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Articles 601 - 630 of 667
Full-Text Articles in Earth Sciences
Hyporheic Exchange And Water Chemistry Of Two Arctic Tundra Streams Of Contrasting Geomorphology, Morgan J. Greenwald, William B. Bowden, Michael N. Gooseff, Jay P. Zarnetske, James P. Mcnamara, John H. Bradford, Troy R. Brosten
Hyporheic Exchange And Water Chemistry Of Two Arctic Tundra Streams Of Contrasting Geomorphology, Morgan J. Greenwald, William B. Bowden, Michael N. Gooseff, Jay P. Zarnetske, James P. Mcnamara, John H. Bradford, Troy R. Brosten
Geosciences Faculty Publications and Presentations
The North Slope of Alaska’s Brooks Range is underlain by continuous permafrost, but an active layer of thawed sediments develops at the tundra surface and beneath streambeds during the summer, facilitating hyporheic exchange. Our goal was to understand how active layer extent and stream geomorphology influence hyporheic exchange and nutrient chemistry. We studied two arctic tundra streams of contrasting geomorphology: a high-gradient, alluvial stream with riffle-pool sequences and a low-gradient, peat-bottomed stream with large deep pools connected by deep runs. Hyporheic exchange occurred to ~50 cm beneath the alluvial streambed and to only ~15 cm beneath the peat streambed. The …
Comparison Of Instantaneous And Constant-Rate Stream Tracer Experiments Through Parametric Analysis Of Residence Time Distributions, Robert A. Payn, Michael N. Gooseff, David A. Benson, Olaf A. Cirpka, Jay P. Zarnetske, W. Breck Bowden, James P. Mcnamara, John H. Bradford
Comparison Of Instantaneous And Constant-Rate Stream Tracer Experiments Through Parametric Analysis Of Residence Time Distributions, Robert A. Payn, Michael N. Gooseff, David A. Benson, Olaf A. Cirpka, Jay P. Zarnetske, W. Breck Bowden, James P. Mcnamara, John H. Bradford
Geosciences Faculty Publications and Presentations
Artificial tracers are frequently employed to characterize solute residence times in stream systems and infer the nature of water retention. When the duration of tracer application is different between experiments, tracer breakthrough curves at downstream locations are difficult to compare directly. We explore methods for deriving stream solute residence time distributions (RTD) from tracer test data, allowing direct, non-parametric comparison of results from experiments of different durations. Paired short- and long-duration field experiments were performed using instantaneous and constant-rate tracer releases, respectively. The experiments were conducted in two study reaches that were morphologically distinct in channel structure and substrate size. …
Sediment And Nutrient Delivery From Thermokarst Features In The Foothills Of The North Slope, Alaska: Potential Impacts On Headwater Stream Ecosystems, W. B. Bowden, M. N. Gooseff, A. Balser, A. Green, B. J. Peterson, John H. Bradford
Sediment And Nutrient Delivery From Thermokarst Features In The Foothills Of The North Slope, Alaska: Potential Impacts On Headwater Stream Ecosystems, W. B. Bowden, M. N. Gooseff, A. Balser, A. Green, B. J. Peterson, John H. Bradford
CGISS Publications and Presentations
Permafrost is a defining characteristic of the Arctic environment. However, climate warming is thawing permafrost in many areas leading to failures in soil structure called thermokarst. An extensive survey of a 600 km2 area in and around the Toolik Lake Natural Research Area (TLNRA) revealed at least 34 thermokarst features, two thirds of which were new since ~1980 when a high resolution aerial survey of the area was done. Most of these thermokarst features were associated with headwater streams or lakes. We have measured significantly increased sediment and nutrient loading from thermokarst features to streams in two well-studied locations …
Finding Concealed Active Faults: Extending The Southern Whidbey Island Fault Across The Puget Lowland, Washington, Brian L. Sherrod, Richard J. Blakely, Craig S. Weaver, Harvey M. Kelsey, Elizabeth Barnett, Lee Liberty, Karen L. Meagher, Kristin Pape
Finding Concealed Active Faults: Extending The Southern Whidbey Island Fault Across The Puget Lowland, Washington, Brian L. Sherrod, Richard J. Blakely, Craig S. Weaver, Harvey M. Kelsey, Elizabeth Barnett, Lee Liberty, Karen L. Meagher, Kristin Pape
CGISS Publications and Presentations
