USGS Science⌕ Search

SEARCH · USGS Science

Results for “Near Surface Geophysics”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22Linked to original sources
USGS2004

Characterization of aquifer heterogeneity using cyclostratigraphy and geophysical methods in the upper part of the Karstic Biscayne Aquifer, Southeastern Florida

This report identifies and characterizes candidate ground-water flow zones in the upper part of the shallow, eogenetic karst limestone of the Biscayne aquifer in the Lake Belt area of north-central Miami-Dade County using cyclostratigraphy, ground-penetrating radar (GPR), borehole geophysical logs, and continuously drilled cores. About 60 miles of GPR profiles were used to calculate depths to shallow geologic contacts and hydrogeologic units, image karst features, and produce qualitative views of the porosity distribution. Descriptions of the lithology, rock fabrics, and cyclostratigraphy, and interpretation of depositional environments of 50 test coreholes were linked to the geophysical interpretations to provide an accurate hydrogeologic framework. Molluscan and benthic foraminiferal paleontologic constraints guided interpretation of depositional environments represented by rockfabric facies. Digital borehole images were used to characterize and quantify large-scale vuggy porosity. Preliminary heat-pulse flowmeter data were coupled with the digital borehole image data to identify candidate ground-water flow zones. Combined results show that the porosity and permeability of the karst limestone of the Biscayne aquifer have a highly heterogeneous and anisotropic distribution that is mostly related to secondary porosity overprinting vertical stacking of rock-fabric facies within high-frequency cycles (HFCs). This distribution of porosity produces a dual-porosity system consisting of diffuse-carbonate and conduit flow zones. The nonuniform ground-water flow in the upper part of the Biscayne aquifer is mostly localized through secondary permeability, the result of solution-enlarged carbonate grains, depositional textures, bedding planes, cracks, root molds, and paleokarst surfaces. Many of the resulting pore types are classified as touching vugs. GPR, borehole geophysical logs, and whole-core analyses show that there is an empirical relation between formation porosity, permeability, formation electrical conductivity, and GPR reflection amplitudes? as porosity and permeability increase, formation electrical conductivity increases and reflection amplitude decreases. This relation was observed throughout the entire vertical and lateral section of the upper part of the Biscayne aquifer in the study area. Further, upward-shallowing brackish- or freshwatercapped cycles of the upper part of the Fort Thompson Formation show low-amplitude reflections near their base that correspond to relatively higher porosity and permeability. This distribution is related to a systematic vertical stacking of rock-fabric facies within the cycle. Inferred flow characteristics of the porosity distribution within the upper part of the Biscayne aquifer were used to identify four ground-water flow classes, with each characterized by a discrete pore system that affects vertical and horizontal groundwater flow: (1) a low-permeability peat, muck, and marl ground-water flow class; (2) a horizontal conduit ground-water flow class; (3) a leaky, low-permeability ground-water flow class; and (4) a diffuse-carbonate ground-water flow class. At the top of the Biscayne aquifer, peat, muck, and marl can combine to form a relatively low-permeability layer of Holocene sediment that water moves through slowly. Most horizontal conduit flow is inferred to occur along touching vugs in portions of the following rock-fabric facies: (1) touchingvug pelecypod floatstone and rudstone, (2) sandy touching-vug pelecypod floatstone and rudstone, (3) vuggy wackestone and packstone, (4) laminated peloid grainstone and packstone, (5) peloid grainstone and packstone, and (6) peloid wackestone and packstone. Gastropod floatstone and rudstone, mudstone and wackestone, and pedogenic limestone rock-fabric facies are the main hosts for leaky, low-permeability units.

Florida↗
USGS2012

Displacement fields from point cloud data: Application of particle imaging velocimetry to landslide geodesy

Acquiring spatially continuous ground-surface displacement fields from Terrestrial Laser Scanners (TLS) will allow better understanding of the physical processes governing landslide motion at detailed spatial and temporal scales. Problems arise, however, when estimating continuous displacement fields from TLS point-clouds because reflecting points from sequential scans of moving ground are not defined uniquely, thus repeat TLS surveys typically do not track individual reflectors. Here, we implemented the cross-correlation-based Particle Image Velocimetry (PIV) method to derive a surface deformation field using TLS point-cloud data. We estimated associated errors using the shape of the cross-correlation function and tested the method's performance with synthetic displacements applied to a TLS point cloud. We applied the method to the toe of the episodically active Cleveland Corral Landslide in northern California using TLS data acquired in June 2005–January 2007 and January–May 2010. Estimated displacements ranged from decimeters to several meters and they agreed well with independent measurements at better than 9% root mean squared (RMS) error. For each of the time periods, the method provided a smooth, nearly continuous displacement field that coincides with independently mapped boundaries of the slide and permits further kinematic and mechanical inference. For the 2010 data set, for instance, the PIV-derived displacement field identified a diffuse zone of displacement that preceded by over a month the development of a new lateral shear zone. Additionally, the upslope and downslope displacement gradients delineated by the dense PIV field elucidated the non-rigid behavior of the slide.

