Graphite electrode water test experiment

1Instruments and equipment

The measuring equipment adopts EDGMD-60 produced by Chongqing Dingfeng Geological Exploration 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.

Main features of EDGMD-60 : The remote detection high-density electrical measurement system adopts fully digital automatic measurement, which can automatically compensate for natural potential, drift and electrode polarization. The instrument has integrated 60 -channel electrode conversion devices and does not require an external electrode converter. It can directly carry out 60 channels of high-density electrical measurement; it also supports cascade expansion of electrode converters, which can be expanded to up to 300 channels.

2Measurement principle

2.1 Basic principles of determination of resistivity of homogeneous objects

For the measurement of resistivity, it is assumed that the resistivity of the earth is uniform in the area to be measured. For measuring the uniform earth resistivity value, in principle, any form of electrode arrangement can be used, that is, supplying power to any two points (A , B) on the earth surface , and then measuring the potential difference between any two points (M , N) . According to the equation (2.1) , the potentials of the two points M and N can be found (Figure 2.1 ).

            (2.1)

The potential difference generated by AB between MN

          (2.2)

From equation (2.2) , the calculation formula of uniform earth resistivity can be obtained as

                    (2.3)

in the formula

K= (2.4)                

Figure 2.1 Measuring the resistivity of uniform earth using a quadrupole arrangement

Equation (2.1) is the basic formula for measuring resistivity on a uniform earth surface using any electrode device ( or electrode arrangement ) . Among them, K is the electrode device coefficient (or electrode arrangement coefficient ) , which is a physical quantity only related to the spatial position of the electrode. Considering the actual needs, in electrical exploration, the power supply electrode and the measuring electrode are generally placed in a straight line. The electrode arrangement shown in Figure 2.1 is called a symmetrical quadrupole arrangement.

For homogeneous geological bodies, the above method can determine the resistivity ( Figure 2.2) .

Figure 2.2 Resistivity measurement curve of uniform geological body

2.2 Theoretical essence of apparent resistivity method measurement

The method of measuring the resistivity of uniform earth is discussed above, and the formula for calculating resistivity is derived. However, under actual field conditions, the geological sections often encountered are electrically uneven and relatively complex. If the above method is still used to measure the apparent resistivity, it is actually equivalent to replacing the originally uneven geoelectric section with an equivalent uniform section. Therefore, the resistivity calculated by equation (2.3) is not the result of a certain rock layer. The true resistivity is the result of the comprehensive influence of the resistivity of various rocks within the electric field distribution range. We call it apparent resistivity and use it to mean,

                    (2.5)

This is the most basic calculation formula in the resistivity method. It can be seen that in the actual work of the resistivity method, the apparent resistivity value is generally measured. Only when the electrode arrangement is located in a formation with a single lithology, the true resistivity value of the formation will be measured.

3 test process

Use EDGMD-60 equipment to connect the graphite electrode wire (the electrode is fixed on the cable) (Figure 3.1 ), connect the positions 1-30 , select four electrodes for the test electrode to be completely immersed in water, the electrode numbers are 27 , 28 , 29 , 30 corresponds to A , M , N , and B respectively. The measurement adopts a custom four-pole measurement method to measure the voltage, current, and resistivity values measured by graphite electrodes in water under different power supply voltages. In order to eliminate the accidental error caused by a single measurement Error, each voltage value in this test was observed ten times, and the relative error and mean square error in statistics were used to measure the accuracy of the test results.

Measurement process: First, prepare a bucket filled with water about 50cm high , put the graphite electrode wire vertically into the water, and the electrode vertical pole distance is 10cm , then connect the cable to the conversion switch, the switch is connected to the host, and the Android tablet or Android tablet is connected via Bluetooth. Mobile phone, open the EMGMD2D program, select the centralized custom measurement mode, make A-27 , M-28 , N-29 , B-30 correspond to the same, detect that the resistance value of the connected electrodes 27-30 is about 600 Ω, and it is not soaked in in water (the resistance value between adjacent electrodes is 15M Ω), then select three different power supply voltages ( 20V , 150V , 286V ) to measure the apparent resistivity values, and record and analyze them for comparison.

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Figure 3.1 EDGMD graphite electrode measurement in water

4 test results

4.1 Power supply voltage 20v

In the morning measurement, the power supply voltage was 20v . The same data was measured repeatedly for 10 times. The apparent resistivity value changes were measured 10 times. The power supply voltage was 974mV , the measurement current was 25mA , and the apparent resistivity value was 24 Ω· m . The three were stable, and the change difference was within the allowable error range (Table 4.1 , Figure 4.1 ).

Table 4.1 Test results when the power supply voltage is 20V

Figure 4.1 Apparent resistivity change curve when the supply voltage is 20V

4.2 Power supply voltage 150v

In the afternoon measurement, the power supply voltage was 150v , and the same data was measured repeatedly for 10 times. The apparent resistivity value changes were measured 10 times. The power supply voltage was 6069mV , the measurement current was 161mA , and the apparent resistivity value was 23.6 Ω· m . The three were stable, and the change difference was within the allowable error range (Table 4.2 , Figure 4.2 ).

Table 4.2 Test results when the power supply voltage is 150V

Figure 4.2 Apparent resistivity change curve when the supply voltage is 150V

4.3 Power supply voltage 286v

In the afternoon, the power supply voltage was 286v , and the same data was measured 10 times to change the apparent resistivity value. When measuring 1-5 times, the power supply voltage was 12421mV , the measurement current was 330mA , and the apparent resistivity value was 23.6 Ω m ; 6-10 times of measurement When the underwater electrode was moved artificially, the voltage and current also changed successively. The resistivity changed slightly and became a little larger. The three were stable, and the change difference was within the allowable error range (Table 4.3 , Figure 4.3 )

Table 4.3 Test results when the power supply voltage is 286V

Figure 4.2 Apparent resistivity change curve when the supply voltage is 286V

5 Conclusion

1. The EDGMD-60 supporting graphite electrode produced by Chongqing Dingfeng Geological Exploration Co., Ltd. can effectively measure the apparent resistivity value in water, and the measurement results are stable and reliable.

2. When measuring resistivity with a graphite electrode in water, it is best to ensure that the position of the graphite electrode is fixed. That is, the position of the electrode cannot be artificially disturbed during measurement, otherwise the measurement results will change slightly, resulting in inaccurate measurement results.

3. When the supply voltage increases, the supply current will also change. The two are also in a linear growth relationship, and the measured apparent resistivity does not change.

4. When power supply is available and under the same measurement conditions, according to the maximum allowable value of the measured power supply current, select a larger voltage as much as possible for power supply measurement.