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At least 199 records · Page 11Linked to original sources
USGS2023

Environmental factors influencing detection efficiency of an acoustic telemetry array and consequences for data interpretation

Background Acoustic telemetry is a commonly used technology to monitor animal occupancy and infer movement in aquatic environments. The information that acoustic telemetry provides is vital for spatial planning and management decisions concerning aquatic and coastal environments by characterizing behaviors and habitats such as spawning aggregations, migrations, corridors, and nurseries, among others. However, performance of acoustic telemetry equipment and resulting detection ranges and efficiencies can vary as a function of environmental conditions, leading to potentially biased interpretations of telemetry data. Here, we characterize variation in detection performance using an acoustic telemetry receiver array deployed in Wellfleet Harbor, Massachusetts, USA from 2015 to 2017. The array was designed to study benthic invertebrate movements and provided an in situ opportunity to identify factors driving variation in detection probability. Results The near-shore location proximate to environmental monitoring allowed for a detailed examination of factors influencing detection efficiency in a range-testing experiment. Detection ranges varied from < 50 to 1,500 m and efficiencies varied from 0 to 100% within those detection ranges. Detection efficiency was affected by distance, wind speed and direction, wave height and direction, water temperature, water depth, and water quality. Conclusions Performance of acoustic telemetry systems is strongly contingent on environmental conditions. Our study found that wind, waves, water temperature, water quality, and depth all affected performance to an extent that could seriously compromise a study if these effects were not taken into consideration. Other unmeasured factors may also be important, depending on the characteristics of each site. This information can help guide future telemetry study designs by helping researchers anticipate the density of receivers required to achieve study objectives. Researchers can further refine and document the reliability of their data by incorporating continuously deployed range-testing tags and prior knowledge on varying detection efficiency into movement and occupancy models.

Massachusetts↗
USGS1955

The Sudbury [Ontario] aeromagnetic map as a test of interpretation methods

The method of interpretation of aeromagnetic maps described by Vacquier, Steenland, Henderson, and Zietz (1951) is applied to several anomalies on the aeromagnetic map of part of Sudbury, Ontario. The resulting average computed depths deviate from the known depths by less than 10 percent. The aeromagnetic map is described and compared with the known surface geology.

Ontario↗
USGS1960

A comprehensive system of automatic computation in magnetic and gravity interpretation

In the interpretation of magnetic and gravity anomalies, downward continuation of fields and calculation of first and second vertical derivatives of fields have been recognized as effective means for bringing into focus the latent diagnostic features of the data. A comprehensive system has been devised for the calculation of any or all of these derived fields on modern electronic digital computing equipment. The integral for analytic continuation above the plane is used with a Lagrange extrapolation polynomial to derive a general determinantal expression from which the field at depth and the various derivatives on the surface and at depth can be obtained. It is shown that the general formula includes as special cases some of the formulas appearing in the literature. The process involves a "once for all depths" summing of grid values on a system of concentric circles about each point followed by application of the appropriate one or more of the 19 sets of coefficients derived for the purpose. Theoretical and observed multilevel data are used to illustrate the processes and to discuss the errors. The coefficients can be used for less extensive computations on a desk calculator.

Geophysics↗
USGS1962

Limiting parameters in the magnetic interpretation of a geologic structure

A prominent aeromagnetic anomaly in Randolph County, Indiana, suggests the existence of a dikelike structure within the Precambrian basement rocks. Because of the ambiguity inherent in magnetic interpretation , a unique solution for the parameters involved (depth of burial, geometric configuration, and magnetic susceptibility of the mass producing the anomaly) is impossible. However, if one of the parameters is known it is sometimes possible to indicate a range of plausible values for each of the remaining parameters . The depth to the surface of the Precambrian rocks in Randolph County is known from drill-hole data to be about 3,000 ft. As the depth of burial is known, limits need be set only on the thickness, angle of dip, and susceptibility contrast of the assumed dike. These limits are determined by a graphical method. Theoretical anomalies over the postulated dike are computed for different dike thicknesses, angles of dip, and susceptibility contrasts. The computed profiles are then fitted to an observed magnetic profile flown at right angles to the trend of the feature. Numerical values for the "goodness of fit" are calculated by using the statistical method of sums of squares. Two plots, one of the index of goodness of fit and the other of susceptibility as functions of dip angle and dike thickness, are made to show graphically the interdependence of the variables and the plausible range of each. The contoured plots of the goodness of fit and the susceptibility contrast show that the range of best fit is between dip angles of 30 and 60 degrees and dike thickness of 1,000 to over 4,000 ft. The contoured plot of the susceptibility contrast, translated into percentage of magnetite, shows this parameter to be independent of the dip angle, varying only with thickness.

