CN108873083A - A kind of artificial field source frequency domain electromagnetism apparent resistivity measurement method - Google Patents

Abstract

The invention discloses an artificial field source frequency-domain electromagnetic apparent resistivity measurement method. Firstly, according to detection or exploration requirements, a horizontal electric dipole field source and an electric field horizontal component measurement system are set up, and a fixed transmitting and receiving device is used for nondestructive testing. Use the electromagnetic signals of different frequency f to record the electromagnetic E _x component of the multi-frequency point of a measuring point. Use the approximate method of first-order difference derivation to calculate the derivative of the electromagnetic field E _x of the corresponding frequency point combination with respect to the frequency point And store the calculated partial derivative information. Using the derivative of the electromagnetic field E _x component to the frequency point Perform apparent resistivity conversion processing. This method overcomes the limitations of using the Carnia apparent resistivity definition criterion to solve the apparent resistivity, and at the same time reduces the risk of unstable calculation results caused by the solution of nonlinear equations in the conventional full-area apparent resistivity calculation. Effectively improve the efficiency and accuracy of apparent resistivity calculation.

Description

A Method for Measurement of Electromagnetic Apparent Resistivity in Frequency Domain of Artificial Field Source

technical field

The present invention relates to an artificial field source frequency domain electromagnetic apparent resistivity measurement method in the field of exploration geophysics. Its special feature is that the method for calculating apparent resistivity in the whole area does not need to solve nonlinear equations, which reduces the inaccurate band of solving nonlinear equations. To define the error of apparent resistivity and improve the accuracy of traditional apparent resistivity calculation.

Background technique

In geophysical exploration, in order to describe the conductivity of underground rocks and ores, resistivity or conductivity is usually used to describe it. However, in actual work, the combined effect of collected underground rocks and ores is not the true resistivity of underground rocks and ores. , which is called the apparent resistivity, which is related to many factors such as the composition and structure of underground rocks and ores, and the layout of collection devices.

Controlled source audio frequency magnetotelluric method (abbreviated as CSAMT) is an electromagnetic sounding method with artificial field source in the frequency domain. Greatly improved the observed signal quality. With the continuous development of electromagnetic prospecting methods, such methods are widely used in oil, natural gas, metal ore, geothermal, hydrological, environmental and other exploration fields. However, the introduction of artificial field sources into the artificial field source frequency electromagnetic method has successfully solved the observation difficulties caused by the randomness of natural sources and weak signal strength, but a series of new problems have also emerged. For example, it still uses the conventional Carnia apparent resistivity definition method for calculation (such as or ), the premise of this definition is that electromagnetic waves propagate as plane waves, so it is necessary to place the observation area in the far area, and use the data in the far area to calculate the apparent resistivity, and the apparent resistivity calculated in the non-far area will produce serious distortion. Therefore, the data in the transition zone and the near zone are discarded, thereby reducing the scope of application of CSAMT. At the same time, working in the field needs to measure two orthogonal electric and magnetic fields in the far area, which reduces the efficiency of field work and increases the cost of field exploration. In addition, the conventional way to define the apparent resistivity of the whole area adopts E_E _x and To solve the problem, this definition method is applicable to most of the measurement areas, and it cannot solve the apparent resistivity obstacle in the non-planar area, but it needs to solve the nonlinear equations to define the corresponding apparent resistivity, resulting in the apparent resistivity accuracy depends on non-planar The solution of linear equations affects the accuracy and efficiency of apparent resistivity calculation.

Contents of the invention

The purpose of the present invention is to overcome the problems described in the background technology and at the same time aim at the deficiencies of the prior art, to provide a method for measuring the electromagnetic apparent resistivity of an artificial field source in the frequency domain, which avoids the plane wave assumption defined by the Carnia apparent resistivity At the same time, it reduces the risk of unstable calculation results brought about by the solution of nonlinear equations in the conventional calculation of apparent resistivity in the whole area, and effectively improves the efficiency of apparent resistivity calculation.

