Electromagnetic exploration method using full-coverage anti-interference artificial source

Abstract

An electromagnetic exploration method using a full-coverage anti-interference artificial source, comprising the steps of: (1) determining the scope and location of the measuring area; (2) field-exploring to determine the location of the transmitting source and the angle of the transmitting antenna; (3) calculating the maximum polarization direction angle of the electric field generated by the antenna at each measuring point; (4) arranging electric field sensors according to the polarization directions; (5) calculating the apparent resistivity of each measuring point. The method of the present disclosure obtains the earth resistivity using the reliable data with high signal-to-noise ratio. The field construction is flexible and convenient, the construction efficiency is high and the cost is low. The present disclosure provides a new development direction for the electromagnetic exploration.

Claims

1. An electromagnetic exploration method using a full-coverage anti-interference artificial source, comprising: determining a scope and a location of a measuring area; field-exploring to determine a location of a transmitting source and an angle of a transmitting antenna; calculating a maximum polarization direction angle of an electric field generated by the antenna at each measuring point; arranging electric field sensors according to polarization directions, and keeping the direction of the electric field sensor consistent with the maximum polarization direction of the electric field; and calculating an apparent resistivity of each measuring point.

2. The electromagnetic exploration method using a full-coverage anti-interference artificial source of claim 1, wherein determining the scope and location of the measuring area comprises ensuring that the target is fully within the measuring area according to a size and a scope of an underground exploration target, wherein ensuring the target is fully within the measuring area comprises ensuring a projection of the target on a surface of earth is within a designed measuring area.

3. The electromagnetic exploration method using a full-coverage anti-interference artificial source of claim 1, wherein field-exploring to determine the location of the transmitting source and the angle of the transmitting antenna comprises: exploring the measuring area in response to determining the scope of the measuring area, selecting a transmitting source location convenient for the field exploration and transportation according to actual terrain condition of the measuring area, determining a location and a direction of the transmitting antenna according to actual terrain condition of the measuring area, and ensuring two ends of the transmitting antenna are grounded.

4. The electromagnetic exploration method using a full-coverage anti-interference artificial source of claim 1, wherein calculating the maximum polarization direction angle of the electric field generated by the antenna at each measuring point comprises: collecting field information of the measuring area, including geological information and existing geophysical data information, establishing a geodetic model according to the geological and the existing geophysical data information of the measuring area, and calculating and simulating a long axis polarization direction of the electric field generated by the transmitting antenna.

5. The electromagnetic exploration method using a full-coverage anti-interference artificial source of claim 4, wherein calculating and simulating the long axis polarization direction of the electric field generated by the transmitting antenna comprises: recording a position information R of each measuring point, calculating a distance r relative to an actual position information T of a center of the transmitting source, and an x-coordinate and a y-coordinate of each measuring point, and calculating electric field of each measuring point according to the following formula: $\begin{array}{cc}{E}_{x0}=\frac{{\mathrm{Id}}_s{}_0}{2r^3}\left[1+\left(\mathrm{irk}+1\right)e^{-ikr}-\frac{3y^2}{r^2}\right]E_{y0}=\frac{{\mathrm{Id}}_s{}_0}{2r^3}\frac{3xy}{r^2}& \left(1\right)\end{array}$ wherein Ex.sub.0 represents a x-direction component of the electric field, wherein Ey.sub.0 represents a y-direction component of respective electric field, wherein I represents a transmitting current, d.sub.s represents a length of the transmitting antenna, and .sub.0 represents a resistivity of the geodetic model established based on the existing geological data, wherein r represents a distance from respective measuring point to the center of the transmitting source, r={square root over (x.sup.2+y.sup.2)}, wherein x represents the x-coordinate and y represents the y-coordinate of the respective measuring point, wherein k represents a wavenumber, $k=\sqrt{\mathrm{.Math.}{}^2-\frac{i}{{}_0}},$ =2f, wherein f represents a transmitting frequency.