Indiana↗
USGS2018

Exploring viable geologic interpretations of gravity models using distance-based global sensitivity analysis and kernel methods

We have explored ways to integrate alternative geologic interpretations into the modeling of gravity data. These methods are applied to the Vaca Fault east of Fairfield, California, USA, where the structure across the fault is in question, and the Vaca Fault is used as a case study to demonstrate the method. The Vaca Fault is modeled using gravity data collected along a 10 km line perpendicular to the strike of the fault. Of particular interest is how the gravity data might inform on the dip of the Vaca Fault and thickness of the nonmarine section and whether spatial autocorrelation of density internal to the geologic units significantly influences the resulting gravity anomaly. We approach these questions by creating a suite of structural geologic models, which we then populate with geostatistically generated densities and from which the respective synthetic gravity anomalies are calculated. We perform distance-based generalized sensitivity analysis to identify which model inputs most leverage the calculated gravity anomaly. We then use multidimensional scaling to transform the gravity anomalies into a metric space and estimate the posterior probabilities of each structural geologic model using a Bayesian approach. We find that the gravity anomalies are particularly sensitive to zones of autocorrelated density values generated from geostatistical modeling. The structural geologic models most likely to produce gravity anomalies that match the observed data are the moderately dipping normal faults, 45° and 60°, although the probability that the fault dips more steeply, including in a strike slip or reverse fault orientation, is approximately 30%. The probability of a thicker nonmarine unit is 67%, more probable than a thinner nonmarine unit. This suggests that the Vaca Fault dips moderately to the east and truncates a thicker nonmarine unit, but that any further process modeling should include alternatives of the geologic structures.

California↗
USGS1973

Revised age and structural interpretations of Nuka Formation at Nuka Ridge, northwestern Alaska: Geologic notes

New collections of foraminifers and macrofossils from Nuka Ridge, the type area of the enigmatic Nuka Formation, demonstrate that the type Nuka Formation is not a continuous stratigraphic sequence as originally described. Paleontologic evidence indicates the presence of a structurally repeated sequence at Nuka Ridge consisting of Upper Mississippian (Meramec and younger) strata overlain by Permian beds which, in turn, are succeeded by Triassic(?) and Cretaceous rocks. Mississippian calcareous mudstone and limestone units are correlated with the Lisburne Group; microcline-bearing arkose units with Mississippian and Permian macrofossils and microfossils represent the Nuka Formation; units of chert are inferred to be of Permian and Triassic age; and wacke at the top of the sequence resembles unnamed wacke assigned to the Cretaceous elsewhere. These new data and their interpretations indicate that the type section of the Nuka Formation is inadequate. The standard for the Nuka Formation is therefore referred to a newly measured section of the middle arkosic unit of the old section.

Alaska↗
USGS2020

How "simple" methodological decisions affect interpretation of population structure based on reduced representation library DNA sequencing: A case study using the lake whitefish

