Summary

In shallow sea area less than 100 meters, it is hard to detect underwater vehicle by using the sonar system. In this situation, to detect an underwater anomalous object, geophysical electrical resistivity method, which uses active source, is introduced. We investigate detection possibility through three dimensional numerical modeling, where survey line length and anomalous object size are set to real scale condition. After confirming detectability by numerical modeling, physical scale modeling takes place in a water tank, where 1:200 scale experimental configuration is designed for the survey line used in numerical modeling. And by performing physical experiments, we validate the detection possibility of an underwater anomalous object using electrical resistivity survey method. Therefore through further study accompanied with onsite real scale experiments in shallow sea area, we expect that electrical resistivity method become an alternative way to detect an underwater object. And then it will be a good solution for underwater vehicle detection and tracking, harbor defence, and so on where the sonar detection is deteriorated due to complex undersea environment. In addition, developed element technologies will be able to be applied to civilian applications such as marine oil, gas, and mineral exploration, monitoring, etc.

Introduction

Electrical resistivity method was developed in the early 1900s, and has been widely used for mineral exploration, environmental monitoring, detection of underground water, archaeological prospecting, and so on (Reynolds, 2011). In recent years, applied research is also being carried out for monitoring purposes: geothermal, permafrost, CO2, landslide, dam, tunnel, and so on(Drahor et all, 2011; Kneisel et all, 2011; Schmidt-hattenberger et all, 2011; Chambers et all, 2011; Ahn et all, 2011; Ryu et all, 2011). Depending on the geometry of electrode configuration, there are several electrode array methods such as wenner, schlumberger, dipole-dipole, etc. And each of array has different spatial resolution, so appropriate array should be selected in consideration of purpose and field condition(Zhou et all, 2002; Dahlin et all, 2004). Recently, electrical resistivity method expands its field of application as well as geophysical survey. The latest geophysical electrical resistivity devices use bipolar DC as an active source, which enables the electrolysis mitigation, self-potential elimination, and efficient high-current and high-voltage transmission. And in the measurement electronics, 24bit sigma delta ADC(Analog-to-Digital Converter) with simultaneously 50/60Hz noise rejection by digital filter is used for high precision(uV or nV) measurement(ABEM, 2012). Therefore, we employ the latest electrical resistivity technologies and examine the detectability of underwater anomalous object. First of all, we confirm detectability by three-dimensional numerical modeling, and then physical scale modeling is carried out to validate the numerical modeling results.

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