The Effect of Pressure and Deviatoric Stress on Rock Magnetism

Experiments were performed on many rock types to examine the effect of pressure and stress on their magnetic properties. The effects of loading path on thermoremanent magnetization (TRM) and magnetic susceptibility were examined in detail. For samples with a TRM, initial loading produced a pronounce...

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Bibliographic Details
Main Author: Martin, III, Randolph J
Format: Report
Language:English
Subjects:
CYCLES
DEMAGNETIZATION
DIFFERENTIAL STRESSES
Geology, Geochemistry and Mineralogy
Geomagnetism
HYDROSTATIC PRESSURE
HYSTERESIS
LOADS(FORCES)
MAGNETIC DOMAINS
MAGNETIC FIELDS
MAGNETIC PROPERTIES
MAGNETIZATION
NUCLEATION
PATHS
PE61102F
PEAK VALUES
PIEZOMAGNETIC EFFECT
PRESSURE
PRESSURIZATION
RECOVERY
REMANENT MAGNETISM
ROCK
ROCK MAGNETISM
ROCK MECHANICS
SENSITIVITY
STRESS ANALYSIS
STRESSES
TEST AND EVALUATION
THERMAL PROPERTIES
TRM(THERMOREMANENT MAGNETISM)
UNLOADING
WUAFOSR2302C1
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Summary:Experiments were performed on many rock types to examine the effect of pressure and stress on their magnetic properties. The effects of loading path on thermoremanent magnetization (TRM) and magnetic susceptibility were examined in detail. For samples with a TRM, initial loading produced a pronounced decrease in magnetization. As the specimen was unloaded, very little recovery in magnetization was observed resulting in a permanent demagnetization at the termination of the cycle. Differential stress produced a larger demagnetization than hydrostatic pressure. Demagnetizations of approx. 20% were observed during pressurization, while the change in magnetization approached 40% of a differential stress of 200 MPA. If the specimen was reloaded over the same path to the same stress, the change in magnetization was much smaller than for the initial cycle, and only a small additional increment of demagnetization was observed at the end of the cycle. If the peak stress was augmented, once the peak stress from the previous cycle was exceeded, the stress sensitivity increased noticeably. Upon unloading, there was a pronounced hysteresis and additional permanent demagnetization at zero stress. The test results may be interpreted in terms of domain characteristics. The piezomagnetic effect is discussed in terms of direct observations of domain behavior during cyclic loading carried out by Boyd et al. They studied the motion and nucleation of magnetic domains as a function of applied stress. Keywords: Rock mechanics; Magnetic fields. Prepared in cooperation with New England Research, Inc., Norwich, VT.