Karst cave prospecting using cross-hole ultra-high density resistivity method
Karst cave prospecting using cross-hole ultra-high density resistivity
method
Summary
The detection experiment of shallow hidden karst development in a
section of Guangzhou Metro under construction was carried out by using the
cross-well ultra-high density resistivity method. The results show that it
is feasible to detect karst caves with the interwell ultra-high-density
resistivity method using the existing survey holes, and the interwell
resistivity distribution characteristics obtained through forward and inversion
calculations can intuitively give the distribution range of karst caves in the
horizontal and vertical directions. But it is not easy to distinguish the
connectivity of the cave, and the depth deviation is the size of the electrode
distance. When detecting, the hole depth of the electrical method is
consistent, the ratio of the hole depth to the hole spacing is greater than 1.5
times, and the electrode spacing is 1-2 m.
Key words: urban geology ; Well electrical method ; ultra-high
density electrical method ; cave
Abstract
The prospecting of shallowly concealed karst caves is studied using the
cross-hole ultra-high density resistivity method. The results are as follows.
It was feasible to prospect Karst caves using existing survey boreholes
according to the cross-hole ultra-high density resistivity method. The
transverse and longitudinal distribution ranges of karst caves can be directed
reflected by the distribution characteristics of the cross-hole resistivity
obtained through forward and inverse calculations. Meanwhile, it was difficult
to distinguish the connectivity between karst caves, and the depth deviation
was the distance between two adjacent electrodes. It is suggested that the
depth of the boreholes used should be consistent, the ratio of the borehole
depth to the borehole space should be higher than 1.5, and the electrode space
should be 1~2 m in practice.
Keywords： urban
geology ; cross-hole resistivity method ; ultra-high
density resistivity method ; Karst cave
0 Preface
Limestone is developed
underground in many cities in my country. When shield construction is used for
urban subway construction, the construction parameters of the soil layer and
limestone layer are different, which not only makes shield construction more difficult
and costs more, but also due to the development of karst caves in limestone,
Construction safety accidents such as shield machine tipping and burying often
occur. Therefore, in the construction of rail transit, geological surveying and
mapping, remote sensing technology, static penetration, isotope tracer,
reconnaissance holes, engineering geophysical prospecting and other
technologies are used to analyze the areas that the subway passes through. It
is of great engineering significance and value to conduct
reconnaissance. Among them, reconnaissance holes are the most practical,
most intuitive, and highest-precision method for detecting karst caves, but
reconnaissance holes can only obtain the karst development within the depth
range of the hole. This will greatly increase the cost of exploration, so
engineering geophysical prospecting has become the preferred method for the
exploration of concealed karst. Engineering geophysical exploration mainly
includes: high-density resistivity method, ground penetrating radar, cross-hole
electromagnetic method, ground seismic reflection wave method, cross-hole
seismic method and microgravity method, among which, ground seismic reflection
wave method, cross-hole seismic method, microgravity method The resolution of
the method is low, and it is greatly affected by the environment. It is mainly
used for the detection of the interface between the soil layer and the
limestone, and the karst development zone; The degree of water content in the
medium has a great influence [ 1 ] , resulting in limited vertical and
horizontal detection distances, usually no more than 10 m; while the
high-density resistivity method can only detect caves with a diameter of more
than 5 m because the electrode distance is usually ≥ 5 m.
In order to reduce the detection
cost, based on the existing survey holes, the hidden karst before the planning
of a subway line in Guangzhou was detected by the cross-well ultra-high density
resistivity method, which avoided the impact of the hidden karst on the
construction, and also provided similar projects. learn from.
1 Detection principle
The ultra-high-density
resistivity method exploration is still based on the basic principle that the
electric field distribution on the surface is related to the resistivity
distribution of the underground rock-soil medium under the action of an
artificial DC electric field[2, 3, 4], and its main features are as follows :
A 64-electrode arrangement is
adopted, and the electrode spacing is 1-2 m. During data collection, the
program automatically divides each array of 64 electrodes into odd groups and
even groups, with 32 electrodes in each group; one of the two groups of
electrodes is selected as the power supply electrode A and B, and during
one power - on process At the same time,
measure the potential difference of other electrodes relative to a certain
electrode M , and obtain 61 potential difference ( MN 1 , MN 2 , MN 3 ,..., MN 60 , MN 61 )
data ( Fig. 1 ). 32 electrodes in an odd group and 32
electrodes in an even group are paired with each other as power supply
electrodes, that is, there are 32×32=1 024 power supply and power-off processes
in one arrangement, and 61 potential difference data can be collected at the
same time for each power supply. The total data volume It should be 32×32×61=62
464.