6. The electromagnetic exploration method using a full-coverage anti-interference artificial source of claim 5, wherein calculating the maximum polarization direction angle of the electric field generated by the antenna at each measuring point comprises calculating the maximum polarization direction angle of the electric field according to the following formula: $\begin{array}{cc}=\frac{1}{2}\mathrm{arc}\tan \left(\frac{2.Math.E_{x0}.Math..Math.E_{y0}.Math.\cos \left({}_y-{}_x\right)}{E_{x0}^2-E_{y0}^2}\right)& \left(4\right)\end{array}$ wherein .sub.x represents phase of electric field component E.sub.x0, and ${}_x=\mathrm{atan}\left(\frac{\mathrm{imag}\left(E_x\right)}{\mathrm{real}\left(E_x\right)}\right),$ wherein .sub.y represents phase of electric field component E.sub.y0, and ${}_y=\mathrm{atan}\left(\frac{\mathrm{imag}\left(E_y\right)}{\mathrm{real}\left(E_y\right)}\right),$ wherein represents the maximum polarization direction of the electric field, which is an included angle between the long axis polarization direction of the electric field and x-coordinate of respective measuring point.

7. The electromagnetic exploration method using a full-coverage anti-interference artificial source of claim 6, wherein arranging electric field sensors according to the polarization directions comprises: according to the electric field maximum polarization direction angles obtained at each measuring point, arranging the electric field sensors at each measuring point to ensure that the included angle between the sensor direction and the x-axis is , and in response to arranging the electric field sensors, transmitting signals, and recording the electric field of each measuring point on measuring line by a plurality of receivers.

8. The electromagnetic exploration method using a full-coverage anti-interference artificial source of claim 7, wherein calculating the apparent resistivity of each measuring point comprises: using iterative method to obtain earth resistivity when the difference between calculated electric field and measured electric field is the smallest, wherein is calculated according to the following formula:
P=|E.sub.mE.sub.x cos E.sub.y sin |=Min.(3) wherein E.sub.x represents the electric field component x and E.sub.y represent the electric field component, wherein E.sub.x and E.sub.y are calculated according to the following formula: $\begin{array}{cc}{E}_x=\frac{{\mathrm{Id}}_s}{2r^3}\left[1+\left(\mathrm{irk}+1\right)e^{-ikr}-\frac{3y^2}{r^2}\right]E_y=\frac{{\mathrm{Id}}_s}{2r^3}\frac{3\mathrm{xy}}{r^2}& \left(4\right)\end{array}$ wherein an actual formula is: $\begin{array}{cc}p=.Math.E_m-\frac{{\mathrm{Id}}_s}{2r^3}\left[1+\left(\mathrm{irk}+1\right)e^{-ikr}-\frac{3y^2}{r^2}\right]\cos -\frac{I{d}_s}{2r^3}\frac{3xy}{r^2}\sin .Math.=\mathrm{Min}.& \left(5\right)\end{array}$

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a conceptual diagram illustrating an example field construction of the conventional CSAMT method.

(2) FIG. 2 is a conceptual diagram illustrating example locations of the transmitting source and the measuring points in the measuring areas of an embodiment of the present disclosure.

(3) FIG. 3 is a conceptual diagram illustrating the maximum polarization angle of the electric field around the transmitting source of an embodiment of the present disclosure.

(4) FIG. 4 is a conceptual diagram illustrating an example field arrangement of the electric field sensor (non-polarizing electrode) at measuring point 1 of an embodiment of the present disclosure.

(5) FIG. 5 is a conceptual diagram illustrating the coverage scope of effective signals of the conventional CSAMT method.

(6) FIG. 6 is a conceptual diagram illustrating the coverage scope of effective signals of an embodiment of the present disclosure.

(7) FIG. 7 a conceptual diagram illustrating a comparison between the electric field effective signals of the present disclosure and that of the prior art.

DETAILED DESCRIPTION

(8) Figures and detailed embodiments are combined hereinafter to further elaborate the technical solution of the present disclosure.