In order to achieve the above-mentioned technical purpose, the technical solution adopted in the present invention is a method for measuring electromagnetic apparent resistivity in the frequency domain of an artificial field source. Cartesian coordinate system is adopted in the field actual data collection. According to the theory of electromagnetic field, the electric field component E of the horizontal electric dipole The expression of _x in the Cartesian coordinate system is:

In the formula, E _x represents the electric field component in the x direction in the Cartesian coordinate system, σ is the conductivity of the uniform half space, φ is the angle between the observation point and the positive direction of the dipole distance, and r is the distance between the observation point and the center of the dipole distance , k is the wave number, under quasi-static conditions, μ is the magnetic permeability, ω=2πf is the angular frequency, and f is the frequency. i represents the serial number of the frequency point.

Taking the partial derivative with respect to the angular frequency ω, we can get:

Simplifies to:

where _ρ0 is the resistivity of the medium.

make p is the induction number, m is the complex intermediate variable, therefore, it is further simplified as:

in,

Finally, the partial derivative of the electromagnetic E _x with respect to the angular frequency ω is obtained:

in, Take the modulus to get:

Then, take the base-e logarithm of the left and right sides of the formula, and add Substituting and simplifying, we get:

Therefore, the apparent resistivity ρi is calculated by using the derivative of the electromagnetic E _x in the x direction of the Cartesian coordinate system to the frequency f as:

Therefore, as long as the derivative of E _x to the frequency f is obtained, the apparent resistivity can be calculated, and there is no need to solve the nonlinear equation to define the apparent resistivity of the whole area, which effectively improves the accuracy of the apparent resistivity calculation and improves the understanding of underground geology. Accuracy of exception interpretation.

A method for measuring electromagnetic apparent resistivity in the frequency domain of an artificial field source, comprising the following steps:

Step S1: First, according to the requirements of detection or exploration, set up a horizontal electric dipole field source and electric field horizontal component measurement system. For non-destructive testing, use fixed transmitting and receiving devices; for exploration, use one transmitting source, multiple measuring points, and multiple surveying lines to observe Mode; the emission source emits multi-frequency electromagnetic signals through horizontal electric dipoles.

Step S2: use the electromagnetic signal receiver to record the potential difference V between the two measuring electrodes. Calculate the electromagnetic E _x component of a single frequency point by the ratio of the distance d, and then use electromagnetic signals of different frequencies f to record the electromagnetic E _x component of multiple frequency points at a measuring point.

Step S3: Use the obtained electromagnetic E _x components of a series of frequencies to combine adjacent frequency points, and use the approximate method of first-order difference derivation to calculate the derivative of the electromagnetic field E _x of the corresponding frequency point combination with respect to the frequency point And store the calculated partial derivative information.

Step S4: Using the derivative of the electromagnetic field E _x component to the frequency point To perform apparent resistivity conversion processing, the solution expression adopted is:

Among them, μ is the magnetic permeability, r is the distance between the center of the dipole moment and the observation point, ω=2πf is the angular frequency, I is the magnitude of the emission current, dL is the length of the dipole emission source, ρ _i represents the i-th Frequency point apparent resistivity.

The electromagnetic signal receiver in step S1 is used to record the potential difference between the two measuring electrodes.

A method for measuring electromagnetic apparent resistivity in the frequency domain of an artificial field source provided by the present invention has positive effects:

(1) The present invention fully considers the calculation of the apparent resistivity in the non-plane wave propagation area when obtaining the apparent resistivity, and expands the measurement range of the traditional artificial field source electromagnetic method, thereby being able to carry out survey measurements in the non-plane wave area and the far zone;

(2) The present invention only needs to measure the electromagnetic field signal of multi-frequency points and single components, and does not need to measure the electric field and magnetic field perpendicular to each other to calculate the apparent resistivity. The electromagnetic field measurement equipment is bulky to improve guidance, improve the efficiency of field exploration data collection, and reduce the cost of field exploration;

(3) The present invention adopts the derivative of the electric field E _x component to the frequency to define the apparent resistivity, without needing to follow the traditional apparent resistivity calculation method, avoiding the shortcomings such as poor apparent resistivity calculation accuracy caused by the inaccurate solution of the nonlinear equation, Effectively improve the efficiency and accuracy of the apparent resistivity solution.

(4) The method for defining the apparent resistivity calculated by the present invention is simple and the calculation is fast, and it is very suitable for non-destructive testing instruments that use resistivity to detect and measure numbers.

