SEARCH · Geology Reports

Results for “Algorithms”

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

SourceResearch connectionFrom yearThrough year

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

At least 1,261 records · Page 70Linked to original sources

USGS2024

Database and time series of nearshore waves along the Alaskan coast from the United States-Canada border to the Bering Sea

Alaska’s Arctic coast has some of the highest coastal erosion rates in the world, primarily driven by permafrost thaw and increasing wave energy. In the Arctic, a warming climate is driving sea ice cover to decrease in space and time. A lack of long-term observational wave data along Alaska’s coast challenges the ability of engineers, scientists, and planners to study and address threats and effects from wave-driven erosion and flooding. To overcome the lack of available observational wave data in the nearshore in this study by the U.S. Geological Survey, waves were downscaled with the Simulating WAves Nearshore numerical wave model (SWAN) for the hindcast period of 1979 to 2019 from the United States-Canada border to the Bering Sea utilizing nine model domains. For each domain, the model was forced at the open boundary with 2,500 representative “sea states,” which are likely combinations of significant wave heights, mean wave periods, mean wave directions, and wind speeds and directions. The sea states were obtained from the European Centre for Medium-Range Weather Forecasts “ERA5” dataset for reanalysis of winds and waves using a multivariant maximum-dissimilarity algorithm. The SWAN runs created a downscaled wave database at each grid point, which was used to reconstruct the 40-year time series in the nearshore along the 5- and 10-meter isobaths at locations approximately 400 m apart and corresponding to transects spaced approximately 50 m alongshore, as developed for USGS shoreline-change assessments. Reconstructed time series were compared to observations to validate the numerical model and the downscaled wave database method and showed overall good agreements.

Alaska↗

USGS2024

Total phosphorus and suspended-sediment concentrations and loads from two main tributaries to Upper Klamath Lake, Oregon, 2014–20

Total phosphorus (TP) and suspended-sediment concentrations (SSC) and loads were computed at two U.S. Geological Survey (USGS) streamgages in the upper Klamath River Basin on the Sprague (USGS site ID 11501000) and Williamson (USGS site ID 11502500) Rivers using high temporal resolution turbidity and streamflow data to develop surrogate regression models. Regression models were updated and validated for TP at the Williamson River site, and additional data improved a prior published TP model, increasing the coefficient of determination (R 2 ) from 0.73 to 0.88. A new TP regression model was developed for the Sprague River site using 2 years of data and showed promising results with an R 2 of 0.93. Suspended-sediment concentration (SSC) surrogate models were also updated at these sites using a longer period of record than the TP models and improved characterization of sediment transport conditions at these monitoring sites. Computations of TP loads were compared to the annual loading capacity dictated by the total maximum daily load (TMDL) for Upper Klamath Lake and showed that the combined TP load of the Williamson and Sprague Rivers approaches the annual loading capacity in water years with high annual streamflow. TP loads were also compared to loads computed by the Klamath Tribes using a long-term dataset and a regression and interpolation algorithm (RIA). The comparison showed that the two methods report similar annual loads, with the surrogate regression method generally reporting lower loads than the RIA, and the RIA annual loads falling within the range of uncertainty of the surrogate regression model results. Determining the effect of habitat and stream restoration on basin-scale TP and suspended-sediment loading is challenging using the surrogate regression method at these sites given the short period of record that TP and suspended-sediment load (SSL) data are available. However, long-term analysis by the Klamath Tribes in their larger monitoring network could provide insight into the impact of restoration at smaller spatial scales compared to the basin-wide assessment produced in this study.

