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abstract:
Because the traditional geological engineering survey method has a large residual error in practical application, the application of cross-hole elastic wave CT in geological engineering survey is put forward. Three-dimensional detection of geological engineering exploration area is carried out by using cross-well elastic wave CT to obtain geological engineering rock mass information. On this basis, the geological three-dimensional space exploration data are imported into the grid model, and the data are analyzed by grid, forming a three-dimensional model map of geological engineering, understanding the spatial distribution and development of geological engineering rock mass, thus completing geological engineering investigation. Experiments show that the residual value of geological engineering investigation results based on cross-hole elastic wave CT is smaller than that of traditional methods, and it is more suitable for geological engineering investigation.
Key words:
Cross-well elastic wave CT; Geological engineering investigation; Three-dimensional space; Grid model; Spatial distribution;
Introduction:
Cross-well elastic wave CT can be divided into seismic cross-well elastic wave CT and ultrasonic CT. On the premise of not damaging the measured object, elastic wave rays penetrate the measured object, and acoustic wave rays are received by the signal receiving system at the other end of the object. Generally speaking, cross-hole elastic wave CT is a multi-sound receiving system, that is, elastic wave rays are emitted at a single point on an object, received in a fan-shaped arrangement at the other end of the object, and then moved synchronously from top to bottom along the other end of the object for scanning observation [1]. With the advantages of high efficiency, non-destructive, high precision and high sensitivity, cross-hole elastic wave CT has been widely used in medical, biological, aerospace and other measurement fields. In foreign countries, cross-hole elastic wave CT technology has been applied to the field of geological exploration, and has achieved certain application results, but there is no application experience in this field in China. At present, the traditional manual survey method is still used in geological engineering survey. In practical application, the traditional survey methods mainly rely on the survey experience and technology of personnel, so it is difficult to obtain accurate internal information of rock mass, which can realize two-dimensional surface detection, but can not realize three-dimensional surface detection. With the increasing difficulty and requirements of geological engineering survey, the traditional survey methods can no longer meet the requirements of geological engineering survey in accuracy and efficiency. Therefore, the application research of cross-hole elastic wave CT in geological engineering investigation is put forward, which provides a theoretical basis for the application of cross-hole elastic wave CT in geological engineering investigation.
1, geological engineering exploration method based on cross-hole elastic wave CT
The main content of geological engineering investigation is to understand the distribution and development of deep geological rock mass through investigation, and then infer the spatial position and fracture development of deep geological rock mass. In this paper, cross-hole elastic wave CT technology is used as the main measurement method of geological engineering, and three-dimensional exploration of geological engineering is carried out to obtain rock mass information of geological engineering, and the geological engineering investigation results are obtained by analyzing rock mass information. The following will describe in detail the geological engineering investigation method based on cross-well elastic wave CT.
1. 1. Geological 3D space exploration based on cross-hole elastic wave CT
Usually, the strata of geological engineering sites are mainly silty soil and gravelly soil, and the longitudinal wave velocity Vp of these two soils is between1200m/s and1500m/s. The bedrock of engineering sites is usually shale, and the longitudinal wave velocity Vp of moderately weathered shale is between 2400m/s and 3500m/s.. The Vp of P-wave acoustic beam in the deep rock mass development area of the site is 1200m/s ~ 1500m/s, and there are obvious acoustic beam differences among different geology. Therefore, according to the requirements of geological engineering exploration, cross-well elastic wave CT technology is selected for 3D geological exploration to obtain data information of deep rock mass for subsequent 3D geological exploration data analysis. The detection process is as follows [2].
According to the actual situation and detection requirements, this paper adopts LOKP-S2S 1 cross-hole elastic wave CT scanner as detection equipment. First, use HIS62 geological core drilling rig to drill holes on the surface in a rotating way. In order to obtain geological three-dimensional spatial data more accurately, the drilling spacing should be relatively small, usually depending on the actual situation, ranging from 5.5 m to 7.5 m, the drilling aperture size should be controlled between 100mm ~ 130mm, and the drilling depth should be between 35 m and 55 m to avoid geological damage. After the hole is formed, the PVC sleeve wall with a diameter of 75 mm is put into the hole for the cross-hole elastic wave CT scanner to work.
After the completion of drilling, two relatively close drilling holes are selected as elastic wave CT emission drilling holes, and the cross-hole elastic wave CT scanner explores geology by exciting high-frequency elastic waves. Then the borehole on the other side is selected as the elastic wave CT receiving borehole, and the cross-hole elastic wave CT scanner has a built-in sensing device for receiving elastic wave signals reflected from the geology. In the detection process, it is necessary to turn on the noise monitoring function of the cross-hole elastic wave ct scanner and select the time range with relatively small noise as the signal receiving time period to ensure the accuracy of the three-dimensional scanning detection data of the cross-hole elastic wave CT scanner. The elastic wave CT signals received by the sensing device are read by the computer and stored uniformly for the analysis of geological three-dimensional space detection data.
