Thomas J. Burbey

Evaluation of two-dimensional resistivity methods in a fractured crystalline-rock terrane

Abstract A series of two-dimensional resistivity profiles collected in the Blue Ridge Province of southwest Virginia and results from numerical modeling of synthetic data reveal substantial differences in depth of investigation, resolution, and sensitivity using Wenner, Wenner–Schlumberger, dipole–dipole, and pole–pole data collection techniques. Resistivity profiles were collected using short (2-m electrode spacing, 48-m profile length), intermediate (6-m electrode spacing, 144-m profile length), and long (10-m electrode spacing, 240-m profile length) arrays over shallow unconsolidated soils and regolith overlying crystalline bedrock. Pole–pole data were only collected with the short array. Numerical modeling was used to simulate both vertical and horizontal structures similar to subsurface conditions in the study area. All of the apparent resistivity data were inverted into earth models using a computer program that uses an l1 norm smoothness constrained inversion technique. Earth models generated from both field data and numerical modeling acquired by the dipole–dipole technique consistently indicated more detail and greater depth of investigation than the other techniques. The dipole–dipole method uniquely imaged thin saturated sands and isolated high resistivity bodies beneath the 48-m-length array, significant horizontal and vertical resistivity variations including a thick transitional resistivity zone in the 144-m-length array, and anomalous low resistivity zones in crystalline bedrock in the 240-m-length array. Earth models created from the Wenner and Wenner–Schlumberger apparent resistivity data had shallower depths of investigation and revealed significantly less geologic detail than the profiles generated from the dipole–dipole surveys. The earth model from the pole–pole data had the greatest depth of investigation but low resolution and limited geologic detail when compared to the dipole–dipole survey.

Aquifer Characterization in the Blue Ridge Physiographic Province Using Resistivity Profiling and Borehole Geophysics: Geologic Analysis

Existing hydrogeological models used to describe the recharge and movement of groundwater in the Blue Ridge Province in the eastern United States generally rely on a simplified two-layer system consisting of regolith overlying a fractured bedrock aquifer. The regolith in these models is assumed to be relatively homogeneous and contains a water table aquifer that supplies water to the underlying variably fractured homogeneous crystalline bedrock. The interconnected fractures in the bedrock serve as conduits for predominantly downward vertical and limited horizontal flow. Recent acquisition of two-dimensional surface resistivity profiles and borehole geophysical logs has revealed new insights into this geologically complex province. The shallow regolith contains large unsaturated areas and also localized sand and clay prone facies with water table and confined aquifer conditions residing locally. Within the crystalline bedrock are anomalous lower resistivity intervals that are associated with ancient fault ...

Use of time–subsidence data during pumping to characterize specific storage and hydraulic conductivity of semi-confining units

Abstract A new graphical technique is developed that takes advantage of time–subsidence data collected from either traditional extensometer installations or from newer technologies such as fixed-station global positioning systems or interferometric synthetic aperture radar imagery, to accurately estimate storage properties of the aquifer and vertical hydraulic conductivity of semi-confining units. Semi-log plots of time–compaction data are highly diagnostic with the straight-line portion of the plot reflecting the specific storage of the semi-confining unit. Calculation of compaction during one-log cycle of time from these plots can be used in a simple analytical expression based on the Cooper–Jacob technique to accurately calculate specific storage of the semi-confining units. In addition, these semi-log plots can be used to identify when the pressure transient has migrated through the confining layer into the unpumped aquifer, precluding the need for additional piezometers within the unpumped aquifer or within the semi-confining units as is necessary in the Neuman and Witherspoon method. Numerical simulations are used to evaluate the accuracy of the new technique. The technique was applied to time–drawdown and time–compaction data collected near Franklin Virginia, within the Potomac aquifers of the Coastal Plain, and shows that the method can be easily applied to estimate the inelastic skeletal specific storage of this aquifer system.

