1. Resistivity method :
1) Resistivity of rock, ore :

Resistivity ρ is an electrical parameter that describes the good or bad conductivity of a material. The better the conductivity of a material, the better the conductivity of the material.

The smaller its resistivity value is. Natural rocks (ores) are composed of minerals. In order to understand the characteristics and changing patterns of resistivity of rocks (ores), it is necessary to study the resistivity of various minerals. According to the quality of conductive properties, solid minerals can be divided into metallic conductive minerals, semiconductor conductive minerals, and solid ion conductive minerals. The resistivity value of minerals changes within a certain range. The same mineral can have different resistivity values, and different minerals can also have the same resistivity value. Therefore, the resistivity of rocks and ores composed of minerals must also be larger. range of change.

The resistivity variation range of various types of rocks and ores is as follows: ( ρ _s ) in ohm meters ( Ω m) 

   Measuring the radicalization rate of rock ore in a certain area of Sichuan    

2) Factors affecting the resistivity of rocks and ores:

There are many factors that affect the resistivity of rocks and ores. In addition to the content of conductive minerals, they also include the structure, structure, porosity, water content and water salinity of rocks and ores, temperature, pressure, etc. In the general survey and exploration of metal minerals, the content and results of good conductive minerals in rocks and ores
are the main influencing factors. In the general survey and exploration of hydrology, engineering geology and sedimentary area structures, the porosity, water saturation and mineralization of rocks Temperature is the decisive factor. In geothermal research and deep geological structure research, temperature changes have become the main factor.

3) , the concept of apparent resistivity:

Resistivity expression: ρ =K Δ U/I , its application conditions are: the ground is an infinite horizontal plane, and the underground is filled with uniform and isotropic conductive media. However, in fact, the terrain is undulating, the underground medium is uneven, various rocks overlap each other, faults and fissures are crisscrossed, or filled with ore bodies. The resistivity value calculated from the above formula is generally neither the resistance of the surrounding rock. rate, nor the resistivity of the ore body, we call it the apparent resistivity, expressed by ρ _S , that is,
ρ _s =K △U _MN /I _AB    unit (Ω·m) ohm·meter
. In the formula: △ U _MN is The receiving electrode MN receives the primary field potential.


I _AB power supply current, A and B are power supply electrodes, the power supply current calculation unit is A (ampere),

M and N are receiving electrodes.
Electric field of two point sources:

M point potential U _M ^AB =I* ρ _s /2 π (1/AM –1/BM) 

Potential at point N U ^AB _N = I* ρ _s /2 π (1/AN –1/BN) 

            K is the device coefficient

Among them, AM , AN , BM and BN represent the horizontal distance between A , B and M and N respectively.

       Figure 1-1 Principle diagram of   resistivity method

4 ), Resistivity method classification:

Profile method: The power supply electrodes ( A , B ) supply power to _the underground, and the potential difference (ΔUMN) is observed between the measuring electrodes ( M , N ) , and the apparent resistivity is calculated. Each electrode can be used simultaneously along the selected measuring line. (or just measuring electrodes) move forward and observe point by point to detect a certain depth underground

The change of the geoelectric section along the horizontal direction within a certain degree.
   Depth sounding method: Mainly used to detect the underground distribution of near-horizontal layered rocks. The electrode distance is gradually expanded at the same measuring point, the detection depth is from shallow to deep, and the changes in apparent resistivity in the vertical direction are detected. By analyzing the electrical sounding Curve to understand the geological .

5 ) Application of resistivity method:

Conduct geological mapping to determine bedrock relief

Determine the tendency of tectonic fracture zones;

Find metallic and non-metallic ores;

Look for groundwater.
1. Apparent resistivity is represented by ρ _S :

ρ _s =K △ U _MN /I _AB   unit ( Ω· m) ohm·meter

In the formula: △ U _MN 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 and _N are receiving electrodes.

K is the device coefficient.

2. Induced polarization method (IP method )
1 ), induced polarization method theory:

In the actual work of electrical exploration, we found that when a certain electrode arrangement is used to supply or cut off current to the earth, the change of potential difference with time can always be observed between the measuring electrodes. In this process similar to charging and discharging, Process, the phenomenon of additional electric field that slowly changes with time due to electrochemical effects is called the induced polarization effect ( referred to as the induced polarization effect ) . The induced polarization method ( or induced polarization method ) is a type of electrical prospecting method that is based on the differences in the electrostatic effects of rocks and ores to achieve ore prospecting or solve certain hydrogeological problems. The characteristic that the polarization effect increases with the increase in the content of electronically conductive minerals in rocks and ores is the physical - chemical basis for the successful application of the polarization method in general prospecting of metal ores.