(9) An electromagnetic exploration method using a full-coverage anti-interference artificial source of the present disclosure, comprising the steps of:

(10) (1) Determining the scope and location of the measuring area: according to the size and scope of the underground exploration target, ensuring that the target is fully within the measuring area, namely, ensuring that the projection of the target on the earth's surface is within the designed measuring area;

(11) (2) Field-exploring to determine the location of the transmitting source and the angle of the transmitting antenna: after the scope of the measuring area is determined, carefully exploring the measuring area and surrounding areas; according to the actual terrain condition, selecting a transmitting source location convenient for the field exploration and transportation, thus allowing the transmitting source to be conveniently transported to the particular location by truck; according to the actual terrain condition, determining the location and direction of the transmitting antenna, thereby ensuring that the two ends A and B of the transmitting antenna are grounded well, wherein the length of the transmitting antenna AB is normally 1-3 km, which can be increased according to the actual situation to generate signals with large transmitting moment.

(12) FIG. 2 shows the designed locations of the transmitting antenna and the measuring areas. A coordinate system taking the direction of the transmitting antenna as the x-direction, the direction perpendicular to the transmitting antenna as the y-direction, and the center coordinates of the transmitting source as the origin point is established. The actual position of the center of the transmitting source is recorded as T. Taking the true north direction as 0 and the clockwise direction as the positive direction, the actual direction of the transmitting antenna is recorded as T.sub.Direction.

(13) In this embodiment, the length of the transmitting antenna is 1 km, the transmitting frequency is 512 Hz, the transmitting current is 10 A, the perpendicular distance between the measuring line 1 and the transmitting antenna is 5 km, and the actual earth resistivity is 1000 ohm.Math.m.

(14) (3) Calculating the maximum polarization direction angle of the electric field generated by the antenna at each measuring point: collecting the field information of the measuring area, including outcrop, borehole and other geological information, as well as the existing geophysical data information; establishing a geodetic model according to the collected geological and geophysical data of the measuring area, wherein in this embodiment, the earth is assumed to be homogeneous, and its resistivity is 500 ohm.Math.m; calculating and simulating the long axis polarization direction of the electric field generated by the transmitting antenna, comprising the following steps: At each measuring point, recording the position information R of the receiving measuring point; calculating the distance r relative to T, and the x-coordinate and y-coordinate of the measuring point; calculating the electric field values in x-direction and y-direction of the measuring point according to the following formula:

(15) $\begin{array}{cc}{E}_{x0}=\frac{{\mathrm{Id}}_s{}_0}{2r^3}\left[1+\left(\mathrm{irk}+1\right)e^{-ikr}-\frac{3y^2}{r^2}\right]E_{y0}=\frac{{\mathrm{Id}}_s{}_0}{2r^3}\frac{3xy}{r^2}& \left(3\right)\end{array}$
wherein Ex.sub.0 represents the x-direction component of the electric field, and Ey.sub.0 represents the y-direction component of the electric field, wherein I represents the transmitting current, d.sub.s represents the length of the transmitting antenna, and .sub.0 represents the resistivity of the geodetic model established based on the existing geological data, wherein in this embodiment, the resistivity of the geodetic model is 500 ohm.Math.m, wherein r represents the distance from the position of the receiving measuring point to the center of the transmitting source, r={square root over (x.sup.2+y.sup.2)}, and x and y are coordinates of the position of the receiving measuring point, wherein k represents the wavenumber,

(16) $k=\sqrt{\mathrm{.Math.}{}^2-\frac{i}{{}_0}},$
=2f, and f represents the transmitting frequency, wherein the calculation formula of the maximum polarization direction angle of the electric field is:

(17) 0$\begin{array}{cc}=\frac{1}{2}\mathrm{arc}\tan \left(\frac{2.Math.E_{x0}.Math..Math.E_{y0}.Math.\cos \left({}_y-{}_x\right)}{E_{x0}^2-E_{y0}^2}\right)& \left(4\right)\end{array}$
wherein .sub.x represents the phase of the electric field component E.sub.x0, and

(18) ${}_x=\mathrm{atan}\left(\frac{\mathrm{imag}\left(E_x\right)}{\mathrm{real}\left(E_x\right)}\right),$
wherein .sub.y represents the phase of the electric field component E.sub.y0, and

(19) ${}_y=\mathrm{atan}\left(\frac{\mathrm{imag}\left(E_y\right)}{\mathrm{real}\left(E_y\right)}\right),$
wherein represents the included angle between the polarization direction of the long axis and the direction of x-coordinate, namely, the maximum polarization direction; calculating the electric field maximum polarization angles of all measuring points around the transmitting antenna as shown in FIG. 3 (the clockwise direction is taken as the negative direction).

