Application of high-density resistivity method in landfill detection

1 Introduction

With the rapid development of my country's economy, domestic waste landfills have entered their peak period, and landfill leakage pollution incidents have occurred frequently, posing a serious threat to the groundwater ecosystem and the human environment. Therefore, how to determine the leakage of landfills Location and dynamic monitoring are currently the main problems to be solved. Among them, monitoring is to actually detect leaks and determine the specific location, and then repair and remediate them in time based on the results; detection is to conduct precise detection at the location or leakage area and then determine the location of the leakage, the form of pollution diffusion, and the scope of influence. Among the many detection methods currently being studied, the high-density resistivity method has the advantages of rapidity, low cost, large samples, rich and continuous information, and real-time dynamic monitoring of groundwater pollution diffusion trends. In this work, the high-density resistivity method was used to detect the landfill, and the surrounding rock boundary and bottom boundary of the landfill as well as the spatial shape of the landfill were carefully depicted. The measurement area was analyzed based on the measurement results. No leakage occurred.

2Instruments and equipment

This measurement equipment uses the EDGMD-60 produced by Chongqing Dingfeng Geological Exploration Co., Ltd. (Figure 1 ). Its main functions are: 1. Underground profile resistivity change detection, 2. Short-range Bluetooth wireless data transmission 3. Long-range 4G wireless data transmission ( Can remotely monitor and automatically control real-time data collection ) .

Main features of EDGMD-60 : The remote detection high-density electrical measurement system adopts fully digital automatic measurement, which can automatically compensate for natural potential, drift and electrode polarization. The instrument has integrated 60 -channel electrode conversion devices and does not require an external electrode converter. It can directly carry out 60 channels of high-density electrical measurement; it also supports cascade expansion of electrode converters, which can be expanded to up to 300 channels.

Figure 1 Instruments and equipment for high-density resistivity method

3 Principles of high-density electrical method

The high-density electrical method adopts an array exploration method. It is an electrical exploration method based on studying the difference in apparent resistivity of rocks and ores as the physical property basis, and studies the abnormal resistivity differences between underground media. Measure the distribution of underground stable current fields in two dimensions. During measurement, multiple electrodes are laid out at one time. These electrodes are arranged (see Figure 2 ). Each electrode is both a power supply electrode and a measurement electrode. Through the programmable multi-channel electrode converter, different electrode combinations and different electrode spacings can be selected to complete the rapid collection of field data.

Figure 2 Schematic diagram of high-density resistivity Farwenner setup

4Characteristics of landfills (leakage)

Landfills mostly choose to dump garbage directly in some low-lying areas or remote ravines. The bedrock of the site often has varying degrees of cracked fracture zones or joint-intensive zones along the direction of the ravine, or there is a permeable layer. When atmospheric rainfall occurs, Under the action of leaching, pollutants seep into the bottom of the landfill. Once the anti-seepage treatment at the bottom of the landfill is not up to standard, leakage pollution will occur in the landfill, which will bring many environmental problems and potential safety hazards to people's drinking water.

The leakage contaminated liquid enters the underground leakage channel from the leakage point and spreads outward. The leakage channel is often the underground fissure crushing zone, joint-intensive zone or permeable layer connected to the leakage point. The seepage direction is mostly towards the landfill. Downstream, the seepage diffusion rate is related to the hydraulic gradient, permeability coefficient and hydrodynamic dispersion coefficient. Therefore, the rainy season is a serious period for seepage pollution.

5Analysis of actual detection results

In this work, three high-density electrical measurement lines were arranged in a domestic landfill (Figure 3 ).

Two-dimensional apparent resistivity cross-section diagram, the measurement parameters are Wenner device: point distance 3m , 60 electrodes , measuring line length 177m , maximum isolation coefficient 19 , 570 effective measuring points on the section , DC power supply voltage 400V . It can be seen from the interpretation and analysis of each measuring line in Figure 4 that the high-density electrical method can accurately depict the resistivity shape of the surface cover soil, boundary surrounding sandstone and landfill within the measurement range.

Figure 3 High-density electrical engineering layout diagram

Figure 4 Apparent resistivity section of two-dimensional section of high-density electrical survey lines ( G1 , G2 , G3 )

Since the three survey lines are parallel and at the same intervals (line spacing 40m ) in space, the starting points of the survey lines are at the same vertical position. Therefore, the three survey lines can be customized in space and then used for three-dimensional visualization processing using Volxer software to see the measurement targets. The three-dimensional spatial shape of the body, the low-resistance (blue) isosurface represents the boundary of the landfill, and the high-resistance (red) isosurface represents the abnormal shape of the landfill area (Figure 5 ).

