High-density electrical pipeline measurement application and cross-hole CT method

1Instruments and equipment

The measuring equipment adopts EDJD-3 produced by Chongqing Dingfeng Geological Exploration Instrument Co., Ltd. Its main functions are: 1. Underground profile resistivity change detection, 2. Short-range Bluetooth wireless data transmission 3. Long-range 4G wireless data transmission.

Figure 1.1 High-density outdoor measurement working diagram

The measurement location is located in the lawn area of the company's campus. It has two main purposes: the first is to conduct precise measurements of underground water pipes; the second is to design two boreholes based on the measurement results for future in-hole electrical CT and seismic measurements. Preparation for cross-bore CT testing.

2Measurement parameters

Use the Wenner device to run the poles horizontally at equal intervals on the same layer for measurement. Roll the first layer with the minimum pole distance, and then gradually increase the pole distance until the maximum pole distance is measured, which is equivalent to measuring from the shallower layer to the deepest layer. Then roll the collection until all data points on the survey line are collected.

Parameter settings: 48 channels of centralized distributed high-density resistivity method instrument , pole distance 1m , measuring line length 47m , 15 measurement layers, Wenner device measurement, 360 single profile measurement points .

Figure 2.1 High-density Wenner device and electric pole running method

3 Measurement result processing and analysis

For the measurement results, the Zuo Di method (Figure 3.1 ) is used for data fitting and inversion.

Figure 3.1 Zodif method inversion result diagram

According to the measurement inversion results and the actual situation, the parameters are optimized for further processing of the data (Figure 3.2 ), and obvious anomalies are extracted. According to the apparent resistivity cross-section diagram, it can be seen that the high resistivity at a depth of 0-3m is the backfill soil capping layer. There is a low resistivity anomaly at a depth of 17m and 4m along the measurement line. There is also a low resistance anomaly at a depth of 25m and 6m . Resistance anomaly. Based on the known distribution and burial depth of drainage pipes, the first low-resistance anomaly along the survey line is the rainwater pipe, the second low-resistance anomaly is the sewage pipe, and both pipes contain a certain amount of water. Produce measured low-resistance abnormal morphology.

Figure 3.2 Comprehensive interpretation results chart

The boreholes are designed to avoid sewerage pipes and surface electric wires and pipelines. According to the requirements of the cross-hole experiment and actual conditions, the designed hole spacing is 12m . The specific location is shown in Figure 3.3 , and the design depth is 50m . This can ensure the depth requirements of the experimental measurement (seismic The detector generally has 12 channels with a pole spacing of 1m and the effective measurement length is 11m . The electrical method generally has 30 channels with a pole spacing of 1m and the effective length is 29m .) There is also a low resistance anomaly between the two holes. Based on the actual effective data of the first cross-hole measurement (Seismic cross-hole CT , resistivity cross-hole CT ) are the benchmark reference and the standard for calibration of future instrument tests.

Figure 3.3 Design drilling diagram

4 Measurement methods in wells

4.1 Seismic cross-bore CT

 The seismic cross-bore CT method is a cross-bore geophysical prospecting method that observes the changes in travel time, energy (amplitude) and waveform when elastic waves pass through geological bodies, and reconstructs images of the internal structure of geological bodies through computer processing. Because of its high resolution characteristics, mainly used for the detection of underground fine structures and targets. As shown in Figure 4.1 , install the seismic source in the transmitting hole and install the geophone in the receiving hole.

It mainly conducts comprehensive detection analysis based on the results of surface detection. Based on the exploration drilling depth of about 50m , the effective hole spacing is designed to be 10-20m , and the measurement accuracy is meter level.

Figure 4.1 Schematic diagram of cross-hole CT signal excitation and observation .

4.2 High-density resistivity method cross-hole CT

Electrical resistivity tomography method ( ERT method for short ) is a method that uses the DC electric field generated by power supply electrodes to detect underground resistivity distribution. The electrode is the most basic device of the ERT method and has two main functions. The first function is to serve as a power supply electrode to form a DC electric field by passing current into the underground. The second function is to serve as a measuring electrode to measure the potential difference at different measuring points, thereby calculating the apparent resistivity and apparent resistance data of the underground medium.

Compared with the ground measurement solution, the measurement solution using cross-hole ERT does not require laying out ground electrodes. It only needs to lay out all the electrodes in the well. Through different electrodes to supply power and measure the potential difference, it can perform high-precision detection of the inter-well area. Imaging. The schematic diagram of the cross-hole ERT measurement device is shown in Figure 4.2 .

Figure 4.2 Cross-hole ERT measurement scheme

Currently, commonly used devices in cross-hole ERT solutions include monopole - monopole devices, monopole - dipole devices, AB-MN devices, AM-BN devices, etc.

5 Conclusion

1. The EDJD-3 high-density electrical instrument with a pole pitch of 1m can be used to conduct precise measurements of underground drainage pipes, and the depth and position positioning are relatively accurate. 2. Design two boreholes based on the measurements. The distance between the holes is 12m and there is an abnormal low-resistance drainage pipe in the middle. It can be used as a reference standard for CT measurements in wells .
   

3. The purpose of this drilling design is to promote the electrical cross-hole and seismic cross-hole measurement methods. The company's existing electrical method host and seismic host are used to complete the complete set of measurement systems, which can carry out cross-hole measurements.