The southern Whidbey Island fault zone (SWIF), as previously mapped using borehole data, potential field anomalies, and marine seismic reflection surveys, consists of three subparallel, northwest trending strands extending ~100 km from near Vancouver Island to the northern Puget Lowland. East of Puget Sound, the SWIF makes landfall between the cities of Seattle and Everett but is concealed beneath a thick mantle of young glacial deposits and vegetation. A ~20-km-wide, northwest trending swath of subparallel, low-amplitude aeromagnetic anomalies crosses this region of the Puget Lowland and is on strike with the SWIF. The most prominent aeromagnetic anomaly, the Cottage Lake …
Application Of Time-Lapse Ert Imaging To Watershed Characterization, Carlyle R. Miller, Partha S. Routh, Troy R. Brosten, James P. Mcnamara
Application Of Time-Lapse Ert Imaging To Watershed Characterization, Carlyle R. Miller, Partha S. Routh, Troy R. Brosten, James P. Mcnamara
Geosciences Faculty Publications and Presentations
Time-lapse electrical resistivity tomography (ERT) has many practical applications to the study of subsurface properties and processes. When inverting time-lapse ERT data, it is useful to proceed beyond straightforward inversion of data differences and take advantage of the time-lapse nature of the data. We assess various approaches for inverting and interpreting time-lapse ERT data and determine that two approaches work well. The first approach is model subtraction after separate inversion of the data from two time periods, and the second approach is to use the inverted model from a base data set as the reference model or prior information for …
A View Toward The Future Of Subsurface Characterization: Cat Scanning Groundwater Basins, Warren Barrash
A View Toward The Future Of Subsurface Characterization: Cat Scanning Groundwater Basins, Warren Barrash
CGISS Publications and Presentations
In this opinion paper we contend that high-resolution characterization, monitoring, and prediction are the key elements to advancing and reducing uncertainty in our understanding and prediction of subsurface processes at basin scales. First, we advocate that recently developed tomographic surveying is an effective and high-resolution approach for characterizing the field-scale subsurface. Fusion of different types of tomographic surveys further enhances the characterization. A basin is an appropriate scale for many water resources management purposes. We thereby propose the expansion of the tomographic surveying and data fusion concept to basin-scale characterization. In order to facilitate basin-scale tomographic surveys, different types of …
Influence Of Morphology And Permafrost Dynamics On Hyporheic Exchange In Arctic Headwater Streams Under Warming Climate Conditions, Jay P. Zarnetske, Michael N. Gooseff, W. Breck Bowden, Morgan J. Greenwald, Troy R. Brosten, John H. Bradford, James P. Mcnamara
Influence Of Morphology And Permafrost Dynamics On Hyporheic Exchange In Arctic Headwater Streams Under Warming Climate Conditions, Jay P. Zarnetske, Michael N. Gooseff, W. Breck Bowden, Morgan J. Greenwald, Troy R. Brosten, John H. Bradford, James P. Mcnamara
CGISS Publications and Presentations
We investigated surface-subsurface (hyporheic) exchange in two morphologically distinct arctic headwater streams experiencing warming (thawing) sub-channel conditions. Empirically parameterized and calibrated groundwater flow models were used to assess the influence of sub-channel thaw on hyporheic exchange. Average thaw depths were at least two-fold greater under the higher-energy, alluvial stream than under the lowenergy, peat-lined stream. Alluvial hyporheic exchange had shorter residence times and longer flowpaths that occurred across greater portions of the thawed sediments. For both reaches, the morphologic (longitudinal bed topography) and hydraulic conditions (surface and groundwater flow properties) set the potential for hyporheic flow. Simulations of deeper thaw, …
Water, Inertial Damping, And The Complex Shear Modulus, Paul Michaels
Water, Inertial Damping, And The Complex Shear Modulus, Paul Michaels
CGISS Publications and Presentations
The author proposes an alternative to the traditional representation of soil damping. Rather than using damping ratio, this author advocates using viscosity as the specific soil property, especially for saturated permeable soils. Thus represented, the imaginary part of the complex shear modulus will vary directly with frequency. The point is particularly relevant in cases where the water table may change, thus affecting the dynamic design of foundations or structures composed of soils.