California↗
USGS2012

Sediment entrainment by debris flows: In situ measurements from the headwaters of a steep catchment

Debris flows can dramatically increase their volume, and hence their destructive potential, by entraining sediment. Yet quantitative constraints on rates and mechanics of sediment entrainment by debris flows are limited. Using an in situ sensor network in the headwaters of a natural catchment we measured flow and bed properties during six erosive debris-flow events. Despite similar flow properties and thicknesses of bed sediment entrained across all events, time-averaged entrainment rates were significantly faster for bed sediment that was saturated prior to flow arrival compared with rates for sediment that was dry. Bed sediment was entrained from the sediment-surface downward in a progressive fashion and occurred during passage of dense granular fronts as well as water-rich, inter-surge flow. En masse failure of bed sediment along the sediment-bedrock interface was never observed. Large-magnitude, high-frequency fluctuations in total normal basal stress were dissipated within the upper 5 cm of bed sediment. Within this near surface layer, concomitant fluctuations in Coulomb frictional resistance are expected, irrespective of the influence of pore fluid pressure or fluctuations in shear stress. If the near-surface sediment was wet as it was overridden by a flow, additional large-magnitude, high-frequency pore pressure fluctuations were measured in the near-surface bed sediment. These pore pressure fluctuations propagated to depth at subsonic rates and in a diffusive manner. The depth to which large excess pore pressures propagated was typically less than 10 cm, but scaled as ( D / f i ) 0.5 , in which D is the hydraulic diffusivity and f i is the frequency of a particular pore pressure fluctuation. Shallow penetration depths of granular-normal-stress fluctuations and excess pore pressures demonstrate that only near-surface bed sediment experiences the full dynamic range of effective-stress fluctuations, and as a result, can be more easily entrained than deeper sediment. These data provide robust tests for mechanical models of entrainment and demonstrate that a debris flow over wet bed sediment will be larger than the same flow over dry bed sediment.

Colorado↗
USGS2026

Hydrogeologic conceptualization of the Arroyos subarea and Northeast Air Force Research Laboratory subarea of the Air Force Research Laboratory, Edwards Air Force Base, Antelope Valley, California, 2017–21

Research and testing within the Air Force Research Laboratory at Edwards Air Force Base in the southwestern Mojave Desert has been conducted since the 1950s. This testing requires large quantities of groundwater for cooling and operations involving hazardous constituents that have entered the groundwater. To better understand the potential movement of contaminants through the groundwater system, the U.S. Geological Survey entered into a cooperative agreement with the U.S. Air Force Civil Engineer Center with the purpose of (1) gaining a more detailed understanding of the hydraulic connection between the unconsolidated basin-fill sediment and the underlying weathered and crystalline bedrock and (2) investigating faults and their potential effect on groundwater flow. This study was specific to the Arroyos subarea and Northeast Air Force Research Laboratory subarea of the Air Force Research Laboratory. The refined hydrogeologic characterization was derived by evaluating surface geologic maps, a regional gravity model, new subsurface geophysical surveys, lithology data from boreholes, and groundwater-level data. Refinement of the hydrogeologic characterization of the study area involved developing a map of basin-fill sediment thickness, estimating the geometry and depth to bedrock, determining the presence of weathered bedrock material, and identifying the location of faults and subsurface structures. The thickness of basin-fill sediment was derived from a regional gravity model. Basin-fill sediment ranges from less than 20 feet (ft) thick to about 150 ft thick and increases substantially toward a deep structural basin in the northwestern part of the study area. The top of bedrock estimated from the new surface geophysical surveys is generally deeper than the top of bedrock from the regional gravity model. These differences in interpreted bedrock tops indicate that there may be a zone of weathered bedrock in the subsurface, overlying crystalline bedrock. Additional subsurface data evaluating the geometry and thickness of weathered bedrock are necessary for a comprehensive evaluation of weathered bedrock in the study area. Previously unmapped extensions of the Leuhman Fault, Spring Fault Zone, and Boron Gate Fault were identified in the new surface geophysical surveys. Evaluation of groundwater flow between bedrock and basin-fill sediment, and the effects of faults on groundwater flow, was done using groundwater-level data from 2020. The groundwater table was in crystalline bedrock throughout most of the Arroyos and Northeast Air Force Research Laboratory subareas, with a few exceptions where it was in basin-fill sediment and weathered bedrock. Basin-fill sediment, and likely weathered bedrock, was partially saturated in parts of the study area where basin-fill sediment was as thick as 150 ft. Weathered bedrock material may be present near the Spring Fault Zone, and if so, was likely saturated in 2020. The groundwater-level data near and within Spring Fault Zone indicate that the Spring Fault Zone acts as a vertical-along-strike conduit for, and as an across-strike impediment to, groundwater flow. In addition, steep groundwater-level gradients in the southeastern part of the study area indicate that a subsurface structure affecting groundwater flow could be related to the Leuhman Fault. Sufficient groundwater-level data were not available to conclusively determine if the Arroyos, Rich, and Boron Gate Faults, and their inferred extensions from the geophysical surveys, affect the movement or direction of groundwater flow.

