NS13A-01
A Comparison Between Deterministic and Stochastic Methods for Inverting Spectral Induced Polarization Data
Cole-Cole model parameters (e.g., chargeability and time constant), extracted from spectral induced polarization (SIP) data, are being increasingly used to characterize subsurface properties. However, fitting Cole-Cole models (especially nested Cole-Cole models) to SIP data is challenging because of nonlinearity and non-uniqueness of the Cole-Cole models. This study compares conventional deterministic approaches (i.e., iterative based estimation methods) with Markov chain Monte Carlo (MCMC) based stochastic approaches for estimating Cole- Cole model parameters. The results of those case studies show that although deterministic methods are able to provide single optimal solutions under certain criteria (e.g., the least squares of misfit) and require minimal computing power, they suffer from two main limitations. The first limitation is that the optimal solutions heavily depend on the choice of the initial values. Different initial values may yield different inversion results, and in many cases, the deterministic methods even cannot converge for the chosen initial values. The second limitation is that those methods provide inadequate or inaccurate information about uncertainty in the estimation. On the contrary, the MCMC-based stochastic approaches are insensitive to the choice of the initial values and can provide extensive information about uncertainty in the estimation. From the drawn large number of samples, we can obtain exhaustive information about unknown parameters, such as the mean, the median, the mode, and even entire probability distribution of each unknown Cole-Cole model parameter. Although MCMC-based stochastic methods typically require that the forward models be run for thousands of times, this is not an issue given the current computer power. Through presentation of extensive synthetic and laboratory case studies, we will illustrate the benefits of the different methods when used individually and in combination with each other.
NS13A-02
Spectral Induced Polarization of Sandstones: Temperature Effects
There is growing interest in the use of spectral induced polarization (SIP) for a wide range of environmental applications, in particular those focused on hydrogeological investigations. Recent experimental work has demonstrated that the mean relaxation time of electrical impedance spectra measured in sandstones is linked to the grain surface area and strongly correlated to some measure of a dominant pore throat size. Such empirically derived relationships lead to potential models of SIP - hydraulic conductivity, which has immense value for the hydrological community. Furthermore, the links between surface area and electrical response may lead to other, equally exciting, applications, such as in characterizing geochemical reactivity of sediments. However, there is a need to understand the fundamental behavior of SIP in such porous media in order for such models to be applied usefully. In an attempt to address this, we focus here on the influence of temperature on the SIP behavior of a range of sandstones. Classical models of dielectric dispersion in colloids have proposed direct inverse relationships between relaxation time and temperature. Through a series of experimental trials we have studied this behavior: examining the impedance spectra (in the 1 mHz to 1 kHz range) of four different sandstones over a temperature range of 5 to 30 degrees Celsius. Analysis of the spectra with the widely used Pelton Cole-Cole model has confirmed hypothesized effects on a mean relaxation time but revealed that the responses to temperature change is a function of physical properties of the sandstone. In addition, the analysis has illustrated how temperature effects on surface complex conductivity of the sandstones differ as a function of pore fluid and formation factor. The results add to the growing experimental evidence of controls on spectral impedance in porous media and help ascertain generalized petrophysical models for a wide range of applications.
NS13A-03
Low Frequency Electrical Responses From Simulated ZVI PRBs With High Carbonate Concentration
We conducted column experiment to investigate the electrical signatures from iron corrosion and mineral precipitation occurring in two simulated PRBs with high carbonate groundwater conditions. Our results reveal consistent increase of electrical resistivity and decrease of polarization magnitude for all the columns. Solid phase analysis identified calcium carbonate (calcite and aragonite) being a major mineral phase at all the channels with magnetite being another major phase at the 2-3 channels closest to the influent ends. Our electrical data correlate well with the reaction process and suggested calcium carbonate being the major mineral phase controlling the change of electrical signatures in a system with both calcium carbonate and iron oxides being major phases. Our results demonstrate that electrical methods have sufficient sensitivity in indirectly sensing the iron corrosion and mineral precipitation process occurring in ZVI PRBs and are potential tools for long term performance monitoring of PRBs.
NS13A-04 INVITED
The Contribution of Robert F. Corwin to Self-Potential and Geotechnical Geophysics
Throughout his career, Robert F. Corwin developed innovative geophysical methods to solve geotechnical problems. Most notable is his work on self-potential (SP) where his focus was a blend of electricity and water, a potentially lethal brew, to solve very practical problems. Corwin's work in SP started with the idea of applying the technique to marine mineral exploration; this early work is characterized by a theme that ran through his career: understanding the effects that can influence measurements, developing methodologies to obtain consistent and reliable data, and interpreting those data in a conservative and believable manner supported by the facts. He expanded the electricity-water connection to geothermal fluids and the SP signals produced by them. He was involved in geothermal exploration throughout the western U.S. including Alaska and Mexico. In addition to developing reliable field techniques he worked on interpretational methods that made SP interpretation quantitative. Corwin's most significant contribution was the study of leaky dams using SP. Water leakage produces an SP anomaly because of the electrokinetic properties of geologic materials. Through a series of SP studies for the U.S. Bureau of Reclamation and the U.S. Army Corps of Engineers he developed a methodology for making and interpreting SP measurements that helped locate, assess, and remediate leakage. This success led to numerous surveys throughout Canada for regional power authorities. Corwin returned to marine geophysical studies throughout his career including SP measurements to locate moveable concrete mats placed in the Mississippi River to control bank erosion. Because of changes in river flow, these large articulated mats were often undercut, moved, and reburied causing hazardous bank conditions. SP and electrical resistivity measurements were found to accurately locate the mats. Corwin also worked on electrical resistivity measurements of the ocean floor. Starting with stationary seafloor measurements to characterize bottom cover he eventually developed a moving measurement system, which private companies routinely use to survey routes and potential hazards on the ocean floor before they lay deep ocean cables.
