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At least 1,513 records · Page 84Linked to original sources
USGS1998

Freshwater resources and saline water near the Sac and Fox Nation tribal lands, eastern Lincoln County, Oklahoma

The purpose of this project was to evaluate the freshwater resources and possible sources of high-chloride and high-sulfate concentrations in parts of the aquifer near the Sac and Fox Nation tribal land in eastern Lincoln County, Oklahoma. Water-quality sampling and borehole geophysical data indicate the potential for fresh ground water on tribal land generally is greatest in the Vanoss Formation, in the SE1/4 sec. 21, T. 14 N., R. 06 E. and in the NE1/4 sec. 22, T. 14 N., R. 06 E. These locations avoid the flood-prone areas and borehole geophysical resistivity logs indicate the altitude of the base of fresh ground water is below 650 ft. The altitude of the base of fresh ground water is indicated to be generally near the surface under the W1/2 sec. 22, T. 14 N., R. 06 E., the SE1/4 sec. 22, SE1/4 SE1/4 NE1/4 sec. 21, and NE1/4 NW1/4 NW1/4 sec. 27. Conditions are more favorable for placement of fresh ground-water wells in sec. 34, T. 14 N., R. 06 E., where the tribe has leased water rights, than on tribal land in secs. 15, 16, 21, and 22, T. 14 N., R. 06 E. Sandstones overlain by or enclosed in thick clay and shale sequences are likely to be somewhat isolated from the flow system and retain some of the residual brine. Borehole geophysical logs suggest that sandstones near CH1, CM1, and WT1 have more clay and shale content than the sandstones near L2. Greater amounts of clay in the sandstones will retard the flushing of residual brines from the sandstones and could result in a shallow base of fresh water near CH1, CM1, and WT1. For these reasons and because circulation of fresh ground water is limited by discharge to the Deep Fork, general water quality under tribal land would probably be poorer than in the area where the tribe has leased water rights. Samples have chloride or sulfate concentrations greater than 250 milligrams per liter in the W1/2 sec. 22, T. 14 N., R. 06 E. Six cluster well samples from tribal land have chloride or sulfate concentrations above the suggested maximum contaminant levels set by U.S. Environmental Protection Agency. Water-quality data indicate there may be more than one source for the salinity in the very saline and briny samples near the tribal land. Two possible sources for chloride and sulfate in water-quality samples are shallow brines and deep oil brines. Probable sources of shallow brines in the study area are: 1) solution of minerals by fresh water moving through the aquifer and 2) residual brines deposited with the sediment. There are no salt or gypsum beds in the Vanoss, Ada, or Vamoosa Formations, but there may be nodules and finely disseminated minerals present in the formations. Residual brines could remain in sand stones and shales that have low hydraulic conductivity and have not been diluted by freshwater recharge. Data suggest both sources have mixed with the fresh ground water from the Vanoss Formation. This is indicated by the relations of the bromide/chloride concentration ratio to chloride concentration, delta deuterium to delta 18oxygen, and by delta 18oxygen to chloride molality relation.

Water-Resources Investigations Report↗
USGS2022

Categorizing active marine acoustic sources based on their potential to affect marine animals

Marine acoustic sources are widely used for geophysical imaging, oceanographic sensing, and communicating with and tracking objects or robotic vehicles in the water column. Under the U.S. Marine Mammal Protection Act and similar regulations in several other countries, the impact of controlled acoustic sources is assessed based on whether the sound levels received by marine mammals meet the criteria for harassment that causes certain behavioral responses. This study describes quantitative factors beyond received sound levels that could be used to assess how marine species are affected by many commonly deployed marine acoustic sources, including airguns, high-resolution geophysical sources (e.g., multibeam echosounders, sidescan sonars, subbottom profilers, boomers, and sparkers), oceanographic instrumentation (e.g., acoustic doppler current profilers, split-beam fisheries sonars), and communication/tracking sources (e.g., acoustic releases and locators, navigational transponders). Using physical criteria about the sources, such as source level, transmission frequency, directionality, beamwidth, and pulse repetition rate, we divide marine acoustic sources into four tiers that could inform regulatory evaluation. Tier 1 refers to high-energy airgun surveys with a total volume larger than 1500 in 3 (24.5 L) or arrays with more than 12 airguns, while Tier 2 covers the remaining low/intermediate energy airgun surveys. Tier 4 includes most high-resolution geophysical, oceanographic, and communication/tracking sources, which are considered unlikely to result in incidental take of marine mammals and therefore termed de minimis . Tier 3 covers most non-airgun seismic sources, which either have characteristics that do not meet the de minimis category (e.g., some sparkers) or could not be fully evaluated here (e.g., bubble guns, some boomers). We also consider the simultaneous use of multiple acoustic sources, discuss marine mammal field observations that are consistent with the de minimis designation for some acoustic sources, and suggest how to evaluate acoustic sources that are not explicitly considered here.

