Underground metal detection system and detection method thereof

Abstract

The invention relates to an underground metal detection system and a detection method thereof. A transmitting coil is connected to a transmitting circuit; a receiving coil is connected to a receiving circuit and is also connected to a signal acquisition and data analysis terminal; a decoupling coil and a precision potentiometer are used to realize hardware decoupling between the transmitting coil and the receiving coil; in terms of physical structure, the decoupling coil and the transmitting coil are coaxially nested and wound independently of each other; in terms of electrical structure, the decoupling coil and the precision potentiometer are connected to the receiving circuit, and the precision potentiometer is fine-tuned to make the output voltage of the decoupling circuit and the primary induced voltage generated by the mutual inductance between the transmitting coil and the receiving coil equal and opposite, and completely offset, so as to eliminate the influence of the primary induced voltage, so that the induced voltage detected on the receiving coil is completely generated by the underground metal conductor, so as to facilitate the direct acquisition and analysis of the feedback waveform of the underground metal, so as to obtain its accurate position information.

Description

Technical Field

[0001] The present invention belongs to the field of underground metal detection, and relates to a method for solving the interference of a transmitting coil on a receiving signal during underground metal detection by using hardware decoupling, and more specifically, to an underground metal detection system and a detection method thereof. Background Art

[0002] Underground metal detection plays an important role in many fields such as military defense, communication engineering, energy transmission and geological survey. In order to efficiently determine the location information and distribution of underground metals and ensure that underground metal structures are not damaged during the detection process, trenchless detection technology is usually required. The underground metal detection system based on transient electromagnetic method (TEM) is a common time domain electromagnetic survey tool with the characteristics of non-destructive and no need for excavation.

[0003] The basic principle of transient electromagnetic detection is to conduct geological analysis of the formation through the interaction between underground metal conductors and electromagnetic fields. In the process of transient electromagnetic detection, an excitation signal is applied to the transmitting coil to generate an induced electromagnetic field and propagate to the surrounding space. Under the action of the primary induced electromagnetic field, the underground metal conductor will generate a secondary induced electromotive force on the receiving coil due to the eddy current effect. Changes in the shape, size, dielectric parameters, underground distribution form, including burial depth, of the underground metal conductor will cause changes in the secondary induced electromotive force. Therefore, by collecting and analyzing the secondary induced voltage response waveform generated by the eddy current effect of the underground metal conductor on the receiving coil, the information of the underground metal conductor can be obtained.

[0004] In practical applications, due to the electromagnetic coupling between the transmitting and receiving coils and the transition process of the transmitting circuit, the collected signals at both ends of the receiving coil not only have secondary induced voltages, but also alias the primary induced voltage generated by the mutual inductance between the transmitting coil and the receiving coil. The amplitude of the primary induced voltage is usually much larger than the secondary induced voltage, and it cannot be directly used as the feedback signal of the target conductor. It is necessary to further analyze and process the feedback signal through a complex inversion algorithm, resulting in a large final detection error. Therefore, the method of solving the interference of the transmitting coil on the receiving signal during transient detection is of great significance to achieve accurate positioning of underground metals. Summary of the invention

[0005] The present invention is to solve the following technical problems: in the traditional underground metal detection process, due to the electromagnetic coupling between the transmitting coil and the receiving coil and the existence of the circuit transition process, the collected signal at both ends of the receiving coil is subject to electromagnetic interference from the primary induced voltage generated by the mutual inductance between the transmitting coil and the receiving coil, resulting in detection difficulties. Therefore, the present invention provides a device and method for improving the accuracy of underground metal detection to achieve efficient detection.

[0006] The specific technical solutions are:

[0007] An underground metal detection system mainly includes: a transmitting coil, a receiving coil, a decoupling coil, a precision potentiometer, and a signal acquisition and data analysis terminal; the transmitting coil is connected to a transmitting circuit; the receiving coil is connected to the receiving circuit and is also connected to the signal acquisition and data analysis terminal; the decoupling coil and the precision potentiometer are used to realize hardware decoupling between the transmitting coil and the receiving coil.

[0008] The decoupling coil and the transmitting coil are coaxially nested and independent of each other; the decoupling coil and the precision potentiometer are connected to the receiving circuit, and are fine-tuned by the precision potentiometer so that the output voltage of the decoupling circuit and the primary induced voltage generated by the mutual inductance between the transmitting coil and the receiving coil are equal and opposite, and completely offset, so that the induced voltage detected on the receiving coil is completely generated by the underground metal conductor.

