John H. Bradford

An Evaluation of the Hydrologic Relevance of Lateral Flow in Snow at Hillslope and Catchment Scales

David Eiriksson et al. — Thu, 28 Mar 2013 10:35:45 PDT

Lateral downslope ﬂow in snow during snowmelt and rain-on-snow (ROS) events is a well-known phenomenon, yet its relevance to water redistribution at hillslope and catchment scales is not well understood. We used dye tracers, geophysical methods, and hydrometric measurements to describe the snow properties that promote lateral ﬂow, assess the relative velocities of lateral ﬂow in snow and soil, and estimate volumes of downslope ﬂow. Results demonstrate that rain and melt water can travel tens of metres downslope along layers within the snowpack or at the snowpack base within tens of hours. Lateral ﬂow within the snowpack becomes less likely as the snowpack becomes saturated and stratigraphic boundaries are destroyed. Flow along the base can be prevalent in all snowpack conditions. The net result of lateral ﬂow in snow can be the deposition of water on the soil surface in advanced downslope positions relative to its point of origin, or direct discharge to a stream. Although both melt and ROS events can redistribute water to downslope positions, ROS events produced the most signiﬁcant volumes of downslope ﬂow. Direct stream contributions through the snowpack during one ROS event produced up to 12% of streamﬂow during the event. This can help explain rapid delivery of water to streams during ROS events, as well as anomalously high contributions of event water during snowmelt hydrographs. In catchments with a persistent snowpack, lateral redistribution of water within the snowpack should be considered a relevant moisture redistribution mechanism.

The Emergence and Future of Near-Surface Geophysics

William E. Doll et al. — Tue, 15 Jan 2013 08:36:15 PST

Over the past 30 years, geophysical methods have assumed a much more prominent and integral role in many investigations where subsurface features have environmental and engineering importance. In fact, the field once referred to as "environmental and engineering geophysics" has broadened to include other applications (e.g., archeology, forensics), and is now commonly referred to more generally as "near-surface geophysics." It is difficult to precisely define near-surface geophysics, and the definition will likely depend on whom you ask. However, we define it as the use of geophysical methods to investigate the zone between the surface and hundreds of meters into the Earth's crust. Applications include, but are not limited to, potable water management, engineered infrastructure and construction, site clearance, gas storage, natural-hazard mitigation, mining, forensics, and archaeology. Although the same physical principles are relevant for any target depth, the high degree of near-surface heterogeneity, rapid change in physical properties, and proximity to the free surface often dictates that dominant processes and therefore key assumptions differ between the near-surface and deeper investigations. While near-surface geophysics shares many technical and cultural attributes of oil and gas exploration, the majority of near-surface geophysicists practice under different economic drivers and conditions.

The Search for the Lost Graves of the Chinese Miners in Hailey, Idaho, USA

John Bradford — Wed, 03 Oct 2012 14:15:43 PDT

Thousands of Chinese immigrants traveled to Idaho, USA during the gold mining boom in the latter half of the 19th century. In the former mining town of Hailey, a separate section of the cemetery was established to accommodate Chinese laborers. In the 1930's, a fire destroyed the wooden grave markers in the Chinese section of the cemetery. As part of an effort to establish a memorial recognizing the contributions of these workers, a ground-penetrating radar (GPR) survey was commissioned in 2009 to identify the graves so that permanent markers could be placed. Interpretation of the pseudo-3D GPR survey identified 120 unmarked graves in the Chinese section of the cemetery. Efforts are currently underway to place markers on these gravesites and to raise funds for a permanent memorial to the Chinese laborers.

Advances in Near-Surface Seismology and Ground-Penetrating Radar

Richard D. Miller et al. — Wed, 03 Oct 2012 10:51:20 PDT

Advances in Near-surface Seismology and Ground-penetrating Radar (SEG Geophysical Developments Series No. 15) is a collection of original papers by renowned and respected authors from around the world. Technologies used in the application of near-surface seismology and ground-penetrating radar have seen significant advances in the last several years. Both methods have benefited from new processing tools, increased computer speeds, and an expanded variety of applications. This book, divided into four sections - "Reviews," "Methodology," "Integrative Approaches," and "Case Studies" - captures the most significant cutting-edge issues in active areas of research, unveiling truly pertinent studies that address fundamental applied problems. This collection of manuscripts grew from a core group of papers presented at a postconvention workshop, "Advances in Near-surface Seismology and Ground-penetrating Radar," held during the 2009 SEG Annual Meeting in Houston, Texas. This is the first cooperative publication effort between the near-surface communities of SEG, AGU, and EEGS. It will appeal to a large and diverse audience that includes researchers and practitioners inside and outside the near-surface geophysics community.