California↗
USGS1989

Test holes for monitoring surface-water/ground-water relations in the Cottonwood Creek area, Shasta and Tehama Counties, California, 1984-85

Ten test holes were drilled to obtain hydrogeologic information for an investigation of stream-aquifer interaction near proposed damsites on Cottonwood Creek and South Fork Cottonwood Creek, California. At each site, one deep well was completed below the first confining clay encountered in the upper Tehama Formation to determine hydraulic gradients between water-bearing deposits in the Tehama Formation and overlying channel deposits. At three sites along Cottonwood Creek, two shallow wells were drilled at each site on a line perpendicular to the stream channel to determine if groundwater in channel deposits is moving toward or away from the stream channel and to monitor water levels. Geophysical logs were correlated with lithologic logs compiled from analyses of drill cuttings to determine depths for setting well screens. After pumping to confirm hydraulic connection between each well and the Tehama Formation, water levels were monitored monthly from June 1984 to June 1985; at two sites, water levels were above the altitude of the stream channel bottom during all streamflow conditions. Tritium dating indicates two wells have water more than 100 years old; one well has either a mixture of old and new water or an intermediate-aged water. (USGS)

Water-Resources Investigations Report↗
USGS2010

Topographic complexity and roughness of a tropical benthic seascape

Topographic complexity is a fundamental structural property of benthic marine ecosystems that exists across all scales and affects a multitude of processes. Coral reefs are a prime example, for which this complexity has been found to impact water flow, species diversity, nutrient uptake, and wave-energy dissipation, among other properties. Despite its importance, only limited assessments are available regarding the distribution or range of topographic complexity within or between benthic communities. Here, we show substantial variability in topographic complexity over the entire inner-shelf seascape of a tropical island. Roughness, estimated in terms of fractal dimension, served as a proxy for topographic complexity, and was computed for linear transects (D T ), as well as the benthic surface (D S ). Spatial variability in both D T and D S was correlated with the known distribution of benthic cover types in the seascape. Transect roughness values ranged from 1.0 to 1.7, with features along the shelf edge being markedly anisotropic with an along-shore bias, whereas regions with high scleractinian coral cover were nearly isotropic and exhibited minimal directional bias. Surface-roughness values ranged from 2.0 in predominantly hardbottom areas with low coral cover to 2.5 in areas with high coral cover. Quantifying roughness across the substrates and biological communities for an entire seascape provides a synoptic view of its spatial variability at scales appropriate for numerous research efforts, including ecosystem studies, parameterizing hydrodynamic models, and designing monitoring programs.

Geophysical Research Letters↗
USGS1997

Tertiary and Quaternary tectonic faulting in southernmost Illinois

Tertiary and/or Quaternary tectonic faulting is documented in three areas of southernmost Illinois: the Fluorspar Area Fault Complex (FAFC) in Pope and Massac Counties, the Ste. Genevieve Fault Zone (SGFZ) in Alexander and Union Counties, and the Commerce Fault Zone (CFZ) in Alexander County. In the FAFC, faults that strike NE and NNE displace Mounds Gravel (late Miocene to early Pleistocene) and, locally, the Metropolis terrace gravel (Pleistocene; pre-Woodfordian). No Woodfordian or younger deposits are deformed. Faults typically outline narrow, linear grabens that formed under tension with a component of strike slip. North-south to NW-trending vertical faults near the southeast end of the SGFZ displace Eocene sediments. Again, faults outline narrow grabens and show indications of strike slip. Deformed Quaternary sediments have not been observed. The CFZ, which trends northeast, displaces Mounds Gravel in Illinois and units as young as Peoria Silt (Woodfordian) in Missouri. Quaternary movement has been interpreted as right-lateral strike-slip. The CFZ coincides with a subtle gravity and magnetic lineament and seems to reflect a major feature in the basement. Surface expression in Illinois is subtle, but mafic and ultramafic intrusions, hydrothermal alteration and small faults align with the Commerce geophysical lineament. Earthquake foci in Missouri and Illinois lie on or close to the CFZ; some focal mechanisms fit the fault trend. Among these structures, only the CFZ exhibits slip that conforms to the current stress field (principal compressive stress axis E-W to ENE-WSW). Possibly, the stress field changed during Neogene time. Alternatively, high fluid pressures or local stress concentrations may have induced slip on less favorably oriented fractures. Tighter constraints are needed on timing, magnitude, and direction of Neogene displacement.

