A differential transient electromagnetic device and a method for detecting unexploded ordnance using the same

Abstract

This application belongs to the field of geophysical exploration technology, specifically disclosing a differential transient electromagnetic method device and its unexploded ordnance detection method. In the differential transient electromagnetic method device, the transmitter injects a transmission current into the transmission loop, generating a primary field excitation signal. Under the influence of the primary field excitation signal, the subsurface anomaly target generates a secondary field excitation signal. The receiving coil assembly, based on a differential coil structure, synchronously cancels the primary field excitation signal and the uniform background signal in the induced voltage, amplifying the local asymmetric differential signal generated by the subsurface anomaly target. The receiver performs time-series analysis on the local asymmetric differential signal, extracts differential signal features, and achieves the location and identification of the subsurface anomaly target. This application is suitable for the detection of shallowly buried small bodies such as UXOs. The overall deployment range of the device is small, facilitating rapid deployment and mobile observation in the field.

Description

Technical Field

[0001] This application belongs to the field of geophysical exploration technology, and more specifically, relates to a differential transient electromagnetic method device and a method for detecting unexploded ordnance. Background Technology

[0002] Unexploded ordnance (UXO) refers to unexploded ordnance such as shells, bombs, and landmines. These targets are usually buried underground or underwater, posing an unpredictable risk of explosion. They not only threaten personal safety but also hinder the reuse of land resources and engineering construction. To ensure public safety and environmental governance, the effective detection and removal of UXO has become an important issue of widespread international concern.

[0003] Currently, common UXO detection methods mainly include magnetic methods, ground-penetrating radar, metal detectors, and transient electromagnetic methods. Among them, the magnetic method utilizes the remanent magnetization or induced magnetic anomalies of ferromagnetic targets for detection. Its advantages are simple equipment and fast detection speed, but it is easily affected by geomagnetic background, geological conditions, and ferrous debris, making it difficult to distinguish between different targets.

[0004] Ground-penetrating radar identifies underground anomalies through electromagnetic wave reflection imaging. Its advantages include high resolution and suitability for shallow exploration. However, signal attenuation is severe in highly conductive strata (such as clay or aquifers), limiting the detection depth.

[0005] Metal detectors are based on the principle of electromagnetic induction. They are portable and suitable for near-surface operations, but their shallow working depth makes quantitative target analysis difficult. Against this backdrop, transient electromagnetic methods are gradually becoming an important means of UXO detection.

[0006] Transient electromagnetic method (TEM) is a non-contact geophysical exploration method based on the principle of electromagnetic induction. By passing a pulsed current through a transmitting loop laid on the ground, the rapid change of the current excites a transient primary field, which induces eddy currents in underground conductors. The eddy currents further generate a secondary field. By recording the decay characteristics of the secondary field over time through a receiving coil assembly, electrical parameter information at different underground depths can be obtained. This method has been widely used in mineral exploration, hydrogeological and engineering geological surveys.

[0007] As shown above, transient electromagnetic methods can reflect the characteristics of underground conductive structures and can be used to identify the spatial distribution and electrical differences of underground targets. However, in unexploded ordnance (UXO) detection, due to the small size and limited burial depth of the target, its induced response signal often decays rapidly in the early stages, resulting in weak intensity. Simultaneously, primary and environmental electromagnetic interference easily masks the secondary field generated by the target, leading to a low signal-to-noise ratio in the received data and making it difficult to accurately reveal target information. Existing transient electromagnetic receiving coil assembly deployment methods mainly include single-loop coils, concentric loops, and multi-station deployments. Single-loop coils have a simple structure but suffer from strong primary and background interference and a low signal-to-noise ratio. Concentric loops provide a strong signal against a one-dimensional background but have limited resolution for local disturbances. Multi-station deployments can improve coverage and positioning capabilities, but require significant fieldwork and are significantly affected by environmental noise.

