Application of Geophysical Exploration Technology in Geological Disaster Investigation in Three Gorges Reservoir Area

Li Hongtao Sun Dangsheng Yang Qinhai Yang Jinping

(Institute of Hydrogeology Engineering Geology Technology and Method, China Geological Survey, Baoding, Hebei, 07 105 1)

This paper briefly introduces the geophysical prospecting techniques and methods commonly used in the investigation of geological disasters in the Three Gorges reservoir area and some typical engineering examples, in order to play a certain demonstration role in the future work and further provide advanced and effective testing means for the investigation of geological disasters.

Keywords: Geophysical methods for geological hazards in Three Gorges reservoir area

1 preface

1997 From 2004 to 2004, the Institute of Hydrogeology Engineering Geology and Technology of China Geological Survey undertook many applied research topics and geophysical exploration tasks, such as Research and Demonstration on Prevention and Control of Major Geological Disasters in the New Resettlement Site in the Three Gorges Reservoir Area, Geophysical Exploration in Sanmashan Community in Fengjie, Geophysical Exploration in Huangtupo Landslide in Badong and Guantangkou Landslide in Wanzhou, and Reservoir Bank Investigation in Counties of Chongqing 14. A large number of comprehensive geophysical surveys have been carried out in Badong, Wushan, Fengjie, Wanzhou, Fengdu and Shizhu in the Three Gorges reservoir area. This paper summarizes the practical experience and understanding of geophysical prospecting technology in the prevention and control of geological disasters in the Three Gorges reservoir area, with a view to playing a certain demonstration role in the future work and further providing advanced and effective testing means for geological disaster exploration.

2 Geophysical exploration techniques and methods

2. 1 shallow high-resolution seismic exploration

2. 1. 1 working technical methods

(1) extended permutation method

Considering the complexity of topographic and geological conditions in the reservoir area, Fengjie and Wushan, as an important test method, adopted the extended layout method before laying seismic profiles. Its function is to understand the time sequence arrangement of various waves in the seismic wave group in the survey area, analyze the seismic phase, determine the instrument parameters and observation system for collecting data, take appropriate excitation and reception measures, and estimate the velocity parameters of the formation medium. The array observation system adopts different offset distances such as 0m, 10m, 20m, 30m, 40m and 50m, and the track distance is 2m or 3m.

(2)*** depth point multiple horizontal stacking method (CDP)

CDP horizontal stacking method is to collect the reflection wave of the same reflection point at different excitation points and receiving points, and extract the trace set of * * * reflection points on the interface from the obtained seismic records. After velocity scanning and static and dynamic correction, the geological interface and structural information are given in the form of time section. This method can improve the signal-to-noise ratio and has a significant effect on suppressing interference waves. The selection of offset in CDP profile observation system is determined according to the relationship between interference waves such as surface wave and acoustic wave and reflected wave of target layer, which are 30m, 40m and 69m respectively. Track spacing is 2m, 3m and 5m. The horizontal stacking times are mostly 6 times, and some are 3 times.

(3) Seismic high-density mapping method

High-density imaging technology uses single excitation and single reception to collect signals with equal offset, and its working mode is similar to sonar method in water, so it is also called land sonar method. The collected signal is amplitude compressed, color modulated and displayed as a color image. The offset of high-density mapping method is 2m, and the point distance is1m.

2. 1.2 field data acquisition equipment

SWS- 1A multifunctional surface wave instrument of Beijing Institute of Hydroelectric Geophysical Exploration and MARK6 portable multi-channel seismograph of ABEM Company in Sweden are used for seismic exploration. 38HZ high sensitivity digital detector with CDP portable cover cable for receiving detector. According to the depth of the exploration target layer and the construction conditions of the exploration area, two kinds of seismic sources, namely hammering and explosive blasting, are used respectively. The hammer that hammers the earthquake source weighs 24 pounds and the hammer pad is 20 mm thick. In order to increase the effective signal and suppress random interference, vertical stacking is adopted, and the stacking times are generally 5 times. The explosive source is usually excited in the hole, the hole depth is 1 ~ 2m, and the charge is 100 ~ 200g.

