The study of normal geomagnetic background fields is the premise and basis for obtaining local geomagnetic anomalies caused by changes in underground pressure during gas injection and extraction in an underground gas storage (UGS) area. Measurements of the total magnetic field in an array consisting of sixty-three stations were acquired at a UGS and its surrounding areas. We completed field measurements at the stations using proton precession magnetometers (PPMs) with a sensitivity of 0.15 nT @ 1 Hz in April 2017 and March 2018. The geomagnetic background field model of the UGS area is calculated using the Taylor polynomial fitting method based on the data of all measured stations. A fourth-order Taylor polynomial model is appropriate for the UGS area according to a comprehensive comparative analysis of the mean square error and boundary distortion effect. The geomagnetic field distribution in the core area of the UGS can be better described by enlarging the network of repeat stations. The results provide a reference model for extracting the variations in the local magnetic field (LMF) caused by the underground pressure variations in the UGS area.

Hutubi underground gas storage (UGS), as the largest UGS in China, operates periodically by performing gas extraction in winter and injection in summer every year. To detect and study the geomagnetic anomalies that may be caused by gas pressure changes in a controlled manner due to the injection and extraction of gas, repeat measurements of geomagnetic total intensity were conducted across the UGS area. We calculated and analyzed the geomagnetic anomalies in the UGS area using data from two field surveys conducted from July to October in 2018. The fourth-order Taylor polynomial model (TPM) could best describe the background field of geomagnetic in the UGS area. Relative variations in the local geomagnetic field (LMF) above the UGS field ranged from –2.5 to 4.1% during gas injection. The complex anomaly pattern in the high-density array is similar to the piezomagnetic field calculated by the Mogi model. This paper attempts to provide a method to determine the regional geomagnetic anomalies on a small scale.

The tectonic processes leading up to an earthquake and the occurrence of the earthquake itself will cause local changes in the geophysical field (geomagnetic field, stress field, etc.). In this paper, the variation characteristics of the tectonic stress field (TSF) and local geomagnetic field (LGF) before and after the Yangbi Ms 6.4 earthquake are studied. The regional stress tensor damping inversion method was used to invert the TSF using focal mechanism solutions (FMSs). The change characteristics of the TSF before and after the earthquake were analyzed. An annual variation model of the LGF was constructed, and the variation of the horizontal vector was analyzed. The azimuth and plunge of the maximum principal compressive stress axis of the TSF in the epicentral region before and after the earthquake were −4.4° and 2.7°, 172.7° and 6.6°, respectively. The variations in the declination, inclination and total intensity of the epicenter one year before and one month after the earthquake were −0.20′ (0.07′), 0.29′ (−0.12′), and −1.7 nT (−1.9 nT), respectively. The epicenter is located at the boundary of the “weak variation region” of the horizontal vector. This research is of great significance concerning the TSF background and incubation mechanism of earthquakes.