Using the integrated geophysical methods detecting active faults: A case study in Beijing, China

    Using the integrated geophysical methods detecting active faults: a case study in Beijing, China Wang Zhihui, Cai Xiangmin, Yan Jiayong, Wang Jiming, Liu Yu, Zhang Lei PII: DOI: Reference:

S0926-9851(17)30101-5 doi:10.1016/j.jappgeo.2017.01.030 APPGEO 3200

To appear in:

Journal of Applied Geophysics

Received date: Revised date: Accepted date:

4 May 2016 28 September 2016 24 January 2017

Please cite this article as: Zhihui, Wang, Xiangmin, Cai, Jiayong, Yan, Jiming, Wang, Yu, Liu, Lei, Zhang, Using the integrated geophysical methods detecting active faults: a case study in Beijing, China, Journal of Applied Geophysics (2017), doi:10.1016/j.jappgeo.2017.01.030

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ACCEPTED MANUSCRIPT Using the integrated geophysical methods detecting active faults: a case study in Beijing, China

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Keywords: active paleomagnetism

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Wang Zhihui1,2, Cai Xiangmin3,4, Yan Jiayong1,2*, Wang Jiming3, Liu Yu4, Zhang Lei4 1 MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China 2 China Deep Exploration Center-SinoProbe Center, Chinese Academy of Geological Sciences, Beijing 100037, China 3 Beijing Bureau of Geology and Mineral Exploration and Development, Beijing 100195, China 4 Beijing Institute of geological survey, Beijing 102206, China

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Active faults in urban have a potential damage to citizens, because they can induce not only earthquakes, but also damage pavements, utilities, homes, businesses, factories and other manmade structures because of the slow, secular and differential slippage. Consequently, the researches of detecting active faults are of great significance. This paper proposes a set of geophysical methods to detect active faults by an example in Beijing, including gravity, controlled source audio-frequency magnetotellurics（CSAMT）, seismic reflection, DC resistivity and paleomagnetism (Natural remanent magnetization in rocks) to locate faults and discuss their activities. In proposed methods, gravity interpretation helps us obtain the distribution and characteristics of buried faults beneath the plain, the results of CSAMT, seismic reflection and DC resistivity reveal features and characteristics of faults from the deep to shallow part; paleomagnetism associated with radiocarbon dating help us analyze the fault slip rate; 3D seismic reflection interpretation shows the structure of two faults in the three-dimensional subsurface and the interaction of each other. Also, a few acquisition parameters, data processing methods and significant suggestions are mentioned.

ACCEPTED MANUSCRIPT Introduction

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The concept of the active fault was put forward by Lawson et al. (1908), Wool(1916), Willis(1923) and Lee(1926) successively. From then on, it has been paid more attention by a few of scholars from different countries. Apart from inducing earthquake, active faults are the source of heavy damage to pavements, utilities, homes, businesses, factories and other manmade structures (Saribudak, 2006). Traditional methods to identify these faults include aerial photographs and field mapping, subsurface borehole data on both the footwall and hanging wall of the faults. However active faults are generally covered by different thickness of Quaternary unconsolidated sediments in the basin or plain, the objects of detecting are buried faults. In this case, those traditional methods are usually out of work, but geophysical exploration such as resistivity, gravity, magnetics, conductivity and seismic reflection play an important tool for detecting active faults. Active faults are closely related to land subsidence, ground fissures (Jia and Guo, 2007) and karst collapse in Beijing. Some of faults are currently active and distributing throughout the Beijing plain (Wang et al., 1990; Xu et al., 1992; Che et al., 1994; Xiang et al., 1996; Jiang et al., 2000; Zhao et al., 2004; Qiu et al., 2007; Zhang et al., 2008; Li et al., 2010; He et al., 2013; Zhao et al., 2015; Jiao et al., 2006; Bai et al. 2014; Zhang et al., 2014a, 2014b, 2014c). And many authors have proposed geophysical methods in detecting active faults in Beijing (Shao and Zhang, 1979; Su et al., 2000; Zhang et al., 2006; Gao et al., 2007; Wang et al., 2007; Zhang and Zhao, 2007; Li et al., 2008; Liu et al., 2010; Hu et al., 2011; Hou et al., 2011; Wang et al., 2011; Yang et al., 2011; He, 2013; Xia et al., 2013; Zhao C. B. et al., 2013; Zhao Y. et al., 2013; Dong et al., 2014; Li et al., 2014; Tian, 2014; Yong et al., 2014). In this paper, we choose some typical profiles to discuss the results and applicability of different geophysical methods in the project of special geological survey and monitoring active faults in Beijing plain.

