CN110673216A - Single-hole resistivity karst cave detection method - Google Patents
Single-hole resistivity karst cave detection method Download PDFInfo
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- CN110673216A CN110673216A CN201911031422.4A CN201911031422A CN110673216A CN 110673216 A CN110673216 A CN 110673216A CN 201911031422 A CN201911031422 A CN 201911031422A CN 110673216 A CN110673216 A CN 110673216A
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- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
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Abstract
The invention discloses a single-hole resistivity karst cave detection method, which comprises the following steps: 1. installing a detection device: embedding each electrode on a PVC (polyvinyl chloride) threading pipe, and connecting a copper core cable connected with the electrodes with an electrical resistivity tester and the electrodes on the earth surface after the copper core cable penetrates through the middle of the PVC threading pipe; 2. placing a detection device: putting the assembled PVC threading pipe into the advanced drilling hole; and/3, acquiring data, calculating apparent resistivity, drawing a coordinate graph, and finally obtaining the area where the karst cave is located through the coordinate graph. The invention adopts the drilling to go deep into the underground for detection, has the characteristics of large detection plane range, large detection depth, more obtained effective data and the like, and can eliminate partial interference in the stratum. Meanwhile, the apparent resistivity is calculated by using a basic resistivity calculation theory, the calculation process is simple, and the detection efficiency is improved.
Description
Technical Field
The invention relates to a single-hole resistivity karst cave detection method, and belongs to the technical field of underground karst cave detection.
Background
The karst cave is an underground cavity formed by limestone after being eroded by underground water for a long time and is divided into a soil cave, a non-filled karst cave, a semi-filled karst cave, a full-filled karst cave and a beaded karst cave. The soil layer conditions of the foundation of the building are closely related to the safety of the building, and the existence of the karst cave at the lower part of the foundation of the building can leave serious potential safety hazards for the building. Therefore, measures are needed to explore possible caverns, and to find out the type, size and position of the caverns, so as to process the caverns in advance. However, the traditional drilling detection range is very limited, and the ordinary resistivity rule is easy to cause that the resistivity depth measurement at the position of the karst cave is not obvious and accurate detection cannot be realized due to the fact that the karst cave is deep or the volume is small. And the surface wave method, the acoustic logging method, the transient electromagnetic method, the seismic radar method and the like have many defects in the aspects of economy and exploration range.
At present, the commonly adopted common resistivity method has the working principle that: two or two groups of iron power supply electrodes A, B are driven into the ground, dry cell or accumulator is used as power supply to supply power to underground, stable current field is established underground, instrument is used to observe the intensity of supply current I, two or two groups of copper measuring electrodes M, N are driven into the ground, the potential difference delta V between two electrodes is observed M, N, and resistivity formula is usedM, N apparent resistivity values at the interpolar midpoints were calculateds(unit: Ω · m). Then all the instruments move forward along the measuring line simultaneously, and the rho is measured and calculated point by pointsValues, p, along the survey line or region, are obtainedsAnd (4) a change rule of the value. In the formula, k is a device coefficient in a certain arrangement calculated from the distance between the feeding electrode A, B and the measuring electrode M, N, and k has a different value depending on the arrangement. The rho obtained from each observationsThe values are plotted as a graph, which is the original curve observed in the field. The original curve is processed indoor arrangement, and is processed forward and backward by a microcomputer, and is interpreted, and is drawn into a map, so that the approximate spatial distribution position of the karst cave in the ground can be obtained. From the 20 s of the 20 th centuryResearch, and later on, with the continuous updating of instruments and the continuous improvement of method theories and technologies, gradually derives a plurality of branch methods to be widely applied in various fields around the world. However, the analysis of the existing karst cave detection method is combined, so that the method has relative defects in the aspects of economy, applicability and complexity, and is mainly reflected in the following aspects:
1. the common resistivity method measuring points are arranged on the ground surface, and the distribution of current density in the ground is rapidly reduced along with the increase of the depth, so that the survey depth of electrical prospecting is limited.
2. When the current density passing through the karst cave with large burial depth is small, even if the current density has a remarkable difference with the resistivity of the overlying strata, the current density close to the ground is difficult to be influenced, so that the potential difference measured on the ground is not greatly influenced, and the accurate judgment cannot be carried out.
3. When very high resistivity insulating layers or low resistivity good conductors are present in the subsurface, this will prevent the current from being distributed deep in the subsurface, creating a so-called shielding effect, in which case the effective depth of investigation will be greatly reduced.
4. The common resistivity method needs to process a large amount of data after collecting data, and can perform forward and backward calculation and drawing imaging.
Disclosure of Invention
The invention aims to provide an economical and efficient single-hole resistivity karst cave detecting method to overcome the defects of the prior art.