[0009] The detection method of the underground metal detection system comprises the following steps:

[0010] (1) The transmitting circuit applies an excitation current to the transmitting coil. When the pulse current is turned off, a pulse magnetic field is generated around the transmitting coil and spreads to the surrounding space. Under the action of the primary pulse magnetic field, induced eddy currents are generated in the underground metal. As the metal loses heat, the induced eddy currents gradually decrease, exciting a new transient magnetic field, which is called the secondary field.

[0011] (2) The attenuation of the secondary magnetic field generates a secondary induced voltage on the receiving coil. Since the secondary field is mainly generated by the induced current in the underground conductor, it contains relevant information about the underground metal conductor. Changes in the shape, size, dielectric parameters and underground distribution of underground metal, including burial depth, will cause changes in the secondary field, and the received induced electromotive force will also be different. By studying the spatiotemporal characteristics of the received induced electromotive force, the data analysis terminal can analyze the information of the underground metal conductor.

[0012] The present invention utilizes a decoupling coil and a precision potentiometer to realize hardware decoupling between a transmitting coil and a receiving coil; in terms of physical structure, the decoupling coil and the transmitting coil are coaxially nested and wound independently of each other; in terms of electrical structure, the decoupling coil and the precision potentiometer are connected to a receiving circuit, and the precision potentiometer is used for fine adjustment to make the output voltage of the decoupling circuit and the primary induced voltage generated by the mutual inductance between the transmitting coil and the receiving coil equal and opposite, and completely cancel each other out, so as to eliminate the influence of the primary induced voltage, so that the induced voltage detected on the receiving coil is completely generated by the underground metal conductor, which is convenient for directly acquiring and analyzing the feedback waveform of the underground metal to obtain its accurate position information. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 A schematic diagram of the structure of the present invention;

[0014] Figure 2 The time domain waveform of the electromagnetic field in traditional transient electromagnetic detection;

[0015] Figure 3 An equivalent model of the underground metal detection system of the present invention;

[0016] Figure 4 The receiving circuit equivalent model of the present invention;

[0017] Figure 5 The transmitting circuit equivalent model of the present invention;

[0018] Figure 6 The relative positions of the transmitting coil and the decoupling coil of the present invention;

[0019] Figure 7 XY top view of the transmitting coil and decoupling coil of the present invention. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical scheme and effect of the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in each embodiment of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0021] Figure 1 FIG. 1 is a schematic diagram of the electromagnetic measurement structure of the transient electromagnetic method of the present invention. Figure 1 As shown, an underground metal detection system mainly includes: a transmitting coil 1, a receiving coil 2, a decoupling coil 7, a precision potentiometer, a signal acquisition 4 and a data analysis terminal 6; the transmitting coil 1 is connected to the transmitting circuit 3; the receiving coil 2 is connected to the receiving circuit 5, and is also connected to the signal acquisition 4 and the data analysis terminal 6;

[0022] The decoupling coil 7 and the precision potentiometer are used to realize hardware decoupling between the transmitting coil 1 and the receiving coil 2; the decoupling coil 7 and the transmitting coil 1 are coaxially nested and wound independently of each other; the decoupling coil 7 and the precision potentiometer are connected to the receiving circuit 5, and are fine-tuned by the precision potentiometer so that the output voltage of the decoupling circuit and the primary induced voltage generated by the mutual inductance between the transmitting coil 1 and the receiving coil 2 are equal and opposite, and completely offset, so that the induced voltage detected on the receiving coil 2 is completely generated by the underground metal conductor.

[0023] The detection method of the underground metal detection system, during the detection process, first applies an excitation current to the transmitting coil 1. According to the principle of electromagnetic induction, when the pulse current is turned off, a pulse magnetic field will be generated around the transmitting coil and diffuse to the surrounding space. Under the action of the primary pulse magnetic field, induced eddy currents will be generated in the underground metal. With the heat loss of the metal, the induced eddy currents gradually decrease, and a new transient magnetic field is excited, which is called the secondary field. The attenuation of the secondary magnetic field will generate a secondary induced voltage on the receiving coil 2. Since the secondary field is mainly generated by the induced current in the underground conductor, it contains relevant information about the underground metal conductor. Changes in the shape, size, dielectric parameters and underground distribution of underground metal, including burial depth, will cause changes in the secondary field, and the received induced electromotive force will also be different. Therefore, by studying the spatiotemporal characteristics of the received induced electromotive force, the information of the underground metal conductor can be analyzed.