Oregon↗

USGS1972

Hydrograph simulation models of the Hillsborough and Alafia Rivers, Florida: a preliminary report

Mathematical (digital) models that simulate flood hydrographs from rainfall records have been developed for the following gaging stations in the Hillsborough and Alafia River basins of west-central Florida: Hillsborough River near Tampa, Alafia River at Lithia, and north Prong Alafia River near Keysville. These models, which were developed from historical streamflow and and rainfall records, are based on rainfall-runoff and unit-hydrograph procedures involving an arbitrary separation of the flood hydrograph. These models assume the flood hydrograph to be composed of only two flow components, direct (storm) runoff, and base flow. Expressions describing these two flow components are derived from streamflow and rainfall records and are combined analytically to form algorithms (models), which are programmed for processing on a digital computing system. Most Hillsborough and Alafia River flood discharges can be simulated with expected relative errors less than or equal to 30 percent and flood peaks can be simulated with average relative errors less than 15 percent. Because of the inadequate rainfall network that is used in obtaining input data for the North Prong Alafia River model, simulated peaks are frequently in error by more than 40 percent, particularly for storms having highly variable areal rainfall distribution. Simulation errors are the result of rainfall sample errors and, to a lesser extent, model inadequacy. Data errors associated with the determination of mean basin precipitation are the result of the small number and poor areal distribution of rainfall stations available for use in the study. Model inadequacy, however, is attributed to the basic underlying theory, particularly the rainfall-runoff relation. These models broaden and enhance existing water-management capabilities within these basins by allowing the establishment and implementation of programs providing for continued development in these areas. Specifically, the models serve not only as a basis for forecasting floods, but also for simulating hydrologic information needed in flood-plain mapping and delineating and evaluating alternative flood control and abatement plans.

Florida↗

USGS1976

FORTRAN computer program for seismic risk analysis

A program for seismic risk analysis is described which combines generality of application, efficiency and accuracy of operation, and the advantage of small storage requirements. The theoretical basis for the program is first reviewed, and the computational algorithms used to apply this theory are described. The information required for running the program is listed. Published attenuation functions describing the variation with earthquake magnitude and distance of expected values for various ground motion parameters are summarized for reference by the program user. Finally, suggestions for use of the program are made, an example problem is described (along with example problem input and output) and the program is listed.

Open-File Report↗

USGS1982

A hybrid structure for the storage and manipulation of very large spatial data sets

The map data input and output problem for geographic information systems is rapidly diminishing with the increasing availability of mass digitizing, direct spatial data capture and graphics hardware based on raster technology. Although a large number of efficient raster-based algorithms exist for performing a wide variety of common tasks on these data, there are a number of procedures which are more efficiently performed in vector mode or for which raster mode equivalents of current vector-based techniques have not yet been developed. This paper presents a hybrid spatial data structure, named the ?vaster' structure, which can utilize the advantages of both raster and vector structures while potentially eliminating, or greatly reducing, the need for raster-to-vector and vector-to-raster conversion. Other advantages of the vaster structure are also discussed.

Open-File Report↗

USGS1987

Interpretation of detailed aerial gamma-ray survey, Jabal Ashirah area, southeastern Arabian Shield, Kingdom of Saudi Arabia

A detailed aerial gamma-ray spectrometric survey of the Jabal Ashirah area in the southeastern Arabian Shield has been analyzed using computer-classification algorithms. The analysis resulted in maps that show radiometric map units and gamma-ray anomalies indicating the presence of possible concentrations of potassium and uranium. The radiometric-unit map was interpreted to 'produce a simplified radiolithic map that was correlated with the mapped geology. The gamma-ray data show uranium anomalies that coincide with a tin-bearing granite, but known gold and nickel mineralization do not have any associated gamma-ray signatures.

Open-File Report↗

USGS1994

A comparison of change detection methods using multispectral scanner data

Change detection methods were investigated as a cooperative activity between the U.S. Geological Survey and the National Bureau of Surveying and Mapping, People's Republic of China. Subtraction of band 2, band 3, normalized difference vegetation index, and tasseled cap bands 1 and 2 data from two multispectral scanner images were tested using two sites in the United States and one in the People's Republic of China. A new statistical method also was tested. Band 2 subtraction gives the best results for detecting change from vegetative cover to urban development. The statistical method identifies areas that have changed and uses a fast classification algorithm to classify the original data of the changed areas by land cover type present for each image date.

