CN113093290B - Method for detecting weak secondary field signal under same-frequency strong magnetic interference background - Google Patents
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Abstract
The invention discloses a method for detecting weak secondary field signals under the background of same-frequency strong magnetic interference, which is used for calibrating the positions of various magnetoelectric sensors in a detection device; generating sine wave excitation signals with different frequencies through a signal generating circuit, and introducing the sine wave excitation signals into a transmitting coil to generate a primary magnetic field; receiving secondary field signals generated by underground media at each measuring point by using a magnetoelectric sensor array; extracting electromagnetic characteristics of the underground medium by analyzing the frequency spectrum characteristics of the induction electromagnetic field received by each measuring point magnetoelectric sensor array; and according to the obtained electromagnetic characteristics, calculating the spatial distribution of the underground medium in an inversion mode. According to the invention, according to the magnetic field distribution of the transmitting coil, the double-composite magnetoelectric sensor and the bias magnetic field magnetic circuit are integrally designed, so that the multiplication of a weak signal of a secondary field is realized while the primary field is counteracted.
Description
Technical Field
The invention belongs to the field of weak magnetic field signal detection, and particularly relates to a method for detecting a weak secondary field signal under a same-frequency strong magnetic interference background.
Background
The frequency domain electromagnetic induction detection method is one of geophysical exploration methods and is widely applied to the fields of geological survey, mineral exploration, underground metal detection, archaeology and the like. The frequency domain electromagnetic induction detecting instrument adopts a transmitting coil to transmit an alternating primary magnetic field signal (namely a primary field), and uses a receiving coil or a magnetic sensor to receive a secondary magnetic field signal (namely a secondary field) generated by the action of the primary field and an abnormal body to acquire the information of the abnormal body to be detected.
Since the primary field of the transmitting coil is strong, and the secondary field signal is weak and has the same frequency as the primary field signal, it is necessary to cancel or suppress the interference of the primary field signal in order to accurately detect the secondary field signal. In reference 1 (Qu X, liY, fang G, et al. Opaque frequency domain electronic system for short metal targets detection [ J ]. Progress In electronics Research,2017, 53, 167-175.), the receiving device uses two coils: one is used as a receiving coil, the other is used as a compensating coil, the areas of the transmitting coil, the receiving coil and the compensating coil are equal, the distance between the transmitting coil and the receiving coil is 1.67m, the distance between the transmitting coil and the compensating coil is 1.06m, the ratio of the transmitting coil to the compensating coil is about 0.63, and the compensating coil and the receiving coil attenuate a primary field and then connect into an amplifying circuit by a physical difference method. However, the instrument volume under the method is large, the size of the transmitting coil and the distance between the transmitting coil and the receiving coil are limited, and the inversion and data processing requirements are high. In reference 2 (GEM-3. The invention with the patent number of 202010405202.X discloses a deep sea transient electromagnetic method detection receiving device and a control method thereof, and has important reference significance for frequency domain primary field interference cancellation. According to the device, a receiving coil is placed above or below a transmitting coil, a first receiving area and a second receiving area of the receiving coil use a plane where the transmitting coil is located as a reference plane, the orthographic projection of the first receiving area on the reference plane is located in the transmitting coil, the orthographic projection of the second receiving area on the reference plane is located outside the transmitting coil, and the magnetic flux passing through the first receiving area and the magnetic flux of the second receiving area when the transmitting coil generates a primary field are equal through adjustment of a controller, so that the primary field is eliminated or weakened. However, the receiving coil obtains the change rate (dB/dt) parameter of the magnetic field by measuring the secondary induction voltage, and has poor detection capability on the deep target.
Disclosure of Invention
The invention aims to provide a method for detecting weak secondary field signals under the background of same-frequency strong magnetic interference, which solves the problems of large primary field interference and inaccurate positioning of the conventional instrument.
