ML073550300
| ML073550300 | |
| Person / Time | |
|---|---|
| Issue date: | 12/18/2007 |
| From: | Dias A NRC/ACNW&M |
| To: | |
| Coleman, N. 301-415-7656 | |
| References | |
| Download: ML073550300 (11) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON NUCLEAR WASTE & MATERIALS WASHINGTON, DC 20555 - 0001 December 18, 2007 MEMORANDUM TO: ACNW&M Members FROM:
Antonio F. Dias, Chief
/RA/
Nuclear Waste & Material Branch, ACRS/ACNW&M
SUBJECT:
TRANSMITTAL OF A TRIP REPORT BY ACNW&M STAFF ON THE FALL MEETING OF THE AMERICAN GEOPHYSICAL UNION, DECEMBER 2007 Enclosed is the trip report by Senior Staff Scientist Neil Coleman regarding his attendance at the American Geophysical Union (AGU) meeting. He chaired session V11D (Volcanic Hazards and Monitoring) and gave a talk titled Evaluating Consequences of Volcanism for Spent Nuclear Fuel at Yucca Mountain, Nevada. Member William Hinze also attended this meeting and gave a talk titled Igneous Activity at Yucca Mountain - Technical Basis for Decisionmaking. Both presentations summarized key points from the Committees white paper on igneous activity at Yucca Mountain. Consultant Bruce Marsh (Johns Hopkins University) was a co-author for both presentations and the published abstracts. This fall meeting of the AGU was held in San Francisco, CA from December 10-14, 2007.
Enclosure:
As stated cc w/att:
F. Gillespie S. Jones ACNW&M Staff
December 18, 2007 MEMORANDUM TO: ACNW&M Members FROM:
Antonio F. Dias, Chief Nuclear Waste & Material Branch, ACRS/ACNW&M
SUBJECT:
TRANSMITTAL OF A TRIP REPORT BY ACNW&M STAFF ON THE FALL MEETING OF THE AMERICAN GEOPHYSICAL UNION, DECEMBER 2007 Enclosed is the trip report by Senior Staff Scientist Neil Coleman regarding his attendance at the American Geophysical Union (AGU) meeting. He chaired session V11D (Volcanic Hazards and Monitoring) and gave a talk titled Evaluating Consequences of Volcanism for Spent Nuclear Fuel at Yucca Mountain, Nevada. Member William Hinze also attended this meeting and gave a talk titled Igneous Activity at Yucca Mountain - Technical Basis for Decisionmaking. Both presentations summarized key points from the Committees white paper on igneous activity at Yucca Mountain. Consultant Bruce Marsh (Johns Hopkins University) was a co-author for both presentations and the published abstracts. This fall meeting of the AGU was held in San Francisco, CA from December 10-14, 2007.
Enclosure:
As stated cc w/att:
F. Gillespie S. Jones ACNW&M Staff G:\\Coleman\\TripReport AGU meeting (Dec 2007).doc OFFICE ACNW&M ACNW&M ACNW&M NAME NColeman (SUNSI) WHinze ADias DATE 12/ 18 /07 12/ 18 /07 12/ 18 /07 OFFICIAL RECORD COPY
Trip Report - Attendance at the 2007 Fall Meeting of the American Geophysical Union, San Francisco, California William Hinze, ACNW&M Member Neil Coleman, ACNW&M Senior Staff Scientist William Hinze and Neil Coleman attended the first four days of the Fall Meeting of the American Geophysical Union during December 10-13, 2007. Neil Coleman chaired session V11D (Volcanic Hazards and Monitoring) and gave a talk titled Evaluating Consequences of Volcanism for Spent Nuclear Fuel at Yucca Mountain, Nevada. Member William Hinze gave a talk titled Igneous Activity at Yucca Mountain - Technical Basis for Decisionmaking. Both presentations summarized key points from the Committees white paper on igneous activity at Yucca Mountain. Consultant Bruce Marsh (Johns Hopkins University) was a co-author for both presentations and their published abstracts. Our other focus at this meeting was to gather information that would help support current interests and planned activities of the ACNW&M. To that end we attended presentations on topics such as volcanism, performance confirmation, unsaturated and saturated zone hydrology, geophysics, bioremediation, and tectonics.