[0008] Existing technologies have proposed methods such as coil arraying and combining digital filtering to enhance the detection capability and signal-to-noise ratio of shallow, small targets. However, these improvements still have limitations and are prone to failure in complex electromagnetic backgrounds. Therefore, existing technologies cannot simultaneously meet the requirements of improving the signal-to-noise ratio and high-resolution detection of shallow, small targets. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this application aims to provide a differential transient electromagnetic method device and its unexploded ordnance detection method, which solves the problem that existing methods using transient electromagnetic methods for unexploded ordnance detection struggle to simultaneously improve the signal-to-noise ratio and meet the high-resolution detection requirements of shallow, small targets.

[0010] The first aspect of this application relates to a differential transient electromagnetic method apparatus, comprising: a transmitter, a transmitting loop, a receiving coil assembly, and a receiver;

[0011] The receiving coil assembly is located in the internal region of the transmitting loop. The receiving coil assembly contains 2n coils that are symmetrical about the center of the transmitting loop. The symmetrical coils are in the same direction. The forward winding ends of two symmetrical coils are connected and connected to the receiver, while their reverse winding ends are both grounded.

[0012] The transmitter injects transmission current into the transmission loop, which generates a primary field excitation signal. The underground anomaly target generates a secondary field excitation signal under the action of the primary field excitation signal. The receiving coil assembly, based on a differential coil structure, synchronously cancels the primary field excitation signal and the uniform background signal in the induced voltage, amplifying the local asymmetric differential signal generated by the underground anomaly target. The receiver performs time-series analysis on the local asymmetric differential signal, extracts differential signal features, and realizes the location and identification of the underground anomaly target.

[0013] In some embodiments, the receiving coil assembly includes receiving frames A, B, C, and D arranged orthogonally and symmetrically around the center of the transmitting loop; receiving frames B and D are symmetrically arranged, as are receiving frames A and C; a first rod and a second rod are provided on the transmitting loop; both the first and second rods are provided with lead holes; the lead hole of the first rod is used for the positive and negative winding ends of receiving frames B and D to pass through; the lead hole of the second rod is used for the positive and negative winding ends of receiving frames A and C to pass through; wherein the first and second rods are insulating rods.

[0014] In some implementations, the transmit loop is a multi-turn circular coil wound with copper wire, used to generate a changing primary field excitation signal at the moment of turn-off by injecting transmit current.

[0015] In some implementations, the emission current injected into the emission return line is a step-type current waveform.

[0016] In some implementations, the spacing between adjacent receiving frames in the receiving coil assembly is adjusted according to the target detection depth and sensitivity requirements.

[0017] In some implementations, the symmetrical differential coils in the receiving coil assembly can be extended into a two-dimensional or three-dimensional receiving array.

[0018] In some implementations, the locally asymmetric differential signal is: ;in, , , and These represent the induced voltages received by receiver frames A, B, C, and D at time t.

[0019] The second aspect of this application relates to a method for detecting unexploded ordnance based on the above-mentioned differential transient electromagnetic method device, specifically including the following steps:

[0020] Step 1: Inject the launch current into the launch return line to generate a primary field excitation signal at the moment of shutdown. The unexploded ordnance generates a secondary field excitation signal under the action of the primary field excitation signal.

[0021] Step 2: When the secondary field excitation signal passes through the receiving coil assembly, the receiving coil assembly receives the induced voltage. Based on the setting of the differential coil, the primary field excitation signal and the uniform background signal are synchronously canceled in the induced voltage, and the local asymmetric differential signal generated by the underground unexploded ordnance is amplified.

[0022] Step 3: Perform time series analysis on the local asymmetric differential signal, extract differential signal features, and realize the location and identification of underground unexploded ordnance.

[0023] In some implementations, local asymmetric differential signals are acquired by setting time channels, the attenuation response of shallow unexploded ordnance is analyzed by the early channel, and the electrical structure information of deep unexploded ordnance is analyzed by the late channel.

[0024] In some implementations, after step two, the local asymmetric differential signal is low-pass filtered, and the filtered differential signal is digitized before timing synchronization.

