Application of a Drone Magnetometer System to Military Mine Detection in the Demilitarized Zone.

We propose a magnetometer system fitted on an unmanned aerial vehicle (UAV, or drone) and a data-processing method for detecting metal antipersonnel landmines (M16) in the demilitarized zone (DMZ) in Korea, which is an undeveloped natural environment. The performance of the laser altimeter was improved so that the drone could fly at a low and stable altitude, even in a natural environment with dust and bushes, and a magnetometer was installed on a pendulum to minimize the effects of magnetic noise and vibration from the drone. At a flight altitude of 1 m, the criterion for M16 is 5 nT. Simple low-pass filtering eliminates magnetic swing noise due to pendulum motion, and the moving average method eliminates changes related to the heading of the magnetometer. Magnetic exploration was conducted in an actual mine-removal area near the DMZ in Korea, with nine magnetic anomalies of more than 5 nT detected and a variety of metallic substances found within a 1-m radius of each detection site. The proposed UAV-based landmine detection system is expected to reduce risk to detection personnel and shorten the landmine-detection period by providing accurate scientific information about the detection area prior to military landmine-detection efforts.

Passive Source Localization Using Acoustic Intensity in Multipath-Dominant Shallow-Water Waveguide.

The array invariant technique has been recently proposed for passive source localization in the ocean. It has successfully estimated the source-receiver horizontal range in multipath-dominant shallow-water waveguides. However, it requires a relatively large-scale hydrophone array. This study proposes an array invariant method that uses acoustic intensity, which is a vector quantity that has the same direction as the sound wave propagating through a water medium. This method can be used to estimate not only the source-receiver horizontal range, but also the azimuth to an acoustic source. The feasibility of using a vector quantity for the array invariant method is examined through a simulation and an acoustic experiment in which particle velocity signals are obtained using a finite difference approximation of the pressure signals at two adjacent points. The source localization results estimated using acoustic intensity are compared with those obtained from beamforming of the acoustic signals acquired by the vertical line array.

Measurements of mid-frequency bottom-interacting signals and geoacoustic inversion in Jinhae Bay, Southeast Korea.

Measurements of bottom-interacting signals over a frequency range of 4-12 kHz were made at two different sites in shallow water off the southeast coast of Korea. Although the distance between the sites was less than 20 km, the geological properties of surficial sediments were different. The surficial sediment at the site located on the north side was dominated by mud, and the measured bottom loss showed typical characteristics of sediment with a sound speed lower than the water sound speed. In addition, some interesting arrivals, which seemed to be reflected from a sub-sediment interface, were observed immediately following bottom arrivals in the frequency range of 4-8 kHz of some datasets. In contrast, sediment at the site located to the south was dominated by sandy mud, and the bottom loss showed typical characteristics of sediment with a higher sound speed. Geoacoustic parameters of surficial sediment were estimated using a simulated annealing method with an objective function comparing the measured bottom loss data with the Rayleigh reflection model. Surficial layer thickness was estimated based on arrival-time differences between bottom reflected and sub-bottom reflected signals. Inversion results are discussed in comparison with a bottom model constructed based on seismic data and core samples.