CGISS Publications and Presentations

Stochastic Cost Optimization of DNAPL Remediation - Field Application

Ungtae Kim et al. — Tue, 18 Jun 2013 08:32:24 PDT

A stochastic remediation design optimization methodology implemented in the program Stochastic Cost Optimization Toolkit (SCOToolkit) was successfully applied to evaluate remediation options at the East Gate Disposal Yard (EGDY) at the former Fort Lewis, now Joint Base Lewis-McChord (JBLM), Washington. Non-optimized forward simulations based on calibrated parameters and their uncertainty inferred from data prior to actual thermal source remediation system implementation at the site indicated a low probability of the actual thermal system design meeting remediation criteria in a reasonable time frame. Calibration using additional data collected during thermal treatment reduced prediction uncertainty, but still predicted a high probability of taking more than 100 years to reach compliance criteria using the actual thermal treatment design with no additional remedial action. Stochastic optimization of the thermal treatment design indicated larger treatment areas were needed to capture source mass due to uncertainty in source delineation. The expected cost for the enlarged thermal treatment system was estimated to be $22M, which is nearly twice that of the actual system, suggesting that additional characterization to reduce source delineation uncertainty or consideration of an alternative strategy that is less sensitive to delineation uncertainty may be warranted. Stochastic optimization of whey injection was investigated to accelerate source zone dense nonaqueous phase liquid (DNAPL) dissolution and enhance dissolved plume biodecay. The optimized design indicated a 93% probability of meeting compliance criteria by 2100 with an expected net present value cost of $4.7M. Whey injection substantially shortened the remediation time compared to no whey injection. The results indicate that the proposed stochastic cost optimization approach is able to reduce expected remediation costs, increase the probability of achieving remediation objectives, and identify data characterization needs.

Recognizing and Modeling Variable Drawdown Due to Evapotranspiration in a Semiarid Riparian Zone Considering Local Differences in Vegetation and Distance from a River Source

Brady Johnson et al. — Mon, 13 May 2013 12:48:34 PDT

Riparian zones in semiarid regions often exhibit high rates of evapotranspiration (ET) in spite of low-soil moisture content due to the presence of phreatophytic vegetation that is able to withdraw water from shallow aquifers. This work seeks to better deﬁne the relationship between ET, the saturated zone and the river boundary by comparing observed water table drawdown records to analytically modeled drawdown in fully penetrating wells of an unconﬁned aquifer in response to daily ET ﬂux. ET at the Boise Hydrogeophysical Research Site (BHRS), a riparian zone in a temperate, semiarid environment, is calculated using a radiation-based method to provide ET values at four different wells with different vegetation densities. Analytically modeled drawdown response to ET forcing shows that drawdown magnitude increases with increasing distance from the river edge even as the surﬁcial ET forcing remains constant. This behavior is also observed in well hydrographs and shows the buffering effect that ﬂow from the river has on drawdown in fully penetrating riparian wells in highly permeable, unconﬁned aquifers. Relative contributions of river water to aquifer storage are calculated for ET-induced diurnal ﬂuctuations of the water table at increasing distances from the river boundary. Failure to account for these spatial differences in drawdown related to the river source may explain some errors associated with estimating ET from well hydrographs alone.

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

David Eiriksson et al. — Thu, 28 Mar 2013 08:39:38 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.

Malargüe Seismic Array: Design and Deployment of the Temporary Array

K. van Wijk — Mon, 14 Jan 2013 14:05:33 PST

We present the goals and the current status of the Malargüe seismic array. Our main goal is imaging and monitoring the subsurface below the Malargüe region, Mendoza, Argentina. More specifically, we focus on the Planchon-Peteroa Volcano and an area just east of the town of Malargüe. We start our project installing a temporary array of 38 seismic stations, which records continuously for one year. The array consists of two subarrays: one array located on the flanks of the volcano; the other spread out on a plateau just east of the Andes. The imaging targets, like the Moho and the Nazca slab, are relatively deep. Yet, the array has a dense station spacing, allowing exploration-type processing. For high-resolution imaging, also a dense source spacing is required. This we aim to achieve by creating virtual sources at the receiver positions, with a technique called seismic interferometry (SI). The array is designed such that a recent improvement of SI can be applied to the recordings. Other goals are to collect high-quality core-phase measurements and to characterize sources of microseism noise in the Southern Hemisphere. Furthermore, we plan to collaborate with researchers from the Pierre Auger Collaboration to study coupling of seismic, atmospheric, and cosmic signals using data from our instruments and from the Pierre Auger detectors.