Journal of Marine Science and Engineering↗
USGS2014

Seismic investigation of gas hydrates in the Gulf of Mexico: 2013 multi-component and high-resolution 2D acquisition at GC955 and WR313

The U.S. Geological Survey led a seismic acquisition cruise at Green Canyon 955 (GC955) and Walker Ridge 313 (WR313) in the Gulf of Mexico from April 18 to May 3, 2013, acquiring multicomponent and high-resolution 2D seismic data. GC955 and WR313 are established, world-class study sites where high gas hydrate saturations exist within reservoir-grade sands in this long-established petroleum province. Logging-while-drilling (LWD) data acquired in 2009 by the Gulf of Mexico Gas Hydrates Joint Industry Project provide detailed characterization at the borehole locations, and industry seismic data provide regional- and local-scale structural and stratigraphic characterization. Significant remaining questions regarding lithology and hydrate saturation between and away from the boreholes spurred new geophysical data acquisition at these sites. The goals of our 2013 surveys were to (1) achieve improved imaging and characterization at these sites and (2) refine geophysical methods for gas hydrate characterization in other locations. In the area of GC955 we deployed 21 ocean-bottom seismometers (OBS) and acquired approximately 400 km of high-resolution 2D streamer seismic data in a grid with line spacing as small as 50 m and along radial lines that provide source offsets up to 10 km and diverse azimuths for the OBS. In the area of WR313 we deployed 25 OBS and acquired approximately 450 km of streamer seismic data in a grid pattern with line spacing as small as 250 m and along radial lines that provide source offsets up to 10 km for the OBS. These new data afford at least five times better resolution of the structural and stratigraphic features of interest at the sites and enable considerably improved characterization of lithology and the gas and gas hydrate systems. Our recent survey represents a unique application of dedicated geophysical data to the characterization of confirmed reservoir-grade gas hydrate accumulations.

Gulf Of Mexico↗
USGS2019

Use of a towed electromagnetic induction (tTem) system for shallow aquifer characterization – An example from the Mississippi Alluvial Plain

The Mississippi Alluvial Plain (MAP) aquifer system is a vital resource that supports agriculture in one of the most productive regions of the country. The U.S. Geological Survey Water Availability and Use Science Program (WAUSP) is conducting a multi-discipline investigation of the MAP aquifer system. The investigation is utilizing borehole, surface, and airborne geophysical methods to improve the characterization and understanding of the aquifer. The combination of geophysical data collected over a range of spatial scales with varying depths of investigation and resolution is key to determining the distribution of sand and clay within the aquifer. Mobile geophysical methods that enable continuous measurements over large areas improve aquifer characterization with their increased spatial coverage. In support of the MAP investigation, a new towed Time-Domain Electromagnetic (tTEM) imaging system developed by Aarhus University was used near Shellmound, Mississippi (MS), to delineate the distribution of coarse- and fine-grained sediments underlying the site. The tTEM results compare favorably with the results of airborne EM (AEM) surveys flown in the study and improve the resolution of sand and gravel distribution within the tTEM depth of investigation.