[0024] Figure 2 This is a time domain waveform of the electromagnetic field in traditional transient electromagnetic detection. The primary induced electromotive force is generated by the mutual inductance between the transmitting coil 1 and the receiving coil 2, which has nothing to do with the underground metal conductor and does not contain target information. Therefore, the feedback signal needs to be further analyzed and processed through a complex inversion algorithm, resulting in a large final detection error before it can be used to study the location information of underground metal.

[0025] Figure 3 and Figure 4 The system of the present invention can be explained by classical circuit theory, and the core part is simplified to four coils, namely, the transmitting coil 1, the conductive equivalent coil, the receiving coil 2 and the decoupling coil 7. Figure 3 As shown. d1 and d2 are the connectors of the two lead wires of the decoupling coil, r1 and r2 are the connectors of the two lead wires of the receiving coil, and p1 is the sliding connector of the precision potentiometer. Figure 5 The system of the present invention can be explained by classical circuit theory, and the core part is simplified to four coils, namely, the transmitting coil 1, the conductive body equivalent coil, the receiving coil 2 and the decoupling coil 7, as shown in FIG. Figure 3 As shown. 10 is the equivalent mutual inductance between the transmitting coil 1 and the underground metal conductor, M 20 is the equivalent mutual inductance between the receiving coil 2 and the underground metal conductor, M 12 is the equivalent mutual inductance between the transmitting coil 1 and the receiving coil 2, M 13is the equivalent mutual inductance between the decoupling coil 7 and the transmitting coil 1. i1(t) and i2(t) are the excitation current applied in the transmitting coil 1 and the induced current generated by the eddy current effect of the underground metal conductor, respectively. v1(t) and v2(t) are the primary induced electromotive force and the secondary induced electromotive force generated by the transmitting coil 1 at both ends of the receiving coil 2, respectively. v3(t) is the induced electromotive force generated by the transmitting coil 1 at both ends of the decoupling coil 7. According to the law of electromagnetic induction, we can get:

[0026]

[0027] Where i′1(t) is the off current on the transmitting coil 1, τ = L c / R c is the equivalent time constant of the metal conductor to be detected. i3(t) is the induced current in the decoupling coil 7. According to formula 1-2, the induced eddy current in the underground metal conductor decays exponentially with time, generating a secondary field in the surrounding space. The secondary induced electromotive force on the receiving coil 2 is proportional to the rate of change of the secondary field with time, that is:

[0028]

[0029] Wherein, δ(t) is the unit pulse function corresponding to the excitation current i1(t). According to the above principle, v2(t) contains the information of the conductor to be measured and is the signal to be collected. Without considering the decoupling coil 7, the measured value v(t) of the induced electromotive force on the receiving coil 2 is:

[0030]

[0031] in, is the magnetic flux passing through the receiving coil 2, including the magnetic flux of the primary field and the secondary field, S is the effective area of ​​a single-turn receiving coil 2, and N is the number of turns of the receiving coil 2. Figure 3 As an equivalent model of the receiving circuit 5, the receiving coil 2 in the receiving device is assumed to be equivalent to the resistor R2 and the inductor L2. When the decoupling coil 7 is present, the measured voltage across the receiving coil 2 is v0(t), which satisfies:

[0032] v0(t)=v1(t)+v2(t)+v3(t) 1-6

[0033] Substituting equation 1-5 into equation 1-6, we get equation 1-7 as follows:

[0034]

[0035] The decoupling coil 7 and the receiving coil 2 are connected in the same circuit to satisfy M 13 =-M 12 By adjusting the precision potentiometer R t, a voltage division effect is realized in the receiving circuit 5, so that:

[0036]

[0037] Therefore, v0=v2(t), that is, the observed value v(t) is the pure secondary induced electromotive force, from which the magnetic induction intensity B(t) value can be calculated and relevant information about the underground metal conductor can be obtained.

[0038] Figure 5 It is the equivalent model of the transmitting circuit 3. In the transmitting circuit 3, the 220V AC mains charges the energy storage capacitor through the voltage regulator and the rectifier circuit, and controls the duration of the rising edge, falling edge and flat top of the excitation waveform by adjusting the switching state of the insulated gate bipolar transistor.

[0039] Figure 6 The figure below shows the relative positions of the transmitting coil and the decoupling coil and a brief structural diagram of the object. The left figure shows the geometrical schematic diagram of the coaxial structure of the coil, and the right figure shows the brief structural diagram of the object. d1 and d2 are the joints of the two lead wires of the decoupling coil, and t1 and t2 are the joints of the two lead wires of the transmitting coil. The decoupling coil and the transmitting coil are coaxially fixed on the bracket, and the support ① is used to determine the positions of the decoupling coil and the transmitting coil and control the relative distance between the two coils.

