Application of Physical Exploration Technology and Methods for Geological Hazard Investigation in Three Gorges Reservoir Area

Li Hongtao, Sun Dangsheng, Yang Qinhai, Yang Jinping

(Institute of Hydrogeology Engineering Geology Technology and Methods, China Geological Survey, Baoding, Hebei, 071051)

AbstractThis paper briefly describes the often-used methods of physical exploration technology in geological disaster investigation in the Three Gorges Reservoir area, as well as some typical examples of engineering, in order to bring a certain demonstration effect to the work in the future. Further provide advanced and effective testing means for geologic hazard investigation.

Keywords three gorges reservoir area geological disasters Survey physical exploration technology methods

1 Preface

From 1997 to 2004, China Geological Survey Institute of Hydrogeology Engineering Geology Technology and Methods undertook the comprehensive geophysical survey of the Three Gorges Reservoir area immigrants relocation to the new site of the major geologic hazards prevention and control study and demonstration, Fengjie Sanmashan district physical exploration survey, Badong Huangtu slope, Wanzhou Guantangkou landslide, and the Baidong Huangtu slope, as well as some typical engineering examples. Batong Huangtu Slope, Wanzhou Guantangkou Landslide Physical Exploration Survey, Chongqing 14 districts and counties bank survey and a number of applied research projects and physical exploration survey tasks. 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 experience of the application of geophysical exploration technology methods in the Three Gorges Reservoir Area geohazard prevention and control project, in order to bring a certain demonstration effect for future work, and further provide advanced and effective testing means for geohazard investigation.

2 Geophysical exploration technology methods

2.1 Shallow high-resolution seismic exploration

2.1.1 Working technology methods

(1) Expanded arrangement method

Considering the complexity of the topographic and geological conditions in the reservoir area, in Fengjie and Wushan, before arranging seismic profiles, as an important test method, both adopted the unfolding arrangement method. Its function is to understand the temporal arrangement of various waves in the seismic wave group in the survey area, to analyze the seismic phase, to determine the instrument parameters and observation system for data acquisition, to take appropriate excitation and reception measures, and to estimate the velocity parameters of the ground medium. The observation system of the unfolding arrangement method adopts different offset distances such as 0m, 10m, 20m, 30m, 40m, 50m, etc., and the channel spacing is 2m or 3m.

(2)***Depth Point Multiple Horizontal Superposition (CDP)

The CDP method of horizontal superposition is to collect the reflection waves from the same reflection point on different excitation and reception points, and then extract the reflection waves from the same reflection point on the interface of the multiple seismic records obtained by the method. interface on the *** reflection point channel set, after velocity scanning, dynamic and static correction, superposition processing, to give the geological interface and tectonic information in the form of time profile, this method can improve the signal-to-noise ratio, the suppression of interference waves have a significant role.CDP profile observation system in the selection of offset distance, is based on the relationship between the surface waves, acoustic waves and other interference waves and the purpose of the layer of the reflection wave to determine the relationship between the surface waves, acoustic waves and other interference waves, respectively, the use of 30m, 40m and 69m. 2m, 3m and 5m are used for the channel spacing. 6 times for most of the horizontal superposition times and 3 times for some of them.

(3) Seismic high-density imaging method

High-density imaging technology using a single excitation, single reception and other offset distance signal acquisition, its mode of operation is similar to the sonar method in the water, so it is also known as the land sonar method. The collected signals are amplitude compressed, color modulated, and displayed in the form of a color image. The offset distance of the high-density image method is 2m, and the point distance is 1m.

2.1.2 Field Data Acquisition Equipment

Seismic exploration adopts the SWS-1A multifunctional surface wave instrument of Beijing Institute of Hydropower Physical Exploration and the MARK6 lightweight multichannel seismic instrument of ABEM Company of Sweden. Receiving geophone with 38HZ high sensitive digital geophone with CDP lightweight cover cable. According to the depth of the target layer and the construction conditions of the survey area, two types of seismic sources, hammering and dynamite blasting, were used. Hammering source hammer weight 24 pounds, hammer pad thickness of 20mm, in order to increase the effective signal, suppress random interference, using vertical stacking, stacking times are generally 5 times. Explosive seismic source is generally excited in the gun hole, hole depth 1 ~ 2m, the amount of 100 ~ 200g.