Time domain charge and discharge waveforms (applicable to all time domain instruments)

    figure 1

2. Classification of induced polarization method:

According to the excited polarization characteristics of rocks and minerals ( stones ) , the excited polarization of rocks and minerals ( rocks ) is often divided into two categories: "surface polarization" and "body polarization". The characteristic of surface polarization is that excited polarization occurs at the interface between polarized bodies and surrounding rock solutions, such as dense metal ores and graphite ores. The characteristic of bulk polarization is that polarized units (tiny metal minerals or rock particles) are distributed throughout the polarized body, such as disseminated metal ores, mineralized rocks and ion-conducting rocks. "Surface polarization" and "body polarization" only have relative meanings. The polarized bodies actually existing underground will not be ideal surface polarized bodies or body polarized bodies, but are closer to a certain typical polarization mode.

The induced polarization method can be divided into DC (time domain) induced polarization method and AC (frequency domain) induced polarization method according to different power supply and measurement content.

DC excited polarization supplies DC current normal to the ground. After power supply, it gradually increases from zero. During the charging process and after power outage, the secondary potential difference Δ U ₂ gradually decays to zero. During the discharge process, under the condition of stable current flowing, the potential difference observed when the power supply time is T is actually Δ The sum of U ₁ and Δ U ₂ , as shown in Figure 1 - 2 , is called the total field potential difference . During the power supply process, the total field potential difference ( Δ U ) between the M and N electrodes is observed, and the potential difference Δ U ₂ of the induced electric field is observed after the power is turned off. Generally , two parameters, η _S and ρ _S , can be obtained simultaneously at each measuring point. , that is, two profile curves can be obtained simultaneously on one survey line. When there is no ore body underground , there is no significant change in η _S and ρ _S along the survey line .are all normal background values of the surrounding rock ; when there is an ore body underground , in addition to changes in ρ _S , due to the increase in the excited polarization current density above the ore body, the η _S profile curve appears at a maximum value above the ore body. ( Figure 1-3b ) . _ For disseminated ore bodies where electronically conductive minerals are scattered, the resistivity value is usually not significantly different from the surrounding rock and cannot be identified by the resistivity method. However, each small metal particle in the ore body can be polarized by a stable current field ( called (Body polarization ) , which produces an electrostatic effect, and the apparent polarizability η _S is obviously abnormal. Therefore , the induced polarization method is effective in finding both dense metal ores and disseminated metal ores. 　　     

Figure 1-3 Principle diagram of  induced polarization method

3. Application of induced polarization method:

◆Search for disseminated ores with low content, metallic and non-metallic solid minerals exploration: general survey of sulfide polymetallic ores, the advantage is that it can find non-ferrous metal ores such as copper, lead, zinc, molybdenum; search for non-magnetic or weakly magnetic minerals Ferrous metal mines, precious metal mines, rare metal mines and radioactive mineral deposits, etc. condition.

◆ Looking for groundwater: After the water-bearing sand layer is charged, the secondary potential stimulated by charging can be observed at the moment of power outage. The decay rate of this secondary potential slows down as the water content increases. In practice, there have been many successful examples of using this method to delineate groundwater enrichment zones and determine well locations. The biggest advantage of IP sounding is that it reflects water intuitively and is less affected by terrain. The most commonly used method for finding water by induced polarization is the symmetrical quadrupole vertical sounding device, which usually uses the Wenner device and maintains the equal ratio of MN/AB=1/3 .

4. The apparent polarizability is:

After the power is turned off, observe the potential difference Δ U ₂ of the induced electric field and define the apparent polarizability

In the formula: Δ U ₂ The secondary electric field is observed after the power is turned off; Δ U is the primary field;

Usually Δ U ₂ is much smaller than Δ U , so η _S is often expressed as a percentage. Generally, two parameters, η _S and ρ _S , can be obtained simultaneously at each measuring point , that is, two profile curves can be obtained simultaneously on one measuring line.

3. Natural electric field method ( SP)

1 ) , natural electric field method theory and application fields:

A method of using the natural electric field in the earth as a field source to prospect for minerals and solve other geological problems.

