| Unexploded ordnance mainly refers to unexploded landmines,ordnance,shells,grenades and missiles left after the war or other military activities.These explosives are usually scattered on the surface or underground very shallow position,which pose a great threat to people's life and property.At the same time,unexploded ordnance will last in force for decades,and a large number of unexploded ordnance will be left in partial war,all of which have aggravated the difficulty of unexploded ordnance clearance.How to accurately detect unexploded ordnance is essential for unexploded ordnance clearance.So far,geophysical methods including magnetic method,electromagnetic method and ground penetrating radar have been widely used in unexploded ordnance detection.The electromagnetic method is very insensitive to the earth's response due to its strong anti-interference ability and therefore plays an important role in the detection of unexploded ordnance.As a time-domain electromagnetic induction detection method,the transient electromagnetic method can effectively identify the complete and dangerous unexploded ordnance.Based on this method,a series of exploration systems including airborne systems,vehicular systems and portable systems have been developed.Among them,the portable transient electromagnetic detection system can adapt to various terrains such as mountainous and woodland,so it is important to study the theory,system design and data processing based on this system.This article focuses on the theory and method of unexploded ordnance detection with portable transient electromagnetic system,the design and optimization of the system and data processing.The main contents of this dissertation are as follows:In this paper,we study the time-domain response characteristics of unexploded ordnance under the excitation of transient electromagnetic system.According to the theoretical response of solid iron sphere,the law of the sphere response decay with time is analyzed,and a parameterized model with power function decay at early times and exponential decay at later times is established.The characteristics of vertical and horizontal axis of unexploded ordnance were measured respectively by long straight solenoid and rectangular coils to improve the accuracy of measurement results.Finally,the longitudinal response and the transverse response of cast iron pipes with different parameters and 60 mm,82mm and 120 mm shells were measured.The variation of the characteristic response parameters with the length,diameter and mass of the target were analyzed.Secondly,the forward model of unexploded ordnance is studied based on three-dimensional induced dipole.By decomposing response into the characteristic response of the target,the characteristics of the sensor and the relative position of the target and the sensor,an orthogonal dipole model of unexpanded response is established.Based on the dipole model,the line response and the two-dimensional response of the axisymmetric target under the condition of single-component emission and central three-component receiving are calculated theoretically.The response of the target with the position,attitude,characteristic response and other parameters is analyzed.At the same time,the line response and two-dimensional response were also measured and compared with the calculated ones.The accuracy of the three-dimensional induced dipole model was discussed based on the comparison between the calculated data and the measured data.The design and optimization of sensors for portable transient electromagnetic system is analyzed in detail.By establishing the relationship between the primary field and the size,power and mass of the transmitting coil,the design and optimization of the transmitting coil are discussed in detail in this paper.The results show that increasing the coil diameter,mass and power can increase the primary field strength.Decreasing the number of turns and the increasing the clamp voltage can reduce the turn-off time of the emission current.By establishing the relationship between receiver noise and its size,the design and optimization of receiving coil are discussed in detail.The results show that the noise level of receiving coil can effectively be reduced by increasing the size and reducing the resonance frequency.Based on the researches above,portable transient electromagnetic sensor with a transmitting coil diameter of 0.5m,a coil turns of 30,a receiving coil side length of 0.06 m and a resonant frequency of 50 kHz is designed.The experimental results show that this system can detect the 82 mm mortar shells with a depth of 1m accurately.Finally,the inversion theory of unexploded ordnance response based on DE algorithm is studied.By analyzing the process and parameter design rules of DE algorithm,this algorithm is applied to the inversion of unexploded ordnance responses to overcome the problem of local convergence of traditional inversion algorithms.On this basis,the target responses under various conditions is calculated and inverted by DE algorithm.The results show that under the ideal conditions,the DE algorithm can accurately estimate the position,attitude and characteristic response of the target.When the signal-to-noise ratio is high,the DE algorithm can still estimate the parameters of the target accurately.When SNR is low,the DE algorithm can only estimate the position of the target and cannot estimate the attitude and the characteristic response of the target accurately.The experimental results show that DE algorithm can accurately estimate the target position,attitude and characteristic response.In this paper,the theory and method of detection unexploded ordnance by portable transient electromagnetic systems have been discussed in the aspects of theory research,system design optimization and data inversion.A portable transient electromagnetic detection system was successfully designed.The DE algorithm was used to inverse the unexploded ordnance response detected by this system.The inversion result was in good agreement with the true value,which proves the feasibility of the system and the data processing algorithm. |
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