Additional details about these topics are given below. We also refer readers to abstracts from a session titled Biogeophysics. This is a rapidly developing, multi-disciplinary field of study that holds considerable promise for applications to waste disposal sites. See abstracts on the web at: http://www.agu.org/cgi-bin/sessions5?meeting=fm07&part=NS11B&maxhits=400.
Volcanism F. Perry (Los Alamos National Laboratory) gave a talk titled Twilight of a Volcanic Field: 11 Million Years of Basaltic Volcanism in the Southwestern Nevada Volcanic Field, USA.
Following the end of major caldera-forming silicic volcanism in the Southwestern Nevada Volcanic Field (SNVF), at least 10 episodes of alkalic basaltic volcanism have occurred over the last ~11 Ma. An understanding of the past behavior of the volcanic field may help to forecast future eruptive behavior for use in hazard assessment for the proposed repository at Yucca Mountain. A program of geophysics, drilling, Ar-Ar dating and geochemistry conducted since 2004 by Los Alamos National Laboratory and the U.S. Geological Survey, combined with previous and ongoing petrogenetic and physical volcanology studies, sheds more light on the early and middle evolution of the volcanic field, much of which has been buried in alluvial basins. Volumes of erupted basalt in this volcanic field have greatly declined over time, from as much as 50 km3 in the Miocene to about 0.5 km3 in the Pleistocene. The volume decrease was accompanied by a drastic decrease in extension rate, suggesting a close link between magmatism and tectonism. Neodymium and strontium isotopic analyses indicate that enriched lithospheric mantle has been the source of basalt throughout the history of the field. Decreasing eruption volumes are accompanied by an approximate doubling of Ce/Yb ratios, indicating that the volume decrease reflects a decrease in degree of partial melting of the lithospheric source.
Eruption style has also changed with time, reflecting an increase in magma volatile content consistent with decreased amounts of partial melting of a volatile-bearing source. These observations are consistent with a model in which the lithospheric mantle source was hottest during the period of major silicic volcanism (Miocene) and the presence of an active plate subduction system. After the breakdown of subduction, continued thermal input into the lithosphere ended and the lithosphere began to cool. Melt accumulation in non-convecting, static lithosphere is probably related to mantle heterogeneities enriched in hydrous minerals that are partially melted. During regional extension, these zones are relatively weak and preferentially deform, forming melt bands of increased porosity that concentrate melt and lead to
2 dike generation. Decreasing regional extension results in less melt accumulation and decreasing eruption volumes. Without a new source of heat and limited lithospheric extension, it is likely that the next million years of volcanic activity in the field will likely be characterized by eruptions of the type that have occurred during the past million years of activity: infrequent eruptions of small-volume (<0.1 km3), volatile-rich alkali basalt magmas within the most tectonically active southern and western margins of the volcanic field.
E. Gaffney (Itasca Consulting Group) gave a presentation titled Deformation of Magma-Filled Bodies during Solidification. As magma or lava solidifies, volatiles are concentrated in the residual liquid. The result will be expansion (including venting) or pressurization. An approximate calculation indicates that an alkali basalt with 4 wt% volatiles would attain 12 MPa with 50% crystallization at constant volume. Such pressures would easily be enough to break through the roof of a typical lava tube. If confined in a tunnel deeper in the ground, even in a relatively weak rock, crystallization would be virtually isochoric (where the volume remains constant). However, in a sill at depths of only a few hundred meters, expansion could result in more nearly isobaric (constant pressure) crystallization. In either event, before cooling enough to become a brittle solid, the outer portions of the magma would reach a viscoplastic state that could seal and trap any remaining vapor phase. This would allow pressures to increase further as solidification progressed. The computer code PELE, which was developed to calculate the progress of solidification, was used to calculate isochoric and isobaric equilibrium crystallization of alkali basalt and obtain pressures and viscosities as a function of temperature. For an initial pressure of 6 MPa and 0.85 weight percent water, the liquidus is 1433 K. The isochoric pressure reaches 11 MPa at 1293 K with 57% of the mass crystallized; the bulk viscosity is about 3 MPa-s, but that of the residual liquid is only 1 kPa-s. At the same temperature, the isobaric path results in 60% crystallization and a viscosity of about 10 kPa-s. A tabular body with these properties would be easily deformed by sagging of the roof if the viscoplastic seal were breached, resulting in a saucer shape. With 91% of the mass crystallized, the isochoric pressure exceeds 28 MPa at 1173 K. By that time, the bulk viscosity of the nearly crystallized mass is on the order of 1025 Pa-s, effectively solid, and the viscosity of the residual liquid (there is also a vapor phase) is about 50 kPa-s. When these results were combined with those of a calculation of the cooling of magma in a horizontal cylinder, it was found that the interior of the magma is at high-pressure (>10 MPa) and quite fluid ( < 1 kPa-s) for long enough times to allow considerable deformation of any structures that might be entombed in magma in a deep tunnel.
