David P. Lesmes

Influence of pore fluid chemistry on the complex conductivity and induced polarization responses of Berea sandstone

The spectral induced-polarization (IP) response of rocks and soils is a complex function of pore solution chemistry, sample microgeometry, and surface chemical properties. We measure the complex conductivity and the time domain IP responses of Berea sandstone as a function of pore fluid ionic strength and pH. Complex conductivity is measured over the frequency range 10−3 to 106 Hz, and chargeability is computed using a time window of 0.16 to 1.74 s. The field IP parameters: phase, percent frequency effect, and chargeability are functions of both the surface and bulk electrical properties of the sample and are observed to decrease with increasing solution conductivity. Dividing these parameters by the sample resistivity yields normalized IP parameters (quadrature conductivity, metal factor, normalized chargeability) that are proportional to the imaginary component of the complex surface conductivity. Normalized IP parameters increase with ionic strength up to concentrations of 10−1 M NaCl and show a reduced response at pH 3, the point of zero charge for quartz-dominated systems. For concentrations >10−1 M NaCl, the normalized parameters decrease with increasing concentration. This decrease in surface polarization may indicate a decrease in the effective mobility of polarizing charges at high solution concentration. Our data indicate that normalized IP parameters are directly related to the physiochemical parameters that control the surface conductivity responses of rocks and soils. Normalization of IP measurements in environmental investigations should increase the effectiveness of IP surveys, especially in high-conductivity environments.

IP interpretation in environmental investigations

The induced polarization (IP) response of rocks and soils is a function of lithology and fluid conductivity. IP measurements are sensitive to the low‐frequency capacitive properties of rocks and soils, which are controlled by diffusion polarization mechanisms operating at the grain‐fluid interface. IP interpretation typically is in terms of the conventional field IP parameters: chargeability, percentage frequency effect, and phase angle. These parameters are dependent upon both surface polarization mechanisms and bulk (volumetric) conduction mechanisms. Consequently, they afford a poor quantification of surface polarization processes of interest to the field geophysicist. A parameter that quantifies the magnitude of surface polarization is the normalized chargeability, defined as the chargeability divided by the resistivity magnitude. This parameter is proportional to the quadrature conductivity measured in the complex resistivity method. For nonmetallic minerals, the quadrature conductivity and normalize...

Dielectric spectroscopy of sedimentary rocks

A physiochemical model for the complex dielectric response of sedimentary rocks is used to invert broadband dielectric spectra for effective grain size distributions. The complex dielectric response of each grain within the “water-wet” granular matrix is obtained by superimposing the polarization of the electrochemical double layer, which is assumed to surround each grain, with the complex dielectric response of the dry mineral grain. The effective complex dielectric response of the water-wet matrix (grains and surface phase) is obtained by volume averaging over the entire distribution of particle sizes. The complex dielectric response of the total mixture (water-wet matrix and bulk pore solution) is obtained using the Bruggeman-Hanai-Sen effective medium theory. Studies of Berea sandstone show that the grain size distribution, obtained by inverting the real part of the complex dielectric spectra, is similar to the grain size distribution obtained from optical images of the sample in thin section. The current model, however, does not account for surface roughness effects or the polarization of counterions over multiple grain lengths; therefore the grain size distribution obtained by dielectric spectroscopy is broader than the image-derived distribution. The dielectric-derived grain size distribution can be fit with two separate power laws that crossover at R ≅ 1 μm, which corresponds to a relaxation frequency of 2 kHz. The low-frequency dielectric response (f 2 kHz) is primarily controlled by the clay size grains and surface roughness, which has a fractal dimension of d = 2.48±0.07. At very low frequencies (f < 0.1 Hz) the dielectric response appears to be controlled by the electrochemical polarization of counterions over multiple grain lengths. A more general model should account for the effects of surface roughness and grain interactions on the dielectric response. It would also be useful to develop a simplified version of this model, perhaps similar in form to the empirically derived Cole-Cole response, which could be more easily used to model and interpret electrical geophysical field surveys (e.g., induced-polarization, ground-penetrating radar, and time domain reflectometery measurements).

Textural controls on low-frequency electrical spectra of porous media

The results from several laboratory studies of the relationships between electrical polarization and physical properties of porous media have prompted interest in the potential use of low-frequency electrical spectra to qualitatively or quantitatively map variation in hydrogeologic properties in the field. Compiling several published and unpublished data sets, supported by new measurements, we have examined the low-frequency electrical spectra of a range of natural and artificial porous media to assess the generality of proposed relationships between electrical and physical properties. Our work confirms a significant positive correlation between the magnitude of electrical polarization (quantified as imaginary conductivity at a specific frequency) and the surface-area/pore-volume ratio Spor . Analyzing the parameters of ageneralized Cole-Cole resistivity relaxation model fitted to many electrical spectra, we observe two apparent controls on the electrical relaxation. For samples with abundant relatively l...

