SQUID-TEM Detection Environmental Electromagnetic Interference Shielding Method and System

Abstract

This application belongs to the field of ground superconducting electromagnetic detection in geophysical exploration, and specifically relates to a method and system for shielding electromagnetic interference in the SQUID-TEM detection environment. The method includes: identifying the interference frequency range; selecting the type of shielding material and the initial minimum shielding thickness; establishing a three-dimensional simulation model of the shielding body for electromagnetic interference in the field operation area based on the finite element method; calculating the shielding effectiveness of the shielding body under different shielding thicknesses; extracting the frequency domain response curve of the shielding effectiveness as a function of the shielding thickness; finding the optimized minimum shielding thickness that meets the shielding effectiveness standard from the frequency domain response curve; preparing the shielding body according to the optimized minimum shielding thickness and the type of shielding material; and placing the shielding body outside the SQUID sensor and front-end electronics unit to complete the shielding deployment, effectively improving the reliability and accuracy of the identification and detection data of underground anomaly targets.

Description

Technical Field

[0001] This application belongs to the field of ground superconducting electromagnetic detection in geophysical exploration, and in particular relates to a method and system for shielding electromagnetic interference in the SQUID-TEM detection environment. Background Technology

[0002] Superconducting Quantum Interference Devices (SQUIDs) employ step-edge grain boundary Josephson junction technology, enabling them to effectively sense extremely weak magnetic signals on the order of fT. They are among the most sensitive magnetic vector sensors known, featuring low noise and wide bandwidth, and are widely used in extremely weak magnetic field measurements in oil and gas exploration, metal mineral exploration, and military deep-sea exploration. Ground-based superconducting transient electromagnetic SQUID-TEM detection systems, built upon SQUID sensors (TEM stands for transient electromagnetic method), acquire data from field experimental areas through readout circuits and receivers. This allows for the effective observation of changes in the magnetic vector field of subsurface anomalies, thereby obtaining richer magnetic anomaly information, improving the resolution and location accuracy of deep anomaly detection, and reducing ambiguity in data inversion interpretation. They offer significant advantages in magnetic anomaly interpretation. The fT-level sensing sensitivity of SQUID sensors has significantly improved the exploration depth and accuracy of SQUID-TEM systems, making it a key development direction for geophysical exploration equipment.

[0003] The SQUID-TEM detection system faces complex electromagnetic interference from wireless communication devices in the field experimental environment. This interference is characterized by wide bandwidth and strong interference, such as low-frequency high-voltage electricity and low-voltage power frequency and its harmonics in free space, AM and FM broadcast communications, radio positioning, aviation mobile signals, aviation radio navigation signals, satellite meteorological (air-to-ground) signals, and space operations (air-to-ground) electromagnetic interference. This interference will severely reduce the detection system's resolution for magnetic anomalies and the accuracy of data interpretation, and can easily overwhelm the effective target signals in electromagnetic detection. Due to site limitations and fixed measurement area locations in field experiments, it is impossible to avoid electromagnetic interference by moving the observation location. Without shielding protection, strong electromagnetic interference in the environment will cause the SQUID sensor's slew rate to become too large or exceed the output measurement voltage amplitude, leading to loss of lock and unstable operation. The measurement data will not be able to effectively extract underground medium parameters.

[0004] In summary, research on shielding methods to address external electromagnetic interference during field operations of the SQUID-TEM observation system is of great significance for achieving refined and effective electromagnetic detection and improving the accuracy of data interpretation. Summary of the Invention

[0005] This application provides a method and system for shielding SQUID-TEM from electromagnetic interference in the detection environment, which solves the problem that strong electromagnetic interference in the environment without shielding can cause the SQUID sensor to have an excessive slew rate or exceed the output measurement voltage amplitude, resulting in loss of lock and unstable operation.

[0006] The first aspect of this application provides a method for shielding electromagnetic interference in a SQUID-TEM detection environment, including: Confirm the interference frequency range; Select the type of shielding material and the initial minimum shielding thickness based on the required interference frequency range and the shielding efficiency standards. A three-dimensional simulation model of the shielding body for electromagnetic interference in the field operation area was established based on the finite element method. The interference frequency range, radiation direction, distance between the interference source and the measuring device, shielding material thickness, single interference source and multiple interference sources were simulated. With the type of shielding material and the initial minimum shielding thickness as constraints, the shielding effectiveness of the shielding body under different shielding thicknesses was calculated. The frequency domain response curve of the shielding effectiveness as a function of thickness was extracted. The optimized minimum shielding thickness that meets the shielding effectiveness standard was found from the frequency domain response curve. The shielding body is prepared based on the optimized minimum shielding thickness and the type of shielding material. The shielding body is then placed on the outside of the SQUID sensor and the front-end electronics unit to complete the shielding deployment.

[0007] Furthermore, the interference frequency range was confirmed to include: Environmental electromagnetic interference is equivalent to the simultaneous injection of DC current components and AC current components into the Josephson junction. Measure the IV characteristic curve of a Josephson junction or a SQUID sensor containing the Josephson junction, and obtain the step voltage of the Shapiro step induced by the AC current component in the IV characteristic curve; Based on the quantized relationship between the step voltage and the interference frequency, the frequency corresponding to the smallest observable step voltage is taken as the lower limit of the interference frequency. The minimum value among the characteristic frequency of the Josephson junction, the microwave coupling cutoff frequency, the measured upper limit of the environmental electromagnetic interference spectrum, and the upper limit of the detection system signal bandwidth is taken as the upper limit of the interference frequency.

