Lee Slater

Cross-hole electrical imaging of a controlled saline tracer injection

Electrical imaging of tracer tests can provide valuable information on the spatial variability of solute transport processes. This concept was investigated by cross-borehole electrical imaging of a controlled release in an experimental tank. A saline tracer (conductivity 8×103 ms/m volume 270 l) was injected into a tank facility (dimensions 10×10×3 m) consisting of alternating sand and clay layers. Injection was from 0.3 m below the surface, at a point where maximum interaction between tank structure and tracer transport was expected. Repeated imaging over a two-week period detected non-uniform tracer transport, partly caused by the sand/clay sequence. Tracer accumulation on two clay layers was observed and density-driven spill of tracer over a clay shelf was imaged. An additional unexpected flow pathway, probably caused by complications during array installation, was identified close to an electrode array. Pore water samples obtained following termination of electrical imaging generally supported the observed electrical response, although discrepancies arose when analysing the response of individual pixels. The pixels that make up the electrical images were interpreted as a large number of breakthrough curves. The shape of the pixel breakthrough-recession curve allowed some quantitative interpretation of solute travel time, as well as a qualitative assessment of spatial variability in advective-dispersive transport characteristics across the image plane. Although surface conduction effects associated with the clay layers complicated interpretation, the plotting of pixel breakthroughs was considered a useful step in the hydrological interpretation of the tracer test. The spatial coverage provided by the high density of pixels is the factor that most encourages the approach.

Relationship between spectral induced polarization and hydraulic properties of saturated and unsaturated sandstone

There is growing interest in the use of geophysical methods for hydrological model parameterization. Empirical induced polarization (IP)–hydraulic conductivity (K) relationships have been developed, but these are only applicable to sediments in which the IP response shows limited variation with electrical current frequency. Here we examine the spectral IP response of samples taken from a UK sandstone aquifer and compare measured parameters with physical and hydraulic properties. We demonstrate the limited value of existing IP-K models due to the inherent IP frequency dependence of these samples. Our results show how the mean relaxation time, τ, is a more appropriate measure of IP response for these sediments. A significant inverse correlation between the surface area to pore volume ratio and τ is observed, suggesting that τ is a measure of a characteristic hydraulic length scale. This is supported by a measured strong positive correlation between log τ and log K. Our measurements also reveal evidence of a relationship between τ and a dominant pore throat size, which leads to postulations about the parallelism between the spectral IP behavior and unsaturated hydraulic characteristics. Additional experiments show how the relaxation time is affected by degree of fluid saturation, indicating that saturation levels must be accounted for if our empirical relationships are applied to vadose zone studies. Our results show clear evidence of the potential value of frequency-based IP measurements for parameterization of groundwater flow models.

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...

Crosshole IP imaging for engineering and environmental applications

Induced polarization (IP) imaging is a promising tool in engineering and environmental studies. Application of this technique for near-surface investigations has previously been limited by incomplete understanding of the physicochemical controls on the IP response, together with a lack of appropriate methods for data inversion. As laboratory studies have shown, description of IP in terms of complex electrical conductivity enables access to various structural characteristics pertinent to practical issues such as subsurface lithology definition, hydraulic permeability estimation, or hydrocarbon contaminant mapping. In particular, analysis in terms of real and imaginary conductivity components offers improved lithological characterization, since surface polarization effects are separated from electrolytic and surface conduction effects. An Occam-type IP inversion algorithm based on complex algebra is described which accounts for these advances in IP interpretation by directly solving for complex conductivity. Results from crosshole applications at two case study sites demonstrate the suitability of the IP imaging approach for subsurface characterization. In the first case study, the imaging results correlate with the observed complex sequence of Quaternary sediments at a waste disposal site. Characterization of the polarizability of these sediments offers significant value in lithological differentiation. In the second case study, the results of IP imaging at a hydrocarbon-contaminated site illustrate the potential of the method in environmental studies. The hydrocarbon location is clearly evident from the IP image, and a markedly different response is observed at an uncontaminated region of the site. By adopting empirical structural‐electrical relationships, images of textural and hydraulic properties are estimated as a step toward improved quantitative characterization. The success of the method for these contrasting applications supports further investigation into understanding the physical and chemical processes that control observed IP.