2.1.3 Data processing

CSP.3.3 seismic data processing system is used for the data processing of CDP profile data. In view of the characteristics of the terrain in this area, which is characterized by large slopes and dramatic undulations, topographic corrections were made before and after stacking. The processing content also includes gain control, noise and interference wave removal, filtering, velocity analysis, dynamic correction and horizontal stacking, etc. The final output of the CDP horizontal iterative two-range reflection wave time profile containing topographic lines, and the resultant geologically interpreted map is completed under AutoCAD 14.0. The processing flow is shown in Figure 1.

Figure 1 Flow chart of shallow seismic data processing

2.2 Surface wave exploration

Transient surface wave (Rayleigh wave) exploration is used. When the surface is vertically excited with a seismic source, direct longitudinal waves, refracted longitudinal waves, reflected longitudinal waves and Rayleigh waves, as well as a variety of converted waves, are generally generated. Theoretical analysis and experiments show that, among all these waves, the energy of Rayleigh waves is the strongest, accounting for about 67%. Rayleigh wave is a kind of surface wave propagating along the surface, its propagation wavefront surface is a cylinder, the depth of propagation is about a wavelength. The use of Rayleigh wave dispersion characteristics, that is, different wavelengths of Rayleigh wave propagation characteristics reflect the characteristics of different depths of the geological body, the detection of geological media structure.

2.2.1 Instrumentation

Face wave exploration adopts SWS-1A multifunctional face wave instrument of Beijing Institute of Hydropower Physical Exploration, and the receiving detector adopts 4Hz low-frequency detector, and the face wave profile adopts 12-channel arrangement, with the channel distance of 1m, the point distance of 5m, and the offset distance of 0m, 5m, 10m, 15m, and 20m.

2.2.2.2 The surface waves are detected using the frequency dispersion characteristic of Rayleigh waves, i.e. different wavelength Rayleigh wave propagation characteristics reflect the characteristics of the geological medium structure. >

2.2.2 Data processing

The surface wave profile is processed by the FKSWSA surface wave processing system, which performs velocity and wave number (wavelength) filtering through multi-channel three-dimensional Fourier transforms in the time-space (T-X) domain and frequency-wave number (F-K) domain. wave number (wavelength) filtering in the time-space (T-X) and frequency-wave number (F-K) domains, eliminating non-face wave signals, effectively extracting surface wave information, plotting surface wave dispersion curves, and inverting and interpreting surface wave data.

FKSWSA surface wave processing system is characterized by fitting processing, i.e., the set stratigraphic structure parameters and the calculated stratigraphic parameters are judged by the correlation coefficient to determine the best stratigraphic structure inversion results.

2.3 Seismic tomography (CT)

Seismic tomography, similar to imaging techniques in other fields of science and technology, is a boundary projection inversion method. From the kinematic and dynamic characteristics of seismic waves, seismic tomography can be divided into two categories: ray tomography and fluctuation equation tomography. They respectively determine the change of information such as travel time, amplitude, phase and period of seismic waves, invert the three-dimensional velocity structure or attenuation characteristics of the geological medium, and represent the results in images.

Seismic CT data acquisition uses a combination of inter-well and well ground methods. The well ground method is to excite the elastic wave along the ground between two holes and receive it in the hole; the inter-well method is to excite it in one hole and receive it in the other hole. Receiving point distance of 2m and 1m, gun distance of 2m or depending on the conditions in the well to determine the composition of the upper and lower cross the observation system, in order to ensure that the ray coverage of the test area, improve the imaging accuracy.

2.3.1 Instrumentation

SWS-1A multifunctional surface wave instrument or MARK6 lightweight multi-channel seismometer.

Reception is by tandem airbag geophone coupled to the well wall.

An explosive seismic source is used with electric detonator excitation.