S. Biswas (Southwest Research Institute) gave a presentation titled Magnetic Modeling of Buried Basalt Near the Potential Repository at Yucca Mountain, Nevada, Constrained by New Paleomagnetic, Rock Magnetic, and Petrographic Studies. Probability estimates for igneous disruption of the potential repository at Yucca Mountain, Nevada, are affected by uncertainties in the number and age of basaltic volcanoes possibly buried in the area. To reduce these uncertainties, the U.S. Department of Energy (DOE) sponsored studies, including a high-resolution aeromagnetic survey of the Yucca Mountain region, to identify potential sites of buried basaltic intrusions and their characteristics. The survey was conducted using a helicopter with an average sensor elevation of 40-50 m above terrain. Based on the resulting anomaly map, a subset of seven anomalies (A, G, I, JF5, JF6, O, and Q) was identified for additional testing. These seven anomaly sites were cored to determine whether buried basalt was the source of the anomalies. Basalt was found in four of the seven boreholesA, G, JF5, and Q.
Basalt samples from these four boreholes were collected for additional analyses, including radiometric age determinations and mineral identification. The authors presented petrographic, paleomagnetic, and geophysical modeling results from an independent review of the DOE aeromagnetic data and analysis of core samples. Experiments included measurements of (i) natural remanent magnetization, (ii) alternating field demagnetization and thermal
3 demagnetization to isolate the inclination of the characteristic remanent magnetization,(iii) room-temperature bulk susceptibility, (iv) temperature dependence of low-field susceptibility to 700 °C
[1,292 °F], and (v) hysteresis and coercivity. The paleomagnetic measurements characterized the magnetic properties of the samples and were used as input for the models. The modeling approach included development of two-dimensional forward models along optimally oriented profiles for each of the four magnetic anomalies. Magnetic source bodies were developed as geologically reasonable polygons with known or inferred magnetic properties for each forward model. Geometry of the source polygons was derived from a combination of known subsurface geologic conditions from the boreholes, extrapolation of nearby geologic structures to the subsurface, analogs to similar features in the region, and general geologic principles. Forward modeling consisted of trial and error alteration of the polygons (both properties and geometries) until the models produced a magnetic response curve that closely matched the observed magnetic profile. Results from the geophysical modeling indicate that the profiles of anomalies G, JF5, and Q in the Yucca Mountain region can be modeled as two-dimensional bodies of relatively thin (10-140 m) basaltic rock. The study of Anomaly A coupled with observations from the core and from thin sections suggests that this anomaly is best modeled as a relatively thick
(>500 m) basaltic rock suggestive of a sill. Abundant relict amphibole phenocrysts in anomaly G basalt are not seen in contemporaneous surface exposures in nearby Crater Flat, indicating that at least two distinct magma types were erupted at approximately 3.8 Myr.
M. Necsoiu (Southwest Research Institute) gave a presentation titled Physical Properties of Volcanic Material (Tephra) Using Visible Near-Infrared Spectroscopy. Sunset Crater, Arizona, offers the opportunity to study relationships between visible near-infrared reflectance and physical properties of volcanic material at a 900-year-old tephra deposit in a semiarid climate.