Description of drawings

Fig. 1 is the basic flowchart of a kind of artificial field source frequency domain electromagnetic apparent resistivity measurement method provided according to the present invention;

Fig. 2 is a schematic diagram of field layout of an artificial field source frequency domain electromagnetic apparent resistivity measurement method provided according to the present invention;

Fig. 3 is a kind of artificial field source frequency domain electromagnetic apparent resistivity measurement method theoretical model apparent resistivity calculation result according to the present invention, this theoretical model is: three-layer H curve, the resistivity of the first layer of the model is 100Ω·m, the thickness 100m, the resistivity of the second layer is 25Ω·m, the thickness is 200m, the resistivity of the third layer is 100Ω·m, the thickness is infinite; the transmitting and receiving distance is 8000m.

Fig. 4; It is the calculation result of the theoretical model apparent resistivity of a kind of artificial field source frequency domain electromagnetic apparent resistivity measurement method provided by the present invention, this theoretical model is: three-layer H curve, the resistivity of the first layer of the model is 100 Ω m, The thickness is 100m, the resistivity of the second layer is 25Ω·m, the thickness is 200m, the resistivity of the third layer is 100Ω·m, and the thickness is infinite;

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments

See shown in Figure 1, a kind of artificial field source frequency domain electromagnetic apparent resistivity measurement method provided by the present invention, it comprises the following steps:

Step S1: First, according to the exploration requirements, set the appropriate field source position, and lay out multiple survey lines and corresponding observation points in the exploration area, place electromagnetic signal receivers at the observation points, and place the signal transmitter at the field source position to monitor the exploration The area transmits electromagnetic signals at multiple frequencies;

Step S2: use the electromagnetic signal receiver to record the potential difference V between the two measuring electrodes. Calculate the electromagnetic E _x component of a single frequency point by the ratio of the distance d, and then use the current signals of different frequencies f to record the electromagnetic E _x component of a multi-frequency point at a measuring point;

Step S3: Use the obtained electromagnetic E _x components of a series of frequencies to combine adjacent frequency points, and use the approximate method of first-order difference derivation to calculate the derivative of the electromagnetic field E _x of the corresponding frequency point combination with respect to the frequency point And store the calculated derivative information;

Step S4: Using the derivative of the electromagnetic field E _x component to the frequency point To perform apparent resistivity conversion processing, the solution expression adopted is:

Among them, μ is the magnetic permeability, r is the distance between the center of the dipole moment and the observation point, ω=2πf is the angular frequency, I is the magnitude of the emission current, dL is the length of the dipole emission source, ρ _i represents the i-th Frequency point apparent resistivity.

The following is an example of the method for measuring the electromagnetic apparent resistivity of the artificial field source in the frequency domain of the present invention.

The theoretical model is a three-layer H curve. The first layer of the theoretical model has a resistivity of 100Ω·m and a thickness of 100m, a second layer of resistivity of 25Ω·m and a thickness of 200m, and a third layer of resistivity of 100Ω·m and an infinite thickness. 8000m and Frequency measurement range is 0.1Hz ~ 10KHz. Artificial field sources are dipole sources and bipolar sources.

Figure 3 and Figure 4 respectively show the calculation results of the apparent resistivity of two artificial field sources without transmitting and receiving distances.

Fig. 3 and Fig. 4 respectively compare the calculation results of apparent resistivity under far zone conditions and the calculation results of full zone apparent resistivity provided by the present invention. The calculation result of Fig. 3 shows that the calculation result provided by the present invention has a good correspondence with the result under the far zone condition at 2Hz～10KHz, and the result is very accurate; the result of Fig. 4 shows that when the transmitting and receiving distance is far, the The calculation results given are in good correspondence with the results under far zone conditions in the range of 0.1Hz to 10KHz, and the results are very accurate. The above results also show from another aspect that the method for measuring the electromagnetic apparent resistivity of the artificial source in the whole frequency domain is correct, and at the same time, it can well indicate the relevant information of the underground geological structure, avoiding iterative calculation of the traditional full-area apparent resistivity The numerical error caused by inaccuracy is of great significance to improve the accuracy of underground geological anomaly interpretation.