The Emergence and Future of Near-Surface Geophysics

William E. Doll et al. — Mon, 14 Jan 2013 10:48:55 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.

Causal Instrument Corrections for Short-Period and Broadband Seismometers

Matthew M. Haney et al. — Wed, 03 Oct 2012 14:47:33 PDT

Of all the filters applied to recordings of seismic waves, which include source, path, and site effects, the one we know most precisely is the instrument filter. Therefore, it behooves seismologists to accurately remove the effect of the instrument from raw seismograms. Applying instrument corrections allows analysis of the seismogram in terms of physical units (e.g., displacement or particle velocity of the Earth’s surface) instead of the output of the instrument (e.g., digital counts). The instrument correction can be considered the most fundamental processing step in seismology since it relates the raw data to an observable quantity of interest to seismologists. Complicating matters is the fact that, in practice, the term “instrument correction” refers to more than simply the seismometer. The instrument correction compensates for the complete recording system including the seismometer, telemetry, digitizer, and any anti‐alias filters.

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

John Bradford — Wed, 03 Oct 2012 11:52:26 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.

Seeing Through the Noise: Seismic Reflection Profiling in Urban Areas

Lee M. Liberty et al. — Tue, 02 Oct 2012 11:50:19 PDT

Studies for urban hazard or resource assessment often take place in densely populated areas characterized by considerable cultural noise. These site conditions can severely compromise seismic reflection data quality. We have collected vibroseis and hammer (weight drop) seismic reflection data in a range of geologic conditions to image stratigraphy and structures in the upper one km along regional highways, city streets,and power line access roads. In addition to the challenges of safety and outreach, acquisition efforts along busy streets and highways often encounter poor receiver coupling and large-amplitude coherent noise from traffic and power lines. Although higher quality seismic reflection data may be obtained by simply choosing alternate sites with less cultural noise, modifications to the acquisition and processing step scan minimize the effects of cultural noise and poor coupling where profiling is most relevant. Flagging crews, flyers and public announcements assist with outreach and safety concerns, and the local news media are often enthusiastic about publicizing geologic studies. Recording long-record vibroseis data reduces the effects of noise by itself,but data quality can be further optimized by recording uncorrelated,unstacked data and applying precorrelation amplitude adjustments and filters. Recording individual hammer shots likewise allows gains or mutes to normalize or remove traffic noise prior to vertical stacking. Large numbers of receiver channels allow attenuation of random noise and velocity filtering to remove coherent noise. Because ground roll and normal moveout (NMO) corrections minimize near-surface coverage, asymmetric source-receiver geometry allows for additional near-surface fold while muting large amplitude ground rolland NMO stretch. Source and geophone coupling on road shoulder scan degrade signal quality due to variable materials and topography,but these problems are often addressed with static corrections. Our experience is that high-quality seismic data can be obtained in noisy urban areas, but many recorded channels and a careful attention to acquisition and processing procedures can significantly improve the results.

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

John H. Bradford — Wed, 26 Sep 2012 15:46:33 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 14:54:23 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.