Imaging Thermal Stratigraphy In Freshwater Lakes Using Georadar, John H. Bradford, Cody R. Johnson, Troy Brosten, James P. Mcnamara, Michael N. Gooseff
Imaging Thermal Stratigraphy In Freshwater Lakes Using Georadar, John H. Bradford, Cody R. Johnson, Troy Brosten, James P. Mcnamara, Michael N. Gooseff
CGISS Publications and Presentations
Thermal stratification exerts significant control over biogeochemical processing in freshwater lakes. Thus, the temporal and spatial distribution of the thermal structure is an important component in understanding lake ecosystems. We present the first reported observations of lake thermal stratification from surface based georadar measurements acquired over two small freshwater lakes. This method is very useful because it can provide rapid acquisition of 2D or 3D lotic stratification.
Tomography Of The Darcy Velocity From Self-Potential Measurements, A. Jardani, A. Revil, A. Bolève, A. Crespy, J.-P. Dupont, Warren Barrash, Bwalya Malama
Tomography Of The Darcy Velocity From Self-Potential Measurements, A. Jardani, A. Revil, A. Bolève, A. Crespy, J.-P. Dupont, Warren Barrash, Bwalya Malama
CGISS Publications and Presentations
An algorithm is developed to interpret self-potential (SP) data in terms of distribution of Darcy velocity of the ground water. The model is based on the proportionality existing between the streaming current density and the Darcy velocity. Because the inverse problem of current density determination from SP data is underdetermined, we use Tikhonov regularization with a smoothness constraint based on the differential Laplacian operator and a prior model. The regularization parameter is determined by the L-shape method. The distribution of the Darcy velocity depends on the localization and number of non-polarizing electrodes and information relative to the distribution of the …
Incorporating Geostatistical Constraints In Nonlinear Inverse Problems, T. C. Johnson, Partha S. Routh, Tom Clemo, Warren Barrash, William P. Clement
Incorporating Geostatistical Constraints In Nonlinear Inverse Problems, T. C. Johnson, Partha S. Routh, Tom Clemo, Warren Barrash, William P. Clement
CGISS Publications and Presentations
In this paper we present a method of incorporating semivariogram constraints into nonlinear inversion problems. That is, we describe a method of sampling the space of inverse solutions that honor a specified semivariogram or set of semivariograms and also explain a set of state data. The approach can be considered a method of conditional simulation where model conditioning is based upon state data (as opposed to parameter data). The difference between this approach and other simulation approaches is that the simulation is posed as an optimization problem with the joint objective of matching the semivariograms and honoring the state data. …
Instantaneous Spectral Analysis: Time-Frequency Mapping Via Wavelet Matching With Application To Contaminated-Site Characterization By 3d Gpr, John H. Bradford, Yafei Wu
Instantaneous Spectral Analysis: Time-Frequency Mapping Via Wavelet Matching With Application To Contaminated-Site Characterization By 3d Gpr, John H. Bradford, Yafei Wu
CGISS Publications and Presentations
Spectral decomposition, by which a time series is transformed from the 1D time/amplitude domain to the 2D time/spectrum domain, has become a popular and useful tool in seismic exploration for hydrocarbons. The windowed, or short-time Fourier transform (STFT) was one early approach to computing the time-frequency (t-f) distribution. This method relies on the user selecting a fixed time window, then computing the Fourier spectrum within the time window while sliding the window along the length of the trace. The primary limitation of the STFT is the fixed window which prevents either time localization of high frequency components (if …
Transient Storage As A Function Of Geomorphology, Discharge, And Permafrost Active Layer Conditions In Arctic Tundra Streams, Jay P. Zarnetske, Michael N. Gooseff, Troy R. Brosten, John H. Bradford, James P. Mcnamara, W. Breck Bowden
Transient Storage As A Function Of Geomorphology, Discharge, And Permafrost Active Layer Conditions In Arctic Tundra Streams, Jay P. Zarnetske, Michael N. Gooseff, Troy R. Brosten, John H. Bradford, James P. Mcnamara, W. Breck Bowden
