Research on three-dimensional high-density electrical measurement
1Instruments and equipment
The measurement equipment used this time is the EDGMD-2C cascade three-dimensional high-density measurement system produced by Chongqing Dingfeng Geological Exploration Instrument Co., Ltd. (Figure 1.1 ).
EDJD-3 main parameters and features:
1. Receiving part
◼Voltage channel: ±60V ,
◼Voltage measurement accuracy: ±0.1 % ±1 word
◼Maximum voltage sampling resolution: 5nV
◼Input impedance : ≥ 50 MΩ
◼Apparent polarizability measurement accuracy: ±0.2 % ±1 word
◼ SP compensation range: ±10V
◼Current channel: 6A
2. Launch part
◼Maximum transmit power: 9KW
◼Maximum power supply voltage: 1500V
◼Maximum supply current: ±6A
◼Power supply waveform: pulse width 1 ~ 60 seconds, duty cycle 1 : 1 , bipolar
◼Operating temperature: -10 ℃ ~ +50 ℃, 95 % RH
◼Storage temperature: -20 ℃～+60 ℃
Figure 1.1 Three-dimensional high-density electrical measurement system interface
2 Principles of three-dimensional high-density electrical method, device selection and measurement line layout
2.1 Principle
The principle of three-dimensional high-density electrical method measurement is the same as that of traditional resistivity method. It is based on the conductivity difference of rock and soil media, and achieves the purpose of solving certain geological problems by observing and studying the distribution rules of artificially established stable current fields in the ground. Due to the ubiquity of differences in conductivity of rock and soil, the resistivity method has been widely used in fields related to rock and soil.
2.2 Device
The three-dimensional high-density electrical method uses three-dimensional grid layout (snake-shaped wiring). Two-pole devices, three-pole devices and dipole devices are often used in electrical survey (other device types can only obtain little data information at the edge of the measurement network) ).
2.3 Layout
This work mainly selects the diode device from the EDGMD-2C cascade three-dimensional high-density measurement system for measurement (Figure 2.1 ). The measurement parameters are: pole distance 1m , serpentine wiring distance 2m , 12 single measuring lines with a length of 11m , and 5 measuring lines with a total length of 8m . The area of the measuring surface is 11 × 8=88m 2 .
Figure 2.1 Field layout of EDGMD-2C three-dimensional high-density measurement system
Bipolar devices usually use the E-Scan method (Figure 2.2a ), in which case each electrode in turn serves as a power supply electrode and the other electrodes in turn serve as measurement electrodes. The computer program numbers the electrodes. When each electrode is used as a power supply electrode, the electrode with a larger number than it is used as a measuring electrode. The number of measurement data that can be obtained from n electrodes is maxn =n(n − 1) / 2 . In order to reduce the measurement data without greatly reducing the exploration quality, another measurement method called "over-diagonal scanning" is used (Figure 2.2b ).
Figure 2.2 Electrode position diagram of three-dimensional high-density electrical two-pole device
3Data processing and analysis
3.1 Data processing and inversion
This measurement uses the acquisition mode of the diode device in Figure 2.2b . Based on the experience of many field measurements, it is shown that the measurement method in Figure 2.2b is more efficient while ensuring the integrity of the measurement data during data collection. , the measurement results are also more true and reliable; Figure 2.2a , the collection mode of the two-pole device, the amount of data collected is too large, the time is lengthy, the same abnormality is measured multiple times, and a single data mutation will affect the measurement results.
Data processing uses Res3dinvx64 three-dimensional high-density inversion software for inversion calculations. The output data format is voxler software format. Three-dimensional visualization processing is used. The shape and shape of detected abnormal bodies can be determined more intuitively through volume diagrams, equipotential surface diagrams, and slice diagrams. Location. Figure 3.1 shows the results of three-dimensional measurement inversion. Combined with actual geological conditions, the anomalies in the high-resistance anomaly equipotential surface diagram of the measurement results are anomalies caused by isolated rock blocks. The measurement results are relatively intuitive and consistent with the actual geological conditions. .