Mississippi↗
USGS2008

High resolution shallow geologic characterization of a late Pleistocene eolian environment using ground penetrating radar and optically stimulated luminescence techniques: North Carolina, USA

Geophysical surveys, sedimentology, and optically-stimulated luminescence age analyses were used to assess the geologic development of a coastal system near Swansboro, NC. This area is a significant Woodland Period Native American habitation and is designated the "Broad Reach" archaeological site. 2-d and 3-d subsurface geophysical surveys were performed using a ground penetrating radar system to define the stratigraphic framework and depositional facies. Sediment samples were collected and analyzed for grain-size to determine depositional environments. Samples were acquired and analyzed using optically stimulated luminescence techniques to derive the depositional age of the various features. The data support a low eolian to shallow subtidal coastal depositional setting for this area. Li-DAR data reveal ridge and swale topography, most likely related to beach ridges, and eolian features including low-relief, low-angle transverse and parabolic dunes, blowouts, and a low-relief eolian sand sheet. Geophysical data reveal dominantly seaward dipping units, and low-angle mounded features. Sedimentological data reveal mostly moderately-well to well-sorted fine-grained symmetrical to coarse skewed sands, suggesting initial aqueous transport and deposition, followed by eolian reworking and bioturbation. OSL data indicate initial coastal deposition prior to ca. 45,000 yBP, followed by eolian reworking and low dune stabilization at ca. 13,000 to 11,500 yBP, and again at ca. 10,000 yBP (during, and slightly after the Younger Dryas chronozone).

Southeastern Geology↗
USGS2011

Saline aquifer mapping project in the southeastern United States

In 2009, the U.S. Geological Survey initiated a study of saline aquifers in the southeastern United States to evaluate the potential use of brackish or saline water from the deeper portions of the Floridan aquifer system and the underlying Coastal Plain aquifer system (Fig. 1). The objective of this study is to improve the overall understanding of the available saline water resources for potential future development. Specific tasks are to (1) develop a digital georeferenced database of borehole geophysical data to enable analysis and characterization of saline aquifers (see locations in Fig. 1), (2) identify and map the regional extent of saline aquifer systems and describe the thickness and character of hydrologic units that compose these systems, and (3) delineate salinity variations at key well sites and along section lines to provide a regional depiction of the freshwater-saltwater interfaces. Electrical resistivity and induction logs, coupled with a variety of different porosity logs (sonic, density, and neutron), are the primary types of borehole geophysical logs being used to estimate the water quality in brackish and saline formations. The results from the geophysical log calculations are being compared to available water-quality data obtained from water wells and from drill-stem water samples collected in test wells. Overall, the saline aquifer mapping project is helping to improve the understanding of saline water resources in the area. These aquifers may be sources of large quantities of water that could be treated by using reverse osmosis or similar technologies, or they could be used for aquifer storage and recovery systems.

Florida, Georgia↗
USGS2018

Discovery of a blind geothermal system in Southern Gabbs Valley, western Nevada, through application of the play fairway analysis at multiple scales

The Great Basin region is capable of generating much greater amounts of geothermal energy than currently produced. Most geothermal resources in this region are blind, and thus favorable characteristics for geothermal activity must be synthesized and methodologies developed to discover new commercial-grade systems. The geothermal play fairway concept involves integration of multiple parameters indicative of geothermal activity to identify promising areas for new development. In the Nevada play fairway project, geologic, geochemical, and geophysical parameters were initially synthesized to produce a new geothermal potential map of 96,000 km2. Southern Gabbs Valley in western Nevada is a particularly promising site selected for detailed study. It contains favorable structural settings, including Quaternary fault intersections and a displacement transfer zone. Geologic, geophysical, and geochemical techniques were employed to define the most likely sites for high permeability and select targets for temperature-gradient holes. Permeability models were revised to reflect results of detailed analyses and generate new detailed play fairway maps. The most promising site lies in an area of multiple fault intersections in a broader displacement transfer zone directly north of the Petrified Springs dextral fault, as revealed by gravity, magnetic, and MT data. A 2-m temperature anomaly and warm temperature gradient wells (~120oC at ~150 m depth) confirm the presence of a geothermal system and provide initial validation of the play fairway methodology. The system is blind, with no surface hot springs, fumaroles, or paleo-geothermal deposits. Lessons learned in the detailed studies include: 1) initially identified sites commonly include multiple favorable settings at a finer scale; 2) promising sites in Cenozoic basins cannot be recognized without detailed geophysical surveys; and 3) play fairway analysis is critical at multiple scales.

