SUMMARY
Induced polarization (more precisely the magnitude and phase of impedance of the subsurface) is measured using a network of electrodes located at the ground surface or in boreholes. This ...method yields important information related to the distribution of permeability and contaminants in the shallow subsurface. We propose a new time‐lapse 3‐D modelling and inversion algorithm to image the evolution of complex conductivity over time. We discretize the subsurface using hexahedron cells. Each cell is assigned a complex resistivity or conductivity value. Using the finite‐element approach, we model the in‐phase and out‐of‐phase (quadrature) electrical potentials on the 3‐D grid, which are then transformed into apparent complex resistivity. Inhomogeneous Dirichlet boundary conditions are used at the boundary of the domain. The calculation of the Jacobian matrix is based on the principles of reciprocity. The goal of time‐lapse inversion is to determine the change in the complex resistivity of each cell of the spatial grid as a function of time. Each model along the time axis is called a ‘reference space model’. This approach can be simplified into an inverse problem looking for the optimum of several reference space models using the approximation that the material properties vary linearly in time between two subsequent reference models. Regularizations in both space domain and time domain reduce inversion artefacts and improve the stability of the inversion problem. In addition, the use of the time‐lapse equations allows the simultaneous inversion of data obtained at different times in just one inversion step (4‐D inversion). The advantages of this new inversion algorithm are demonstrated on synthetic time‐lapse data resulting from the simulation of a salt tracer test in a heterogeneous random material described by an anisotropic semi‐variogram.
For the study of charmonium resonances above and including the
χ
c0
, Fermilab experiment E-835 required an intense and stochastically cooled antiproton beam with kinetic energies from
8
GeV
(the ...injection energy of the Accumulator) down to
4
GeV
. We developed a scheme in which the momentum compaction factor of the machine was changed as the antiprotons were decelerated, so that the energies of interest to the experiment were kept above transition. The scheme was used during the E-835 10-month run of the year 2000. Here we describe the design criteria, operational procedures and diagnostic tools we used to exploit the machine as an efficient antiproton decelerator. The machine performance during data taking is also discussed, in relation to the main experimental requirements.
We present the results of a lab-scaled feasibility study to assess the performance of electrical resistivity tomography for detection, characterization, and monitoring of fuel grade ethanol releases ...to the subsurface. Further, we attempt to determine the concentration distribution of the ethanol from the electrical resistivity tomography data using mixing-models. Ethanol is a renewable fuel source as well as an oxygenate fuel additive currently used to replace the known carcinogen methyl tert-butyl ether; however, ethanol is preferentially biodegraded and a cosolvent. When introduced to areas previously impacted by nonethanol-based fuels, it will facilitate the persistence of carcinogenic fuel compounds like benzene and ethylbenzene, as well as remobilize them to the ground water. These compounds would otherwise be retained in the soil column undergoing active or passive remediation processes such as soil vapor extraction or natural attenuation. Here, we introduce ethanol to a saturated Ottawa sand in a tank instrumented for four-dimensional geoelectrical measurements. Forward model results suggest pure phase ethanol released into a water saturated silica sand should present a detectable target for electrical resistivity tomography relative to a saturated silica sand only. We observe the introduction of ethanol to the closed hydraulic system and subsequent migration over the duration of the experiment. One-dimensional and three–dimensional temporal data are assessed for the detection, characterization, and monitoring of the ethanol release. Results suggest one-dimensional geoelectrical measurements may be useful for monitoring a release, while three-dimensional geoelectrical field imaging would be useful to characterize, monitor, and design effective remediation approaches for an ethanol release, assuming field conditions do not preclude the application of geoelectrical methods. We then attempt to use predictive mixing models to calculate the distribution of ethanol concentration within the measurement domain. For this study we examine four different models: a nested parallel mixing model, a nested cubic mixing model, the complex refractive index model (CRIM), and the Lichtenecker-Rother (L-R) model. The L-R model, modified to include an electrical formation factor geometry term, provided the best agreement with expected EtOH concentrations.
•Evaluate popular mixing models to estimate biofuel (ethanol) concentration from 4D electrical resistivity tomography.•Observe the nature of EtOH as injected into a closed hydraulic system.•Estimated EtOH concentration within 5% from temporal ERT data with a formation factor-based Lichtenecker-Rother model.