The emergence of hydrogeophysics for improved understanding of subsurface processes over multiple scales

Abstract Geophysics provides a multidimensional suite of investigative methods that are transforming our ability to see into the very fabric of the subsurface environment, and monitor the dynamics of its fluids and the biogeochemical reactions that occur within it. Here we document how geophysical methods have emerged as valuable tools for investigating shallow subsurface processes over the past two decades and offer a vision for future developments relevant to hydrology and also ecosystem science. The field of “hydrogeophysics” arose in the late 1990s, prompted, in part, by the wealth of studies on stochastic subsurface hydrology that argued for better field‐based investigative techniques. These new hydrogeophysical approaches benefited from the emergence of practical and robust data inversion techniques, in many cases with a view to quantify shallow subsurface heterogeneity and the associated dynamics of subsurface fluids. Furthermore, the need for quantitative characterization stimulated a wealth of new investigations into petrophysical relationships that link hydrologically relevant properties to measurable geophysical parameters. Development of time‐lapse approaches provided a new suite of tools for hydrological investigation, enhanced further with the realization that some geophysical properties may be sensitive to biogeochemical transformations in the subsurface environment, thus opening up the new field of “biogeophysics.” Early hydrogeophysical studies often concentrated on relatively small “plot‐scale” experiments. More recently, however, the translation to larger‐scale characterization has been the focus of a number of studies. Geophysical technologies continue to develop, driven, in part, by the increasing need to understand and quantify key processes controlling sustainable water resources and ecosystem services.

A 3D ERT study of solute transport in a large experimental tank

A high resolution, cross-borehole, 3D electrical resistivity tomography (ERT) study of solute transport was conducted in a large experimental tank. ERT voxels comprising the time sequence of electrical images were converted into a 3D array of ERT estimated fluid conductivity breakthrough curves and compared with direct measurements of fluid conductivity breakthrough made in wells. The 3D ERT images of solute transport behaviour were also compared with predictions based on a 3D finite-element, coupled flow and transport model, accounting for gravity induced flow caused by concentration differences. The tank (dimensions 185×245×186 cm) was filled with medium sand, with a gravel channel and a fine sand layer installed. This heterogeneous system was designed to complicate solute transport behaviour relative to a homogeneous sand tank, and to thus provide a challenging but insightful analysis of the ability of 3D ERT to resolve transport phenomena. Four ERT arrays and 20 piezometers were installed during filling. A NaCl tracer (conductivity 1.34 S/m) was injected and intensively monitored with 3D ERT and direct sampling of fluid chemistry in piezometers. We converted the bulk conductivity estimate for 250 voxels in the ERT imaged volume into ERT estimated voxel fluid conductivity by assuming that matrix conduction in the tank is negligible. In general, the ERT voxel response is in reasonable agreement with the shape of fluid conductivity breakthrough observed in six wells in which direct measurements of fluid conductivity were made. However, discrepancies occur, particularly at early times, which we attribute to differences between the scale of the image voxels and the fluid conductivity measurement, measurement errors mapped into the electrical inversion and artificial image roughness resulting from the inversion. ERT images revealed the 3D tracer distribution at 15 times after tracer injection. The general pattern and timing of solute breakthrough observed with ERT agreed with that predicted from the flow/transport modelling. However, the ERT images indicate a vertical component of tracer transport and preferential flow paths in the medium sand. We attribute this to transient vertical gradients established during tracer injection, and heterogeneity caused by sorting of the sand resulting from the filling procedure. In this study, ERT provided a unique dataset of 250 voxel breakthrough curves in 1.04 m3. The use of 3D ERT to generate an array of densely sampled estimated fluid conductivity breakthrough curves is a potentially powerful tool for quantifying solute transport processes.