The technical scheme for realizing the purpose of the invention is as follows: a method for detecting weak secondary field signals under the background of same-frequency strong magnetic interference comprises the following specific steps:
and 7, calculating the spatial distribution of the underground medium in an inversion mode according to the obtained electromagnetic characteristics.
Preferably, the transmitting coil is a rectangular transmitting coil or a circular transmitting coil.
Preferably, the magnetoelectric sensor array includes a plurality of groups of magnetoelectric sensors that set up along transmitting coil, and two magnetoelectric sensors are a set of, place respectively in the equal position of the inside and outside both sides magnetic field size of transmitting coil.
Preferably, when the transmitting coil is a rectangular transmitting coil, the connecting line of the position where each group of magnetoelectric sensors is located is perpendicular to the side line of the coil; when the transmitting coil is a circular transmitting coil, the position of each group of magnetoelectric sensors should be collinear with three points of the center of the coil.
Preferably, the magnetoelectric sensitive unit of the magnetoelectric sensor is a three-layer composite structure of magnetostrictive material/piezoelectric material/magnetostrictive material, epoxy resin glue is used for bonding the layers, and a permanent magnet is used for applying a bias magnetic field.
Preferably, the sensitive direction of the magnetoelectric sensor is the length direction of the magnetoelectric sensor, and the length direction of the magnetoelectric sensor is perpendicular to the plane where the transmitting coil is located.
Preferably, the subsurface medium comprises the earth or a survey target layer/volume.
Compared with the prior art, the invention has the following remarkable advantages:
(1) The invention designs a novel magnetoelectric sensor array structure according to the magnetic field distribution of transmitting coils, provides a design method for integrating a double-composite magnetoelectric sensor and a bias magnetic field magnetic circuit, and realizes multiplication of weak signals of a secondary field while realizing primary field offset;
(2) The invention can realize the direct measurement of the signal of the secondary induction magnetic field (B) by using the magnetoelectric sensor, has essential difference compared with the parameter of the change rate (dB/dt) of the magnetic field obtained by measuring the secondary induction voltage by the traditional coil sensor, and can obtain higher signal-to-noise ratio and larger detection depth;
(3) The magnetoelectric sensor used in the invention is convenient for designing a sensor array, can acquire a plurality of groups of detection data at the same measuring point, and improves the inversion positioning precision.
The present invention is described in further detail below with reference to the attached drawings.
Drawings
FIG. 1 is a schematic diagram of a rectangular transmitting coil and a magnetic flux density measurement line.
Fig. 2 is a graph of the magnitude of the magnetic flux density on a rectangular coil side.
Fig. 3 is a schematic diagram of a weak secondary field signal detection system architecture based on a broadband magnetic sensor under the background of same-frequency strong magnetic interference.
Fig. 4 is a detection schematic diagram of the present invention.
FIG. 5 is a flow chart of the present invention.
Fig. 6 is a schematic structural view of a magnetoelectric sensor in embodiment 1 of the present invention.
Fig. 7 is a schematic diagram of the operation of the magnetoelectric sensor array in embodiment 1 of the present invention.
Fig. 8 is a schematic view of an integrated structure of the magnetoelectric sensor in embodiment 2 of the present invention.
Fig. 9 is a schematic diagram of the operation of the magnetoelectric sensor array in embodiment 2 of the present invention.
Reference numerals: 1. a magnetostrictive material; 2. a piezoelectric material; 3. a permanent magnet; 4. a base with a boss; 5. a wire; 6. a primary field; 7. a bias magnetic field; 8. a secondary field; 9. the direction of the current; 10. the piezoelectric material polarization direction.
Detailed Description
As shown in fig. 3 and 5, a method for detecting weak secondary field signals under the background of same-frequency strong magnetic interference includes the following steps:
In a further embodiment, the transmitting coil is a rectangular transmitting coil or a circular transmitting coil;
and 2, determining a detection route and a measuring point of the area to be detected, and placing a detection device which is calibrated at the starting point of the detection route.
