2.1 Purpose of the survey
2.2 Survey Method
Detection is carried out using environmental geophysical exploration, electrical resistivity tomography and electromagnetic.
The survey uses the GD-20 multi-channel workstation and the GEM-2 induction electromagnetic instrument, and together with the XRF, the measurement point information is collected by RTK.
3.2 Line Survey
Reasonable layout of the line is an important factor in obtaining high-quality data. This time, the high-density resistivity method and the induction electromagnetic method are comprehensively tested, and the area where the large-area suspected heavy metal soil pollution is distributed is detected by geophysical techniques. 16 lines and 140,587 square meters of induction electromagnetic test.
4.Electrical Resistivity Tomography Works
The resistivity method is a geophysical method based on the difference in dielectric resistivity. The detection principle of the DC resistance method is to establish an artificial electric field by using a direct current to pass a pair of current poles A and B into the ground. By the difference in conductivity between the interlayer media, another potential pair M, N can be used to measure the potential difference between the electric field between M and N, thereby obtaining the apparent resistivity of the formation and estimating the conductivity distribution of underground formation.
It is often presented as a cross-sectional equivalent image. Physical units often use resistivity (ohm-m) to represent different levels of color with different tones.
The main method of induction electromagnetic method (EM) is frequency domain electromagnetic method (FDEM), which generates an induced magnetic field in the formation through the instantaneous current of the coil, and calculates the apparent conductivity of the material by recording the phase difference of the secondary magnetic field. In a fixed case, the two-dimensional tangent plane distribution of the conductivity of the material at a specific depth can be delineatedt.
6.1 Background Area
6.2 EM Results
In the EM measurement area of the river bottom sediment, most of the regional conductivity is biased to be greater than the background conductivity, and only the local area is less than 50 mS/m. It is speculated that the sediment in the EM measurement area of the local block is contaminated by heavy metals.
6.3 ERT Results
The overall electrical properties can be roughly divided into three layers, and the electrical resistivity in the longitudinal direction should exhibit a “high-low-high” change. It is inferred that the surface layer is a gravel accumulation layer of river gravel, and the middle layer is 1.3 to 4 meters, which is presumed to be a river sediment layer and is a contaminated layer. The relatively high-resistance layer of the bottom layer is presumed to be a coarse-grained material of the river. At the same time, the results of the resistivity distribution of the anomaly zone are basically consistent with the results of the induction electromagnetic method.
The anomaly is mainly distributed in the shallow layer, the blue region, the resistivity is less than 10 Ohm-m, the draw depth is about 0-2 meters, and the average depth is about 2.7-7 meters, and there is a relatively high resistivity layer with a resistivity greater than 50 Ohm-m. The layer is distributed throughout the measurement area and the level is continuous. Combined with the analysis of geological data, the high-resistance layer is presumed to be a bedrock layer and is a water-repellent layer. At the same time, the results of surface resistivity distribution in this area are basically consistent with the results of induction electromagnetic method. A large range of heavy metal contaminated soil is mainly distributed on the east side of the site.
7.Advantages of geophysical methods