A Novel Detection Method for Underwater Moving Targets by Measuring Their ELF Emissions with Inductive Sensors.

Jinhong Wang,Li Li,Lianping Chen,Bin Li

doi:10.3390/s17081734

Abstract

In this article, we propose a novel detection method for underwater moving targets by detecting their extremely low frequency (ELF) emissions with inductive sensors. The ELF field source of the targets is modeled by a horizontal electric dipole at distances more than several times of the targets’ length. The formulas for the fields produced in air are derived with a three-layer model (air, seawater and seafloor) and are evaluated with a complementary numerical integration technique. A proof of concept measurement is presented. The ELF emissions from a surface ship were detected by inductive electronic and magnetic sensors as the ship was leaving a harbor. ELF signals are of substantial strength and have typical characteristic of harmonic line spectrum, and the fundamental frequency has a direct relationship with the ship’s speed. Due to the high sensitivity and low noise level of our sensors, it is capable of resolving weak ELF signals at long distance. In our experiment, a detection distance of 1300 m from the surface ship above the sea surface was realized, which shows that this method would be an appealing complement to the usual acoustic detection and magnetic anomaly detection capability.

Highlights

With the rapid development of ocean exploitation and military applications around the world, there has been a significant rise in the requirement for the measurement, surveillance and warning of the ship’s physical fields
Since the extremely low frequency (ELF) emissions are generated by the periodic modulation of the corrosion currents of the targets with their propellers’ rotating, we assume the ELF source can be modeled by a horizontal electric dipole (HED) with electric moment of Il at distances longer than several times of the target length, where I is the current in the shaft after modulation, and l is approximately equal to the distance from the propeller to the auxiliary anode
We assumed that the ELF field source of the underwater moving targets could be modeled as a HED submerged in sea

Summary

Introduction

With the rapid development of ocean exploitation and military applications around the world, there has been a significant rise in the requirement for the measurement, surveillance and warning of the ship’s physical fields. The corrosion related shaft-rate electromagnetic field source of the underwater moving targets has generated great interest It produces extremely low frequency (ELF) emissions, which arise from the modulation of the corrosion currents of the targets and are expected to be of substantial value in poorly maintained platforms [4,5,6,7,8,9]. These emissions offer unique advantages over MAD signals as follows: first, at ELF detection frequencies, the usual electromagnetic noise interference should be much less of an issue than in the MAD frequency band; second, ELF emissions have relative low propagation attenuation which can be detected at long distances or deep locations in seawater. The methods of signal processing and noise rejection will be considered in the phase work

Theoretical Modeling and Analysis

Numerical Integration Technique

Test Environment Description

Experimental Results and Discussions

Conclusion