Induced polarization measurements on unsaturated, unconsolidated sands

Induced polarization (IP) measurements were obtained on unsaturated, unconsolidated sediments during (1) evaporative drying and (2) pressure drainage followed by subsequent imbibition (water reentry). Porous ceramic discs were used with existing laboratory IP instrumentation to permit accurate IP measurements on unsaturated samples. Polarization magnitude during evaporative drying approximates a power law dependence on saturation. Saturation exponents for the polarization term were consistently less than Archie conduction exponents, although no clear relationship between the exponents was observed. The polarization measured over a pressure drainage and imbibition cycle exhibits a complex (yet similar between tested samples) saturation dependence, being a function of saturation range and saturation history. Polarization is observed to increase with saturation over certain saturation intervals, yet decrease with saturation over others. High polarization observed during sample imbibition is consistent with a...

Controls on induced polarization in sandy unconsolidated sediments and application to aquifer characterization

Resistivity and induced polarization (IP) measurements (0.1–1000 Hz) were made on clay‐free unconsolidated sediments from a sandy, alluvial aquifer in the Kansas River floodplain. The sensitivity of imaginary conductivity σ″, a fundamental IP measurement, to lithological parameters, fluid conductivity, and degree of saturation was assessed. The previously reported power law dependence of IP on surface area and grain size is clearly observed despite the narrow lithologic range encountered in this unconsolidated sedimentary sequence. The grain‐size σ″ relationship is effectively frequency independent between 0.1 and 100 Hz but depends on the representative grain diameter used. For the sediments examined here, d90, the grain diameter of the coarsest sediments in a sample, is well correlated with σ″. The distribution of the internal surface in the well‐sorted, sandy sediments investigated here is such that most of the sample weight is likely required to account for the majority of the internal surface. We fin...

Investigating peatland stratigraphy and hydrogeology using integrated electrical geophysics

Hydrology has been suggested as the mechanism controlling vegetation and related surficial pore-water chemistry in large peatlands. Peatland hydrology influences the carbon dynamics within these large carbon reservoirs and will influence their response to global warming. A geophysical survey was completed in Caribou Bog, a large peatland in Maine, to evaluate peatland stratigraphy and hydrology. Geophysical measurements were integrated with direct measurements of peat stratigraphy from probing, fluid chemistry, and vegetation patterns in the peatland. Consistent with previous field studies, ground-penetrating radar (GPR) was an excellent method for delineating peatland stratigraphy. Prominent reflectors from the peat-lake sediment and lake sediment-mineral soil contacts were precisely recorded up to 8 m deep. Two-dimensional resistivity and induced polarization imaging were used to investigate stratigraphy beneath the mineral soil, beyond the range of GPR. We observe that the peat is chargeable, and that IP imaging is an alternative method for defining peat thickness. The chargeability of peat is attributed to the high surface-charge density on partially decomposed organic matter. The electrical conductivity imaging resolved glaciomarine sediment thickness (a confining layer) and its variability across the basin. Comparison of the bulk conductivity images with peatland vegetation revealed a correlation between confining layer thickness and dominant vegetation type, suggesting that stratigraphy exerts a control on hydrogeology and vegetation distribution within this peatland. Terrain conductivity measured with a Geonics EM31 meter correlated with confining glaciomarine sediment thickness and was an effective method for estimating variability in glaciomarine sediment thickness over approximately 18 km 2 . Our understanding of the hydrogeology, stratigraphy, and controls on vegetation growth in this peatland was much enhanced from the geophysical study.

On the estimation of specific surface per unit pore volume from induced polarization: A robust empirical relation fits multiple data sets

We analyze the relationship between induced polarization (IP) parameters and the specific surface area normalized to the pore volume ( Spor ) for an extensive sample database. We find that a single linear imaginary conductivity- Spor relation holds across a range of single-frequency IP data sets composed of sandstones and unconsolidated sediments that lack an appreciable metallic mineral content. We also apply a recent approach defined as Debye decomposition (DD) to determine normalized chargeability ( mn ) , a global estimate of polarization magnitude from available spectral IP (SIP) data sets. A strong linear relationship between mn and Spor is also found across multiple data sets. However, SIP model parameters determined for samples containing metallic minerals are approximately two orders of magnitude greater than for the model parameters estimated for the nonmetallic sample database. We propose a concept of “polarizability of the mineral-fluid interface per unit Spor ” to explain this difference, whi...