Geothermal Resources Council Transactions↗
USGS2019

The relationship of channel planform and point bar architecture on a reach of the Wabash River near Grayville, Illinois

The erosional and depositional characteristics of meandering rivers lead to the formation and maintenance of point bars along the inner banks of meander bends. Point bars are composed of sediment layers in patterns resulting from the rate and style of channel migration, hydrodynamics, and sediment transport and deposition within the river system (e.g. Jackson, 1976; Dietrich and Smith, 1984; Dietrich, 1987, Abad and Garcia, 2009). The distribution of the sediments preserved in the internal architecture of a river point bar provides a record of channel planform evolution. Geophysical methods are used to gain a large-scale visualization of the subsurface and aid in the interpretation of historic channel patterns (Best et al., 2003; Woodward et al., 2003; Sambrook Smith et al., 2006). Comparing known surficial extents of the point bar to features identified in the subsurface can also enhance the understanding of historic channel planform. This study investigates two point bars along bends with different styles of migration, Maier and TB3, in a well-documented reach of the Wabash River near Grayville, IL. Evidence from historic aerial photography, modern lidar, photogrammetry, and geophysical surveys were used to determine the relationship between the point bar architecture and channel planform. Airborne lidar was flown in 2011 and is used to create the 2011 point bar surface. In 2017, a terrestrial lidar and Real Time Kinematic-Global Navigation Satellite System (RTK-GNSS) topographic survey were combined to create the surface for TB3. In 2018, a photogrammetric survey collected with a small unmanned aerial system (sUAS) was used to create a structure-frommotion (SfM) derived surface for Maier bend. The 2018 survey-based point bar surfaces were differenced from the 2011 point bar surfaces to get a Digital Elevation Model (DEM) of difference (DoD) to visualize areas of erosion and deposition. In addition, geophysical surveys using ground penetrating radar (GPR) were conducted in transverse and streamwise lines across the point bars in 2018. Elevation profiles from the 2011 point bar surfaces are extracted and overlain onto the 2018 GPR images to determine how the point bar is preserving the structure of sediments previously deposited. Results from this study provide an update to current models of point bar architecture.

Illinois↗
USGS2021

Model structural uncertainty quantification and hydrogeophysical data integration using airborne electromagnetic data

A irborne electromagnetic (AEM) data are used to estimate large - scale model structural geometry, i.e. the spatial distribution of different lit hological units based on assumed or estimated resistivity - lithology relationships, and the uncertainty in those structures given imperfect measurements. Geophysically derived estimates of model structural uncertainty are then combined with hydrologic obse rvations to assess the impact of model structural error on hydrologic calibration and prediction errors. Using a synthetic numerical model, we describe a sequential hydrogeophysical approach that: (1) uses Bayesian Markov chain Monte Carlo (McMC) methods to produce a robust estimate of uncertainty in electrical resistivity parameter s , (2) combines geophysical parameter uncertainty estimates with borehole observations of lithology to produce probabilistic estimates of model structural uncertainty over the e ntire AEM survey area using geostatistical sequential indicator simulation algorithms, and (3) uses model structural estimates along with hydrologic observations to quantify both hydrologic parameter and prediction uncertainty using a second McMC sampling algorithm. Results of simulations will be presented that illustrate the complete workflow from geophysical parameter uncertainty analysis to the impact of model structural uncertainty on hydrologic parameter estimates.