Open-File Report↗

USGS1996

Catalog of earthquake hypocenters for Augustine, Redoubt, Iliamna, and Mount Spurr volcanoes, Alaska: January 1, 1991 - December 31, 1993

The Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, the Geophysical Institute of the University of Alaska, Fairbanks, and the Alaska Division of Geological and Geophysical Surveys, has maintained a program of seismic monitoring at potentially active volcanoes in the Cook Inlet region since 1988. The principal objectives of this program include the seismic surveillance of the Cook Inlet volcanoes and the investigation of seismic processes associated with active volcanism. This catalog reflects the status and evolution of the seismic monitoring program, and presents the basic seismic data for the time interval January 1, 1991, to December 31, 1993. For an interpretation of these data the reader should refer to several recent articles on volcano related seismicity in the Cook Inlet region (e.g. Jolly and others, 1994; Power and others, 1995; and McNutt and others, 1995). A similar catalog covers the period from October 12, 1989 to December 31, 1991 (Power and others 1993). The AVO seismic monitoring program has undergone significant changes during the catalog period. The changes included 1) new seismic stations placed at Mount Spurr and Redoubt Volcano, resulting in increased earthquake detection capability and improved earthquake locations, 2) the addition of several regional stations to the seismic data acquisition system which improved location quality near the volcano and enhanced our ability to scale eruptions and measure magnitudes of the largest volcanic earthquakes, 3) installation of a new event detection algorithm XDETECT (Rogers, 1993), which offered increased data collection capabilities , 4) modifications to the earthquake location program HYPOELLIPSE (Lahr, 1989) which now allows distinct velocity models and station corrections at each volcanic center, and 5) the addition of seismic stations at Augustine and niamna volcanoes to the data acquisition/location system. The 1992 eruptions at Mount Spurr's Crater Peak vent provided the highlight of the catalog period. The crisis included three sub-plinian eruptions, which occurred on June 27, August 18, and September 16-17, 1992. The three eruptions punctuated a complex seismic sequence which included volcano-tectonic (VT) earthquakes, tremor, and both deep and shallow long period (LP) earthquakes. The seismic sequence began on August 18, 1991, with a small swarm of volcano-tectonic events beneath Crater Peak, and spread throughout the volcanic complex by November of the same year. Elevated levels of seismicity persisted at Mount Spurr beyond the catalog time period.

Alaska↗

USGS1997

A manual for a laboratory information management system (LIMS) for light stable isotopes

The reliability and accuracy of isotopic data can be improved by utilizing database software to (i) store information about samples, (ii) store the results of mass spectrometric isotope-ratio analyses of samples, (iii) calculate analytical results using standardized algorithms stored in a database, (iv) normalize stable isotopic data to international scales using isotopic reference materials, and (v) generate multi-sheet paper templates for convenient sample loading of automated mass-spectrometer sample preparation manifolds. Such a database program is presented herein. Major benefits of this system include (i) an increase in laboratory efficiency, (ii) reduction in the use of paper, (iii) reduction in workload due to the elimination or reduction of retyping of data by laboratory personnel, and (iv) decreased errors in data reported to sample submitters. Such a database provides a complete record of when and how often laboratory reference materials have been analyzed and provides a record of what correction factors have been used through time. It provides an audit trail for stable isotope laboratories. Since the original publication of the manual for LIMS for Light Stable Isotopes, the isotopes 3 H, 3 He, and 14 C, and the chlorofluorocarbons (CFCs), CFC-11, CFC-12, and CFC-113, have been added to this program.