[0008] Furthermore, based on the quantized relationship between the step voltage and the interference frequency, it can be expressed as: , in, For the first Step voltage, To reduce Planck's constant, For elementary charge, For the total series, This refers to the frequency of external electromagnetic interference.

[0009] Furthermore, based on the required interference frequency range and the standards for shielding energy efficiency, the type of shielding material and the initial minimum shielding thickness are selected, including: Obtain the interference frequency range, distance to the interference source, and preset shielding effectiveness standards; Based on Schelkunoffs' electromagnetic shielding theory, an expression for the shielding effectiveness of a shielding body, including reflection loss, absorption loss, and multiple reflection losses within the material, is established. Based on the interference frequency and the distance to the interference source, the field type of the shield is determined. The field type includes the far-field region, the near-field electric field region, and the near-field magnetic field region, and the corresponding reflection loss calculation method is selected accordingly. Select shielding materials based on the field type and interference frequency range; Obtain the relative permeability and relative conductivity of the selected shielding material to copper within the interference frequency range, and calculate the maximum skin depth based on the relative permeability and relative conductivity of copper. Based on the maximum skin depth, reflection loss, and multiple reflection losses within the material, the initial minimum shield thickness that meets the shielding effectiveness standard is calculated.

[0010] Furthermore, based on the field type and interference frequency range, the shielding materials selected include: When the shield is in the far field region, a conductor material with a conductivity higher than the first conductivity threshold is selected as the main body of the shielding layer, and the shielding mechanism dominated by reflection loss is utilized. When the shield is in the near-field electric field region and the interference frequency is below the first frequency threshold, a high-conductivity metal material is selected to utilize the electric field reflection effect. When the shield is in the near-field magnetic field region and the interference frequency is lower than the second frequency threshold, a high permeability material is selected as the first shielding layer, and a high conductivity material is composited on its outer or inner side as the second shielding layer. The high permeability material is used to provide magnetic shielding dominated by absorption loss, and the high conductivity material is used to suppress residual high-frequency interference. When the interference frequency range covers low-frequency magnetic field interference and high-frequency electromagnetic interference, a multi-layer composite shielding structure is adopted.

[0011] Furthermore, based on the maximum skin depth, reflection loss, and multiple reflection losses within the material, the initial minimum shielding thickness that meets the shielding effectiveness standard is calculated, including: Based on the preset shielding effectiveness standard, and combined with reflection loss and multiple reflection loss inside the material, the minimum absorption loss required is calculated. Assuming the shield thickness reaches a skin depth or more, and ignoring multiple reflection losses within the material; Based on the relationship between absorption loss and shield thickness and skin depth, the minimum initial shield thickness that meets the shielding effectiveness standard is obtained by substituting the maximum skin depth and minimum absorption loss.

[0012] Furthermore, when the reflection loss is calculated using different formulas depending on the field type where the shield is located, the minimum shield thickness is calculated using the corresponding reflection loss value: For the far-field region, far-field reflection loss is used; For the near-field electric field region, near-field electric field reflection loss is used; For the near-field magnetic field region, near-field magnetic field reflection loss is used; By using the reflection loss value of the corresponding field area, the minimum shield thickness adapted to different field area types can be obtained.

[0013] A second aspect of this application provides a SQUID-TEM detection environment electromagnetic interference shielding system, comprising: A shielding enclosure is used to house the SQUID sensor and front-end electronics unit of the SQUID-TEM detection system. The shielding enclosure is designed based on the required interference frequency range and the standards for shielding effectiveness. The type of shielding material and the initial minimum shielding thickness are selected accordingly. A three-dimensional simulation model of the shielding enclosure for electromagnetic interference in the field operation area is established using the finite element method. Simulations are performed on the interference frequency range, radiation direction, distance between the interference source and the measuring device, shielding material thickness, and single and multiple interference sources. With the type of shielding material and the initial minimum shielding thickness as constraints, the shielding effectiveness of the enclosure under different shielding thicknesses is calculated. The frequency domain response curve of the shielding effectiveness as a function of thickness is extracted, and the optimized minimum shielding thickness that meets the shielding effectiveness standards is found from the frequency domain response curve.

[0014] Compared with existing technologies, the advantages of this application are as follows: This application proposes that when external electromagnetic interference exists, introducing a co-frequency alternating current into the Josephson junction will cause a step effect on the IV characteristic curve. Based on the characteristic parameters of the SQUID sensor, the required shielding frequency range in the field can be determined. For different frequency bands of electromagnetic interference, based on Schelkunoffs electromagnetic shielding theory and finite element software simulation analysis, an effective shielding body can be designed. The shielding body parameters can be optimized according to electromagnetic interference under different environments. Through urban space electromagnetic interference shielding effectiveness testing and field experiments, effective electromagnetic response data are obtained. This application shows that it can effectively improve the reliability and accuracy of identification and detection data of underground anomaly targets, and promote the application of the SQUID-TEM detection system in the field of measurement. Attached Figure Description