CGISS Publications and Presentations
Transient storage of solutes in hyporheic zones or other slow-moving stream waters plays an important role in the biogeochemical processes of streams. While numerous studies have reported a wide range of parameter values from simulations of transient storage, little field work has been done to investigate the correlations between these parameters and shifts in surface and subsurface flow conditions. In this investigation we use the stream properties of the Arctic (namely, highly varied discharges, channel morphologies, and subchannel permafrost conditions) to isolate the effects of discharge, channel morphology, and potential size of the hyporheic zone on transient storage. We repeated …
Frequency-Dependent Attenuation Analysis Of Ground-Penetrating Radar Data, John H. Bradford
Frequency-Dependent Attenuation Analysis Of Ground-Penetrating Radar Data, John H. Bradford
CGISS Publications and Presentations
In the early 1990s, it was established empirically that, in many materials, ground-penetrating radar (GPR) attenuation is approximately linear with frequency over the bandwidth of a typical pulse. Further, a frequency-independent Q* parameter characterizes the slope of the band-limited attenuation versus frequency curve. Here, I derive the band-limited Q* function from a first-order Taylor expansion of the attenuation coefficient. This approach provides a basis for computing Q* from any arbitrary dielectric permittivity model. For Cole-Cole relaxation, I find good correlation between the first-order Q* approximation and Q* computed from linear fits to the attenuation coefficient …
Modified Kubelka-Munk Equations For Localized Waves Inside A Layered Medium, Kasper Van Wijk
Modified Kubelka-Munk Equations For Localized Waves Inside A Layered Medium, Kasper Van Wijk
CGISS Publications and Presentations
We present a pair of coupled partial differential equations to describe the evolution of the average total intensity and intensity flux of a wave field inside a randomly layered medium. These equations represent a modification of the Kubelka-Munk equations, or radiative transfer. Our modification accounts for wave interference (e.g., localization), which is neglected in radiative transfer. We numerically solve the modified Kubelka-Munk equations and compare the results to radiative transfer as well as to simulations of the wave equation with randomly located thin layers.
A Field Comparison Of Fresnel Zone And Ray-Based Gpr Attenuation-Difference Tomography For Time-Lapse Imaging Of Electrically Anomalous Tracer Or Contaminant Plumes, Timothy C. Johnson, Partha S. Routh, Warren Barrash, Michael D. Knoll
A Field Comparison Of Fresnel Zone And Ray-Based Gpr Attenuation-Difference Tomography For Time-Lapse Imaging Of Electrically Anomalous Tracer Or Contaminant Plumes, Timothy C. Johnson, Partha S. Routh, Warren Barrash, Michael D. Knoll
CGISS Publications and Presentations
Ground-penetrating radar (GPR) attenuation-difference tomography is a useful tool for imaging the migration of electrically anomalous tracer or contaminant plumes. Attenuation-difference tomography uses the difference in the trace amplitudes of tomographic data sets collected at different times to image the distribution of bulk-conductivity changes within the medium. The most common approach for computing the tomographic sensitivities uses ray theory, which is well understood and leads to efficient computations. However, ray theory requires the assumption that waves propagate at infinite frequency, and thus sensitivities are distributed along a line between the source and receiver. The infinite-frequency assumption in ray theory leads …
Physical Modeling And Analysis Of P-Wave Attenuation Anisotropy, Kasper Van Wijk
Physical Modeling And Analysis Of P-Wave Attenuation Anisotropy, Kasper Van Wijk
CGISS Publications and Presentations