2. 1.3 data processing

The data processing of CDP profile data adopts CSP.3.3 seismic data processing system. According to the characteristics of large terrain slope and large fluctuation in this area, terrain correction was carried out before and after stacking. The processing contents also include gain control, noise and interference wave removal, filtering, velocity analysis, dynamic correction and horizontal stacking. Finally, the CDP horizontal superimposed bidirectional reflection time profile with topographic lines is output, and the geological interpretation map is completed under AutoCAD 14.0. The processing flow is shown in figure 1.

Figure 1 Flowchart of Shallow Seismic Data Processing

2.2 Surface wave exploration

Transient surface waves (Rayleigh waves) are used for exploration. When an earthquake is vertically excited by a seismic source on the surface, it generally produces direct longitudinal waves, refracted longitudinal waves, reflected longitudinal waves, Rayleigh waves and various converted waves. Theoretical analysis and experiments show that of all these waves, Rayleigh wave has the strongest energy, accounting for about 67%. Rayleigh wave is a kind of surface wave propagating along the surface. Its wavefront is a cylinder, and its propagation depth is about one wavelength. The dispersion characteristics of Rayleigh waves, that is, the propagation characteristics of Rayleigh waves with different wavelengths reflect the characteristics of geological bodies with different depths, are used to detect the geological medium structure.

2.2. 1 instruments and equipment

SWS- 1A multifunctional surface wave instrument of Beijing Institute of Hydroelectric Geophysical Exploration is used for surface wave exploration, and 4Hz low-frequency geophone is used as the receiver. The surface wave profile is arranged in 12 channel, with track distance 1m, point distance 5m and offset of 0m, 5m, 10m, 15m and 20m respectively.

data processing

FKSWSA surface wave processing system is used for surface wave profile. Through multi-channel three-dimensional Fourier transform, velocity and wave number (wavelength) are filtered in time-space (T-X) domain and frequency-wave number (F-K) domain, so as to eliminate non-surface wave signals, effectively extract surface wave information, draw surface wave dispersion curves, and carry out inverse interpretation of surface wave data.

The feature of FKSWSA surface wave processing system is that it can be fitted, that is, the best inversion result of formation structure can be determined by judging the correlation coefficient between the set formation structure parameters and the calculated formation parameters.

2.3 seismic tomography (CT)

Seismic tomography is a boundary projection inversion method, which is similar to imaging technology in other scientific and technological fields. According to the kinematic and dynamic characteristics of seismic waves, seismic tomography can be divided into two types: ray tomography and wave equation tomography. They measure the changes of travel time, amplitude, phase and period of seismic waves, invert the three-dimensional velocity structure or attenuation characteristics of geological media, and display the results with images.

Seismic CT data acquisition adopts the combination of well and ground. Well-to-ground mode is to excite elastic waves along the ground between two holes and receive them in the holes; The cross-well mode is that one hole is excited and the other hole is received. The distance between receiving points is 2m and 1m, and the offset is 2m, or the situation in the well is determined, so as to form an observation system that crosses up and down, ensure that the radiation covers the test area and improve the imaging accuracy.

2.3. 1 instrument

SWS- 1A multifunctional surface wave instrument or MARK6 portable multichannel seismograph.

The receiver is coupled with the borehole wall through a series airbag detector.

Using explosion source, electric detonator excitation.

data processing

The data processing of Windows seismic tomography system adopts CST. Each imaging area is divided into 2m×2m units, and the density of ray nodes on each unit block is10x10. The results are displayed by wave velocity isoline, and the image output is realized by Winsurf6.04. The processing flow is shown in Figure 2.

Fig. 2 Processing Flow of Seismic Tomography Data

2.4 eh-4 conductivity imaging

Eh-4 conductivity imaging method belongs to a magnetotelluric testing method combining partially controllable sources with natural fields. Different from the direct current method, it does not increase the exploration depth by extending the cable and increasing the polar distance, but obtains the depth information of the measuring point through its frequency conversion. EH-4 During the investigation of the ground subsidence pit in Sanwantang, Baotaping, fengjie county, a north-south profile was laid at the bottom of the pit, with a point distance of 5m and a dipole distance of 15m, which was consistent with the profile direction. A section is arranged on the south surface of the collapse pit, with a point distance of 5m and an electric dipole distance of 10m.