[0025] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art:

[0026] This application provides a differential transient electromagnetic method device. The receiving coil assembly adopts multiple sets of differential coils, which can directly capture the signal of anomalies. By setting one or more sets of differential coils, the primary field excitation signal and the uniform background signal can be synchronously canceled, significantly improving the signal-to-noise ratio. The dual-quadrant symmetrical layout structure ensures that the target will generate asymmetric interference to at least one differential pair regardless of which position it is in the array, thereby avoiding the problem of the central blind zone of the coil. The differential structure amplifies the local asymmetric response, which is especially suitable for the detection of shallowly buried small bodies such as UXO. The overall deployment range of the device is small, which is convenient for rapid deployment and mobile observation in the field, meeting the needs of UXO detection. Multiple symmetrical differential coils can be expanded into a two-dimensional or three-dimensional receiving array, further improving spatial resolution and imaging capabilities. Attached Figure Description

[0027] Figure 1 This is a schematic flowchart of the bomb detection method based on the differential transient electromagnetic method provided in the embodiments of this application.

[0028] Figure 2 This is a schematic diagram of the differential coil structure provided in the embodiments of this application.

[0029] Figure 3 This is a schematic diagram of the internal workings of the receiver provided in an embodiment of this application.

[0030] Figure 4(a) is a schematic diagram of the forward modeling results of the left receiving coil assembly of the unexploded ordnance tilted according to an embodiment of this application.

[0031] Figure 4(b) is a schematic diagram of the differential results of the signal from the left receiving coil assembly of the unexploded ordnance tilted according to an embodiment of this application.

[0032] Figure 5(a) is a schematic diagram of the forward modeling result of the signal from the left receiving coil assembly when the non-explosive shell is vertical, provided in the embodiment of this application.

[0033] Figure 5(b) is a schematic diagram of the differential results of the left receiving coil assembly signal when the non-explosive bomb is vertical, according to the embodiment of this application. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0035] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A existing alone, A and B existing simultaneously, and B existing alone. In this application, the symbol " / " indicates that the related objects are in an "or" relationship, for example, A / B means A or B.

[0036] In this application, the terms “first” and “second” are used to distinguish different objects, rather than to describe a specific order of objects.

[0037] In this application, the term "electrical connection" can refer to a direct circuit connection or a signal transmission via a communication protocol.

[0038] In the embodiments of this application, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or design. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0039] In the description of the embodiments in this application, unless otherwise stated, "multiple" means two or more.

[0040] The embodiments of this application are described below with reference to the accompanying drawings.

[0041] This application provides a differential transient electromagnetic method device, including a transmitter, a transmitting loop 1, a receiving coil assembly 2, and a receiver 3;

[0042] The transmitter is used to transmit current; the transmitting loop 1 is used to generate a primary field excitation signal; the receiving coil assembly 2 is arranged in the internal area of ​​the transmitting loop and the symmetrical coils are in the same direction; the receiver 3 is used to acquire and process the received signal;

[0043] The transmitting loop 1 is a multi-turn circular coil with a diameter of 1.6m, made of copper wire. Its function is to generate a rapidly changing electromagnetic field by injecting current and at the moment of turn-off. The transmitting current is driven by the transmitter and can generate an approximately step-type current waveform.

[0044] The receiving coil assembly 2 includes four circular receiving coils with a diameter of 0.4m, denoted as A, B, C, and D respectively; the four receiving coils are arranged orthogonally and symmetrically around the center of the transmitting loop, that is, at a 90° angle to each other, forming a four-quadrant symmetrical structure; the spacing between adjacent coils can be adjusted according to the target detection depth and sensitivity requirements, and generally does not exceed half the radius of the transmitting loop;

[0045] An 11-turn copper wire, evenly wound, is passed through the lead hole at the left or right end of the BD rod on the transmit return line 1. The wires are twisted together to form + and - leads, which are then connected to the transmitter. The copper wires in the B receiver frame are twisted together and led out from the middle of the BD rod to connect to receiver 3. Similarly, the D receiver frame is connected to the receiver in the middle. The B and D receiver frames form a lateral differential pair. The differential method involves connecting the positive winding end (+) as the output and grounding the negative winding end (-), thus achieving a ++ connection and a -- ground connection. Similarly, the A and C receiver frames have the same structure. The BD and AC rods need to be insulated.

