2. Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
Urban geophysical exploration differs greatly from the traditional geophysical survey. The urban geological environment is a natural environment which has been transformed by human activities, whereas the actual situation will be more complicated. In addition to natural geological bodies such as strata and structures, there are underground complexes such as underground pipe network systems, underground rail transit systems, and underground storage. Moreover, dynamic evolution, and even more significant changes will occur in the urban geophysical field in the process of urbanization (Chen Yong et al., 2003). Therefore, urban geophysical survey has the characteristics of shallow detection depths, high demands for detection accuracy, more interference factors and limited site (Li Wanlun et al., 2018). At present, there is not much experience in ultra-shallow seismic explorations with a depth of only a few tens of meters (or even a dozen meters) in the target layer at home and abroad, and theoretical research is not deep enough. The depth range of dozens of meters from the surface to the underground is a blind area of various artificial seismic prospecting, but it is of great significance to the research of fault location and activity, therefore, many shallow seismic prospecting practices and technical research was carried out in recent years to meet the needs of engineering construction and disaster prevention and mitigation (Zhu Jinfang et al., 2005). In the exploration of urban active faults, the methods for detecting active faults in Quaternary overburden areas can be divided into geological methods and geophysical methods. Shallow or ultra-shallow seismic methods are one of the important means for geophysical methods to investigate urban active faults (Deng Qidong et al., 2002, 2003). Due to the shallow geological conditions and the characteristics of the new tectonic movement, shallow or ultra-shallow seismic methods are not exactly the same as those used in conventional seismic prospecting. The reasonable data acquisition scheme is an important foundation to obtain high-resolution seismic profile, and the study of the propagation velocity of shallow seismic waves provides a theoretical basis for the determination of velocity reference value in data processing. In recent years, the study of the shallow reflection seismic method in urban active fault surveys and its examples were published, and the previous research results reflect the continuous improvement of the research level of shallow seismic method (Williams R. A. et al., 1995; Pan Jishun et al., 2002; Fang Shengming et al., 2002; Liu Baojin et al., 2002, Xu Mingcai et al., 2005, Duan Shengquan et al., 2005, Yang Xiaoping et al., 2007). In the applications of shallow seismic prospecting methods to determine the location of concealed active fault and the depth of its upper breakpoint. Some geophysicists have made useful explorations on the aspects of seismic wave excitations, data acquisition methods, indoor data processing and interpretation methods, and gained a lot of valuable experience. As a result, the quality of shallow seismic exploration profiles and fault location accuracy of urban active faults are continuously improving (He Zhengqin et al., 2010; Li Dahu et al., 2010; Yang Qiyan et al., 2015, 2016; Xu Hangang et al., 2016).
Qingdao is located in the southwestern part of the Jiaodong Peninsula and on the coast of the Yellow Sea. The neotectonic movement is dominated by uplifting, and the lower part of the Tertiary and Quaternary systems are missing. In the late Quaternary period, affected by tectonic action and sea levels rising, the Late Pleistocene-Holocene river and sea deposits, which were mainly composed of sand clay more than ten to twenty meters thick, were directly covered by weathered bedrock (mostly the granite). The southeastern part is mainly low-mountain hills, composed of granite. The bedrock forms a weathered crust of a certain thickness under long-term weathering, and weathered-slope deposits accumulate in the gentle zone such as the foothills. In the coastal harbor area, there are late Quaternary fluviomarine deposits. The northwestern part is mainly late Quaternary fluviomarine plains or bays, with Quaternary deposits having a thickness of 10m-30m. Therefore, seismic exploration in Qingdao belongs to the category of ultra-shallow seismic exploration. There are several NNE faults in the area, where activity is not very strong, the geological and geomorphological manifestations are not very clear, many sections are hidden under the Quaternary, and the exact location and activity of the faults are not very reliable (Fig. 1). Compared with the favorable detection environment in other regions, the Quaternary system in Qingdao is very thin, with large changes in lateral physical properties and poor vertical stratification. For different seismic geological conditions and detection purposes, reasonable field acquisition methods and acquisition parameters must be used (Williams R. A. et al., 1995). Therefore, data acquisition parameters should be carefully tested before formal data collection in a new work area (He Zhengqin et al., 2007). By comparing the different observing systems and collecting parameters in the Wanggezhuang fault in Qingdao, the high resolution seismic profile is obtained in the depth range of several meters to more than 20m, which provides a reliable scientific basis for the location of concealed active fault, the determination of breakpoint on fault, the layout of drilling section, and the determination of fault activity.