2.4. 1 instruments and equipment

Eh-4 conductivity imaging system is jointly produced by GEOMETRLCS and EMI. It is an advanced electromagnetic exploration instrument in the world at present.

Data processing of eh-4

Includes field data processing and subsequent processing. Field data processing is mainly one-dimensional analysis, which is used to check the quality of field collected data and adjust parameters. Subsequent processing includes data analysis, one-dimensional data processing and display, and quasi-two-dimensional processing. Data analysis software is used to identify noise sources, estimate and adjust the signal level of transmitter, and analyze the quality of data acquisition. One-dimensional data processing and display is data re-processing after data analysis, and new power spectrum can be obtained, which can delete data with serious noise to reduce divergence and increase signal correlation. Two-dimensional processing is quasi-two-dimensional inversion by EMAP method, which can effectively eliminate the influence of static correction, construct resistivity profile, give the gray map of interpretation results on site, and make color mapping by computer two-dimensional inversion.

2.5 Acoustic logging technology

Based on measuring the sound velocity and amplitude of rocks and minerals, acoustic logging is an effective method to divide the lithology of bedrock, the degree of weathering and fracture, determine the location of fracture zone, the interface between bedrock and overburden, and determine the middle and low velocity layers of overburden and bedrock.

Single-hole full-wave train acoustic wave test adopts double receiving probes with a spacing of 50cm and a spacing of 30cm between transmitting source and receiving source. Acoustic information is transmitted and received along the borehole wall in the borehole (bare hole). During logging, the probe is lowered to the bottom of the well and tested upward at a certain distance. The data acquisition and storage of full-wave train are completed by computer, and P-wave and S-wave are picked up indoors through playback and data processing. According to the interference point, amplitude and frequency spectrum analysis of waveform in full wave train acquisition, the first arrival time of P-wave and S-wave is determined, the velocity of P-wave and S-wave is calculated, and the result map is drawn.

The instrument used in the test is a super jet -4D full-wave acoustic logging tool (Institute of Hydrogeology Engineering Geology and Technology, China Geological Survey).

There are two kinds of downhole probes: dry well wall-attached probe and water coupling probe.

3 Application result analysis

3. 1 landslide accumulation body

Landslide accumulation body is a kind of loose accumulation body with multiple causes and stages. Most of them are landslides, collapses, debris flow accumulations and karst collapse accumulations formed under structural and gravity unloading and karst action. The purpose of geophysical exploration is to understand the thickness and deep structural characteristics of accumulation body, and the main working methods are arrangement method, CDP profile and surface wave method.

3. 1. 1 Deep structural characteristics of Jingtan Road-Xiangyun Road-Jixian Road, Wushan New Town Site

It is very difficult to detect earthquakes because of the large topographic relief in this area and the artificial backfilling of gullies. Figure 3 (section F) reflects the deep structural characteristics of Jingtan Road-Xiangyun Road-Jixian Road. It can be seen that the buried depth of the intact bedrock is 40 ~ 50m, and a deep ditch with a depth of 30m is formed between Xiangyun Road and Jixian Road. Figure 4 (part H) spans the water source and gully, and the gully is obvious. On the time profile, the cutting and weathering of the valley area are in the form of multiphase axis, which reflects the complexity of valley deposition. The detection results clearly reflect the bedding characteristics of the accumulation body.

3. 1.2 Fine structure characteristics of landslide accumulation body

In order to further reveal the fine structural characteristics of landslide accumulation, surface wave detection is used to understand the shallow geological structure. Figure 5 lists typical dispersion curves and their geological interpretation results. It can be seen that surface wave exploration can provide detailed information about shallow strata and their velocity distribution. The results show that the interior of landslide accumulation body can be divided into three layers:

Fig. 3 shallow seismic exploration results of Jingtan road-Jixian road (section f) in Wushan new site.

The first layer: 0 ~ 3. 15m, which is a gravel clay layer with shear wave velocity of 330 ~ 470 m/s.

The second layer: 3 ~ 8m, containing gravel and soil layer, with shear wave velocity of 470 ~ 770m/s. ..