[0015] Figure 1 This is a flowchart of a method for shielding against electromagnetic interference in the environment using a SQUID-TEM system, provided in an embodiment of this application. Figure 2 This is a target frequency band magnetic field curve diagram of transient electromagnetic detection provided in the embodiments of this application; Figure 3 These are IV characteristic curves of the Josephson junction provided in the embodiments of this application, (a) is the DC IV characteristic curve, and (b) is the AC IV characteristic curve. Figure 4 These are frequency response curves of reflection loss, absorption loss, skin depth, and shielding effectiveness provided in the embodiments of this application, where (a) is reflection loss, (b) is absorption loss, (c) is skin depth, and (d) is shielding effectiveness. Figure 5 This is a finite element simulation result diagram of the interference source-shield provided in the embodiments of this application; Figure 6 This is an attenuation curve diagram of the field experiment results without shielding provided in the embodiments of this application; Figure 7 This is an attenuation curve diagram of the field experiment with shielding provided in the embodiments of this application. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0017] A superconducting Josephson junction (JJ), also known as a superconducting tunnel junction, is an SIS structure consisting of two superconducting layers sandwiching an extremely thin non-superconducting barrier layer. S represents the superconductor, and I represents the non-superconducting barrier layer. The thickness of the intermediate barrier layer must be less than the coherence length of the Cooper pair (approximately on the order of 1 nanometer), thus allowing for weak coupling of the macroscopic quantum wave functions of the two superconductors. In circuits, the Josephson junction is typically represented by the Resistively Capacitance Shunted Junction (RCSJ) model, which is equivalent to an ideal Josephson junction in parallel with a junction resistance R and a junction capacitance C. The resistance simulates quasi-particle tunneling current and leakage current, while the capacitance simulates displacement current.

[0018] SQUID sensors are composite devices consisting of one or two Josephson junctions and a superconducting loop (a closed superconducting coil). When external electromagnetic interference is present, it is equivalent to simultaneously injecting DC and AC currents into the Josephson junction. The presence of AC current in the junction current produces a step effect in the AC IV characteristic curve. The SQUID-TEM detection system is based on a high-sensitivity DC SQUID sensor, which introduces a current greater than the superconducting loop's critical current into the superconducting loop pickup circuit. of DC bias current The external magnetic field is sensed by the pickup loop and coupled to the SQUID sensor. The ideal Josephson junction follows the following equation: , , in, It is the phase difference of the quantum wave function at the junction center. For time, To reduce Planck's constant, For elementary charge, For junction current, Based on the Josephson effect, the SQUID sensor exhibits macroscopic quantum interference phenomena, outputting a sinusoidal superconducting voltage, which is the junction voltage. ( (where the superconducting voltage phase is the average value) is: , In the formula It is a superconducting ring resistor. For the self-inductance of the superconducting ring, For magnetic flux quanta, This refers to the external magnetic flux. Under constant DC bias, the change in external magnetic flux is measured by observing the output voltage of the SQUID sensor.

[0019] See Figure 1 As shown, this application provides a method for shielding electromagnetic interference in a SQUID-TEM detection environment. It provides a shielding body for the SQUID sensor, placing the SQUID sensor and its front-end electronics unit within the shielding body. This method receives target frequencies and shields against interference frequencies. The method includes: Confirm the interference frequency range; Select the type of shielding material and the initial minimum shielding thickness based on the required interference frequency range and the shielding efficiency standards. A three-dimensional simulation model of the shielding body for electromagnetic interference in the field operation area was established based on the finite element method. The simulation was carried out on the interference frequency range, radiation direction, distance between the interference source and the measuring device, shielding material thickness, and single and multiple interference sources. With the type of shielding material and the initial minimum shielding thickness as constraints, the shielding effectiveness of the shielding body under different shielding thicknesses was calculated. The frequency domain response curve of the shielding effectiveness as a function of thickness was extracted, and the optimized minimum shielding thickness that meets the shielding effectiveness standard was found from the frequency domain response curve.

[0020] The shielding body is prepared based on the optimized minimum shielding thickness and the type of shielding material. The shielding body is then placed on the outside of the SQUID sensor and the front-end electronics unit to complete the shielding deployment.

[0021] Among them, the SQUID sensor has a value of less than 100. The SQUID sensor features high sensitivity, low noise, and wide bandwidth, with a frequency response range in the DC to MHz range. However, in field experimental environments, it faces strong electromagnetic interference, such as FM radio signals in the 88-108MHz band, electromagnetic waves and digital communication signals above 100MHz used for air-to-ground operations, micro-meteorology, and space research. This interference can cause the Josephson junction of the high-sensitivity SQUID sensor to become unlocked, resulting in a dynamic range of the measured signal exceeding the SQUID sensor's measurement range or a rate of change of the measured signal exceeding the system slew rate. This can lead to the SQUID sensor losing lock and being unable to effectively measure experimental data. External electromagnetic interference manifests as the introduction of high-frequency alternating current into the DC bias current of the Josephson junction, affecting the operating point of the SQUID sensor. When both DC and alternating currents are present, the current between the Josephson junctions is as follows: , make , , , , , The angular frequency of the applied alternating current. For the amplitude of the applied AC current, For junction capacitance, To normalize the applied alternating current, To normalize the DC bias current, the simplified equation is: , in, , The normalized angular frequency of the applied alternating current. , The damping coefficient is related to the junction capacitance. Junction resistance related.

[0022] See Figure 3 The following is a graph showing the IV characteristics of the Josephson junction. Figure 3 (a) in the figure is the DC IV characteristic curve. Figure 3 (b) is the AC IV characteristic curve. When AC current is applied to the RCSJ model, the AC IV characteristic curve shows a stepped voltage, which is distributed according to a certain pattern. Mathematical calculations prove that the stepped voltage occurs at: , See Figure 2 As shown, the curves of the real part, imaginary part, and amplitude conform to the characteristics of transient electromagnetic detection target signals, and the frequency band of the transient electromagnetic detection target signal... Electromagnetic waves in the frequency range between Hz and above MHz are considered interference bands.