2.3.2 Data processing

Data processing was performed using the CST for Windows seismic tomography imaging system. Each imaging area was dissected into 2m×2m cells, and the density of ray nodes on each cell block was 10×10. The results are presented as wave velocity contour chromatograms, and the image output is realized by Winsurf 6.04. The processing flow is shown in Fig. 2.

Figure 2 Seismic Stratigraphic Imaging Data Processing Flow

2.4 EH-4 Conductivity Imaging

EH-4 conductivity imaging method is a partially controllable source and natural field combined with a kind of geomagnetic testing method. Different from the DC method, it is not through the extension of the cable and increase the pole distance to increase the depth of exploration, but in the measurement point, through its frequency to obtain the depth information.EH-4 in Fengjie County Baotaping Sanwantang ground collapse pit investigation, in the bottom of the pit arranged a north-south profile, the point spacing of 5m, the electroconductivity pole spacing of 15m, and the direction of the profile is the same. A profile was arranged on the surface of the south side of the collapse pit, with a point distance of 5m and an electric dipole distance of 10m.

2.4.1 Instrumentation

The EH-4 conductivity imaging system is jointly produced by GEOMETRLCS and EMI Company of the U.S. It is one of the most advanced imaging systems in the world at present, and it is the first one in the world to be used in the field of electrical conductivity imaging. It is one of the more advanced electromagnetic exploration instruments in the world.

2.4.2 EH-4 data processing

Including on-site data processing and follow-up processing of two major parts. Field data processing is mainly one-dimensional analysis, which is used to check the quality of data collected in the field and adjust parameters. The follow-up processing includes data analysis, one-dimensional data processing and display and simulated two-dimensional processing. The data analysis software is used to identify noise sources, estimate and adjust the signal level of the transmitter, and analyze the quality of data acquisition. One-dimensional data processing and display is the reprocessing of information after the new power spectrum is obtained after data analysis, which removes noise-heavy data to reduce divergence and increase signal correlation. Two-dimensional processing is the use of EMAP method for the proposed two-dimensional inversion, effectively eliminating the static effect, the construction of resistivity cross-section map, in the field to give the interpretation of the results of the grayscale map, through the computer two-dimensional inversion, color into the map.

2.5 Acoustic Logging Technology

Acoustic logging is based on the determination of acoustic velocity and amplitude of rock and ore, and it is a more effective method in dividing the bedrock lithology, the degree of weathering and fragmentation, and determining the location of fracture zones, the interface between bedrock and overburden, as well as determining the low-velocity layer in the overburden, bedrock, and other aspects.

Single-hole full-wave acoustic wave test is to use a hair double-receiving probe tube, transmitter-receiver source distance of 50cm, spacing 30cm, in the borehole (bare hole) along the wall transmitter-receiver acoustic information, logging will be down to the bottom of the well, according to a certain distance up the test, the computer to complete the full-wave data acquisition and data storage, the indoor through the playback and data processing. Pick up longitudinal and transverse waves, in the full wave train acquisition waveform according to the waveform interference points, amplitude, spectrum analysis, to determine the longitudinal and transverse, the wave to the beginning of the time, calculate the longitudinal wave, transverse wave velocity drawing results.

The instrument used for the test is SSJ-4D full wave train acoustic wave logger (Institute of Hydrogeology Engineering Geology Technology and Methods, China Geological Survey).

The downhole probe is divided into two types: dry hole against the wall type and water coupling.

3 Analysis of application results

3.1 Landslide accumulation

The landslide accumulation is a kind of loose accumulation with multiple genesis and phases. Most of them are landslides, avalanches, debris flow accumulations and karst collapse accumulations formed under tectonic and gravitational unloading and karst action. The purpose of geophysical exploration is to understand the thickness of the accumulation body and the deep structural characteristics, and the main working methods used are unfolding arrangement method, CDP profile and surface wave method.

