Geology topics

W.F. Isherwood

Publications and source records attributed to W.F. Isherwood.

3 recordsLinked to original sources

USGS2012

Aeromagnetic and gravity surveys in the Coso Range, California

The effect of an underlying magma reservoir cannot be identified within the complex gravity pattern in the Coso Range, California. Rather, linear gravity contours, which suggest a regional tectonic origin, enclose the location of most of the volcanic activity of the Coso Range. Faults along the edges of northwest trending, magnetic blocks probably provided paths of minimum resistance to the ascending viscous magma that was extruded as rhyolite domes. Dense, magnetic rocks associated with a complex mafic pluton 9 km in diameter form a relatively impermeable north border of the Pleistocene volcanic field. A heat flow high nearly coincides with the west half of a 6‐km‐diameter magnetic low. A 2‐km‐diameter outcrop of a pre‐Cenozoic silicic pluton, which has low magnetization compared to the surrounding metamorphic rocks, presumably typifies the rocks that underlie the magnetic low and heat flow high. Hydrothermal fluids may have destroyed some magnetite in the more magnetic wall rock, further reducing the magnetic intensity.

California↗

USGS2012

Geodetic analysis of reservoir depletion at the Geyser steam field in northern California

Reservoir depletion at the Geysers from 1974 to 1977 is evident in measured changes in gravity, surface strain, and pore pressure drainage. The drainage area increased about 20%, the maximum gravity decrease was about −120 μGal, and the maximum elevation change was about 6 cm during this period. Since the net mass withdrawal is known, it may be combined with the gravity change to estimate a drainage volume. The maximum drainage volume is 25 km 3 . Because the depth of the caprock and extent of the pore pressure drainage is known, this volume limit implies that no significant mass withdrawal occurs below a depth of 4 km. The ratios of surface elevation changes to horizontal contraction coinciding with the drainage area imply an equant drainage geometry, assuming that fluid production produces negative dilatation. Using the same cylindrical geometry used to model the gravity, negative dilatational strain rates of 4 to 5 × 10 −5 /yr were found to produce the observed surface displacements. The likelihood of boiling in the system to produce steam combined with apparent large bulk moduli (from seismic velocities) and small pore pressure declines suggest that most of the reservoir contraction is due to cooling.

Journal of Geophysical Research Solid Earth↗

USGS1978

Evaluation of Baltazor known geothermal resources area, Nevada

By virtue of the Geothermal Steam Act of 1970, the U.S. Geological Survey is required to appraise geothermal resources of the United States prior to competitive lease sales. This appraisal involves coordinated input from a variety of disciplines, starting with reconnaissance geology and geophysics. This paper describes how the results of several geophysical methods used in KGRA evaluation were interpreted by the authors, two geophysicists, involved with both the Evaluation Committee and the research program responsible for obtaining and interpreting the geophysical data to be used by the committee. ?? 1979.

Geothermics↗