Correction for Geomagnetic Secular Variation Effects: A New Method (Case Study of the Altai Geodynamic Network)

––The study focuses on detection of geomagnetic secular variation and the respective correction of tectonomagnetic data. A new technique is proposed for picking the secular variation component in the Earth’s main magnetic field recorded by precise measurements at 100 to 500 km sites on the surface. Long-period field variations presumably arise from fluid motions in the liquid core, at depths of 3000 km, whereas the sizes of observation networks are within 500 km. The sources of secular variation, irrespective of their configuration, are much deeper than those of tectonomagnetic anomalies located above the Curie surface depths of ~10 to 20 km. Therefore, the surfaces that represent the space distribution of secular variation must be smoother than the respective surfaces for tectonomagnetic anomalies. The problem is thus to separate the regional and local signals from the two types of sources located at different depths. The new method is tested using data of yearly geomagnetic measurements at more than 30 repeat stations of a ~120 km long geodynamic network in Gorny Altai spanning the period from 2004 through 2018. The secular variation pattern is reconstructed by quadratic interpolation. The precise data corrected for secular variation of the main field reveal previously hidden tectonomagnetic anomalies up to 12 nT. The 3 nT positive anomaly falls within the zone of surface deformation caused by the Mw = 7.3 Chuya earthquake of 27 September 2003.

IMPROVING ALGORITHM OF DETERMINING THE COORDINATES OF THE VERTICES OF THE POLYGON TO INVERT MAGNETIC ANOMALIES OF TWO-DIMENSIONAL BASEMENT STRUCTURES IN SPACE DOMAIN

In this paper, we present an improved algorithm based on Murthy and Rao’s algorithm to invert magnetic anomalies of two-dimensional basement structures. Here, the magnetic basement interface is approximated by a 2N-sided polygon with assumption that the bottom of the basement is the Curie surface. The algorithm is built in Matlab environment. The model testing shows that the proposed method can perform computations with fast and stable convergence rate. The obtained result also coincide well with the actual model depth. The practical applicability of the method is also demonstrated by interpreting three magnetic profiles in the southeast part of the continental shelf of Vietnam.

Crustal magnetic anomaly and Curie surface beneath Tarim Basin, China, and its adjacent area

Based on the geomagnetic field model NGDC-720-V3 (National Geophysical Data Center), the distribution of crustal magnetic anomalies and the Curie surface beneath Tarim Basin, China, and its adjacent area were investigated. The results show that the spatial distribution of the magnetic anomalies in Tarim Basin coincides with the regional tectonic structure, i.e., a basin sandwiched between mountain ranges. Shallow parts of the Curie surface are located in uplifted zones of the basin and correspond well to high values of heat flow. In contrast, deep parts of the Curie surface, having low values of heat flow, are distributed along the depression zones of the basin. The Curie point depth is consistent with the measured heat flow value, with a correlation coefficient of 0.65.