Reduced representation (RRL) sequencing approaches (e.g., RADSeq, genotyping by sequencing) require decisions about how much to invest in genome coverage and sequencing depth, as well as choices of values for adjustable bioinformatics parameters. To empirically explore the importance of these “simple” methodological decisions, we generated two independent sequencing libraries for the same 142 individual lake whitefish (Coregonus clupeaformis) using a nextRAD RRL approach: (1) a larger number of loci at low sequencing depth based on a 9mer (library A); and (2) fewer loci at higher sequencing depth based on a 10mer (library B). The fish were selected from populations with different levels of expected genetic subdivision. Each library was analyzed using the STACKS pipeline followed by three types of population structure assessment (FST, DAPC and ADMIXTURE) with iterative increases in the stringency of sequencing depth and missing data requirements, as well as more specific a priori population maps. Library B was always able to resolve strong population differentiation in all three types of assessment regardless of the selected parameters, largely due to retention of more loci in analyses. In contrast, library A produced more variable results; increasing the minimum sequencing depth threshold (-m) resulted in a reduced number of retained loci, and therefore lost resolution at high -m values for FST and ADMIXTURE, but not DAPC. When detecting fine population differentiation, the population map influenced the number of loci and missing data, which generated artefacts in all downstream analyses tested. Similarly, when examining fine scale population subdivision, library B was robust to changing parameters but library A lost resolution depending on the parameter set. We used library B to examine actual subdivision in our study populations. All three types of analysis found complete subdivision among populations in Lake Huron, ON and Dore Lake, SK, Canada using 10,640 SNP loci. Weak population subdivision was detected in Lake Huron with fish from sites in the north-west, Search Bay, North Point and Hammond Bay,showing slight differentiation. Overall, we show that apparently simple decisions about library construction and bioinformatics parameters can have important impacts on the interpretation of population subdivision. Although potentially more costly on a per-locus basis, early investment in striking a balance between the number of loci and sequencing effort is well worth the reduced genomic coverage for population genetics studies. More conservative stringency settings on STACKS parameters lead to a final dataset that was more consistent and robust when examining both weak and strong population differentiation. Overall, we recommend that researchers approach “simple” methodological decisions with caution, especially when working on non-model species for the first time.

Michigan, Ontario, Saskatchewan↗
USGS2008

Comparisons of shear-wave slowness in the Santa Clara Valley, California using blind interpretations of data from invasive and noninvasive methods

Many groups contributed to a blind interpretation exercise for the determination of shear-wave slowness beneath the Santa Clara Valley. The methods included invasive methods in deep boreholes as well as noninvasive methods using active and passive sources, at six sites within the valley (with most investigations being conducted at a pair of closely spaced sites near the center of the valley). Although significant variability exists between the models, the slownesses from the various methods are similar enough that linear site amplifications estimated in several ways are generally within 20% of one another. The methods were able to derive slownesses that increase systematically with distance from the valley edge, corresponding to a tendency for the sites to be underlain by finer-grained materials away from the valley edge. This variation is in agreement with measurements made in the boreholes at the sites.

Bulletin of the Seismological Society of America↗
USGS2015

Interpretation of S waves generated by near-surface chemical explosions at SAFOD

A series of near-surface chemical explosions conducted at the San Andreas Fault Observatory at Depth (SAFOD) were recorded by high-frequency downhole receiver arrays in separate experiments in November 2003 and May 2005. The 2003 experiment involved &sim;100&thinsp;&thinsp;kg shots detonated along a 46-km-long line (Hole&ndash;Ryberg line) centered on SAFOD and recorded by 32 three-component geophones in the pilot hole between 0.8 and 2.0 km depth. The 2005 experiment involved &sim;36&thinsp;&thinsp;kg shots detonated at Parkfield Area Seismic Observatory (PASO) stations (at &sim;1&ndash;8&thinsp;&thinsp;km offset) recorded by 80 three-component geophones in the main hole between the surface and 2.4 km depth. These data sample the downgoing seismic wavefield and constrain the shallow velocity and attenuation structure, as well as the first-order characteristics of the source. Using forward modeling on a velocity structure designed for the near field, both observed P - and S -wave energy for the PASO shots are identified with the travel times expected for direct and/or reflected phases. Larger-offset recordings from shots along the Hole&ndash;Ryberg line reveal substantial SV and SH energy, especially southwest of SAFOD from the source as indicated by P -to- S amplitude ratios. The generated SV energy is interpreted to arise chiefly from P -to- S conversions at subhorizontal discontinuities. This provides a simple mechanism for often-observed low P -to- S amplitude ratios from nuclear explosions in the far field, as originating from strong near-field wave conversions.