3.1.1 Deep Structural Characteristics of Jintan Road-Xiangyun Road-Jixian Road in Wushan New Town Site

This area has brought great difficulties to seismic detection due to the large undulation of the terrain and the artificial backfilling of the gullies and other factors. Figure 3 (Section F) reflects the deep structural characteristics in the direction of Jintan Road-Xiangyun Road-Jixian Road. It can be seen that the complete bedrock is buried at a depth of 40-50m, and a deep trench as deep as 30m is formed between Xiangyun Road and Jixian Road. Figure 4 (Profile H) cross-cutting the headrace gully, the washout pattern is obvious. In the time profile, where in the part of the alluvial gully, due to cutting and weathering in a multi-homogeneous axial morphology, reflecting the complexity of the alluvial gully accumulation. The detection results clearly reflect the paraphyletic character of the accumulation.

3.1.2 Fine Structure Characteristics of Landslide Accumulators

In order to further suggest the fine structure characteristics of landslide accumulators, surface wave probing was used to understand the shallow geological structure. Figure 5 lists the typical dispersion curves and their geological interpretation results, and it can be seen that the surface wave survey can well provide the shallow stratigraphic details and its velocity distribution information. The results show that the interior of the landslide pile can be divided into three layers:

Figure 3 Results of shallow seismic survey at Wushan New Site Jintan Road-Jixian Road (Profile F)

The first layer: 0-3.15m, gravel-bearing clay layer, with a transverse wave velocity of 330-470m/s.

The second layer: 3-8m, gravel-bearing clay layer, with a transverse wave velocity of 470-770m/s.

The second layer: 3-8m, gravel-bearing clay layer. The transverse wave velocity is 470-770m/s.

The third layer: 8-16m, is broken rock layer, the transverse wave velocity is 770-970m/s.

3.1.3 Interpretation of the results

The depth of the landslide pile is about 40m, but there is a groove as deep as 70m between Xiangyun Road and Jixian Road. The bottom surface of the landslide accumulation is obviously in the direction of the rock layer, and the dip angle is up to 30°. In the landslide accumulation body, it can be subdivided into 3 layers, and its wave speed does not exceed 1000m/s, which indicates that its rock integrity is poor.

3.2 Landslides

The technical method adopted for landslide investigation is mainly CDP profile method, and the objects of investigation include Huangtuopo landslide in the new urban area of Badong County, Xiufeng Temple landslide in Wushan, Guantangkou landslide in Wanzhou District of Chongqing Municipality, and Cubic Bank landslide in the section of Yangtze River Bridge-Shangtuokou in Wanzhou District. In this paper, only some of the representative results are summarized as follows.

3.2.1 Badong County new city loess slope landslide

(1) seismic time profile wave group characteristics

Badong loess slope landslide *** made 9 profiles, this paper lists 2 profiles to be analyzed. From the time profile in Figure 6 (D profile), Figure 7 (C profile), it can be seen that there are one or two groups of reflected waves in the same phase axis, in which the T1 wave group is more stable, the time is about 30-60ms, its depth is 30-51m, this layer can be considered as the interface between the Quaternary landslide accretionary body and the underlying bedrock, the T2 wave group time is about 50-90ms, its depth is 52-76m, this layer can be considered as the bedrock weathering, and this layer can be considered as the bedrock weathering, and this layer can be considered as the bedrock weathering. This layer can be considered as the interface between the weathered bedrock and the intact bedrock. From Fig. 6 (section D) and Fig. 7 (section C), we can see that there is no indication of large fault traces, but the fissures (joints) are more developed, which form the rock fragmentation, and from the characteristics of the reflected wave, it forms the sign of the disorganized and weak reflections or the wrong breaks of the wave group.

Figure 4 Results of shallow seismic survey on Xiangyun Road (Profile H), the new site of Wushan

Figure 5 Results of wave survey on Jintan Road-Jixian Road, the new site of Wushan

Figure 6 Time profile of shallow seismic survey on the landslide of Huangtu Slope in Badong (Profile D)

Figure 7 Time profile of shallow seismic survey on the landslide of Huangtu Slope in Badong (Profiles C1 and C2) Time profile

(2) Geological interpretation

The seismic exploration results of Badong Huangtu slope landslide basically identified the thickness and spatial distribution range of the Quaternary loose accumulator and the thickness and distribution range of the landslide accumulator in the work area. The inferred geological interpretation map visualizes the bedrock burial depth and undulation pattern, and the distribution range of its burial depth is generally in the range of 50~90m. The anomalous distribution zone and location of bedrock weak structural surface in the work area were identified, and 21 bedrock fracture zones and fissure development zones were inferred by *** interpretation***.