It is the earliest electrical exploration method used by people. It does not require artificial methods to supply power to the ground. As for the reasons for the generation of natural electric fields, there are still different opinions. The underground phreatic surface cuts the electronically conductive ore body. Oxidation occurs in the upper part of the phreatic surface and reduction occurs in the lower part, causing uneven electric double layers on the surface of the upper and lower ends of the ore body, thereby forming a natural current inside and outside the ore body. Usually, negative natural potential anomalies can be observed on the surface above the ore body, which can be used to achieve ore prospecting purposes. Another view is that the ore body itself does not participate in chemical reactions and only serves to transfer electrons. In addition, some people have proposed the electrode potential theory and the wave difference battery theory. For ionic conductors, when groundwater flows in rock pores, because the aqueous solution often contains a large number of positive and negative ions, and the rock particles attract negative ions, the water takes away a large number of positive ions, forming a natural electric field. When working in the field, place the electrode N very far away (at ∞), and measure the natural potential U point by point along the measuring line with the measuring electrode M ( both M and N poles are non-polarized electrodes) . The measurement results can be drawn into U profile curves and plane contour maps. The natural electric field method does not require artificial power supply, so the instruments and equipment are lighter and the production efficiency is high.

2) Application scope of natural electric field method

Used to find electronically conductive metal ore deposits and non-metallic deposits;

Carry out geological mapping;

Determine the groundwater flow rate and flow direction to find groundwater and other hydrogeological issues.

3) Working method of natural electric field method

(1) Potential method: observe the potential of all measuring points relative to a certain fixed point (base point), that set the fixed electrode on the base point, then move the movable electrode point by point along the measuring line, and observe the potential difference relative to the fixed electrode .

(2) Gradient method: observe the potential difference between two adjacent measuring points. The measuring electrodes are placed on two adjacent measuring points of the same N measuring line, the potential difference between them is observed, and then they move synchronously or crosswise along the measuring line. That is, after each observation, the following electrode is moved Move in front of the previous electrode and continue moving in a crosswise manner. This pole running method can avoid the accumulation of extreme differences between the two electrodes.

(3) After sorting out the observation results, use the following formula to calculate the natural potential value of each point

Potential value = reading + base point difference - ( range + range distribution ) where: the potential value is the potential value of the measuring point relative to the total base point

   The reading is the potential value of the measuring point relative to the base point.

   The base point difference is the potential difference between the sub-base points and the total base point.

   The range is the potential difference between the movable electrode and the fixed electrode at startup.

The range distribution is the change value of the range from the start of work to the end of work, and each point is calculated in chronological order.

Linear distribution of values.

4. Charging method
1). Basic theory of charging method :

When the resistivity of an ideal good conductor is much smaller than the resistivity of the surrounding rock (< 200 times), it can be approximately regarded as an ideal conductor. When it is located in a general conductive medium, after power is supplied ( or charged ) to any point on it , the current It spreads throughout the ideal conductor and then flows perpendicular to the surface of the conductor to the surrounding medium. When current flows through an ideal conductor, no voltage drop occurs, so the ideal conductor is called an equipotential body. The charging electric field of an ideal conductor has nothing to do with the location . It only depends on the size of the charging current, the shape, origin, size, position of the charging conductor and the electrical distribution of the surrounding medium. By observing the distribution of the charging electric field in this way, we can infer the distribution of electrical properties of the entire underground good conductor and surrounding rock. The principle of charging method is shown in Figure 1-4 .

                     Figure 1-4 Charging method principle diagram 

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 relationships between known adjacent ore bodies;

Find blind ore bodies near known mines;

Use a single well to determine the flow direction and velocity of groundwater;

Study landslides and track underground metal pipelines, etc.

Application conditions of charging method:

The object being studied (charging body) has at least one location exposed or exposed so that a charging point can be set up;

The charging body should be a good conductor relative to the surrounding rock;

The larger the charging body is and the shallower it is buried, the more ideal the charging method will be. The maximum research depth of the charging method is generally half the extended length of the charging body.

3) Connection method between power supply electrode and charging body:

  The positive electrode of the power supply electrode must be connected to the charging body. Since the conditions under which the power supply body is exposed are different , the connection methods are also different. When used to evaluate the detailed investigation stage of metal ores, 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 drilled into the ore body and connected in parallel . , connected to the positive pole of the power supply. When it is difficult to drive the iron electrode, you can use heavy objects 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 needs to be used as the power supply electrode, and the brush electrode is placed on the ore body in the well. When lowering the pipeline, if the exposed point of the pipeline can be found on the ground, the pole can be directly connected to the exposed point of the pipeline. The negative electrode should be placed in a low-lying and humid place 1000 to 1500m away from the measurement area to reduce resistance and increase power supply current.

4). Main observation methods and methods in the charging method:

1 ) Potential method: Fix a measuring electrode N at the edge far away from the measuring area as the potential

The other measuring electrode M moves point by point along the measuring line to observe the zero value point relative to the N pole.

Potential difference, as the potential value U of the measuring point where the M pole is located , while observing the supply current I , calculate

Calculate the normalized potential value U/I .

( 2 ) Potential gradient method: Keep the measuring electrode MN at a certain distance and move together along the measuring line.

Observe the potential difference △ U and the 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 potential difference ΔU .