Data Series 830
Available Geophysical Data and Quality/Noise IssuesThe following section describes the data available to the public, and the problems that arise from differences in data density and acquisition parameters. When modeling is planned, sharp contrasts in data quality and station spacing can lead to artificial results. Magnetic DataMagnetic data represented in this report were obtained from several sources Pan-American Center for Earth and Environmental Sciences, 2011; U.S. Geological Survey, 1999, 2002; Hill and others, 2009). Unfortunately, these data were acquired in discrete patches aimed at specific targets or objectives, each with differently tuned acquisition parameters. Later they were spliced into a larger whole for viewing at regional scales. The splicing process inevitably contributes digital artifacts at the seams, making these data difficult to model except within a single dataset. The different magnetic datasets acquired and presented here included both hand-digitized analog data and digitally acquired data, with flight-line spacings ranging from 1 to 10 km and flight elevations ranging from 170 meters (m) above ground level (AGL) (draped-flight data acquisition, nominally at a fixed distance above ground level) to 3,200 m above mean sea level (AMSL) (level-flight data acquisition, nominally at a fixed altitude above sea level). The merging process for these disparate datasets required upward continuation and releveling of all data to a common datum (3,200 m ASL). There is an anisotropic frequency content in some of these data, especially north of the Newberry area, due to very wide flight-line spacing and differing flight-line orientations. Different methods were also used for removal of the regional magnetic field in each dataset, and sometimes these are poorly documented. To the eye, the spliced magnetic data at nearly all scales presented appear seamless. However, later in this document one can see how certain types of processing amplify a sharp seam at 44° N., and other subtler but distinct seams at 43° 30’ N., 121° 00’ W., and 121° 30’ W. As a result, two-dimensional (2-D) modeling can be done effectively only within each small, discrete digital dataset but not crossing seam boundaries. The grid-spacing of the spliced magnetic dataset is 67 m, which honors the close along-line data sampling in some of the best datasets but leads to anisotropy in others that have wider survey line spacings. Gravity DataGravity data used in this paper were acquired from public-domain internet sources such as Pan-American Center for Earth and Environmental Sciences at the University of Texas, El Paso (2011) and U.S. Geological Survey (1999, 2002). Gravity data stations in Oregon have been acquired and collected over the past half-century by numerous entities, including the U.S Geological Survey, the U.S. Air Force, mining and oil companies, and universities (Gettings and Griscom, 1988). Data were typically acquired along roads, or wherever ready access to precise elevations such as benchmarks could be found. Raw gravity data are first corrected for drift and base reference, then for latitude and elevation (geoid effects), and finally for terrain effects using a standard average rock density. For this report the historical standard average density of 2.67 grams per cubic centimeter (g/cm3) is used (Blakely, 1996; U.S. Geological Survey, 1999, 2002) to provide a complete Bouguer anomaly. No isostatic correction was made, as this would have little effect this far east of the continental margin. The data acquired for central Oregon are often dense in flat-lying areas (typically acquired along roads) and sparse in rugged mountainous areas. The uneven and irregular station density makes it difficult to carry out meaningful 2-D modeling with gravity data except along a few of these roads, unless additional targeted profiles are collected. The gravity data density is not nearly as complete as the magnetic data. The grid-spacing of the gravity dataset is 447 m, but there are gaps between stations in some places in excess of 12 km. In the few places where this occurs, the reader will see blank white areas in the geophysical images. Radiometric DataNURE (National Uranium Resource Evaluation program) radiometric data for the Newberry region were obtained from Hill and others (2009) and U.S. Geological Survey (2010). Data were acquired by a national survey that included most of the 2-degree quadrangles in the continental United States and 3-degree quadrangles in Alaska between 1977 and 1983. Airborne surveys using paired detector crystals of different compositions were nominally flown 122 m above the ground. These data were calibration-corrected at a test site in Grand Junction, Colorado. The southern half of the Newberry area was surveyed as one 2-degree quadrangle flown at 10-km spacing, whereas another quadrangle in the northern half was flown with 5-km spacing, for reasons now lost in history. Atmospheric pressure changes appear to be incompletely compensated for, and the metadata are insufficiently detailed to explain fully what was done (U.S. Geological Survey, 2010). Along with the wide and inconsistent flight-line spacing, these issues mean that the data have limited value. However, they still validate geologic observations on the ground such as the 1,300 B.P. eruption of the Big Obsidian Flow and its related eastward-blown tephra (MacLeod and others, 1982; Dave Sherrod, oral commun., 2011). The grid-spacing of this dataset is 115 m, to honor the along-flight-line data; however, this also leads to strong apparent anisotropy due to the wide flight-line spacing. For purposes of this data series, the images for the 2-degree quadrangles covering Newberry Volcano were splined and concatenated.
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Suggested citation: Wynn, Jeff, 2014, Gravity, magnetic, and radiometric data for Newberry Volcano, Oregon, and vicinity: U.S. Geological Survey Data Series 830, https://dx.doi.org/10.3133/ds830. U.S. Department of the Interior U.S. Geological Survey |