The technical scheme of the invention is as follows: a single-hole resistivity karst cave detection method comprises the following steps:
s1, installing a detection device: embedding the electrodes on the PVC threading pipes, and connecting a copper core cable connected with the electrodes with a resistivity measuring instrument and the electrodes on the earth surface after penetrating through the PVC threading pipes, wherein the resistivity measuring instrument adopts an American R8 resistivity tester;
s2, placing a detection device: putting the assembled PVC threading pipe into the advanced drilling hole;
s3, collecting data: recording the measured potential value and current value by an American R8 resistivity tester by adopting three different running grade modes respectively;
and S4, respectively calculating apparent resistivities of the three running grade modes: using the basic resistivity calculation theory:
wherein k is a geometric parameter, and k is a geometric parameter,
three different k values can be calculated in three different running modes respectively;
wherein r isC1P1Is the distance between the C1 electrode and the P1 electrode, rC2P1Is the distance between the C2 electrode and the P1 electrode, rC1P2Is the distance between the C1 electrode and the P2 electrode, rC2P2Is the distance between the C2 electrode and the P2 electrode;
and S5, drawing a coordinate graph: apparent resistivity value rho on the y-axisaThe x axis is the coordinate of the distance from the measuring point to the ground, and rho is obtained through calculationaSubstituting and drawing a graph; the region where the resistivity suddenly decreases is the region where the karst cave is located.
In the method, the electrodes are embedded and fixed on the PVC threading pipe in groups at equal intervals.
The three different running level modes comprise a running level mode a, a running level mode b and a running level mode c; wherein the electrodes in the running stage mode a are arranged in the sequence of C1, P1, P2 and C2; the b electrodes are arranged in the sequence of C2, C1, P1 and P2; the running grade mode C electrodes are arranged in the sequence of C1, P1, C2 and P2; the distance between the electrodes can be determined according to the field condition, the electrodes are arranged at equal intervals, and the interval can be selected within the range of 0.2-0.5 m.
Due to the adoption of the technical scheme, the invention has the advantages that:
1. the detected electrode device is placed in the advanced drilling hole, so that the effective depth of detection is ensured;
2. the electrode device with multiple measuring points goes deep into the ground, so that more effective data can be obtained, and the detectable plane range is enlarged;
3. complex data processing and calculation are not needed;
4. inversion calculation and imaging are not needed;
5. the detection device is simple to install and short in detection time;
6. the detection device can be repeatedly used, and is economical and reasonable.
Drawings
FIG. 1 is a flow chart of the detection of the present invention;
FIG. 2 is an enlarged partial view of the electrode arrangement;
FIG. 3 is a schematic structural view in the practice of the present invention;
FIG. 4 is a schematic diagram of three run level approaches;
fig. 5 is an apparent resistivity and depth plot.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be further described in detail with reference to the accompanying drawings and examples.
The embodiment of the invention is that an existing project is located in a karst area, engineering geology, hydrogeology conditions and site environment geology conditions of a site foundation are found, and soil layers of the site are ① mucky soil with the average depth of 2.1m, ② fine sand with the average layer thickness of 2.49m, ③ coarse sand with the average thickness of 3.25m, ④ silty clay with the average thickness of 2.61m, ⑤ strongly weathered limestone, medium-weathered limestone and micro-limestone, wherein a drilling soil sample has cavities at the underground depths of 15m and 20 m.
Karst caves may exist underground the field, and further detection is needed. Referring to fig. 1, the embodiment adopts a single-hole resistivity karst cave detection method, and the specific implementation steps are as follows:
1. installing a detection device: a copper outer hexagon bolt with the model number of M10 multiplied by 90 is adopted as the electrode 3; a PVC threading pipe 1 with the outer diameter of 75mm and the wall thickness of 3mm is adopted. 20 PVC threading pipes 1 are used in total, each length is 1.2m, and the PVC threading pipes are connected by equal-diameter joints. Holes with the aperture of 10mm are respectively punched on two sides of each PVC threading pipe 1 along the diameter direction, and the distance between the holes along the length direction is 0.4 m. The electrode 3 is passed through the hole and fixed by a copper hexagonal nut. Referring to fig. 2, a copper core cable 2 connected with electrodes 3 passes through the middle of a PVC threading pipe 1 and is connected with a US R8 resistivity tester 4 on the ground surface, the electrodes 3 are embedded on the PVC threading pipe 1 in a plurality of groups, wherein the electrodes a in a running grade mode are arranged in the sequence of C1, P1, P2 and C2; the b electrodes are arranged in the sequence of C2, C1, P1 and P2; the running grade mode C electrodes are arranged in the sequence of C1, P1, C2 and P2;
2. placing a detection device: the assembled PVC threading pipe 1 is put into a pilot borehole with the diameter of 100mm and the depth of 24m, as shown in figure 3:
3. collecting data: recording the measured potential value and current value by the resistivity tester 4 of the American R8 in three different running grade modes; three different grading modes: as shown in FIG. 4, the electrodes are arranged at equal intervals, and the pitch in this embodiment is 0.4m, wherein C1 and C2 are power supply electrodes, P1 and P2 are measuring electrodes, and the distance between the two electrodes is one electrode pitch. When the PVC threading pipe 1 has n electrode points, taking the measuring point 1 as an example: c1 inputs current at electrode point 1, C2 outputs current at electrode point 4, P1 inputs potential at electrode point 2, and P2 outputs potential at electrode point 3, so that first data can be obtained; under the condition that the electrode distance of the measuring point 2 is kept unchanged, the electrode points 2, 3, 4 and 5 are used for obtaining second data; measuring point 3 uses electrodes 3, 4, 5, 6 to obtain third data … … and so on, and measuring point n-3 uses electrodes n-3, n-2, n-1, n to obtain the n-3 th data. The total number of data is n-3. In this embodiment, 60 electrode points are arranged in total, and each run stage mode has 57 data, and the total number of data obtained is 171 in all three modes.