[0046] The four circular coils in a symmetrical differential configuration can directly capture signals from anomalous objects. Through two sets of differential coils in the longitudinal and transverse directions, the primary field and uniform background signals are synchronously canceled, significantly improving the signal-to-noise ratio. The four-quadrant symmetrical layout ensures that no matter which direction the target is located in the array, it will generate asymmetric interference to at least one differential pair, thereby avoiding the problem of the coil's central blind zone. The differential structure amplifies the local asymmetric response, making it particularly suitable for the detection of shallowly buried objects such as UXO. The overall deployment range of the device is small, facilitating rapid deployment and mobile observation in the field, meeting the needs of UXO detection. Multiple symmetrical differential coils can be expanded into a two-dimensional or three-dimensional receiving array, further improving spatial resolution and imaging capabilities.

[0047] Example

[0048] like Figure 1 As shown, this application provides a UXO symmetrical differential coil detection method for achieving differential measurement. Figure 2 The four receiving coils are paired to form a differential unit; specifically, receiving coil A and receiving coil C form a longitudinal differential pair, and receiving coil B and receiving coil D form a transverse differential pair; the differential method is that the positive winding end (+) is connected as the output, and the negative winding end (-) is grounded, thereby realizing the connection method of +++ connected and -- grounded; the differential induced voltage is output to receiver 3 for synchronous acquisition;

[0049] The differential output signal can be represented as:

[0050]

[0051] in, , , and These are the induced voltages received by each receiving coil at time t. Since the induced response of the primary field is equal at symmetrical positions, differential operation can effectively cancel the primary field and uniform background noise, highlighting the secondary field signal generated by the local anomaly (UXO).

[0052] like Figure 3 As shown, receiver 3 includes an input module, a dynamic amplification module, a filtering module, an analog-to-digital conversion module, and a control module. The input module is used to receive the voltage signal output by the differential coil. The filtering module is used to perform low-pass filtering on the original signal to remove high-frequency interference. The analog-to-digital conversion module is used to digitize the analog signal. The control module is used to realize timing synchronization and data storage.

[0053] According to Faraday's law of electromagnetic induction, the induced voltage generated by a single receiving coil can be expressed as:

[0054]

[0055] in, This refers to the number of coil turns. The magnetic flux passing through the coil; The vertical component of the secondary field generated for the target; The vacuum permeability; S Where is the receiving area of ​​a single coil; for the differential coil structure of this application, the differential output signal is:

[0056]

[0057] in, Let A be the receiving area of ​​the receiving wireframe; Let C be the receiving area of ​​the receiving wireframe.

[0058] Symmetric components under a uniform background field cancel each other out, while asymmetric components caused by local UXO targets are preserved.

[0059] In actual observations, the time channel is set to... s to A total of 24 logarithmically distributed time points were collected; the rapid decay response of shallow small targets can be analyzed through the early channels, while the late channels can reflect the electrical structure information of deeper targets.

[0060] To verify the validity of this application, a composite target consisting of a cylinder and a cone was buried at a depth of -1m in a uniform half-space background model. The induced voltage of the target under different conditions was detected in both vertical and inclined states (the cylinder has a radius of 0.05m and a height of 0.5m; the cone has a base radius of 0.05m, a height of 0.1m, a top radius of 0.01m, and a conductivity of typical steel). Three-dimensional numerical simulation was performed using the above-mentioned transmitter-receiver coil arrangement and compared with a common single receiver coil. The typical response curves obtained are shown in Figures 4(a), 4(b), 5(a), and 5(b) (inclined and vertical).

