Design And Implementation Of Dense Geomagnetic Observatory Array

As an important fundamental geophysical field,the geomagnetic field is widely used in geoscientific research,mineral exploration,military navigation,space weather monitoring,earthquake monitoring and forecasting,and other fields.In the research of seismic-magnetic relationship,more and more geomagnetic earthquake precursor anomaly information is discovered,but the reliability of the anomaly information is often limited by the number of observation points.The encrypted observation in the key seismic monitoring area currently mainly relies on mobile geomagnetic measurements,which cannot obtain timely information on changing magnetic field anomalies and affects the timeliness of seismic magnetic precursor anomaly information acquisition.In view of the high construction cost,large land acquisition area and construction difficulties of the existing traditional geomagnetic stations,it is necessary to develop a low-cost buried dense geomagnetic observation array.In this paper,we designed and constructed the world’s largest dense geomagnetic observation array in the southern section of the Xiaojiang Fault zone in Yuxi,Yunnan Province.The construction and operation of the station array show that the construction of measurement points is economical and fast,the network structure is simple and easy to implement,the observation data quality is excellent,and the spatial and temporal resolution of geomagnetic observation data is effectively improved.It provides strong support for the research of intensive geomagnetic monitoring and seismic-magnetic relationship in seismic hazard areas,active rupture zones and other key seismic monitoring areas.The research conducted in this paper and the innovative results achieved are as follows.(1)Research on the technical solution for the construction of station array measurement points under the low-cost targetThe observation environment requirements of station array measurement points are analyzed,and the process and method of measurement point survey for dense station array are proposed.From the perspective of temperature change,the burial depth requirement of simple buried instruments is theoretically analyzed,and the technical scheme of simple buried measurement point construction is proposed in combination with the cost requirement of measurement point construction.The observation facilities should be buried 1m deep below the permafrost layer,with a bin diameter of 1m,and covered with a 0.5m thick soil layer for insulation after wrapping with multi-layer waterproof plastic.The operation results of the measurement points show that the daily temperature difference of more than 98%of the measurement points is within 2℃,and the temperature protection effect is good.The evaluation of the overall operation status of the station array shows that the simple buried observation method produces high data continuity rate and stable background noise,which meets the needs of geomagnetic observation.(2)Design a wireless self-assembling network system for station array measurement points in the field without a public network environmentFor the environment that field observation does not have public network conditions,the self-assembled network transmission system based on mesh network is designed by using the flexible networking and simple structure of wireless selfassembled network technology.The goal of point-to-point transmission distance of more than ten kilometers and transmission bandwidth of not less than 100 kbps is achieved.To facilitate the unified management of measurement point instruments,management nodes are designed to unify power supply,timing and data acquisition for the instruments.The station array monitoring and management center is built to realize real-time data transmission and dynamic monitoring of station array measurement points.(3)Study the pre-processing and correction techniques for observation data anomaliesIn view of the actual situation that the dense observation array is susceptible to environmental disturbances,we identify and pre-process the observation anomalies based on the different data patterns exhibited by the environmental disturbances at the measurement points.Due to the simple installation of the measurement points,there are inevitable effects such as daily temperature difference and instrument burial error,so we use the linear fitting method to correct the temperature drift,and use the attitude model and genetic algorithm to correct the attitude error to process the data of the station array.After the correction,the correlation between the station array observation data and the reference geomagnetic station observation data is effectively improved.(4)Preliminary application of the observation data to the study of the electrical structure of the regional subsurface mediumBased on the high spatial and temporal resolution geomagnetic observation data from the station array,a preliminary application study of the electrical structure of the regional subsurface medium with an observation time of 2 months is conducted.Due to the long-term continuous observation of the array,the subsurface electrical structure model can be output in any time length such as day,month and year,and the time resolution is improved compared with the traditional mobile observation mode.The spatial resolution of the subsurface dielectric structure obtained by inversion in the dense array observation mode is about 2.5 km,which cannot be achieved in the traditional geomagnetic station observation mode,and the spatial resolution is significantly improved.The results show that the subsurface electrical structure remains stable during the seismic calm period.Large-scale subsurface low-resistance bodies are developed in the region,and the electrical structures at the two ends of the southern section of the Xiaojiang Fault zone are completely different.