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Geological setting and background

Figure 1. Seismological setting overlaid with survey areas and lines. The historical earthquake data (date from A.D.294 to A.D.2014) were loaded from China Seismic Network Website ( http://www.ceic.ac.cn)

The Beijing plain is located in the northwestern part of the North China Plain and surrounded by the Yanshan Mountains to the north and the Taihang Mountains to the west. The long histories of development and evolution in tectonics have formed the features of basin-range tectonics and multi-sets of fault systems with different ranges. Generally, they can be divided as NNE-NE, NNW-NW and approximately E-W trending basins and faults (Ran et al., 2001). The most prominent active-faults trend in the northeast and extend about tens of kilometers. Then, it is the NW-trending active faults which are perpendicular to the former and smaller on the surface than the former and discontinuous. These faults control the regional tectonic-geomorphology, Quaternary geology and neotectonics, and caused several historically strong earthquakes (Figure 1).

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Figure 2. Different geophysical methods survey line(area) location

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Geophysical methods 1. Gravity

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Gravity exploration is usually used for obtaining the characteristics of tectonic and geological setting, especially on the greenfield, due to the relatively large density differences between Quaternary and others strata. In our survey, gravity explorations were conducted to obtain the overview of distribution and characteristics of active faults in Beijing plain. In data processing of extracting structural information, we attribute such effort to the edge enhancement and detection of gravitational fields (Yan et al., 2015). Figure 3 gives an overview of the structural edge from the shallow to the deep in the whole plain. It shows the dip angle at the different part and the dynamical process, and also gives the suggestion with gravity profile (point spacing 50m) to arrange others geophysical methods’ lines. As indicated by red rectangle in Figure 3, Xiadian faults have low dip angle structures in the northeast and southwest part, but steep dip angle structures in the middle segment. Another representative survey was conducted on the intersection of two faults. The object is to gain the horizontal characteristics and intersected relationship of the two faults, simultaneously, contribute to assign the 3D seismic survey area and decrease the survey area, because 3D seismic is so expensive for engineering geological investigation. Figure 4 represents Bouger gravity (a) and its first vertical derivative (b), obviously, Huangzhuang-Gaoliying fault were truncated by Nankou-Sunhe fault and the strike in the intersection has been changed from North-North-East to South-North direction, which certified the previous research result and also was certified by the following 3D seismic reflection results.

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Figure 3. Multi-scale edge detection of gravity anomalies in Beijing plain and its neighborhoods

Figure 4. The result of gravity exploration overlaid with interpreted faults. Bouger gravity (a) and its first vertical derivative

ACCEPTED MANUSCRIPT 2. CSAMT

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CSAMT is a frequency-domain electromagnetic sounding technique (Goldstein and Strangway, 1975) which uses a fixed grounded dipole or horizontal loop as an artificial signal source. And it is similar to the natural-source magnetotellurics (MT) and audio-frequency magnetotellurics (AMT) techniques; the chief differences center around the use of the artificial CSAMT signal source at a finite distance (Zonge and Hughes, 1991). As CSAMT can provide a stable, dependable signal, and greater detecting depth than DC resistivity, it is applied to map the subsurface structures of less than 2km to 3km depth in detecting active faults after gravity exploration. In our survey, we adapted scalar measurements with equatorial dipole array (Edwards, 1977) and simultaneously measured the electric field in x-direction (Ex) and the magnetic field in y-direction (Hy) with the electrode distance of 50m. Figure 5 illustrates the resistivity inversion result together with a geological interpretation and two drills profiles. The horizontal coordinate indicates station number, while the vertical coordinate represents the detection depth. It is obviously observed that the value of resistivity increases with the depth, low-resistivity in shallow part and high-resistivity in deep part. On the right hand of station 158, the resistivity cross section shows two different layers. The upper layer represents unconsolidated sediments in the Quaternary formation and the thickness of the upper layer varies from 500m to 900m. The resistivity anomaly in the lateral variations indicates that sediments are inhomogeneous in horizon and suddenly changes at station 198, which shows that a fault (F1) exists. The lower layer is a high-resistivity layer, composed of bedrock of Ji-xian formation. On the left hand of station 158, the resistivity cross section shows three different layers. The upper layer as well as the right represents unconsolidated sediments in the Quaternary formation, and the thickness varies slightly, from 300m to 350m. As the thickness on both sides of station 158 is apparently different, the other fault (F2) was suggested. Two holes (one at station 120 and the other at station 204) had been drilled after the 2D resistivity inversion analysis was executed. The results of two cores verified the CSAMT interpretation of the Quaternary thickness and improved the interpretation accuracy of detecting faults. As to geological interpretation of Jurassic and Ji-xian formation depicted in Figure 4, we referred to the previous geological knowledge. The result also indicates that the crustal in the region was uplifted and some strata are eroded before the Jurassic period and the subsidence on the leftward of station 158 occurred at the Jurassic period and was uplifted again before the Cretaceous period. Until the Quaternary, the whole region occurred subsidence. Because of the difference of sedimentation rate, the sag was formed between station 158 and 198 and controlled by the two faults.