This is an analog area for latent eruption and post-eruption surface processes near the potential high-level waste repository at Yucca Mountain, Nevada. Quantitative and qualitative analyses were performed on tephra modified by eolian processes to study the effects of grain size, shape, texture, and weathering on spectral response. Reflectance spectra were collected from homogeneous sample splits separated by sieve fraction. Principal component analysis (PCA) was applied to decompose data by finding maximum variances, so the complexity of tephra samples could be easily interpreted. Partial least squares (PLS) was used to develop a linear calibration model between grain size and spectral reflectance of sieve fractions. The model was used to estimate grain-size distributions of other tephra samples collected from other locations in the study area. The trends observed in the spectral reflectance of these samples showed that a complex relationship exists between reflectance and geometry/grain size of the analyzed fractions. The first and second principal components were useful to separate the samples based on shape, texture, and the amount of weathering. The grain size of a homogeneous sample affects the reflectance properties such that an increase in grain size produces a decrease in reflectance. This trend is noticeable for grain-size sieve fractions less than 0.6 mm. The authors seek to improve their understanding of the relationships between physical properties and spectral response of volcanic material in the visible and near-infrared regions. These relationships are being used to support investigations of the extent of tephra deposit remobilization and redistribution, resuspension, rates of weathering and erosion, and grain-size characteristics.
M. Morrissey (Colorado School of Mines) gave a talk titled Expected Behavior of Basaltic Magma with the Proposed High Level Nuclear Repository at Yucca Mountain, Nevada. The expected series of eruptive events for a future igneous event in Yucca Mountain within the next 1 Myr is comparable to that at the Lathrop Wells basalt center and other Crater Flat Quaternary volcanoes. Lathrop Wells and Crater Flat Quaternary volcanoes are generally comprised of a single scoria cone with one or two lava flow fields extending from the base. The lava flow fields
4 associated with scoria cones all appear to extend from the base of a scoria cone and to be comprised of lava terraces. A three-dimensional model of the plumbing system for a possible future igneous event was presented, based on the characteristic features of eruptive deposits at Lathrop Wells and other Crater Flats Quaternary volcanoes. The model also describes consequences related to the interaction between magma and the repository. The repository is expected to be 200-300 m below the surface and comprised of parallel drifts 5 m in diameter, 0.5-1.0 km in length and spaced 85 m apart. Each drift is to be filled with a series of 1.8 m diameter waste packages made of Alloy 22. The conceptual model of the plumbing system and related consequences are described in six stages. Stage 1 Intersection of dike with drift: One dike will intersect the repository. The width of a future dike in YMR is expected to vary along the length with a maximum value of < 4.0 m at repository depths. The number of drifts that would be intersected by the dike will be 6-24 depending on the lateral extent of the dike through the repository. Stage 2 Initial stage magma-drift interaction: The lateral variation in magma properties would produce two different styles of expected activity upon entering a drift: a mixture of gas and fragments of magma characteristic of a lava fountain at wide portions of the dike, and crystallizing magma relatively depleted in volatiles at the narrowest part of the dike. A spray of pyroclastics is expected inside a drift from a lava fountain that will bombard and coat waste packages with magma. Crystallizing magma relatively depleted in volatiles will be a slowly moving, crystallizing flow that would be expected to behave like a plug sealing the drift. Stage 3 Surface activity: initial cone building stage: Magma that is not diverted into the drift will follow the crack tip and make its way to the surface and erupt at the surface along the fissure as a curtain of lava fountains. A conduit or cone building part of the eruption will develop at the widest part of the dike. Stage 4 Second stage of magma drift interaction: Strombolian activity at the repository depth is expected to occur in only one drift; the drift that is intersected by the widest part the dike. This drift would be inundated with pyroclastic material associated with the early cone-building Strombolian events. Lava is expected to enter an adjacent drift as discrete pulses.
Stage 5 Surface activity: final cone building phase: At the surface, activity will transition to a more violent Strombolian style. Additional discrete pulses of lava may occur. Stage 6 Final stage magma drift interaction: Magma entering drifts at this stage will be either a pyroclastic flow or lava.
