Geophysics and Seismology Commons^™

Application Of Time-Lapse Ert Imaging To Watershed Characterization, Carlyle R. Miller, Partha S. Routh, Troy R. Brosten, James P. Mcnamara

Geosciences Faculty Publications and Presentations

Time-lapse electrical resistivity tomography (ERT) has many practical applications to the study of subsurface properties and processes. When inverting time-lapse ERT data, it is useful to proceed beyond straightforward inversion of data differences and take advantage of the time-lapse nature of the data. We assess various approaches for inverting and interpreting time-lapse ERT data and determine that two approaches work well. The first approach is model subtraction after separate inversion of the data from two time periods, and the second approach is to use the inverted model from a base data set as the reference model or prior information for …

Issues During The Inversion Of Crosshole Radar Data: Can We Have Confidence In The Outcome?, William P. Clement

Geosciences Faculty Publications and Presentations

One method of assessing the confidence in modeled features is to compare the results from different inversion schemes. I use synthetic traveltimes calculated from a model of an unconfined aquifer to determine the reliability of crosshole tomography. I compare the inverted models from straight and curved ray approximations to wave propagation. I investigate the effects of added random noise, regularization, the starting model, and the reference model on the curved ray inversion method. I also investigate the effects of different grid sizes for the forward model and of limited ray coverage through the earth model. Understanding the effects of these …

Crosshole Radar Tomography In A Fluvial Aquifer Near Boise, Idaho, William P. Clement, Warren Barrash

Geosciences Faculty Publications and Presentations

To determine the distribution of heterogeneities in the saturated zone of an unconfined aquifer in Boise, ID, we compute tomograms for three adjacent well pairs. The fluvial deposits consist of unconsolidated cobbles and sands. We used a curved-ray, finite-difference approximation to the eikonal equation to generate the forward model. The inversion uses a linearized, iterative scheme to determine the slowness distribution from the first arrival traveltimes. The tomograms consist of a layered zone representing the saturated aquifer. The velocities in this saturated zone range between 0.06 to 0.10 m/ns. We use a variety of methods to assess the reliability of …

Reflectivity Modeling Of A Ground-Penetrating-Radar Profile Of A Saturated Fluvial Formation, William P. Clement, Warren Barrash, Michael D. Knoll

Geosciences Faculty Publications and Presentations

Major horizons in radar reflection profiles may correlate with contacts between stratigraphic units or with structural breaks such as fault surfaces. Minor reflections may be caused by clutter or, in some cases, may indicate material properties or structure within stratigraphic units. In this study, we examine the physical basis for major and minor reflections observed in a shallow, unconfined, fluvial aquifer near Boise, Idaho, U. S. A. We compare a 2D profile from a surface ground-penetrating-radar reflection transect with the 1D modeled reflection profiles at three wells adjacent to the surface-reflection profile. The 1D models are based on dielectric constant …

Traveltime Inversion Of Vertical Radar Profiles, William P. Clement, Michael D. Knoll

Geosciences Faculty Publications and Presentations

Traveltimes of direct arrivals in vertical radar profiles (VRPs) are tomographically inverted to estimate the earth’s electromagnetic (EM) velocity between a surface transmitter and a downhole receiver. We determine the 1D interval velocity model that best fits the first-arrival traveltimes by using a weighted, damped, least-squares inversion scheme. We assess the accuracy of the velocity model using synthetic traveltimes from a known velocity-distribution model simulating an unconfined aquifer. The inverted velocity profile closely matched the velocity profile of the input model in the synthetic examples. Using vertical radar profile data from an unconfined aquifer near Boise, Idaho, we inverted traveltimes …

Image Interpretation Using Appraisal Analysis, Partha S. Routh, Carlyle R. Miller

Geosciences Faculty Publications and Presentations

In geophysical inversion, a significant effort is invested to obtain images of the Earth from finite data. The first step is to obtain an image i.e. solve the inverse problem. This step alone provides significant challenges that are not addressed inthis paper. The next step is to interpret the image in terms of specific questions. For example, what can we say about the average value of a physical property within a certain region of the model? What scale information can we resolve from the data? These questions are problem dependent and may require that inversion be carried out several times …