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Figure 5. CSAMT resistivity inversion section and geologic interpretation map

3. Seismic reflection

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Seismic reflection exploration has been proved to be the most effective geophysical tool for active faults exploration in China (Liu et al., 2010; Wang et al. 2011). Compared with other geophysical methods, it has a higher precision and resolution in horizontal and vertical direction and the detecting depth attained from deep seismic reflection profiling (Sato et al., 2009; Liu et al, 2009 and 2010) to ultra-high resolution seismic reflection profiling (Karastathis et al., 2007; Kaiser et al., 2009). In our survey, a few hundred of 2D profiles across main faults were conducted to locate active faults accurately above bedrock surface after CASMT exploration and a few square kilometers of true-3D seismic reflection were conducted to solve the two faults’ intersection relationship and retrieve three-dimensional structures. The acquisition parameters and data processing scheme of 2D and 3D seismic reflection are showed in Table 1, Table 2 and Table 3, respectively. Subsequently, some typical results are chosen to illustrate the detection effects. Table 1. Seismic acquisition parameters of 2D seismic reflection for active faults 96

Minimum/Maximum offset

20m/495m

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Number of active channels Nominal source interval

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Recording length

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One vibroseis (28 tons)

Geophone type

Three cascade vertical 60Hz per set

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Orthogonal (8 receiver lines and 12 5m×10m

Number of active channels

96×8

Maximum offset

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Nominal source interval

20m

Shot line interval

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Maximum inline offset

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0.5ms Two set of vibroseis (28 tons) 6 Vertical 60Hz(three cascade and two parallel)

Table 3. 2D and 3D seismic reflection data processing scheme for active faults

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2D scheme

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3D Scheme Prestack Trace editing AGC Refraction statics Surface-consistent amplitude compensation Frequency filter and F-K filter

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Deconvolution

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Velocity analysis

NMO correction

NMO correction

Residual statics

Residual statics

CMP stacking

DMO velocity analysis

Poststack Signal enhancement Band-pass filter

DMO stacking Random noise attenuation Migration Time-variant filter Amplitude balance

3.1 2D seismic reflection A profiling across Shunyi fault is chosen to illustrate the accomplishment of seismic reflection in detecting active faults. As indicated in Figure 6, seismic reflections from Quaternary have well continual appearances, however, the discontinuity exists at station 770 and the displacement between hanging wall and footwall happens as a fault (F1) slipped. Obviously, there is a high-amplitude reflection at 0.6s and it represents

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the bottom of Quaternary interface. At station 534, reflections from both sides have different characteristics in amplitude and phase. Seismic reflections are also disconnecting and have 0.1s time movement. An interesting seismic wave is indicated by white arrow between 0.8 and 1.0s at the lower left corner of Figure 5. We prefer to interpret it as seismic diffraction from the F2 fault plane, because seismic reflections on both sides of have a better continuity on the stack profiling and we can exclude it from seismic reflections according to the prior geological knowledge. However, “diffraction” is not visible in shot records and time- or depth-migration are not applied for this profiling, so it maybe also reflection wave from the side of survey line and can been solved only in 3D seismic survey.

