Gravity observations by the U.S. Geological Survey in Northwest Nevada, Southeast Oregon, and Northeast California, 1984-1986

Abstract

Tables 6, 7, and 8 list principal facts for the gravity data. The information listed for each station fits in 80 columns on a line in a computer file and is reformatted for readability in the tables. Station names consist of 8 or less characters. Locations are described in geographic coordinates expressed in degrees and minutes. Inasmuch as most topographic maps and benchmark lists in the area indicate elevations in feet, elevations are expressed in feet. Values of observed gravity are referenced to the IGSN-71 datum and are expressed in milligals. The free-air gravity anomaly, expressed in units of milligals, uses the GRS-67 formula for normal gravity on the spheroid and includes higher order terms of Swick's (1942, p. 65) formula for the free-air correction. Terrain corrections are subdivided into two columns of values expressed in milligals. The first column is the hand terrain correction determined through Hammer's (1939) F-ring. The second column is the part calculated from digital topography between Hammer's (1939) F-ring (0.89 km) and 166.7 km. Bouguer gravity anomalies ("BOUG ANOM") and values of the isostatic residual gravity ("ISOS ANOM") are expressed in milligals. "ACC CODE" refers to a 4-digit accuracy code. The first digit describes the location and the type of elevation at the station (table 2). The second digit provides an estimate of the elevation accuracy (table 3), which relates to a large source of error (0.06 mGal/ft) when calculating the Bouguer gravity anomaly. The third digit indicates the accuracy of horizontal location (table 4). The fourth digit indicates the accuracy of observed gravity (table 5). This code depends on the type of gravity meter used, the magnitude of drift, and the number and quality of repeat readings. The "DATE" is the most recent date, expressed in month, day, and year (without the leading 19), when gravity was observed at the station. "BASE NAME" is the 5-digit name of the base station to which the station was tied.

Mineral resources of the Rincon Wilderness Study Area, Harney County, Oregon

“…In 1986, the U.S. Geological Survey established 27 gravity stations in and adjacent to the study area. A preliminary gravity map prepared from these data shows that the study area is located in a broad, nose-shaped gravity high that extends westward from a major gravity high centered near the crest of the Pueblo Mountains (Plouff, 1987). Pre-Cenozoic basement rocks that underlie the gravity high in the Pueblo Mountains are denser than basement rocks at comparable depths.…”

Mineral resources of the Upper Leslie Gulch and Slocum Creek Wilderness Study Areas, Malheur County, Oregon

“…Atomic absorption was used to analyze for gold and mercury, and the presence of uranium was determined by ultraviolet fluorescence. Earlier geophysical gravity data were supplemented by data from additional gravity stations within and near the study area (Plouff, 1987).…”

“…The earlier survey was described and generally interpreted in a thesis by Lillie (1977). Additional gravity stations were located within and near the study area by Plouff (1987). Average spacing between stations within and near the study area is 2 to 3 mi.…”

Mineral resources of the Guano Creek Wilderness Study Area, Lake County, Oregon

“…In 1986, the U.S. Geological Survey established 17 gravity stations in and within 2 mi outside the border of the study area (Plouff, 1987). A preliminary gravity map (Donald Plouff, unpub.…”