Aeromagnetic anomalies related to remanent magnetism in volcanic rocks, Nevada Test Site

Structural discontinuities and variations in the resistivity of near‐surface rocks often seriously distort dc resistivity and frequency‐domain electromagnetic (FDEM) depth sounding curves. Reliable interpretation of such curves using one‐dimensional (1-D) models is difficult or impossible. Short‐offset time‐domain electromagnetic (TDEM) sounding methods offer a number of advantages over other common geoelectrical sounding methods when working in laterally heterogeneous areas. In order to test the TDEM method in a geologically complex region, measurements were made on the east flank of Yucca Mountain at the Nevada Test Site (NTS). Coincident, offset coincident, single, and central loop configurations with square transmitting loops, either 305 or 152 m on a side, were used. Measured transient voltages were transformed into apparent resistivity values and then inverted in terms of 1-D models, Good fits to all of the offset coincident and single loop data were obtained using three‐layer models. In most of the area, two well‐defined interfaces were mapped, one which corresponds closely to a contact between stratigraphic units at a depth of about 400 m and another which corresponds to a transition from relatively unaltered to altered volcanic rocks at a depth of about 1000 m. In comparison with the results of a dipole‐dipole resistivity survey, the results of the TDEM survey emphasize changes in the geoelectrical section with depth. Nonetheless, discontinuities in the layering mapped with the TDEM method delineated major faults or fault zones along the survey traverse. Schlumberger resistivity soundings expanded to an AB/2 of 1220 m detect only the first interface mapped by the TDEM method. Schlumberger and large‐offset, frequency‐domain, sounding curves appear to be more distorted by lateral variations than the TDEM sounding curves.

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REMANENT MAGNETISM AS A CONTRIBUTOR TO SOME AEROMAGNETIC ANOMALIES

Geophysics ◽

10.1190/1.1439024 ◽

1962 ◽

Vol 27 (3) ◽

pp. 359-375 ◽

Cited By ~ 21

Author(s):

Kenneth G. Books

Keyword(s):

Remanent Magnetization ◽

Volcanic Rocks ◽

Horizontal Direction ◽

Dominant Factor ◽

Rock Body ◽

North Central ◽

Remanent Magnetism ◽

Aeromagnetic Anomalies ◽

Intrusive Rocks ◽

Central Montana

An investigation of some aeromagnetic anomalies in north‐central Montana shows that remanent magnetization is the dominant factor in anomalies over some rocks and a contributing factor in anomalies over others. In volcanic rocks, remanence is commonly south‐seeking down and has an intensity approximately ten times the induced intensity of magnetization. Remanence is shown to be the dominating factor in anomalies over volcanic rocks by the agreement between the profiles of an observed anomaly over a volcanic rock body and an anomaly calculated from remanence data for the same body. In intrusive rocks, the remanence is north‐seeking down and comparable in intensity to the induced intensity of magnetization. That remanence also contributes to the anomalies over some intrusive rocks is shown by the agreement between the profiles of an observed anomaly over an intrusive body and an anomaly calculated from the resultant of induced and remanent magnetizations. A consistent relationship between the axis of the anomaly, or direction between the anomaly high and low, and the horizontal direction of magnetization suggests that the direction of the anomaly axis can be used to indicate the horizontal direction of magnetization in this area.

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