Anisotropic attenuation can provide sensitive attributes for fracture detection and lithology discrimination. This paper analyzes measurements of the P-wave attenuation coefficient in a transversely isotropic sample made of phenolic material. Using the spectral-ratio method, we estimate the group (effective) attenuation coefficient of P-waves transmitted through the sample for a wide range of propagation angles (from 0° to 90°) with the symmetry axis. Correction for the difference between the group and phase angles and for the angular velocity variation help us to obtain the normalized phase attenuation coefficient A governed by the Thomsen-style attenuation-anisotropy parameters εQ and δQ. …
Issues During The Inversion Of Crosshole Radar Data: Can We Have Confidence In The Outcome?, William P. Clement
Issues During The Inversion Of Crosshole Radar Data: Can We Have Confidence In The Outcome?, William P. Clement
Geosciences Faculty Publications and Presentations
One method of assessing the confidence in modeled features is to compare the results from different inversion schemes. I use synthetic traveltimes calculated from a model of an unconfined aquifer to determine the reliability of crosshole tomography. I compare the inverted models from straight and curved ray approximations to wave propagation. I investigate the effects of added random noise, regularization, the starting model, and the reference model on the curved ray inversion method. I also investigate the effects of different grid sizes for the forward model and of limited ray coverage through the earth model. Understanding the effects of these …
Imaging Complex Structure In Shallow Seismic-Reflection Data Using Prestack Depth Migration, John H. Bradford, Lee M. Liberty, Mitchell W. Lyle, William P. Clement, Scott Hess
Imaging Complex Structure In Shallow Seismic-Reflection Data Using Prestack Depth Migration, John H. Bradford, Lee M. Liberty, Mitchell W. Lyle, William P. Clement, Scott Hess
CGISS Publications and Presentations
Prestack depth migration (PSDM) analysis has the potential to significantly improve the accuracy of both shallow seismic reflection images and the measured velocity distributions. In a study designed to image faults in the Alvord Basin, Oregon, at depths from 25–1000 m, PSDM produced a detailed reflection image over the full target depth range. In contrast, poststack time migration produced significant migration artifacts in the upper 100 m that obscured reflection events and limited the structural interpretation in the shallow section. Additionally, an abrupt increase from ~2500 to >3000 m/s in the PSDM velocity model constrained the interpretation of the transition …
Crosshole Radar Tomography In A Fluvial Aquifer Near Boise, Idaho, William P. Clement, Warren Barrash
Crosshole Radar Tomography In A Fluvial Aquifer Near Boise, Idaho, William P. Clement, Warren Barrash
Geosciences Faculty Publications and Presentations
To determine the distribution of heterogeneities in the saturated zone of an unconfined aquifer in Boise, ID, we compute tomograms for three adjacent well pairs. The fluvial deposits consist of unconsolidated cobbles and sands. We used a curved-ray, finite-difference approximation to the eikonal equation to generate the forward model. The inversion uses a linearized, iterative scheme to determine the slowness distribution from the first arrival traveltimes. The tomograms consist of a layered zone representing the saturated aquifer. The velocities in this saturated zone range between 0.06 to 0.10 m/ns. We use a variety of methods to assess the reliability of …
Ground-Penetrating Radar Theory And Application Of Thin-Bed Offset-Dependent Reflectivity, John H. Bradford, Jacob C. Deeds
Ground-Penetrating Radar Theory And Application Of Thin-Bed Offset-Dependent Reflectivity, John H. Bradford, Jacob C. Deeds
CGISS Publications and Presentations
Offset-dependent reflectivity or amplitude-variationwith- offset (AVO) analysis of ground-penetrating radar (GPR) data may improve the resolution of subsurface dielectric permittivity estimates. A horizontally stratified medium has a limiting layer thickness below which thin-bed AVO analysis is necessary. For a typical GPR signal, this limit is approximately 0.75 of the characteristic wavelength of the signal. Our approach to modeling the GPR thin-bed response is a broadband, frequency-dependent computation that utilizes an analytical solution to the three-interface reflectivity and is easy to implement for either transverse electric (TE) or transverse magnetic (TM) polarizations. The AVO curves for TE and TM modes differ …