Vsp Traveltime Inversion: Near-Surface Issues, Geoff J. M. Moret, William P. Clement, Michael D. Knoll, Warren Barrash

Geosciences Faculty Publications and Presentations

P-wave velocity information obtained from vertical seismic profiles (VSPs) can be useful in imaging subsurface structure, either by directly detecting changes in the subsurface or as an aid to the interpretation of seismic reflection data. In the shallow subsurface, P-wave velocity can change by nearly an order of magnitude over a short distance, so curved rays are needed to accurately model VSP traveltimes. We used a curved-ray inversion to estimate the velocity profile and the discrepancy principle to estimate the data noise level and to choose the optimum regularization parameter. The curved-ray routine performed better than a straight-ray inversion for …

Mae Sae Basin And Wiang Nong Lom: Radiocarbon Dating And Relation To The Active Strike-Slip Mae Chan Fault, Northern Thailand, Spencer H. Wood, Fongsaward S. Singharajwarapan, Tharaporn Bundarnsin, Eric Rothwell

Geosciences Faculty Publications and Presentations

The Mai Sai basin and the Wiang Nong Lom swamp lie north of, and adjacent to, the northeast-trending Mae Chan fault, which is a left -lateral strike-slip fault in northernmost Thailand. The sediment in the Mae Sai basin is about 600 meters thick. The Mae Sai basin has an anticlinal fold below a 390-meter depth. This fold is buried by broadly downwarped sediment. In the Wiang Nong Lom swamp, sediment is about 170 meters thick. Radiocarbon ages from a 2.7-meter auger hole at Ko Mae Mai in the Wiang Nong Lom swamp show that 0.5 meter of sediment accumulated in …

Recent Paleoseismic Investigations In Northern And Western Thailand, Clark H. Fenton, Punya Charusiri, Spencer H. Wood

Geosciences Faculty Publications and Presentations

Recent paleoseismic investigations have identified a number of active faults in Northern and Western Thailand. Northern Thailand is an intraplate basin and range province, comprised of north-south-trending Cenozoic intermontane grabens and half grabens, bounded by north- to northwest-striking normal to normal-oblique faults and northeast-striking left-lateral strike-slip faults. The basin-bounding normal faults are marked by steep, linear range fronts with triangular facets and wineglass canyons and have slip rates of 0.1 to 0.8 mm/yr. Based on limited data, the average vertical displacement-per-event is about 1.0 to 1.5 m. These faults are characterized by recurrence intervals of thousands to tens of thousands …

Tracer/Time-Lapse Radar Imaging Test At The Boise Hydrogeophysical Research Site, Warren Barrash, Michael D. Knoll, David W. Hyndman, Tom Clemo, Edward C. Reboulet, Elisabeth M. Hausrath

Geosciences Faculty Publications and Presentations

A combined tracer and time-lapse radar imaging experiment was conducted in the unconfined coarse fluvial aquifer at the Boise Hydrogeophysical Research Site in August, 2001. Two tracers (bromide and uranine) were injected to form a plume over a 4-m interval that spanned the contact between hydrostratigraphic units with contrasting permeability. The tracer plume traveled 6.9 m to well B6, passing through well A1 instrumented with 20 sampling zones over a 5-m interval that spanned the injection interval. Radar tomographic data were collected periodically on cross-sectional and longitudinal planes, two of which passed through well A1 for quantitative calibration of radar …

Borehole Radar Attenuation-Difference Tomography During The Tracer/Time-Lapse Test At The Boise Hydrogeophysical Research Site, Sarah E. Goldstein, Tim C. Johnson, Michael D. Knoll, Warren Barrash, William P. Clement

Geosciences Faculty Publications and Presentations

A tracer test and time-lapse radar imaging experiment was conducted at the Boise Hydrogeophysical Research Site to investigate the utility of crosswell radar in imaging an electrically conductive tracer plume. A multilevel water sampling system down gradient from the tracer injection well and in the radar imaging plane was used to collect detailed, 1-dimensional, fluid electrical conductivity data during the tracer test. We compare the spatial and temporal position and concentration variations of the plume as indicated by the fluid conductivity data to those suggested by radar level run attenuation differences, shot-receiver attenuation difference crossplots, and an attenuation-difference tomogram. We …