Illinois↗
USGS2006

Hydrogeology and ground-water quality, Chippewa Township, Isabella County, Michigan, 2002-05

The ground-water resource potential of Chippewa Township, Isabella County, Mich. was characterized on the basis of existing hydrogeologic data, water-level records, analyses of water samples, and interpretation of geophysical survey data. Eight ground-water samples were collected and analyzed for major ions, nutrients, and trace-metal composition. In addition, 10 direct current-resistivity soundings were collected throughout Chippewa and Coe Townships to identify potential freshwater in the aquifer system. The aquifer system includes complexly interbedded glaciofluvial, glaciolacustrine, and basal-lodgment tills, which overlie Jurassic or Pennsylvanian sedimentary rocks. In parts of the township, freshwater is present in all geologic units, but in most areas saline water is encountered near the base of Pleistocene glacial deposits and in the Jurassic or Pennsylvanian bedrock. A near-surface sheet of relatively dense basal-lodgment till likely prevents, or substantially retards, significant direct recharge of ground water to glacial and bedrock aquifers in Chippewa and adjacent townships. Glacial sands and gravels form the principal aquifer for domestic wells (97.5 percent of wells in the township). The single community water supply in the township has wells screened in glacial deposits near the base of the glacial drift. Increased withdrawals of ground water in response to increasing demand has led to a slight decline in water quality from this supply. This water-quality decline is related primarily to an increase of dissolved sulfate, which is probably a function of well depth and dissolution of gypsum, a common mineral constituent in the Jurassic “red beds” , which form the uppermost bedrock unit throughout most of the township. One explanation for the increase in sulfate is upconing of saline water from bedrock sources, which may contain saline water.

Michigan↗
USGS2019

Heat and mass transport in a vapor-dominated hydrothermal area in Yellowstone National Park, USA: Inferences from magnetic, electrical, electromagnetic, subsurface temperature and diffuse CO2 flux measurements

Vapor‐dominated hydrothermal systems are characterized by localized and elevated heat and gas flux. In these systems, steam and gas ascend from a boiling water reservoir, steam condenses beneath a low‐permeability cap layer, and liquid water descends, driven by gravity (“heat pipe” model). We combine magnetic, electromagnetic, and geoelectrical methods and CO 2 flux and subsurface temperature measurements in the Solfatara Plateau Thermal Area in the Yellowstone Caldera to address several fundamental questions: (1) What are the structural and/or lithological controls on heat and mass transport in vapor‐dominated areas? (2) What is the geometry and size of convecting multiphase thermal plumes? (3) Are thermal plumes associated with subsurface rock alteration and demagnetization? Magnetic and electromagnetic data inversions suggest an asymmetric 50‐ to 100‐m thick basin of glacial deposits with the thickest part adjacent to the margin of a rhyolite flow. The 3‐D electrical conductivity model in the glacial basin reveals a narrow vertical conductor interpreted as a focused multiphase plume, which coincides at the ground surface with the heat and CO 2 flux maxima. The magnetic data suggest that destruction of magnetic minerals due to rock alteration associated with the hydrothermal plume occurs mainly near the ground surface. We propose a model where the buoyant multiphase plume forms in response to decompression, boiling, and phase separation of pressurized thermal groundwater that discharges from the brecciated base of a rhyolite flow into the basin of glacial deposits. Results from multiphase groundwater flow and heat transport numerical simulations corroborate the first‐order characteristics of this model.