1.2, geological three-dimensional space detection data analysis
The data collected by cross-hole elastic wave CT scanner are all related to elastic waves. In order to understand the spatial position and fracture development of deep rock mass through these data, it is necessary to process and analyze the three-dimensional space detection data. Firstly, the survey area is discretized into grid units of several specifications, and the geological three-dimensional space detection data are imported into the grid units to form a grid model, which is expressed as follows by the formula:
In the formula (1), ti is the grid model of the discrete area of geological engineering, that is, the travel time of the first elastic wave ray; Dij is the length of the i-th elastic wave ray passing through the j-th grid; N is the number of elastic wave rays; M is the number of grids in the discrete regional grid model of geological engineering; V is the velocity of elastic wave; (x, y) is the path of elastic wave ray. By solving the above formula with joint iteration method, the slowness value of geological elastic wave in each small square can be obtained, and the rock mass distribution between boreholes can be obtained by taking their inverse values respectively. Using the above formula, draw the three-dimensional model map of each discrete geological engineering rock mass, and fuse all the discrete regional three-dimensional model maps of rock mass to form the three-dimensional distribution map of geological engineering exploration rock mass. Through this map, we can understand the development and distribution of geological engineering rock mass, determine the specific development position and scope of rock mass, and thus complete the three-dimensional spatial distribution map of geological engineering exploration rock mass.
2. Experimental demonstration and analysis
The experiment takes a geological engineering as the experimental object, and the investigation area of this geological engineering is 6642.5m? The geological soil layer is silty clay, gravel soil, etc. And the wave velocity is different. In this experiment, the design method and traditional method are used for geological engineering investigation, and the distribution of deep rock mass is obtained.
According to the actual situation of geological engineering, there are 10 survey lines in the survey area, with 5 boreholes for each survey line, including 50 boreholes for the * * * meter and 5 pairs of cross-hole elastic wave CCTT profiles. . In the actual experiment, the diameter of the geological borehole in the middle project is 1 10000 mmm, and the depth of the borehole is 45 m. The elastic wave ct of the borehole is used to scan each borehole for three times, and a group of data with the least noise fluctuation in the three scans is taken as the exploration data, and the data 1235.52MB is obtained. Based on this, the geological data are analyzed. Residual value is an effective index for evaluating accuracy, and its value range is 0- 1. The closer the residual value is to 1, the lower the accuracy of the survey results, the greater the error between the survey results and the actual values, and the smaller the degree of conformity. The closer the residual value is to 0, the higher the accuracy of the survey results, the smaller the error between the survey results and the actual values, and the higher the coincidence degree. In the experiment, the residual value is taken as the experimental result, and two geological engineering investigation methods are compared and analyzed. The experimental results are shown in the following figure.
It can be clearly seen from the above figure that the minimum residual value of geological engineering survey method based on cross-well elastic wave CT can reach 0.42δ, and the evaluation residual value is only 0.63δ. However, the maximum residual value of the traditional method is 0.96δ, and the average residual value is 0.89δ, which is much higher than the design method. Therefore, the experiment proves that the geological engineering survey method based on cross-hole elastic wave CT has higher survey accuracy than the traditional method and is more suitable for geological engineering survey.
3. Conclusion
In this paper, the application of cross-hole elastic wave CT in geological engineering investigation is studied. The cross-hole elastic wave CT technology is used to scan and obtain the relevant information of geological engineering, and then it is processed and analyzed to form a new geological engineering investigation method. The application of cross-hole elastic wave ct technology effectively overcomes the difficulty of obtaining accurate information of deep rock mass by traditional survey methods, improves the efficiency and accuracy of geological engineering survey, helps to realize three-dimensional space detection of geological engineering, and has a certain promotion effect on the application of cross-hole elastic wave CT in geological engineering survey.
refer to
[1] Xing Zhiqiang, application of cross-hole elastic wave CT technology in karst exploration of a ship lock engineering site [J] Western exploration project 2018,30 (071)11.
Yu Hui, Huang Yuanliang, Wang Jiqing. Application of CT scanning technology of cross-hole elastic wave in pile foundation investigation in karst area [A]. Proceedings of Pan-Yangtze River Delta Highway Development Forum of Jiangsu Comprehensive Transportation Society.2019 [c]. Jiangsu Comprehensive Transportation Society Highway Branch: Jiangsu Highway Society, 2019.6.
Lu Zechang, Shi Yanchao, Zhang Jincai. Research on 3D modeling technology of engineering geology and its application [J] China Standardization, 2019 (04):104-105.
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