, only one current electrode pair and a limited number of potential electrodes placed at the surface are used to monitor the migration of a saline plume. A coupled forward model for flow, solute transport, and resistivity developed in COMSOL Multiphysics is used to calculate the concentration and potential distribution for a given value of aquifer hydraulic conductivity. In practice, the value of K is unknown and our goal is therefore to estimate it from observed electrical potentials. For this purpose, the coupled flow, transport, and resistivity models are used to determine the K value that minimizes the difference between the observed and simulated electric potentials. Using the COMSOL Optimization Lab, we have been able to estimate K to within 0.5% using only the surface resistivity measurements with an initial tracer concentration of 0.5M. Subsequent sensitivity analyses indicate that injection concentrations above four times that of the background ion concentration in the aquifer, set here to 0.0025M, can yield an estimate within half an order of magnitude from the true hydraulic conductivity thus improving accuracy with increases in concentration. Further sensitivity analyses are being performed to investigate the signal strength in regards to background noise and tracer depth below the surface.
H23A-1015
Experimental Estimation of the Penetration Depth of the GPR Groundwave
The Ground Penetrating Radar (GPR) groundwave is a direct wave that travels between the transmitting and receiving antennas in the shallow subsurface. Several researchers have shown that both variable-offset and common-offset groundwave data can be used to estimate soil moisture, indicating that GPR is a promising tool for field-scale water content estimation. However, the exact penetration depth of the GPR groundwave is unknown, and this uncertainty limits the efficacy of the groundwave technique for large-scale soil water content monitoring. Modeling studies of the GRP groundwave have indicated that the penetration depth is a function of GPR frequency and soil water content. This research experimentally investigates the penetration depth of the GPR groundwave as a function of frequency under controlled conditions within a large tank filled with a fairly homogeneous sandy soil. In this experiment, water was added to part of the sand, and this sand was thoroughly mixed to produce a homogenous soil with a very high water content. The remainder of the sand was oven-dried to produce a homogeneous soil with a very low water content. The wet sand was placed in the bottom 20-cm of a large tank, and variable-offset and common-offset GPR data were collected over the wet sand using 100-, 250-, 500-, and 1000-MHz antennas. Then, a 3-cm layer of the dry sand was placed on top of the wet sand, and the GPR surveys were repeated. Additional layers of dry sand were placed in the tank in 3-cm lifts, and GPR data were collected after each sand layer was added until the GPR data showed no changes in electromagnetic velocity with additional layers of dry sand. The depth of dry sand where the electromagnetic velocity is no longer influenced by the underlying wet sand layer is assumed to represent the penetration depth of the GPR groundwave in this soil. Data analysis of this experiment is ongoing, but preliminary results confirm the theoretical expectation that penetration depth is a function of GPR frequency.
H23A-1016
Comparison of Hydrologic Parameter Estimates Using Sequential and Integrated Data Fusion During a GPR Monitored Infiltration Event
Constraining parameters that govern variably saturated flow is important for applications ranging from quantifying water availability for ecosystems to constraining recharge rates and contaminant fluxes to groundwater. In this study we explore the effectiveness of sequential versus integrated data fusion for estimating unsaturated flow parameters using ground penetrating radar (GPR) data. In Sequential Data Fusion (SDF), geophysical imaging is used to create a map of the geophysical properties of the subsurface. Subsequently these properties are transformed to hydrologic properties that can be used to constrain an independent hydrologic inverse problem. In contrast, Integrated Data Fusion (IDF) uses the geophysical data to directly constrain hydrologic properties of interest without performing the intermediate geophysical imaging step. Our comparison of SDF and IDF is performed for a synthetic study of 2D infiltration into a homogeneous soil from a constant flux point source located at the ground surface. Here we focus on results for the estimation of intrinsic permeability (k) from cross- borehole GPR travel times collected throughout the duration of the infiltration event. The target permeability (k=7.4x10-12m2) is uniform over the 20 meter by 20 meter area modeled in this study; though the soil is homogeneous, we emphasize that water content is both spatially variable and transient. We use TOUGH2 to simulate infiltration, MATLAB to simulate GPR travel times, and PEST to perform the parameter estimation. To quantitatively compare SDF and IDF, we calculate the normalized error in estimated permeability for each method. In our study, we investigated the performance of the data fusion methods under varying survey geometries by changing the antenna spacing. In all cases we have found that IDF significantly outperforms SDF. For large antenna separations (1.7-6.7m) SDF produces an average error in estimated permeability of 78.6% while IDF errors are only 35.4%. As ray density is increased for antenna separations of 1.0-1.5m, average estimation error for SDF drops to 77.3%, but is drastically reduced to only 5.8% for IDF. Also, SDF estimates are consistently biased lower than the target value, while IDF results are unbiased. Our results suggest the IDF is a powerful new approach for hydrologic characterization of the subsurface using geophysical measurements.