Conceptual evaluation of regional ground-water flow in the carbonate-rock province of the Great Basin, Nevada, Utah, and adjacent states

The carbonate-rock province of the Great Basin, mainly in eastern Nevada and western Utah, is characterized by a thick sequence of Paleozoic rocks. Beneath the carbonate rocks are Cambrian elastics and Precambrian basement rocks. Since deposition, however, compression, extension, intrusive and volcanic episodes, and erosion have greatly modified the distribution and thickness of the carbonate rocks and emplaced a variety of rocks and deposits within and above the carbonate rocks. The most notable features are the normal faults caused by Tertiary extension which have formed the northto northeast-trending mountain ranges and adjacent basins that are partly filled with detritus from the mountains. Ground-water flow in the province was conceptualized as relatively shallow flow, primarily through basin-fill deposits and adjacent mountain ranges, superimposed over deeper flow through carbonate rocks. A computer model was developed to simulate this concept. The province was divided into cells with dimensions of 5 miles by 7.5 miles containing two model layers. The upper model layer was used to simulate flow primarily through basin-fill deposits and adjacent mountain ranges to depths of several thousand feet. The lower model layer was used to simulate deep flow beneath the basin fill and mountain ranges. The actual depth to the base of deep flow is unknown. The carbonate rocks may be as much as 30,000 feet thick, and freshwater has been detected to depths of as much as 10,000 feet. Several simplifying and necessary assumptions were made during the conceptualization and simulation of ground-water flow in the province: (1) Flow through fractures and solution openings in consolidated rocks is approximately equivalent to flow through a porous medium; (2) Darcys Law is applicable from a regional perspective; (3) ground-water flow is in a state of equilibrium where recharge equals discharge (prior to ground-water withdrawals), and most of the recharge occurs in or near the mountain ranges; (4) areal distribution of discharge is known as well as amount of discharge from regional springs; and (5) transmissivity is homogeneous and isotropic for an area that is represented by a model cell. Although the assumptions are probably valid for parts of the province, the validity of each assumption is unknown for its entirety. Therefore, the model results should be interpreted with caution and are considered to be conceptual. Transmissivity values in both model layers and vertical leakance between layers were adjusted during repeated simulations until computed water levels approximated the general water-level trends that were interpolated and extrapolated from measured water levels, and areas of simulated discharge approximated areas of known discharge. Because the amount of recharge and discharge are only approximately known and because water-level data are not available for large parts of the area, the computed transmissivity and leakance values are at best approximate. Nonetheless, several inferences can be made regarding flow in the province. Model-derived fluxes indicate that flow in the lower model layer can be divided into five deep-flow regions. These regions are named the (1) Death Valley, (2) Colorado River, (3) Bonneville, (4) Railroad Valley, and (5) Upper Humboldt River regions after the terminal sink for deep ground-water flow in each region. Superimposed over the 5 deep-flow regions are 17 shallow-flow regions as determined from model derived fluxes in the upper model layer. These regions approximately coincide with the delineations of flow systems from topography, water-level data, discharge areas, and water budgets. In general, the simulation of flow in the eastern and northern parts of the province is from south to north toward the Great Salt Lake Desert (Bonneville region) and the Humboldt River, elsewhere, flows are north to south toward either Death Valley or the Virgin and Colorado Rivers. Most of the ground-water discharge in the model simulation is due to evapotranspiration or springs prior to reaching the terminal sinks, particularly in the Colorado River region. This is probably due to low-permeability rocks and (or) basin fill within the area of the sinks. The estimate of recharge, as well as discharge, within the carbonate-rock province is 1.6 million acre-feet per year, which is about 3 percent of the estimated total precipitation. If local flow were included, the estimates of recharge and discharge would be considerably more. The transmissivity and leakance values were determined independently of the geology but are based on limited water-level data and the distribution and amount of recharge and discharge. However, simulation of flow in the upper model layer is generally away from or around outcrop areas of intrusive rocks, ancient volcanoes, Precambrian basement rocks, and fine—grained basin-fill deposits. Flow in the lower model layer is generally diverted around inferred low-permeability barriers as interpreted from aeromagnetic anomalies. Such anomalies may represent Precambrian basement rocks or granitic intrusions. Although the model simulation is entirely conceptual, this report presents estimates of the direction and magnitude of flow from recharge to discharge areas and discusses where the results agree and disagree with hypotheses and hydrologic estimates reported by other investigators. 1