[0061] In the embodiment, the above-mentioned receiving device was moved 23 times along the survey line at an altitude of 1m above the ground surface, with each movement being 0.25m, to obtain 24 detection data above the UXO, which were used to reflect the state of the underground target.

[0062] As shown in Figures 4(a) and 4(b), the horizontal axis represents the spatial position along the survey line, and the vertical axis represents the dBz / dt response amplitude of the received differential signal. Different colored curves correspond to different time channels.

[0063] As can be seen from Figures 4(a) and 4(b), above the target, the curves exhibit a typical positive and negative symmetrical distribution, with one side having a positive peak and the other side having a negative peak, and the zero value in the middle corresponding to the center position of the target projection. This positive and negative anomaly is a direct manifestation of the differential receiving structure's sensitivity to local asymmetric secondary field excitation signals.

[0064] The early time channels (blue and red curves) have large response amplitudes and clear positive and negative values, reflecting the rapid decay characteristics of shallowly buried targets; the late time channels (green, purple, etc. curves) have gradually weakened response amplitudes and tend to flatten out, indicating that the secondary field excitation signal decays significantly after the time delay, and the contribution of the target body is weakened.

[0065] The uniform background and the primary field component are effectively canceled out in the differential processing. The background curve approaches zero when it is far away from the target, highlighting the local abnormal signal of the target. This shows that the device of this application can enhance the recognition ability of the secondary field of the target under a strong primary field background.

[0066] The distribution range of the positive and negative peaks of the curve is approximately concentrated in The peak position corresponds significantly to the target burial point, indicating that the device has good lateral resolution and positioning accuracy in shallow body detection.

[0067] Compared with Figures 4(a) and 4(b), the anomalous bodies in Figures 5(a) and 5(b) are in vertical and tilted states, respectively. Their direct manifestation in the image is whether the intersection of the positive and negative curves is above the 0 value, which can be used to judge the tilt of the UXO.

[0068] Compared with traditional single-coil, concentric coil, and figure-eight coil structures, the four-quadrant symmetrical differential receiving coil device of this application has the following advantages:

[0069] Dual differential suppression: It can simultaneously cancel the primary field and uniform background response, significantly improving the signal-to-noise ratio;

[0070] Blind-zone-free detection: When a target is located at any position in the array, it will inevitably break the symmetry of at least one difference pair, thus being detected;

[0071] Compact structure: The small layout area of ​​the transmit loop and receive coil makes it suitable for shallow UXO detection missions;

[0072] Easy array expansion: Multiple symmetrical differential units can be spliced ​​together to form an array system, enhancing two-dimensional or three-dimensional spatial resolution.

[0073] In summary, the symmetrical differential receiving coil device proposed in this application overcomes the limitations of existing TEM receiving coils, and combines high signal-to-noise ratio with high-resolution detection capability of shallow bodies, thus having broad application prospects.

[0074] It should be understood that expressions such as “comprising” and “may include” used in this application indicate the existence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In this application, terms such as “comprising” and / or “having” are to be interpreted as indicating a particular characteristic, number, operation, constituent element, component, or combination thereof, but not to exclude the existence or possibility of adding one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof.

[0075] Furthermore, in this application, the expression "and / or" includes any and all combinations of the associated listed words. For example, the expression "A and / or B" may include A, may include B, or may include both A and B.

[0076] In the description of the embodiments of this application, it should be noted that, unless otherwise explicitly specified and limited, the term "connection" should be interpreted broadly. For example, "connection" can be a detachable connection or a non-detachable connection; it can be a direct connection or an indirect connection through an intermediate medium.

[0077] Furthermore, the mathematical concepts mentioned in the embodiments of this application, such as symmetry, equality, parallelism, and perpendicularity, are limitations specific to the current technological level, rather than absolute and strict mathematical definitions. Slight deviations are permissible; approximations of symmetry, equality, parallelism, and perpendicularity are all acceptable. For example, "A and B are parallel" means that A and B are parallel or approximately parallel, and the angle between A and B can be between 0 and 10 degrees. "A and B are perpendicular" means that A and B are perpendicular or approximately perpendicular, and the angle between A and B can be between 80 and 100 degrees.

