John H. Bradford

Frequency Dependent Attenuation of GPR Data as a Tool for Material Property Characterization: A Review and New Developments

John H. Bradford — Wed, 28 Sep 2011 09:37:21 PDT

Variations in the spectrum of a ground-penetrating radar signal record characteristics of the material through which the signal has propagated. One measure of this rich source of information is frequency dependent attenuation. These data can help measure variations in clay fraction, the total volumetric water content, or identify the location of groundwater contaminants.

Electromagnetic Induction Calibration Using Antenna Transfer Functions

D. Moghadas et al. — Wed, 28 Sep 2011 09:37:19 PDT

The present commercial Electromagnetic induction (EMI) instruments applied for soil mapping have some limitations. Due to the lack of robustness in calibration of EMI instruments, the quantitative measure of apparent electrical conductivities has not been yet possible. We introduce a novel method, for calibration of EMI systems in which the EMI loop antennas are modeled using antenna transfer functions. We performed EMI measurements over the saline water with different increasing conductivity values under controlled laboratory conditions. Inversion of the model permits to retrieve conductivity of the water appropriately. This approach appears to be promising for estimation of apparent electrical conductivity.

A Radar Transparent Layer in a Temperate Valley Glacier: Bench Glacier, Alaska

Joel Brown et al. — Wed, 10 Aug 2011 14:49:02 PDT

Radar surveys of Bench Glacier, Alaska, collected over five field seasons between 2002 and 2006 reveal a surface layer of radar transparent ice in this temperate valley glacier. The transparent layer covers the up-glacier half of the ablation zone and is defined by a distinct lack of the radar scattering events considered typical of temperate ice. Radar scattering ice underlies the transparent zone, and extends to the surface elsewhere on the glacier. We observed the layering in constant offset radar surveys conducted with characteristic frequencies ranging from 5 MHz to 100 MHz. The radar transparent layer extends from the surface to 20 m depth on average, but up to 50 m in some places. Bench Glacier's transparent layer appears similar to the cold surface layer of polythermal glaciers, however, observations in over 50 boreholes on Bench Glacier suggest there is no cold ice corresponding to the radar transparent layer. We conclude that spatially extensive radar-transparent layers normally used to identify cold ice in polythermal glaciers are present in some temperate glaciers.

Evidence for Composite Hydraulic Architecture in an Active Fault System Based on 3D Seismic Reflection, Time-Domain Electromagnetics and Temperature Data

Scott Hess et al. — Mon, 11 Jul 2011 09:46:55 PDT

Fault hydrology is a topic of scientific and practical importance but considerable uncertainty exists regarding the nature of structural controls on fluid flow. Here we use seismic reflection and time-domain electromagnetic data to develop a three-dimensional model of hydraulic architecture in a predominantly dip-slip normal fault system and we predict the architectural elements based on subsurface fluid flow patterns inferred from near-surface temperature measurements. Our observations indicate the presence of high-permeability flow paths parallel to fault planes in poorly-lithified sediments. These results are best explained using a combination of elements from commonly accepted conceptual models of fault architecture, a finding that exhibits the heterogeneous nature of the geologic materials comprising the site. These insights may be useful as a guide to future studies of active fault systems, where multiple-mode investigations (geophysical, hydrologic, thermal, geochemical) will be required to better understand subsurface fluid/fault interactions.

Comparison of In-Channel Mobile–Immobile Zone Exchange During Instantaneous and Constant Rate Stream Tracer Additions: Implications for Design and Interpretation of Non-Conservative Tracer Experiments

Michael N. Gooseff et al. — Mon, 11 Jul 2011 09:46:52 PDT

The stream tracer experiment, including field tracer application and subsequent analysis of solute transport and storage, is an important tool in stream hydrology and ecology. However, there have been few comparisons of tracer dynamics between the commonly applied methods of instantaneous (IA) and constant rate (CRA) tracer additions. To determine whether there are fundamental differences between the two addition techniques due to surface storage zone loading and flushing during experiments, we compare longitudinal distributions of tracer dynamics of stream in-channel dead zones during IA and CRA experiments. Back-to-back IA and CRA additions were carried out in two morphologically distinct tundra stream reaches in Alaska. Dead zone tracer time series are determined by an aggregate of upstream transport and individual dead zone residence time distributions (RTDs). The dead zone breakthrough curves for both tracer addition techniques were not consistent, neither were aggregate RTDs observed in each dead zone. Flushing patterns of tracer from dead zones reveal that stream flushing after IA additions was slower than after CRA additions. However, whole-stream RTDs were similar between IA and CRA techniques in each reach. The implications of these findings are important to design and interpretation of IA and CRA stream tracer experiments, particularly those with reactive solutes whose transformations may depend on solute concentration. Thus, IA and CRA experiments may yield differing conclusions about non-conservative transport in streams because of the inherent differences in loading of transient storage zones between these two addition techniques, and potential differences in biogeochemical processing that may occur as a consequence.