Reflectivity Modeling Of A Ground-Penetrating-Radar Profile Of A Saturated Fluvial Formation, William P. Clement, Warren Barrash, Michael D. Knoll
Reflectivity Modeling Of A Ground-Penetrating-Radar Profile Of A Saturated Fluvial Formation, William P. Clement, Warren Barrash, Michael D. Knoll
Geosciences Faculty Publications and Presentations
Major horizons in radar reflection profiles may correlate with contacts between stratigraphic units or with structural breaks such as fault surfaces. Minor reflections may be caused by clutter or, in some cases, may indicate material properties or structure within stratigraphic units. In this study, we examine the physical basis for major and minor reflections observed in a shallow, unconfined, fluvial aquifer near Boise, Idaho, U. S. A. We compare a 2D profile from a surface ground-penetrating-radar reflection transect with the 1D modeled reflection profiles at three wells adjacent to the surface-reflection profile. The 1D models are based on dielectric constant …
Investigating The Stratigraphy Of An Alluvial Aquifer Using Crosswell Seismic Traveltime Tomography, Geoff J.M. Moret, Michael D. Knoll, Warren Barrash, William P. Clement
Investigating The Stratigraphy Of An Alluvial Aquifer Using Crosswell Seismic Traveltime Tomography, Geoff J.M. Moret, Michael D. Knoll, Warren Barrash, William P. Clement
CGISS Publications and Presentations
In this study, we investigate the use of crosswell P-wave seismic tomography to obtain spatially extensive information about subsurface sedimentary architecture and heterogeneity in alluvial aquifers. Our field site was a research wellfield in an unconfined aquifer near Boise, Idaho. The aquifer consists of a ~ 20-m-thick sequence of alluvial cobble- and-sand deposits, which have been subdivided into five stratigraphic units based on neutron porosity logs, grainsize analysis, and radar reflection data. We collected crosswell and borehole-to-surface seismic data in wells 17.1 m apart. We carefully considered the impact of well deviation, data quality control, and the choice of inversion …
Traveltime Inversion Of Vertical Radar Profiles, William P. Clement, Michael D. Knoll
Traveltime Inversion Of Vertical Radar Profiles, William P. Clement, Michael D. Knoll
Geosciences Faculty Publications and Presentations
Traveltimes of direct arrivals in vertical radar profiles (VRPs) are tomographically inverted to estimate the earth’s electromagnetic (EM) velocity between a surface transmitter and a downhole receiver. We determine the 1D interval velocity model that best fits the first-arrival traveltimes by using a weighted, damped, least-squares inversion scheme. We assess the accuracy of the velocity model using synthetic traveltimes from a known velocity-distribution model simulating an unconfined aquifer. The inverted velocity profile closely matched the velocity profile of the input model in the synthetic examples. Using vertical radar profile data from an unconfined aquifer near Boise, Idaho, we inverted traveltimes …
Image Interpretation Using Appraisal Analysis, Partha S. Routh, Carlyle R. Miller
Image Interpretation Using Appraisal Analysis, Partha S. Routh, Carlyle R. Miller
Geosciences Faculty Publications and Presentations
In geophysical inversion, a significant effort is invested to obtain images of the Earth from finite data. The first step is to obtain an image i.e. solve the inverse problem. This step alone provides significant challenges that are not addressed inthis paper. The next step is to interpret the image in terms of specific questions. For example, what can we say about the average value of a physical property within a certain region of the model? What scale information can we resolve from the data? These questions are problem dependent and may require that inversion be carried out several times …
Applying Reflection Tomography In The Postmigration Domain To Multifold Ground-Penetrating Radar Data, John H. Bradford
Applying Reflection Tomography In The Postmigration Domain To Multifold Ground-Penetrating Radar Data, John H. Bradford
CGISS Publications and Presentations