3.2.2 Wushan Xufeng Temple landslide

(1) wave group characteristics of the seismic time profile

Wushan Xufeng Temple landslide *** did 8 shallow seismic profile, this paper lists one of the typical seismic profile in Figure 8, from the time profile can be seen, there is a group of one or two groups of reflected waves in the same axis, one of the group is more stable, the time of 50ms (after eliminating the effects of terrain). after eliminating the influence of topography). This layer can be regarded as the interface between the landslide accumulation and the underlying bedrock, and its depth is generally about 30m. It is also reflected for some interfaces with different structural characteristics, such as weathered rock bodies. The time is generally about 75ms, which is inferred to be the interface between the intact bedrock and the weathered rock body or the fragmented rock layer. In addition, in Figure 8, CDP point 120 ~ 140 reflection wave with the same phase axis downward depression or even cusp extinction, combined with the site geology, this location for an ancient temple is located in the location of the seismic reflection wave this phenomenon may be due to the ancient engineering artificial excavation caused by the stratum of the wave impedance interface differences.

Figure 8 Wushan Xufeng Temple D3 shallow seismic survey results

(2) Geological Interpretation

The eight shallow seismic profiles completed for the Wushan Xufeng Temple landslide basically identified the thickness and spatial morphology of the landslide accretion, and the inferred geologic map intuitively reflects the morphology of the bedrock and the thickness change of the overburden. In addition to the bedrock surface, there are some homoclinic axes on the CDP profiles, which are real reflections of the seismic wave geologic information, such as the homoclinic discontinuity reflected in line D3 coincides with the location of the old temple. The 8 profiles of the Xufeng Temple landslide demonstrate that the thickness of the Xufeng Temple landslide accretion is about between 25 and 35m.

3.2.3 Surveys on the Cushion Bank Landslide of Yangtze River Bridge - Shangtuokou Section, Wanzhou District, Chongqing

(1) Wave Group Characteristics of Seismic Profiles

Five CDP shallow seismic profiles were made for the survey of the Cushion Bank Landslide of Shangtuokou Section of the Yangtze River Bridge in Wanzhou ****. Figure 9 and Figure 10 are two typical profiles, from Figure 7 and Figure 8, the wave group characteristics of seismic reflection waves are more obvious, generally continue 1 to 2 phases, from the wave phase, energy, waveforms, continuity and other aspects of the comparison, in which the T1 wave group for the Fourth Series landslide accretionary layer and the underlying bedrock (weathering layer) of the interface, the continuity of the reflected waves of the layer and the phase characteristics of the analysis and judgement of the thickness change of the collapse and sliding accretionary layer, the main basis. The T2 reflection layer is inferred to be the reflection inside the bedrock, which is the main basis for inferring the depth and undulation pattern of the bedrock, and it reflects the lateral change characteristics of the bedrock weathering crust and the lithology of the weak rock layer.

(2) Geological interpretation

The five shallow seismic profiles completed for the landslide on the bank of the Yangtze River Bridge Shangtuokou section basically identified the thickness and spatial pattern of the landslide accumulation. The inferred geological map visualizes the thickness and distribution range of the fourth system landslide accumulation layer, and the average thickness of the landslide accumulation layer is 3.5-9 m. The thickness of the bedrock weathering crust within the scope of the work area is basically determined, and the average thickness of the bedrock weathering crust is about 14-17 m. The thickness of the bedrock weathering crust within the scope of the work area is basically determined. The bedrock depth and undulation pattern were determined. The anomalous distribution and structural characteristics of the bedrock structural surface in the work area have been inferred and interpreted accordingly, ***Interpretation of the inferred bedrock fracture zone and fissure development zone *** counted 11.