NS13A-05 INVITED
Tomography of ground water flow from self-potential data
An inversion algorithm is developed to interpret self-potential (SP) data in terms of distribution of the seepage velocity of the ground water. The model is based on the proportionality existing between the electrokinetic source current density and the seepage velocity of the water phase. As the inverse problem is underdetermined, we use a Tikhonov regularization method with a smoothness constraint based on the differential Laplacian operator to solve the inverse problem. The regularization parameter is determined by the L-shape method. The recovery of the distribution of the seepage velocity vector of the ground water flow depends on the localization and number of non-polarizing electrodes and information relative to the distribution of the electrical resistivity of the ground. The inversion method is tested on two 2D synthetic cases and on two real SP data. The first field test corresponds to the infiltration of water from a ditch. The second one corresponds to large flow at the Cerro Prieto geothermal field in Baja California.
NS13A-06
3-D Inversion of Self-Potential Data to Recover Hydraulic Head
The self-potential method may be used to assess subsurface flow conditions in response to the electrokinetic phenomenon of streaming potential, where fluid flow through porous media generates electrical current flow. Of particular interest is inversion of the electrical potential data to evaluate the 3-D distribution of hydraulic head. As a first step, we have developed an efficient 3-D forward modelling algorithm that enables us to determine the SP distribution resulting from a saturated, or variably saturated, flow model and a known distribution of electrical properties, namely the electrical conductivity and cross-coupling conductivity coefficient. The study region is divided into a discrete rectangular mesh, in which hydraulic head and electrical properties are defined for each cell. Discrete equations are formulated using the method of finite volumes and the forward problem, which is a linear relationship between the electrical potentials and hydraulic head, is solved using a preconditioned biconjugate gradient stabilized method. In the inverse problem we are supplied with measured electrical potentials and our goal is to estimate the causative 3-D model of hydraulic head. We assume fully saturated conditions, such that the electrical properties are not a function of hydraulic head. The earth model is divided into rectangular cells, each of which is assigned constant values of the electrical properties. In practice, these must be obtained through additional analysis, but the cross-coupling conductivity coefficient may usually be estimated since it varies over a small range, and electrical conductivity may be characterized using DC resistivity survey techniques. Since the number of available SP data is usually less than the number of grid cells we are faced with a typical underdetermined inverse problem. We solve the inverse problem by minimizing an objective function that consists of a data misfit and a model objective function. The data misfit is weighted according to the measurement error. A priori information is incorporated into the solution via the model objective function. We use a Tichonov-type regularization technique to balance fitting the data with fitting a chosen reference head model. The algorithm is tested using measured data collected in a laboratory tank experiment that simulates flow under a cut-off wall.
NS13A-07 INVITED
A Description of Seismo-electric and Electro-seismic Coupling
In this invited overview, the basic mechanisms involved in seismo-electric and electro-seismic coupling are presented. The coupling mechanism is electrokinetic, meaning it is due to the nanoscale charge separation present at the water/mineral-grain interface inside of rocks and soils. Two types of seismo-electric response may be distinguished: (1) the electric field contained within a seismic wave and that moves along with the wave as part of the material response but that has no support outside the seismic wave; and (2) the electric field generated when a seismic wave traverses an interface, resulting in the symmetry of the charge separation within the wave to be broken, thus creating a far field quasi-static electric field that falls off like a dipole. It is the interface response that allows interfaces within the earth to be imaged. In electro-seismic response, a quasi-static electric field is established by injecting a time-varying current into the earth. This electric field drives a fluid flow via electro-osmosis. Where the electric field traverses an interface, more fluid may be driven into the interface from one side than can be removed from the other side. This results in fluid accumulation and dilation on one side of the interface, and fluid depletion and contraction on the other. Such a time-varying mechanical dipole creates a seismic wave. Material property dependencies of these couplings will be discussed along with interesting potential applications of the couplings. Numerical modeling of these effects will be presented along with both qualitative and mathematical descriptions of the mechanisms.
NS13A-08 INVITED
Exploring Hydrological and Biogeochemical Processes Associated With Remedial Treatments Using Geophysical Methods
Many remediation approaches induce biogeochemical transformations in subsurface systems, such as mineral dissolution and precipitation, gas evolution, and biomass generation. These processes can in turn alter the permeability and porosity of a subsurface system. Although understanding how hydrological and biogeochemical properties change over space and time in response to remedial treatments is critical for developing and predicting effective remediation strategies, it is hindered by our inability to monitor these processes with sufficient resolution over field-relevant scales. Recent advances in hydrogeophysics and biogeophysics have illustrated the potential that geophysical methods have for characterizing subsurface hydrological properties and for indicating biogeochemical changes associated with remedial treatments. Here, we investigate hydrological and biogeochemical processes associated with laboratory and field bioremediation experiments using advanced characterization and monitoring approaches. Specifically, we explore: the utility of using time-lapse geophysical methods for quantifying biogeochemical reaction end-products; the influence of initial hydrogeochemical heterogeneity on the spatiotemporal distribution of biogeochemical transformations; and subsequent alterations of the flow field caused by the transformations. Our experimental and numerical studies confirm a close coupling between hydrological and biogeochemical processes at both laboratory and field scales, and suggest that geophysical methods have the potential to provide insights into the complex processes in a manner that should be useful for guiding remedial treatments.