Application of microtremor survey method in detection of urban land subsidence

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Application of microtremor survey method in detection of urban land subsidence



Urban land subsidence can easily lead to ground cracks or even subsidence, causing safety accidents. In the urban environment, the human interference is strong, and the safety and environmental protection requirements are high, which makes the traditional geophysical prospecting methods unable to be effectively carried out in the city. Micro-motion prospecting technology is a passive, efficient and environment-friendly geophysical prospecting method, and it can detect underground geological conditions in the city's strong interference environment. Xiegang Primary School in Hefei City was selected as the research site, focusing on the applicability and effectiveness of the fretting method in the detection of land subsidence. According to the collected fretting data, the FK method is used to extract the dispersion curve, and then the inversion is carried out to obtain the underground shear wave velocity structure, and then the underground geological conditions are understood, and finally the drilling is used for verification. The research results show that the micro-motion exploration technology can effectively detect the location and scale of the underground uncompacted soil, thereby eliminating the occurrence of secondary accidents in the subsidence area.

Key words: Fretting exploration ; urban land subsidence ; unclassified entity ; Cause of subsidence ; FK method ; urban geology


Urban land subsidence is liable to induce ground fractures or even collapse,thus causing safety accidents.However,traditional geophysical methods cannot be applied in cities due to the strong human interference and high requirements for safety and environmental protection in the urban environment.The microtremor survey method is a passive,efficient,and environment-friendly geophysical method and it can be used to detect the underground geological conditions in the urban environment with strong interference.Taking Xiegang Primary School in Hefei City as the research site,this paper focuses on the applicability and effectiveness of the microtremor survey method in detecting the causes of land subsidence.Based on the microtremor survey data collected,the dispersion curve was extracted using the F-K method and then the underground shear wave velocity structure was obtained through inversion,thus revealing the underground geological conditions.Finally,verification was conducted through drilling.The results show that the microtremor survey is an effective geophysical method for detecting the information such as the position and scale of underground unconsolidated soil,and thus the secondary accidents in subsidence areas can be eliminated.

Keywords microtremor survey ; urban land subsidence ; unconsolidated soil ; land subsidence reason ; F-K method ; urban geology

0 Preface

Land subsidence is a kind of downward movement of local rock and soil mass that causes the surface elevation to decrease due to the loss of groundwater, the loosening of the underground soil layer, and the consolidation and compression of the ground under the influence of human engineering and economic activities[1], which has occurred in many cities at home and abroad Different degrees of land subsidence 2 , 3 , 4 , 5 , 6 ] lead to damage to residential buildings, secondary geological disasters such as ground fissures and even ground subsidence, which seriously threaten the safety of people's lives and property 7 ] .

The degree of looseness of the underground soil seriously affects the propagation speed of its mechanical wave 8 ] . Micro-motion prospecting technology is also called passive surface wave method (passive surface wave method), which is a kind of surface wave prospecting method. In the 1950s and 1960s, Aki 9 ] and Capon 10 ] used the spatial autocorrelation method (SPAC method) and the frequency-wavenumber method (FK method) to separate the surface wave from the micro-motion signal and extract the surface wave dispersion curve. Afterwards, seismologists conducted a systematic research on this method 11 , 12 , 13 , 14 ] , so that this method can be used to solve practical geological problems. On the basis of predecessors' research, Okada 15 ] systematically proposed the micro-motion exploration method after more than ten years of practice and research, thus making the micro-motion method move from theory to practice. Domestic researchers introduced the micro-motion exploration method in the late 1980s and early 1990s 16 , 17 , 18 ] , and applied it to the research work of geothermal resource exploration and shallow engineering geophysical prospecting 19 , 2021 ] , due to the limitations of early data processing technology, it has been rarely used in the field of geotechnical engineering investigation. After 2010, this method has made great progress, and its application fields have been further expanded. Currently, it is used in geotechnical engineering investigation 22 , 23 ] , geological hazards 24 , 25 , 26 ] , geothermal exploration 27 , 28 ] and urban geological survey 29 , 30 , 31 ] , but there is no research case in land subsidence detection. In this paper, using the relationship between the degree of looseness of the underground soil and the propagation velocity of mechanical waves, the fretting exploration technology is applied to the detection of the cause of land subsidence, and its application effect is studied.

1 Micro-motion exploration technology

1.1 Basic Principles

Micro-motion signal is a kind of natural source signal, which carries rich information of subsurface structure. There are two main sources of signal: one is the vibration brought by human daily production and life, the frequency is generally greater than 1 Hz; the other is various natural phenomena, such as rivers, waves, tides, etc., the frequency is generally less than 1 Hz. The micro-motion signal has great randomness, which can be described by a space-time stationary random process 9 ] . Although the source of the micro-motion is random and the signal of the micro-motion is also random, due to the multiple reflection and refraction of the wave, the micro-motion accumulates information reflecting the inherent characteristics of the site medium during the propagation process. The inherent information that does not change with time can be used to invert the parameters of the underground medium through the analysis of micro-motion signals 30 ] .