Open-File Report↗

USGS1998

A manual for a Laboratory Information Management System (LIMS) for light stable isotopes

Open-File Report↗

USGS1999

SutraPlot, a graphical post-processor for SUTRA, a model for ground-water flow with solute or energy transport

This report documents a graphical display post-processor (SutraPlot) for the U.S. Geological Survey Saturated-Unsaturated flow and solute or energy TRAnsport simulation model SUTRA, Version 2D3D.1. This version of SutraPlot is an upgrade to SutraPlot for the 2D-only SUTRA model (Souza, 1987). It has been modified to add 3D functionality, a graphical user interface (GUI), and enhanced graphic output options. Graphical options for 2D SUTRA (2-dimension) simulations include: drawing the 2D finite-element mesh, mesh boundary, and velocity vectors; plots of contours for pressure, saturation, concentration, and temperature within the model region; 2D finite-element based gridding and interpolation; and 2D gridded data export files. Graphical options for 3D SUTRA (3-dimension) simulations include: drawing the 3D finite-element mesh; plots of contours for pressure, saturation, concentration, and temperature in 2D sections of the 3D model region; 3D finite-element based gridding and interpolation; drawing selected regions of velocity vectors (projected on principal coordinate planes); and 3D gridded data export files. Installation instructions and a description of all graphic options are presented. A sample SUTRA problem is described and three step-by-step SutraPlot applications are provided. In addition, the methodology and numerical algorithms for the 2D and 3D finite-element based gridding and interpolation, developed for SutraPlot, are described. 1

Open-File Report↗

USGS1982

Digital classification of Landsat data for vegetation and land-cover mapping in the Blackfoot River watershed, southeastern Idaho

This paper documents the procedures, results, and final products of a digital analysis of Landsat data used to produce a vegetation and landcover map of the Blackfoot River watershed in southeastern Idaho. Resource classes were identified at two levels of detail: generalized Level I classes (for example, forest land and wetland) and detailed Levels II and III classes (for example, conifer forest, aspen, wet meadow, and riparian hardwoods). Training set statistics were derived using a modified clustering approach. Environmental stratification that separated uplands from lowlands improved discrimination between resource classes having similar spectral signatures. Digital classification was performed using a maximum likelihood algorithm. Classification accuracy was determined on a single-pixel basis from a random sample of 25-pixel blocks. These blocks were transferred to small-scale color-infrared aerial photographs, and the image area corresponding to each pixel was interpreted. Classification accuracy, expressed as percent agreement of digital classification and photo-interpretation results, was 83.0:t 2.1 percent (0.95 probability level) for generalized (Level I) classes and 52.2:t 2.8 percent (0.95 probability level) for detailed (Levels II and III) classes. After the classified images were geometrically corrected, two types of maps were produced of Level I and Levels II and III resource classes: color-coded maps at a 1:250,000 scale, and flatbed-plotter overlays at a 1:24,000 scale. The overlays are more useful because of their larger scale, familiar format to users, and compatibility with other types of topographic and thematic maps of the same scale.

Professional Paper↗

USGS2006

Mapping ground water in three dimensions: An analysis of airborne geophysical surveys of the Upper San Pedro River Basin, Cochise County, southeastern Arizona