Qingdao and its adjacent areas belong to the Jiaodong-Liaoning fault uplift of the North China Platform, and the basement is the Proterozoic metamorphic rock series, which was consolidated by the Lüliang Movement. The platform began to develop in the Paleozoic, and was in the state of structural uplift for a long time, the strata from Paleozoic to Triassic are missing, resulting in a large area of crystalline basement being exposed. Generally speaking, mountains in Qingdao and its adjacent areas are not high, they are mainly low mountains, hills and plains, and high elevation peaks are mostly isolated. Generally, the area is divided into two geomorphic zones, the northwest and the southeast, with the Cangkou fault as the boundary. The northwestern terrain is relatively low, with mostly bays and the Quaternary alluvial plains. The eastern Shandong area is dominated by uplifting movement during the Neotectonic period, and the Quaternary strata in Qingdao and its adjacent areas are poorly developed. In many areas, the bedrock is exposed, and some areas are deposited with Late Pleistocene and Holocene deposits, and sediments of the late Middle Pleistocene are developed in local areas. The southeastern part of the urban area is mainly composed of granite hills, thick weathering and slope accumulation in the gentle areas such as the piedmonts, and there are the late Quaternary fluviomarine deposits in the coastal bays; in the northwest, there are the peneplains composed of the Mesozoic strata and the Quaternary fluviomarine deposits plain (or bay), with a thick Quaternary system, with the maximum thickness reaching nearly 30 meters.2 ULTRA-SHALLOW SEISMIC PROSPECTING 2.1 Characteristics of Ultra-shallow Seismic Prospecting in the Qingdao Area
(1) The objective layer is relatively shallow; the effective reflection wave is mostly within 70ms. In the time window where the reflection wave appears, there are strong noises of various kinds, especially the acoustic wave and the surface wave, in the latter tracks of the reflection wave received, a mixture of refractions and reflections occurs, which seriously affects the effective reflection wave.
(2) It is difficult to obtain effective waves because of the serious interference in the interference band near the source. The amplitude of reflected signals formed by different impedance interfaces is much smaller than the amplitude of interference waves.
(3) The sedimentary structure and stratum thickness in Qingdao and its vicinity have great lateral variations, and the velocity and frequency of reflected wave are greatly affected by these variations.
(4) The overburdens are relatively thin in Qingdao and its adjacent areas. There is usually only a strong weathering interface at the bottom of the Quaternary. Because the weathering degree of the weathering strata is gradual, the wave impedance difference between the weathering bedrock is small, and the vertical stratification in the Quaternary is poor. Weathering crust is often several meters thick at the top of bedrock. Some bedrock surfaces are even shallower. The wave impedance difference between weathering bedrock and overlying strata is not obvious.2.2 Design of Observation System and Selection of Acquisition Parameters
In view of the characteristics of the above-mentioned seismic exploration, this paper selects the Wanggezhuang fault (fault F4 in Fig. 1) as the experimental object, and in order to obtain the seismic section within the depth range of the Wanggezhuang fault from the near surface to the Quaternary bottom interface (about 30m deep), different methods and detection parameters are used on the same seismic line for the same depth range according to the working principle of low to high detection accuracy (crossing the F4 line near ZK3 in Fig. 1). The seismic sections of different detection parameters are named as TEST-a and TEST-b. Seismic data acquisition uses the SUMMIT digital seismograph produced by the German DMT Company. The seismic wave excitation uses manual tamping and manual hammering. The observation system parameters, seismic wave excitation and data acquisition parameters used for the same detection depth but different precision requirements are shown in Table 1.