The atmospheric neutrino flavour ratio measured using a 1.52 kton-year exposure of Soudan 2 is found to be 0.72 ± 0.19
−0.07
+0.05 relative to the expected value from a Monte Carlo calculation. The ...possible background of interactions of neutrons and photons produced in muon interactions in the rock surrounding the detector has been investigated and is shown not to produce low values of the ratio.
It is not known how biofilms affect seismic wave propagation in porous media. Such knowledge is critical for assessing the utility of seismic techniques for imaging biofilm development and their ...effects in field settings. Acoustic wave data were acquired over a two‐dimensional region of a microbial‐stimulated sand column and an unstimulated sand column. The acoustic signals from the unstimulated column were relatively uniform over the 2D scan region. The data from the microbial‐stimulated column exhibited a high degree of spatial heterogeneity in the acoustic wave amplitude, with some regions exhibiting significant increases in attenuation while others exhibited decreases. Environmental scanning electron microscopy showed differences in the structure of the biofilm between regions of increased and decreased acoustic wave amplitude. We conclude from these observations that variations in microbial growth and biofilm structure cause heterogeneity in the elastic properties of porous media with implications for the validation of bioclogging models.
We propose a 2D/3D forward modelling and inversion package to invert direct current (DC)-resistivity, time-domain induced polarization (TDIP), and frequency-domain induced polarization (FDIP) data. ...Each cell used for the discretization of the 2D/3D problems is characterized by a DC-resistivity value and a chargeability or complex conductivity for TDIP/FDIP problems, respectively. The governing elliptic partial differential equations are solved with the finite element method, which can be applied for both real and complex numbers. The inversion can be performed either for a single snapshot of data or for a sequence of snapshots in order to monitor a dynamic process such as a salt tracer test. For the time-lapse inversion, we have developed an active time constrained (ATC) approach that is very efficient in filtering out noise in the data that is not correlated over time. The forward algorithm is benchmarked with simple analytical solutions. The inversion package IP4DI is benchmarked with three tests, two including simple geometries. The last one corresponds to a time-lapse resistivity problem for cross-well tomography during enhanced oil recovery. The algorithms are based on MATLAB® code package and a graphical user interface (GUI).
► We developed a code to invert resistivity, frequency-domain and time domain IP data. ► The code include a new time lapse appraoch called the ATC inversion. ► The forward algorithm is benchmarked against analytical solutions. ► An application is shown for an enhanced oil recovery problem. ► Inverson can be 2D, 3D, 2D plus time, 3D plus time.
Groundwater flow advects heat, and thus, the deviation of subsurface temperatures from an expected conduction‐dominated regime can be analysed to estimate vertical water fluxes. A number of ...analytical approaches have been proposed for using heat as a groundwater tracer, and these have typically assumed a homogeneous medium. However, heterogeneous thermal properties are ubiquitous in subsurface environments, both at the scale of geologic strata and at finer scales in streambeds. Herein, we apply the analytical solution of Shan and Bodvarsson (), developed for estimating vertical water fluxes in layered systems, in 2 new environments distinct from previous vadose zone applications. The utility of the solution for studying groundwater‐surface water exchange is demonstrated using temperature data collected from an upwelling streambed with sediment layers, and a simple sensitivity analysis using these data indicates the solution is relatively robust. Also, a deeper temperature profile recorded in a borehole in South Australia is analysed to estimate deeper water fluxes. The analytical solution is able to match observed thermal gradients, including the change in slope at sediment interfaces. Results indicate that not accounting for layering can yield errors in the magnitude and even direction of the inferred Darcy fluxes. A simple automated spreadsheet tool (Flux‐LM) is presented to allow users to input temperature and layer data and solve the inverse problem to estimate groundwater flux rates from shallow (e.g., <1 m) or deep (e.g., up to 100 m) profiles. The solution is not transient, and thus, it should be cautiously applied where diel signals propagate or in deeper zones where multi‐decadal surface signals have disturbed subsurface thermal regimes.