Performance Confirmation C. Dinwiddie (Southwest Research Institute) gave a talk titled Sensors and Monitoring Techniques for the Deep Unsaturated Zone: Reducing Uncertainty Related to Seepage and Transport in Fractured Rock. Planning for performance confirmation of hydrologic properties and processes in a potential geologic repository for high-level radioactive waste at Yucca Mountain is a requirement stated in Subpart F of 10 CFR Part 63. An important goal of performance confirmation is to acquire information indicating whether natural and engineered barriers are functioning as intended, and whether the conditions encountered are within the limits assumed during a licensing review. Long-term monitoring of hydrologic properties and processes and in situ confirmation of design assumptions will play a key role in the safe operation of the potential geologic radioactive waste repository and in the decision to close the repository. Despite remarkable advances in cyber-infrastructure for linking sensors into spatially distributed environmental networks, the extended time horizon (decades to hundreds of years) for long-term monitoring activities, the harsh thermal and radiative conditions in the near-field environment, the deep fractured unsaturated rock environment at Yucca Mountain, the potential scope of observations, and restricted access to observation ports for maintenance and upgrades each present unprecedented challenges to the design of hydro-environmental monitoring networks. Activities for performance confirmation could include the use of pore water samplers and sensors for measuring water content, matric potential, temperature, relative
5 humidity, and water and gas fluxes. Current sensor technology for deep fractured rock systems (1) lags behind environmental observatory network solutions for surface and near-surface processes, (2) lags behind analogous technology for unconsolidated porous media, (3) cannot be reliably deployed without ongoing maintenance or replacement at relatively frequent intervals, and (4) is not designed to withstand harsh thermal and radiative conditions. Long-term monitoring could require special design considerations, such as measurement redundancy, built-in self-calibration and quality assurance measures, a staged and upgradable monitoring network design, and a focused initiative for development of appropriate technologies. Safe repository operation and long-term stewardship call for concerted efforts to advance sensor technology and for strategic planning to overcome these challenges.
Infiltration and the Unsaturated Zone S. Stothoff (Center for Nuclear Waste Regulatory Analyses) gave a presentation titled Million-Year Estimates of Net Infiltration at Yucca Mountain, Nevada. The performance period for the potential repository at Yucca Mountain, Nevada, may extend to one million years. Assessments of repository performance may use a stylized steady-state representation for deep percolation fluxes after 10,000 years, while considering the uncertainty in the steady-state value. A procedure for estimating million-year-average deep percolation integrates time sequences of areal-average net infiltration estimates over potential future time-varying climate sequences.
Two estimates of million-year-average deep percolation were developed from two independent estimates of future climate sequences, both based on correlating climate (in the form of mean annual precipitation and temperature) during past glacial cycles to the orbital characteristics of the Earth and projecting the orbital characteristics into the future. Correlations based on core data, glacier extent, lake stands, treeline variation, and vegetation species composition provide bases for relating climate to the extent of continental glaciation. Despite the independent assumptions and data sources, both approaches to estimate future climate yield similar estimates of million-year-average future precipitation and temperature. A numerical model for net infiltration, which compares well with regional and site estimates for net infiltration, provides the link between climate and areal-average deep percolation. Analyses using the net infiltration model suggest that million-year-average net infiltration is expected to be ~3 times greater than at present using both sets of climate estimates. The analysis found that the mean and variance of estimated future million-year-average areal-average net infiltration is reduced by less than 10% and less than 20%, respectively, from the case using uncertain time-varying climate to the case with steady and certain climate, regardless of the climate sequence used. The results imply that most of the uncertainty in estimating future net infiltration can be attributed to the uncertainty in estimating net infiltration for individual climate states. Small systematic increases in expected long-term-average net infiltration arise from uncertainty in the climate sequences that may occur over glacial cycles and from climatic variability over glacial cycles.