Measuring Water Content Heterogeneity Using Multifold GPR with Reflection Tomography

John H. Bradford — Tue, 29 Mar 2011 15:26:05 PDT

Continuous multioffset acquisition of ground penetrating radar (GPR) data provides the capability to measure the lateral and vertical distribution of soil moisture. Multioffset data enable measurement of radar velocity, which in turn allows the estimation of soil moisture through an appropriate petrophysical relationship. Although rarely used in GPR investigations, reflection tomography coupled with prestack depth migration has the ability to measure lateral velocity variations with much greater resolution and accuracy than conventional methods of velocity analysis. I used reflection tomography in the post-migration domain to estimate radar velocity and the Topp equation to estimate subsurface moisture distribution in two and three dimensions. At a contaminated site near a former refinery I identified a near-vertical boundary separating coarse-grained sands and gravels from a unit containing a high fraction of silts and clays. At a chlorinated solvent waste site, I found significant heterogeneity in the moisture content distribution despite apparent homogeneity indicated by direct push methods.

Measuring Thaw Depth Beneath Peat-Lined Arctic Streams Using Ground-Penetrating Radar

John H. Bradford et al. — Tue, 29 Mar 2011 15:15:36 PDT

In arctic streams, depth of thaw beneath the stream channel is likely a significant parameter controlling hyporheic zone hydrology and biogeochemical cycling. As part of an interdisciplinary study of this system, we conducted a field investigation to test the effectiveness of imaging substream permafrost using ground-penetrating radar (GPR). We investigated three sites characterized by low-energy water flow, organic material lining the streambeds, and water depths ranging from 0·2 to 2 m. We acquired data using a 200 MHz pulsed radar system with the antennas mounted in the bottom of a small rubber boat that was pulled across the stream while triggering the radar at a constant rate. We achieved excellent results at all three sites, with a clear continuous image of the permafrost boundary both peripheral to and beneath the stream. Our results demonstrate that GPR can be an effective tool for measuring substream thaw depth.

Continuous Profiles of Electromagnetic Wave Velocity and Water Content in Glaciers: An Example from Bench Glacier, Alaska, USA

John Bradford et al. — Tue, 29 Mar 2011 11:01:53 PDT

We conducted two-dimensional continuous multi-offset georadar surveys on Bench Glacier, south-central Alaska, USA, to measure the distribution of englacial water. We acquired data with a multichannel 25MHz radar system using transmitter–receiver offsets ranging from 5 to 150 m. We towed the radar system at 5–10 kmh^–1 with a snow machine with transmitter/receiver positions established by geodetic-grade kinematic differentially corrected GPS (nominal 0.5m trace spacing). For radar velocity analyses, we employed reflection tomography in the pre-stack depth-migrated domain to attain an estimated 2% velocity uncertainty when averaged over three to five wavelengths. We estimated water content from the velocity structure using the complex refractive index method equation and use a three-phase model (ice, water, air) that accounts for compression of air bubbles as a function of depth. Our analysis produced laterally continuous profiles of glacier water content over several kilometers. These profiles show a laterally variable, stratified velocity structure with a low-watercontent (~0–0.5%) shallow layer (~20–30 m) underlain by high-water-content (1–2.5%) ice.