The third stratum: 8 ~ 16m, which is a broken rock stratum with shear wave velocity of 770 ~ 970 m/s.

3. 1.3 Interpretation of results

The buried depth of landslide accumulation body is about 40m, but there is a trench with a depth of 70m between Xiangyun Road and Jixian Road. The bottom of landslide accumulation body is obviously along the direction of rock stratum, with an inclination angle of 30. In the landslide accumulation body, it can be subdivided into three layers, and its wave velocity does not exceed 1000m/s, indicating that its rock mass integrity is poor.

3.2 Landslide

CDP profile method is the main technical method used in landslide exploration. Exploration targets include Huangtupo landslide in Badong county, Xiufengsi landslide in Wushan, Guantangkou landslide in Wanzhou district of Chongqing, and reservoir bank landslide in Shangtuokou section of Yangtze River Bridge in Wanzhou district. This paper only describes some representative achievements as follows.

3.2. 1 Huangtupo Landslide in Xincheng District of Badong County

Characteristics of (1) seismic time profile wave group

Nine sections have been made for Huangtupo landslide in Badong county, and two sections are listed in this paper for analysis. It can be seen from the time profiles of Figure 6 (section D) and Figure 7 (section C) that there are one or two sets of reflection in-phase axes, in which the T 1 wave group is relatively stable, with a time of about 30 ~ 60ms and a depth of 30 ~ 51m.. This layer can be regarded as the interface between Quaternary landslide accumulation and underlying bedrock, and the T2 wave group time is 50. As can be seen from Figure 6 (section D) and Figure 7 (section C), there are no large fault traces, but cracks (joints) are developed, resulting in rock mass fragmentation. Judging from the characteristics of reflected waves, there are signs of chaotic weak reflection or staggered wave groups.

Fig. 4 shallow seismic exploration results of Xiangyun road (h section) in Wushan new site.

Fig. 5 Exploration results of Jingtan Road-Jixian Road Surface Wave in Wushan New Site

Fig. 6 time profile of shallow seismic exploration of huangtupo landslide in Badong (section d)

Fig. 7 time profile of shallow seismic exploration of huangtupo landslide in Badong (C 1 and C2)

(2) Geological interpretation

The seismic exploration results of Huangtupo landslide in Badong basically found out the thickness and spatial distribution range of Quaternary loose accumulation and landslide accumulation in the working area. The inferred geological interpretation map directly reflects the buried depth and undulating shape of bedrock, and its buried depth distribution range is generally about 50 ~ 90 m. The abnormal distribution zone and position of weak structural plane of bedrock in the working area are found out, and the bedrock fracture zone and fracture development zone are interpreted and inferred to be 2 1.

3.2.2 Wushan Xiufeng Temple landslide

Wave group characteristics of (1) seismic time profile

Eight shallow seismic profiles were made for Xiufengsi landslide in Wushan. This paper lists the typical seismic profile of 1, as shown in Figure 8. It can be seen from the time profile that there are one or two sets of reflection in-phase axes, one of which is relatively stable, and the time is about 50ms (after removing the influence of topography). This layer can be considered as the interface between landslide accumulation and underlying bedrock, and the depth is generally about 30 m, which also reflects some interfaces with different structural characteristics, such as weathered rock mass. The time is generally about 75ms, which is inferred as the interface between complete bedrock and weathered rock mass or gravel layer. In addition, in fig. 8, the reflected wave at CDP point 120 ~ 140 is axially depressed or even pointed out. Combined with the geological conditions of the site, this location is where an ancient temple is located. This phenomenon appears in seismic reflection wave, which may be caused by the difference of stratum wave impedance interface caused by artificial excavation of ancient engineering.

Figure 8 D3 Shallow seismic exploration results of Xiufeng Temple in Wushan.

(2) Geological interpretation

Eight shallow seismic profiles of Xiushengsi landslide in Wushan basically find out the thickness and spatial shape of landslide accumulation body, and infer that the geological map directly reflects the changes of bedrock shape and overburden thickness. In addition to the bedrock surface, there are some in-phase axes on CDP profile, which are the true reflection of seismic wave geological information. For example, the discontinuity of the in-phase axis reflected by the D3 line coincides with the location of the old temple. The eight profiles of Xiufengsi landslide show that the thickness of the accumulation body of Xiufengsi landslide is about 25 ~ 35m.