[0023] The interference frequency range was confirmed to include: the environmental electromagnetic interference was equivalent to the simultaneous injection of DC current components and AC current components into the Josephson junction. Based on the quantized relationship between the step voltage and the interference frequency, the frequency corresponding to the smallest observable step voltage is taken as the lower limit of the interference frequency. Measure the IV characteristic curve of a Josephson junction or a SQUID sensor containing the Josephson junction, and obtain the step voltage of the Shapiro step induced by the AC current component in the IV characteristic curve; the step voltage is proportional to the electromagnetic wave frequency, expressed as: ,in, For the first Step voltage, To reduce Planck's constant, For elementary charge, For the total series, To determine the frequency of external electromagnetic interference, the location of the first step voltage is identified, i.e., the corresponding step voltage. The electromagnetic wave frequency is then calculated based on the step voltage, serving as the lower limit of the frequency affected by external electromagnetic interference.

[0024] The minimum value among the characteristic frequency of the Josephson junction, the microwave coupling cutoff frequency, the measured upper limit of the environmental electromagnetic interference spectrum, and the upper limit of the detection system signal bandwidth is taken as the upper limit of the interference frequency.

[0025] The characteristic frequency of a Josephson junction is the ratio of its characteristic voltage to the magnetic flux quantum. ,in, For magnetic flux quanta, The normal-state resistance of the junction, The characteristic frequency of the Josephson junction is such that when the frequency of external electromagnetic interference waves approaches or exceeds this characteristic frequency, the impedance of the Josephson junction changes significantly, the AC Josephson effect weakens, and the amplitude of the Shapiro step drops sharply until it disappears. Therefore, the characteristic frequency of the Josephson junction itself constitutes the highest physical upper limit of its response to external interference.

[0026] The microwave coupling cutoff frequency of a Josephson junction depends on the coupling structure between the Josephson junction or SQUID sensor and the external electromagnetic field, including: the geometry of the antenna or coupling loop, the bandwidth of the input circuit, and the filtering characteristics of the packaging and shielding structure. Even if the Josephson junction itself can respond to a very high frequency, if the external electromagnetic interference energy cannot be effectively coupled to both ends of the junction to form a sufficient alternating current component, an observable Shapiro step cannot be generated. Therefore, the cutoff frequency of the microwave coupling structure is the second limit to the upper limit of the actual interference frequency.

[0027] In real-world measurement environments, external electromagnetic interference is not uniformly distributed across all frequencies. The upper limit of the measured spectrum of environmental electromagnetic interference is obtained by measuring the measurement environment under unshielded or known-shielded conditions using a spectrum analyzer or broadband receiving antenna, and identifying the highest significant interference frequency component. If there is no measurable electromagnetic interference energy above a certain frequency in the environment, then even if the Josephson junction and system can respond to higher frequencies, interference-induced Shapiro steps will not occur at that frequency. Therefore, the actual upper limit of the existence of environmental electromagnetic interference is a necessary constraint.

[0028] A typical detection system includes a cryogenic preamplifier, a room-temperature voltage amplifier, a data acquisition card, and filtering circuitry. The upper limit of the detection system's signal bandwidth is defined as the highest frequency at which the system can transmit AC signals without significant attenuation and maintain the required voltage resolution. If the interference frequency is too high, although the Shapiro step induced on the Josephson junction exists, the stability information of the corresponding AC voltage component or DC step voltage is suppressed by the system bandwidth when transmitted to the final measurement end, and therefore cannot be reliably identified on the IV characteristic curve. Therefore, the upper limit of the detection system's own signal bandwidth constitutes a fourth constraint.

[0029] Each of the four restrictions mentioned above will become a bottleneck for the overall upper limit of the interference frequency. The minimum value among the characteristic frequency of the Josephson junction, the microwave coupling cutoff frequency, the upper limit of the measured spectrum of environmental electromagnetic interference, and the upper limit of the signal bandwidth of the detection system will be taken as the upper limit of the interference frequency.

[0030] Based on Schelkunoffs' electromagnetic shielding theory, when electromagnetic waves propagate to the surface of a shielding material, they attenuate through three different mechanisms: reflection loss at the incident surface, absorption loss that enters the shielding body without being reflected, and multiple reflection loss inside the shielding body. The electromagnetic shielding effectiveness is analyzed from these three behaviors to attenuate electromagnetic waves, and a shielding body that meets the shielding effectiveness is designed to determine the selection criteria for shielding materials.

[0031] In one embodiment, the type of shielding material and the initial minimum shielding thickness of the shielding body are selected according to the required interference frequency range and the shielding efficiency standards, including: Obtain the interference frequency range, distance to the interference source, and preset shielding effectiveness standards; Based on Schelkunoffs' electromagnetic shielding theory, an expression for the shielding effectiveness of a shielding body, including reflection loss, absorption loss, and internal multiple reflection loss, is established. Based on Schelkunoffs' electromagnetic shielding theory, the shielding material is used to reflect or absorb electromagnetic waves to prevent their propagation from one space to another. When an electromagnetic wave passes through a shielding body, its surface absorbs and loses energy, causing the wave propagation to attenuate exponentially from the surface inwards. This attenuation of energy transmission is mainly achieved through three behaviors: reflection, absorption, and internal multiple reflection loss, thus realizing electromagnetic shielding. The shielding effectiveness is... The expression is as follows: , Among them, reflection loss , Let be the wave impedance of the incident wave. The wave impedance and absorption loss of the shielding body , To achieve skin depth, multiple reflections within the material result in losses. , The thickness of the shielding body.

