Introduction to magnetotelluric method for Ground Water Detection

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Introduction to magnetotelluric method for Ground Water Detection

Introduction to magnetotelluric method

Magnetic Methods - Mundell & Associates, Inc.

The magnetotelluric method appeared in the early 1950s and was proposed by Cagniard in France and Tichonov in the Soviet Union. It is a geophysical method using the natural electromagnetic field as the field source, and it is also called the Magnetotelluric Method in China. In a sense, it can be said to be the development of the earth current method. In fact, the purpose of understanding the underground electrical structure is achieved by measuring the ratio of the surface horizontal electric field component E, (or Er) to the orthogonal horizontal magnetic field component H. (or H,) at various frequencies. Theoretically, the detection depth is infinite. As far as the application of the current method is concerned, its detection depth has reached the level of understanding the geoelectric properties of the earth's crust and upper mantle. This method also has the advantages of not being shielded by underground high-resistivity layers and having high resolution for low-resistance layers. These advantages of the method The advantages are valued by geophysicists.

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The earth's natural electromagnetic field is a natural field source with rich energy and wide frequency spectrum. For ease of description, according to the customary application frequency range of the magnetotelluric method, the earth’s natural electromagnetic field involved in the method is divided into: low frequency part (10-4-10 Hz); audio part (10-20×10° Hz): high frequency part (30×10-500×10 Hz). This division method is not strict, and each family is not consistent. The low-frequency part of the earth's natural electromagnetic field mainly comes from the sun and is affected by various factors such as the sun. In space, there are strongly changing solar plasma flows and short-wave radiation produced by solar explosions, as well as plasma clouds with a velocity of 300-800 km/s (that is, the solar wind) composed of high-energy protons. The plasma cloud is blocked by the earth's magnetic field, and a current is induced at the boundary of the magnetic field to form a magnetosphere. The entire magnetosphere forms a slender and streamlined shape like a "water drop". In contrast to plasma in the magnetosphere, which is ionized by ultraviolet and x-rays from the sun, magnetohydrodynamic phenomena in the magnetosphere are ionized at altitudes of 90-140 km due to the Hoff conductivity of the ionosphere Layers generate large horizontal current plates. They also radiate to Electromagnetic energy in the atmosphere. 

Due to the complex factors of solar physics and geophysics, the mechanism of forming the low-frequency natural electromagnetic field is not yet fully understood, and there are different opinions. The disturbance of the natural electromagnetic field outside the earth above 1 Hz, which is shielded by the ionosphere. The electromagnetic field in the audio part mainly comes from lightning and thunderstorms in the far area. The energy generated by lightning and thunderstorms propagates around the globe through the waveguide formed between the earth's surface and the ionosphere (i.e., the earth waveguide). The field source itself constitutes a broad frequency spectrum, but due to the waveguide characteristics, most frequency field sources are easily attenuated. Schuman (Schuman) believes that at the frequencies of 8, 14, 20 and 25 Hz, very strong peaks of energy can be observed. At about 2000 Hz, there is a strong absorption. Below 2000 Hz, the amplitude increases with decreasing frequency. Under normal circumstances, the energy level is higher at low latitudes, higher in summer than in winter, and higher in the afternoon than in the morning. These characteristics illustrate the time and regional constraints of field work.

(2) Basic theory

 The hypothesis that the magnetotelluric field is a planar electromagnetic wave projected vertically from high altitude to the ground put forward by Cagniard is applicable to the actual geophysical exploration work in most cases, accepted by geophysicists, and is the theoretical basis of the magnetotelluric method . Because the magnetotelluric field studied by the magnetotelluric method comes from the electromagnetic waves that pass through the ionosphere, or the electromagnetic waves caused by thunderstorms and lightning at high altitudes, the distance of this field source is large enough compared to the actual research area and exploration depth. When the period of the electromagnetic field is close to the order of 10 seconds, the influence of the size of the field source will appear, and when the period is less than 10 seconds, the influence of the size of the field source can be ignored. In general, the magnetotelluric method treats the earth as a plane problem in theory, and this assumption is applicable when the earth's internal structure is not too deep and the electromagnetic field period is not too large. In any medium, the vectors E and H satisfy the following Maxwell's equations: E root H = P (1)

P. is the apparent resistivity, which is a function of the surface number of the geoelectric agent and the observation period. It is a reflection of the entire geoelectricity in a certain period. The above formulas are derived using the MSKA system of units. If the unit of electric field is millivolt/km, and the unit of magnetic field is gamma, then (31) formula becomes: P.-0.2 T- T- H, (32) Equation (32) is the calculation formula of apparent resistivity by magnetotelluric method. At present, the two-layer and some three-layer theoretical measuring plate curves have been calculated at home and abroad, and the relevant parameters are obtained by comparing the theoretical measuring plate curves with the measured curves. Figure 1 is a two-layer theoretical salt plate curve. * When the number of ground films is the limit, use the asymptotes of the ascending branch and descending branch of the measured curve, or some extreme points, to solve the longitudinal conductance S+ and the top surface depth of the low-resistance layer. At the same time, computer calculation can also be used for automatic fitting. In practice, the apparent resistivity value obtained according to the formula (32) is often scattered in the same frequency band, which is caused by anisotropy, and its impedance is not a scalar quantity. In order to consider the effect of anisotropy, the concept of tensor impedance is proposed. Due to limited space, the tensor impedance analysis method is not described here.

The electrodes for measuring the electric field are usually non-polarized electrodes. Generally, a copper rod is immersed in a porcelain tank containing a colloidal solution of copper sulfate, or a metal copper is immersed in a chloride (CaCl) solution in a bar. Compared with the former, the latter has the advantages of slow leakage and can be used for 6-8 months after adding a solution. Recent experiments in Japan have shown that pure carbon electrodes have a small temperature coefficient and stable polarization potential, and are widely used. At present, it is also used to place a glue under the copper plate to isolate it from the ground to form a parallel plate capacitor. This is the so-called capacitive electrode not connected to the ground. 5×10~~V/C*, so the electrodes should be buried deep underground.

 The choice of electrode distance mainly depends on the intensity of the earth current in the survey area and the sensitivity of the instrument. In areas covered by sedimentary rocks, a polar distance of 0.3-1.3 km is often used, and in areas where crystalline rocks are exposed, the polar distance is as small as several meters. At present, five-channel digital magnetotelluric instruments are mainly used abroad. Its block diagram is shown in the figure below. Generally include three H channels and two E channels (record H., H,, H. and E., E, components).

 The field preamplifier is a shielded differential input chopper carrier amplifier with very low noise and thermally isolated shockproof components. In application, it is necessary to maintain a wide frequency band, low noise and necessary high gain for the signal provided by the coil with high inductance. When recording five-component electromagnetic field signals in the field, two sets of electrodes arranged at right angles and three induction coils perpendicular to each other are used. The connection between electrodes and ground is required to maintain low resistivity and low noise, and the input to each electrical signal channel is the potential difference between a group of electrodes.

 Due to the drastic change of the lateral resistance string near the surface, the direction and magnitude of the electric field on the surface often change within a short distance. At the same time, the two electrode groups adopt the orthogonal form with four arms as equal as possible, and the terrain fluctuation can cause similar Distortion caused by uneven resistor strings. When the terrain undulation is greater than 10% of the electrode distance, it should be avoided as much as possible. At the same time, when selecting measuring points, try to avoid power plants, power lines, cathodic protection circuits, unprotected pipelines, roads and railways, etc.

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