Application of microtremor survey method in detection of urban land subsidence
Summary
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
Abstract
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 , 20, 21 ] , 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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