This report summarizes the results of two airborne geophysical surveys conducted in the upper San Pedro Valley of southeastern Arizona in 1997 and 1999. The combined surveys cover about 1,000 square kilometers and extend from the Huachuca Mountains on the west to the Mule Mountains and Tombstone Hills on the east and from north of the Babocomari River to near the Mexican border on the south. The surveys included the acquisition of high-resolution magnetic data, which were used to map depth to the crystalline basement rocks underlying the sediments filling the basin. The magnetic inversion results show a complex basement morphology, with sediment thickness in the center of the valley ranging from ~237 meters beneath the city of Sierra Vista to ~1,500 meters beneath Huachuca City and the Palominas area near the Mexican border. The surveys also included acquisition of 60-channel time-domain electromagnetic (EM) data. Extensive quality analyses of these data, including inversion to conductivity vs. depth (conductivity-depth-transform or CDT) profiles and comparisons with electrical well logs, show that the electrical conductor mapped represents the subsurface water-bearing sediments throughout most of the basin. In a few places (notably the mouth of Huachuca Canyon), the reported water table lies above where the electrical conductor places it. These exceptions appear to be due to a combination of outdated water-table information, significant horizontal displacement between the wells and the CDT profiles, and a subtle calibration issue with the CDT algorithm apparent only in areas of highly resistive (very dry) overburden. These occasional disparities appear in less than 5 percent of the surveyed area. Observations show, however, that wells drilled in the thick unsaturated zone along the Huachuca Mountain front eventually intersect water, at which point the water rapidly rises high into the unsaturated zone within the wellbore. This rising of water in a wellbore implies some sort of confinement below the thick unsaturated zone, a confinement that is not identified in the available literature. Occasional disparities notwithstanding, maps of the electrical conductor derived from the airborne EM system provide a synoptic view of the presence of water underlying the upper San Pedro Valley, including its three-dimensional distribution. The EM data even show faults previously only inferred from geologic mapping. The magnetic and electromagnetic data together appear to show the thickness of the sediments, the water in the saturated sediments down to a maximum of about 400 meters depth, and even places where the main ground-water body is not in direct contact with the San Pedro River. However, the geophysical data cannot reveal anything directly about hydraulic conductivity or ground-water flow. Estimating these characteristics requires new hydraulic modeling based in part on this report. One concern to reviewers of this report is the effect that clays may have on the electrical conductor mapped with the airborne geophysical system. Although the water in the basin is unusually conductive, averaging 338 microsiemens per centimeter, reasoning cited below suggests that the contribution of clays to the overall conductivity would be relatively small. Basic principles of sedimentary geology suggest that silts and clays should dominate the center of the basin, while sands and gravels would tend to dominate the margins. Although clay content may increase the amplitude of the observed electrical conductors somewhat, it will not affect the depths to the conductor derived from depth inversions. Further, fine-grained sediments generally have higher porosity and tend to lie toward a basin center, a fact in general agreement with the observed geophysical data.

Arizona↗

USGS2008

Evolving magma storage conditions beneath Mount St. Helens inferred from chemical variations in melt inclusions from the 1980-1986 and current (2004-2006) eruptions

