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Resistivity method:
1) Resistivity of rocks and ores:
Resistivity ρ is an electrical parameter that describes the good or bad conductivity of a material. The better the conductivity of a material,
the smaller its resistivity value. Natural rocks (ores) are composed of minerals. In order to understand the characteristics and changing laws of rock (ore) resistivity, it is necessary to study the resistivity of various minerals. According to the good or bad conductivity, solid minerals can be divided into metallic conductive minerals, semiconductor conductive minerals, and solid ion conductive minerals. The resistivity value of minerals varies within a certain range. The same mineral can have different resistivity values, and different minerals can have the same resistivity value. Therefore, the resistivity of rocks and ores composed of minerals must also have a large range of variation.
The resistivity variation range of various rocks and ores is as follows: (ρs) Unit ohm-meter (Ωm)
| igneous rock | 102~1062Ω·m | metamorphic rock | 102~105Ω·m |
| hard shale | 10~500Ω·m | soft shale | 0.5~102Ω·m |
| porous limestone | 100~8000Ω·m | sandstone | 50~30002·m |
| loess layer | 1~200Ω·m | clay | 1~200Ω·m |
| Water-bearing sand and pebble layer | 50~500 | Soft hornbeam | 1~200 |
| water-proof clay layer | 5~30 | rainwater | ＞1000 |
| sand | -50~1000 | saltwater | 12~15 |
| river water | 10~100 | porous limestone | 100~8000 |
| seawater | 0.1~1 | graphite sheet | 101~103 |
| The electrical resistivity of metals is very low | dense limestone | nx107 | |
2) Factors affecting the resistivity of rocks and ores:
There are many factors affecting the resistivity of rocks and ores. In addition to the content of conductive minerals, they also include the structure, texture, porosity, water content and water mineralization, temperature, pressure, etc. of rocks and ores. In the survey and exploration of metal minerals, the content and results of good conductive minerals in rocks and ores are the main influencing factors. In hydrological, engineering geological surveys and sedimentary area structural surveys and explorations, the porosity, water saturation and mineralization of rocks are the decisive factors. In geothermal research and deep geological structure research, temperature changes have become the main factors.
3) The concept of apparent resistivity:
Resistivity expression: ρ=KΔU/I, its application conditions are: the ground is an infinite horizontal plane, and the underground is filled with uniform isotropic conductive media. However, in reality, the terrain is uneven, the underground medium is uneven, various rocks overlap each other, faults and fissures are crisscrossed, or there are ore bodies filled. The resistivity value calculated by the above formula is generally neither the resistivity of the surrounding rock nor the resistivity of the ore body. We call it apparent resistivity, represented by ρS, that is, ρs=K△UMN/IAB unit (Ω·m) ohm·meter
Where: △UMN is the primary field potential received by the receiving electrode MN.
IAB power supply current, A and B are power supply electrodes, the power supply current calculation unit is A (ampere),
M, N are receiving electrodes.
The electric field of two point power sources:
M point potential UMAB =I*ρs /2π(1/AM –1/BM)
N point potential UABN = I*ρs /2π(1/AN –1/BN)
Among them, AM, AN, BM, and BN represent the horizontal distances between A, B and M, N respectively.
2). Application scope and conditions of charging method:
Geological problems solved by charging method:
Determine the shape, occurrence, scale, plane distribution position and depth of the hidden part of the exposed (or exposed) ore body;
Determine the connection relationship between known adjacent ore bodies;
Find blind ore bodies near known mines;
Use single well to determine the flow direction and flow rate of groundwater;
Study landslides and track underground metal pipelines, etc.
Application conditions of charging method:
The object (charged body) under study has been exposed or exposed at least once in order to set up charging points;
The charged body should be a good conductor relative to the surrounding rock;
The larger the scale of the charged body and the shallower the burial, the more ideal the effect of applying charging method. The maximum research depth of charging method is generally half of the extended length of the charged body.
3) Connection method of power supply electrode and charging body:
The positive pole of the power supply electrode must be connected to the charging body. Due to the different conditions of the exposure of the power supply body, the connection method is also different. When evaluating the detailed investigation stage of metal mines, if the metal ore body is exposed on the surface or in wells, pits and other projects, a group of (3 to 10) iron electrodes are usually driven into the ore body, connected in parallel, and connected to the positive pole of the power supply. When it is not easy to drive the iron electrode, a heavy object can be used to press the thin iron wire or copper wire tightly on the surface of the ore body. When the ore body is exposed in the borehole, a special brush electrode is needed as the power supply electrode, and the brush electrode is placed on the ore body in the well. When the pipeline is lowered, if the pipeline exposure point can be found on the ground, the electrode can be directly connected to the exposure point of the pipeline. The negative pole should be set in a low-lying and humid place 1000 to 1500m away from the measurement area to reduce resistance and increase the power supply current.
4). Main observation methods and approaches in the charging method:
① Potential method: Fix a measuring electrode N at the edge far away from the measuring area as the potential zero point, and move the other measuring electrode M point by point along the measuring line to observe its potential difference relative to the N pole as the potential value U of the measuring point where the M pole is located. At the same time, observe the power supply current I and calculate the normalized potential value U/I.
② Potential gradient method: Keep the measuring electrode MN at a certain distance and move it along the measuring line together. Observe the potential difference △U and the power supply current I point by point, and calculate the normalized potential gradient value △U/(MN·I). The recording point is the midpoint of MN, and pay attention to the sign change of the observed potential difference △U.
