1. Introduction
The topics of this artificial seismic exploration consist of deep seismic reflection exploration and shallow artificial seismic exploration. The main purpose of deep seismic reflection exploration is to explore the location and deep development characteristics of faults in the target area, while that of shallow artificial seismic exploration is to explore the location and buried depth of buried faults in the target area, determine the distribution of buried faults in the basin, and preliminarily determine the latest strata of fault faults.
The southern section of the exploration area enters the Qilian Mountains area. This place is of complex topography, deep river cutting and developed V-shaped landform, making it difficult to lay the lines in the field. The riverbed is covered by thick gravel layer, but the floodplain and terrace bedrock are exposed, and the terrace is located at the source with good excitation effect. In the basin, the geomorphic unit is mainly alluvial fan. According to previous research results, the residual thickness of Cenozoic in foreland depression in the southern margin of the basin is nearly 1,000m, and it is mainly composed of gravel layer, which causes great difficulties in drilling and has obvious attenuation effect on seismic wave propagation.
2. Treatment Solution
Aries digital seismograph (Figure 1) and 60Hz triple-combination series geophone are used for this shallow artificial seismic exploration, with track spacing of 4m, excitation point distance of 16m, arrangement length of 480 tracks, asymmetric excitation in the middle, recording length of 2.0s, sampling rate of 0.5ms, coverage times of not less than 40 times, and vibroseis for excitation.
(a) Seismic instrument host (b) Data acquisition station (c) Data transmission cable
Figure 1: Arise Telemetry Digital Seismograph Made in Canada
For deep seismic reflection exploration, French-made 428XL digital seismograph (Figure 2) and 10Hz 12-combination series geophones are used, with track spacing of 2m, excitation point distance of 280m for small shot and 1,120m for large shot, arrangement length of 1,600 tracks, asymmetric excitation in the middle, recording length of 25s, sampling rate of 4ms, coverage times of not less than 50 times, and explosive source for excitation.
(a) Instrument cart | (b) Instrument host |
Figure 2: Main Components of Instruments and Equipment
3. Construction Situation
In this project, 4 shallow artificial seismic exploration survey lines with a total length of 31.308km are arranged in the southern margin of Wuwei Basin. There are 2 deep seismic reflection survey lines with a length of 65km (Figure 3).
Figure 3: Schematic Diagram of Survey Line Position
Note: The blue survey lines in the figure are the shallow artificial seismic survey line.
4. Achieved Accomplishment
The reflection structure characteristics of the lithosphere in the southern margin of Wuwei basin have been identified by the results of deep seismic reflection profile. CSS2 survey line shows that Qilian Mountain block and Alashan block collide with each other in the deep, and Qilian Mountain block has subduction characteristics, with jagged teeth at the junction and multiple groups of jagged reflection interfaces, which is an important symbol of stress release and ductile material migration caused by deep collision of the two blocks. The undulating shape of Mohorovicic discontinuity shown by CSS2 survey line is obvious, and its buried depth is about 47-51km.
The interpretation results of this shallow artificial seismic survey line show that the faults in the southern margin of Wuwei Basin develop along the basin edge, showing multiple groups of reverse faults inclined to SW, which is the most important fault structure system in the southern margin of Wuwei Basin. The faults continue to develop in the basin direction, but no faults of this fault system have developed in the control area of CSS3-1 line in Wuwei, indicating that the main active development areas of this fault are concentrated in the basin edge.
5. FAQ
Q1: Why choose explosive sources instead of vibroseis vehicle as the excitation of deep seismic reflection exploration in this project?
Although the field construction site for deep seismic reflection is relatively flat, which is conducive to the passage of vibroseis vehicles, due to the extremely developed gravel layer along the piedmont of the survey line, the seismic wave attenuation is very serious. In order to ensure the quality of collected data, explosive source excitation is finally selected.
Q2: What are the similarities and differences between deep seismic reflection exploration technology and shallow seismic reflection exploration technology adopted in this project?
Deep seismic reflection technology is one of the effective means to study the fine structure of crust and upper mantle. Its principle is the same as that of shallow artificial seismic exploration. It uses the reflection event composed of elastic wave reflection generated by interfaces with different physical properties to describe the geological structure characteristics such as interfaces and faults. However, the difference is that the purpose of deep reflection seismic profile is to obtain reflection images from middle and lower crust, Mohorovicic discontinuity and upper mantle, so the record length of profile is much longer than that of shallow artificial seismic exploration.
Q3: What are the key technologies for data processing of deep seismic reflection exploration technology?
The key technologies of data processing in deep seismic reflection exploration mainly include static correction, amplitude recovery, pre-stack denoising, deconvolution, frequency division and zoning, velocity analysis, post-stack denoising, migration and so on.
Q4: What advantages does Xi’an Institute have in the field of deep seismic reflection exploration compared with other companies?
In recent years, Xi’an Institute has carried out a number of deep seismic reflection exploration projects in the areas from the eastern plain areas to the western mountainous pebble areas, and accumulated rich experience. Moreover, the data acquisition and processing equipment is advanced, the data processing and interpretation software and hardware are complete, and the technical personnel structure is reasonable. Xi’an Institute has accumulated a good reputation in the industry.
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