Major element, trace element, and volatile concentrations in 187 glassy melt inclusions and 25 groundmass glasses from the 1980-86 eruption of Mount St. Helens are presented, together with 103 analyses of touching FE-Ti oxide pairs from the same samples. These data are used to evaluate the temporal evolution of the magmatic plumbing system beneath the volcano during 1980-86 and so provide a framework in which to interpret analyses of melt inclusions from the current (2004-2006) eruption. Major and trace element concentrations of all melt inclusions lie at the high SiO 2 end of the data array defined by eruptive products of the late Quaternary age from Mount St. Helens. For several major and trace elements, the glasses define a trend that is oblique to the whole-rock trend, indicating that different mineral assemblages were responsible for the two trends. The whole-rock trend can be ascribed to differentiation of hydrous basaltic parents in a deep-seated magma reservoir, probably at depths great enough to stabilize garnet. In contrast, the glass trends were generated by closed-system crystallization of the phenocryst and microlite mineral assemblages at low pressures. The dissolved H 2 O content of the melt inclusions from 1980-86, as measured by the ion microprobe, ranges from 0 to 6.7 wt. percent, with the highest values obtained from the plinian phase of May 18, 1980. Water contents decrease with increasing SiO 2 , consistent with decompression-driven crystallization. Preliminary data for dissolved CO 2 in melt inclusions from the May 18 plinian phase from August 7, 1980, indicate that X H 2 O in a vapor phase was approximately constant at 0.80, irrespective of H 2 O content, suggestive of closed-system degassing with a high bubble fraction or gas streaming through the subvolcanic system. Temperature and f O 2 estimates for touching Fe-Ti oxides show evidence for heating during crystallization owing to release of latent heat. Consequently, magmas with the highest microlite crystallinities record the highest temperatures. Magmas also become progressively reduced during ascent and degassing, probably as a result of redox equilibria between exsolving S-bearing gases and magmas. The lowest temperature oxides have f O 2 ≈ NNO, similar to high-temperature fumarole gases from the volcano. The temperature and f O 2 of the magma tapped by the plinian phase of May 18, 1980, are 870-875°C and NNO+0.8, respectively. The dissolved volatile contents of the melt inclusions have been used to calculate sealing pressures; that is, the pressure at which chemical exchange between inclusion and matrix melt ceased. These are greatest for the May 18 plinian magma (120 to 320 MPa); lower pressures are recorded by samples of the preplinian cryptodome and by all post-May 18 magmas. Magma crystallinity, calculated from melt-inclusion Rb contents, is negatively correlated with sealing pressure, consistent with decompression crystallization. Elevated contents of Li in melt inclusions from the cryptodome and post-May 18 samples are consistent with transfer of Li in a magmatic vapor phase from deeper parts of the magma system to magma stored at shallower levels. The Li enrichment attains its maximum extent at ~150 MPa, which is ascribed to separation of a single vapor phase into H 2 O-rich gas and dense Li-rich brine at the top of the magma column. There are striking correlations between melt-inclusion chemistry and monitoring data for the 1980-86 eruption. Dissolved SO 2 contents of melt inclusions from any given event, multiplied by the mass of magma erupted during that event, correlate with the measured flux of SO 2 at the surface, suggesting that magma degassing and melt-inclusion sealing are closely related in time and space. Textural and chemical evidence indicates that melt inclusions became effectively sealed (physically or kinetically) shortly before eruption. Thus by converting pressure to depth using a density model and edifice-loading algorithm for the volcano, changing depths of magma extraction with time can be tracked and compared to the seismic record. The plinian eruption of May 18, 1980, involved magma stored 5-11 km below sea level; this is inferred to be the subvolcanic magma chamber. The preceding eruptions, including the May 18, 1980, blast, involved magma withdrawal from the cryptodome and conduit down to 5 km below sea level. Subsequent 1980 eruptions tapped magma down to depths of ≤10 km below sea level. Tapping of magma stored deeper than 2 km below sea level stopped abruptly at the end of 1980, coincident with the onset of extensive shallow seismicity and a change from explosive to effusive eruption style from 1981 to 1986. Overall, the 1980-86 eruption is consistent with the evisceration of a thin, vertically extensive body of magma extending from 5 to at least 11 km below sea level and connected to the surface by a thin conduit. In the absence of sustained high magma-supply rates from depth, decompression crystallization of magma ascending through the system leads eventually to plugging of the conduit. The current eruption of Mount St. Helens shares some similarities with the 1981-86 dome-building phase of the previous eruption, in that there is extensive shallow seismicity and extrusion of highly crystalline material in the form of a sequence of flows and spines. Melt inclusions from the current eruption have low H 2 O contents, consistent with magma extraction from shallow depths. Highly enriched Li in melt inclusions suggests that vapor transport of Li is a characteristic feature of Mount St. Helens. Melt inclusions from the current eruption have subtly different trace-element chemistry from all but one of the 1980-86 melt inclusions, with steeper rareearth-element (REE) patterns and low U, Th, and high-fieldstrength elements (HFSE), indicating addition of a new melt component to the magma system. It is anticipated that increasing involvement of the new melt component will be evident as the current eruption proceeds.