Conference Paper↗
USGS2021

California deepwater investigations and groundtruthing (Cal DIG) I: Fault and shallow geohazard analysis offshore Morro Bay

The California Deepwater Investigations and Groundtruthing (Cal DIG) I project focuses on the potential seafloor hazards and impacts of alternative energy infrastructure in the outer continental shelf region offshore of south-central California. This is one of three reports covering a single study area located between Monterey and Point Conception, California in federal waters outside of the State of California three nautical mile limit and in water depths of 400 to 1500 meters. The goal of this report is to provide baseline geologic interpretations of the area for the purpose of improving regional models of shallow geologic hazards and sedimentary processes. Geophysical and geological data from this project will help to address important issues associated with marine spatial planning and potential offshore infrastructure development, such as offshore floating wind turbines. Datasets covered in this report include comprehensive high-resolution sub-bottom (multi-channel and Chirp seismic reflection profiles), seafloor (bathymetry), and sampling (piston, gravity, and vibracore) data collected in 2018-2019 during a series of seven seagoing geological and geophysical surveys. Specifically, this report outlines interpretation of subsurface geologic structure from the geophysical data, details preliminary core analysis results related to fluid, gas, and sediment transport activity, provides interpretations of the current geohazards in the area, and suggests next steps for improving interpretations of geohazard processes. Specific targets of geohazard interest in the study area are geological structures such as faults and folds, seafloor pockmarks within a large field (the Big Sur pockmark field), submarine channels, and mass wasting (slope failure) features. The vast majority of faults and other structures in the study occur within sediment and rock formations we interpret to be pre-Quaternary (older than 2.58 Myr BP), and thus we interpret that these structures are unlikely to present significant current hazard to seabed infrastructure, although we note that the numerous structures mapped in the study area may have the potential to become reactivated. Similarly, we find no new evidence of Holocene (younger than 11,650 years BP) fluid or gas advection in the Big Sur pockmark field. However, such fluid and gas hazards are currently difficult to assess, as additional analyses and sampling of existing core data is needed to better understand pockmark formation processes and potential gas accumulations we have mapped in the subsurface. Mass wasting along the eastern and western edges of the Santa Lucia Bank during earthquakes, as well as sediment transport down the Lucia Chica and San Simeon channels, are among the most significant, although still likely infrequent during the Holocene, hazards to seabed stability in the study area. Further analyses of the existing cores, including radiocarbon dating, stable isotope, and compositional analyses, are again needed to better understand the timing and sources of the numerous sand deposits found throughout the study area, which may have been transported downslope due to mass wasting and/or earthquake shaking processes.

California↗
USGS2022

Updates to and applications of the USGS National Crustal Model for seismic hazard studies

The U.S. Geological Survey (USGS) National Crustal Model (NCM) is being developed to assist in the modeling of seismic hazards across the conterminous United States. The NCM is composed of a grid of geophysical profiles, extending from the Earth’s surface into the upper mantle. It is constructed from a 3D geologic framework and geophysical rules defined by: (1) a petrologic and mineral physics database; (2) a 3D temperature model; and (3) a calibrated rock type- and age-dependent porosity model. Parameters needed to estimate site response for existing ground motion models (GMMs), including the time-averaged velocity in the upper 30 meters ( V S 30 ) and the depths to 1.0 and 2.5 km/s shear-wave velocity ( Z 1.0 and Z 2.5 ), can be extracted from the NCM. As GMMs develop, other metrics could also be extracted or derived from the NCM such as sediment thickness and travel times, fundamental frequency, a fully frequency-dependent site response function, or 3D geophysical volumes for wavefield simulations. Application of the NCM may also benefit other aspects of seismic hazard analysis including better accounting for path-dependent attenuation and geometric spreading, more accurate estimation of earthquake source properties such as hypocentral location and stress drop, and calculation of crustal strength profiles that inform estimates of the base of seismicity.