[0032] Based on the interference frequency and the distance to the interference source, the field type of the shield is determined. The field type includes the far-field region, the near-field electric field region, and the near-field magnetic field region, and the corresponding reflection loss calculation method is selected accordingly. The classification of field types is based on the relative relationship between the distance to the interference source and the wavelength of the electromagnetic wave, where the wavelength is determined by the interference frequency, and a characteristic distance is defined. This is the dividing point between the near-field and far-field regions: , λ is the wavelength.

[0033] In the near-field region, the specific value of the wave impedance depends on the characteristics of the interference source, and is divided into: the near-field electric field region (high impedance field) and the near-field magnetic field region (low impedance field). The reflection loss in the far-field region is: , in, For reflection loss in the far field region, The electrical conductivity is relative to that of copper. ρ is the permeability relative to copper.

[0034] Near-field electric field region reflection loss for: , Near-field magnetic field region reflection loss , in This refers to the distance between the interference source and the shield.

[0035] Based on the field type and interference frequency range, the shielding material is selected, including: when the shield is in the far field, a conductor material with a conductivity higher than a first conductivity threshold is selected as the main body of the shielding layer, utilizing the shielding mechanism dominated by reflection loss; the first conductivity threshold can be set to 20% of the conductivity of copper, that is, brass and higher conductivity conductor materials can be selected. Preferred materials include copper, aluminum, silver, and brass, etc.

[0036] See Figure 4 The provided frequency response curves for reflection loss, absorption loss, skin depth, and shielding effectiveness are as follows: Figure 4 In the graph (a), the frequency response curve of the reflection loss is shown. Figure 4 (b) in the figure is the frequency response curve of the absorption loss. Figure 4 (c) in the figure represents the frequency response curve of the skin depth. Figure 4 (d) in the figure represents the frequency response curve of the shielding effectiveness. It can be seen that the frequency response curves of different shielding materials, such as copper, aluminum and silver, are different. The appropriate shielding material should be selected according to the field type and the interference frequency range.

[0037] When in a near-field magnetic field region and the interference frequency is less than 10 kHz, the reflection loss contributes very little to the overall shielding effectiveness in such scenarios. The shielding effectiveness mainly relies on absorption loss. Therefore, it is necessary to select a high-permeability material with a permeability higher than the first permeability threshold and use its low magnetic resistance to bypass the low-frequency magnetic field, thereby completing the shielding.

[0038] When the shield is in the near-field electric field region and the interference frequency is below the first frequency threshold, a high-conductivity metal material is selected to utilize the electric field reflection effect. The first frequency threshold is an empirical value, such as 100MHz. When the interference frequency is below 100MHz, the wave impedance in the near-field electric field region increases sharply with the decrease of frequency, and the reflection loss can reach a very high level. A high-conductivity metal material can then constitute an effective shield.

[0039] In the corresponding near-field magnetic field region and in scenarios where the interference frequency is less than 10 kHz, the reflection loss of good conductors is very limited due to the very low wave impedance of low-frequency magnetic fields. The shielding effect mainly depends on absorption loss. Therefore, conventional good conductors with high conductivity and low permeability cannot be directly selected. Instead, high-permeability materials with sufficiently high initial permeability, such as silicon steel, permalloy, or ferrite, must be selected. These materials rely on the low magnetic reluctance characteristics brought by high permeability to provide a low-resistance bypass for low-frequency magnetic lines of force, confining the magnetic field inside the shielding material and diverting the interfering magnetic field to achieve effective shielding. Furthermore, the thickness of such high-permeability materials needs to meet the minimum design requirements for absorption loss in order to achieve the expected shielding effect.

[0040] When the shield is in the near-field magnetic field region and the interference frequency is below the second frequency threshold (10MHz), a high permeability material is selected as the first shielding layer, and a high conductivity material is composited on its outer or inner side as the second shielding layer. The high permeability material is used to provide magnetic shielding dominated by absorption loss, and the high conductivity material is used to suppress residual high-frequency interference. High conductivity materials include, for example, copper and aluminum, and high permeability materials include, for example, iron and nickel.

[0041] When the interference frequency range covers low-frequency magnetic field interference and high-frequency electromagnetic interference, a multi-layer composite shielding structure is adopted.

[0042] In such scenarios, high permeability materials can increase the magnetic flux of low-frequency magnetic fields entering the shielding material by concentrating magnetic field lines, thereby significantly improving absorption loss and effectively solving the shielding shortcomings of good conductors, such as low reflection loss and insufficient absorption loss of low-frequency magnetic fields. At the same time, high conductivity materials composited on the outer or inner sides can further attenuate residual high-frequency interference that penetrates the high permeability layer by relying on reflection loss, achieving effective shielding across the entire frequency band without increasing the overall thickness too much.

[0043] In one example, when the shield is far from the interference source in the far-field region, it conforms to plane wave far-field shielding, the electromagnetic wave impedance is independent of the radiation source, and the wave impedance of the incident wave in free space is... The wave impedance of the shielding body, which is related to the properties of the shielding material, is: , It is the permeability. is the electrical conductivity.

[0044] Electromagnetic wave reflection loss is inversely proportional to the wave impedance of the shielding material. Therefore, the higher the conductivity and the lower the permeability of the shielding material, the smaller the wave impedance of the shielding material, and the greater the electromagnetic wave reflection loss. The reflection loss in the far-field region can be simplified as follows: .