C. Manepally (Center for Nuclear Waste Regulatory Analyses) gave a talk titled Evaporation from Near-Drift Fractured Rock Surfaces. The amount of water entering emplacement drifts from a fractured unsaturated rock is an important variable for performance evaluation of a potential repository at Yucca Mountain, Nevada. Water entering the drifts as liquid or gas may enhance waste package corrosion rates and transport released radionuclides. Liquid water in the form of droplets may emerge from fractures, or flow along the drift wall and potentially evaporate and condense at other locations. Driven by pressure and temperature gradients, vapor may be transported along fractures, or liquid water may evaporate directly from the matrix. Within the drift, heat-driven convection may redistribute the moisture leading to condensation at other locations. The geometry of the evaporation front around the drift is not fully understood and this, in turn, influences processes related to rewetting as the thermal pulse
6 dissipates. Existing models focus on processes in the porous media (e.g., two-phase dual-permeability models for matrix and fractures), or on processes in the drift (e.g., gas-phase computational fluid dynamics models). This study focused on the boundary between these two domains, and the corresponding models, where evaporation at the solid rock/drift air interface appears to play an important role. Studies have shown that evaporation from porous media is a complex process sensitive to factors such as (1) hydrological properties of the porous media, (2) pressure gradients in the porous media, (3) texture of the interface or boundary, (4) local vapor and temperature gradients, and (5) convective flow rate and boundary layer transfer.
Experimental observations based on passive monitoring at Yucca Mountain have shown that the formation surrounding the drift is able to provide and transport large amounts of water vapor over a relatively short time.
Tectonics and Hydrology An abstract by R. Blakely (USGS) et al. was titled Tectonic Setting of the Gravity Fault and Implications for Ground-Water Resources in the Death Valley Region, Nevada and California.
The Amargosa trough, extending south from Crater Flat basin to the California-Nevada state line, is believed to be a transtensional basin accommodated in part by strike-slip displacement on the northwest-striking State Line fault and normal displacement on the north-striking Gravity fault. The Gravity fault, lying along the eastern margin of the Amargosa trough, was first recognized in the 1970s on the basis of correlations between gravity anomalies and a prominent spring line in Amargosa Valley. The Gravity fault causes an inflection in water-table levels, similar to other (but not all) normal faults in the area. Pools along the spring line, some of which lie within Death Valley National Park and Ash Meadows Wildlife Refuge, include endemic species potentially threatened by increasing agricultural activities in Amargosa Valley immediately to the west, where water tables are declining. Most of the springs and pools lie east of the Gravity fault, however, and it is important to understand the role that the Gravity fault plays in controlling ground-water flow. The authors have conducted a variety of geophysical investigations at various scales to better understand the tectonic framework of the Amargosa Desert and support new ground-water-flow models. Much of their focus has been on the tectonic interplay of the State Line, Gravity, and other faults in the area using gravity, ground-magnetic, audiomagnetotelluric (AMT), and time-domain electromagnetic (TEM) surveys. With 1250 new gravity measurements from Ash Meadows and Stewart Valley, the authors have developed a revised three-dimensional crustal model of the Amargosa trough constrained by well information and geologic mapping. The model predicts approximately 2 km of vertical offset on the Gravity fault but also suggests a complex structural framework. The fault is conventionally seen as a simple, down-to-the-west normal fault juxtaposing permeable pre-Tertiary carbonate rocks to the east against less permeable Tertiary sediments to the west. The new gravity inversion indicates a more complex footwall: some springs, for example, are associated with a concealed ridge or horst, with a secondary basin lying to the east. Six ground-magnetic transects across the Gravity fault using a truck-towed magnetometer show a characteristic magnetic anomaly reflecting different magnetic properties in rocks east and west of the fault. Ground-magnetic measurements, interpreted in conjunction with existing aeromagnetic data, allow us to map the shallow aspects of the Gravity fault and other faults in Ash Meadows in detail. Three TEM transects across the Gravity fault showed no strong evidence of a faulted contact, although depth of penetration may have been insufficient to reach associated resistivity contrasts. An AMT transect, however, shows a narrow zone of high resistivity directly along the Gravity fault. Although other interpretations are possible, this resistivity anomaly may reflect carbonate-rich cementation along the fault plane, possibly contributing to its influence on ground-water flow.