3.2.3 Reservoir bank landslide survey of Shangtuokou section of Yangtze River Bridge in Wanzhou District, Chongqing.

Wave group characteristics of (1) seismic profile

Five CDP shallow seismic profiles were made in the exploration of reservoir bank landslide in Shangtuokou section of Wanzhou Yangtze River Bridge. Figure 9 and Figure 10 are two typical sections. As can be seen from Figure 7 and Figure 8, the wave group characteristics of seismic reflection wave are obvious, generally lasting 1 ~ 2 phases. Compared with wave phase, energy, waveform and continuity, T 1 wave group is the interface between Quaternary landslide accumulation layer and underlying bedrock (weathered layer). T2 reflection layer is inferred as the reflection inside bedrock, which is the main basis for inferring the buried depth and undulating shape of bedrock, and reflects the lateral variation characteristics of lithology of weathered crust and bedrock weak strata.

(2) Geological interpretation

The thickness and spatial form of landslide accumulation body are basically found out in five shallow seismic profiles of reservoir bank landslide in Tuokou section of Yangtze River Bridge. It is inferred that the geological map directly reflects the thickness and distribution range of the Quaternary landslide accumulation layer, and the average thickness of the landslide accumulation layer is 3.5 ~ 9m. The thickness of bedrock weathering crust in the working area is basically determined, and the average thickness of bedrock weathering crust is about 14 ~ 17m. The buried depth and undulating form of bedrock are determined. The abnormal distribution and structural characteristics of bedrock structural plane in the working area are inferred and explained. * * * Interpretation infers that there are 1 1 bedrock fracture zone and fracture development zone.

3.2.4 Acoustic logging of Guantangkou landslide group in Wanzhou District, Chongqing and Xincheng site in Badong County.

Acoustic logging exploration was carried out in Guantangkou landslide group in Wanzhou, Chongqing and Xincheng landslide body in Badong County, with the purpose of evaluating and dividing lithology and integrity and determining the position of sliding zone and fracture zone in combination with geological survey.

Fig. 9 Shallow seismic exploration results of C-C-C' reservoir bank (bank collapse) protection project of Wanzhou Yangtze River Bridge-Shangtuokou section.

Figure 10 Shallow seismic exploration results of D-D-D' reservoir bank (bank collapse) protection project of Wanzhou Yangtze River Bridge-Shangtuokou section.

There are 13 observation boreholes in Guantangkou landslide group in Wanzhou, and 12 observation boreholes in Huangtupo landslide in Badong. Figures 65, 438+065 and 438+0 are typical sound (wave) velocity-hole depth curves of Guantangkou ZK3, which are based on the original recorded sound wave train, the extracted sound wave time difference-hole depth curve and the calculated sound velocity-. From this, the boundary between bedrock and overburden can be clearly divided, and at the same time, it can be seen that the sound velocity of bedrock is above 3500m/s, and the sound velocity of fracture development zone is low; The upper overburden can be divided into two layers, and the average sound velocity is 1800m/s and 2200m/s, respectively. The change of its velocity shows that the content of rock and soil, the lithology of rock and stratum structure have changed in different degrees. Fig. 12 is a comparison diagram of acoustic wave test curve and borehole histogram. The curve frequency between 20.5 ~ 24m is low, and the amplitude of sound wave is small, reflecting the loose rock mass. Boreholes of 20.5~24m indicate that there is a fracture zone in the intact rock mass (see Figure 12). Figure 13 is a typical sound (wave) velocity-hole depth curve of Badong ZK 1. The wave velocity values of 66.0 ~ 67.5 m and 77.5 ~ 84.5 m are obviously increased to 3800m/s, which is considered to have entered the bedrock, and the frequency of the received waveform becomes lower from the variable area image.