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Figure 6. 2D seismic reflection profiling and geologic interpretation map 3.2 3D seismic reflection 3D seismic reflection exploration has been widely used for oil, gas and coal exploration and has got better results in imaging subsurface structure. Fortunately, we also got supported funds to accomplish this project to solve the two faults’ intersection relationship and retrieve three-dimensional structures. According to the previous seismic geometry and data processing scheme stated, we attained the data volume of pre-time migration (Figure 7a, 7c). Compared to 2D survey line, the view of subsurface in all directions is retrieved and faults are easy to be identified. In order to interpret exactly and get the relation between seismic reflections and strata, a synthetic seismogram based on acoustic logging and density logging was made. As illustrated in Figure 7b, different seismic reflections (T01-T08) correspond to different strata interfaces and have a strong resemblance with seismic record. Although the core did not intersect bedrock, the interface of Quaternary and bedrock can be tracked easily, because strong impedance difference generates high-amplitude.

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Figure 7. 3D seismic pre-migration data volume(a), synthetic seismogram(b) and the volume sliced at

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the interpreted bedrock depth (c)

Two faults can be identified in Figure 8, one fault strike extends towards northwest and the other nearly towards north-south. Also, the north-south fault was intersected by the northwest one, the sag was formed in the northeast of area due to the jointed settlement. Time slices revealed the space configuration of structure and the variation of lithology, for example, the west segment of the northwest fault in time slices of 200ms and 250ms is less distinct than in time slices of 300ms, 350ms and 400ms, and disappears in the time slice of 500ms, which can been interpreted that the lithology varied in depth and seismic reflections originated from Quaternary, bedrock and the interior of bedrock respectively. Whereas a set of reflection wave originate from the interface of Quaternary and bedrock in the time slice of 550ms shows stronger amplitude than others slices. As the white arrow shows in the time slice of 550ms, the high-amplitude from different time slice can locate faults in different depth. The 3D seismic results are not only accordance with gravity exploration and retrieve the subsurface structures, but provide the formation mechanism of the sag in the northeast, which have a great significance to the prevention of geological disasters in this area. Some interesting circular structures indicated by grey arrows are identified in time slices

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of 300ms, 500ms and 550ms, which are considered to be related to buried hill. If it was true, the result will contribute to understand the kinematics mechanism of the two faults.

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Figure 8. The different time slices with 50ms interval

4. DC resistivity

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The multi-electrode resistivity system has been widely applicable to address a variety of hydrogeological, geotechnical and environmental applications (Toll and Hassan, 2012). Compared with traditional instruments, it has several desired advantages (Tsourlos, 1995; Dahlin, 2001): (1) speed up data acquisition process; (2) improve the resolution and confidence through collecting large data sets to construct 2D and 3D images; (3) offer the flexibility to choose a suitable electrode arrangement for a particular problem, which in turn reduces the efforts and laborious electrodes switching using manual systems. Taking into account these advantages and the shortness of seismic reflection imaging at the depth shallower than 50m, we take the multi-electrode resistivity system as a complement of seismic exploration to track the buried growth fault in the shallow part. A series of trials were conducted to choose geometry, electrode spacing and cable length. To balance the resolution and penetration depth, we ultimately chose Wenner geometry, 2m electrode spacing and 120 electrodes. Figure 9 illustrates a typical example, the horizontal coordinate also indicates station number and the vertical coordinate represents the detection depth. It is easy to make out that there are two electrical layers in this section and the value of resistivity increases with the depth. Moreover, electrical characteristics happen to change at station 294. On the left hand, the upper low-resistivity layer and lower high-resistivity

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layer are good in appearance; on the right hand, the lower high-resistivity layer has an irregular shape and the anomaly center is lower than the left one. These all attribute to the fault existed at station 294. However, DC resistivity prospecting as well as others geophysical methods based on potential field theory has the volume effect which decreases the detecting resolution in depth. By numerical modeling of the buried growth fault, we conclude that the interface of different stratum does not just site on the interface of different resistivity values, but below the interface a little. Although this qualitative analysis is not complete in theory and has a certain resolution error, it has got a better detecting depth and resolution than seismic reflection exploration in shallow part.

