The Effect of Pressure and Deviatoric Stress on Rock Magnetism

• 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

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.