ERT method and induction electromagnetic method (EM) are used in the target area， Exploring the distribution and depth of heavy metal pollution in the plant area
1） Exploring the distribution and depth of heavy metal pollution in the plant area
2）Find out if the pollution in the plant area has spread to the downstream.
3）Provide reliable information for post-repair projects
(1) ERT system and ERT principle
GD-20 ERT system
GD-20 series D.C geo-electrical system is a world-leading multi-functional direct current (D.C) method instrument, developed from the latest digital and analog circuitry technology. It is capable of performing electrical resistivity tomography (ERT), self-potential (SP) and induced polarization (IP) measurements. Apart from conventional electrical soundings, it also supports cross-sectional profiling with 2D or 3D ERT arrays. This system is not only capable of conventional surface surveys, but also the geo-electrical measurements in borehole logging, coal mining tunnels, marine and submarine surveys. It is even equipped with online real-time field monitoring mode, enabling our clients to perform unattended and automated surveys remotely for long durations.
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.
Advantage：
1）Deep depth, small terrain effect
2）Graphical representation of underground structures
3）Can be applied to well investigation
4）2D, 3D, 4D analysis
5）More specific quantitative indicators
6）Resistivity 20% or more difference with high resolution
7）Can be combined with other electrical surveys
Disadvantage：
1）Have a certain in-line or lateral interference effect
2）If the background value is too similar to the target resistivity, the identification is difficult.
Resistivity ERT
（2）EM and the EM principle
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 delineated.
It is often presented in the form of a cut plane equivalent image or a single point measurement, depicting the difference in surface electrical distribution, and the conductivity of the physical unit. (mS/m), representing different heights and values in different shades.
Advantage
1）Quick test
2）Single-player and GPS
3）Completely lossless
4）Suitable for large-scale screening
Disadvantage
1）Comprehensive conductivity measurement
2）Measurement depth is limited by frequency
3）Urban measurement is susceptible to interference
Unable to identify specific types of pollutant
The survey selected geophysical exploration of Electrical Resistivity Tomography with 16 test lines and EM for an area of approximately 6,000 square meters. Using GPS positioning for the induction electromagnetic field，the point distance is 0.1-0.3m, and the detection target depth is within 15 meter.
Background EM results show that the overall level of conductivity in the background region is mainly below 20mS/m, that is, the purple region in the figure.
Background EM result
The background ERT results indicate that the resistivity distribution is between about 30 ohm.m and 300 ohm.m. There is no extremely high or very low resistivity anomaly in the area. Based on this, if the results in the plant area are extremely low resistance (<2 ohm.m), it is highly likely to be caused by heavy metal pollution.
Background ERT result
1.EM Result
Areas 1, 3 are the house and construction, and the high conductivity that appears above area 2 is also caused by housing, so these interference items are first excluded. Areas 2, 4, and 5 are the main pollution potential areas, and the area surrounded by the areas 3, 4, and 5 also has a very high pollution potential. This area is complicated by other buildings due to its large number of constructions. Subsequent further determination of the location and distribution of the pollution by ERT. According to the results in the figure, the high-conductivity anomalies appearing in the suspect area are over 200mS/m, even exceeding 250mS/m.。
EM result
2 ERT Result
Setup 13 ERT lines in the plant.
ERT lines
The distribution range of the resistivity of the test line ERT1~ERT3 is basically consistent with the background value, and it is speculated that the result of the line is no abnormal pollution area. At the beginning of the ERT4~5 line, there is a very low anomaly area, the resistivity is about 2 ohm.m, and its depth is 6~7 meters, which is presumed to be a high concentration pollution anomaly area. There are three suspicious low-resistance anomalies in ERT6. The resistivity is not very low as ERT4~5. It is speculated that the pollutants are not concentrated in the bottom of the line, but are mapped from the side to the line profile, or The concentration of pollutants here is low. The three line detection lines of ERT7~ERT9 are located in the high pollution area, and the results show a high degree of agreement with the EM results. The main heavy metal pollution potential is located in the middle of the line ERT7 and ERT8. Corresponding areas, from ERT7 to ERT9, the degree of pollution gradually diminished. Compared with the second half of the line, there are different degrees of light pollution potential at the front end of the three lines, and there is a serious pollution trend within about 13 meters.
Figure 5-7：Line ERT1 result
Figure 5-8：Line ERT2 result
Figure 5-9：Line ERT3、ERT4、ERT5 result
Figure 5-10：Line ERT6 result
Figure 5-11：Line ERT7、ERT8、ERT9 result
The front end of the two lines of ERT10~11 is a medium pollution potential area, which coincides with the front end of ERT4 and ERT5 in this pollution potential area. In the 0-64m of ERT10, the depth is about 7m and the ERT11 is 0-44m, and the depth is about 4m, which is the pollution potential. ERT12 is a line across moderately and heavily polluted areas. It has a light heavy metal pollution potential in the range of about 66-84m and a depth of about 8-18m. It is in the range of 112m to 168m and the depth is about 8m. There is a serious pollution potential. ERT13 is located in the heavily polluted area, and there is a significant low-resistance anomaly area, which is about 42-80m in the horizontal distance and about 6m in the depth of the line.
Figure 5-12：Line ERT10 result
Figure 5-13：Line ERT11 result
Figure 5-14：Line ERT12 result
Figure 5-15：Line ERT13 result
3 Result -- Outside of plant
Figure 5-16：Outside ERT line
Two lines of ERT14 and 15 are outside the plant. The length of Line ERT14 is 98m and line ERT15 is 158m，the spacing is 2m. The resistivity distribution is about 30-100ohm.m，there is two layers. The result of this area is not low resistance（≤2ohm.m）, so the pollution does not extend to the outside area.
Figure 5-17：Line ERT14和ERT15 result
The plane distribution range of the abnormal value of the measurement area is determined by the inductive electromagnetic method, and the distribution range of the abnormal value of the measurement area in the vertical direction is determined by the high-density resistivity method. The two methods are closely matched to obtain the geological body in the space of the measurement area. Characteristic parameter
The traditional way of investigating is to estimate the possible distribution through limited borehole sampling, and to estimate the extent and depth of the pollution distribution through the correlation of the results between the points; the geophysical prospecting is non-destructive. The method of measuring the landfill and stratum structure under the ground, in addition to the characteristics of no drilling, can also obtain continuous profile data through a wide range of measurements.
Geophysical methods investigate contaminated sites, no holes are exposed, prevent further spread of pollution, environmental protection, and no secondary pollution.
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