Induced-polarization detection and mapping of contaminant plumes

Several laboratory and scaled model investigations suggest that organic contaminants affect the surface electrical properties of exposed soils/rocks and therefore produce measurable induced polarization IP signatures. However, there is little field evidence of an IP methodology for contaminant mapping. A2D time-domain IP method is developed for mapping the FS12 contaminant plume at the Massachusetts Military Reservation MMR located in Cape Cod, Massachusetts. The FS-12 plume consists of approximately 265 m 3 of fuel that erupted from a broken underground pipeline in the early 1970s. Benzene and ethylene dibromide EDB are the primary contaminants at FS-12, with concentrations exceeding the allowed maximum concentration levels MCL, while other constituents of the plume did not exceed their MCL. Therefore, the contaminants of interest are benzene and EDB, partly because of their health risk and partly because they present the highest concentrations 2400 and 1000 g/L, respectively among the plume constituents and are therefore more likely to be related to the polarization source. IP data were acquired along a survey line that partially transects the plume extending over contaminated and uncontaminated zones and were inverted to give 2D resistivity and chargeability plots to 100 m depth and a horizontal extent of 400 m. By separately inverting IP data derived from time windows located at short and long decay times, a timedomain gross spectral chargeability difference is produced. Both the chargeability and gross spectral chargeability difference show good agreement with the known location of the plume from monitoring wells, with the IP chargeability section suggesting contaminant distribution detail that cannot otherwise be inferred from the sparse borehole distribution.

Geophysics at the interface: Response of geophysical properties to solid‐fluid, fluid‐fluid, and solid‐solid interfaces

Laboratory studies reveal the sensitivity of measured geophysical properties to solid-fluid, fluid-fluid, and solid-solid interfaces in granular and fractured materials. In granular materials, electrical properties and nuclear magnetic resonance relaxation times exhibit a strong dependence on the size and properties of the solid-fluid interface. The electrical and seismic properties of granular materials and the seismic properties of fractured materials reveal a dependence on the size or geometry of fluid-fluid interfaces. Seismic properties of granular and fractured materials are affected by the effective stress and cementing material at solid-solid interfaces. There have been some recent studies demonstrating the use of field-scale measurements to obtain information about pore-scale interfaces. In addition, a new approach to geophysical field measurements focuses on the geophysical response of the field-scale interface itself, with successful applications in imaging the water table and a redox front. The observed sensitivity of geophysical data to interfaces highlights new ways in which geophysical measurements could be used to obtain information about subsurface properties and processes.

Exploiting the temperature effects on low frequency electrical spectra of sandstone: A comparison of effective diffusion path lengths

A number of recent investigations have highlighted the potential value of using relaxation times derived from electrical spectra to infer key physical properties of permeable rocks. To date, most studies have assumed a grain size or pore throat as a measure of the length scale of the ionic diffusive process, although this has been challenged in recent experimental investigations. We compare the electrical spectra of three sandstones, adopting a new approach in which the temperature of the rock samples is perturbed and the relaxation time measured as a function of temperature. Our results suggest that, for the sandstones tested here, the effective diffusion coefficient should be considered as a function of the electrical tortuosity. These findings may help explain the apparent long relaxation times observed in low-permeability rocks in recent experimental studies. We also highlight the need to account for temperature in related studies of electrical spectra.

A multiscale radar‐stratigraphic analysis of fluvial aquifer heterogeneity

The objective of this study is to use an integrated and multiscale exploration approach to quantify the geometrical parameters that are needed to predict block hydraulic conductivity tensors within fluvial deposits. We use 3‐D ground‐penetrating radar (GPR) data, electromagnetic (EM) surveys, resistivity soundings, and hand‐augered borings to characterize the 3‐D architecture of fluvial deposits on a floodplain of the Piscatiquis River, near South Sebec, Maine. Field‐scale surveys made across the entire floodplain were used to map the depth to the glacially eroded bedrock surface and the thickness of the overlying sediments, which consist of glaciomarine clay, fluvial sands and silts, and overbank deposits. An EM conductivity anomaly in the center of the floodplain defines the position of a ridge in the glaciomarine clay deposit. This ridge separates horizontally bedded sand and silt deposits in the northern half of the floodplain from inclined point‐bar sand and silt deposits in the southern half of the ...

Seismic Measurements of Ground Displacements Caused by Trains and Quarry Blasts at the Superconducting Super Collider Site in Texas

Ground vibrations caused by cultural noise sources such as road traffic, trains, and quarry blasts can significantly affect sensitive scientific experiments and manufacturing processes. When constructing a facility that requires ultra-stable ground conditions it is prudent to first measure the level of background noise at the proposed site. In this study, seismic measurements are used to measure near-surface ground vibrations at the superconducting super collider (SSC) site in the state of Texas. The SSC is an oval shaped ring, which is 85km in circumference. A design limit of the SSC is that the ground vibrations should be less than 12.7μm (5.0×10−4inches) in amplitude. Seismic measurements of ground velocity were made, both at the surface and within boreholes, at several sites around the proposed SSC ring. Through a series of data processing steps the ground velocity measurements were transformed into ground displacements. Ground displacements caused by quarry blasts were sufficiently small at the SSC r...

INDUCED POLARIZATION WITH ELECTROMAGNETIC COUPLING: 3D SPECTRAL IMAGING THEORY: EMSP PROJECT NO. 73836

The principal objective of the project was to develop a non-invasive imaging technique, based on spectral induced polarization (SIP), to characterize in-situ distribution of organic and inorganic contaminants. This was to be an advance over a similar technique offered by the DC resistivity method. The motivation for the choice of IP over resistivity is rooted in the fact that resistivity response is governed by volume distributions of electrical parameters and therefore is relatively insensitive to small changes contributed by the presence of contaminants. IP response on the other hand is governed by the electrochemical properties of the rock-grain pore-fluid interface, which can be significantly altered by the incoming contaminant (ions) over long residence times. Small concentrations of contaminants are the rule rather than the exception thus, the detection threshold for IP, which is more sensitive to small concentrations, is much lower than for resistivity (IP field threshold for PCE/TCE is about 1mg/g). Additionally, the observation that IP depends on the chemistry of the contaminants provided the motivation that a spectral IP response could lead to a database of identifying signatures by which contaminants can be discriminated.