figure 1
Figure 1 Schematic
layout of electrodes for ultra-high density resistivity method
Fig.1 Electrode
layout of ultra-density resistivity method
Compared with the high-density
resistivity method, the ultra-high-density resistivity method can achieve
multi-level and multi-angle testing with one-time electrode layout and software
control. The collected data is not only rich and consistent, but also the
collection speed is increased by hundreds of times, greatly improving work
efficiency. Its richness improves the accuracy and reliability of the
inversion results, and its consistency avoids that in the data collection of
the high-density electrical method, some lay more emphasis on horizontal
resolution, and some lay more emphasis on vertical resolution, resulting in the
collection of data at the same location. The shortcomings of the different
inversion results generated by the data, coupled with the electrode spacing of
1~2 m, can carry out more fine data processing and grid division, improve the
resolution of electrical anomalies, and can more truly reflect the different
electrical properties of the underground. The location and size of the sexually
abnormal body.
2 Examples of cave detection
2.1 Pole
arrangement
Before the planning of a subway
line in Guangzhou, the hidden karst detection was carried out at Qingbu Station
by using the ultra-height resistivity method to avoid the impact of hidden
karst on construction and operation [ 5 , 6 , 7 ] . The
ultra-high-density electrical method is divided into three types:
surface [ 8 ] ,
well-ground and well-well. 32 electrodes were used to observe the current and
voltage data between the two holes, and the electrode distance was 1 m ( Fig. 2 ).
figure 2
Figure 2 Electrode
layout of ultra-high density electrical method between wells
Fig.2 Electrode
layout of cross-hole ultra-density resistivity method
2.2 Data
Collection
In order to save the detection
cost, 12 survey holes were used to arrange 21 inter-well ultra-high-density
electrical measuring lines in the work area . The WDJD-4 multifunctional digital direct current
excitation instrument developed by the Institute of Numerical Control
Technology.
image 3
Fig. 3 Schematic
diagram of layout of super-high-density electrometric holes and measuring lines
between wells at Qingbu Station of Guangzhou Metro
Fig.3 Layout
of resistivity hole and survey line in Qingbu station of Guangzhou Metro
2.3 Data
processing
Each group of cross-well
resistivity profiles takes the point of electrode 32# in the left survey hole
as the origin (0,0), the line where the two survey holes are located as the
x-axis, and the left survey hole along the depth direction
as the y -axis. Establish a Cartesian coordinate system;
conduct resistivity forward and inversion calculations on the collected data to
obtain the difference in resistivity characteristics of the interwell medium;
remove abnormal points, set damping coefficients, grid division, and fitting
errors on the collected original data Threshold, forward simulation processing,
and related work such as adding combined inversion and strengthening inversion
in the least squares method inversion, and finally obtaining a two-dimensional
profile with high accuracy [ 9 , 10 , 11 , 12 ] .
1) The basic equation of
electrical forward modeling:
▽⋅(σ⋅▽U)=−Iδ (r−rc);r,rc∈Ω▽·(p·▽�)=−�d(�−��);�,��∈Oh
Where: σ is the
electrical conductivity, I is the electric field
intensity, δ is the Dirac function, and r c is
the electrode position. According to formula (1), the distribution
characteristics of the electric field intensity at any electrode can be
obtained.
In the formula: m is
the resistivity; λ is the balance factor; d ( m )
and d 0 are the electric field
data of forward modeling and actual measurement respectively; m 0 is
the initial model of inversion; W d and W m are
weighting factors, which are used to control The value of the correction amount
to the model in the iterative process is determined according to the
signal-to-noise ratio of the measured data, generally 0.005~0.2.
The specific steps of
ultra-high-density resistivity data inversion are as follows: firstly, a
theoretical distribution model of resistivity is established; secondly, the
theoretical model is used for forward calculation to obtain the theoretical
resistivity value; thirdly, the relationship between the measured data and the
theoretical value is calculated. After the difference, the difference is
calculated to the subdivided grid according to different algorithms, so as to
correct the theoretical resistivity model and obtain a new theoretical resistivity
distribution model; finally, using this new theory Do the forward calculation
of the model, repeat the above steps, and iteratively fit until the root mean
square error of the fitting is small enough or meets the requirements, and the
inversion ends. The theoretical resistivity distribution model at this
time is considered as the final inversion result.