3.2.4 Sonic logging of Chongqing Wanzhou Guantangkou Landslide Group and Badong County New City Site Landslide Body

The sonic logging exploration of Chongqing Wanzhou Guantangkou Landslide Group and Badong County New City Site Landslide Body is aimed at combining with the geologic investigation to assess the division of lithology and integrity, and to determine the location of the slip zone and the fragmentation zone.

Figure 9 C-C′ shallow seismic survey results of Wanzhou Yangtze River Bridge-Shangtuokou section bank (collapsed bank) protection project

Figure 10 D-D′ shallow seismic survey results of Wanzhou Yangtze River Bridge-Shangtuokou section bank (collapsed bank) protection project

Figure 11 D-D′ shallow seismic survey results of Wanzhou Yangtze River Bridge-Shangtuokou section bank (collapsed bank) protection project. Survey results

Wanzhou Guantangkou landslide group totaled 13 boreholes were observed, Badong Huangtuopo landslide 12 boreholes were observed, Figure 11 shows the typical sound (wave) velocity - borehole depth curve of Guantangkou ZK3, which is the result of the original recordings of the acoustic wave train and its extracted acoustic time difference - borehole depth curves and the calculated sound velocity - borehole depth curves. the sound velocity-hole depth curve plotted. As a result, the boundary between bedrock and overburden can be clearly made, and it can also be seen that: the sound velocity of the bedrock part is above 3500m/s, and the sound velocity of the fissure development zone is somewhat lower; the upper overburden can be divided into two layers of the average sound velocity of 1800m/s and 2200m/s, and the change of their velocities indicates that the content of the blocks and soils, the lithology of the blocks, and the stratigraphic structure have all changed to different degrees. Figure 12 shows the comparison between the sonic test curve graph and the borehole histogram. The low frequency of the curve and the small amplitude of the sonic wave between 20.5 and 24m are a reflection of the loose rock mass. Drill hole 20.5～24m indicates the existence of fissure fragmentation zone inside the intact rock body (see Figure 12). Figure 13 shows the typical sound (wave) velocity-hole depth curve of Badong ZK1. The wave velocity values of 66.0～67.5m and 77.5～84.5m sections are obviously increased to 3800m/s, which is considered to have entered into the bedrock, and the 68.0～77.0m section sandwiched in between is regarded to be a layer of weak interlayer with the frequency of received waveforms becoming lower and the velocity becoming lower from the image of variable area, and is considered to be a layer of weak interlayer and It was verified in the later treatment works.

Figure 11 ZK3 sonic logging results of Guantangkou landslide survey

Figure 12 Comparison between ZK7 sonic test curve graph and borehole histogram

Figure 13 Batong Huangtu Slope ZK1 borehole sonic logging results

The sonic test results of the 13 drilled boreholes in Wanzhou Guantangkou landslide group statistically show the average value of the sonic velocity of the different strata of the lithology in Tables 1 and 2.

Table 1 Main lithological wave velocity of Guantangkou landslide group

Table 2 Main lithological wave velocity of loess slope

Based on the analysis of logging and drilling data, it is deduced that there exists more than one slip zone in Guantangkou landslide. According to the test results, the location of the slip zone, which is inferred and interpreted, is the lithological demarcation part of the upper overburden and the underlying bedrock. From the analysis of the overall distribution of test holes, the front and back edges of the landslide body are shallow, the depth of the front edge is 20m, the depth of the back edge is 30m, and the depth of the middle part of the landslide body is 55m.

Sonic logging in the division of bedrock lithology, weathering and fragmentation, to determine the location of the fragmentation zone, bedrock and cover layer interface, as well as in the cover layer, bedrock to determine the low-velocity layer and so on is a more effective method.