The Lower Cretaceous King Lear Formation, Northwest Nevada: Implications for Mesozoic Orogenesis in the Western U.S. Cordillera

Aaron J. Martin et al. — Tue, 29 Mar 2011 11:01:51 PDT

Cretaceous paleogeography of the U.S. Cordillera west of the Sevier fold-and-thrust belt is poorly known due to the scarcity of Cretaceous supracrustal rocks in the arc and backarc regions. The Lower Cretaceous King Lear Formation, exposed in the Jackson Mountains and the Krum Hills of northwest Nevada, provides a rare and important record of Early Cretaceous tectonism and paleogeography. Our work shows that the King Lear Formation everywhere overlies deformed and metamorphosed Triassic-Jurassic rocks across a major unconformity. The King Lear Formation is dominated by conglomerates and sandstones deposited in subaerial alluvial-fan and gravelly braided river systems, and it can be divided into three new members based on differences in clast provenance: a lower member derived from local arc and backarc rocks; a middle member dominated by externally derived quartzite and chert clasts; and an upper member derived largely from an intrabasinal volcanic complex. Sedimentary features and provenance analysis indicate that clasts in the middle member were derived from nearby exposures of Paleozoic rocks such as the Roberts Mountains and Golconda allochthons, which must have formed a topographically elevated area in central Nevada during the Early Cretaceous. The stratigraphic record provides evidence of deposition in a tectonically active basin. In contrast with some prior studies inferring that deposition was synchronous with contractional deformation, our new field observations, structural data, and a shallow seismic-reflection profile confirm that the King Lear Formation was deposited in an active half-graben in the Jackson Mountains. These results provide new upper age constraints on the timing of shortening deformation in the arc and backarc regions: shortening must have been complete not only before deposition of the King Lear Formation in the middle Early Cretaceous, but also prior to several kilometers of exhumation of its depositional basement. Possible driving forces for extension leading to development of the King Lear basin include relaxation of thick crust in the hinterland of thrust belts to the east, stresses associated with regional tilting and development of dynamic topography, and stresses related to dextral strike-slip deformation to the west.

Profiles of Temporal Thaw Depths Beneath Two Arctic Stream Types Using Ground-Penetrating Radar

Troy R. Brosten et al. — Tue, 29 Mar 2011 11:01:49 PDT

Thaw depths beneath arctic streams may have significant impact on the seasonal development of hyporheic zone hydraulics. To investigate thaw progression over the 2004 summer season we acquired a series of ground-penetrating radar (GPR) profiles at five sites from May–September, using 100, 200 and 400 MHz antennas. We selected sites with the objective of including stream reaches that span a range of geomorphologic conditions on Alaska's North Slope. Thaw depths interpreted from GPR data were constrained by both recorded subsurface temperature profiles and by pressing a metal probe through the active layer to the point of refusal. We found that low-energy stream environments react much more slowly to seasonal solar input and maintain thaw thicknesses longer throughout the late season whereas thaw depths increase rapidly within high-energy streams at the beginning of the season and decrease over the late season period.

Multi-Offset GPR Methods for Hyporheic Zone Investigations

T. R. Brosten et al. — Tue, 29 Mar 2011 11:01:47 PDT

Porosity of stream sediments has a direct effect on hyporheic exchange patterns and rates. Improved estimates of porosity heterogeneity will yield enhanced simulation of hyporheic exchange processes. Ground-penetrating radar (GPR) velocity measurements are strongly controlled by water content thus accurate measures of GPR velocity in saturated sediments provides estimates of porosity beneath stream channels using petrophysical relationships. Imaging the substream system using surface based reflection measurements is particularly challenging due to large velocity gradients that occur at the transition from open water to saturated sediments. The continuous multi-offset method improves the quality of subsurface images through stacking and provides measurements of vertical and lateral velocity distributions. We applied the continuous multi-offset method to stream sites on the North Slope, Alaska and the Sawtooth Mountains near Boise, Idaho, USA. From the continuous multi-offset data, we measure velocity using reflection tomography then estimate water content and porosity using the Topp equation. These values provide detailed measurements for improved stream channel hydraulic and thermal modelling.

Vertical Extension of the Subglacial Drainage System into Basal Crevasses

Joel T. Harper et al. — Mon, 14 Mar 2011 08:17:58 PDT

Water plays a first-order role in basal sliding of glaciers and ice sheets and is often a key constituent of accelerated glacier motion¹, ²,³, ⁴. Subglacial water is known to occupy systems of cavities and conduits at the interface between ice and the underlying bed surface, depending upon the history of water input and the characteristics of the substrate5. Full understanding of the extent and configuration of basal water is lacking, however, because direct observation is difficult. This limits our ability to simulate ice dynamics and the subsequent impacts on sea-level rise realistically. Here we show that the subglacial hydrological system can have a large volume of water occupying basal crevasses that extend upward from the bed into the overlying ice. Radar and seismic imaging combined with in situ borehole measurements collected on Bench Glacier, Alaska, reveal numerous water-filled basal crevasses with highly transmissive connections to the bed. Some crevasses extend many tens of metres above the bed and together they hold a volume of water equivalent to at least a decimetre layer covering the bed. Our results demonstrate that the basal hydrologic system can extend high into the overlying ice mass, where basal crevasses increase water-storage capacity and could potentially modulate basal water pressure. Because basal crevasses can form under commonly observed glaciological conditions, our findings have implications for interpreting and modelling subglacial hydrologic processes and related sliding accelerations of glaciers and ice sheets.