Figure 3 The definition curve of the horizontal electric dipole three-layer H-shaped curve E _xf apparent resistivity (transmitting distance 8000m), which respectively shows the forward modeling of the dipole source and the forward modeling of the bipolar source, and the apparent resistivity of the far zone and the whole zone The calculation curve and the dipole forward modeling adopt the dipole source method to obtain the apparent resistivity curves of the far zone and the whole zone.

Fig. 4 The definition curve of the horizontal electric dipole three-layer H-shaped curve E _xf apparent resistivity (transmitting distance ), showing the dipole forward modeling and bipolar source forward modeling, the calculation curves of the far zone and the whole zone apparent resistivity, and the dipole forward modeling using the bipolar source method to obtain the far zone and the whole zone apparent resistivity curves .

Claims (3)

1. A method for measuring artificial field source frequency domain electromagnetic apparent resistivity is characterized by comprising the following steps: adopting a rectangular coordinate system in field actual data acquisition, and according to an electromagnetic field theory, obtaining an electric field component E of a horizontal electric dipole_xThe expression under the rectangular coordinate system is:

in the formula E_xExpressed as electric field component in x direction under rectangular coordinate system, sigma is uniform half spacePhi is the included angle between the observation point and the positive direction of the dipole moment, r is the distance between the observation point and the center of the dipole moment, k is the wave number, under the quasi-static condition,mu is magnetic conductivity, omega-2 pi f is angular frequency, and f is frequency; i represents the frequency point sequence number;

the angular frequency ω is subjected to partial derivation to obtain:

the simplification is as follows:

where ρ is₀Is the resistivity of the medium;

order top is the induction number, m is the complex intermediate variable, therefore, the simplification is further:

wherein,

finally, an electromagnetic E is obtained_xPartial derivative of angular frequency ω:

wherein,taking the model to obtain:

then, taking the logarithm taking e as the base on the left side and the right side of the formula, and obtaining the logarithm taking e as the baseSubstituting, simplifying and obtaining:

therefore, electromagnetic E in the x-direction using a rectangular coordinate system_xThe apparent resistivity ρ i is calculated as the derivative of the frequency f as:

therefore, only obtain E_xThe derivative of the frequency f enables the calculation of apparent resistivity without the need to solve nonlinear equations to define the total apparent resistivity.

2. The method for measuring artificial field source frequency domain electromagnetic apparent resistivity according to claim 1, characterized in that: the steps are as follows,

step S1: firstly, according to the detection or exploration requirement, a horizontal electric dipole field source and an electric field horizontal component measuring system are arranged, and a transmitting and receiving fixed device is adopted for nondestructive detection; for exploration, an observation mode of one emission source, multiple measuring points and multiple measuring lines is adopted; the emission source emits a multi-frequency electromagnetic signal through a horizontal electric dipole;

step S2: recording the potential difference V between two measuring electrodes by using an electromagnetic signal receiver, wherein the orientations of the measuring electrodes and the power supply electrode are consistent, recording the distance d between the measuring electrodes, and calculating the single-frequency-point electromagnetism E by using the ratio of the potential difference V to the distance d between the measuring electrodes_xComponent, then using electromagnetic signal with different frequency f to record multi-frequency point electromagnetic E of some measuring point_xA component;

step S3: electromagnetic E using a series of frequencies obtained by the calculation_xThe components are combined with adjacent frequency points, and the electromagnetic field E of the corresponding frequency point combination is calculated by adopting an approximation method of first-order difference derivation_xDerivative to frequency pointStoring the calculated partial derivative information;

step S4: using electromagnetic fields E_xDerivative of component to frequency pointAnd (3) performing apparent resistivity conversion treatment, wherein the adopted solving expression is as follows:

wherein mu is magnetic conductivity, r is the distance between the center of dipole moment and the observation point, ω -2 π f is angular frequency, I is the magnitude of emission current, dL is the length of the dipole emission source, ρ_iAnd (3) representing the visual resistivity of the ith frequency point.

3. The method for measuring artificial field source frequency domain electromagnetic apparent resistivity according to claim 2, characterized in that: the electromagnetic signal receiver in step S1 is used to record the potential difference between the two measuring electrodes.