3.3 Karst and Caves

3.3.1 Karst Collapse

Fengjie County, Baotaping sub-district of Zhaojialiangzi west of the bottom of the gully of the Sanwantang gully at the gentle slope, at 2:30 p.m. on May 30, 1997, collapse occurred, the formation of the collapse pit with a length of the short and long axes of 20 to 25m, about 20m deep. The profile was funnel-shaped, with a volume of about 6,000-7,000 m3, and the ground crack on the northeast side was less than 4 m from the newly relocated private house. the collapse attracted a great deal of attention from all walks of life, especially from leaders at all levels of the county party committee. In order to further find out the depth and extension development of the collapse pit, the group conducted a special research and used advanced EH-4 conductivity imaging system, high-resolution seismic survey, high-density resistivity method, audio geodetic field method and inter-well seismic stratigraphic imaging and other comprehensive physical exploration.

(1)EH-4 conductivity imaging

Figure 14 shows the EH-4 survey profile at the bottom of the collapse pit.

Figure 14 EH-4 survey profile of collapse pit bottom in Baotaping, Fengjie

From the figure, it can be seen that the complete bedrock interface is about 55m deep from the bottom of the pit downward, and together with the distance from the bottom of the pit to the surface, the interface of the bottom of the collapse pit is about 70m deep from the surface, and at the same time, this profile also reflects the difference in the degree of weathering and crushing of bedrock on the north and south sides of the pit, with a thick layer of clay layer overlaying the north side, and the bedrock is strongly weathered and crushed On the north side, the clay layer is thick and the bedrock is strongly weathered and crushed, while on the south side, there is a crushed bedrock section, the bottom boundary of which is about 55m away from the surface, and underneath which may be a karst channel. The result of this interpretation is consistent with the result of seismic B section.

(2) High-resolution seismic survey

Figure 15 reflects the deep structural features along the Baotaping collapse gully. The profile starts from the collapse pit, and the survey line is about 200m long and nearly north-south oriented. The geological structure of the area can be divided into four layers:

The first layer: the burial depth is 0-40m, and is dominated by block gravel sandwiched with clay layer.

The second layer: burial depth 40 to 70mm, broken and loose rock.

The third layer: burial depth of 70 to 100mm, a more complete rock body.

The fourth layer: buried depth of 100m or less, complete rock body.

In addition, two cross sections B and C were made from Shunchonggou in a nearly east-west direction (Figs. 16 and 17). The results show that the stratigraphic structure is similar to that revealed in Fig. 15, but the reflection interface on the south side of the collapse pit shows an upward curved arch, similar to the characteristics of the rounded wave, and is locally discontinuous, which is inferred to be a possible karst anomaly. The direction of its connecting line is consistent with the direction of the washout. The development depth of profile B is 55-60m, and that of profile C is 60-65m.

(3) Seismic CT profiles

In order to further find out the extension and development of the collapse pit, three seismic CT profiles were arranged in a targeted way, and the comprehensive analysis of the seismic CT imaging profiles based on the characteristics of the wave velocity images, the distribution of wave velocity contours combined with the drilling data is as follows (see Fig. 18).

Figure 15 Shallow seismic exploration results of BaoTaPing A line in Fengjie

Figure 16 Shallow seismic exploration results of B line in BaoTaPing in Fengjie

Figure 17 Shallow seismic exploration results of BaoTaPing C line in Fengjie

Figure 18 CT imaging map of the drilling holes of shallow seismic 1 line in BaoTaPing in Fengjie

a. The distribution of the longitudinal wave velocity in the whole work area is low, which is in the range of 0.8 to 3.8km/s. The wave velocity distribution of the upper part (50-60m) of the fragmented rocky soil is between 0.8 and 1.6km/s, and the wave velocity of the bedrock part is only 2.0-3.8km/s, which is the fragmented rocky section exposed by the borehole.

b.The velocity distribution of CT imaging shows an uneven shape, indicating that the bedrock part of the work area has developed joints and fissures, and the rock body is broken. The upper part of the fragmented rock-soil accumulation has different morphology and complex structure.

c. A series of interface features dipping from NW to SE can be seen in Fig. 18, which is presumed to be stratigraphic yielding or lithologic contact surfaces. This is consistent with the results of the interpretation of shallow seismic B and C profiles (Fig. 16 and Fig. 17).