Bulletin of the Seismological Society of America↗
USGS2002

On geological interpretations of crystal size distributions: Constant vs. proportionate growth

Geological interpretations of crystal size distributions (CSDs) depend on understanding the crystal growth laws that generated the distributions. Most descriptions of crystal growth, including a population-balance modeling equation that is widely used in petrology, assume that crystal growth rates at any particular time are identical for all crystals, and, therefore, independent of crystal size. This type of growth under constant conditions can be modeled by adding a constant length to the diameter of each crystal for each time step. This growth equation is unlikely to be correct for most mineral systems because it neither generates nor maintains the shapes of lognormal CSDs, which are among the most common types of CSDs observed in rocks. In an alternative approach, size-dependent (proportionate) growth is modeled approximately by multiplying the size of each crystal by a factor, an operation that maintains CSD shape and variance, and which is in accord with calcite growth experiments. The latter growth law can be obtained during supply controlled growth using a modified version of the Law of Proportionate Effect (LPE), an equation that simulates the reaction path followed by a CSD shape as mean size increases.

American Mineralogist↗
USGS1980

Geology and mineral resources of the North Absaroka Wilderness and vicinity, Park County, Wyoming, with sections on mineralization of the Sunlight mining region and geology and mineralization of the Cooke City mining district, and a section on aeromagnetic interpretation

SUMMARY The North Absaroka Wilderness is approximately 560 square miles (1,450 km 2 ) of rugged scenic mountainous terrain that adjoins the eastern boundary of Yellowstone National Park in northwestern Wyoming. The area was studied during 1970, 1971, and 1972 by personnel of the U. S. Geological Survey and the U. S. Bureau of Mines to evaluate its mineral-resource potential as required by the Wilderness Act of 1964. This evaluation is based on a search of the literature courthouse and production records, geologic field mapping, field inspection of claims and prospects, analyses of bedrock and stream-sediment samples, and an aeromagnetic survey. The North Absaroka Wilderness is underlain almost entirely by andesitic and basaltic volcanic rocks of Eocene age. These volcanics rest on deformed sedimentary rocks of Paleozoic and, locally, of Mesozoic age that are exposed at places along the northern and eastern edges of the wilderness. Dikes and other igneous intrusive bodies cut both the volcanic and sedimentary rocks. A nearly flat detachment fault, the Heart Mountain fault, and a related steep break-away fault have displaced middle and upper Paleozoic rocks and some of the older part of the volcanic sequence to the southeast. A much greater thickness of volcanic rocks was found to be involved in Heart Mountain faulting than had previously been recognized; however, most of the volcanic rocks and many of the intrusives were emplaced after Heart Mountain faulting. Local folding and high-angle faulting in mid-Eocene time have deformed all but the youngest part of the volcanic sequence in the southeastern part of the wilderness. This deformation is interpreted as the last pulse of Laramide orogeny. The results of this study indicate that the mineral-resource potential of the wilderness is minimal. Bentonite, petroleum, low-quality coal, and localized deposits of uranium and chromite have been produced in the surrounding region from rocks that underlie the volcanic rocks; but such deposits, if present in the wilderness, would be too deeply buried, too small, or too sporadically distributed to be profitably located and exploited. Copper and gold mines and prospects are present on the fringes of the wilderness, but otherwise the area seems to be devoid of economically valuable concentrations of metallic minerals. No surface evidence of geothermal-energy potential was found. Known mineral deposits in the vicinity of the North Absaroka Wilderness are associated with intrusive rocks. From the Cooke City mining district, just north of the wilderness, replacement deposits in Upper Cambrian carbonate rocks may extend a short distance into the north edge of the wilderness, In the Sunlight mining region, an enclave nearly surrounded by the wilderness, mineralization occurs in veins and is disseminated in volcanic and plutonic rocks. Richer concentrations of metallic minerals may occur in carbonate rocks adjacent to intrusive bodies at depth beneath the volcanic rocks in the Sunlight region. A few small intrusive bodies occur in the wilderness, but no significant associated mineralization was detected. Aeromagnetic data indicate that other intrusives not exposed by erosion may occur in the wilderness; however, no significant metamorphism or alteration is evident at the surface to indicate their presence. Although most of the rocks of the wilderness are of igneous origin, they are all so old (Eocene) that it is unlikely that they retain any original heat. The Pleistocene rhyolitic ash-flow tuffs in the southwestern part of the wilderness were erupted from sources in Yellowstone National Park just to the west; however, in the wilderness these tuffs are too thin to contain any residual heat.

