Purpose of the survey
1. Exploring the distribution and depth of the oil leakage
2.Find out the location of the oil leakage
Survey Method
Adopt GPR and electrical resistivity (Tomography) for geophysical exploration.（As for the investigation of the distribution of the underground oil like LNAP L pollution, the traditional method is to predict the possible distribution through limited borehole sampling or MIP, and to estimate the range and depth of the pollution distribution through the correlation of results between points. But in this way, will face the problem of complicated site filling and insufficient spot sampling. Geophysical exploration is the measurement of underground landfills and formations in a non-destructive manne. In addition to non-required boreholes, but also can be obtained continuous profile data through a large range of measurements. Therefore, it is recommended to use the GPR and ERT in this investigate on By surveying the difference underground material’s resistivity distribution and dielectric constant, we can understand the range and depth of oil pollution distribution to make up for the shortcomings of the traditional investigation, so as to complement each other.
1. Equipment
GD-20 multi-channel workstations and GPR
2.Cable configuration
The site is divided into several small grids totally about 9000 square meters and investigated by GPR. In the tree lawn measured 4 ERT lines in 1 meter spacing; each measuring line is 100 meters totally 400 meters.
Principles of Ground Penetrating Radar GPR
diagram of reflection detection Radar record diagram
The ground penetrating radar uses high-frequency electromagnetic waves in the form of wide-band short pulses, which are sent from the ground to the ground through the transmitting antenna T, reflected by the underground formation or the target body, and then returned to the ground for reception by the ground receiving antenna R. Because underground mediums often have different physical characteristics, such as differences in dielectric, conductivity, and magnetic permeability, they have different wave impedances to electromagnetic waves. When electromagnetic waves entering the ground pass through underground layers or other target bodies, because the impedance of the waves on both sides of the interface is different. The electromagnetic waves will be reflected and refracted at the interface of the medium. The propagation path, electromagnetic wave field strength and waveform of the electromagnetic wave pulse reflected back to the ground will vary with the electrical properties and geometric shape of the medium passing through. According to the received radar wave waveform, intensity, phase and geometric shape analysis, so as to achieve the detection of the target body.
Principles of electrical resistivity tomography ERT
chematic diagram of resistivity method Schematic diagram of ERT
The Electrical Resistivity Method or Direct Current Resistivity Method is based on the difference in dielectric resistivity, observing the power supply current intensity and measuring the potential difference between the electrodes, and then calculating and studying the apparent resistivity. Infer the distribution of underground contaminated soil. The measured result is the ground resistivity profile. The factors that affect the resistivity of the stratum include the constituent minerals, particle size, configuration, and the water content of the stratum and the substances contained in the water. The stratum has obvious resistivity contrast, which is suitable for the DC resistance method.
Results of ERT
From the resistivity grid diagram and two-dimensional section, the red dotted line can be clearly depicted as the abnormal area .This anomaly is characterized by relatively high impedance region and the abnormal feature is the anomaly in the relatively high resistivity area. In addition, there were 4 anomalies (red dotted line) in ERT1, which were presumed to be oil seepage points. The anomalies at the horizontal position of 8-16 meters were those at the site excavation, which were verified to be oil-bearing areas. The measured results were consistent with the excavation verification results. There are 3 anomalies in ERT4, which are supposed to be abnormal characteristics caused by oil products.ERT2 and ERT3 have no obvious abnormal areas caused by contamination. In summary, it is speculated that the pollution depth of the investigation area is about 1.4 meters below, and the oil leakage mode is through the local circulation channel, and the pollution is mainly distributed on the north side of the road.From the resistivity grid diagram and two-dimensional section, the red dotted line can be clearly depicted as the abnormal area .This anomaly is characterized by relatively high impedance region and the abnormal feature is the anomaly in the relatively high resistivity area. In addition, there were 4 anomalies (red dotted line) in ERT1, which were presumed to be oil seepage points. The anomalies at the horizontal position of 8-16 meters were those at the site excavation, which were verified to be oil-bearing areas. The measured results were consistent with the excavation verification results. There are 3 anomalies in ERT4, which are supposed to be abnormal characteristics caused by oil products.ERT2 and ERT3 have no obvious abnormal areas caused by contamination. In summary, it is speculated that the pollution depth of the investigation area is about 1.4 meters below, and the oil leakage mode is through the local circulation channel, and the pollution is mainly distributed on the north side of the road.