Wyoming↗
USGS1976

Tectonic aspects of the Guatemala earthquake of 4 February 1976

The locations of surface ruptures and the main shock epicenter indicate that the disastrous Guatemala earthquake of 4 February 1976 was tectonic in origin and generated mainly by slip on the Motagua fault, which has an arcuate roughly east-west trend across central Guatemala. Fault breakage was observed for 230 km. Displacement is predominantly horizontal and sinistral with a maximum measured offset of 340 cm and an average of about 100 cm. Secondary fault breaks trending roughly north-northeast to south-southwest have been found in a zone about 20 km long and 8 km wide extending from the western suburbs of Guatemala City to near Mixco, and similar faults with more subtle surface expression probably occur elsewhere in the Guatemalan Highlands. Displacements on the secondary faults are predominantly extensional and dip-slip, with as much as 15 cm vertical offset on a single fracture. The primary fault that broke during the earthquake involved roughly 10 percent of the length of the great transform fault system that defines the boundary between the Caribbean and North American plates. The observed sinistral displacement is striking confirmation of deductions regarding the late Cenozoic relative motion between these two crustal plates that were based largely on indirect geologic and geophysical evidence. The earthquake-related secondary faulting, together with the complex pattern of geologically young normal faults that occur in the Guatemalan Highlands and elsewhere in western Central America, suggest that the eastern wedge-shaped part of the Caribbean plate, roughly between the Motagua fault system and the volcanic arc, is being pulled apart in tension and left behind as the main mass of the plate moves relatively eastward. Because of their proximity to areas of high population density, shallow-focus earthquakes that originate on the Motagua fault system, on the system of predominantly extensional faults within the western part of the Caribbean plate, and in association with volcanism may pose a more serious seismic hazard than the more numerous (but generally more distant) earthquakes that are generated in the eastward-dipping subduction zone beneath Middle America.

Science↗
USGS2008

An Integrated Hydrogeologic and Geophysical Investigation to Characterize the Hydrostratigraphy of the Edwards Aquifer in an Area of Northeastern Bexar County, Texas

In August 2007, the U.S. Geological Survey, in cooperation with the San Antonio Water System, did a hydrogeologic and geophysical investigation to characterize the hydrostratigraphy (hydrostratigraphic zones) and also the hydrogeologic features (karst features such as sinkholes and caves) of the Edwards aquifer in a 16-square-kilometer area of northeastern Bexar County, Texas, undergoing urban development. Existing hydrostratigraphic information, enhanced by local-scale geologic mapping in the area, and surface geophysics were used to associate ranges of electrical resistivities obtained from capacitively coupled (CC) resistivity surveys, frequency-domain electromagnetic (FDEM) surveys, time-domain electromagnetic (TDEM) soundings, and two-dimensional direct-current (2D-DC) resistivity surveys with each of seven hydrostratigraphic zones (equivalent to members of the Kainer and Person Formations) of the Edwards aquifer. The principal finding of this investigation is the relation between electrical resistivity and the contacts between the hydrostratigraphic zones of the Edwards aquifer and the underlying Trinity aquifer in the area. In general, the TDEM data indicate a two-layer model in which an electrical conductor underlies an electrical resistor, which is consistent with the Trinity aquifer (conductor) underlying the Edwards aquifer (resistor). TDEM data also show the plane of Bat Cave fault, a well-known fault in the area, to be associated with a local, nearly vertical zone of low resistivity that provides evidence, although not definitive, for Bat Cave fault functioning as a flow barrier, at least locally. In general, the CC resistivity, FDEM survey, and 2D-DC resistivity survey data show a sharp electrical contrast from north to south, changing from high resistivity to low resistivity across Bat Cave fault as well as possible karst features in the study area. Interpreted karst features that show relatively low resistivity within a relatively high-resistivity area likely are attributable to clay or soil filling a sinkhole. In general, faults are inferred where lithologic incongruity indicates possible displacement. Along most inferred faults, displacement was not sufficient to place different members of the Kainer or Person Formations (hydrostratigraphic zones) adjacent across the inferred fault plane. In general, the Kainer Formation (hydrostratigraphic zones V through VIII) has a higher resistivity than the Person Formation (hydrostratigraphic zones II through IV). Although resistivity variations from the CC resistivity, FDEM, and 2D-DC resistivity surveys, with mapping information, were sufficient to allow surface mapping of the lateral extent of hydrostratigraphic zones in places, resistivity variations from TDEM data were not sufficient to allow vertical delineation of hydrostratigraphic zones; however, the Edwards aquifer-Trinity aquifer contact could be identified from the TDEM data.