7 Tectonics and GPS E. Hill (Harvard-Smithsonian Center for Astrophysics) gave a presentation titled Investigation of site-dependent GPS errors and monument stability using a short-baseline network of braced monuments. A short-baseline network has been established at Yucca Mountain, southern Nevada, to help put new constraints on the level at which individual site motion can be detected by continuous GPS stations. This network was used to investigate and quantify aspects of the GPS error budget at a level of precision that would have been hard to imagine only a few years ago. The network consists of three GPS stations (established in 2006) with baseline lengths of
~10, 100, and 1000 meters with respect to site REPO (established in 1999). The four sites together lie along a N-S line, and have nearly identical instrumentation and setup configuration.
The site with the shortest baseline to REPO (REP2) has a shallow-braced (to ~1 m depth) monument, while the remaining two sites (REP3 and REP4) and REPO are deep-braced monuments fixed into bedrock to a depth of ~10 m. This setup enables investigation of processes affecting monument stability in a relatively controlled environment. In addition to the very short baselines, the desert environment of the area further reduces the influence of systematic errors in the results. Furthermore, the network was designed so that baselines to additional BARGEN sites at distances of ~10, 100, and 1000 km permit assessment of baseline-dependent errors over a full five orders of magnitude. Analysis of the data for the short-baseline network, using the GAMIT software, produces baseline time series with a very low level of noise (from tens to hundreds of microns). However, seasonal signals with amplitudes of ~0.1--0.5 mm are clearly discernible in some of the baseline time series, even for the shortest baselines.
These signals are primarily in the horizontal time series and are less evident in the vertical.
Values for the RMS scatter of the daily baseline time series about simple models for the seasonal cycle are 0.04--0.16 mm for the east component, 0.06--0.18 mm for the north, and 0.10--0.48 mm for the vertical, with the higher values for the longer baselines. These numbers indicate the high precision of these measurements, but also raise an intriguing question about the cause of the seasonal signals.
Bioremediation and Geophysics An invited abstract by S. Hubbard (Lawrence Berkeley National Laboratory) et al. was titled Exploring Hydrological and Biogeochemical Processes Associated With Remedial Treatments Using Geophysical Methods. Many remediation approaches induce biogeochemical transformations in subsurface systems, such as mineral dissolution and precipitation, gas evolution, and biomass generation. These processes can alter the permeability and porosity of a subsurface system. Although understanding how hydrological and biogeochemical properties change over space and time in response to remedial treatments is critical to develop and predict effective remediation strategies, it is hindered by an inability to monitor these processes with sufficient resolution over field scales. Recent advances in hydrogeophysics and biogeophysics have illustrated the potential that geophysical methods have for characterizing subsurface hydrological properties and for indicating biogeochemical changes associated with remedial treatments. The authors investigated hydrological and biogeochemical processes associated with laboratory and field bioremediation experiments using advanced characterization and monitoring approaches. They explored the use of time-lapse geophysical methods for quantifying biogeochemical reaction end-products; the influence of initial hydrogeochemical heterogeneity on the spatiotemporal distribution of biogeochemical transformations; and subsequent alterations of the flow field caused by the transformations. The experimental and numerical studies confirm a close coupling between hydrological and biogeochemical processes at both laboratory and field scales, and suggest that geophysical methods have the potential to
8 provide insights into the complex processes in a manner that should be useful for guiding remedial treatments.
An abstract by E. Woolery (University of Kentucky) et al. was titled Integrated Geophysical and Geological Fault Assessment at a Hazardous-Waste Landfill: Fluorspar Area Fault Complex, Central United States. Federal and Commonwealth of Kentucky regulations require proposed hazardous waste facilities undergo a surface-fault rupture hazard assessment prior to issuing construction permits. Permanent ground deformation may expose below-ground structures such as landfills and settling ponds, as well as above-ground structures such as tanks and incinerators to rupture and topple failure, and thus potential uncontrolled contaminant release.