The specific process of micro-motion exploration is: according to the field exploration target, select the appropriate acquisition system, including station arrangement, station number, etc.; then extract the surface wave dispersion curve from the micro-motion signal, and the surface wave dispersion curve There are many methods, the most commonly used are the spatial autocorrelation method and the frequency-wavenumber method; after obtaining the dispersion curve, the subsurface apparent S-wave velocity can be calculated by inversion or empirical formula, so as to obtain the structure of the underground soil layer. information 32 ] . The specific process is shown in Figure 1.

figure 1

Fig. 1   Schematic diagram of micro-motion exploration technology principle

Fig.1   Schematic diagram of the principle for microtremor survey

1.2 Research on extraction method of dispersion curve

The core step of fretting data processing is the extraction of dispersion curve. At present, the most commonly used methods for extracting surface wave dispersion curves in fretting exploration are SPAC method and FK method. Due to exploration in urban environment, it is difficult to find a circular array layout site that can meet the SPAC processing method, so linear arrays are generally used, and FK is more suitable for the processing method. The main processing flow is shown in Figure 2 .

figure 2

Fig. 2   Schematic flow chart of extracting surface wave dispersion by FK method

Fig.2   Flow chart of surface wave dispersion extraction by F-K method2 Application Cases

Hefei Xiegang Primary School was completed in 2005. The teaching building covers an area of ​​2,550 m 2 . The main part is a 5-story frame structure, with a total longitudinal length of 39.4 m, a total horizontal width of 20.4 m, and a building height of 18 m. Since the seismic strengthening of the teaching building in 2010, the use conditions and environment have not changed, and the use load has not changed, and there is no history of renovation.

Since 2015, ground deformation has been found in and around the campus, and ground subsidence monitoring has been carried out. Since 2019, the ground deformation has been particularly obvious. Many buildings in the school (enclosed walls, equipment rooms, etc.) have cracked, and the ground deformation of the playground is particularly serious. , The surrounding residential areas hardened, and the road surface cracked to varying degrees. There are no obvious cracks in the main teaching building, but the ground part in contact with it sinks obviously, and cracks appear in some tiles on the wall. The situation of ground subsidence and cracking is shown in Fig. 3.

image 3

Fig. 3   Settlement and cracking in the study area

Fig.3   Settlement and cracking in the study area


According to the ground cracking situation at the site, the author laid out the micro-motion exploration section, and the distribution of measuring points is shown in Figure 4. The micro-motion data is collected, which is mainly used to study the relevant information of the underground rock and soil mass that causes ground cracks.

Figure 4

Fig. 4   Fretting exploration points and distribution of verification boreholes

Fig.4   Location of microtremor survey and verification boreholes

2.1 Micro-motion field construction and data processing method selection

Considering the detection effect and the limitation of site conditions comprehensively, the micro-motion data acquisition array in this study adopts a 7-point linear arrangement, that is, the detection points are in the form of -4 m—-2 m—-1 m—0 m—1 m—2 m— 4 m symmetrical 7-point calculation. The schematic diagram of the data acquisition system is shown in Figure 5 .

Figure 5

Fig.5   Schematic diagram of 7-point linear micro-motion array

Fig.5   Schematic diagram of 7-points linear microtremor array


The method of extracting the surface wave dispersion curve of the micro-motion exploration data adopts the FK method, which has no strict requirements on the array formation. According to the processing method, the data processing program is compiled, and the micro-motion data collected in the field are mapped and the dispersion curve is extracted to obtain the dispersion curve of each exploration point. As shown in Figure 6 , it is the original data and dispersion curve of the No. 12 point of the No. 7 section in the test.

Figure 6

Fig.6   Original waveform (a) and dispersion curve (b) of point 12 on section 7

Fig.6   The original waveform (a) and dispersion curve (b) for the 12th point of No.7 profile

2.2 Geological overview of the study area

The geomorphic unit of the study area is the Jianghuai wavy plain, and the micro-geomorphology is the second-order terrace of the Nanfei River 34 ] . During the geological history period, due to denudation and accumulation, a wavy plain with alternate hills and depressions and undulating ridges and farms was formed, and the micro-landforms showed gentle slopes, hill slopes and slope hills, which are the main landform types with the widest area in and around the study area.

