Application of high-density electrical method and active phased array radar method in karst detection in Guangzhou Metro

1 Introduction

Covering karst has a certain degree of concealment and is difficult to detect; covering karst has greater potential risks. If the development area of karst caves is located within the pile foundation construction range, it will directly have various impacts on the pile foundation construction, such as circulation in the pile holes. Liquid loss, displacement of weak soil around pile holes, and difficulty in punching piles can easily cause foundation settlement and deformation. When encountering surface water infiltration or rapid changes in groundwater levels, it is easy to induce sudden collapse, etc.

Therefore, when constructing underground structures in areas with strong karst development, it is necessary to first determine the development of karst caves in order to provide a basis for measures to deal with karst.

This work uses high-density electrical method and active phased array radar method for detection. The work area is located in Tianmei New Village, Huadu District, Guangzhou City (Figure 1.1 ). Due to the narrow lanes in the village, some subway lines are located directly under the houses. , normal drilling measurements cannot be carried out. This work is designed to have two north-south directions that intersect obliquely with the subway line. Exploration boreholes are designed based on the measurement results. The drilling work is reduced as much as possible based on scientific and accurate exploration results, and then a reasonable hole spacing is selected. Cross-hole CT measurements were performed on the boreholes to accurately characterize the development of underground karst.

Figure 1.1 Measurement line engineering layout diagram

2Instruments and equipment

2.1 High-density electrical method

In terms of its basic principle, the high-density resistivity method is exactly the same as the traditional resistivity method. Therefore, it is still a type of electrical exploration method based on the difference in conductivity of rock and soil bodies, and studies the distribution of conductive current in the detected rock and soil bodies under the action of an applied electric field.

This measurement equipment uses EDJD-3 produced by Chongqing Dingfeng Geological Exploration Co., Ltd. Its main functions are: 1. Underground profile resistivity change detection, 2. Short-range Bluetooth wireless data transmission 3. Long-range 4G wireless data transmission.

Figure 2.1 High-density electrical measurement working diagram

2.2 Active phased array radar method

Pseudo-randomly coded acoustic wave technology has a high signal-to-noise ratio, which is not only suitable for urban strong interference environments, but its high-frequency signals can also overcome the problem of reduced detection accuracy with increasing depth through phase-controlled focusing. The effect of conventional seismic exploration requires Only professional engineers can interpret it, and the effect of focused imaging of encoded sound waves can clearly see that underground interface sound waves are easier to propagate underground than electromagnetic waves. Phased array acoustic wave focusing imaging, the sound waves are superimposed in the same direction, and can achieve a larger detection depth. Phased array acoustic wave focusing imaging can reach a detection depth of 50m in the overlay . Using active source wireless communication protocol, it only takes half a minute to collect data, which is highly efficient.

Figure 2.2 Active phased array acoustic radar measurement working diagram

3Geological overview

This work is located in the Guanghua depression area of Guangzhou City. The regional structure is stable and controlled by the Guangcong fault and long-term erosion and denudation. Since the Guanghua compound syncline is made of soluble limestone, karst ditches, karst troughs, karst caves, karst depressions, karst funnels and underground rivers have been formed under the erosion and dissolution of surface water and groundwater. Later structural subsidence buried the karst terrain underground.

According to regional geological data and existing survey data, the strata distributed at the site are mainly Cenozoic Quaternary (Q) strata and Carboniferous (C) strata. The soil layers are mainly the Holocene System of the Fourth System (Q4) and the Upper Pleistocene (Q3) , and the Middle Pleistocene (Q2) and Lower Pleistocene (Q1) are missing ; the Holocene is composed of artificial fill soil (Qm l 4) and silt layer and silty sandy soil layer (Qm c 4) . The Upper Pleistocene is mainly composed of (Qa l+pl 3) alluvial-pluvial soil layer and residual soil layer. The lower bedrock is mainly limestone of the Datang Stage Shidengzi Member (C1ds) of the Lower Carboniferous System .

4Geophysical characteristics

The difference in resistivity and density between the detection target and the surrounding rock and soil media is the prerequisite for high-density electrical detection and active phased array radar detection. According to the results of field measurements, it can be seen that the density of the karst area is definitely higher than that of the bedrock; the resistivity of the on-site cave cover is relatively low, the bedrock is mainly micro-weathered limestone, and the resistivity of porous and fissured bedrock is low. The resistivity measured on site The values are shown in Table 4.1 .

