We were commissioned to conduct a geophysical survey in the water area of the upper reservoir section of a reservoir in Fujian (hereinafter referred to as the "reservoir area") using the marin high-density resistivity method (ERT) of geophysical exploration, in order to find out the water body and sedimentation in the reservoir area. The delineated survey area is about 145,732 square meters, as shown in the red area in the figure below.
The water area from this section belongs to the water inlet and is the main source of water supply for the reservoir. After the reservoir is impounded, the flow rate of the reservoir water is slowed down, and the sediment transport capacity of the water body is reduced, and the sediment carried by it is partially or completely deposited in the river channel, forming siltation. The accumulation over the years will gradually shorten the reservoir capacity and shorten the service life of the reservoir.
Purpose of the survey
The main purposes of this survey are: to find out the depth of the water body in the reservoir area; to find out the layer thickness of the sedimentation in the reservoir area.
Equipment
The main testing Geophysical survey equipment and instruments invested in this project are shown in the table below:
| serial number | device name | Specifications | quantity |
| 1 | GD series high density resistivity meter | GD-10 | 1(set) |
| 2 | Water quality conductivity meter | / | 1(set) |
| 3 | BP-150 Power Supply | GD series | 1(set) |
| 4 | BP-450 Power Supply | GD series | 1(set) |
| 5 | CS-60 | centralized | 1(set) |
| 6 | Resistivity Box | GD | 1(set) |
Table 1 Main testing equipment invested in the project
Survey-line schem
The on-site data collection of the geophysical method survey will start on December 1, 2021 and end on December 9, 2021. A total of 8 high-density electrical survey lines on the water and 1 underwater are laid out. In this work, the mobile phone Ovie map is used to locate the first and last coordinates, and the tape measure is used to measure the distance of the points. Finally, a map is drawn according to the known coordinate points in the field. During the test, the report was used to record the disturbance and abnormal conditions around each survey line in detail, and at the same time, the relevant regional geological data was collected.
Figure 2 ERT line layout in the survey area
Figure 3 Field work diagram
2D result profile
Figure 4 Results of ERT 1-9 high-density resistivity method
In order to visually observe the distribution of underground resistivity in the reservoir area, the profile results of the high-density electrical survey line are formed into a grid map to obtain the comprehensive high-density resistivity distribution result of the reservoir area, and the anomalies are projected to the map of the survey area. And show it from different perspective, as shown below.
Fig. 5 Results of grid projection of resistivity profile
From the grid projection map of high-density resistivity profile (Fig. 4 ERT 1-9), it can be seen that the overall resistivity of the survey line in the water body of the reservoir area can be roughly divided into three layers, and the resistivity in the longitudinal direction shows a "medium-low-high" variation. Based on the characteristics of the resistivity medium, it is speculated that the first layer is the water body in the reservoir area, the resistivity is between 80-150 Ohm-m, the average depth is about 1-10 meters, and the deepest water body is about 10 meters; the second layer is the sedimentary layer, the resistivity is between 0-50 Ohm-m, the layer thickness is about 1-13 meters, and the deepest point of the sedimentary layer is about 23 meters, located at the ERT3 & ERT9 survey line in the south of the survey area; The third layer is below the depth of the sedimentary layer and is bedrock with resistivity between 80-400 Ohm-m. Moreover, in the middle of the first layer of water body and the second layer of sedimentary layer, there is a transition zone of about 2-3 meters thick bottom mud and sedimentary layer; The 5-meter-thick interlayer is presumed to be the rock surface weathered layer or the original riverbed; at the same time, combined with the resistivity projection maps of the two 300-meter-long survey lines,it can be seen that the bedrock at the bottom of the test reservoir area is relatively complete as a whole, and there is no obvious low-resistance anomaly. There is no abnormal area of low resistance in strip or group. It is presumed that there is no fracture or fracture zone at the bottom of the survey area and no karst development area.
2D/3D imaging
In order to more intuitively display the water depth and deposition thickness of the reservoir area, the data values of the resistivity profile of each survey line were calculated using the Surfer software for 2D contour calculation and 3D interpolation calculation, and a 3D model of the underground deposition resistivity distribution in the reservoir area and 2D flat map were established .
Figure 6 Plan view of water depth range
Figure 7 Plan view of sediment thickness range
Figure 8 3D model diagram
It can be seen from the plan view of the water depth range in Figure 6 that the depth of the water body in the reservoir area is about 0.5 to 10 meters. The depth of the water above 10 meters on the south side of the reservoir area is caused by interpolation at the edge of the survey line data. The specific depth is caused by interpolation. It needs to be re-measured and calculated, and it is not counted as the true depth.
It can be seen from the plan view of the range of deposition thickness in Fig. 7 that the overall deposition thickness of the reservoir area is about 1-13 meters. The west bank has the shallowest bedrock surface at the bottom, which forms river floodplain deposition, so the deposition thickness is the thinnest, and the deposition in the middle of the water body is the thinnest. The thickest, with an average thickness in the range of 8 to 13 meters.
The siltation distribution in the reservoir area can be clearly depicted from the 3D model diagram, and the resistivity morphology is hill-like accumulation, which is thick as a whole. At the same time, the comprehensive resistivity grid map estimates that the water depth of the reservoir area is about 0.5-10 meters, the deposition thickness is about 1-13 meters, and the total deposition volume is about 1,352,727 cubic meters.
Through the collection of on-site geological data and combined with the on-site conditions, the following inferences are made on the water flow direction and deposition cause in the survey area:
1. The upper tributaries on the north side of the survey area merge into the main channel. Due to the slowing down of water flow, a large amount of sediment carried by them is deposited here and gradually develops into an inland delta;
2. River floodplain deposition is gradually formed on the east and west sides of the main channel due to river erosion (the position where the overall deposition is the thinnest);
3. Due to the influence of the backwater of the reservoir, siltation in the backwater area is also formed at this location in the main channel.
Through this high-density electrical marine survey , the distribution and depth of water bodies and sediments in the reservoir area have been roughly identified. The main results are as follows:
1. The resistivity characteristics of the reservoir area can be roughly divided into three layers, and the electrical distribution law of "medium-low-high" changes longitudinally;
2. The water body in the reservoir area, the resistivity is between 80-140 Ohm-m, the average depth is about 0.5-10 meters, and the deepest water body is about 10 meters, which are located in the south of the reservoir area;
3. Sedimentation in the reservoir area, the resistivity is between 0-50 Ohm-m, the layer thickness is about 1-13 meters, and the deepest sediment is 23 meters, located in the southern area of the reservoir area;
4. The calculated siltation area of the reservoir area is about 145,732 square meters, and the silt volume is about 1,352,727 cubic meters;
5. The underlying bedrock is limestone, which is relatively complete as a whole, with no obvious low-resistivity anomalies developing downward, and no banded or clustered low-resistivity anomalies. It is speculated that there are no cracks or broken zones at the bottom of the lake and no karst development areas. ;
6. The high-density electrical method can be used to detect the geological strata very well, and it can find out the geological conditions such as the stratification of the rock layer, the surrounding structure and the fissure, and also can estimate the storage capacity of the reservoir area well, efficient and non-destructive method.
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