On the ultra-shallow seismic records with different group intervals (Fig. 2), Fig. 2(a) and Fig. 2(b) are the common shot points of the ultra-shallow seismic records corresponding to TEST-a and TEST-b, respectively. Comparing with Fig. 2(a) and Fig. 2(b), it can be seen that a clear set of reflected waves can be obtained by using larger group intervals, smaller offset and larger energy sources, but the near source is seriously affected by interference waves, and the effective reflected waves can't be obtained in the ultra-shallow layer. The use of smaller group intervals, larger offsets and smaller energy can obtain effective reflection waves in ultra-shallow layers, and also can reduce the influence of interference waves, but the energy is weak, and the reflected wave signal is relatively weak.
With smaller group intervals and weaker source, the ultra-shallow effective reflection wave can be obtained with a good vertical stratification. Due to the short arrangement receiving, the smaller the group interval is, the slower the attenuation of the reflected wave energy is. Therefore, the influence of the source interference wave is also smaller, but the surface wave is relatively developed. It can be seen that the smaller group interval and smaller offset are beneficial to protect the reflected wave and delay the attenuation of the reflected wave, but also bring about the problem of surface wave development. Under the premise of ensuring the detection depth requirement, the weak source is beneficial to avoid the influence of source interference waves.2.3 Methods for Ultra-shallow Seismic Data Processing
Seismic data processing uses the Promax seismic reflection processing system. In the process of data processing, due to the serious near source interference to the original records of reflection waves, the strong far source reflection wave energy and weak interference energy, the teleseismic reflection wave records with high signal to noise ratio are selected to improve the resolution of seismic data. The data processing flow and methods mainly include: trace editing, static correction, true amplitude recovery, surface consistency amplitude processing, surface consistency deconvolution, two-dimensional random noise attenuation, f-k filtering, normal moveout correction (NMO), common midpoint stack (CMP), post-stack bandpass filtering, and post-stack denoising.
In this paper, different filtering and processing methods are used to improve the signal-to-noise ratio of seismic data. Fig. 3 shows the comparative analysis results and spectrum analysis results obtained by f-k filtering of the same recorded data under different conditions. Fig. 3(a) shows the original data and the corresponding spectral analysis results. It can be seen that in ultra-shallow seismic exploration, the seismic reflection wave is affected by shallow refraction, surface wave and acoustic wave, and the reflected wave signal is relatively weak (red circle in the figure), and the signal-to-noise ratio is not high. The spectrum analysis shows that the main frequency of the seismic data has two frequency bands: 50Hz and 100Hz. Fig. 3(b) shows the result of applying f-k filtering to the original data and its spectrum analysis. It can be seen from the figure that after applying f-k filtering, the surface wave and acoustic wave in the data are effectively weakened, the reflected wave signal is strengthened, and the signal-to-noise ratio is also improved. The spectrum analysis shows that the data frequency is 100Hz. At the same time, the results also show that the surface wave frequency range is close to the effective seismic reflection wave, so it is difficult to filter, and the data signal-to-noise ratio is low. Fig. 3(c) avoids the interference of the near-surface wave to the reflected wave by cutting off the first few signals of the data. With f-k filtering, the reflected wave signal is significantly enhanced, and the signal-to-noise ratio is further improved. The comparison results show that in shallow or ultra-shallow seismic explorations, with high signal-to-noise ratio data, the f-k filtering of seismic data can effectively remove interference waves and improve the signal-to-noise ratio of seismic reflection waves.