Washington↗

USGS2008

Nutrient Concentrations and Their Relations to the Biotic Integrity of Nonwadeable Rivers in Wisconsin

Excessive nutrient [phosphorus (P) and nitrogen (N)] input from point and nonpoint sources is frequently associated with degraded water quality in streams and rivers. Point-source discharges of nutrients are fairly constant and are controlled by the U.S. Environmental Protection Agency's (USEPA) National Pollutant Discharge Elimination System. To reduce inputs from nonpoint sources, agricultural performance standards and regulations for croplands and livestock operations are being proposed by various States. In addition, the USEPA is establishing regionally based nutrient criteria that can be refined by each State to determine whether actions are needed to improve water quality. More confidence in the environmental benefits of the proposed performance standards and nutrient criteria would be possible with improved understanding of the biotic responses to a range of nutrient concentrations in different environmental settings. To achieve this general goal, the U.S. Geological Survey and the Wisconsin Department of Natural Resources collected data from 282 streams and rivers throughout Wisconsin during 2001 through 2003 to: (1) describe how nutrient concentrations and biotic-community structure differ throughout the State, (2) determine which environmental characteristics are most strongly related to the distribution of nutrient concentrations and biotic-community structure, (3) determine reference conditions for water quality and biotic indices for streams and rivers in the State, (4) determine how the biotic communities in streams and rivers in different areas of the State respond to differences in nutrient concentrations, (5) determine the best regionalization scheme to describe the patterns in reference conditions and the corresponding responses in water quality and the biotic communities (primarily for smaller streams), and (6) develop algorithms to estimate nutrient concentrations in streams and rivers from a combination of biotic indices. The ultimate goal of this study is to provide the information needed to guide the development of regionally based nutrient criteria for Wisconsin streams and rivers. In this report, data collected, primarily in 2003, from 42 nonwadeable rivers are used to describe nutrient concentrations and their relations to the biotic integrity of rivers in Wisconsin. In a separate report by Robertson and others (2006a), the data collected from 240 wadeable streams are used to describe these relations in streams in Wisconsin. Reference water-quality conditions for nonwadeable rivers were found to be similar throughout Wisconsin (approximately 0.035 milligrams per liter (mg/L) for total P (TP), 0.500 mg/L for total N (TN), 4 micrograms per liter for suspended chlorophyll a (SCHL), and greater than 110 centimeters for Secchi-tube depth (SD)). For each category of the biotic community (SCHL, macroinvertebrates, and fish), a few indices were more strongly related to differences in nutrient concentrations than were others. For the indices most strongly related to nutrient concentrations, reference conditions were obtained with a regression approach, from values corresponding to the worst 75th-percentile value from a subset of minimally impacted streams (streams having reference nutrient concentrations), and from the best 25th-percentile value of all the data. Concentrations of TP and TN in nonwadeable rivers increased as the percentage of agricultural land in the basin increased; these increases resulted in increased SCHL concentrations and decreased SDs. The responses in SDs and SCHL concentrations to changes in nutrient concentrations were similar throughout most of the State except in rivers in the southeastern part, where SCHL concentrations were lower than would be expected given their nutrient concentrations. Rivers in the southeastern part of the State had high concentrations of total suspended sediment compared to the SCHL concentrations. Many biotic indices responded to increases in nu

Professional Paper↗

USGS2010

Integrated satellite observations of the 2006 eruption of Augustine Volcano: Chapter 20 in The 2006 eruption of Augustine Volcano, Alaska

Satellite observations played an important role in monitoring the 2006 eruption of Augustine Volcano. It represented the first opportunity for observers to use, in an operational setting, new Web-based tools and techniques developed by the Alaska Volcano Observatory remote sensing group. The 'Okmok Algorithm' was used to analyze thermal infrared satellite data and highlight changes in the style and phases of activity. Temperature measurements were used to estimate ash cloud heights, which compared favorably to radar and ground-based observations, although larger discrepancies were seen when compared to pilot reports. Brightness temperature difference techniques were used to locate and track the 14 ash clouds produced during the explosive phase of the eruption. Stacking of these analyses allowed the creation of composite maps showing the distribution of airborne ash. The data from these maps were further combined with information from local reports and samples of ashfall to create a prototype of a concentration map that could be used to assess the potential hazard an eruption represents to aircraft, infrastructure and human health.

