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Geophysics Unit of Menlo Park, CA (GUMP)

U.S. Geological Survey - Western Region - Geology and Geophysics

Aeromagnetic Survey Across Crater Flat and Yucca Mountain

INTRODUCTION

An aeromagnetic survey was flown by the U.S. Geological Survey (USGS) over Crater Flat and part of Yucca Mountain, Nevada, to help in the interpretation of the subsurface geologic structure at a potential location of a nuclear waste repository. The USGS airplane was in the Yucca Mountain vicinity to fly an extensive aeromagnetic and VLF survey of the Beatty area just to the west. The lines reported here followed the trace of a proposed seismic profiles across Crater Flat and Yucca Mountain. Magnetic surveys are used to help locate and identify the sources of anomalies in the Earth's magnetic field. Magnetic anomalies may be related to near-surface geology or to geologic structural features within the Earth's crust. Magnetic data may reveal the existence of faults, the distribution of stratigraphic units, the presence of intrusive bodies, the thickness and shape of sedimentary basins, and the depth to the bottom of magnetic sources. Magnetic anomalies will tend to form along boundaries where there is a vertical offset of beds.

MAJOR ANOMALIES

The gridded and contoured aeromagnetic survey data (fig.1) show a number of magnetic features that can also be seen on a detailed aeromagnetic map (fig. 2) of the Timber Mountain area. A broad magnetic low in the western third of profiles 1, 2, and 3 may be due to reversely magnetized tuffs. These tuffs are Miocene in age and consist of quartz- and hornblende-bearing rhyolitic ash-flow tuffs (Carr and others, 1986). However, this anomaly may be related to a reversely magnetized basalt flow that was penetrated in drill-hole USW VH-2 (Carr and Parrish, 1985).

 

Link to Yucca Mtn Home Page


MAGNETIC MAP (Click on image for a full-size version, 75 Kbytes.)
Aeromagnetic map along seismic reflection lines across Crater Flat and Yucca Mountain. Red colors, magnetic highs; blue colors, magnetic lows.

A broad magnetic high occurs just south of Black Cone, on profiles 1, 2, and 3 and 4, 5, and 6 (fig. 1). The source of this high is unknown but may be due to buried normally magnetized volcanic rocks if they thicken towards the center of the anomaly. A hole drilled over this anomaly revealed about 300 m (984 ft) of Topopah Spring Tuff of the Paintbrush Group and over 140 m (459 ft) of densely welded Bullfrog Member of the Crater Flat Tuff. Both of these units have magnetic properties that could cause the anomaly. Physical property measurements show that both these units are normally magnetized. A deep north trending low in the middle of lines 4, 5, and 6 is ascribed by Kane and Bracken (1983) to a possible offset in underlying horizontal tuffs. Magnetic highs over Yucca Mountain, at the northeast end of lines 10,11, and 12 and the northwest on two-thirds of lines 13, 14, and 15, generally correlate with exposures of the Topopah Spring Tuff of the Paintbrush Group(Sawers and others, 1994). Linear magnetic features in this area may reflect offsets in flat-lying volcanic units. Such offsets may only represent lithologic causes, such as variations in thickness or magnetic properties of the volcanic units, or they could be due to tectonic elements, such as faults. Joint interpretation of these data together with seismic and other data still to come should help resolve the nature of these possible offsets.

MAGNETIC MAP (Click on image for a full-size version, 43 Kbytes.)
Aeromagnetic map of the Timber Mountain survey. Red colors, magnetic highs; blue colors, magnetic lows.

By Robert F. Sikora

See the article: Sikora, R.F., Campbell, D.L, and Kucks, R.P., 1995, Aeromagnetic surveys across Crater Flat and part of Yucca Mountain, Nevada: U.S. Geological Survey Open-File Report 95-812, 13 p. PDF (1.3 Mb).

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