(20) (4) Arranging electric field sensors according to the polarization directions: according to the electric field maximum polarization direction angles obtained at each measuring point, arranging the electric field sensors at each measuring point to ensure that the included angle between the sensor direction and the x-axis is . Taking the receiving measuring point whose coordinates are x=5 km and y=5 km as an example, the obtained electric field maximum polarization angle at the receiving measuring point is about 71, and the included angle between the direction of the electric field sensor at the actual measuring point in the field and the x-axis should be 71. The arrangement of the sensors in the field is shown in FIG. 4; after the sensors of all measuring points are arranged, transmitting the signals, and recording the electric field values of each measuring point on the measuring line by a plurality of receivers;

(21) (5) Calculating the apparent resistivity of each measuring point: at this point, the measured electric field value of each measuring point is obtained, wherein the amplitude of the measured electric field value is large, and the data quality is much better than that obtained using the existing technology; using the iterative method to obtain the earth resistivity when the difference between the calculated electric field and the measured electric field (E.sub.m) is the smallest:
p=|E.sub.mE.sub.x cos E.sub.y sin |=Min.(5)
wherein E.sub.x and E.sub.y represent the electric field component x and electric field component y calculated by forward modelling:

(22) $\begin{array}{cc}{E}_x=\frac{{\mathrm{Id}}_s}{2r^3}\left[1+\left(\mathrm{irk}+1\right)e^{-ikr}-\frac{3y^2}{r^2}\right]E_y=\frac{{\mathrm{Id}}_s}{2r^3}\frac{3\mathrm{xy}}{r^2}& \left(4\right)\end{array}$
wherein the actual formula is:

(23) $\begin{array}{cc}p=.Math.E_m-\frac{{\mathrm{Id}}_s}{2r^3}\left[1+\left(\mathrm{irk}+1\right)e^{-ikr}-\frac{3y^2}{r^2}\right]\cos -\frac{I{d}_s}{2r^3}\frac{3xy}{r^2}\sin .Math.=\mathrm{Min}.& \left(5\right)\end{array}$

(24) The aforesaid formula can be calculated as all values are known except for the variable earth resistivity. After calculation, the earth resistivity is 1000 ohm.Math.m.

(25) It can be seen that, through adopting the technical solution of the present disclosure, reliable and effective signals with high signal-to-noise ratio are obtained, environmental noise interference is effectively suppressed, and data quality is stable and reliable. Although there is a big difference between the actual earth resistivity and the initial geodetic model with resistivity set to be 500 ohm.Math.m in this embodiment, the actual earth resistivity can still be obtained. Its objective is to verify whether there is a big difference between the resistivity of the initial model and the actual earth resistivity.

(26) FIG. 5 shows the coverage scope of field strength of the conventional method, wherein the brightness of the figure represents the strength of effective signals. It can be seen that the effective signal strength of the conventional method is weak and the quality of the observational data is poor. Even worse, there are low effective signal areas, which may lead to the failure of field construction.

(27) FIG. 6 shows the coverage scope of effective signals of the present disclosure. It can be found that the present disclosure achieves a high-intensity full-coverage of effective signals around the transmitting source. According to the present disclosure, the field construction efficiency is greatly improved, the construction cost is lowered, and through the effective suppression of noise, the field data with high reliability is obtained.

(28) FIG. 7 shows a comparison between the calculated effective electric field value observed on the measuring line 1 of the present disclosure and the effective electric field value of the conventional method. It can be noticed that the electric field value of the conventional method is significantly lower than that of the present disclosure, which results in a poor data quality. Moreover, there is an obvious low electric field value band, which makes effective signals submerged in the noise, or even leads to the failure of construction. The effective electric field value of the present disclosure is obviously larger than that of the conventional method. The present disclosure effectively suppresses the noise, greatly improves the data quality, and ensures that the effective data with high signal-to-noise ratio can be collected in the field.