4. Calculating apparent resistivity:
the potential values at the C1 and C2 midpoint in the soil layer medium are:
wherein r isC1And rC2The distance of the point from the C1 and C2 current electrodes, respectively;
the potential difference between the two points is:
using the basic resistivity calculation theory:
wherein k is a geometric parameter, and k is a geometric parameter,
wherein r isC1P1Is the distance between the C1 electrode and the P1 electrode, rC2P1Is the distance between the C2 electrode and the P1 electrode, rC1P2Is the distance between the C1 electrode and the P2 electrode, rC2P2Is the distance between the C2 electrode and the P2 electrode;
in this embodiment, the run level mode a: r isC1P1＝0.4m，rC2P1＝0.8m，rC1P2＝0.8m，rC2P2Substituting 0.4m into the formula to obtain
Run level mode b: r isC1P1＝0.4m，rC2P1＝0.8m，rC1P2＝0.8m，rC2P2Substituting 1.2m into the formula to obtain
Step c: r isC1P1＝0.4m，rC2P1＝0.4m，rC1P2＝1.2m，rC2P2Substituting 0.4m into the formula to obtain
5. Drawing a coordinate graph: apparent resistivity value rho on the y-axisaThe x axis is the coordinate of the distance from the measuring point to the ground, and rho is obtained through calculationaSubstituting and drawing a graph; as shown in FIG. 5, the resistivity calculated by three different run-level methods has no big difference, and two points which are suddenly reduced in the graph are the areas where the karst caves are located。
In conclusion, the method adopts the drilling to go deep into the underground detection, has the characteristics of large detection plane range, large detection depth, more obtained effective data and the like, and can eliminate partial interference in the stratum. Meanwhile, the apparent resistivity is calculated by using a basic resistivity calculation theory, the calculation process is simple, and the detection efficiency is improved.
Claims (3)
1. A single-hole resistivity karst cave detection method is characterized by comprising the following steps:
s1, installing a detection device: embedding the electrode on the PVC threading pipe, and connecting a copper core cable connected with the electrode with an electrical resistivity measuring instrument on the earth surface after penetrating through the middle of the PVC threading pipe;
s2, placing a detection device: putting the assembled PVC threading pipe into the advanced drilling hole;
s3, collecting data: recording the measured potential value and current value by a resistivity measuring instrument in three different running-level modes;
and S4, respectively calculating apparent resistivities of the three running grade modes: using the basic resistivity calculation theory:
wherein k is a geometric parameter, and k is a geometric parameter,
wherein r isC1P1Is the distance between the C1 electrode and the P1 electrode, rC2P1Is the distance between the C2 electrode and the P1 electrode, rC1P2Is the distance between the C1 electrode and the P2 electrode, rC2P2Is the distance between the C2 electrode and the P2 electrode;
and S5, drawing a coordinate graph: apparent resistivity value rho on the y-axisaThe x axis is the coordinate of the distance from the measuring point to the ground, and rho is obtained through calculationaSubstituting and drawing a graph; the region where the resistivity suddenly decreases is the region where the karst cave is located.
2. The method for detecting karst cave according to the resistivity of single hole in claim 1, wherein: the electrodes are embedded and fixed on the PVC threading pipe in a plurality of groups at equal intervals.
3. The method for detecting karst cave according to the resistivity of single hole in claim 1, wherein: the three different running level modes comprise a running level mode a, a running level mode b and a running level mode c; wherein the electrodes of running stage mode a are arranged in the sequence of C1, P1, P2 and C2; the electrodes of the running stage mode b are arranged in the sequence of C2, C1, P1 and P2; the electrodes of running grade mode C are arranged in the sequence of C1, P1, C2 and P2.
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| CN117031552A (en) * | 2023-08-16 | 2023-11-10 | 河海大学 | Resistivity tomography measuring system and method based on multichannel electrode design |
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