Results of GPR A
The area A is located in the factory. Three GPR test lines were tested, and the test lines were followed by EW, with an interval of 2.5 meters. From GPR in A region. The color order represents the amplitude, and the “blue-green-red” indicates that the amplitude varies from weak to strong. On the whole, the survey line is 40-70 meters in the horizontal position, the buried depth is about 1-1.4 meters, and there is strong reflective layer (the black dotted line in the figure) .The amplitude of the wave is strong, and the range of anomalies decreases with the increase of depth. It is speculated that the anomaly may be caused by oil contamination.
Results of GPR D
The area D is located in the factory, and total of 15 GPR survey lines are measured by grid method, including 9 in NS direction, 6 in EW direction, and NS and EW in the survey lines, with grid spacing of 4m×4m. From the results of GPR D, the color order represents the amplitude, and the “blue-green-red” indicates that the amplitude varies from weak to strong. It can be seen from the results that the south side of area D is mainly field excavation and oil pollution area. The strong reflection anomaly area can be clearly depicted in figure a6, as shown in the black dotted line, and the anomaly range gradually decreases with the increase of depth.
Results of GPR T
The area T is located next to the security booth in the factory, and 12 GPR lines were tested by grid method, including 8 lines in NS direction, 4 lines in EW direction, and lines in NS and EW, with grid spacing of 5m×4m.From the results of sections with equal depth of GPR in the t-zone, the color order represents the amplitude, and the “blue-green-red” indicates that the amplitude varies from weak to strong. According to the results, the abnormal area without obvious strong reflector was found in this region, and there was no evidence of oil contamination.
Results of GPR T
The area G is located on the road outside the factory. A total of 15 GPR lines were measured by grid method, including 10 in the SN direction and 5 in the EW direction and spacing is 5m×3m.From the results of GPR slices of equal depth in G region , the color order represents the amplitude, and the “blue-green-red” indicates that the amplitude varies from weak to strong. It can be seen from the results that at the horizontal position of 2-12m and at the depth of 26-45m, the buried depth is below 0.6m, there are two obvious abnormal areas of the strong reflection layer, as shown in the black dashed line in the figure. This anomaly has strong energy and large amplitude. Radar wave is characterized by increased reflection, enlarged wave peak, and multi-layer reflection. It is speculated that this anomaly which is caused by oil product pollution, and there is no obvious anomaly in the rest.
Results of GPR F
The area F is located on the road outside the factory. A total of 15 GPR lines were measured by grid method, including 11 in the SN direction and 4 in the EW direction and spacing is 5m×3m.From the section F results of GPR there is no obvious abnormal area in this zone. Some scattered strong reflection anomalies occur in the shallow soil. Since the data is really impossible to judge, the remaining scattered strong reflection should be caused by the building interference. Meanwhile, the strong reflection of the black box in the shallow layer of 0.8-1.2 meters is the underground pipeline.
Results of GPR U
The area U is located on the road outside the factory area, using a grid method, a total of 15 GPR survey lines were tested, including 11 in the NS direction and 4 in the WE direction, and the survey lines were routed between NS and WE with a grid spacing of 5m × 3m.From the results of GPR in the U area, it can be seen that the obvious anomaly appears near the buried depth of 1.2 meters. The anomaly is characterized by strong amplitude and discontinuous, presumably caused by oil pollution shown in the Black dotted line. There are no obvious abnormalities in the rest.
This area is located in the loading and unloading platform of the factory area, using a grid method, a total of 21 GPR survey lines were tested, including 18 in the SN direction and 4 in the WE direction. Spacing is 5m*5m.From the GPR in this area, we can see that there is no obvious abnormal area of the strong reflection layer on the whole. There are some scattered and discontinuous strong reflections in the shallow soil, which are caused by interference from buildings.
The oil pollution distribution complete result chart
Combining all the survey lines, it is speculated that the possible contamination location is shown by the black dotted line in the figure. The main strong reflection anomalies still appear on the downstream side of the workshop, and there are strong reflection anomalies displayed on the upper and lower sides of the workshop, and the abnormalities on the road are mainly distributed on the north side of the road. At the same time, combined with the ERT measurement results, it can be seen that the oil leakage channel is a partial channel method, so it is speculated that the oil leakage may start from the workshop. Due to the difference in stratum and ground, the dominant channel of the partial flow through is formed The leaked oil uses this dominant channel and other channel formed by the underground pipeline to spread in the direction of the road, and the leaked oil may spread in the east-west direction through the pipe corridor under the road.
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