[0045] From the skin depth formula Absorption loss The expression is: , It can be seen that the absorption loss is related to the thickness of the shield. External electromagnetic interference frequency The electrical conductivity of the material relative to copper Magnetic permeability relative to copper Proportional to the frequency of external interference and the thickness of the shielding, the absorption loss increases with the use of high electrical conductivity and high magnetic permeability materials.

[0046] Skin depth The skin depth decreases as the frequency increases. At the same interference frequency, the skin depth decreases as the conductivity and permeability of the shielding material increase. Reducing the required thickness of the shielding material can reduce the eddy current effect.

[0047] Choose metallic materials with high electrical conductivity and low magnetic permeability, such as copper and aluminum.

[0048] Obtain the relative permeability and relative conductivity of the selected shielding material to copper within the interference frequency range, and calculate the maximum skin depth based on the relative permeability and relative conductivity of copper. The skin depth formula is as follows: , Take the maximum value, which corresponds to the maximum skin depth; Based on the maximum skin depth, reflection loss, and multiple reflection losses within the material, the initial minimum shield thickness to meet the shielding effectiveness standard is calculated, including: Based on the preset shielding effectiveness standard, and combined with reflection loss and multiple reflection loss inside the material, the minimum absorption loss required is calculated. Assuming the shield thickness reaches a skin depth or more, and ignoring multiple reflection losses within the material; Based on the relationship between absorption loss and shield thickness and skin depth: Substituting the maximum skin depth and minimum absorption loss, we obtain the initial minimum shield thickness that meets the shielding effectiveness standard.

[0049] The above-mentioned reflection loss uses different calculation formulas according to the field type. For the far field region, the far field reflection loss is used; for the near field electric field region, the near field electric field reflection loss is used; and for the near field magnetic field region, the near field magnetic field reflection loss is used. The minimum shielding thickness suitable for different field types is obtained by using the reflection loss value of the corresponding field region.

[0050] See Figure 5 The diagram showing the finite element simulation results of the interference source-shield shows that a shield model for field experiments was established using the finite element method. Through simulations of the field electromagnetic interference frequency band, radiation direction, distance between the interference source and the measuring device, material thickness of the shield, and single and multiple interference source models, the working parameters of the electromagnetic interference shield suitable for different environments were further optimized. A three-dimensional simulation model of the shielding structure for electromagnetic interference in the field operation area was established based on the finite element method, with the ground resistivity set as follows: Air conductivity is In one example, the interference frequency range is determined to be 30MHz-130MHz, and the wavelength of the interfering electromagnetic wave is calculated. ; Based on Schelkunoffs far-field shielding, the solution domain of the shield is assumed to be 10m × 10m × 10m. The interference source is selected as a magnetic dipole or an electric dipole, and the distance between the magnetic dipole and the shield is... When there is a single interference source, it satisfies Conditions: When there are multiple interference sources, the interference sources can be distributed on both sides of the shield. The far-field plane wave shielding conditions are still met. For interference frequencies in the range of 30MHz-130MHz, models of single-point dipoles, multi-point dipoles, radiation distance, radiation direction, shield thickness, and energy amplitude parameters of magnetic and electric dipole moments are established respectively. The shielding effectiveness SE of the shield is calculated, and the shield parameters are designed and optimized according to the characteristics of the interference sources.

[0051] The above simulation process uses the selected type of shielding material and the initial minimum shielding thickness obtained from preliminary calculation as constraints to simulate the shielding effectiveness of the shielding body under different shielding thicknesses, extracts the frequency domain response curve of the shielding effectiveness as a function of the shielding thickness, and finds the optimized minimum shielding thickness that meets the shielding effectiveness standard from the frequency domain response curve.

[0052] The optimized minimum shielding thickness is compared with the initial minimum shielding thickness obtained from preliminary calculation. If the difference between the two is within the preset allowable range, the optimized result is directly used to complete the shielding design. If the difference exceeds the preset threshold, the constraints of the shielding material parameters are readjusted based on the optimized minimum shielding thickness, and the above simulation calculation and extraction screening process is repeated until the final shielding thickness parameters that meet both the shielding performance index requirements and the requirements of lightweight engineering and cost control are obtained, thus completing the optimization of the shielding.

[0053] The following is the verification process: A portable, low-noise, passive, small meter-wave loop receiving antenna is connected to a high-sensitivity radio receiver as the receiving device. A low-noise preamplifier is added at the antenna feed point to reduce electrical interference and improve the signal-to-noise ratio. Simultaneously, the more turns the receiving antenna has, the greater the low-frequency gain. The receiving antenna uses a 4-turn Möbius loop coaxial cable structure. When the wavelength is very large compared to the antenna size, it helps to eliminate most of the energy concentrated in the low-frequency band electrical noise and prevents the electrical balance from being altered by the transmission line. The receiving antenna, paired with an impedance converter, employs a low-loss, wideband, miniature balun design, requiring no manual tuning. It is suitable for FM broadcasting, aviation bands, and amateur radio HAM bands, and can achieve a response of up to 300MHz in the VHF band. The multi-antenna receiver has three antenna interfaces and an external reference clock input port, is USB powered, has built-in filtering, a single observation bandwidth of up to 10MHz, a receiving bandwidth of 1kHz-2GHz, and covers the complete RF bands of VLF, LF, MW, HF, VHF, UHF, and the 1-2GHz L-band. The system receives FM radio signals from the city via an antenna. The absolute power and signal-to-noise ratio of the FM radio signals are observed using a radio receiver. Signal amplitude is received at the same location both inside and in the surrounding space to verify the shielding effectiveness. Through urban spatial electromagnetic interference shielding effectiveness testing, effective electromagnetic response data can be measured. The test results demonstrate that the shielding effectively prevents external electromagnetic waves from interfering with the SQUID-TEM system.