Acquisition and processing of multifold ground-penetrating radar (GPR) data enable detailed measurements of lateral velocity variability. The velocities constrain interpretation of subsurface materials and lead to significant improvement in image accuracy when coupled with prestack depth migration (PSDM). Reflection tomography in the postmigration domain was introduced in the early 1990s for velocity estimation in seismic reflection. This robust, accurate method is directly applicable in multifold GPR imaging. At a contaminated waste facility within the U.S. Department of Energy’s Hanford site in Washington, the method is used to identify significant lateral and vertical velocity heterogeneity associated with infilled waste pits. Using …
Seeing Through The Noise: Seismic Reflection Profiling In Urban Areas, Lee M. Liberty, Thomas L. Pratt, Scott S. Hess
Seeing Through The Noise: Seismic Reflection Profiling In Urban Areas, Lee M. Liberty, Thomas L. Pratt, Scott S. Hess
CGISS Publications and Presentations
Studies for urban hazard or resource assessment often take place in densely populated areas characterized by considerable cultural noise. These site conditions can severely compromise seismic reflection data quality. We have collected vibroseis and hammer (weight drop) seismic reflection data in a range of geologic conditions to image stratigraphy and structures in the upper one km along regional highways, city streets,and power line access roads. In addition to the challenges of safety and outreach, acquisition efforts along busy streets and highways often encounter poor receiver coupling and large-amplitude coherent noise from traffic and power lines. Although higher quality seismic reflection …
Crosshole Radar Tomography In An Alluvial Aquifer Nearboise, Idaho, William P. Clement, Warren Barrash
Crosshole Radar Tomography In An Alluvial Aquifer Nearboise, Idaho, William P. Clement, Warren Barrash
CGISS Publications and Presentations
To determine the distribution of heterogeneities in an unconfined aquifer in Boise, ID, we compute radar tomograms for three adjacent well pairs. The fluvial deposits consist of unconsolidated cobbles and sands. We used a curved‐ray, finite‐difference approximation to the eikonal equation to generate the forward model. The inversion uses a linearized, iterative scheme to determine the slowness distribution from the first arrival traveltimes. The tomograms consist of a sequence of layers representing the saturated aquifer. The velocities in this saturated zone range between 0.06 to 0.10 m/ns. We use a variety of methods to assess the reliability of our velocity …
Relating Damping To Soil Permeability, Paul Michaels
Relating Damping To Soil Permeability, Paul Michaels
CGISS Publications and Presentations
Published comparisons of complex moduli in dry and saturated soils have shown that viscous behavior is only evident when a sufficiently massive viscous fluid (like water) is present. That is, the loss tangent is frequency dependent for water saturated specimens, but nearly frequency independent for dry samples. While the Kelvin-Voigt (KV) representation of a soil captures the general viscous behavior using a dashpot, it fails to account for the possibly separate motions of the fluid and frame (there is only a single mass element). An alternative representation which separates the two masses, water and frame, is presented here. This Kelvin-Voigt-Maxwell-Biot …
Comparison Of Viscous Damping In Unsaturated Soils, Compression And Shear, Paul Michaels
Comparison Of Viscous Damping In Unsaturated Soils, Compression And Shear, Paul Michaels
CGISS Publications and Presentations
Geophysical down-hole surveys can be used to measure the small strain dynamic properties of soils by the effects these properties have on wave propagation. The relevant effects include amplitude decay (corrected for beam divergence) and velocity dispersion. In this paper, down-hole data collected during the GeoInstitute's Denver 2000 field day are presented and analyzed as a Kelvin-Voigt solid. Findings for these unsaturated soils include viscous damping and stiffness which differ significantly for shear and compressional waves. A strong viscous damping is observed in compression, but weak damping is presented in shear. Lumped parameter constitutive models are discussed which mathematically represent …