Figure 5 Three-dimensional visualized abnormal contour map

6. Establish model analysis and detection end

6.1 Two-dimensional cross-section model

Taking the G1 line as an example of interpretation results, a two-dimensional profile model was established to analyze the accuracy of the actual detection results. Res2dmod software was used to establish a forward modeling model. The parameters were: background field value of 300 Ω· m , surrounding rock resistance (bottom bedrock) rate value 60 Ω· m , landfill resistivity value 700 Ω· m , surface cover soil resistivity value 150 Ω· m ; measuring line length 180m , pole distance 3m , maximum layer depth 30m .

6.1.1 Forward analysis

Figure 6 shows that based on the forward measurement of the actual model measurement, the low-resistance boundaries on both sides can be displayed, the actual depth of detection is doubled, the overall abnormality is stretched laterally, and the high-resistance separated in the middle is connected together, and the horizontal resolution is relatively low. Poor, it affects the surface cover map and the bottom bedrock low resistance display is not obvious.

Figure 6 Two-dimensional section forward modeling

6.1.2 Inversion analysis

Use Res2dinv software to save the forward modeling results in the inversion data format for inversion calculation (Figure 7 ). It can be seen from the inversion results that the inversion shows that the abnormal results are consistent with the established model, and the details can also be reflected, that is The actual burial depth varies greatly. When using this software for inversion processing, you need to add a depth correction coefficient based on actual conditions.

Figure 7 Two-dimensional section forward modeling

6.2 Three-dimensional model

The number of layers in this model is 5 (Figure 8 ). The regional background value is 300 Ω· m , 60 Ω· m low-resistance anomaly is set at 0~5m on the first layer, 60 Ω· m low-resistance anomaly is set on both sides of 5~10m on the second layer, and 700 is set in the middle area of 10~15m on the third layer. Ω· m high resistance anomaly, no anomaly is set at 15~18m on the fourth layer, and a low resistance anomaly of 60 Ω· m is set at 18~25m on the fifth layer .

Figure 8 3D forward layered model

In this work, the RES3DMOD software was used in combination with the theoretical model to establish a three-dimensional model for forward and inverse calculations. The measurement parameters were 30 × 30 mesh and the polar distance was 3m .

Figure 9 Three-dimensional layered model forward calculation results diagram

After forward and inversion calculations, the maximum effective depth of the model is 25m . In order to better fit the original state of the model, Res3dinvx64 three-dimensional inversion software was used to perform inversion calculations. From the inversion results, it can be seen that the relative position and shape of the high-resistance area wrapped in the low-resistance area in the transverse and longitudinal directions can be compared with The forward model is consistent.

This model only simulates the high-resistivity landfill area under ideal conditions. There are certain differences with the actual measurement results, but the general trend is consistent. Both the high-resistance landfill area and the low-resistance surrounding rock boundary can be reflected, indicating that the two-dimensional The actual measurement results of the profile are true and reliable. It is recommended that true three-dimensional full measurement be used for data collection when conditions permit.

Figure 10 Three-dimensional model inversion calculation results diagram

7 Conclusion

1.       The EDGMD-60 produced by Chongqing Dingfeng Geological Exploration Co., Ltd. (which can be used for regular detection and measurement as well as real-time dynamic monitoring) can be effectively measured in the landfill, and the measurement results are stable and reliable.

2.       It can be seen from the measurement results that this measurement accurately depicts the boundary of the surrounding rock, the bottom boundary of the landfill, and the spatial shape of the landfill. According to the measurement results, no leakage occurred in the measurement area.

3.       Since the data are collected and measured on the same day during the same work, the apparent resistivity value of each measuring line can be a relatively fixed value. Professional software can be used to perform three-dimensional visualization processing of the two-dimensional data, and the data can be put together to establish longitudinal interpolation. The spatial position relationship can more accurately reflect the abnormal boundary and depict the abnormal shape three-dimensionally and intuitively.

4.       The results of two-dimensional measurement and three-dimensional visualization can only reflect the abnormal boundaries and volumes of isosurfaces displayed through interpolation results, and there is a certain deviation from the actual form. The reason for the analysis should be that on the one hand, there is no longitudinal data and the quantity is incomplete; On the other hand, the complex and uneven distribution of components within the landfill itself results in differences in resistivity.

5.       Based on the profile measurement results and the actual situation, establish a two-dimensional profile forward and inversion model for analysis. The actual measurement results can be matched, and the electrical method can also effectively characterize the anomalies, which is the corresponding software inversion. There is a deviation in depth, and a correction coefficient should be added to the actual measurement process to make the measurement results more scientific and accurate.

The three-dimensional forward and inverse models are consistent with the actual landfill. It is recommended to use the three-dimensional full measurement mode when measurement conditions are met. There are more measurement data points than two-dimensional measurement to obtain more information, and the details can be described more clearly and accurately.