Integrated Hydrostratigraphic Interpretation of 3D Seismic-Reflection and Multifold Pseudo-3D GPR Data

John H. Bradford — Wed, 26 Sep 2012 16:20:43 PDT

To map the 3D distribution of major hydrologic boundaries in a shallow aquifer near Boise, Idaho, 3D seismic-reflection data and multifold, pseudo-3D ground-penetrating- radar (GPR) data were analyzed. The seismic data covered a 75-X 70-m area and imaged horizons from 18 to 150 m deep. The 10-fold, 50-MHz GPR data were acquired on a 20- X 30-m grid using a multichannel GPR system and offsets ranging from 2 m to 20 m. By correlating the well-resolved GPR depth image with a clay-aquitard seismic reflection, the seismic-velocity model was improved substantially and the accuracy of the final interpretation was improved. The resulting clay-aquitard surface differed by 0.12+0.46 m from the depth to clay measured in wells. By integrating the interpretations of the GPR and seismic data, a 3D map of major hydrostratigraphic boundaries was produced.

Permittivity Structure Derived from Group Velocities of Guided GPR Pulses

Matthew M. Haney et al. — Wed, 26 Sep 2012 16:20:42 PDT

On a 2D profile of subsurface permittivity structure derived from guided GPR pulses recorded in the Kuparuk River watershed, Alaska, the transition from a stream channel to a peat layer is interpreted. Although multichannel data are used, guided waves are analyzed using single-channel analysis, which sidesteps assumptions regarding lateral homogeneity within receiver arrays. As a result, 2D structure is obtained along a profile using an inversion procedure. These data were processed in three steps: (1) picking group traveltimes, (2) performing tomography in the lateral direction, and (3) inverting local group-velocity dispersion curves. When the permittivity profile obtained from the guided waves is compared to a GPR reflection profile, it is clear that the guided waves capture shallow structure near a stream channel that is not imaged accurately on the reflection profile. This demonstrates the utility of using guided waves to provide information on shallow structure that cannot be obtained from reflections.

Electromagnetic Induction Antenna Modelling Using a Linear System of Complex Antenna Transfer Functions

Davood Moghadas et al. — Tue, 17 Jul 2012 07:41:50 PDT

The quantitative retrieval of soil apparent electrical conductivity using electromagnetic induction (EMI) has remained limited due to strong simplifications regarding EMI antenna modelling. In this paper, a new technique for EMI antenna modelling is applied for the common-offset EMI systems. The EMI system is efficiently described using global transmission and reflection coefficients and Green’s functions are used to describe wave diffusion for horizontal and vertical dipole modes. We performed EMI measurements along a 180-metre-long transect with two different instrument heights above the soil surface, as well as with different orientations and frequencies. To ensure proper retrieval of the soil apparent electrical conductivity, the reference values were obtained from electrical conductivity data measured from 11 undisturbed soil cores taken along the EMI transect. The apparent electrical conductivity values calculated by applying the proposed model have a good agreement with reference values, however some discrepancies can be observed that are mainly attributed to the presence of local heterogeneities and also errors due to the variations in the height of the EMI instruments above the ground. The proposed method appears to be promising for quantitative retrieval of soil apparent electrical conductivity and resolving calibration issues that are typically encountered using EMI. In addition, the model calibration (antenna transfer functions determination) was successfully accomplished using conductivity values measured from the soil cores.