Estimating 3D Variation in Active-Layer Thickness Beneath Arctic Streams Using Ground-Penetrating Radar

Troy R. Brosten et al. — Mon, 14 Mar 2011 08:17:56 PDT

We acquired three-dimensional (3D) ground-penetrating radar (GPR) data across three stream sites on the North Slope, AK, in August 2005, to investigate the dependence of thaw depth on channel morphology. Data were migrated with mean velocities derived from multi-offset GPR profiles collected across a stream section within each of the 3D survey areas. GPR data interpretations from the alluvial-lined stream site illustrate greater thaw depths beneath riffle and gravel bar features relative to neighboring pool features. The peat-lined stream sites indicate the opposite; greater thaw depths beneath pools and shallower thaw beneath the connecting runs. Results provide detailed 3D geometry of active-layer thaw depths that can support hydrological studies seeking to quantify transport and biogeochemical processes that occur within the hyporheic zone.

Improved GPR Interpretation Through Resolution of Lateral Velocity Heterogeneity: Example from an Archaeological Site Investigation

Joel Brown et al. — Fri, 11 Mar 2011 09:18:39 PST

In a typical common-offset ground-penetrating radar (GPR) survey, lateral velocity contrasts may go undetected leading to misinterpretation. Resolution of lateral velocity heterogeneity requires multi-fold acquisition and analysis. Further, pre-stack depth migration (PSDM) is required to produce accurate images in the presence of large lateral velocity gradients. In an archaeological investigation conducted near Boise, Idaho, we delineated a portion of what we interpret to be an abandoned dump site. Using multi-fold acquisition with reflection tomography, we identified an abrupt lateral velocity increase of ~ 40% resulting in a substantial velocity pull-up in the time domain. PSDM corrected for the velocity pull-up enabling a more accurate interpretation and identification of additional structures of potential historical significance. The migration velocity model provided additional constraints on materials which enhanced our understanding of the subsurface.

Comparison of Instantaneous and Constant-Rate Stream Tracer Experiments through Parametric Analysis of Residence Time Distributions

Robert A. Payn et al. — Mon, 01 Nov 2010 14:51:12 PDT

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. Frequency- and time domain deconvolution techniques were used to derive RTDs from the resulting tracer concentrations. Comparisons of results between experiments of different duration demonstrated few differences in hydrologic retention characteristics inferred from short- and long-term tracer tests. Because non-parametric RTD analysis does not presume any shape of the distribution, it is useful for comparisons across tracer experiments with variable inputs and for validations of fundamental transport model assumptions.

Hyporheic Exchange and Water Chemistry of Two Arctic Tundra Streams of Contrasting Geomorphology

Morgan J. Greenwald et al. — Mon, 01 Nov 2010 14:51:09 PDT

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 thaw bulb was deeper than the hyporheic exchange zone in both stream types. The hyporheic zone was a net source of ammonium and soluble reactive phosphorus in both stream types. The hyporheic zone was a net source of nitrate in the alluvial stream, but a net nitrate sink in the peat stream. The mass flux of nutrients regenerated from the hyporheic zones in these two streams was a small portion of the surface water mass flux. Although small, hyporheic sources of regenerated nutrients help maintain the in-stream nutrient balance. If future warming in the arctic increases the depth of the thaw bulb, it may not increase the vertical extent of hyporheic exchange. The greater impacts on annual contributions of hyporheic regeneration are likely to be due to longer thawed seasons, increased sediment temperatures or changes in geomorphology.

