Figure 1.2 plant aerial photo layoutE
1.2 Working purpose and task
The underground pollution of the site is entrusted to be investigated, and the purposes of the geophysical exploration task are:
High resolution resistivity method (i.e. ERT electrical method) is used for the soil pollution detection.
1.3 Project implementation and measuring line layout
According to site survey and interview content, non-destructive geophysical method is selected for investigating areas with high pollution potential and serious pollution diffusion, which are located inside production area ② and near sewage treatment tank respectively. Construction is relatively difficult, due to the limited time and cement hardened pavement. Three measuring lines are arranged for analysis as shown in the red area of Fig. 1.3.
FIG . 1 . 3 survey line arrangement region
A horizontal measuring line is arranged in the production area ② printing and dyeing workshop. Lawn in the workshop is badly damaged, and cement hardening integrity is very poor, which cause the printing and dyeing materials directly contact the foundation soil. The first location selected of the original measuring line is the inside of the groove which is always used for wastewater or material transportation or is a barrier area. If there are structural cracks, the wastewater and other possible pollutants will permeate into the soil, polluting soil at the bottom. However, the groove hasn’t been used for a long time, many material wastes, sludge and water are accumulated inside the groove (Fig. 1.5), and there is a hardened base at the bottom of the groove, which is not suitable for electric drill construction and electrode arrangement. Therefore, a transverse resistivity measuring line (Fig. 1.4 and Fig. 1.6) is arranged in the area where serious damages to buildings can be seen nearby to investigate whether there is any pollutant intrusion. The resistivity measuring line has 32 electrodes in total, and the electrode interval is 1m, so the total length is 31m, which is named as line 1 with the gate side as the starting point.t
Figure 1.6 resistivity line 1 actual layout
At the same time, resistivity measuring line 2 (Fig. 1.8) and resistivity measuring line 3 (Fig. 1.9) are arranged in the area around the wastewater treatment tank in a crossed mode, as shown in Fig. 2.5. The reason of the measuring line is mainly for the wastewater treatment tank to collect all waste water and residues of printing and dyeing. However, as a larger pollution source, if the discharge is irregular or the reaction tank itself has leakage, the wastewater will immerse into the soil and cause pollution. Within a limited period of time, focus should be put on the areas which have the most possibility of pollution leakage. For example, two resistivity measuring lines are arranged in the middle and the outer part of the two waste water tanks to investigate the pollution situation, as shown in Fig. 1.7. The resistivity line 2 in the figure extends to the inside of the left pool with a unknown function, which is presumably the rainwater accumulation formed by the buildings. The measuring line has 35 electrodes in total with the electrode interval of 1 m, starting from the end of the water tank. The resistivity measuring line 3 extends to the inside of the production area ⑥, and it has 29 electrodes in total with the electrode spacing of 1m, starting from area nearby the production area ②. The two measuring lines meet at the 11th electrode of the resistivity line 1 and the 6th electrode of the resistivity measuring line 2.
Figure 1.7 resistivity line 2 , 3 aerial photo
Figure 1.8 Survey line 2 layout
Figure 1.9 Survey line 3 layoutc
2 working methods and techniques
Geophysical exploration technology is a non-invasive and non-destructive testing technology (NDT), which does not cause invasive damage to the detected object, and has the advantages of large detection range (horizontal and perpendicular range), high speed, high accuracy and etc. The different formation lithology and adhesive state results in different conductive properties which is generally represented by resistivity. Therefore, the resistivity corresponding to different formation lithology and moisture content is indirectly known by detecting the change of formation resistivity in vertical and horizontal directions.
This investigation uses ERT section method to conduct large-area hydrogeological investigation, so as to judge the overall geological structure characteristics in the industrial zone and the abnormal area of formation materials, distinguish the pollution accumulation area and diffusion, and reduce the penetration of the water barrier layer during subsequent well setting and soil sampling operation, to avoid the pollution diffusion to different aquifers.
2. 1 Survey network arrangement
The principle of the geophysical exploration measuring line arrangement is to be according to the actual situation of the factory, the orientation should be determined by a compass, the distance of the point position should be measured by a tape.
Data acquisition with ERT resistivity method: connect the measuring instrument –arrange the cables and electrodes according to the location of the measuring line-connect the cables and converters – input parameters and measure ground resistance – enter data acquisition mode after grounding well – collect data. Other field data collection is carried out in accordance with the relevant regulations or specifications issued by the state.
(A) equipment requirements
ERT resistivity equipment can be divided into:
This geophysical method of this method is based on the site topography, ground features and the target to be measured. At present, pole – pole array, wenner array, Schlumberger array and other various array and other methods are commonly used in domestic, and the array methods should be decided according to the actual situation.
(B) Operation Procedure
The factory operation procedure with ERT resistivity method is as follows:
（6）Connecting the cable leader, ERT cable and the mainframe
(7) Measuring the ground resistance of the electrodes to ensure that electricity flows into the ground and, if necessary, filling saline water at grounding position of the electrode bar or replacing the original electrode bar with several electrodes used in parallel.
(8) Connecting the battery with relevant joints.
(9) The electrical current is used to measure the resistance reaction of different electrodes (corresponding to different depths).
(10) When measuring, the operator must pay attention to the reaction between each electrode on the mainframe, and adjust the measuring parameters or measure again when necessary.