Monitoring Glacier Surface Seismicity in Time and Space Using Rayleigh Waves

T. D. Mikesell et al. — Thu, 07 Jun 2012 08:27:38 PDT

Sliding glaciers and brittle ice failure generate seismic body and surface wave energy characteristic to the source mechanism. Here we analyze continuous seismic recordings from an array of nine short-period passive seismometers located on Bench Glacier, Alaska (USA) (61.033°N, 145.687°W). We focus on the arrival-time and amplitude information of the dominant Rayleigh wave phase. Over a 46-hour period we detect thousands of events using a cross-correlation based event identification method. Travel-time inversion of a subset of events (7% of the total) defines an active crevasse, propagating more than 200 meters in three hours. From the Rayleigh wave amplitudes, we estimate the amount of volumetric opening along the crevasse as well as an average bulk attenuation ( Q ¯ = 42) for the ice in this part of the glacier. With the remaining icequake signals we establish a diurnal periodicity in seismicity, indicating that surface run-off and subglacial water pressure changes likely control the triggering of these surface events. Furthermore, we find that these events are too weak (i.e., too noisy) to locate individually. However, stacking individual events increases the signal-to-noise ratio of the waveforms, implying that these periodic sources are effectively stationary during the recording period.

High-Resolution Study of Layering Within the Percolation and Soaked Facies of the Greenland Ice Sheet

Joel Brown et al. — Mon, 16 Apr 2012 15:39:29 PDT

Within the percolation and soaked facies of the Greenland ice sheet, the relationship between radar-derived internal reflection horizons and the layered structure of the firn column is unclear. We conducted two small-scale ground-penetrating radar (GPR) surveys in conjunction with 10 m firn cores that we collected within the percolation and soaked facies of the Greenland ice sheet. The two surveys were separated by a distance of ∼50 km and ∼340 m of elevation leading to ∼40 days of difference in the duration of average annual melt. At the higher site (∼1997 m a.s.l.), which receives less melt, we found that internal reflection horizons identified in GPR data were largely laterally continuous over the grid; however, stratigraphic layers identified in cores could not be traced between cores over any distance from 1.5 to 14.0 m. Thus, we found no correlation between firn core stratigraphy observed directly and radar-derived internal reflection horizons. At the lower site (∼1660 m a.s.l.), which receives more melt, we found massive ice layers >0.5 m thick and stratigraphic boundaries that span >15 m horizontally. Some ice layers and stratigraphic boundaries correlate well with internal reflection horizons that are laterally continuous over the area of the radar grid. Internal reflection horizons identified at ∼1997 m a.s.l. are likely annual isochrones, but the reflection horizons identified at ∼1660 m a.s.l. are likely multi-annual features. We find that mapping accumulation rates over long distances by tying core stratigraphy to radar horizons may lead to ambiguous results because: (1) there is no stratigraphic correlation between firn cores at the 1997 m location; and (2) the reflection horizons at the 1660 m location are multi-annual features.

Georadar-Derived Estimates of Firn Density in the Percolation Zone, Western Greenland Ice Sheet

Joel Brown et al. — Mon, 19 Mar 2012 14:58:37 PDT

Greater understanding of variations in firn densification is needed to distinguish between dynamic and melt-driven elevation changes on the Greenland ice sheet. This is especially true in Greenland’s percolation zone, where firn density profiles are poorly documented because few ice cores are extracted in regions with surface melt. We used georadar to investigate firn density variations with depth along a ~70 km transect through a portion of the accumulation area in western Greenland that partially melts. We estimated electromagnetic wave velocity by inverting reflection traveltimes picked from common midpoint gathers. We followed a procedure designed to find the simplest velocity versus depth model that describes the data within estimated uncertainty. On the basis of the velocities, we estimated 13 depth-density profiles of the upper 80 m using a petrophysical model based on the complex refractive index method equation. At the highest elevation site, our density profile is consistent with nearby core data acquired in the same year. Our profiles at the six highest elevation sites match an empirically based densification model for dry firn, indicating relatively minor amounts of water infiltration and densification by melt and refreeze in this higher region of the percolation zone. At the four lowest elevation sites our profiles reach ice densities at substantially shallower depths, implying considerable meltwater infiltration and ice layer development in this lower region of the percolation zone. The separation between these two regions is 8 km and spans 60 m of elevation, which suggests that the balance between dry-firn and melt-induced densification processes is sensitive to minor changes in melt.

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.