Ground-Penetrating-Radar Reflection Attenuation Tomography with an Adaptive Mesh

Emily A. Hinz et al. — Mon, 25 Oct 2010 15:39:27 PDT

Ground-penetrating radar (GPR) attenuation-difference analysis can be a useful tool for studying fluid transport in the subsurface. Surface-based reflection attenuation-difference tomography poses a number of challenges that are not faced by crosshole attenuation surveys. We create and analyze a synthetic attenuation-difference GPR data set to determine methods for processing amplitude changes and inverting for conductivity differences from reflection data sets. Instead of using a traditional grid-based inversion, we use a data-driven adaptive-meshing algorithm to alter the model space and to create amore even distribution of resolution. Adaptive meshing provides a method for improving the resolution of the model space while honoring the data limitations and improving the quality of the attenuation difference inversion. Comparing inversions on a conventional rectangular grid with the adaptive mesh, we find that the adaptively meshed model reduces the inversion computation time by an average of 75% with an improvement in the root mean square error of up to 15%. While the sign of the conductivity change is correctly reproduced by the inversion algorithm, the magnitude varies by as much as much as 50% from the true values. Our heterogeneous conductivity model indicates that the attenuation difference inversion algorithm effectively locates conductivity changes, and that surface-based reflection surveys can produce models as accurate as traditional crosshole surveys.

Transient Storage as a Function of Geomorphology, Discharge, and Permafrost Active Layer Conditions in Arctic Tundra Streams

Jay P. Zarnetske et al. — Mon, 18 Oct 2010 12:54:40 PDT

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 stream tracer experiments in five morphologically diverse tundra streams in Arctic Alaska during the thaw season (May–August) of 2004 to assess transient storage and hydrologic characteristics. We compared transient storage model parameters to discharge (Q), the Darcy-Weisbach friction factor (f), and unit stream power (ω). Across all studied streams, permafrost active layer depths (i.e., the potential extent of the hyporheic zone) increased throughout the thaw season, and discharges and velocities varied dramatically with minimum ranges of eight-fold and four-fold, respectively. In all reaches the mean storage residence time (t_stor) decreased exponentially with increasing Q, but did not clearly relate to permafrost active layer depths. Furthermore, we found that modeled transient storage metrics (i.e., t_stor, storage zone exchange rate (α_OTIS), and hydraulic retention (R_h)) correlated better with channel hydraulic descriptors such as f and ω than they did with Q or channel slope. Our results indicate that Q is the first-order control on transient storage dynamics of these streams, and that f and ω are two relatively simple measures of channel hydraulics that may be important metrics for predicting the response of transient storage to perturbations in discharge and morphology in a given stream.

Depth Characterization of Shallow Aquifers with Seismic Reflection, Part I—The Failure of NMO Velocity Analysis and Quantitative Error Prediction

John H. Bradford — Mon, 18 Oct 2010 08:13:31 PDT

As seismic reflection data become more prevalent as input for quantitative environmental and engineering studies, there is a growing need to assess and improve the accuracy of reflection processing methodologies. It is common for compressional-wave velocities to increase by a factor of four or more where shallow, unconsolidated sediments change from a dry or partially watersaturated regime to full saturation. While this degree of velocity contrast is rare in conventional seismology, it is a common scenario in shallow environments and leads to significant problems when trying to record and interpret reflections within about the first 30 m below the water table. The problem is compounded in shallow reflection studies where problems primarily associated with surface-related noise limit the range of offsets we can use to record reflected energy. For offset-to-depth ratios typically required to record reflections originating in this zone, the assumptions of NMO velocity analysis are violated, leading to very large errors in depth and layer thickness estimates if the Dix equation is assumed valid. For a broad range of velocity profiles, saturated layer thickness will be overestimated by a minimum of 10% if the boundary of interest is <30 m below the water table. The error increases rapidly as the boundary shallows and can be very large>(>100%) if the saturated layer is <10 m thick. This degree of error has a significant and negative impact if quantitative interpretations of aquifer geometry are used in aquifer evaluation such as predictive groundwater flow modeling or total resource>estimates.

Depth Characterization of Shallow Aquifers with Seismic Reflection, Part II—Prestack Depth Migration and Field Examples

John H. Bradford et al. — Mon, 18 Oct 2010 08:13:29 PDT

It is common in shallow seismic studies for the compressional-wave velocity in unconsolidated sediments to increase by a factor of four or more at the transition from dry or partial water saturation to full saturation. Under these conditions, conventional NMO velocity analysis fails and leads to large depth and layer thickness estimates if the Dix equation is assumed valid. Prestack depth migration (PSDM) is a means of improving image accuracy. A comparison of PSDM with conventional NMO processing for three field examples from differing hydrogeologic environments illustrates that PSDM can significantly improve image quality and accuracy.