H23A-1017
Comparison of Soil Moisture Content Estimated with Air-Launched and Ground-Coupled GPR Techniques
Air-launched Ground Penetrating Radar (GPR) data are commonly used in assessment of pavement conditions for highway and bridge maintenance. These data can be collected over large distances very quickly and have relatively simple data processing requirements, so data processing is usually automated. In contrast, ground- coupled GPR data must be collected more slowly, and ground-coupled data used for soil moisture content estimation usually require a user who is experienced in GPR data interpretation. Thus, air-launched GPR data are used relatively easily for commercial applications, while ground-coupled GPR data used for soil moisture estimation have thus far primarily been employed in research applications. This experiment compares GPR data collected over wet and dry soil in the air-launched and ground-coupled modes to determine whether air-launched data could be used for accurate, field-scale soil moisture estimation. In this experiment, GPR data are collected using 250-, 500-, and 1000-MHz antennas in a large tank with controlled soil moisture conditions. The dielectric constant is determined using reflection coefficient theory for the common-offset air-launched data and electromagnetic velocity analysis of the variable-offset ground-coupled data. Data were initially collected using both the ground-coupled and air-launched modes over wet sand. Then, a 3-cm layer of dry sand was placed over the wet sand, and air-launched and ground-coupled GPR surveys were repeated. Repeated layers of sand, each 3-cm thick, were added until both air-launched and ground-coupled data show no change in dielectric constant with the addition of more dry sand. Analysis of these GPR data will allow comparisons of dielectric constants estimated from both ground-coupled and air-launched data for different frequencies and will indicate the suitability of air-launched GPR techniques for soil moisture estimation. These data will also provide experimental results for the depth of penetration of air-launched GPR signals for different frequencies.
H23A-1018
A direct sequential cosimulation algorithm and its application in hydrogeophysics
After interpretation, geophysical surveys often provide exhaustive maps of secondary information (electric resistivity for example) that are extremely useful to guide the interpolation of a primary variable (hydraulic conductivity for example). However, most often the relation between the variables are modeled with a linear statistical relationship (often between the log of the variables) or with conditional expectation. Here, we argue that these approaches are only special and limited cases of a broader approach in which the relation between the two variables should be modeled by a joint probability density function. An advantage of considering the joint relation in this manner is that it allows to model relatively easily situations in which the conditional probabilities (for example the probability of having a certain hydraulic conductivity knowing the resistivity) is multimodal. In this framework, we present a direct conditional co-simulation technique that allows creating collocated conditional simulations of the primary variable from a set of data points of the primary variable, an exhaustive map of the secondary variable, a numerical description of the joint pdf between the two variables, and a model of spatial correlation for the primary variable. Spatial cross-correlations are neglected at this stage of the work. The approach is illustrated with data from a coastal karstic aquifer located in the Sultanate of Oman.
H23A-1019
Characterizing Fault Zone Permeability Through Integrated Geophysical and Hydrological Data, Elkhorn Fault, Park County, Colorado
Fault zones are ubiquitous in ground-water aquifers and can play a significant role in fluid transport at local to regional scales. Fault-zone permeability structure controls whether the fault behaves as a barrier, conduit, or combined barrier-conduit for fluid flow. Much work has been done to measure local- to well-scale permeability of faults. However, hydrogeologic heterogeneity prevents local-scale measurements from accurately characterizing the regional hydrogeologic impact of fault zones. Near-surface geophysical techniques provide an estimate the distribution and continuity of subsurface fault zone properties, allowing for the identification of structural heterogeneities that may cause heterogeneities in permeability. This research focuses on the integration of geophysical and hydrological techniques to characterize the regional hydrogeologic effects of the Elkhorn Fault in Park County, Colorado. The Elkhorn fault is a Laramide-aged thrust fault with a sedimentary foot wall and fractured Proterozoic, crystalline hanging wall. Magnetic, gravity, and resistivity data are used to constrain the location and geometry of the fault as well as to aid in the interpretation of the structural and hydrogeologic complexity of the fault and its associated damage zone. The combined geophysical interpretation provides a framework for the development of a physical domain in which pumping test and time-series hydrologic data can be used to evaluate permeability. This interpretation also serves as a guideline for the placement of wells, facilitating direct hydrological testing of the in-situ permeability of different components of the fault zone. By utilizing geophysical techniques, hydrological data may be more effectively collected and interpreted, leading to the development of ground-water-flow models that more accurately depict the regional hydrogeologic effect of the Elkhorn fault.
H23A-1020
Stereological analysis of fractures in the Roselend tunnel and permeability determination
Fractures are often present in geological formations over a large range of scales. They determine the macroscopic mechanical, hydraulic, and transport properties of many natural rocks. In hard rock environments such as granitic media, the role of fractures in flow and transport is enhanced since the matrix can be considered as impervious. This work shows that field measurements of fractures in conjunction with records of water fluxes in an underground tunnel, can be used for estimating the permeability of a fractured medium. Orientations and length of fracture traces, and fluxes of percolating water were manually recorded along the 128 meter-long Roselend dead-end tunnel drilled in the granite of the Méraillet massif (French Alps). The thickness of rock overburden increases from 7 m at the tunnel entrance to 55 m at the tunnel dead end. The tunnel has a roughly cylindrical shape with a 2.4 m diameter. The fractures can be classified in two families: large fractures which intersect the tunnel, and small fractures which partially intersect it. Three different zones in the tunnel are distinguished with mild, low and high water fluxes, starting from the entrance. A stereological analysis of the trace length probability densities of small fractures provides the fracture diameter probability density distribution which is best approximated by a power law. Large fractures are assumed monodisperse, with a 5 m estimated radius. The generated fracture networks obtained by combining large and small fractures do percolate while networks consisting of small fractures only do not percolate. Various assumptions can be made on the permeability of each fracture. It is usual to assume that the fracture permeability is a power law of its lateral extent. The macroscopic permeabilities of the generated fracture networks can be systematically computed with our numerical technique based on the meshing of each fracture and a finite-volume formulation for the Darcy equation. Observed water fluxes are best modelled when the fracture permeability is a power law of its lateral extent with an exponent equal to 3. Ref: D. Patriarche, E. Pili, P.M. Adler, J.F. Thovert, Water Resour. Res., in press.