Research On Detection Technology Of Underwater Ferromagnetic Targets Applying Power Frequency Electromagnetic Field

Large-scale power transmission networks with different structures have been set up all over the world,which can emit stable power-electromagnetic waves that are very little attenuated by propagation media such as the ocean into the surrounding space.The local power frequency electromagnetic anomaly caused by underwater targets magnetized by the geomagnetic field can be used for targets detection and positioning.Therefore,the power frequency electromagnetic wave has a good application prospect in the field of underwater targets detection and positioning.This thesis focuses on the method for detecting and positioning underwater ferromagnetic targets applying power frequency electromagnetic fields,carries out the research on the mechanism of the target detection.Under the theoretical guidance of the mechanism research,a simulation model of the interaction between the power frequency electromagnetic wave and the underwater target is constructed.Based on the simulation results,a set of multi-scale method for detecting and positioning underwater ferromagnetic targets by using power frequency electromagnetic wave are proposed.According to the measured weak power frequency magnetic field data,a normalized difference method is proposed to enhance the target magnetic anomaly characteristics.The main work of the thesis is as follows:(1)Starting with the basic theories of Maxwell's equations and the low of electromagnetic induction,the generation mechanism of power frequency electromagnetic waves in distributed transmission networks and the propagation characteristics in different media,as well as the law of electromagnetic anomalies caused by ferromagnetic objects are studied.A simulation example of the interaction between a current-carrying wire and a metal disc and field tests of the magnetic anomaly detection of moving vehicles are carried out,revealing the feasibility of power frequency electromagnetic detection of underwater ferromagnetic targets.(2)With high-voltage/ultra-high-voltage/ultra-high-voltage transmission networks of China as the excitation source,the air,the sea water,the land,the seabed,and the ionosphere as the transmission medium,a power frequency electromagnetic simulation calculation model is established.Two field tests of the background power frequency magnetic field in the Shuibuya Reservoir and Jinshahe Reservoir are carried out.By comparing the simulation results with the satellite inversion data and the field measured data,the rationality of the simulation model is verified.(3)Taking the underwater vehicle as the detection object,the interaction model between the underwater ferromagnetic target and the power frequency electromagnetic wave is further constructed,and then the distribution of electromagnetic abnormal signals under different observation altitudes is analyzed.(4)Based on the simulation data of the power frequency electromagnetic anomaly caused by underwater ferromagnetic targets,a set of multi-scale underwater targets detection and positioning scheme applying power frequency electromagnetic fields is proposed,which solves the problem of large-scale aerial detection and positioning of underwater targets,reduces the suspected target range,and gives the approximate direction of underwater moving targets.Based on the measured magnetic field data,a normalized difference method is proposed to enhance the target's weak magnetic anomaly signal characteristics,so that the signal-to-noise ratio is increased by at least 20 dB.