Scientific Investigations Report↗
USGS2023

Predevelopment water levels, groundwater recharge, and selected hydrologic properties of aquifer materials, Hinkley and Water Valleys, California

Hydrologic and geophysical data were collected to support updates to an existing groundwater-flow model of Hinkley Valley, California, in the Mojave Desert about 80 miles northeast of Los Angeles, California. These data provide information on predevelopment (pre-1930) water levels, groundwater recharge, and selected hydrologic properties of aquifer materials. A predevelopment groundwater-level map, drawn using water-level measurements from 48 wells collected as early as 1918, showed groundwater movement from recharge areas along the Mojave River to evaporative discharge areas near the margin of Harper (dry) Lake in Water Valley. During predevelopment conditions, depth to water ranged from near land surface along the Mojave River to above land surface near Harper (dry) Lake, consistent with flowing wells in Water Valley at that time. Depths to water in much of Hinkley Valley downgradient from the Lockhart fault were less than 20 feet below land surface. By 2017, water-level declines as a result of agricultural pumping, were as much as 60 feet near the Hinkley compressor station. Areal recharge from infiltration of precipitation on the valley floor is negligible. Average annual recharge as infiltration of runoff from upland drainages to Hinkley and Water Valleys averages 64.7 acre-feet per year. In most years recharge does not occur; in years when it occurs, recharge to Hinkley Valley is typically about 296 acre-feet. In contrast, average recharge as infiltration of streamflow from the Mojave River from 1931 to 2015 was between 13,400 and 17,100 acre-feet per year; in some years recharge from the Mojave River exceeded 100,000 acre-ft. Estimates of predevelopment groundwater movement through Hinkley Gap and groundwater discharge to Harper (dry) Lake ranged from 570 to 1,900 and 820 to 2,460 acre-feet per year, respectively; at the time of this study in 2017, groundwater movement through Hinkley Gap was estimated to be about 83 acre-feet per year. Hydraulic-conductivity values estimated from slug-test data for 95 monitoring wells ranged from less than 0.1 to 680 feet per day (ft/d); values generally decreased with depth. Median hydraulic-conductivity values calculated from nuclear magnetic resonance (NMR) data for Mojave River alluvium and near-shore lake deposits were 73 and 11 ft/d, respectively; median hydraulic-conductivity values for locally derived alluvium and weathered bedrock were 6 and 2 ft/d, respectively. Hydraulic-conductivity values, estimated from NMR data for formerly saturated deposits overlying the 2017 water table, were as high as 300 ft/d near the Hinkley compressor station. Downgradient from the Hinkley compressor station, formerly saturated deposits had hydraulic-conductivity values of about 150 ft/d, which were higher than values in saturated material. Coarse-textured, permeable material in formerly saturated deposits above the 2017 water table may have allowed groundwater, released from the Hinkley compressor station that may have contained Cr(VI), to move rapidly downgradient. The Lockhart fault is an impediment to groundwater flow within Hinkley Valley. Groundwater-flow directions from horizontal point-velocity probe data were deflected to the west on the upgradient side of the fault compared to the nominal direction of groundwater flow estimated from water-level data. Younger groundwater was present on the upgradient and downgradient sides of the fault, and older groundwater with unadjusted carbon-14 ages as old as 5,650 years before present was in water from wells within splays of the Lockhart fault, consistent with limited groundwater movement across the fault. As a result, groundwater and Cr(VI) released from the Hinkley compressor station moved to the northwest along the downgradient side of the fault. Coupled well-bore flow and depth-dependent water-quality data show water from wells C-01 and IW-03 within the Q4 2015 (October–December 2015) regulatory Cr(VI) plume was yielded from thin layers within the aquifer that are composed of well-sorted lake-margin (beach) deposits that likely have high lateral and longitudinal connectivity. Collectively, data show highly permeable deposits above the regional water table and thin permeable deposits within saturated portions of the upper aquifer that may have conducted groundwater and Cr(VI) downgradient when releases from the Hinkley compressor station first occurred.

California↗
USGS2009

Preliminary Physical Stratigraphy and Geophysical Data From the USGS Dixon Core, Onslow County, North Carolina