2.2 Array technology
The instrument used in this site investigation is the ERT system independently developed by Shenzhen Gomative Technology Co., Ltd: GD-10 SUPREME 3D PLUS Multi-electrode Res/IP Imaging System Distributed System.Test using the electrical resistivity method
Figure 2. 2 GD-10 electrical analyzer system
The electrode devices of ERT in this site investigation are wenner – Schlumberger array and edge – gradienter array. The minimum interval coefficient is 1, and the electrode point distance is 1 m. The total number of electrodes array is determined according to the site, and the detection depth is within 10m. Before measuring, the first thing is to detect the electrode grounding resistance. If the grounding resistance is normal, data collection and measurement can be carried out. The schlumberger resistivity method is one of the common methods in resistivity sounding.
2.4 Data processing and analysis
The measured data of the factory area is transmitted to a computer, and processed by special processing software, to convert the measured data into a picture through operations such as abrupt point processing and terrain correction. The processing flow is as follows: data acquisition → data communication → parameter processing → image formation → image and text processing → archiving → printout.
Collect the relevant data of the area and its adjacent areas, and combine the data of the area to analyze and study the resistivity information collected by ERT resistivity method, so as to understand the soil pollution in the area in detail; The appropriate parameters are selected for section inversion and interpretation, and the interpretation results are analyzed and relevant section drawings and plan drawings are drawn. Prepare text reports.
3 data processing and interpretation
3.1 Result Analysis of Measuring line 1
Figure 3.1 Resistivity results of measuring line 1 of wenner-sclumberger and edge-gradient arrays
There is no geological soil type data on site. It is known that the main soil composition of Hangzhou is clay, normally the resistivity of which is about 60 Ω–m (Figure 3.2), or as low as 30 Ω–m if wet. Therefore, as shown in the above figure, the whole strata substances present a ultra-low resistivity, which can be inferred that there is a serious pollution intruded into the field; the measuring results of both arrays show high resistivity between electrodes 6-8 where are located at 5-7m of the whole measuring line, and it can be inferred that there might be holes or pipelines in the building structure; while the resistivity below is as low as a fewΩ–m, so we can figure out that the accumulated pollutants of the upper pipelines diffuses into subsoil and causes the low resistivity phenomenon in depth of 1m-2.6m in this area. From the ground surface, it can be seen in the Figure 3.1 that there is hardening cement and pavement distress, and the black soil components and some production waste can be seen on the surface, which may cause materials or waste materials pollution to permeate through the damaged pavement to the strata. In addition, there are abnormal areas at 15-20m of the measuring line, which is indicated that the situation in the ground surface (Figure 3.4) is similar to that of the place of 5-7m. The pavement is damaged and covered with black unknown soil which may be speculated as waste accumulation. Therefore, the low resistivity of underground is also caused by the pollution leakage of waste accumulation.
Figure 3. 2 soil resistivity table
Figure 3.5 Resistivity results of measuring line 2 of wenner-sclumberger and edge-gradient arrays
Measuring line 2 mainly stands beside wastewater treatment plant, and the resistivity imaging result shown in Figure 3.5 indicates that the surface of measuring line 10-13m of the measuring line has a high resistivity, the reasons of which can be determined as pipelines or hole, and the depth of the hole is about 1m. We also drilled to the hole when made the electrode arrangement at this position on construction day (Figure 3.6). On account of the sewage pond nearby, therefore, it can be judged that the underground pipelines are used for wastewater delivery. So the low resistivity (10-60Ω–m) 2m beneath the area is caused by wastewater diffusion due to the damaged pipelines or other reasons. Similarly, it is speculated that the ultra-low resistivity after the measuring line 13m nearby the pipeline is also the pollution of surface soil and deeper soil caused by the wastewater transportation. In addition, similar to 10m of measuring line, the condition is high resistivity in the surface oil and low resistivity in the intermediate soil layer at about20m of the measuring line, which could be assumed that voids exist in this area. However, this place brick structure (Figure 3.7), so there are large pores in the soil contact area, as a result, if the surface is polluted, the pollution is easy to spread into primary soil, and the resistivity profiles can be inferred that the intermediate strata in the area are seriously contaminated. The high resistivity of the surface of the whole measuring line are mostly caused by the pipelines or concrete reinforcement, while the abnormally low resistivity of the soil under the line can be judged as that most of the soil is polluted and the polluted depth is more than 5m, and the more seriously polluted area is the at 13-20m of the measuring line with the depth range of 1m-2.5m.
Figure 3.6 The hole reached by drill at 10-13m of the Measuring line
Figure 3.7 Bricks and waste accumulation area at about 20m of the measuring line
Figure 3.7 Resistivity results of measuring line 3 of wenner-sclumberger and edge-gradient arrays
The measuring lines 3 and 2 are measured crosswise, and share one electrode which is the 11th electrode of the measuring line 2 and the 6th electrode of the measuring line 3 respectively. There are pipelines or hole at about 10m of the measuring line, so that the high resistivity area under 5m of the measuring line 3 can be drawn as that there is indeed a pipeline, and the direction of which is parallel to that of the measuring line 3 and perpendicular to that of the measuring line 2, because the high resistivity tomography in measuring line 2 is similar to a circle of a certain semi-diameter, while the high resistivity area in measuring line 3 is distributed in a large scale and long distance. So it can also be inferred that the pipeline may be to deliver the wastewater of production area ② to the wastewater treatment pond. This underground pipeline can be confirmed again by radar. At about 10m of the measuring line also presents high resistivity, which is abnormal and it not yet possible to determine what exactly is going on, so it may needs a comprehensively application of other geophysical means. The overall measuring line has a low resistivity, so it can be judged that the soil is also contaminated at this location, but the contamination is slighter than the above two measuring lines. The key potential contaminated area is at 12-13m of the measuring line, which is located between two sewage treatment ponds and may be contaminated by surface sewage intrusion.