In a further embodiment, the magnetoelectric sensor array comprises a plurality of groups of magnetoelectric sensors, two magnetoelectric sensors are in a group and are respectively placed at positions with equal magnetic field sizes at the inner side and the outer side of the transmitting coil, so that output signals of the two magnetoelectric sensors are equal, and a primary field can be cancelled after difference. And a plurality of groups of magnetoelectric sensors are arranged at different positions along the transmitting coil to form a magnetoelectric sensor array.
As shown in fig. 4, an alternating current is passed through the transmitting coil, thereby generating an alternating magnetic field, referred to as a primary field. When an abnormity or an object exists below the measuring point, the abnormity or the object can generate an induced eddy current and a secondary field under the excitation of a primary field according to the electromagnetic induction principle, and a magnetoelectric sensor array is used for receiving a secondary field signal. Therefore, the directions of the secondary fields at the positions of the magnetoelectric sensor arrays are the same, and the magnitudes are approximately the same.
Specifically, the magnetoelectric sensitive unit of the magnetoelectric sensor is a three-layer composite structure of magnetostrictive material/piezoelectric material/magnetostrictive material, epoxy resin glue is used for bonding layers, a permanent magnet is used for applying a bias magnetic field, the size is about centimeter level, the structure is simple, and the manufacture is convenient.
The invention applies a certain bias magnetic field to the magnetoelectric composite material by utilizing the linear magnetoelectric effect of the magnetoelectric composite material, so that the magnetostrictive material is magnetized along the length direction and works in a linear region, namely the amplitude of the output magnetoelectric voltage is in direct proportion to the amplitude of an alternating magnetic field.
Specifically, the sensitive direction of the magnetoelectric sensor is the length direction of the magnetoelectric sensor, and the length direction of the magnetoelectric sensor is perpendicular to the plane where the transmitting coil is located when the magnetoelectric sensor is arranged.
FIG. 1 is a schematic diagram of a rectangular transmitting coil with 1m side length, on which a measuring line from the center point of the rectangular coil to the symmetrical position of 0.5m outside the coil exists. According to the current direction of the rectangular coil given in the figure, according to the ampere rule, the direction of the magnetic field inside the coil is inward of the vertical coil plane, and the direction of the magnetic field outside the coil is outward of the vertical coil plane.
Fig. 2 is a magnetic flux density curve graph obtained by simulation on the measuring line when the rectangular transmitting coil is electrified with 1kHz and 1A current, and it can be known that there are positions with equal magnetic field magnitude at the inner side and the outer side of the coil and positions with equal magnetic field magnitude which are symmetrical about the coil by using the intersection point of the measuring line and the coil as the origin.
In a further embodiment, the magnetoelectric sensors may be arranged individually or integrally, and when the magnetoelectric sensors are arranged individually, the bias magnetic field required for the operation of the magnetoelectric sensors is also applied by the permanent magnets inside each sensor, respectively. When the magnetoelectric sensors are integrally arranged, bias magnetic fields required by the work of the two magnetoelectric sensors are integrally applied by the permanent magnet, and the bias magnetic fields of the magnetoelectric sensors on the two sides of the coil are applied in opposite directions.
Specifically, when a rectangular transmitting coil is used, a connecting line of the position where each group of magnetoelectric sensors is located is perpendicular to a coil side line; when a circular transmitting coil is adopted, the position of each group of magnetoelectric sensors is collinear with three points of the circle center of the coil.
And 3, controlling a signal generating circuit to generate sine wave excitation signals with different frequencies through an embedded processor, and introducing the sine wave excitation signals into the transmitting coil to generate a primary magnetic field. According to the principle of electromagnetic induction, a subsurface medium, which includes the earth or a survey target layer/volume, is excited by a primary magnetic field to generate an induced electromagnetic field (secondary field).
And 4, receiving the secondary field signals by using the magnetoelectric sensor array to obtain the secondary field signals at different positions.
And 5, after the measurement is finished, moving the detection device to the next measuring point, and repeating the step 3 and the step 4 until all the preset measuring points are detected.