Table 4.1 Resistivity values of rock formations in the measurement area

5Data processing and interpretation

5.1 High-density electrical method data interpretation

This measurement uses a high-density electrical method with a pole distance of 2m , 96 channels, and a line length of 190m . The Wenner device is used for measurement. Since the ground surface is cement pavement, copper electrodes are used and bentonite plus copper sulfate adhesive is used to fix the electrodes. The measured ground resistance is 5-10k Ω. The measurement results (Figure 5.1 ) are inverted and calculated using the least squares method and the Zodi inversion method. The least squares method depicts shallow anomalies relatively thinly, but the deep anomaly effect is not precise enough, and the measurement The depth is shallow. The Zodi inversion method has better results. The shallow caprock is consistent with the actual geological conditions, and deep anomalies can also be reflected. However, the mapping effect is not obvious enough, and the anomaly description is not precise and intuitive enough. Therefore, the mapping data of the Zodi inversion method are used. , remove the shallow data on both sides and use surfer to form a map, the anomaly characterization will be more precise, and the places that are not obvious on the inversion map can also be visually highlighted.

5.1 High-density electrical method apparent resistivity section interpretation result chart

5.2 Phased array radar method data interpretation

The active phased array acoustic radar method uses 2m point spacing and high-density electrical method to measure key locations. There are a total of 6 measurements per time, the length of a single measurement is 10m , 8 channels are designed , and the measurement line length is 80m . Professional software is used to process the data. After processing and then using surfer to visualize the two-dimensional map, the preliminary interpretation result (Figure 5.2 ) is relatively messy. According to the actual geological conditions, the optimized parameter refinement processing results are shown in Figure 5.3 .

Figure 5.2 Preliminary interpretation results of active phased array acoustic radar measurements

Figure 5.3 Fine interpretation results of active phased array acoustic radar measurements

5.3 Comprehensive data interpretation

Based on a comprehensive comparison of the measurement results of the two methods (Figure 5.4 ), the shallow layer with a burial depth of 0-10m is a sandy soil mixed fill cover layer, the middle part is a strongly weathered layer area, and the bottom bedrock is developed. The high-density electrical method shows abnormalities. The low-resistance area corresponds to the low-frequency area measured by the phased array radar. The delineated anomalies are consistent with the trend shape and spatial location. The delineated areas are inferred to be caves, which need to be drilled and verified before grouting, so as to construct shield tunnels in the subway. to avoid affecting the safety of surface buildings.

Figure 5.3 Comprehensive interpretation results chart

5 Conclusion

1. The high-density electrical method uses copper electrodes fixed to cement pavement with adhesives, which can effectively measure geological bodies below the earth's surface.

2. The active phased array acoustic radar method is an emerging geophysical exploration method with high measurement efficiency, short collection time, and relatively messy initial measurement data. The measurement results after optimizing parameters can also effectively measure geological bodies.

3. By comprehensively comparing the measurement results of the two methods, the shallow caprock shows consistent correspondence, the deep caves have low resistance and low density (low frequency value) and are obviously different from the surrounding rock, and their spatial locations are consistent.

4. Drilling is required to verify the results of the comprehensive interpretation, and then based on the verified drill holes, select appropriate hole spacing for cross-hole CT measurements to characterize the karst development of the blind areas on the surface.

5. Due to the complex conditions in the city, the electrical method requires longer measuring lines ( 200-300m ) to meet the detection depth requirements, and when laying out the measuring lines, it is also necessary to consider preventing vehicle crushing and provide protection. Although the detection efficiency is Low, but effective measurement can still be carried out in places where drilling construction is not possible. The detection accuracy depends on the pole distance and the length of the measuring line; the phased array acoustic radar method is simple to construct and requires three or four people to work normally and perform a single measurement. It only requires a length of about 5-10m , rolling collection and measurement. The single collection time is short and is less affected by the surrounding environment. However, data processing and interpretation require some known geological conditions to optimize model parameters to achieve better measurement results.

6. In places where detailed exploration and drilling are not available or where only surface geophysical prospecting work can be done, the two methods are combined for measurement. Comprehensive processing and comparison can meet the measurement requirements.