Figure 1 1 ZK3 landslide survey Acoustic Logging Results in Guantangkou

Figure 12 ZK7 Comparison between Acoustic Testing Curve and Borehole Histogram

Fig. 13 logging results of ZK 1 hole in Badong loess slope

According to the acoustic wave test results of Guantangkou landslide group 13 well in Wanzhou, the average sound velocity of different formation lithology is shown in Table 1 and Table 2.

Table 1 main lithologic wave velocity of Guantangkou landslide group

Table 2 Lithologic Wave Velocity of Main Strata of Huangtupo Landslide

According to the analysis of logging data and borehole data, it is inferred that there is more than one slip zone in Guantangkou landslide. According to the test results, the position of the sliding zone is the lithologic boundary between the upper overburden and the underlying bedrock. From the analysis of the overall distribution position of the test boreholes, the front and rear edges of the landslide are shallow, with the buried depth of 20m in the front edge, 30m in the rear edge and 55m in the middle of the landslide.

Acoustic logging is an effective method to divide the lithology, weathering and crushing degree of bedrock, determine the location of crushing zone, the interface between bedrock and overburden, and determine the middle and low velocity layers of overburden and bedrock.

3.3 Karst and Cave

3.3. 1 karst collapse

At 2: 30pm on May 30th, 1997, the gentle slope at the bottom of Sanwantanggou on the west side of Zhaojialiangzi in Baotaping Community, fengjie county collapsed, forming a collapse pit with a long axis of 20 ~ 25m and a depth of about 20m. The profile is funnel-shaped, with a volume of about 6000~7000m3, and the ground fissure on the northeast side is less than 4m away from the newly demolished house. The collapse has aroused great concern from all walks of life, especially the leaders at all levels of the county party Committee. In order to further find out the depth and scope of the collapse pit, the research group conducted a special investigation, and adopted advanced EH-4 conductivity imaging system, high-resolution seismic exploration, high-density resistivity method, audio geoelectric field method and cross-hole seismic tomography and other comprehensive geophysical exploration means.

(1) eh-4 conductivity imaging

Figure 14 shows the EH-4 measurement profile at the bottom of the subsidence pit.

Figure 14 Electrical Exploration Profile at the Bottom of Baotaping Collapse Pit in Fengjie

As can be seen from the figure, the complete bedrock interface is about 55m deep from the pit bottom, and the distance from the pit bottom to the surface is about 70m deep. At the same time, the profile also reflects the difference of weathering and crushing degree of bedrock on the north and south sides of the collapse pit. The clay layer on the north side is thick, and the bedrock is strongly weathered and broken. There is a bedrock fracture section on the south side, and the bottom boundary is about 55m away from the surface, which may be the karst development channel below it. This interpretation result is consistent with that of seismic section B.

(2) High resolution seismic exploration

Figure 15 reflects the deep structural characteristics of Baotaping collapse ditch. The profile starts from the collapse pit, and the survey line is about 200 meters long, nearly north-south direction. The geological structure of this area can be divided into four layers:

The first layer: the buried depth is 0 ~ 40m, and it is mainly composed of gravel, with clay layer.

The second layer: the buried depth is 40 ~ 70 mm, which is broken loose rock mass.

The third layer: the buried depth is 70 ~ 100 mm, which is a relatively complete rock mass.

The fourth layer: the buried depth is below 100m, which is a complete rock mass.

In addition, two nearly east-west cross sections B and C (Figures 16 and 17) were obtained from the gully. The detection results show that the stratum structure is similar to that revealed in Figure 15, but the reflection interface on the south side of the collapse pit is arched upward, similar to diffraction wave, and it is discontinuous locally, which may be a karst anomaly. The connecting line direction is consistent with the gully direction. Development depth b is 55 ~ 60m, and section c is 60 ~ 65m.

(3) seismic wave CT profile

In order to further find out the extension and development of the subsidence pit, three seismic CT profiles are arranged in a targeted manner. According to the wave velocity image characteristics, wave velocity isoline distribution and borehole data of seismic CT imaging profile, the comprehensive analysis is as follows (see attached figure 18).

Figure 15 shallow seismic exploration results of Baotaping A line in Fengjie.

Figure 16 shallow seismic exploration results of Baotaping B line in Fengjie.

Figure 17 shallow seismic exploration results of Baotaping C line in Fengjie.