Figure 3.1 Three-dimensional measurement results diagram
3.2 Forward modeling and inversion of theoretical models
In order to better verify the accuracy of the three-dimensional high-density electrical measurement results, this work combined with the theoretical model and used the RES3DMOD software to establish a three-dimensional model for forward and inverse calculations. Since the actual measurement range is small, this forward simulation uses proportional amplification for theoretical simulation. The measurement parameters are 30 × 30 mesh, the pole distance is 2m , and two nodes are set between each two electrodes, so that the lattice The grid is more dense.
After many simulations, forward modeling and inversion calculation of the effective depth of the model, it was found that the best measurement effect can only be achieved when the maximum depth A of the model is one-third of the maximum side length B within the measurement area; if it is greater than A , deep anomalies cannot be detected; if it is less than the A value, the simulated maximum depth becomes shallower.
Combined with the actual measurement results, the model parameters were selected as 5 layers (Figure 3.2 ): the first layer is 0-5m , the background field value is 40 Ω· m , and five small 80 Ω· m high-resistance blocks are set inside ; The second layer is 5-10m , the background field value is 50 Ω· m , and a small 120 Ω· m high-resistance block is set inside ; the third layer is 15-20m , the background field value is 50 Ω· m , and a large one is set inside 120 Ω· m high-resistance block; the fourth layer is 15-20m , the background field value is 50 Ω· m , and a small 120 Ω· m is set insideHigh resistance area; fifth layer 20-25m , background field value 50 Ω· m .
According to the forward calculation results (Figure 3.3 ), the computer software is designed to calculate 9 layers by default, with a maximum depth of 18m . The depth of the model corresponding to each layer and the size of the abnormal location can be displayed. In order to better fit the original state of the model, the normal The inversion data is saved in a three-dimensional inversion format, and the Res3dinvx64 three-dimensional inversion software is used for inversion calculations. From the inversion results (Figure 3.4 ), it can be seen that the first high-resistance anomaly from top to bottom on the right side of the first layer is related to the inversion It is not particularly highlighted during calculations (mainly due to the small size of the shallow anomaly body there, the anomaly is less than the length of one polar distance ( 2m ) in the longitudinal direction), and the high-resistance anomaly equipotential surface diagram corresponds to the theoretical model (Fig. 3.2 ) The position is accurate, the corresponding depth is not much different, the abnormal shape is single, and the actual cuboid model changes into a relatively smooth ellipsoid shape.
Figure 3.2 Three-dimensional forward layered model
Figure 3.3 Three-dimensional layered model forward calculation results diagram
Figure 3.4 Three-dimensional layered model inversion calculation results diagram
4 Conclusion
1. The EDGMD-2C cascade high-density measurement system produced by Chongqing Dingfeng Geological Exploration Co., Ltd. can effectively measure abnormalities. It adopts disposable serpentine wiring, has a large measurement range, many measurement data values, and the measurement results are scientific and accurate.
2. When measuring with a three-dimensional high-density diode measuring device, through forward modeling and inverse calculation of the model and comparative analysis of the actual measurement results: the maximum measurement depth of the simulation device should be about one-third of the maximum measurement side length for the most accurate measurement; When the size of the anomaly is less than one pole length, the specific characteristics of the anomaly cannot be shown; the size, position, shape, and location of the theoretical model are consistent with the inversion results.
3. Compared with the two-dimensional high-density electrical method, the three-dimensional high-density electrical method can eliminate the influence of side effects. The measurement results are more intuitive, the abnormal characteristics of the target body are relatively single, and the number of solutions is reduced. It is suitable for exploring unknown locations in a certain area. , isolated geological body.
4. Use three-dimensional measurement as much as possible when the site conditions are available to obtain more data information; there are many factors that affect the measurement results. Only by selecting reasonable measurement devices and measurement parameters based on the target object can the best measurement results be achieved.