[0054] A field detection experiment was conducted using the SQUID-TEM detection system. A bipolar trapezoidal wave was emitted from a magnetic source, and the electromagnetic response signal was measured with and without shielding. The B-field attenuation curve of the SQUID sensor was observed, and the measurement data was interpreted to obtain resistivity-depth imaging of the test area. The shielding effectively prevented external electromagnetic waves from interfering with the electromagnetic interference of the SQUID-TEM system.

[0055] In another aspect, this application provides a SQUID-TEM detection environment electromagnetic interference shielding system, comprising: A shielding enclosure is used to house the SQUID sensor and front-end electronics unit of the SQUID-TEM detection system. The shielding enclosure is designed based on the required interference frequency range and the standards for shielding effectiveness. The type of shielding material and the initial minimum shielding thickness are selected accordingly. A three-dimensional simulation model of the shielding enclosure for electromagnetic interference in the field operation area is established using the finite element method. Simulations are performed on the interference frequency range, radiation direction, distance between the interference source and the measuring device, shielding material thickness, and single and multiple interference sources. With the type of shielding material and the initial minimum shielding thickness as constraints, the shielding effectiveness of the enclosure under different shielding thicknesses is calculated. The frequency domain response curve of the shielding effectiveness as a function of thickness is extracted, and the optimized minimum shielding thickness that meets the shielding effectiveness standards is found from the frequency domain response curve.

[0056] The shielding enclosure adopts a box-type or cylindrical structure. Its internal dimensions are determined based on the external dimensions of the SQUID sensor and front-end electronics unit, ensuring that the SQUID sensor and its associated circuitry can be completely housed within the shielding enclosure while maintaining adequate operating space and ventilation channels. The wall thickness of the shielding enclosure is the minimum shielding thickness calculated based on the electromagnetic interference shielding design.

[0057] After the shielding structure is manufactured, a shielding effectiveness test must be conducted to verify whether the actual shielding effect meets the design standards.

[0058] The tests were conducted in an urban environment, using a typical FM radio signal (frequency range 88 MHz to 108 MHz) as the interference source. The test setup included a portable, low-noise, passive, miniature meter-wave loop receiving antenna (using a 4-turn Möbius strip coaxial cable structure, paired with a low-loss, wideband miniature balun) and a high-sensitivity radio receiver.

[0059] The test steps are as follows: Place the receiving antenna at a designated position outside the shield and record the received power of radio signals at each known frequency; then place the receiving antenna inside the shield at the same geometric position as outside, and record the received power of the same radio signals; the difference between the two received power values ​​is the measured shielding effectiveness of the shield at that frequency. Test multiple frequency points separately to obtain the measured curve of shielding effectiveness changing with frequency, and compare it with the frequency domain response curve obtained from simulation to verify the accuracy of the simulation results.

[0060] The shield was applied to a field experiment of the SQUID-TEM detection system. The experiment used a ground-based loop source device, in which a bipolar trapezoidal wave was emitted by a magnetic source. The SQUID sensor was placed inside the shield, and the attenuation curve of the secondary field generated by the underground anomaly was measured.

[0061] Comparison of measurement results with and without shielding: See Figure 6 As shown, the measurement data from measuring points 1-10 indicate that without shielding, the SQUID sensor is affected by environmental electromagnetic interference, cannot maintain stable operation, and the attenuation curve recorded by the receiver exhibits jumps and noise, making it unsuitable for resistivity-depth imaging of underground anomalies; see [link to relevant documentation]. Figure 7 As shown, when protected by a shield, the measurement data from measuring points 1 to 7 indicate that the SQUID sensor can work stably and obtain a smooth attenuation curve. The data interpretation of the induced field of the measurement data yields clear apparent resistivity-depth imaging results, verifying the effective suppression capability of the shield against interference in complex electromagnetic environments in the field.

[0062] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method of SQUID-TEM probing ambient electromagnetic interference shielding, characterized by, include: Confirm the interference frequency range, including: Environmental electromagnetic interference is equivalent to the simultaneous injection of DC current components and AC current components into the Josephson junction. Measure the IV characteristic curve of a Josephson junction or a SQUID sensor containing the Josephson junction, and obtain the step voltage of the Shapiro step induced by the AC current component in the IV characteristic curve; Based on the quantized relationship between the step voltage and the interference frequency, the frequency corresponding to the smallest observable step voltage is taken as the lower limit of the interference frequency. The minimum value among the characteristic frequency of the Josephson junction, the microwave coupling cutoff frequency, the upper limit of the measured spectrum of environmental electromagnetic interference, and the upper limit of the signal bandwidth of the detection system is taken as the upper limit of the interference frequency. Select the type of shielding material and the initial minimum shielding thickness based on the required interference frequency range and the shielding efficiency standards. A three-dimensional simulation model of the shielding body for electromagnetic interference in the field operation area was established based on the finite element method. The interference frequency range, radiation direction, distance between the interference source and the measuring device, shielding thickness, and single and multiple interference sources were simulated. With the type of shielding material and the initial minimum shielding thickness as constraints, the shielding effectiveness of the shielding body under different shielding thicknesses was calculated. The frequency domain response curve of the shielding effectiveness as a function of the shielding thickness was extracted, and the optimized minimum shielding thickness that meets the shielding effectiveness standard was found from the frequency domain response curve. The shielding body is prepared based on the optimized minimum shielding thickness and the type of shielding material. The shielding body is then placed on the outside of the SQUID sensor and the front-end electronics unit to complete the shielding deployment.