And 6, extracting the electromagnetic characteristics of the underground medium from the spectrum characteristics of the induction electromagnetic field received by each measuring point magnetoelectric sensor array.
And 7, calculating the spatial distribution of the underground medium in an inversion way according to the obtained electromagnetic characteristics.
The invention adopts the magnetoelectric sensor to replace a receiving coil, directly measures the parameters of the magnetic field (B), improves the signal-to-noise ratio of deep secondary field signals and further improves the detection capability of deep targets.
The magnetoelectric sensor is used as a novel magnetic sensor, a magnetostrictive/piezoelectric composite material is used as a sensitive unit, and the B field can be directly measured. Under the action of a magnetic field, the magnetostrictive material generates stress or strain, the stress or strain is transmitted to the piezoelectric material through interlayer coupling, the piezoelectric material generates an electric field due to the piezoelectric effect, a voltage signal is output, and the magnetic-mechanical-electrical energy conversion is realized. Compared with traditional magnetic field sensors such as a magnetoresistive sensor, a fluxgate sensor and a SQUID, the magnetoelectric sensor has the advantages of wide frequency band, low power consumption, wide range, small volume, low cost and the like, and has unique advantages in the field of electromagnetic detection.
Example 1
A method for detecting weak secondary field signals under the background of same-frequency strong magnetic interference comprises the following specific steps:
And 2, determining a detection route and a measuring point of the area to be detected, and placing a detection device which is calibrated at the starting point of the detection route.
And 3, controlling a signal generating circuit to generate sine wave excitation signals with different frequencies through an embedded processor, and introducing the sine wave excitation signals into the transmitting coil to generate a primary magnetic field.
And 4, receiving the secondary field signals by using the magnetoelectric sensor array to obtain the secondary field signals at different positions.
And 5, after the measurement is finished, moving the detection device to the next measuring point, and repeating the step 3 and the step 4 until all the preset measuring points are detected.
And 6, extracting the electromagnetic characteristics of the underground medium from the spectrum characteristics of the induction electromagnetic field received by each measuring point magnetoelectric sensor array.
And 7, calculating the spatial distribution of the underground medium in an inversion way according to the obtained electromagnetic characteristics.
In this embodiment, the magnetoelectric sensor array is composed of a single magnetoelectric sensor, and each magnetoelectric sensor is applied with a bias magnetic circuit separately.
As shown in fig. 6, the magnetoelectric sensitive unit of the magnetoelectric sensor of the present invention is a three-layer composite of magnetostrictive material/piezoelectric material/magnetostrictive material, both sides are permanent magnets, which can provide uniform bias magnetic field, and are fixed on the base with a boss.
Fig. 7 is a schematic diagram illustrating the operation of the magnetoelectric sensor array in this embodiment, in which a pair of permanent magnets in the magnetoelectric sensing unit are magnetized along the length direction and are symmetrically disposed on two sides of the magnetostrictive unit in parallel. The same polarity of the same end of the permanent magnet is the same, and a tiny space is reserved between the permanent magnet unit and the magnetostriction unit so as to ensure that the magnetostriction unit can freely stretch and contract. The pair of permanent magnets are magnetic sources, and magnetic lines of force respectively emit from the N poles of the pair of permanent magnets and return to the S poles of the permanent magnets through the magnetostrictive material to form a symmetrical magnetic circuit structure, so that an optimal bias magnetic field is provided for the magnetostrictive material to exert the optimal performance of the magnetostrictive material.
In this embodiment, the transmitting coil is placed on a non-metal non-magnetic platform, no metal target is present near the platform, a current with a certain frequency and amplitude is introduced, and then the position of the magnetoelectric sensor is calibrated. The sensitive direction of the magnetoelectric sensor is the length direction, and the length direction of the magnetoelectric sensor is perpendicular to the plane of the transmitting coil when the magnetoelectric sensor is arranged. The directions of bias magnetic fields applied by the inner and outer magnetoelectric sensors are opposite.