In summary, near the collapse pit of Zhaojialiangzi in Baotaping, no large cavern was found in the bedrock part at the location of the CT profile. However, the results of high-resolution seismic and audio geodetic field show that in the downstream direction of the collapse pit, there is an anomalous zone of SN tectonic fragmentation in Shungou, which forms a groundwater channel, and plays a role of dissolving and migrating the medium of the stratum, with a depth of 50-60m.3.3.2 Caves

In order to cooperate with the "Demonstration Study on Geological Hazard Prevention, Treatment and Utilization of Chongqing Wushan New Town", there are two types of caves in the bedrock of Zhaojialiangzi, Wushan New Town.

In order to cooperate with the "Chongqing Wushan New Town Geological Hazard Prevention and Utilization Demonstration Study", three pairs of seismic wave CT were made in the foundation of Zhoujiabao unified house in Wushan New Town, and the CT imaging map of Zhoujiabao Drill Hole ZB5-ZB6 in Wushan County is shown in Fig. 19. Its velocity distribution is between 0.71 and 3.40 km/s, which is low compared with the intact chert, and the shallow karst is extremely developed.The rock body below 310m elevation is relatively intact, but its wave velocity is still low, which is inferred to be interpreted as a high number of fissures or small caverns, especially at the bottom of the ZB5-ZB6 profile, where there is a red area with a diameter of about 3m, which is inferred to be a cavern. From the elevation of 310m in hole ZB5 to the elevation of 280m in hole ZB6, there are 6 bead-like distribution of relatively independent closed red areas, which are inferred to be caverns formed by tectonic influence.

Figure 19 CT imaging map of borehole ZB5-ZB6 in Zhoujia Bao, Wushan County

4 Conclusion

Geological hazards are affected and controlled by a variety of natural and man-made complex factors, and their distribution, formation, occurrence, development and change are very complicated, especially in the Three Gorges Reservoir area, where the geologic and geographic conditions are complex and geological hazards are numerous, The distribution of geological hazards is wide and frequent. With the help of traditional geotechnical methods alone, it is no longer possible to accomplish the tasks of investigation, monitoring, forecasting and prevention, and the new technical methods are a powerful weapon to improve the conventional geological investigation methods, to realize the modernization of geological work, and a powerful means to make new progress and breakthroughs in geological work. In the whole process of the migration and relocation of the Three Gorges Reservoir area, the complexity of the geological problems brought great pressure to the migration and relocation, and also provided a broad place for the application of the new technology of investigation.

In the whole process of geological disaster investigation and prevention and rational development and utilization in the reservoir area, geophysical survey has been more widely used. Especially in the geological disaster investigation, the application of new technology of investigation is unprecedented no matter from the type of geological disaster involved, the type of method selected and its suitability and the work invested, the results achieved are also multi-faceted, outstanding, over the years I have used the advanced CT chromatography imaging, shallow seismic detection, surface wave exploration, high-density mapping, acoustic wave detection, EH-4 and other methods, to investigate the Three Gorges Reservoir, and to provide a more comprehensive survey of the geological disasters in the reservoir area, and the rational development and utilization of the geophysical survey. 4 and other methods, we have carried out demonstration studies on the distribution law of karst in the Three Gorges Reservoir area, the structure of collapse pits, landslides, and the distribution of human defense works, etc., which provide scientific basis for the prevention of geologic hazards, and are of important practical value and significance for guidance. However, due to the limitation of the geological interpretation of the theoretical basis of the physical exploration method and the complex geological conditions and poor working environment in the Three Gorges Reservoir Area, some of the results of the physical exploration work are often unsatisfactory. This requires us to be persevering, through the rational and effective use of geophysical exploration of new technologies (including the correct choice of physical exploration methods and their optimal combination of forms according to different geological conditions and purposes) on the existing physical exploration methods of the work layout, data acquisition and interpretation of the processing method proposed to improve, in order to adapt to the Three Gorges Reservoir Area, the special working environment.