Figure 18 Fengjie 1 Baotaping shallow earthquakes borehole CT image.

A the longitudinal wave velocity distribution in the whole work area is low, ranging from 0.8 km/s to 3.8 km/s, the wave velocity distribution in the upper (50-60m) broken rock and soil is between 0.8 km/s and kloc-0/.6 km/s, and the wave velocity in the bedrock part is only 2.0-3.8 km/s, which is the broken rock mass section exposed by drilling.

The uneven velocity distribution of b. b. CT imaging shows that the bedrock joints and cracks in the working area are developed and the rock mass is broken. The debris on the upper part of stone and soil has different accumulation forms and complex structure.

C from figure 18, we can SEe a series of interface features inclined from NW to se, which are presumed to be stratum occurrence or lithologic contact surface. This is consistent with the interpretation results of sections B and C in shallow earthquakes (Figure 16, Figure 17).

To sum up, near the collapse pit of Zhaojialiangzi in Baotaping, no large karst cave was found in the bedrock at the position of CT profile. However, the results of high-resolution earthquake and audio-frequency geoelectric field show that there is a SN- trending structural fracture anomaly zone along the ditch in the downstream direction of the subsidence pit, forming a groundwater channel, which plays a role in dissolving and migrating the formation medium, with a depth of 50 ~ 60 m.

In order to cooperate with the research on the development of shallow karst in the project of "Demonstration Research on the Prevention and Utilization of Geological Disasters in Wushan New Town, Chongqing", three pairs of seismic wave CT were made in the foundation of Zhoujiabao series buildings in Wushan New Town. Figure 19 shows the CT image of borehole ZB5—ZB6 in Zhoujiabao, Wushan County. Its velocity distribution is between 0.7 1 ~ 3.40 km/s, which is lower than that of complete limestone and shallow karst is extremely developed. The rock mass below 3 10m elevation is relatively complete, but its wave velocity is still not high. It is inferred that there are many cracks or small caves, especially there is a red area with a diameter of about 3m at the bottom of ZB5—ZB6 section, which is inferred as a cave. From the elevation of ZB5 hole 3 10m to the elevation of ZB6 hole 280m, there are six relatively independent and closed red areas, which are distributed like beads, which are presumed to be karst caves formed by structural influence.

Fig. 19 CT image of borehole ZB5—ZB6 in Zhoujiabao, Wushan County.

4 conclusion

Geological disasters are influenced and controlled by natural and man-made complex factors, and their distribution, formation, occurrence, development and changes are very complicated, especially in the Three Gorges reservoir area, where the geological and geographical conditions are complex and geological disasters are numerous, widely distributed and frequent. The tasks of exploration, monitoring, prediction and prevention cannot be accomplished simply by traditional geological techniques. New technology is a powerful weapon to improve conventional geological exploration methods and realize the modernization of geological work, and it is also a powerful means to make new progress and breakthrough in geological work. In the whole process of resettlement in the Three Gorges reservoir area, due to the complexity of geological problems, it has brought great pressure to resettlement and provided a broad space for the application of new exploration technologies.

Geophysical exploration has been widely used in the whole process of geological disaster exploration, prevention and rational development and utilization in reservoir areas. Especially in the investigation of geological disasters, the application of new exploration technology, no matter the types of geological disasters involved, the types of methods selected, the scope of application and the work invested, is unprecedented, and the achievements obtained are various and outstanding. Over the years, I have used advanced methods such as CT tomography, shallow seismic exploration, surface wave exploration, high-density imaging, acoustic wave detection and EH-4 to analyze the karst distribution law, collapse pits and landslide structures in the Three Gorges reservoir area. However, due to the limitation of multi-solution of geological interpretation determined by the theoretical basis of geophysical exploration methods, as well as the complex geological conditions and harsh working environment in the Three Gorges reservoir area, there are often some unsatisfactory results in some geophysical exploration work. This requires us to make rational and effective use of new geophysical exploration techniques (including correctly selecting geophysical exploration methods and their best combination forms according to different geological conditions and purposes) and improve the working arrangement, data acquisition, interpretation and processing methods of existing geophysical exploration methods to adapt to the special working environment of the Three Gorges reservoir area.