The Influence of Geologic Structures on Deformation due to Ground Water Withdrawal

A 62 day controlled aquifer test was conducted in thick alluvial deposits at Mesquite, Nevada, for the purpose of monitoring horizontal and vertical surface deformations using a high-precision global positioning system (GPS) network. Initial analysis of the data indicated an anisotropic aquifer system on the basis of the observed radial and tangential deformations. However, new InSAR data seem to indicate that the site may be bounded by an oblique normal fault as the subsidence bowl is both truncated to the northwest and offset from the pumping well to the south. A finite-element numerical model was developed using ABAQUS to evaluate the potential location and hydromechanical properties of the fault based on the observed horizontal deformations. Simulation results indicate that for the magnitude and direction of motion at the pumping well and at other GPS stations, which is toward the southeast (away from the inferred fault), the fault zone (5 m wide) must possess a very high permeability and storage coefficient and cross the study area in a northeast-southwest direction. Simulated horizontal and vertical displacements that include the fault zone closely match observed displacements and indicate the likelihood of the presence of the inferred fault. This analysis shows how monitoring horizontal displacements can provide valuable information about faults, and boundary conditions in general, in evaluating aquifer systems during an aquifer test.

The Value of Subsidence Data in Ground Water Model Calibration

The accurate estimation of aquifer parameters such as transmissivity and specific storage is often an important objective during a ground water modeling investigation or aquifer resource evaluation. Parameter estimation is often accomplished with changes in hydraulic head data as the key and most abundant type of observation. The availability and accessibility of global positioning system and interferometric synthetic aperture radar data in heavily pumped alluvial basins can provide important subsidence observations that can greatly aid parameter estimation. The aim of this investigation is to evaluate the value of spatial and temporal subsidence data for automatically estimating parameters with and without observation error using UCODE-2005 and MODFLOW-2000. A synthetic conceptual model (24 separate cases) containing seven transmissivity zones and three zones each for elastic and inelastic skeletal specific storage was used to simulate subsidence and drawdown in an aquifer with variably thick interbeds with delayed drainage. Five pumping wells of variable rates were used to stress the system for up to 15 years. Calibration results indicate that (1) the inverse of the square of the observation values is a reasonable way to weight the observations, (2) spatially abundant subsidence data typically produce superior parameter estimates under constant pumping even with observation error, (3) only a small number of subsidence observations are required to achieve accurate parameter estimates, and (4) for seasonal pumping, accurate parameter estimates for elastic skeletal specific storage values are largely dependent on the quantity of temporal observational data and less on the quantity of available spatial data.

An integrated risk assessment model of township-scaled land subsidence based on an evidential reasoning algorithm and fuzzy set theory.

Land subsidence risk assessment (LSRA) is a multi-attribute decision analysis (MADA) problem and is often characterized by both quantitative and qualitative attributes with various types of uncertainty. Therefore, the problem needs to be modeled and analyzed using methods that can handle uncertainty. In this article, we propose an integrated assessment model based on the evidential reasoning (ER) algorithm and fuzzy set theory. The assessment model is structured as a hierarchical framework that regards land subsidence risk as a composite of two key factors: hazard and vulnerability. These factors can be described by a set of basic indicators defined by assessment grades with attributes for transforming both numerical data and subjective judgments into a belief structure. The factor-level attributes of hazard and vulnerability are combined using the ER algorithm, which is based on the information from a belief structure calculated by the Dempster-Shafer (D-S) theory, and a distributed fuzzy belief structure calculated by fuzzy set theory. The results from the combined algorithms yield distributed assessment grade matrices. The application of the model to the Xixi-Chengnan area, China, illustrates its usefulness and validity for LSRA. The model utilizes a combination of all types of evidence, including all assessment information--quantitative or qualitative, complete or incomplete, and precise or imprecise--to provide assessment grades that define risk assessment on the basis of hazard and vulnerability. The results will enable risk managers to apply different risk prevention measures and mitigation planning based on the calculated risk states.

