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Feature Article

Low-Frequency Synthetic Aperture Sonar For Detecting Explosives in Harbors
MUD System Trials Show Potential for Revealing Objects Buried Under Mud, Sand

By Dr. Guus Beckers
Senior Consultant
Dr. Robbert van Vossen
Research Scientist
The Hague, Netherlands

Lt. Gert Vlaming
Project Officer
Underwater Technology Research
Defence Materiel Organisation
The Hague, Netherlands

Naval mines and underwater improvised explosive devices in harbors can halt or block maritime transport, causing local disruptions that can impact national and regional economies considerably. Securing harbors against such threats is implemented in the Netherlands through cooperation between civil and military authorities.

Existing commercial high-frequency sonar systems can detect and classify underwater explosives, however, these systems are not capable of detecting objects that have become covered by mud or sand because high-frequency sound does not penetrate into the harbor bottom. To detect and identify buried objects, alternative techniques have to be applied. Magnetometry and low-frequency sonar are two candidate solutions.

The Netherlands Organisation for Applied Scientific Research (TNO) has made progress in detecting buried objects, particularly in mud, with the development of a low-frequency side-looking synthetic aperture sonar (SAS) named the Mine Underground Detection (MUD) system. By implementing low-frequency sonar within a side scan system, the TNO has found that mud-penetrating detection with a considerable swath width becomes possible. Even at a speed of 2 to 4 knots, significant area coverage rates can be obtained.

The wet-end part of the MUD system with two orthogonal receiver antennas is shown.

Low-Frequency SAS System
The MUD is designed to be flexible, so setup parameters, such as the frequency band and the tilt angle of the system, can be varied. Its hardware is composed of acoustical sources (1 to 4 kilohertz, 4 to 9 kilohertz and 11 to 26 kilohertz) and two receiving arrays—a horizontal line array and a vertical line array—each comprised of 16 hydrophones. The components are mounted on a frame that is adapted for operation from a Royal Netherlands Navy diver-support vessel.

In order to monitor the position and attitude of the sonar system, two nonacoustical sensors were placed on top of the support frame: a PolaRx3 real-time kinematic GPS from Septentrio Satellite Navigation (Leuven, Belgium) and an iXSea (Cambridge, Massachusetts) PHINS inertial navigation system for recording the accelerations and rotation angles of the system in the three directions in real time. Accurate positioning is essential for advanced signal processing and SAS processing.

Digital acquisition of all data streams was synchronized. The MUD software has a signal-processing chain consisting of data conditioning, matched filtering, multipath suppression and SAS processing for resolution enhancement and signal-to-reverberation optimization.

MUD Trials
The MUD system was evaluated in the Haring'vliet estuary, 30 kilometers south of Rotterdam, Netherlands. In this area, TNO created a test that contained 20 targets, including objects of different materials made in the shape of sea mines, the NATO Undersea Research Centre's EVA cylinder (an epoxy-filled, 400-kilogram, nonmetallic object), boulders, Mk82 bombs (500 pounds), Mk84 bombs (2,000 pounds), 155 mm shells, heavy power cable and a chain. Targets were distributed along three parallel lines at a depth of 13 meters, with the average distance among them at 25 meters. Their position was measured with a hand-held GPS.

The bottom of the test location consisted of sand that is covered with a layer of mud, which varies in thickness. The largest part of the test site was designed to have most of the targets buried, however, some were to be left slightly raised above the estuary's bottom for reference. Most targets were buried on impact, but divers assisted in burying some targets by removing the sediment below them. The divers verified burial depths of at least 10 to 20 centimeters for a majority of the targets, 44 centimeters for the EVA cylinder and unknown burial depths for a few targets that were deployed in an area where the mud was more than 1 meter thick. A settlement period of six months was taken into account before the MUD trial runs, which took place in April 2011.

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Dr. Guus Beckers received his Ph.D. in medical informatics at the Erasmus University Rotterdam after an applied physics education at the Delft University of Technology. He joined TNO in 1996 and acts as program manager and consultant, overseeing the organization's activities on mine countermeasures.

Dr. Robbert van Vossen received his Ph.D. in seismic signal processing at Utrecht University in 2005. In 2007, he joined the sonar department at TNO, where he is project manager and research scientist specializing in sonar signal processing and data analysis.

Lt. Gert Vlaming received his master's degree in earth sciences at Utrecht University in 2006. After graduating, he joined the Royal Netherlands Navy. He is currently project officer for underwater technology at the Netherlands Defense Materiel Organisation.

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