Love Wave Propagation in Viscoelastic Media

G. Vijaya Raghavendra Chakravarthy et al. — Fri, 21 Sep 2012 12:58:12 PDT

Surface wave measurements have been used to compute the dynamic soil properties for near surface site characterization and the dynamic design of foundations. Much of this work has been done with the Rayleigh waves which are dependent on both the shear and the compressive wave properties of the soil. Love waves,on the other hand, are sensitive only to the shear wave response of the soil. This shear only sensitivity greatly simplifies determining the damping and the stiffness of a near surface soil profile. Further, the mechanism of damping can be related to purely inertial interactions of the soil frame and the pore fluids, free from compressive factors. Traditionally, soils have been represented by elastic models. While elastic models are adequate in representing dry or impermeable soils, they fail to account for the observed down-hole body wave dispersion in permeable, water saturated soils. To overcome this limitation, a viscoelastic model can be used. In this work, a viscoelastic representation of the Love wave propagation is derived for the forward problem. The solution to this forward problem yields the dispersion and the attenuation curves. Also computed are the complex motion-stress vectors fora vertically heterogeneous, viscoelastic medium, with the shear viscosity as a specific material property. The viscoelastic constitutive model will lead to an improved representation of Love wave propagation in permeable,water saturated soils where the concept of the effective viscosity becomes inappropriate.

Extension of the Spatial Autocorrelation (SPAC) Method to Mixed-Component Correlations of Surface Waves

Matthew M. Haney et al. — Wed, 19 Sep 2012 15:03:48 PDT

Using ambient seismic noise for imaging subsurface structure dates back to the development of the spatial autocorrelation (SPAC) method in the 1950s. We present a theoretical analysis of the SPAC method for multicomponent recordings of surface waves to determine the complete 3 × 3 matrix of correlations between all pairs of three-component motions, called the correlation matrix. In the case of isotropic incidence, when either Rayleigh or Love waves arrive from all directions with equal power, the only non-zero off-diagonal terms in the matrix are the vertical–radial (ZR) and radial–vertical (RZ) correlations in the presence of Rayleigh waves. Such combinations were not considered in the development of the SPAC method. The method originally addressed the vertical–vertical (ZZ), RR and TT correlations, hence the name spatial autocorrelation. The theoretical expressions we derive for the ZR and RZ correlations offer additional ways to measure Rayleigh wave dispersion within the SPAC framework.

Expanding on the results for isotropic incidence, we derive the complete correlation matrix in the case of generally anisotropic incidence. We show that the ZR and RZ correlations have advantageous properties in the presence of an out-of-plane directional wavefield compared to ZZ and RR correlations. We apply the results for mixed-component correlations to a data set from Akutan Volcano, Alaska and find consistent estimates of Rayleigh wave phase velocity from ZR compared to ZZ correlations. This work together with the recently discovered connections between the SPAC method and time-domain correlations of ambient noise provide further insights into the retrieval of surface wave Green’s functions from seismic noise.

Western Limits of the Seattle Fault Zone and Its Interaction with the Olympic Peninsula, Washington

A. P. Lamb et al. — Mon, 30 Jul 2012 12:57:27 PDT

We present evidence that the Seattle fault zone of Washington State extends to the west edge of the Puget Lowland and is kinematically linked to active faults that border the Olympic Massif, including the Saddle Mountain deformation zone. Newly acquired high-resolution seismic reflection and marine magnetic data suggest that the Seattle fault zone extends west beyond the Seattle Basin to form a >100-km-long active fault zone. We provide evidence for a strain transfer zone, expressed as a broad set of faults and folds connecting the Seattle and Saddle Mountain deformation zones near Hood Canal. This connection provides an explanation for the apparent synchroneity of M7 earthquakes on the two fault systems ∼1100 yr ago. We redefine the boundary of the Tacoma Basin to include the previously termed Dewatto basin and show that the Tacoma fault, the southern part of which is a backthrust of the Seattle fault zone, links with a previously unidentified fault along the western margin of the Seattle uplift. We model this north-south fault, termed the Dewatto fault, along the western margin of the Seattle uplift as a low-angle thrust that initiated with exhumation of the Olympic Massif and today accommodates north-directed motion. The Tacoma and Dewatto faults likely control both the southern and western boundaries of the Seattle uplift. The inferred strain transfer zone linking the Seattle fault zone and Saddle Mountain deformation zone defines the northern margin of the Tacoma Basin, and the Saddle Mountain deformation zone forms the northwestern boundary of the Tacoma Basin. Our observations and model suggest that the western portions of the Seattle fault zone and Tacoma fault are complex, require temporal variations in principal strain directions, and cannot be modeled as a simple thrust and/or backthrust system.

