Magnetic variations in the reconnaissance of sedimentary basins: Field procedure and generalized inversion of short‐period data from the Rio Grande rift

Abstract

The magnetic variation (MV) technique employs magnetic transients from natural sources in the magnetosphere to delineate geologic structures in the earth’s interior based on their electrical properties. By measuring only the magnetic field at each site, and not the electric field as required for magnetotelluric (MT) studies, a site can be set up quickly, and often in places which might be quite unsuitable for MT measurements. This generally allows one to execute MV surveys in culturally developed areas or in rugged and logistically difficult terrain at much closer site spacings than those used for conventional MT surveys. Procedures for acquiring and processing MV field data are straightforward, as are methods for inverting data to obtain plausible geophysical models. Using a new 2-D generalized inverse algorithm which employs singular value damping of the Lanczos (or SVD) inverse, we apply the MV technique to determine the basement topography beneath a sequence of 11 remote referenced MV sites from an east‐west profile transecting one of the sedimentary basins of the Rio Grande rift—the San Antonio graben in the southern portion of the Socorro Basin in central New Mexico. Band‐limited data at 50 and 63 s were adequate to delineate the major features of the basin: its lateral margins, its asymmetrical cross‐section with basement dipping sharply to the west, and a large vertical displacement along the western boundary fault of the graben. Our results suggest, therefore, that reconnaissance surveys can be optimized to capture only those data needed to resolve such features. This strategy significantly affects the cost‐effectiveness of the method as a complement to other geophysical techniques.