Simulations Of Undersea Electronmagnetic Anomaly Targets

In recent years,with the rapid advances in development and utilization of ocean and marine economy,seabed resources survey and ocean security maintenance is becoming a pressing issue.How to find and recognize invading undersea targets in real-time constitutes a great need of our country.The undersea magnetic anomaly detection technique has the advantages of passiveness,good concealment,wide detection range,strong anti-interference ability in shallow sea and high-precision positioning,and can effectively compensate for the defects and deficiencies of traditional acoustic detection methods.Therefore,the detection and identification of underwater magnetic anomaly targets has great research value for ocean security maintenance.In this thesis,the anti-corrosion current electromagnetic field of an underwater magnetic anomaly target is studied.The anti-corrosion current electromagnetic field is generated by the sacrificial anode passive protection system,the applied current cathodic protection system,and the extremely low-frequency current formed by the ship’s propeller modulation.Firstly,the air-seawater-seabed three-layered media was selected as the basic model for theoretical derivation according to the actual application environment.Then,starting from the characteristics of anti-corrosion current,the anti-corrosion current source can be equivalently replaced by a plurality of superposed horizontal electric dipoles.According to the actual situation,the expressions of the vector magnetic potential and the magnetic field expressions of the anti-corrosion current source in the air-seawater-seabed threelayered media are deduced respectively in the case of direct current and low frequency alternating current.Based on the obtained expressions,the anomalous field distribution of a typical underwater anti-corrosion current source in space can be accurately calculated,which provides the reference signal sample for undersea or aviation magnetic anomaly detection.To efficiently calculate the Sommerfeld integrals in the electromagnetic potential or field expressions,the acknowledged discrete complex image method(DCIM)is used.By fitting the integrands with a set of complex exponential functions,the Sommerfeld integrals can give semi-analytical expressions thanks to the Sommerfeld identity.Finally,pre-processing of experimental raw data is performed to obtain valid measured data for comparisons with theoretical evaluations.Two sets of experiment raw data are used.One is the aeromagnetic data with the sensors carried by an airplane to fly over a sea area.The other is a seafloor experiment with the sensors deployed on the seafloor.It is found that the simulated results are in reasonable agreements with the measured data,which illustrates that the theoretical formulae are trustful.