This method takes the natural magnetotelluric field as the working field source, measures the electric field component generated by the natural magnetotelluric field on the ground at the selected frequency, and studies the electrical change of the underground geoelectric profile. The observation instrument is 8327 microcomputer frequency selector with microprocessor. The main observation methods are as follows.
1. Measurement method of moving electrodes M and N along the measuring line
Record the potential difference between the electrodes m and n with the central point o of MN as the recording point (Figure 5-4-7).
2. Measurement method of vertical line movement of electrodes M and N
Record the potential difference between the electrodes M and N, with the center point O of MN as the recording point (Figure 5-4-8).
The former Zhengzhou local school has carried out the experiment and measurement of natural electric field frequency selection method in more than ten counties in five cities of Henan Province. This paper only introduces the effect of finding underground caves and pipelines. Han Rongbo, 1987. Application of natural electric field frequency selection method in urban geophysical exploration. See: Proceedings of the Geophysical and Geochemical Information Network of the Ministry of Geology and Mineral Resources, Urban Engineering Application Geophysical and Geochemical Technology Exchange Conference.
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Fig. 5-4-7 Schematic diagram of measuring device when electrodes M and N move along measuring line.
Fig. 5-4-8 Schematic diagram of the measuring device when the electrodes M and N move perpendicular to the measuring line.
(1) Find a bomb shelter
Fig. 5-4-9 is an abnormal profile measured in the air-raid shelter of Gongren Road in Zhengzhou City. The cave gradually inclines eastward from the mouth of the cave, with a width of 4m at the bottom and a semicircle at the top. The distance from the bottom of the cave to the highest point of the cave roof is 4m. The survey line is arranged in the north-south direction, with a distance of 15m from the borehole. It can be seen from the abnormal curve of the profile that when the working frequency is 25Hz and 67Hz, the abnormal response on the air-raid shelter is obvious, and when the frequency is 170Hz, the abnormal response is poor, which may be caused by high frequency and ground interference.
(2) looking for underground drainage pipes
Figure 5-4- 10 shows the abnormal section of the underground drainage pipe on the north side of Zhengzhou Air Force Hospital. The pipeline is a circular pipeline made of cement, with a diameter of about 2m and a north-south direction. The top of the pipeline is 2m from the ground. On the east-west profile, several working frequencies have obvious abnormal reflections, and the high-value abnormal position is consistent with the projected position of the pipeline on the ground.
Figure 5-4-9 Observation Results of Natural Electric Field Frequency Selection Method in Air-raid Shelter
(According to Han Rongbo 1987)
Fig. 5-4- 10 observation results of natural electric field frequency selection method for drainage pipeline
(According to Han Rongbo 1987)
(3) Looking for underground cast iron pipes
The above list is an example of finding a high-resistance body, and the existence of a high-resistance body and its projected position on the ground are determined by using the characteristics of high-value anomalies above the high-resistance body. Let's take an example of finding a low-resistance body, and use the characteristics of low-value anomalies above the low-resistance body to determine the existence and position of the low-resistance body.
Figure 5-4- 1 1 is one of the measured results when an arsenal in Henan is looking for underground cast iron pipes. The outside diameter of cast iron pipe is 24 cm, and it runs nearly north and south. The pipeline is buried in the loess with stones, and the average buried depth is about1.5m. I use magnetic method and electrical method to do my work, but because of the influence of various ferromagnetic substances, electricity, obstacles and uneven surface, I didn't get any results, so I chose the natural electric field frequency selection method to do my work. The results show that there is a low-value anomaly above the pipeline, especially when the working frequency is 170Hz, which is proved to be a cast iron pipeline by excavation.