| The purpose of high density electrical resistivity method (ERT) is to determine the resistivity distribution of the sounding soil volume. Artificially generated electric currents are supplied to the soil and the resulting potential differences are measured. Potential difference patterns provide information on the form of subsurface heterogeneities and of their electrical properties. It can image the spatial resistivity distribution using a large number of four electrode measurements. As a non-invasive method, ERT can also image temporal variations in the resistivity at the same location, and the electrode spacing can be adjusted to provide information about the resistivity atvarious scales. In this paper, through a lot of theoretical analysis and experiments, the application of ERT in MSW landfill to detect leachate and gas distribution were studied. First of all, a lot of lab experiments was carried out to research on the electrical characteristics of waste and we used the forward and inversion software to carry on the numerical simulation of the distribution forms of the waste leachate and gas. Secondly, the electrical monitoring system of the large scale landfill unit was independently designed and we proposed a set of methods to deal with abnormal data. Finally, through a large number of testing in the large landfill unit in different conditions, We obtained pattern and distribution of the landfill leachate and gas, and relevant results was applied to a simple waste landfill. Based on a large number of theoretical analysis and experiments, the main research results of this paper include:(1) In view of the electrical characteristics of solid waste, a lot of indoor tests were carried out. The test results show that there is a great difference between the resistivity of each component, distributing in 10-1～105Ω.m. In rock-soil medium, the landfill waste has a low resistance characteristic, and the resistivity characteristic range is 100～102Ω.m. The resistivity of each component of the waste after being saturated with leachate is less than 30 Ω.m. Subsequently, an orthogonal experiment on the resistivity of waste was carried out. The results show that the resistivity decreases with the increase of mass water content under the same porosity ratio.(2) The RES2MOD forward modeling software was used to build several geoelectric models of municipal solid waste leachate and gas distribution. The inversion analysis for the several models above was carried out by RES2DINV program. Numerical simulation results show that when there are multiple anomalies in the underground space, the apparent resistivity section measured by instruments (i.e. the forward modeling results), will be affected by the side of the anomalies. And it is difficult to fully reflect the distribution Characteristics of underground anomalies. The inversion results is able to comprehensively reflect the distribution of underground anomalies. The distribution pattern of abnormal body is closer to the theoretical model, but it also has generated illusion. In practice, therefore, it is necessary to sampling for electrical resistivity testing which can provide reliable parameters for inversion, in order to obtain more reliable interpretation results.(3) Based on the large scale landfill unit, the waste under different water levels was tested and studied. Under the condition of gravity water emptying, the apparent resistivity of solid waste is in the range of 10-1～103Ω.m, which is mainly distributed between 0～150Ω.m. Based on the correlation between resistivity and waste saturation, we speculated that there is a large area of stagnant water zone and gas penetration area in waste body. Second, for solid waste in high water level, the apparent resistivity of MSW under the leachate level is smaller and distributes more uniformly in a range of 20～37Ω.m; Above the leachate level, the apparent resistivity of MSW is larger and has a certain extent of dispersion, in a range of 48～121Ω.m. Therefore, the ERT can be used to determine the depth of the landfill leachate level.(4) In the continuous monitoring of five weeks, the resistivity of the waste below the leachate level is basically stable at 22～27Ω.m. According to the ERT test results, it is found that the area of several high resistance parts in waste body showed different changes with time, which are consistent with the test results of U tube. We speculated that the occurrence of these phenomena must be related to the accumulation and dissipation of the gas under the leachate level.(5) In a simple landfill site electrical detection test, the results show that the low resistance area tested by ERT and the area of the solid waste distribution has a good corresponding relationship. The location and scope of the MSW can be effectively reflected, which is able to provide favorable support for the late treatment of landfill site. In addition, by comparing the ERT-V1 test results with the nearby BZ07 and CZ09 drilling results, we can know the distribution of the depth of the soil layer in situ. However, when the geological conditions are more complex, the diameter of the cylinder is too small to make a comprehensive representative. In contrast, the ERT results can reflect the distribution range and trend of the underground soil layer more effectively. |
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