1. Introduction
There are dense cities in Guanzhong area, which is close to the east side of China's north-south earthquake tectonic zone. It is formed and developed at the intersection of four tectonic systems, namely the latitudinal tectonic belt, Cathaysian tectonic belt, Qiluhe E-type tectonic system and Longxi rotational shear tectonic system. It has a complex geological tectonic background, and the active faults in Weihe fault subsidence zone have the characteristics of early formation and high activity intensity (Figure 1). At present, there is no systematic research data on active faults in this area, which makes it impossible to scientifically identify their latest active age and evaluate their earthquake risk and harmfulness.
Figure 1: ATM image of Guanzhong area
2. Treatment solution
By using the shallow and deep reflection seismic exploration methods of P-wave reflection, 24 seismic exploration lines were arranged on 21 presumed faults in Guanzhong area (Figure 2). Aries digital seismograph (Figure 3) was used in seismic exploration. 60Hz triple combination serial geophones were used for shallow exploration and 10Hz six-series and double-parallel combination geophones were used for deep exploration. The shallow exploration was excited by two KZ-28 vibroseis (Figure 4) and the deep exploration was excited by explosive source (Figure 5).
Figure 2: Schematic diagram of fault distribution and seismic exploration line layout in Guanzhong area
(a) Seismic instrument host (b) Data acquisition station (c) Data transmission cable
Figure 3: Arise telemetry digital seismograph made in Canada
Figure 4: Vibroseis excitation system
Figure 5: Hole-forming and explosive excitation site
3. Work situation
In this shallow seismic exploration, there were 24 seismic survey lines, with a total length of 235.157km and a total of 11,320 physical points. The geographical coordinates of the construction area are in the range of N 34º0´-34º38´ and E 107º15´-110º8´, covering the major part of Guanzhong area and a part of Qinling Mountains, with an area of about 24,000 km2.
4. Achieved accomplishment
In this project, through shallow and deep seismic exploration technology, especially frequency division and time-sharing processing technologies used in the process of data processing technology, the activity characteristics, shallow and deep tectonic characteristics, buried depth and strike position of upper breakpoints of the Weihe faults and the northern Qinling faults in Guanzhong area were finally found out, and the relationship between shallow and deep buried old faults and main faults of various seismic lines in the area were found out (Figure 6). The results have been verified by field trench exploration, and the effect is good (Figure 7), which guarantees the seismic risk assessment in Guanzhong area.
Figure 6: Interpretation of the seismic time profile along line L1-1
Figure 7: Development of the active faults exposed by the exploratory trench
5. FAQ
Q1: What work does CCTEG Xi’an Research Institute mainly carry out in active fault exploration?
A: In terms of active fault exploration, CCTEG Xi’an Research Institute mainly carries out 2D/3D P/S-wave reflection exploration, refraction wave exploration, microseismic monitoring, etc. The exploration depth spans from several meters to tens of kilometers, and the working area covers lands and waters.
Q2: What are the outstanding advantages of CCTEG Xi’an Research Institute in this field?
A: CCTEG Xi’an Research Institute is one of the first major units in China to carry out special topics related to seismic activity fault exploration. Since it stepped into this field, it has successfully completed seismic exploration projects in over ten provinces and autonomous regions nationwide, and has a series of seismic exploration acquisition equipment, processing and interpretation software and hardware in the forefront of the world. At the same time, as a central enterprise, CCTEG Xi’an Research Institute also has a complete set of quality, safety and health management systems for implementing such projects.
Q3: Why didn’t we use other geophysical methods to explore the active faults in Guanzhong Area?
A: Based on years of practical research, P-wave seismic reflection multiple covering is the most effective and direct method to explore the underground buried faults. It has obvious advantages in suppressing interference and improving SNR of data by using multiple coverage technology, and has outstanding advantages in dividing sedimentary stratigraphic sequences with certain thickness and exploring geological structures such as buried active faults.
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