According to the previous engineering geological data, as shown in Figure 7, miscellaneous fills are widely distributed in the shallower than 2 m underground in the area, and the thickness of miscellaneous fills is not uniform, and can reach about 3 m in some areas; sporadically distributed in the study area The silty clay layer has strong heterogeneity in the underground soil; according to previous drilling, the layer of strongly weathered rock is about 20 m underground.

Figure 7

Fig.7   Engineering geological section of the study area

Fig.7   Engineering geological profile of the study area


The groundwater types in the study area are mainly stagnant water and confined water. Among them, the stagnant water in the upper layer mainly occurs in the miscellaneous fill soil, and the main sources of recharge are atmospheric precipitation, surface water, and seepage water from surrounding pipelines. The main discharge methods of groundwater are evaporation and runoff. The pressurized water mainly occurs in the underground sandy clay, silt and fine sand interbedded with sand, and the recharge source is mainly lateral recharge, the water volume is average, and it is pressure-bearing.

2.3 Analysis of detection results

Judging from the obtained fretting detection profiles, there are many fretting anomalies in Section 7 ( Fig. 8 ), and the coincidence degree with the field ground fractures is relatively high. The following first analyzes the fretting exploration of Section 7 and the drilling of verification drill hole JZ5. In case of ground conditions.

Figure 8

Fig. 8   Anomaly map (a) of micro-motion exploration No. 7 profile and physical photo (b) of drill hole JZ5 verification

Fig.8   Contour maps of No.7 profile(a) and physical picture of JZ5 borehole(b)


There are many low-velocity anomalies in the No. 7 section of micro-motion exploration. Considering the impact on the ground structures, the anomalies below 20 m underground are mainly investigated. Among them, the 7-2 anomaly is near 10 m underground between points 17 and 18, and the scale is not large. Large, about 1.5 m thick.

In order to verify this abnormal situation, JZ5 borehole was drilled into the soil layer 12 m here. Judging from the soil layer encountered by the hole ( Fig. 9 ), there is a medium-soft plastic clay layer about 10 m underground with a thickness of about 2 m. abnormal low speed. Analyzing the fretting exploration results of section No. 5 ( Fig. 9 ), there is a wide range of low-velocity anomalies at about 22 m underground at points 11 and 12. According to the engineering geological data collected in the early stage, it is difficult to find such anomalies here. A wide range of low-speed abnormalities, but the specific cause is difficult to explain. In order to verify the real situation of this anomaly, the control borehole JZ4 in the study area was specially located here. After drilling verification, a soft sandy clay layer was drilled at about 23 m underground, and a watery sandy layer was drilled at 25 m underground. layer. It can be inferred that in this study area, the low-velocity anomalies distributed at a depth of 20 m underground may be caused by water-bearing sand layers. This situation is not found in the engineering geological data collected in the early stage, and it also verifies the effectiveness of micro-motion exploration in shallow exploration from another aspect.

Figure 9

Fig. 9   Anomaly map (a) of Micro-motion Exploration No. 5 profile and verification physical photo of drill hole JZ4 (b)

Fig.9   Contour maps of No.5 profile(a) and physical picture of JZ4 borehole(b)


According to the comprehensive analysis of micro-motion exploration and drilling survey results, the shallow backfill layer has a greater impact on the ground. On the one hand, the shallow backfill soil is not dense enough, and it is easy to wash away part of the material after surface runoff penetrates into this layer; On the one hand, the leakage of underground pipelines around the school leads to a large amount of water seeping into the ground for a long time, causing the existence of impermeable entities under the erosion of groundwater, resulting in abnormal wave speeds, which are reflected in the fretting section.

A total of 9 boreholes were laid out in this survey drilling work, with a total drilling depth of about 107 m. Some of the boreholes were laid out due to serious ground cracks. A total of 6 boreholes were used to verify the anomalies in the previous geophysical prospecting. It was planned to verify 8 anomalies. The actual drilling results 3 anomalies were verified, and the detection success rate was about 37%.

3 Conclusions and Prospects

Through this study, the applicability and effectiveness of micro-motion exploration technology in the detection of urban land subsidence are fully affirmed. Compared with engineering excavation and drilling, micro-motion exploration technology can detect underground uncompacted soil more quickly and efficiently, which is undoubtedly crucial for the detection of urban land subsidence. During the whole research process, the surface wave information of natural source is received, no artificial source is needed, it is safe and environmentally friendly, and the exploration cost is saved at the same time. From the drilling verification results, it basically verifies the effectiveness of micro-motion exploration technology applied to urban land subsidence detection.

Due to the dense buildings in urban areas, it is generally difficult to meet the space requirements of the two-dimensional fretting formation, so this time the FK method is mainly studied. But in theory, SPAC and ESPAC have better effects 24 ] , which will be compared and improved in future research.

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