Geophysical Studies of the Crump Geyser Known Geothermal Resource Area, Oregon, in 1975

The U.S. Geological Survey (USGS) conducted geophysical studies in support of
the resource appraisal of the Crump Geyser Known Geothermal Resource Area
(KGRA). This area was designated as a KGRA by the USGS, and this designation
became effective on December 24, 1970. The land classification standards for a
KGRA were established by the Geothermal Steam Act of 1970 (Public Law 91-581).
Federal lands so classified required competitive leasing for the development of
geothermal resources.

The author presented an administrative report of USGS geophysical studies
entitled "Geophysical background of the Crump Geyser area, Oregon, KGRA" to a
USGS resource committee on June 17, 1975. This report, which essentially was a
description of geophysical data and a preliminary interpretation without
discussion of resource appraisal, is in Appendix 1. Reduction of sheets or
plates in the original administrative report to page-size figures, which are
listed and appended to the back of the text in Appendix 1, did not seem to
significantly degrade legibility. Bold print in the text indicates where minor
changes were made. A colored page-size index and tectonic map, which also show
regional geology not shown in figure 2, was substituted for original figure 1.
Detailed descriptions for the geologic units referenced in the text and shown
on figures 1 and 2 were separately defined by Walker and Repenning (1965) and
presumably were discussed in other reports to the committee. Heavy dashed lines
on figures 1 and 2 indicate the approximate KGRA boundary.

One of the principal results of the geophysical studies was to obtain a gravity
map (Appendix 1, fig. 10; Plouff, and Conradi, 1975, pl. 9), which reflects the
fault-bounded steepness of the west edge of sediments and locates the maximum
thickness of valley sediments at about 10 kilometers south of Crump Geyser.
Based on the indicated regional-gravity profile and density-contrast
assumptions for the two-dimensional profile, the maximum sediment thickness was
estimated at 820 meters. A three-dimensional gravity model would have yielded a
greater thickness. Audiomagnotelluric measurements were not made as far south
as the location of the gravity low, as determined in the field, due to a lack
of communication at that time. A boat was borrowed to collect gravity
measurements along the edge of Crump Lake, but the attempt was curtailed by
harsh, snowy weather on May 21, 1975, which shortly followed days of hot
temperature.

Most of the geophysical data and illustrations in Appendix 1 have been
published (Gregory and Martinez, 1975; Plouff, 1975; and Plouff and Conradi,
1975), and Donald Plouff (1986) discussed a gravity interpretation of Warner
Valley at the Fall 1986 American Geophysical Union meeting in San Francisco.
Further interpretation of possible subsurface geologic sources of geophysical
anomalies was not discussed in Appendix 1. For example, how were apparent
resistivity lows (Appendix 1, figs. 3-6) centered near Crump Geyser affected by
a well and other manmade electrically conductive or magnetic objects? What is
the geologic significance of the 15-milligal eastward decrease across Warner
Valley? The explanation that the two-dimensional gravity model (Appendix 1,
fig. 14) was based on an inverse iterative method suggested by Bott (1960) was
not included. Inasmuch as there was no local subsurface rock density
distribution information to further constrain the gravity model, the
three-dimensional methodology suggested by Plouff (1976) was not attempted.

Inasmuch as the associated publication by Plouff (1975), which released the
gravity data, is difficult to obtain and not in digital format, that report is
reproduced in Appendix 2. Two figures of the publication are appended to the
back of the text. A later formula for the theoretical value of gravity for the
given latitudes at sea level (International Association of Geodesy, 1971)
should be used to re-compute gravity anomalies. To merge the observed-gravity
values printed in that report with later measurements, an empirically
determined constant gravity datum shift should be applied.

[Summary provided by the USGS.]