Regulations prohibit placing new hazardous waste facilities within 61 m of a Holocene-active fault. However, identifying and characterizing active faults in areas lacking geomorphic expression is challenging, as exemplified near the New Madrid seismic zone and Fluorspar Area fault complex (FAFC). In the mid-continent, surface manifestations of active faults are impeded by a thick sequence of relatively weak, water-saturated Mississippi embayment sediment overlying bedrock. The soft sediment overburden and long recurrence interval between large earthquakes conceal neotectonic structures in bedrock and fail to produce significant or noticeable geomorphic features. A proposed hazardous-waste landfill in western Kentucky is located in the upper Mississippi embayment and above the late Proterozoic-early Cambrian FAFC, an area also coincident with diffuse microseismicity. Integrated geophysical and geological methodologies were essential for a surface-fault rupture assessment. Nearly 1 km of SH-wave seismic reflection data were collected and interpreted for evidence of late Quaternary deformation. Five significant high-angle anomalies were interpreted to extend within 7 m of the ground surface, near the upper limit of the seismic sampling. Eighty-six densely spaced, continuous cores, each 9.1 m deep, intersected these features. Stratigraphic and chronological analyses were performed on the cores to assess the presence or absence of structure, and to determine the near-surface extent and age of the features. The upper 10 m of sediment ranged between nearly16 ka and greater than 125 ka. Interpretations of geologic cross sections show that the most abrupt elevation changes were constrained to post-date a 53.6 to 75.5 ka loess deposit; however, no perceptible displacement was found at the base of a younger loess dated between 16.6 and 23.5 ka. Collectively, these analyses indicate an absence of Holocene deformation beneath the proposed landfill site.
Geophysics/Geothermal An abstract by C. Bouligand (USGS) et al. was titled A new method for mapping depth to the Curie-temperature isotherm in the Great Basin from aeromagnetic anomalies. They have revisited the problem of using aeromagnetic data to map depth to the Curie-temperature isotherm and tested a new methodology in an attempt to provide an independent estimate of heat flow in the Great Basin. Such methods assume that the depth-extent of crustal magnetic sources corresponds to the temperature at which rocks lose their spontaneous magnetization (e.g., 580°C for magnetite). They usually operate in the Fourier domain by analyzing the shape of the power-density spectrum calculated from aeromagnetic anomalies and critically depend on assumptions about the distribution of crustal magnetization. Early methods assumed that crustal magnetization is a completely random function of position characterized by a flat power-density spectrum. In this study the authors attempted to incorporate more realistic geologic models for crustal magnetization and applied the method to newly released aeromagnetic compilations for Nevada and North America. They assume that crustal magnetization has fractal properties so that the power-density spectrum of the magnetization is proportional to the wavenumber raised to a power -, where is related to the geologic terrane. In this case, the theoretical power spectrum, as derived by Maus et al. (Geophys. J. Int., 129, 163-168, 1997), depends on three
9 independent parameters: the depths to the top and bottom of the magnetic source layer and the fractal exponent. The authors estimate these parameters by first calculating a three-dimensional matrix representing the misfit between the power spectrum computed from observed data and a variety of theoretical spectra calculated from a range of realistic parameter values. They then search the matrix for the set of parameters that leads to the minimum misfit.
This operation was performed on overlapping sliding windows that were swept across the entire magnetic map. A matrix was developed for each window, thereby providing lateral variations in the depth to the bottom of magnetic sources. The authors tested this methodology on synthetic aeromagnetic data and applied it to aeromagnetic compilations from the Great Basin.
Preliminary results obtained by assuming is constant throughout the Great Basin show spatial variations in the depth to the bottom of magnetic sources that, in general, do not depend on the assumed value of or on the size of the window. However, the observed variations also do not correlate to a large extent with observed surface heat-flow anomalies. They may reflect real variations in crustal magnetic thickness, due either to undulations of the depth to the Curie-temperature isotherm not reflected in surface heat-flow measurements, or to lateral variations of shallower magnetic interfaces. Alternatively, they may be artifacts caused by variations in geologic terrane (i.e., variations in ). Future studies will attempt to include explicitly, using mapped geology as a guide to help distinguish which of the observed patterns reflect real variations in depth to Curie-temperature isotherm.