Conference Paper↗
USGS2024

Recent applications of the USGS National Crustal Model for Seismic Hazard Studies

The U.S. Geological Survey is developing the National Crustal Model (NCM) for seismic hazard studies to facilitate modeling site, path, and source components of seismic hazard across the conterminous United States. The NCM is composed of a 1km grid of geophysical profiles, extending from the Earth’s surface into the upper mantle. It is constructed from a threedimensional (3D) geologic framework and geophysical rules that use (1) a petrologic and mineral physics database; (2) a 3D temperature model; and (3) a calibrated rock type- and age-dependent porosity model. Parameters needed to estimate site response for existing ground motion models (GMMs), including the time-averaged velocity in the upper 30 meters (VS30), the depths to 1.0 and 2.5 km/s shear-wave velocity (Z1.0 and Z2.5), and sediment thickness, can be computed from the NCM. As GMMs continue to improve in the future, other metrics could also be extracted or derived from the NCM, such as fundamental period, site attenuation (ko), a fully frequency-dependent site response function, or 3D geophysical volumes for wavefield simulations. Application of the NCM may also benefit other aspects of seismic hazard analysis, including better accounting for path-dependent attenuation and geometric spreading, more accurate estimation of earthquake source properties such as hypocentral location and stress drop, and calculation of crustal strength profiles that inform estimates of the base of seismicity.

conterminous United States↗
USGS2025

Preliminary depth to basement modeling at Salton Sea, California

The San Andreas Fault – Imperial Fault (SAF-IF) transtensional step-over zone along the southern margin of the Salton Sea hosts substantial geothermal production and lithium brine resources. Recent volcanism at the Salton Buttes and active seismicity along the SAFIF fault system highlight active tectonic and magmatic processes that pose natural hazards and may impact energy and mineral production. Characterizing the subsurface architecture and extent of concealed alteration associated with this tectono-magmatic system enhances understanding of these active processes, associated hazards, and resources. We have compiled a gravity database, consisting of new and re-processed existing data, from which we have constructed a new isostatic residual gravity anomaly map of the Salton trough. We have used this new gravity dataset together with a compilation of publicly available borehole data to develop new depth to basement inversion models for the region. These depth to basement models help to constrain basin geometries, inform alteration mapping, and reveal variations in basement rocks. Due to the concealed nature of the complex tectonic framework at the Salton trough, it is necessary to utilize geophysical methods for subsurface characterization. These new depth to basement models are a first step toward constructing 2D and 3D geophysical and geologic models of the Imperial Valley and Salton Sea geothermal area. This analysis complements other geophysical initiatives, including magnetotelluric (MT) modeling (Tokmakoff et al., 2024), magnetic mapping (Glen and Earney, 2023, 2024) and potential field modeling, and seismic studies focused on hazard and resource investigations in the Imperial Valley.

California↗
USGS2026

Open-source gravity reduction workflows for geothermal resource assessment

Potential-field geophysical data such as gravity can enhance understanding of geothermal resources at all stages of the resource life cycle, including assessment, exploration, development, and monitoring, and at multiple scales, from the reservoir scale to regional scale. However, to make gravity data useful for geothermal resource characterization, several processing steps are required to isolate the effects of density variations in the Earth’s crust to enable the identification of structural features associated with geothermal resources. Although this process is well-established, standard computational implementations for processing gravity data that are FAIR (Findable, Accessible, Interoperable, and Reproduceable) are still lacking. This paper details ongoing efforts at the U.S. Geological Survey (USGS) to develop a standard set of open-source Python tools for gravity data reduction that align with the FAIR principles. This workflow makes use of existing open-source tools for geophysical data processing with the goal of maximizing opportunities for rapid improvements, interoperability, and adaptability to other types of geophysical data.

Conference Paper↗
USGS2011

A trans-dimensional Bayesian Markov chain Monte Carlo algorithm for model assessment using frequency-domain electromagnetic data

A meaningful interpretation of geophysical measurements requires an assessment of the space of models that are consistent with the data, rather than just a single, ‘best’ model which does not convey information about parameter uncertainty. For this purpose, a trans-dimensional Bayesian Markov chain Monte Carlo (MCMC) algorithm is developed for assessing frequencydomain electromagnetic (FDEM) data acquired from airborne or ground-based systems. By sampling the distribution of models that are consistent with measured data and any prior knowledge, valuable inferences can be made about parameter values such as the likely depth to an interface, the distribution of possible resistivity values as a function of depth and non-unique relationships between parameters. The trans-dimensional aspect of the algorithm allows the number of layers to be a free parameter that is controlled by the data, where models with fewer layers are inherently favoured, which provides a natural measure of parsimony and a significant degree of flexibility in parametrization. The MCMC algorithm is used with synthetic examples to illustrate how the distribution of acceptable models is affected by the choice of prior information, the system geometry and configuration and the uncertainty in the measured system elevation. An airborne FDEM data set that was acquired for the purpose of hydrogeological characterization is also studied. The results compare favorably with traditional least-squares analysis, borehole resistivity and lithology logs from the site, and also provide new information about parameter uncertainty necessary for model assessment.