Wyoming↗
USGS1980

Interpretation of an aerial radiometric and magnetic survey of the Salmo-Priest study area (RARE E6-981 A1-981), Pend Oreille County, Washington and Boundary County, Idaho

An aerial radiometric and magnetic survey of the Salmo-Priest study area (RARE E6-981) (index map) was made by the U.S. Geological Survey during October, 1978. Geophysical data from the survey will be used in the assessment of the mineral-resource potential of the study area. The Salmo-Priest area is located in northeastern Washington and northern Idaho and the north edge is the United States-Canadian boundary. This report presents the data obtained by the survey and an interpretation of the data.

Idaho, Washington↗
USGS1986

Correlation of coal beds in the Fruitland Formation as interpreted from geophysical logs, east-central San Juan County, New Mexico

Correlation of coal beds in the Fruitland Formation (Upper Cretaceous) in the subsurface, from interpretation of geophysical logs, is an outgrowth of unpublished studies of deep coal on the Navajo Reservation by the author in the Toadlena 30 x 60 minute quadrangle, New Mexico, and by W. J. Mapel in the Farmington 30 x 60 minute quadrangle, New Mexico. The lines of sections of this report extend eastward from the reservation into east-central San Juan County, which is in the western part of the San Juan Basin. In this area, the rocks dip gently northeast toward the central part of the basin. The thick coal is less than 1,500 feet beneath the surface. Of the 53 logs in the sections, 48 are from oil- and gas-test holes, and 5 are from coal-test holes (see table 1). Stratigraphic relations of the Upper Cretaceous Lewis Shale, Pictured Cliffs Sandstone, and Fruitland Formation, shown on the cross sections and briefly reviewed below, have been previously described for this or other parts of the San Juan Basin by Hayes and Zapp (1955), Beaumont (1971), Fassett and hinds (1971), O;Sullivan and other (1972), and Molenaar (1983), among others.

New Mexico↗
USGS1979

Geologic interpretation of aeromagnetic map of the Seward and Blying Sound quadrangles, Alaska

An aeromagnetic survey of the Seward and northern part of the Blying Sound quadrangles was flown by Geometrics, Inc., during 1975-77 to provide magnetic data to aid in an appraisal of the mineral resources. Background details of the regional geology and major structures are described in a companion report by Tysdal and Case (1979), and interpretation of the gravity data are described in a report by Case and others (1979). Preliminary reports of a paleomagnetic investigation have been reported by Hillhouse and Gromme (1977), and a more complete analysis of the results is in preparation. The aeromagnetic survey was flown at a nominal height of 300 m above the surface and flight lines were spaced about 1.7 km apart. Total magnetic field was measured by a proton-precession magnetometer. The International Geomagnetic Reference Field (IGRF), updated to 1976, was removed, and the residual values were computer-contoured to produce the aeromagnetic map (sheet 1). Detailed aeromagnetic maps at scale 1:63,360 and a map at scale 1:250,000 have been released as Open File Reports 78-1080 through 1083 (U.S. Geological Survey, 1978). Magnetic anomalies in the Seward-Blying Sound region are somewhat less complex than in other parts of south-central Alaska, primarily because of the huge volume of relatively nonmagnetic flysch that constitutes the Valdez and Orca Groups. Several rather isolated groups of mafic-ultramafic rocks are significantly more magnetic than the flysch assemblages, and cause distinctive patterns of magnetic anomalies. A few of the granitic plutons are magnetic, but most appear to have little or no magnetic expression.