Traditional methods used to invert gravity data are generally based on smoothness as a regularizer and the time-lapse inversion of gravity data is traditionally based on sequential inversion. A ...smooth density model can be in contradiction with the known geology of the target and sequential time-lapse inversion may lead to the presence of artefacts in the sequence of tomograms due to the propagation of errors from the initial dataset and its associated tomogram. We propose a deterministic time-lapse algorithm to invert a sequence of gravity data combining two features: an image focusing technique and the use of a time-dependent regularizer using an Active Time Constrained (ATC) approach. These two features are included directly in the objective function to minimize. The ATC inversion of time-lapse gravity data is efficient in filtering out noise-contaminated data as long as the noise is not correlated over time. Our approach can also be used to incorporate prior information regarding the density model we want to retrieve. The forward solver is based on a commercial finite element solver with a high flexibility in meshing irregular domains, a feature that is important to include realistic topography from digital elevation maps, for instance, and to describe the density distribution of geological structures with complex geometries. We benchmark the accuracy of the forward modelling code using an analytical expression and test the effectiveness of the focusing algorithm. We show the advantage of our approach in the case of the water flooding of an oil reservoir in order to detect and monitor the position of the oil-water encroachment front. We also test the model against synthetic data describing the evolution of a hydrothermal system feed by a magmatic source and the collapse of a mine. In all these cases, the approach we follow is successful in monitoring density changes.
Brook trout
spawn in fall and overwintering egg development can benefit from stable, relatively warm temperatures in groundwater-seepage zones. However, eggs are also sensitive to dissolved oxygen ...concentration, which may be reduced in discharging groundwater (i.e., seepage). We investigated a 2 km reach of the coastal Quashnet River in Cape Cod, Massachusetts, USA, to relate preferred fish spawning habitats to geology, geomorphology, and discharging groundwater geochemistry. Thermal reconnaissance methods were used to locate zones of rapid groundwater discharge, which were predominantly found along the central channel of a wider stream valley section. Pore-water chemistry and temporal vertical groundwater flux were measured at a subset of these zones during field campaigns over several seasons. Seepage zones in open-valley sub-reaches generally showed suboxic conditions and higher dissolved solutes compared to the underlying glacial outwash aquifer. These discharge zones were cross-referenced with preferred brook trout redds and evaluated during 10 years of observation, all of which were associated with discrete alcove features in steep cutbanks, where stream meander bends intersect the glacial valley walls. Seepage in these repeat spawning zones was generally stronger and more variable than in open-valley sites, with higher dissolved oxygen and reduced solute concentrations. The combined evidence indicates that regional groundwater discharge along the broader valley bottom is predominantly suboxic due to the influence of near-stream organic deposits; trout show no obvious preference for these zones when spawning. However, the meander bends that cut into sandy deposits near the valley walls generate strong oxic seepage zones that are utilized routinely for redd construction and the overwintering of trout eggs. Stable water isotopic data support the conclusion that repeat spawning zones are located directly on preferential discharges of more localized groundwater. In similar coastal systems with extensive valley peat deposits, the specific use of groundwater-discharge points by brook trout may be limited to morphologies such as cutbanks, where groundwater flow paths do not encounter substantial buried organic material and remain oxygen-rich.
A laboratory study was conducted to investigate the effect of microbial growth and biofilm formation on compressional waves, and complex conductivity during stimulated microbial growth. Over the 29 ...day duration of the experiment, compressional wave amplitudes and arrival times for the control (nonbiostimulated) sample were observed to be relatively uniform over the scanned 2‐D region. However, the biostimulated sample exhibited a high degree of spatial variability in both the amplitude and arrival times, with portions of the sample exhibiting increased attenuation (∼80%) concurrent with an increase in the arrival times, while other portions exhibited decreased attenuation (∼45%) and decreased arrival times. The acoustic amplitude and arrival times changed significantly in the biostimulated column between days 5 and 7 of the experiment, consistent with a peak in the imaginary conductivity (σ″) values. The σ″ response is interpreted as recording the different stages of biofilm development with peak σ″ representing maximum biofilm thickness and decreasing σ″ representing cell death or detachment. Environmental scanning electron microscope imaging confirmed microbial cell attachment to sand surfaces and showed apparent differences in the morphology of attached biomass between regions of increased and decreased attenuation. The heterogeneity in the elastic properties arises from the differences in the morphology and structure of attached biofilms. These results suggest that combining acoustic imaging and complex conductivity techniques can provide a powerful tool for assessing microbial growth or biofilm formation and the associated changes in porous media, such as those that occur during bioremediation and microbial enhanced oil recovery.
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