Alaska↗

USGS2023

Magnetotelluric investigation of northern Harrat Rahat, Kingdom of Saudi Arabia

Volcanism within the harrats (Arabic for “volcanic field”) of the Kingdom of Saudi Arabia includes at least one historical eruption occurring close to the holy city of Al Madīnah in 1256 C.E. As part of a volcanic- and seismic-hazard assessment of northern Harrat Rahat, magnetotelluric (MT) data were collected to investigate the structural setting of the area, the presence or absence of melt within the crust, and the mantle-derived magmatic source. Collected MT data were modeled in both two dimensions, where anisotropy can be estimated, and three dimensions. Interpretation of the preferred resistivity model includes a shallow sediment-filled graben beneath northern Harrat Rahat lavas, a melt-free upper crust, and a region of decompression melting in the asthenosphere below 60–70 kilometers depth. Models in two dimensions image the lower crust as anisotropic, demonstrating that a series of elongate conductivity anomalies with a strike of N. 10° E. within the lower crust of the three-dimensional model are artifacts of inverting anisotropic data with an isotropic modeling algorithm. Careful examination of the resistivity models, in combination with regional geological and geophysical data, suggests an anisotropic lower crust that is free of large zones of melt. Azimuthal anisotropy in the lower crust extends well beyond the limits of Harrat Rahat volcanic rocks, with a conductive direction oriented N. 10° E. and an anisotropy factor of 2–5 between the most and least conductive directions. Enhanced conductivity is likely caused by interconnected grain-boundary graphite, where the direction of anisotropy reflects either frozen-in fabric from the Neoproterozoic stabilization of the Arabian Shield or ductile deformation driven by channelized asthenospheric flow coupled with a thin rigid mantle lid. Asthenospheric melt is interpreted to transect the crust largely through diking, with limited melt storage and short residence times within the crustal column.

northern Harrat Rahat↗

USGS2023

Thickness of the Saudi Arabian crust

As part of a joint Saudi Geological Survey (SGS) and U.S. Geological Survey (USGS) project, we analyzed P-wave receiver functions from seismic stations covering most of the Kingdom of Saudi Arabia to map the thickness of the crust across the Arabia Plate. We present an update of crustal-thickness estimates and fill in gaps for the western Arabian Shield and the rifted margin at the Red Sea (the coastal plain), as well as the eastern Arabian Platform. We applied a conventional H-k stacking algorithm and included careful attention to stacking weights, two forms of sedimentary corrections for stations located on the Arabian Platform, and additional processing for noisy stations. We obtained useful results at 154 stations from 898 teleseismic events over a 2-year period from 1995–1997 (for non-SGS stations) and a 6-year period from 2008–2014 (for SGS stations). Average crustal thickness (that is, depth to the Mohorovičić discontinuity [Moho] below the surface) beneath the Red Sea coastal plain (the rift margin) is 29 kilometers (km), beneath the volcanic fields (known in Arabic as harra [plural] or harrat [singular]) is 35 km, beneath the Arabian Shield (excluding harrats) is 37 km, and beneath the Arabian Platform is 38 km. Crustal thinning appears not to extend east of the rift escarpment, suggesting uniform extension that is no broader at depth than at the surface. In contrast to some previous claims that the Arabian Platform crust is thicker than that of the Arabian Shield, we find no statistically significant difference between their whole crustal thicknesses. However, the average subsedimentary crustal thickness (that is, the crystalline crust) for stations on the Arabian Platform is 34 km, 3 km thinner than the crust of the Arabian Shield. Individual station P-wave (pressure) velocity and S-wave (shear) velocity ratios ( V P / V S ) are highly variable for the Arabia Plate, ranging from 1.60 to 1.97 and averaging 1.75, with a standard deviation of 0.07. There are no statistically significant differences between V P / V S ratios of the different geologic regions of Saudi Arabia. Similar V P / V S ratios, coupled with similar crustal thicknesses for harrats and the Arabian Shield, indicate that Cenozoic magmatism has contributed negligibly to crustal growth.

Professional Paper↗