[0078] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A differential transient electromagnetic method device, characterized in that, include: Transmitter, transmit loop, receive coil assembly, and receiver; The receiving coil assembly is located in the internal region of the transmitting loop. The receiving coil assembly contains 2n coils that are symmetrical about the center of the transmitting loop. The symmetrical coils are in the same direction. The forward winding ends of two symmetrical coils are connected and connected to the receiver, while their reverse winding ends are both grounded. The transmitter is used to inject transmission current into the transmission loop, which is used to generate a primary field excitation signal. The underground anomaly target is used to generate a secondary field excitation signal under the action of the primary field excitation signal. The secondary field generates an induced voltage when it passes through the receiving coil. The receiving coil assembly is used to synchronously cancel the primary field excitation signal and the uniform background signal in the induced voltage based on the differential coil structure, thereby amplifying the local asymmetric differential signal generated by the underground anomaly target. The receiver is used to perform time-series analysis on local asymmetric differential signals, extract differential signal features, and realize the location and identification of underground anomaly targets; The receiving coil assembly includes receiving frames A, B, C, and D arranged orthogonally and symmetrically around the center of the transmitting loop; receiving frames B and D are symmetrically arranged, as are receiving frames A and C; a first rod and a second rod are provided on the transmitting loop; both the first and second rods are provided with lead holes; the lead hole of the first rod is used for the positive and negative winding ends of receiving frames B and D to pass through; the lead hole of the second rod is used for the positive and negative winding ends of receiving frames A and C to pass through; wherein, the first and second rods are insulating rods.

2. The differential transient electromagnetic method apparatus according to claim 1, characterized in that, The transmitting loop is a multi-turn circular coil wound with copper wire, used to generate a changing primary field excitation signal at the moment of turn-off by injecting transmitting current.

3. The differential transient electromagnetic method apparatus according to claim 1 or 2, characterized in that, The emission current injected into the emission return line has a step-type current waveform.

4. The differential transient electromagnetic method apparatus according to claim 1, characterized in that, The spacing between adjacent receiving frames in the receiving coil assembly can be adjusted according to the target detection depth and sensitivity requirements.

5. The differential transient electromagnetic method apparatus according to claim 1, characterized in that, The symmetrical differential coils in the receiving coil assembly can be extended into a two-dimensional or three-dimensional receiving array.

6. The differential transient electromagnetic method apparatus according to claim 1, characterized in that, The locally asymmetric differential signal is: ;in, , , and These represent the induced voltages received by receiver frames A, B, C, and D at time t.

7. A method for detecting unexploded ordnance based on the differential transient electromagnetic method device according to any one of claims 1 to 6, characterized in that, Includes the following steps: Step 1: Inject the launch current into the launch return line to generate a primary field excitation signal at the moment of shutdown. The unexploded ordnance generates a secondary field excitation signal under the action of the primary field excitation signal. Step 2: When the secondary field excitation signal passes through the receiving coil assembly, the receiving coil assembly receives the induced voltage. Based on the setting of the differential coil, the primary field excitation signal and the uniform background signal are synchronously canceled in the induced voltage, and the local asymmetric differential signal generated by the underground unexploded ordnance is amplified. Step 3: Perform time series analysis on the local asymmetric differential signal, extract differential signal features, and realize the location and identification of underground unexploded ordnance.

8. The method for detecting unexploded ordnance according to claim 7, characterized in that, By setting time channels to acquire local asymmetric differential signals, the attenuation response of shallow unexploded ordnance is analyzed through the early channel, and the electrical structure information of deep unexploded ordnance is analyzed through the late channel.

9. The method for detecting unexploded ordnance according to claim 7, characterized in that, After step two, the local asymmetric differential signal is low-pass filtered, and the filtered differential signal is digitized before timing synchronization.

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