A New Zonation Algorithm with Parameter Estimation Using Hydraulic Head and Subsidence Observations

Parameter estimation codes such as UCODE_2005 are becoming well-known tools in groundwater modeling investigations. These programs estimate important parameter values such as transmissivity (T) and aquifer storage values (Sa ) from known observations of hydraulic head, flow, or other physical quantities. One drawback inherent in these codes is that the parameter zones must be specified by the user. However, such knowledge is often unknown even if a detailed hydrogeological description is available. To overcome this deficiency, we present a discrete adjoint algorithm for identifying suitable zonations from hydraulic head and subsidence measurements, which are highly sensitive to both elastic (Sske) and inelastic (Sskv) skeletal specific storage coefficients. With the advent of interferometric synthetic aperture radar (InSAR), distributed spatial and temporal subsidence measurements can be obtained. A synthetic conceptual model containing seven transmissivity zones, one aquifer storage zone and three interbed zones for elastic and inelastic storage coefficients were developed to simulate drawdown and subsidence in an aquifer interbedded with clay that exhibits delayed drainage. Simulated delayed land subsidence and groundwater head data are assumed to be the observed measurements, to which the discrete adjoint algorithm is called to create approximate spatial zonations of T, Sske , and Sskv . UCODE-2005 is then used to obtain the final optimal parameter values. Calibration results indicate that the estimated zonations calculated from the discrete adjoint algorithm closely approximate the true parameter zonations. This automation algorithm reduces the bias established by the initial distribution of zones and provides a robust parameter zonation distribution.

Assessing Hydrofracing Success from Earth Tide and Barometric Response

Identifying fracture pathways and connectivity between adjacent wells is vital for understanding flow characteristics, transport properties, and fracture characteristics. In this investigation, a simple, straightforward methodology is presented for assessing hydrofracing success and identifying possible fracture connectivity between neighboring boreholes, using water-level barometric response and tide signatures of individual fractures in a crystalline-rock setting. Water levels and barometric pressure heads were collected at two wells 27 m apart both prior to, and after, hydrofracing one of the wells at the fractured-rock research site in Floyd County, Virginia. Vastly different barometric and tidal signatures existed at the two wells prior to hydrofracing as well EX-1 had no discernable fractures, while W-03 was connected to an identified fault-zone aquifer and produced a notable water-level earth tide and barometric signatures. After hydrofracing EX-1, new fractures were induced and the resulting water-level tidal signature and barometric efficiencies were nearly identical to the W-03 well. Aquifer testing conducted from both wells verified this connectivity along the fault-zone aquifer. The small phase difference between the tidal responses in the two wells can be accounted for by the calculated differences in transmissivity and casing diameter.

Combining periodic hydraulic tests and surface tilt measurements to explore in situ fracture hydromechanics

Fractured bedrock reservoirs are of socio-economical importance, as they may be used for storage or retrieval of fluids and energy. In particular, the hydromechanical behavior of fractures needs to be understood as it has implications on flow and governs stability issues (e.g., microseismicity). Laboratory, numerical, or field experiments have brought considerable insights to this topic. Nevertheless, in situ hydromechanical experiments are relatively uncommon, mainly because of technical and instrumental limitations. Here we present the early stage development and validation of a novel approach aiming at capturing the integrated hydromechanical behavior of natural fractures. It combines the use of surface tiltmeters to monitor the deformation associated with the periodic pressurization of fractures at depth in crystalline rocks. Periodic injection and withdrawal advantageously avoids mobilizing or extracting significant amounts of fluid, and it hinders any risk of reservoir failure. The oscillatory perturbation is intended to (1) facilitate the recognition of its signature in tilt measurements and (2) vary the hydraulic penetration depth in order to sample different volumes of the fractured bedrock around the inlet and thereby assess scale effects typical of fractured systems. By stacking tilt signals, we managed to recover small tilt amplitudes associated with pressure-derived fracture deformation. Therewith, we distinguish differences in mechanical properties between the three tested fractures, but we show that tilt amplitudes are weakly dependent on pressure penetration depth. Using an elastic model, we obtain fracture stiffness estimates that are consistent with published data. Our results should encourage further improvement of the method.