2. The method of claim 1, wherein the SQUID-TEM detects electromagnetic interference in an environment. Based on the quantized relationship between the step voltage and the interference frequency, it can be expressed as: , in, For the first Step voltage, To reduce Planck's constant, For elementary charge, For the total series, This refers to the frequency of external electromagnetic interference.

3. The electromagnetic interference shielding method for SQUID-TEM detection environment according to claim 1, characterized in that, Based on the required interference frequency range and the standards for shielding efficiency, select the type of shielding material and the initial minimum shielding thickness, including: Obtain the interference frequency range, distance to the interference source, and preset shielding effectiveness standards; Based on Schelkunoffs' electromagnetic shielding theory, an expression for the shielding effectiveness of a shielding body, including reflection loss, absorption loss, and multiple reflection losses within the material, is established. Based on the interference frequency and the distance to the interference source, the field type of the shield is determined. The field type includes the far-field region, the near-field electric field region, and the near-field magnetic field region, and the corresponding reflection loss calculation method is selected accordingly. Select shielding materials based on the field type and interference frequency range; Obtain the relative permeability and relative conductivity of the selected shielding material to copper within the interference frequency range, and calculate the maximum skin depth based on the relative permeability and relative conductivity of copper. Based on the maximum skin depth, reflection loss, and multiple reflection losses within the material, the initial minimum shield thickness that meets the shielding effectiveness standard is calculated.

4. The electromagnetic interference shielding method for SQUID-TEM detection environment according to claim 3, characterized in that, Depending on the field type and interference frequency range, the shielding materials selected include: When the shield is in the far field region, a conductor material with a conductivity higher than the first conductivity threshold is selected as the main body of the shielding layer, and the shielding mechanism dominated by reflection loss is utilized. When the shield is in the near-field electric field region and the interference frequency is below the first frequency threshold, a high-conductivity metal material is selected to utilize the electric field reflection effect. When the shield is in the near-field magnetic field region and the interference frequency is lower than the second frequency threshold, a high permeability material is selected as the first shielding layer, and a high conductivity material is composited on its outer or inner side as the second shielding layer. The high permeability material is used to provide magnetic shielding dominated by absorption loss, and the high conductivity material is used to suppress residual high-frequency interference. When the interference frequency range covers low-frequency magnetic field interference and high-frequency electromagnetic interference, a multi-layer composite shielding structure is adopted.

5. A method for shielding electromagnetic interference in a SQUID-TEM detection environment according to claim 3, characterized in that, Based on the maximum skin depth, reflection loss, and multiple reflection losses within the material, the initial minimum shield thickness to meet the shielding effectiveness standard is calculated, including: Based on the preset shielding effectiveness standard, and combined with reflection loss and multiple reflection loss inside the material, the minimum absorption loss required is calculated. Assuming the shield thickness reaches a skin depth or more, and ignoring multiple reflection losses within the material; Based on the relationship between absorption loss and shield thickness and skin depth, the minimum initial shield thickness that meets the shielding effectiveness standard is obtained by substituting the maximum skin depth and minimum absorption loss.

6. The electromagnetic interference shielding method for SQUID-TEM detection environment according to claim 5, characterized in that, When the reflection loss is calculated using different formulas depending on the field type where the shield is located, the minimum shield thickness is calculated using the corresponding reflection loss value: For the far-field region, far-field reflection loss is used; For the near-field electric field region, near-field electric field reflection loss is used; For the near-field magnetic field region, near-field magnetic field reflection loss is used; By using the reflection loss value of the corresponding field area, the minimum shield thickness adapted to different field area types can be obtained.

7. A SQUID-TEM detection environment electromagnetic interference shielding system, characterized in that, include: A shield is used to house the SQUID sensor and front-end electronics unit of the SQUID-TEM detection system. The shielding body is selected based on the required interference frequency range and the shielding efficiency standards. The shielding material type and initial minimum shielding thickness are selected, and a three-dimensional simulation model of the shielding body for electromagnetic interference in the field operation area is established based on the finite element method. The interference frequency range, radiation direction, distance between the interference source and the measuring device, shielding material thickness, single interference source and multiple interference sources are simulated. With the shielding material type and initial minimum shielding thickness as constraints, the shielding efficiency of the shielding body under different shielding thicknesses is calculated. The frequency domain response curve of the shielding efficiency as a function of thickness is extracted, and the optimized minimum shielding thickness that meets the shielding efficiency standards is found from the frequency domain response curve. The shielding body is required to shield the following interference frequency ranges: Environmental electromagnetic interference is equivalent to the simultaneous injection of DC current components and AC current components into the Josephson junction. Measure the IV characteristic curve of a Josephson junction or a SQUID sensor containing the Josephson junction, and obtain the step voltage of the Shapiro step induced by the AC current component in the IV characteristic curve; Based on the quantized relationship between the step voltage and the interference frequency, the frequency corresponding to the smallest observable step voltage is taken as the lower limit of the interference frequency. The minimum value among the characteristic frequency of the Josephson junction, the microwave coupling cutoff frequency, the measured upper limit of the environmental electromagnetic interference spectrum, and the upper limit of the detection system signal bandwidth is taken as the upper limit of the interference frequency.

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