In October through November 2006, scientists from the U. S. Geological Survey (USGS) Eastern Region Earth Surface Processes Team (EESPT) and the Raleigh (N.C.) Water Science Center (WSC), in cooperation with the North Carolina Geological Survey (NCGS) and the Onslow County Water and Sewer Authority (ONWASA), drilled a stratigraphic test hole and well in Onslow County, N.C. The Dixon corehole was cored on ONWASA water utility property north of the town of Dixon, N.C., in the Sneads Ferry 7.5-minute quadrangle at latitude 34deg33'35' N, longitude 77deg26'54' W (decimal degrees 34.559722 and -77.448333). The site elevation is 66.0 feet (ft) above mean sea level as determined using a Paulin precision altimeter. The corehole attained a total depth of 1,010 ft and was continuously cored by the USGS EESPT drilling crew. A groundwater monitoring well was installed in the screened interval between 234 and 254 ft below land surface. The section cored at this site includes Upper Cretaceous, Paleogene, and Neogene sediments. The Dixon core is stored at the NCGS Coastal Plain core storage facility in Raleigh. The Dixon corehole is the fourth and last in a series of planned North Carolina benchmark coreholes drilled by the USGS Coastal Carolina Project. These coreholes explore the physical stratigraphy, facies, and thickness of Cretaceous, Paleogene, and Neogene Coastal Plain sediments in North Carolina. Correlations of lithologies, facies, and sequence stratigraphy can be made with the Hope Plantation corehole, N.C., near Windsor in Bertie County (Weems and others, 2007); the Elizabethtown corehole, near Elizabethtown, N.C., in Bladen County (Self-Trail and others, 2004b); the Smith Elementary School corehole, near Cove City, N.C., in Craven County (Harris and Self-Trail, 2006; Crocetti, 2007); the Kure Beach corehole, near Wilmington, N.C., in New Hanover County (Self-Trail and others, 2004a); the Esso#1, Esso #2, Mobil #1, and Mobil #2 cores in Albermarle and Pamlico Sounds, N.C. (Zarra, 1989); and the Cape Fear River outcrops in Bladen County, N.C. (Farrell, 1998; Farrell and others, 2001). This report contains the lithostratigraphic summary recorded at the drill site, core photographs, geophysical data, and calcareous nannofossil biostratigraphic correlations.

Open-File Report↗
USGS2014

Volcanic activity on Umnak and Great Sitkin Islands, 1946–1948

As part of the United States Geological Survey's volcano program in the Aleutian volcanic arc during 1946–1948, the volcanoes and thermal areas on Umnak and Great Sitkin Islands were studied. In addition to observation of the volcanoes, temperatures of fumaroles were measured and some products of the volcanic activity were collected and analyzed. After an eruption in 1945, Cone A in Okmok Caldera on northeastern Umnak Island remained relatively quiet during the period 1946–1948. The fumarole gases contained water vapor, carbon dioxide, and sulfur dioxide. The incrustations associated with the sulfur dioxide fumaroles are the sulfate minerals halotrichite, gypsum, and soda alum. Temperatures of lava fumaroles at the source of the December, 1945 lava flow from Cone A dropped from an average of 320°C on July 19, 1946, to 90°C on September 5, 1946. With exception of the large central fumarole which reached red heat in 1947, the extra crater fumaroles of Cone A ranged from 96° to 98° C in temperature Cone C, another cone in Okmok Caldera was in the solfataric stage and emitted water vapor, hydrogen sulfide, and probably carbon dioxide. Incrustations associated with fumaroles on this cone consisted of sulfur, pyrite, and aragonite. The temperatures of the fumaroles ranged from 95° to 96°C and were probably kept within this narrow range partly by the heat supplied during condensation of steam. Sixteen large thermal springs emerging from the base of Cone D in Okmok Caldera had an aggregate discharge of approximately 115 cfs and gave off approximately 21,000 kcal of heat per second on September 7, 1946. Water samples of thermal springs on Umnak Island contained as high as 159 ppm of boron expressed as B 2 0 3 and a few parts per million of arsenic and antimony. Several of the thermal springs in southwestern Umnak Island were slightly superheated with respect to the boiling point of water and behaved as small geysers. As has been found elsewhere, the discharge, temperature, and composition of the emanations from a spring is modified by surface conditions of topography and drainage. Thermal springs and fumaroles on Great Sitkin Island occurred at an altitude of about 2000 ft near the head of the west fork of Big Fox Creek. All the fumaroles were at or near a temperature of 100°C. The large crater fumarole in the center of the 1945 crater dome was unapproachable. No change in the activity of Great Sitkin volcano was observed between 1946 and August 1948.

Alaska↗
USGS2015

Hydrothermal response to a volcano-tectonic earthquake swarm, Lassen, California

The increasing capability of seismic, geodetic, and hydrothermal observation networks allows recognition of volcanic unrest that could previously have gone undetected, creating an imperative to diagnose and interpret unrest episodes. A November 2014 earthquake swarm near Lassen Volcanic National Park, California, which included the largest earthquake in the area in more than 60 years, was accompanied by a rarely observed outburst of hydrothermal fluids. Although the earthquake swarm likely reflects upward migration of endogenous H 2 O-CO 2 fluids in the source region, there is no evidence that such fluids emerged at the surface. Instead, shaking from the modest sized (moment magnitude 3.85) but proximal earthquake caused near-vent permeability increases that triggered increased outflow of hydrothermal fluids already present and equilibrated in a local hydrothermal aquifer. Long-term, multiparametric monitoring at Lassen and other well-instrumented volcanoes enhances interpretation of unrest and can provide a basis for detailed physical modeling.

