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G.R. Olhoeft

Publications and source records attributed to G.R. Olhoeft.

28 records · Page 2Linked to original sources
USGS1985

Low-frequency electrical properties

In the interpretation of induced polarization data, it is commonly assumed that metallic mineral polarization dominantly or solely causes the observed response. However, at low frequencies, there is a variety of active chemical processes which involve the movement or transfer of electrical charge. Measurements of electrical properties at low frequencies (such as induced polarization) observe such movement of charge and thus monitor many geochemical processes at a distance. Examples in which this has been done include oxidation-reduction of metallic minerals such as sulfides, cation exchange on clays, and a variety of clay-organic reactions relevant to problems in toxic waste disposal and petroleum exploration. By using both the frequency dependence and nonlinear character of the complex resistivity spectrum, these reactions may be distinguished from each other and from barren or reactionless materials.

Geophysics↗
USGS1984

Interpretation of core and well log physical property data from drill hole UPH-3, Stephenson County, Illinois

Laboratory and well log physical property measurements show variations in the mineralogy with depth in UPH-3. Gamma ray values generally decrease with depth in the drill hole, corresponding to a decrease in the felsic mineral components of the granite. Correspondingly, an increase with depth in mafic minerals in the granite is indicated by the magnetic susceptibility, and gamma ray measurements. These mineralogic changes indicated by the geophysical well logs support the hypothesis of fractionation during continuous crystallization of the intrusive penetrated by UPH-3. Two fracture zones, and an altered zone within the granite penetrated by drill hole UPH-3 are defined by the physical property measurements. An abnormally low magnetic susceptibility response in the upper portion of the drill hole can be attributed to alteration of the rock adjacent to the sediments overlying the granite. Fracture zones can be identified from the sonic velocity, neutron, and resistivity measurements. A fracture zone, characterized by low resistivity values and low neutron values, is present in the depth interval from 1150 to 1320 m. Low magnetic susceptibility and high gamma ray values indicate the presence of felsic-micaceous pegmatites within this fracture zone. An unfractured region present from a depth of 1380 m to the bottom of the hole is characterized by an absence of physical property variations. The magnetic susceptibility and gamma ray measurements indicate a change in the amount of mafic minerals at the base of this otherwise homogenous region of the drilled interval. Abrupt changes and repeated patterns of physical properties within the drill hole may represent interruptions in the crystallization process of the melt or they may be indicative of critical temperatures for specific mineral assemblages within the intrusive.

Open-File Report↗
USGS1979

Initial report of the Petrophysics Laboratory

Current national requirements for energy and mineral resources, and for protection of the quality of the environment, have focused attention on the need to develop new methods and interpretation techniques to be used in understanding geologic systems. Geophysical surveys play a significant role in acquiring this information, particularly by providing data in the third dimension--at depth beneath the surface.

Circular↗
USGS1977

Laboratory measurements of complex resistivity characteristics of oil shales from Saterdal No.1 corehole, Piceance Creek Basin, Colorado

Laboratory measurements of the complex electrical resistivity characteristics of more than 120 oil shale samples from Saterdal #1 corehole, Piceance Creek Basin, Colorado, indicate the following: (1) The presence of kerogen in wet Green River Formation marls increases their d.c. resistivities from about 300 ohm-m to more than 10 5 ohm-m, makes them isotropic, and causes a substantial frequency dependence. (2) Microanisotropy coefficient increases with water content from about 2 to as much as 200. The effect seems to be due to an extreme porosity and/or permeability anisotropy, involving a drop in longitudinal resistivity from about 10 8 ohm-m for dry samples to as low as 10 5 ohm-m for wet ones, while transverse resistivities stay relatively constant at about 10 8 ohm-m. (3) Intermediate grade oil shales show the most extreme anisotropy effect; both very rich and very lean oil shales are more nearly isotropic. (4) Both wet and dry oil shales show a pronounced frequency effect for frequencies >10Hz, and may make good IP targets.

Colorado↗