The sensitivity of each magnetoelectric sensor is K, and the components of the magnetic field at the positions of the two magnetoelectric sensors in the sensitive direction of the magnetoelectric sensors are B 1 、B 2 . Firstly, determining the position of a sensor in each group of sensors and calibrating, wherein the magnetoelectric sensor outputs V 1 ＝KB 2 . Then the position of another magnetoelectric sensor is adjusted to make the output signal amplitudes of two magnetoelectric sensors equal, i.e. V 1 ＝V 2 Obtaining | B 1 |＝|B 2 I, the components of the magnetic fields at the positions of the two magnetoelectric sensors in the sensitive direction of the magnetoelectric sensors are equal in magnitude. The output of the two sensors is reversely connected in series and then can cancel the primary field after difference, namely V = V 1 -V 2 And =0. After the calibration is finished, the magnetic field intensity is only in proportional relation with the amplitude of the emission current and is irrelevant to the frequency; the directions of magnetic fields sensed by the two magnetoelectric sensors at any moment are opposite, and the magnetic fields are increased or decreased simultaneously, so that a primary field can be counteracted under the condition of transmitting current with any amplitude and frequency.
After the position calibration of the magnetoelectric sensor is completed, the magnetoelectric sensor is fixed with the transmitting coil, and the relative position of the magnetoelectric sensor and the transmitting coil is ensured to be unchanged.
During detection, when no abnormity or target body exists below a measuring point, the output signals of the two magnetoelectric sensors are equal in size, and a primary field can be cancelled after the difference of the signals of the two sensors is subtracted, namely V = V 1 -V 2 ＝0。
When an anomaly or target exists below the measuring point, the anomaly or target can generate an induced eddy current and a secondary field. The magnetic fields of the two magnetoelectric sensors at the positions of the secondary fields are respectively B ″ 1 、B` 2 According to the magnetic field distribution principle, two magnetoelectric signals are transmitted in the secondary fieldThe magnetic field directions of the positions of the sensors are the same, and the magnitudes are approximately equal, namely B ″ 1 ≈B` 2 . And because the magnetoelectric sensors are positioned at the inner side and the outer side of the transmitting coil, the component directions of the magnetic fields at the positions of the two magnetoelectric sensors in the sensitive direction of the magnetoelectric sensors are opposite.
The directions of the secondary fields and B in FIG. 7 2 The directions are the same, and the magnetic fields of the two magnetoelectric sensors at the positions in the total magnetic field are respectively B 1 -B` 1 、B 2 +B` 2 Then, the output voltages of the sensors are respectively: v 1 ＝K(B 1 -B` 1 )、V 2 ＝K(B 2 +B` 2 ). According to the polarization direction of the piezoelectric material, the outputs of the two sensors are reversely connected in series, and the total output voltage after difference is V = V 1 -V 2 ＝K(B 1 -B` 1 )-K(B 2 +B` 2 )＝-K(B` 1 +B` 2 )＝-2KB` 1 The sensitivity multiplication of the sensor is realized, and the amplification of secondary field signals is realized.
In the embodiment, a group of magnetoelectric sensors only need to be arranged at the positions with the same magnetic field size at the inner side and the outer side of the coil, so that the distance between the magnetoelectric sensors and the coil can be flexibly adjusted according to requirements.
Example 2
The working device, method steps and principle of the present embodiment are the same as those of embodiment 1, except that the magnetoelectric sensor uses an integrated bias magnetic circuit.
As shown in fig. 8, the present embodiment uses an integrated bias magnetic field application structure. The two sides are magnetoelectricity sensitive units, the middle two are permanent magnets, and the two permanent magnets are fixed on the same base. The two permanent magnets are magnetized along the length direction of the two permanent magnets and are parallelly placed in the positions close to the outer sides of the two magnetoelectric sensitive units, and the outer side surfaces of the two permanent magnets are flush with the outer end surfaces of the two piezoelectric materials; a certain distance is left between the two permanent magnets, so that the coil can conveniently penetrate through the permanent magnets.