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

Davood Moghadas et al. — Mon, 16 Jul 2012 10:32:27 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.

Stochastic Cost Optimization of DNAPL Remediation - Method Description and Sensitivity Study

Jack Parker et al. — Tue, 26 Jun 2012 13:59:22 PDT

A modeling approach is described for optimizing the design and operation of groundwater remediation at DNAPL sites that considers uncertainty in site and remediation system characteristics, performance and cost model limitations, and measurement uncertainties that affect predictions of remediation performance and cost. The performance model simulates performance and costs for thermal source zone treatment and enhanced bioremediation with statistical compliance rules and real-time operational system monitoring. An inverse solution is employed to estimate model parameters, parameter covariances, and residual prediction error from site data and a stochastic cost optimization algorithm determines design and operation variables that minimize expected net present value cost over Monte Carlo realizations. The method is implemented in the program SCOToolkit. A series of applications to a hypothetical problem yielded expected cost reductions for site remediation as much as 85% compared to conventional non-optimized approaches, while also increasing the probability of achieving “no further action” status in a specified timeframe by more than 60%. Optimizing monitoring frequency for compliance wells used to make no further action determinations as well as operational monitoring used to make decisions on individual remediation system components reveals tradeoffs between increased direct costs for sampling and analysis versus decreased construction and operating costs that arise because more data increases decision reliability. Optimizing protocols for operational monitoring and heating unit shutdown protocols for thermal source treatment (incremental versus all-or-none shutdown, soil versus groundwater sampling, number and frequency of samples) produced cost savings of more than 20%. Defining compliance based on confidence limits of a moving time window regression decreased expected cost and lowered failure probability compared to using measured extreme values over a lookback period. Uncertainty in DNAPL source delineation was found to have a large effect on the cost and probability of achieving remediation objectives for thermal source remediation.

Scanning for Velocity Anomalies in the Crust and Mantle with Diffractions from the Core-Mantle Boundary

Elmer Ruigrok et al. — Fri, 22 Jun 2012 12:26:08 PDT

A novel method, based on differential arrival times of diffractions from the core-mantle boundary, swiftly scans for seismic velocity anomalies in the crust and mantle below an array of seismometers. The method is applied to data from the USArray and the large-scale structural features in the western United States are resolved. High lateral resolution is achieved, but structure is averaged over depth. As such, this method is complementary to surface-wave and tomographic body-wave methods, where averaging takes place in the lateral sense. Processing and data-volume requirements involved are minimal. Therefore, this method can be applied during the early stages of array deployment, before the necessary data is acquired to obtain accurate inversion images. The quick scanner can be used to identify features of interest, upon which the array could be refined.

Three-Dimensional Stochastic Estimation of Porosity Distribution: Benefits of Using Ground-Penetrating Radar Velocity Tomograms in Simulated-Annealing-Based or Bayesian Sequential Simulation Approaches

Baptiste Dafflon et al. — Fri, 22 Jun 2012 11:57:40 PDT

Estimation of the three-dimensional (3-D) distribution of hydrologic properties and related uncertainty is a key for improved predictions of hydrologic processes in the subsurface. However it is difficult to gain high-quality and high-density hydrologic information from the subsurface. In this regard a promising strategy is to use high-resolution geophysical data (that are relatively sensitive to variations of a hydrologic parameter of interest) to supplement direct hydrologic information from measurements in wells (e.g., logs, vertical profiles) and then generate stochastic simulations of the distribution of the hydrologic property conditioned on the hydrologic and geophysical data. In this study we develop and apply this strategy for a 3-D field experiment in the heterogeneous aquifer at the Boise Hydrogeophysical Research Site and we evaluate how much benefit the geophysical data provide. We run high-resolution 3-D conditional simulations of porosity with both simulated-annealing-based and Bayesian sequential approaches using information from multiple intersecting crosshole gound-penetrating radar (GPR) velocity tomograms and neutron porosity logs. The benefit of using GPR data is assessed by investigating their ability, when included in conditional simulation, to predict porosity log data withheld from the simulation. Results show that the use of crosshole GPR data can significantly improve the estimation of porosity spatial distribution and reduce associated uncertainty compared to using only well log measurements for the estimation. The amount of benefit depends primarily on the strength of the petrophysical relation between the GPR and porosity data, the variability of this relation throughout the investigated site, and lateral structural continuity at the site.

