Karst cave prospecting using cross-hole ultra-high density resistivity method

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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

Diagram

Description automatically generated

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)=− (rrc);r,rc∈Ω·(p·)=−d(��);,��Oh

Where: σ is the electrical conductivity, I is the electric field intensity, δ is the Dirac function, and 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 0 are the electric field data of forward modeling and actual measurement respectively; 0 is the initial model of inversion; d and 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

Section
No.

Corresponding survey hole
(cable I—cable II)

Cable I
hole depth/m

Cable II
hole depth/m

Hole spacing
/m

1

540—180

40.0

42.5

19.47

2

542—186

45.0

39.0

11.98

3

541—539

39.0

38.0

14.73

Electrode distance: 1 m

Sampling interval: 2 s

 

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; According 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; 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. According 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.  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: According 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; According 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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