As shown in fig. 9, 2 permanent magnets and magnetostrictive material form a symmetrical integrated magnetic circuit, which can also provide a sufficiently large bias magnetic field. The magnetic circuit is formed by two permanent magnets as magnetic field sources, magnetic lines of force are emitted from the N pole of the first permanent magnet and return to the S pole of the second permanent magnet through the magnetostrictive unit, and then the magnetic lines of force are emitted from the N pole of the permanent magnet and return to the S pole of the first permanent magnet. The directions of the bias magnetic fields applied by the two magnetoelectric sensors are opposite, and the same is true as the embodiment 1.
In the embodiment, the outputs of the magnetoelectric sensitive units on the inner side and the outer side of the coil are reversely connected in series and are differentiated to realize the cancellation of the interference of the primary field with the same frequency and the multiplication of the sensitivity to the secondary field.
In this embodiment, the two magnetoelectric sensors are axisymmetrical structures, and the coil passes through the middle, so the two magnetoelectric sensors need to be placed at positions which are symmetric about the coil and have the same magnetic field.
Compared with embodiment 1, in this embodiment, the position of the magnetoelectric sensor and the coil are relatively unchanged, and the magnetic circuits of the two magnetoelectric sensors are integrally designed, so that the installation is convenient.
Claims (5)
1. A method for detecting weak secondary field signals under the background of same-frequency strong magnetic interference is characterized by comprising the following specific steps:
step 1, starting a detection device, wherein the detection device comprises a magnetoelectric sensor array and an emission coil, and calibrating the position of each magnetoelectric sensor in the detection device;
step 2, determining a detection route and a measuring point of the area to be detected, and placing a detection device which is calibrated at the starting point of the detection route;
step 3, generating sine wave excitation signals with different frequencies through a signal generating circuit, and introducing the sine wave excitation signals into the transmitting coil to generate a primary magnetic field;
step 4, receiving a secondary field signal generated by the underground medium by using a magnetoelectric sensor array;
step 5, after the measurement is finished, moving the detection device to the next measuring point, and repeating the step 3 and the step 4 until all the preset measuring points are detected;
step 6, extracting the electromagnetic characteristics of the underground medium by analyzing the frequency spectrum characteristics of the induction electromagnetic field received by each measuring point magnetoelectric sensor array;
step 7, calculating the spatial distribution of the underground medium in an inversion way according to the obtained electromagnetic characteristics;
the transmitting coil is a rectangular transmitting coil or a circular transmitting coil; the magnetoelectric sensor array comprises a plurality of groups of magnetoelectric sensors arranged along the transmitting coil, and two magnetoelectric sensors are in a group and are respectively placed at the positions with equal magnetic field sizes at the inner side and the outer side of the transmitting coil.
2. The method for detecting weak secondary field signals under the background of same-frequency strong magnetic interference according to claim 1, wherein when the transmitting coil is a rectangular transmitting coil, the position connecting line where each group of magnetoelectric sensors is located is perpendicular to the coil side line; when the transmitting coil is a circular transmitting coil, the position of each group of magnetoelectric sensors should be collinear with three points of the center of the coil.
3. The method for detecting weak secondary field signals under the background of same-frequency strong magnetic interference according to claim 1, wherein a magnetoelectric sensitive unit of the magnetoelectric sensor is a three-layer composite structure of magnetostrictive material/piezoelectric material/magnetostrictive material, epoxy resin glue is used for bonding layers, and a permanent magnet is used for applying a bias magnetic field.
4. The method according to claim 1, wherein the sensitive direction of the magnetoelectric sensor is the length direction of the magnetoelectric sensor, and the length direction of the magnetoelectric sensor is perpendicular to the plane of the transmitting coil.
5. The method according to claim 1, wherein the underground medium comprises the earth or a detection target layer/body.
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