[0040] Figure 7 The XY plane top view of the transmitting coil and the decoupling coil. This article takes the traditional solenoid coil as an example. It should be emphasized that the specific coil structure in the detection system includes but is not limited to the structure in this application.

[0041] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A detection method of an underground metal detection system, wherein the underground metal detection system comprises a transmitting coil, a receiving coil, a decoupling coil, a precision potentiometer, and a signal acquisition and data analysis terminal; The transmitting coil is connected to the transmitting circuit; the receiving coil is connected to the receiving circuit and is also connected to the signal acquisition and data analysis terminal; The decoupling coil and the precision potentiometer are used to realize hardware decoupling between the transmitting coil and the receiving coil; The decoupling coil and the transmitting coil are coaxially nested and independent of each other; the decoupling coil and the precision potentiometer are connected to the receiving circuit, and the precision potentiometer is used for fine adjustment so that the output voltage of the decoupling circuit and the primary induced voltage generated by the mutual inductance between the transmitting coil and the receiving coil are equal and opposite in value; It is characterized in that The method comprises the following steps: (1) The transmitting circuit applies an excitation current to the transmitting coil. When the pulse current is turned off, a pulse magnetic field is generated around the transmitting coil and spreads to the surrounding space. Under the action of the primary pulse magnetic field, induced eddy currents are generated in the underground metal. As the metal loses heat, the induced eddy currents gradually decrease, exciting a new transient magnetic field, which is called the secondary field. (2) The attenuation of the secondary magnetic field generates a secondary induced voltage on the receiving coil. Since the secondary field is mainly generated by the induced current in the underground conductor, it contains relevant information about the underground metal conductor. The shape, size, dielectric parameters and underground distribution of the underground metal, including the change of the burial depth, will cause the change of the secondary field, and the received induced electromotive force will also be different. By studying the spatiotemporal characteristics of the received induced electromotive force, the data analysis terminal analyzes the information of the underground metal conductor. Specific: M 10 is the equivalent mutual inductance between the transmitting coil and the underground metal conductor, M 20 is the equivalent mutual inductance between the receiving coil and the underground metal conductor, M 12 is the equivalent mutual inductance between the transmitting coil and the receiving coil, M 13 is the equivalent mutual inductance between the decoupling coil and the transmitting coil; i1(t) and i2(t) are the excitation current applied in the transmitting coil and the induced current generated by the eddy current effect of the underground metal conductor respectively; v1(t) and v2(t) are the primary induced electromotive force and the secondary induced electromotive force generated by the transmitting coil at both ends of the receiving coil respectively, and v3(t) is the induced electromotive force generated by the transmitting coil at both ends of the decoupling coil; According to the law of electromagnetic induction, we get: Where i′1(t) is the off current on the transmitting coil, τ = L c / R c is the equivalent time constant of the metal conductor to be detected; i3(t) is the induced current in the decoupling coil; according to formula 1-2, the induced eddy current in the underground metal conductor decays exponentially with time, generating a secondary field in the surrounding space, and the secondary induced electromotive force on the receiving coil is proportional to the rate of change of the secondary field with time, that is: Among them, δ(t) is the unit pulse function corresponding to the excitation current i1(t); v2(t) contains the information of the conductor to be measured and is the signal that needs to be collected; when the decoupling coil is not considered, the measured value v(t) of the induced electromotive force on the receiving coil is: in, is the magnetic flux passing through the receiving coil, including the magnetic flux of the primary field and the secondary field, S is the effective area of ​​the single-turn receiving coil, and N is the number of turns of the receiving coil; suppose the receiving coil in the receiving device is equivalent to the resistor R2 and the inductor L2; when there is a decoupling coil, the measured voltage across the receiving coil is v0(t), which satisfies: v0(t)=v1(t)+v2(t)+v3(t) 1-6 Substituting equation 1-5 into equation 1-6, we get equation 1-7 as follows: The decoupling coil and the receiving coil are connected in the same circuit to satisfy M 13 =-M 12 ; By adjusting the precision potentiometer R t , realize the voltage division in the receiving circuit, so that: Therefore, v0=v2(t), that is, the observed value v(t) is the pure secondary induced electromotive force, from which the magnetic induction intensity B(t) value is calculated and the information of the underground metal conductor is obtained.

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