In resistivity method, all parameters to be measured are apparent resistivity ρs values. But it is not a directly measured value, but is calculated by apparent resistivity formula according to the corresponding δ umn and I values, so the task of resistivity instrument is to measure the potential difference δ umn and current I.

1. Requirements for electrical measuring instruments

In order to facilitate observation and ensure accuracy, it is required that the power output is stable and the voltage is continuously adjustable. The requirements for the receiver are as follows:

1) has high sensitivity. The higher the sensitivity of the instrument, the smaller the measurable δδUMN value. Under the condition that ρs remains unchanged, ρδUMN is in direct proportion to i. Therefore, improving the sensitivity of the instrument can reduce the power supply current, reduce the weight of the power supply, reduce the number of power supply electrodes, and use thin power supply wires, thus making the whole equipment lighter.

2) Strong anti-interference ability. The instrument requires a strong ability to suppress 50 Hz industrial interference signals and all kinds of accidental interference to ensure the high sensitivity of the instrument.

3) High stability. Instruments used in the field are required to adapt to various climatic conditions, so they should be able to maintain stable performance in a considerable range of temperature and humidity.

4) High input impedance. In order to maintain the required accuracy under the changing field grounding conditions, the instrument is required to have a high input impedance.

2. Automatic compensator and its working principle

There are many instruments commonly used in electrical prospecting, but the current negative feedback automatic compensator is one of them. This instrument is also suitable for other DC methods. We will not discuss the specific circuit here, but simply introduce the principle of automatic compensation. Figure 4-68 shows the principle circuit diagram of the automatic compensator.

Figure 4-68 Principle Circuit Diagram of Electronic Automatic Compensation (Potential) Instrument

When the instrument is connected to the measuring electrodes M and N, let the potential difference to be measured be Δ δUMN. At this time, i 1 enters a potential difference Δ u across the resistor R through the input loop of the instrument, and then there is a potential difference Δ u at its output due to the action of the amplifier K. When the current i2 flows through the feedback resistor r K in the output loop, a potential difference Δ u = i2rk is generated across the RK. Because the above processes are almost completed at the same time, and the phase difference between the δ U output and the δ U input is 180 (that is, they are in opposite directions), for the amplifier, the input voltage is not only δδUMN, but also δ UK. On the other hand, when the input resistance r is large (about 10 ~ 15ω), the observation error caused by grounding resistance RMN can be ignored. The potential drop of i 1 on RK is very small and can be ignored. So there is

δ u in = δ umn-δ UK (4-38)

It can be seen from this formula that if Δ u can be substituted into ≈0, Δ umn ΔΔ uk and umn can be determined by observing Δ uk. Therefore, the method of "automatic compensation" is adopted. That is, as long as Δ umn > Δ uk in the line, Δ u will enter the amplifier, resulting in an increase in the feedback current i2, and Δ uk will also increase. If this cycle goes on, Δ uk will quickly tend to Δ δUMN. When Δ UK increases to be in dynamic equilibrium with Δ δUMN, i2 no longer increases. At this time,

δUMN≈δUK = i2RK

therefore

General geophysical exploration

That is, the feedback current i2 is proportional to the potential difference to be measured (RK is a fixed value). Therefore, the size of i2 can directly represent the size of δδUMN. Therefore, as long as the current scale of the μA microammeter connected in series in the output circuit is changed to the corresponding potential difference scale, the value of the measured potential difference δδUMN (generally millivolts) can be directly read out.

In order to measure the current I sent to the ground through the power supply electrodes A and B, a standard resistor R0 can be connected in series in the power supply circuit, and the potential difference δ U0 across R0 can be measured by an automatic compensator. Generally, R0 is 0. 1ω, then according to ohm's law I = δ u0/r0, the measured potential difference is multiplied by 10, and the milliamperes of current I can be obtained.

3.DWD-2 A microcomputer electric measuring instrument and its working principle.

DWD-2A microcomputer electric measuring instrument is an intelligent electric measuring instrument controlled by microprocessor. The instrument adopts the principle of direct amplification measurement to observe the potential difference (δUMN). Its structural block diagram is shown in Figure 4-69.

Figure 4-69 DWD-2A Microcomputer Electric Tester Structure Block Diagram

The instrument adopts 80C39 single chip microcomputer to complete the automatic control and data processing of the whole machine. The input switch of the instrument is controlled by the computer Δ V/I control signal (UmnIAB is measured when the signal is zero) and 1 is measured. The zero control signal controls the input short-circuit switch. When the signal is 1, the instrument input is short-circuited. Check the amplifier zero point. Otherwise, enter the measurement state. The amplifier circuit of the instrument consists of four operational amplifiers. The first three stages are used as input stages to form a differential amplifier, and the input signal at the MN end is compared with the polarization compensation signal sent by the D/A converter for polarization compensation. The fourth stage operational amplifier is a gain, computer-controlled program-controlled amplifier. The principle of polarization compensation of the instrument is: before power supply, measure the extreme difference (including natural potential) between MnS, then carry out A/D conversion, and calculate its size by computer. After D/A conversion, D/A inversion and D/A attenuation, the compensation signal is output to the input stage for polarization compensation. The instrument controls the on-off of V-MOS switch in high voltage circuit through automatic power supply control circuit to realize power supply.