H23C-1509
Using Earthquake-triggered Groundwater Fluctuations to Estimate the Anomalous Stress- source Area
Earthquakes are usually triggered in the stress-concentrated area. To explain the stress-pore pressure relationship, poroelastic theory is one of the commonly used models. In this model, stress and pore pressure are coupled. We applied an analytical solution of the poroelastic model with point force source to estimate the stress-concentrated area based on earthquake-triggered groundwater fluctuations. A classification system of groundwater level anomalies is developed and step groundwater fluctuations are used for data analysis. The collected anomalous data shows that the detectable distance of groundwater level anomaly increases as the earthquake magnitude increases. The maximum detectable distance is about 250 km and the minimum detectable earthquake magnitude is about 5.0 based on the groundwater fluctuations. Semi-analytical and empirical models are also developed to construct the relations among earthquake magnitude, epicentral distance, and groundwater level anomaly. Using the principle of superposition, the stress-concentrated areas can be estimated from two earthquake events in Taiwan. The results show that earthquake-triggered groundwater fluctuations can be a potential tool to estimate the stress-concentrated area and may be used to indicate the possible area of an epicenter.
H23C-1510
Detection and Localization of Hydromechanical Disturbances in a Sandbox using the Self-potential Method
The self-potential response during hydromechanical disturbance of water-infiltrated porous material was investigated in the laboratory. Foursandbox experiments were performed to understand the electrokinetic response associated with the injection of a pulse of water and the abstraction of a small volume of pore water. The resulting self-potential signals are measured using 32 Ag/AgCl very sensitive and non-polarising medical electrodes. The injected/abstracted volumes of water are responsible for hydro-mechanical disturbances. In turn, these disturbances generate dipolar electrical anomalies of electrokinetic nature with a distinct electrical signature and an amplitude of few V. The source function is a product of a dipolar Green's function and a source intensity function that depends solely on the product between the streaming potential coupling coefficient of the sand and the pore fluid overpressure with respect to the hydrostatic pressure. Numerical modeling is performed to solve the coupled hydro-mechanical problem and to determine the distribution of the resulting streaming potential in the course of the experiments. We use 2D and 3D algorithms based on the intercorrelation method and wavelet analysis of potential fields to show that the source corresponds to a vertical dipole. These methods are also used to localize the position of the source of the hydromechanical disturbance from the self-potential collected at the top surface of the tank. Applications concern the detection and localization of fracturing in active volcanoes.
H23C-1511
The Effects of Gravity-Induced Stresses on Hydrogeologic Properties Near Ridges: Examples From two Rift Basins
Twenty Mesozoic rift basins have been identified along the northeast coast of North America. At two of these, the Minas Basin in Nova Scotia and the Newark Basin in New York, steep topographic ridges formed initially by extensional tectonic processes have been evaluated for groundwater potential by obtaining geophysical logs in nearby water wells. The two locations have common lithologic and topographic characteristics, whereby a basalt cap overlying sedimentary rocks is exposed along a ridge flank. Despite these similarities, the basalts in Nova Scotia serve as a local water supply whereas the wells in New York are non-producing. Analyses of acoustic televiewer logs and measurements from flowmeter/pumping tests locate and distinguish the permeable fractures from the general fracture population. In addition, a complementary finite-element stress model is used to simulate ridge geometry from topographic maps and incorporates rock elastic properties as determined from processing full-waveform sonic logs. Results derived from field data and modeling indicate that gravity-induced stresses near ridges result from a lack of buttressing along the free faces and are amplified by spreading effects due to a contrast in Poisson's ratio between the basalts and the sedimentary rocks. These conditions act to enhance the permeability of fractures striking subparallel to the ridge axis in Nova Scotia. Conversely, the predominant fracture population in New York strikes perpendicular to the ridge axis and to the maximum horizontal principal stress. Consequently, there are few fractures preferentially aligned with the stress field and no hydraulically conductive features are recognized in these rocks. The hydrogeologic properties of the basalts near the ridge flanks at these two sites are markedly different because of the orientation of pre-existing fractures and their response to imposed stresses as derived from topography and lithostratigraphy.
H23C-1512
Evaluation of the Effects of Cementation on Specific Storage of Granular Porous Media Using Discrete Element Models
In recent years, population growth, climate change and surface water contamination have resulted in increased development of deep groundwater resources. As this trend continues, quantitative data of aquifer properties (i.e. specific storage) will be crucial for ensuring the sustainability of groundwater resources. Specific storage is the volume of water expelled due to the compressibility of the surrounding granular skeleton. The compressibility is believed to be controlled by the microscale properties of the surrounding granular skeleton (e.g. grain size, shape and mineralogy) and cementation. In general, cementation is assumed to reduce specific storage by reducing compressibility. However, data which adequately quantifies the relationship between cementation and storage does not exist due to unknown variations in mineralogy, amount and distribution of the cement. The goal of this research is to address this problem by quantifying the effects of cementation of granular porous media on specific storage using discrete-based numerical models. We modeled the modification of specific storage by varying microscale parameters (cement stiffness and percent of cementation) in a series of discrete element models (DEM). Modifications to microscale properties produce bulk-scale responses that are represented in a series of stress/strain curves for each model. The stress/strain curves are used to determine the stiffness for each model and to calculate percent of change in specific storage as a function of the microscale properties. Additionally, we are fabricating analog granular assemblages which will be used to better understand fluid storage in weakly cemented clastic materials. The assemblages will be used to evaluate the differences between natural and synthetic systems and to improve and interpret the DEM models. Preliminary results suggest that cementation has a strong control on the elastic storage properties of cemented granular media. We present results which quantify the proportion of cement needed to effectively reduce the specific storage. We anticipate being able to use the bulk-scale results from the DEM models as an input parameter for continuum-based coupled fluid-solid deformation models. The coupled models will allow us to further investigate elastic and inelastic poroelastic properties, (specific storage and aquifer compaction, respectively), with implications for carbon sequestration during which reactive transport (i.e. precipitation) is assumed to be important for fluid storage.
H23C-1513
Conditions of Fissuring in a Pumped-Faulted Aquifer System
Earth fissuring associated with subsidence from groundwater pumping is problematic in many arid-zone heavily pumped basins such as Las Vegas Valley. Long-term pumping at rates considerably greater than the natural recharge rate has stressed the heterogeneous aquifer system resulting in a complex stress-strain regime. A rigorous artificial recharge program coupled with increased surface-water importation has allowed water levels to appreciably recover, which has led to surface rebound in some localities. Nonetheless, new fissures continue to appear, particularly near basin-fill faults that behave as barriers to subsidence bowls. The purpose of this research is to develop a series of computational models to better understand the influence that structure (faults), pumping, and hydrostratigraphy has in the generation and propagation of fissures. The hydrostratigraphy of Las Vegas Valley consists of aquifers, aquitards and a relatively dry vadoze zone that may be as thick as 100m in much of the valley. Quaternary faults are typically depicted as scarps resulting from pre- pumping extensional tectonic events and are probably not responsible for the observed strain. The models developed to simulate the stress-strain and deformation processes in a faulted pumped aquifer-aquitard system of Las Vegas use the ABAQUS CAE (Complete ABAQUS Environment) software system. ABAQUS is a sophisticated engineering industry finite-element modeling package capable of simulating the complex fault- fissure system described here. A brittle failure criteria based on the tensile strength of the materials and the acting stresses (from previous models) are being used to understand how and where fissures are likely to form. , Hypothetical simulations include the role that faults and the vadose zone may play in fissure formation