Alaska↗
USGS2000

Puerto Rico: Marine sediments, terrestrial and seafloor imagery, and tectonic interpretations

Introduction: Puerto Rico is an island situated in the plate boundary zone between the Caribbean and the North American Plates. This is a geologically fascinating, tectonically active region, where the Caribbean Plate has over-ridden the North American Plate and is now sliding past it with strike-slip motion. The details of this tectonic interaction are poorly understood, largely because much of the region lies under water, making it difficult to study. The geologic setting of Puerto Rico has created or contributed to several pressing societal issues, related to human safety, environmental health, and economic development. Because the island lies on an active plate boundary, earthquakes are a constant threat and the densely populated coastal areas are vulnerable to tsunamis. Coastal erosion is a concern in many coastal areas, but is particularly serious to an island economy which relies heavily on a thriving tourist industry. In Puerto Rico, illegal mining of beach sands for use as aggregate has exacerbated coastal erosion and left coastal communities more exposed to the ravages of storms and tsunamis. The serious need for an affordable local source of aggregate to use in the growing construction industry on the island has created a strong interest in locating and evaluating offshore sand and gravel deposits. During the last 20 years the U.S. Geological Survey, often in cooperation with the Puerto Rico Department of Natural Resources and/or the University of Puerto Rico, has conducted a variety of projects to address the geologic aspects of these pressing societal issues. The papers and data presented here are the results of these projects. Some have been previously published elsewhere; others, such as the database of surficial sediments of the Puerto Rico insular shelf, interpretation of side-looking airborne radar imagery of the island, and the GIS data, are presented here for the first time. The purpose of this CD is to make the results of these diverse studies available in a readily accessible digital form, so that managers, planners, and other researchers can utilize the results.

Puerto Rico↗
USGS2001

Implications for the formation of the Hollywood Basin from gravity interpretations of the northern Los Angeles Basin, California

Gravity data provide insights on the complex tectonic history and structural development of the northern Los Angeles Basin region. The Hollywood basin appears to be a long (> 12 km), narrow (up to 2 km wide) trough lying between the Santa Monica Mountains and the Wilshire arch. In the deepest parts of the Hollywood basin, the modeled average thickness ranges from roughly 250 m if filled with only Quaternary sediments to approximately 600 m if Pliocene sediments are also present. Interpretations of conflicting drill hole data force us to consider both these scenarios. Because of the marked density contrast between the dense Santa Monica Mountains and the low-density sediments in the Los Angeles Basin, the gravity method is particularly useful in mapping the maximum displacement along the Santa Monica-Hollywood-Raymond fault zone. The gravity-defined Santa Monica–Hollywood fault zone deviates, in places, from the mapped active fault and fold scarps located with boreholes and trenching and by geomorphological mapping by Dolan and others (1997). Our models suggest that the Santa Monica–Hollywood fault zone dips northward approximately 63°. Three structural models are considered for the origin of the Hollywood basin: pull-apart basin, flexural basin, and a basin related to a back limb of a major fold. Although our preferred structural model involves flexure, the available geologic and geophysical data do not preclude contributions to the deepening of the basin from one or both of the other two models. Of particular interest is that the distribution of red-tagged buildings and structures damaged by the Northridge earthquake has a strong spatial correlation with the axis of the Hollywood basin defined by the gravity data. Several explanations for this correlation are explored, but two preferred geologic factors for the amplification of ground motion besides local site effects are (1) focussing of energy by a fault along the axis of the Hollywood basin and (2) focussing effects related to differential refraction of seismic rays across the basin.

California↗
USGS2002

Geologic interpretation of aeromagnetic maps in the Santa Cruz basin: Patagonia Mountains area, south-central Arizona

High resolution aeromagnetic data for the Santa Cruz basin - Patagonia Mountains region of south-central Arizona (USGS, 2000) can be combined with geologic mapped regions to reconfirm known geologic features, extend known features, and suggest new possibilities. These include mapping of lava flows, mapping of igneous intrusions, mapping of faults, and the mapping of basement rocks which lie beneath sediments. The processed maps of interest for a geologic interpretation include the reduced-to-the-pole magnetic map (Phillips, Plate 3), the depth estimate by the horizontal gradient method (Phillips, Plate 6), and the basin depth estimates map.

Arizona↗