Figure 9. The resistivity inversion section and geologic interpretation map

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5. Paleomagnetism

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All geophysical methods stated above are intent to locate the fault and unable to provide the information of the fault slip rate. Paleomagnetism associated with radiocarbon dating has been certified as an effective tool for researching the fault slip rate (Zhao et al., 2013; Zhang et al., 2014a, 2014b, 2014c; Bai et al., 2014) To get the vertical displacement between hanging wall and footwall at different periods and discuss the mechanics of the fault since Quaternary, two cores at least are usually conducted on the hinging wall and the foot wall respectively after locating faults accurately using the integrated geophysical methods. We take drill 17 and drill 18 as an example. The sample, test and thermal demagnetization have been depicted by Zhang et.al (2014b). The contrast of drill 17, drill 18 and the geomagnetic polarity time scale (Cande and Kent, 1992) is shown in Figure 10. The Late Miocene-Early Pleistocene boundary (M/G) depth of drill 17 (hinging wall) and drill 18 (foot wall) are 309m and 192.5m respectively; the Early-Middle Pleistocene boundary (B/M) depth is 98m and 68m. Based on the result of Optically Stimulated Luminescence (OSL) and the sediment characteristics, the lower boundary of the Late Pleistocene of drill 17 and drill 18 are defined at the depth of 25.7m and 21.6m respectively; the Pleistocene–Holocene boundary at the depth of 1.0m and 0.8m. Accordingly, we got a conclusion that Nankou-Sunhe fault moved vertically at the speed of 0.065mm/a, 0.046 mm/a, 0.03mm/a, and 0.11mm/a during the Early, Middle, Late Pleistocene and the Holocene, which shows the fault slip rate is accelerating. Some attention should been paid on this fault and some actions should been taken to reinforce buildings and protect Infrastructures across this fault. As the slip rate is calculated relative to the differential movement of the hanging wall and footwall, It is important to exclude others uncertainties to ensure the accuracy of the calculation. The first and the most important are to ensure the number of samples

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and testing accuracy. Then, the comparative analysis of lithology of borehole section should be careful, especially for the alluvial fan area. Because the sediment source with the same lithology may be different. The last is to eliminate uncertainties caused by geological structure, for example, the calculation is not exact when the stratum is not horizontal. A method to degrade this effect is that decreasing the distance between bores.

Figure 10. Geomagnetic polarity timescale and polarity events of drill 17 and drill 18(after Zhang et al., 2014b)

ACCEPTED MANUSCRIPT Discussion

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The integrated method helped us to solve the involved problem, but there are some problems and suggestions to be discussed as follow yet. (1) Seismic reflection has better resolution than other geophysical methods, but is limited by the exploration depth range due to the absence of deep seismic reflection profiling and ultra-shallow seismic reflection profiling. This mainly attribute to the shortage of supported funds and dynamite forbidden in Beijing. Many papers published have proven that the shallow information can been gained by little receiver spacing (20cm) and a specific explosive device (hundreds of frequency) used in exploration. The result equivalent to the ground-penetrating radar can image in the subsurface of 1m to 6m range (Barlach F., 2015). (2) There are less physical properties of rock sample from Quaternary, so we cannot confirm and constrain each geophysical interpretation result. (3) Survey lines are not usually orthogonal with faults strike because of manmade structures, which obstacles the interpretation accuracy. (4) We like to take faults as a tree, and it usually has some secondary faults as branches. To research the activity of a fault, we must get the deep structure and its dynamic characteristics to make sure the extent of the fault in the ground and locate the main fault and each fracture, rather than detecting merely the shallow part.

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Conclusion

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We propose a series of geophysical methods to locate faults from region/depth to local/shallow area and discussed the activity of main faults by paleomagnetism associated with radiocarbon dating. In the basis of these results, (1) we improve the accuracy of locating active faults and re-locate some faults, (2) make a great deal of insight into the structure and tectonic of main faults, especially for the intersection of Nankou-Sunhe fault and Huangzhuang-Gaoliying fault. (3) draw the conclusion that different faults are accelerating to slip since the Holocene.However, some methods still have the potential to be improved as mentioned in the previous section and the processing technology of denoising in a heavily urban environment should be improved. There are all forward us to do better in the future.

Acknowledgement Thanks are firstly given two anonymous reviewers and editors for their help comments on this manuscript. Then we would like to express our gratitude to Prof. Luo Shuiyu, Xu Mingcai and Huang Lijun from Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Science, Prof. Xu Deshu from Beijing Explo-Tech Engineering Co., Ltd., Liu Guimei from Surveying and Maping Co., Ltd of Hebei Jiuhua, Niu pengcheng from Research Institute of Coal Geophysical Exploration of China National Administration of Coal Geology, Ding Lianjing from Beijing Institute of Geo-exploration Technology, and others who were involved in the whole process of geophysical data acquisition and processing. Thanks are also given to all staff from Beijing Institute of Geological Survey for their support to complete this manuscript. This work is supported by the National Natural Science Foundation of China under Grant 41574133 and

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area-Interpretation of the flower structures from seismic sounding data

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