Geophysical Journal International↗
USGS2015

A random-walk algorithm for modeling lithospheric density and the role of body forces in the evolution of the Midcontinent Rift

This paper develops a Monte Carlo algorithm for extracting three-dimensional lithospheric density models from geophysical data. Empirical scaling relationships between velocity and density create a 3D starting density model, which is then iteratively refined until it reproduces observed gravity and topography. This approach permits deviations from uniform crustal velocity-density scaling, which provide insight into crustal lithology and prevent spurious mapping of crustal anomalies into the mantle. We test this algorithm on the Proterozoic Midcontinent Rift (MCR), north-central U.S. The MCR provides a challenge because it hosts a gravity high overlying low shear-wave velocity crust in a generally flat region. Our initial density estimates are derived from a seismic velocity/crustal thickness model based on joint inversion of surface-wave dispersion and receiver functions. By adjusting these estimates to reproduce gravity and topography, we generate a lithospheric-scale model that reveals dense middle crust and eclogitized lowermost crust within the rift. Mantle lithospheric density beneath the MCR is not anomalous, consistent with geochemical evidence that lithospheric mantle was not the primary source of rift-related magmas and suggesting that extension occurred in response to far-field stress rather than a hot mantle plume. Similarly, the subsequent inversion of normal faults resulted from changing far-field stress that exploited not only warm, recently faulted crust but also a gravitational potential energy low in the MCR. The success of this density modeling algorithm in the face of such apparently contradictory geophysical properties suggests that it may be applicable to a variety of tectonic and geodynamic problems.

Iowa, Kansas, Michigan, Minnesota, Nebraska, Wisco↗
USGS2016

Evidence for nonuniform permafrost degradation after fire in boreal landscapes

Fire can be a significant driver of permafrost change in boreal landscapes, altering the availability of soil carbon and nutrients that have important implications for future climate and ecological succession. However, not all landscapes are equally susceptible to fire-induced change. As fire frequency is expected to increase in the high latitudes, methods to understand the vulnerability and resilience of different landscapes to permafrost degradation are needed. We present a combination of multiscale remote sensing, geophysical, and field observations that reveal details of both near-surface (<1 m) and deeper (>1 m) impacts of fire on permafrost. Along 11 transects that span burned-unburned boundaries in different landscape settings within interior Alaska, subsurface electrical resistivity and nuclear magnetic resonance data indicate locations where permafrost appears to be resilient to disturbance from fire, areas where warm permafrost conditions exist that may be most vulnerable to future change, and also areas where permafrost has thawed. High-resolution geophysical data corroborate remote sensing interpretations of near-surface permafrost and also add new high-fidelity details of spatial heterogeneity that extend from the shallow subsurface to depths of about 10 m. Results show that postfire impacts on permafrost can be variable and depend on multiple factors such as fire severity, soil texture, soil moisture, and time since fire.

Alaska↗
USGS1985

A study of the depth of weathering and its relationship to the mechanical properties of near-surface rocks in the Mojave Desert

Weathered granite extends 70 m deep at Hi Vista in the arid central Mojave Desert of southern California. The low strength of this granite is due to the alteration of biotite and chlorite montmorillonite. Deep weathering probably occurs in most granites, although we cannot rule out some anomalous mechanisms at Hi Vista. Geophysical instruments set in these slightly altered rocks are limited by the unstable behavior of the rocks. Thus, tectonic signals from instruments placed in shallow boreholes give vague results. Geophysical measurements of these weathered rocks resemble measurements of granitic rocks near major faults. The rheology of the rocks in which instruments are placed limits the useful sensitivity of the instruments. ?? 1985 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