Geologic And Tectonic History Of The Western Snake River Plain, Idaho And Oregon, Spencer H. Wood, Drew M. Clemens

Geosciences Faculty Publications and Presentations

The western Snake River Plain is a Neogene-aged intracontinental rift basin, about 70 km wide and 300 km long, trending northwest across the southern Idaho batholith. Its southeastern end merges with the northeast-trending eastern plain, a structural downwarp associated with extension along the track of the Yellowstone hot spot. Orientation of the western plain rift is parallel to several regional northwest-trending crustal discontinuities, such as the Olympic-Wallowa lineament and the Brothers fault zone, suggesting that the rift failed along zones of lithospheric weakness, as the lithosphere was softened by the passing hot spot. Crustal refraction data and gravity show that …

Vertical Variation In Groundwater Chemistry Inferred From Fluid Specific-Conductance Well Logging Of The Snake River Plain Basalt Aquifer, Idaho National Engineering Laboratory, Southeastern Idaho, Spencer H. Wood, William Bennecke

Geosciences Faculty Publications and Presentations

Well logging of electrical fluid specific conductance (C_s) shows that permeable zones yielding ground water to intrawell flows and the water columns in some wells at INEL have highly different chemistry, with as much as a two-fold variation in C_s). This suggests that dedicated pump sampling of ground water in the aquifer may not be representative of the chemistry of the waste plumes migrating southwest of the nuclear facilities. Natural background C_s in basalt-aquifer ground water of this part of the Snake River Plain aquifer is less than 325µS/cm (microSiemans/cm), and total dissolved solids in …

Chronology Of Late Pleistocene And Holocene Volcanics, Long Valley And Mono Basin Geothermal Areas, Eastern California, Spencer H. Wood

Geosciences Faculty Publications and Presentations

Hydration-rind ages based on hydration-rind thicknesses of obsidian and an assumed hydration rate of 5 microns²/1000 yrs have been determined for the 26 exposed Mono domes and coulees. Hydration-rind thickness data give good estimates of relative age differences between the domes, but determination of absolute ages will depend upon calibration to radiometric ages.

The first extrusion of highly differentiated, sparsely porphyritic rhyolite occurred an estimated 32,000 to 40,000 yrs ago and consists of a small dome at the northwest end of the contiguous chain. The next major extrusive event occurred about 24,000 yrs ago and is represented by …

Proximal Air-Fall Deposits Of Eruptions Between May 24 And August 7, 1980 -- Stratigraphy And Field Sedimentology. U.S. Department Of The Interior, Geological Survey, Richard B. Waitt Jr., Vicki L. Hansen, Andrei M. Sarna-Wojcicki, Spencer H. Wood

Geosciences Faculty Publications and Presentations

During each of the magmatic eruptions of Mount St. Helens on May 25, June 12, and August 7, a vertical eruptive column rose intermittently to altitudes of 12-15 km, from which pumice, lithic fragments, and crystals settled downwind in lobes that generally become thinner and finer away from the volcano. Each ejecta lobe is asymmetric according to several criteria, including (1) the axes of maximum thickness and of maximum pumice size are not midway between the two margins of the lobe, (2) the axis of maximum pumice size does not correspond to the axis of thickness, and (3) the median …

Areal Distribution, Thickness, Mass, Volume, And Grain Size Of Air-Fall Ash From The Six Major Eruptions Of 1980, Andrei M. Sarna-Wojcicki, Susan Shipley, Richard B. Waitt Jr., Daniel Dzurisin, Spencer H. Wood

Geosciences Faculty Publications and Presentations

The airborne-ash plume front from the Mount St. Helens eruption of May 18 advanced rapidly to the northeast at an average velocity of about 250 km/hr during the first 13 min after eruption. It then traveled to the east-northeast within a high-velocity wind layer at altitudes of 10-13 km at an average velocity of about 100 km/hr over the first 1,000 km. Beyond about 60 km, the thickest ash fall was east of the volcano in Washington, northern Idaho, and western Montana. A distal thickness maximum near Ritzville, Wash., is due to a combination of factors: (1) crude sorting within …