A Field Proof-of-Concept of Aquifer Imaging Using 3-D Transient Hydraulic Tomography with Modular, Temporarily-Emplaced Equipment

Michael Cardiff et al. — Thu, 07 Jun 2012 11:46:53 PDT

Hydraulic tomography is a field scale aquifer characterization method capable of estimating 3-D heterogeneous parameter distributions, and is directly sensitive to hydraulic conductivity (K), thus providing a useful data source for improving flow and transport models. We present results from a proof-of-concept field and modeling study in which we apply 3-D transient hydraulic tomography (3DTHT) to the relatively high-K and moderately heterogeneous unconfined aquifer at the Boise Hydrogeophysical Research Site. Short-duration (20 min) partially penetrating pumping tests, for which observed responses do not reach steady state, are used as the aquifer stimulation. To collect field data, we utilize a system of temporarily emplaced packer equipment to isolate multiple discrete intervals in boreholes. To analyze the data, we utilize MODFLOW combined with geostatistical inversion code based on the quasilinear approach of Kitanidis (1995). This combination of practical software allows inversion of large datasets (>250 drawdown curves, and almost 1000 individual data points) and estimation of K at >100,000 locations; reasonable runtimes are obtained using a single multicore computer with 12 GB of RAM. The K heterogeneity results from 3DTHT are cross-validated against K characterization from a large set of partially penetrating slug tests, and found to be quite consistent. The use of portable, modular equipment for field implementation means that 3DTHT data collection can be performed (including mobilization/demobilization) within a matter of days. Likewise, use of a practical, efficient and scalable numerical modeling and inversion strategy means that computational effort is drastically reduced, such that 3-D aquifer property distributions can be estimated quickly.

Monitoring Glacier Surface Seismicity in Time and Space Using Rayleigh Waves

T. D. Mikesell et al. — Wed, 06 Jun 2012 11:56:00 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.

Hotspot: The Snake River Geothermal Drilling Project - An Overview

Lee M. Liberty — Thu, 31 May 2012 14:59:36 PDT

The Snake River volcanic province (SRP) overlies a thermal anomaly that extends deep into the mantle; it represents one of the highest heat flow provinces in North America, and an area with the highest calculated geothermal gradients. This makes the SRP one of the potentially highest producing geothermal districts in the United States. Elevated heat flow is typically highest along the margins of the topographic Snake River Plain and lowest along the axis of the plain, where thermal gradients are suppressed by the Snake River aquifer. Beneath this aquifer, however, thermal gradients rise again and may tap even higher heat flows associated with the intrusion of mafic magmas into a geophysically-imaged mid-crustal sill complex. The primary goal of this project is to evaluate geothermal potential in three distinct settings: (1) the high sub-aquifer geothermal gradient associated with the intrusion of mafic magmas and the release of crustal fluids from the associated wall rocks, (2) the valley-margin settings where surface heat flow may be driven by the up-flow of hot fluids along buried caldera ring-fault complexes, and (3) a more traditional fault-bounded basin with thick sedimentary cover. All settings are found within the central or western Snake River Plain and represent previously untested targets for geothermal exploration. Our first drill site tests the extent of geothermal resources along the axis of the plain, beneath the Snake River aquifer, in an area where elevated groundwater temperatures imply a significant flux of conductive or advective heat flow from below. Our second drill site assesses the geothermal potential of up-flow zones along a buried caldera margin, in an area of known geothermal potential (Twin Falls geothermal district). Further studies will include seismic reflection-refraction surveys, gravity-magnetic surveys, and downhole geophysical logs.