California↗
USGS1997

Multiphase groundwater flow near cooling plutons

We investigate groundwater flow near cooling plutons with a computer program that can model multiphase flow, temperatures up to 1200°C, thermal pressurization, and temperature‐dependent rock properties. A series of experiments examines the effects of host‐rock permeability, size and depth of pluton emplacement, single versus multiple intrusions, the influence of a caprock, and the impact of topographically driven groundwater flow. We also reproduce and evaluate some of the pioneering numerical experiments on flow around plutons. Host‐rock permeability is the principal factor influencing fluid circulation and heat transfer in hydrothermal systems. The hottest and most steam‐rich systems develop where permeability is of the order of 10 −15 m 2 . Temperatures and life spans of systems decrease with increasing permeability. Conduction‐dominated systems, in which permeabilities are ≤10 −16 m 2 , persist longer but exhibit relatively modest increases in near‐surface temperatures relative to ambient conditions. Pluton size, emplacement depth, and initial thermal conditions have less influence on hydrothermal circulation patterns but affect the extent of boiling and duration of hydrothermal systems. Topographically driven groundwater flow can significantly alter hydrothermal circulation; however, a low‐permeability caprock effectively decouples the topographically and density‐driven systems and stabilizes the mixing interface between them thereby defining a likely ore‐forming environment.

Journal of Geophysical Research B: Solid Earth↗
USGS2000

Martian oceans, valleys and climate

The new Mars Global Surveyor altimetry shows that the heavily cratered southern hemisphere of Mars is 5 km higher that the sparely cratered plains of the northern hemisphere. Previous suggestions that oceans formerly occupied that northern plains as evidenced by shorelines are partly supported by the new data. A previously identified outer boundary has a wide range of elevations and is unlikely to be a shoreline but an inner contact with a narrow range of elevations is a more likely candidate. No shorelines are visible in the newly acquired, 2.5 metre/pixel imaging. Newly imaged valleys provide strong support for sustained or episodic flow of water across the Martian surface. A major surprise, however, is the near absence of valleys less than 100 m across. Martian valleys seemingly do not divide into ever smaller valleys as terrestrial valleys commonly do. This could be due to lack of precipitation or lack of surface runoff because of high infiltration rates. High erosion rates and supports warm climates and presence of large bodies of water during heavy bombardment. The climate history and fate of the water after heavy bombardment remain cotroversial.

Astronomy and Geophysics↗
USGS2022

P- and S-wave velocity estimation by ensemble Kalman inversion of dispersion data for strong motion stations in California

This study uses an ensemble Kalman method for near-surface seismic site characterization of 154 network earthquake monitoring stations in California to improve the resolution of S -wave velocity ( V S ) and P -wave velocity ( V P ) profiles—up to the resolution depth—coupled with better quantification of uncertainties compared to previous site characterization studies at this network. These stations were part of the Yong et al . site characterization project, with selected stations based on future recordings of ground motions that are expected to exceed 10 per cent peak ground acceleration in 50 yr. To estimate V S and V P from experimental dispersion data, Yong et al . investigated these stations using linearized (local search and iteration) routines, and Yong et al . later studied a subset of these stations using nonlinear (global search and optimization) routines. In both studies, the selection of model parameters—that is, discretization of the V S and V P profiles with only five fixed thickness layers—was mainly based on trial and error. In contrast, this paper uses an approximate Bayesian method to assimilate experimental dispersion data and sequentially update an ensemble of particle estimates that span the V S and V P parameter spaces. Doing so, we systematically determine the most probable profiles conditioned on the experimental dispersion data, the introduced noise levels, and a priori knowledge in the form of physical constraints. We consider two configurations to discretize the soil depth from the surface to half of the maximum discernible wavelength obtained from the experimental dispersion data, namely refined and coarse models, and two initial models for each configuration to study solution multiplicity. Our results suggest that using the refined model for the top surface layers improves the resolution of near-surface site characteristics and the model’s success rate in capturing dispersion data at high frequencies. All models result in similar V S but distinct V P profiles, with increasing uncertainty at deeper layers, suggesting that the fundamental mode of Rayleigh wave dispersion data is not adequate to constrain the P -wave velocity profile and the S -wave velocity close to the resolution depth.

California↗