2.4 Result
analysis
Taking three survey lines as an
example, the inversion profile is compared with the stratigraphic histogram
obtained from the reconnaissance hole. The reconnaissance hole parameters
corresponding to the three survey lines are shown in Table 1 .
Table
1. Interwell ultra-high density resistivity survey hole
parameters
Table
1 Parameters of resistivity holes
Fig. 4 shows the resistivity profile, geological
inference profile and formation columnar diagram of 540-180-hole
ultra-high-density electrical inversion. It can be seen from Fig. 4 that: (1) According to the stratigraphic columnar
diagram of the 180 reconnaissance hole, limestone is seen at 28.7 m, but there
are fractures developed, and 30.0-34.0 m and 34.7-40.0 m are karst caves, which
are fully filled, and the main fillings are soft plastic Clay, 40.0-42.5 m is
limestone with fractures; ②According
to the inversion resistivity profile, the interwell ultra-high-density
electrical method clearly reflects the anomaly of low resistivity caused by
karst caves, and there are karst caves at 29.0-38.0 m, but the inversion
interpretation The rising deviation of the top of the cave is 1.0 m, and the
deviation of the bottom of the cave is 2.0 m, but the resolution of multi-layer
caves is low; ③ limestone interlayers less than
1.0 m cannot be distinguished.
Figure 4
Fig. 4 Inversion
resistivity profile, stratigraphic histogram and geological inference profile
from holes 540 to 180
Fig.4 Inversion
resistivity profile, stratigraphic histogram and geological inference profile
between No.540 hole and No.180 hole
Figure 5 shows the ultra-high density electrical
inversion resistivity profile, geological inference profile and stratigraphic
column diagram for holes 542 to 186. It can be seen from Fig. 5 that: (1) According to the stratigraphic columnar
map of the 186 reconnaissance hole, limestone is found at 24.5 m, and there are
karst caves at 27.0-30.2 m, 31.3-33.9 m, and 35.2-37.1 m. ②According to the inversion resistivity
profile, there are karst caves at 28.0-35.5 m, because the depth of hole 186 is
not enough, the anomaly of low resistance caused by karst caves is obvious; but
the deviation of the top of the karst cave from the inversion interpretation is
1.0 m, and at the same time, the bottom of the karst cave is rising The
deviation is 1.6 m. ③ The resolution of multi-layer
caves is low.
Figure 5
Fig. 5 Inversion
resistivity profile, stratigraphic histogram and geological inference profile
of holes 542~186
Fig.5 Inversion
resistivity profile, stratigraphic histogram and geological inference profile
between No.542 hole and No.186 hole
Figure 6 shows the ultra-high-density electrical
inversion resistivity profile, geological inference profile and stratigraphic
columnar diagram of holes 541 to 539. It can be seen from Fig. 6 that: ①According to the columnar map of the 539 reconnaissance hole, limestone
is found at 23.0 m, and there are karst caves at 24.7-27.0 m, 28.5-30.0 m, and
31.8-34.3 m; ②According to the inversion
resistivity profile, 23.5-34.5 m There are karst caves in m, but the inversion
interpretation shows that the top of the cave rises with a deviation of 1.2 m,
and the bottom of the cave falls with a deviation of 0.2 m.
Figure 6
Fig. 6 Inversion
resistivity profile, stratigraphic histogram and geological inference profile
of holes 541~539
Fig.6 Inversion
resistivity profile, stratigraphic histogram and geological inference profile
between No.541 hole and No.539 hole
3 Conclusion
1) It is feasible to use
reconnaissance holes for interwell ultra-high-density electrical cave
detection, and it saves drilling costs. The ultra-high-density electrical
method has high sensitivity to the detection of filled karst caves, is more
effective in judging high and low resistivity anomalies, and has good data
reproducibility. The horizontal and vertical distribution ranges enrich the
detection methods of caves.
2) According to the
corresponding relationship between caves and resistivity, the development range
of caves can be delineated, but for caves and rock interlayers whose diameter
is smaller than the electrode distance, it is easy to be "covered";
it is not easy to distinguish the connectivity of caves.
3) On the whole, the inversion
resistivity profile has a good quantitative explanation for the undulation of
the interwell rock surface and the development of karst caves, and the depth
deviation is basically the size of the electrode distance.
4) When detecting, it is best to
use the same hole depth for the electrical method to avoid blind detection or
single-point anomalies, which will affect the accuracy of inversion; in
addition, the ratio of hole depth to hole spacing should be greater than 1.5
times to ensure the detection effect.
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