Fig. 4 and Fig. 5 show the seismic reflection time section and geological interpretation section of the lines TEST-a and TEST-b after data processing (in the figure, west points left, and east points right). In the seismic reflection time section of Fig. 4, rich formation boundary reflection wave groups appear in the time range of more than 100ms, there is a set of undulating reflection wave groups, composed of 2-3 strong phases; in the seismic reflection time section of Fig. 5, the rich formation interface reflection wave groups appear in the time range of 60ms or more, and there are two reflected wave groups with undulating changes, composed of 2-3 strong phases. According to the lateral variation characteristics of the reflected wave in-phase axis on the section and the relationship between the upper and lower reflected wave groups, a breakpoint on the section of this profile is explained, which is denoted as F in Fig. 4 and Fig. 5. The fault F is located at CDP423# on the line TEST-a and CDP87# on the line TEST-b, the reflected wave is in the same phase, and the fault is a west-dipping, high dip-angle reverse fault. According to the geological data of the survey area, the fault is judged as the Wanggezhuang fault.
Comparing the seismic reflection stack profiles of lines TEST-a and TEST-b, it is found that the line TEST-b has clear reflection in-phase axis in the ultra-shallow part, especially in the part over 30ms, which indicates that the small group interval can reveal more details of fault structure in ultra-shallow seismic explorations.3 DRILLING GEOLOGICAL SECTION AND COMPREHENSIVE SOLUTION
Around the fault breakpoint near CDP423# on the line TEST-a and CDP87# on the line TEST-b in the north of Wanggezhuang, a set of 7 boreholes were drilled, with depths ranging from 19.1m to 23.5m and hole spacings of 5m-10m. It is found that the fault offsets at the top of the bedrock and the bottom of the Quaternary by about 4m between ZK2 and ZK6, with the west side uplifting and the east dropping (Fig. 6). It can be seen from Fig. 6 that except for the bottom horizon, all layers of the Quaternary are continuously distributed, and the structure of the fault is not changed. According to the results of the previous Quaternary stratigraphic division (including the dating results), the stratum is of the upper Pleistocene series. Therefore, the Wanggezhuang fault was still active in the early Pleistocene.
(1) From the perspective of urban active fault detection, this paper discusses the preconditions for acquiring effective detection results of ultra-shallow seismic prospecting in complex tectonic settings by using the relevant theories and methods of geology, geophysics and other disciplines. Comprehensive analysis of seismic prospecting data in the Qingdao area shows that: ① In ultra-shallow seismic exploration, it is difficult to distinguish the reflection wave from the interference waves in the near-source track records, and a certain offset is needed to distinguish them clearly. ② The offset can't be too large, because the target layer is very shallow. If the array length is too long, the record is mostly refraction wave, so it is necessary to use short arrangement receiving and small group interval for data acquisition.
(2) On the basis of systematical study of ultra-shallow seismic data acquisition technology, data processing and interpretation methods, taking typical sections which can clearly distinguish the fault position as an example, it is proved that the ultra-shallow seismic reflection method can obtain the reflected signals of the depths of only more than ten meters on the basis of selecting suitable observation systems and data processing methods.The reflection signals of strata and most of the reflection profiles can clearly reveal the location and characteristics of ultra-shallow faults. The results of seismic exploration for the Wanggezhuang fault in Qingdao show that the method of ultra-shallow seismic reflection has good application prospects in the detection of urban active faults, especially in areas with shallow coastal overburden.
(3) Based on the study and interpretation of the ultra-shallow seismic reflection time sections of the Wanggezhuang fault in Qingdao, the exact location, geometric structure and activity of the Wanggezhuang fault are studied with the combined borehole profiling data of typical fault points. The study shows that the Wanggezhuang fault may also be active since the late Pleistocene.
This paper has been published in Chinese in the journal of Technology for Earthquake Disaster Prevention, Volume 13, Number 2, 2018.
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2. 中国地震局地球物理研究所，北京 100081