Oil Spill Detection System Based on Marine X-band Radar

Sea Technology, Apr 2007 by Egset, Cathrine N, Nøst, Elisabeth

A verified system for automatic detection and real-time presentation of position, extent and drift of oil spills

The success of oil spill response operations at sea is highly dependent on continuous and reliable remote-sensing information. Accurate data on position, area and movement of oil on the sea surface is paramount in ensuring that mechanical oil recovery equipment, or chemical dispersant spraying equipment, stays within areas of combatable oil thicknesses at any given time.

This capability has to be independent of the prevailing visibility and light conditions. In modern oil spill emergency response, remote sensing of oil spills at sea is an integral part of the overall operation. This is due to the very limited visual over-view possible from low levels. In low-visibility situations, even advanced aerial and satellite surveillance sensors provide inadequate data.

Miros oil spill detection (OSD) system is the only operationally proven ship-based system that can ensure continuous oil spill detection in sea states Beaufort 2 to 6 independent of visibility and light conditions.

It utilizes raw data from standard X-band navigation radar and performs complex state-of-the-art digital processing to provide tactical presentation of the location and extent of oil spills.

Miros OSD

The Miros OSD system is an add-on to the Miros Wavex system, which derives scaled directional wave spectra and sea surface currents from X-band radar images. Additional environmental data, such as wind velocity and direction, are collected by the system and used for quality control. The Wavex system can be connected to most standard marine X-band radars.

The principle of oil detection is based on the fact that areas covered by oil reflect less microwave power due to the dampening of the sea surface capillary waves. Normally, wind speed must be 2 meters per second or more for capillary waves to be generated. The detection range is dependent on the installation height of the radar, which has to be at least 15 meters.

The collected data from the radar are subject to extensive processing in order to extract as much information as possible from the sea surface within the radar coverage area. The data are compensated for any movement or rotation of the system's installation site. Objects (such as ships) and noise present in the radar image are removed using various advanced object and noise removal techniques. The post-processing results in Back Scatter Intensity (BSI) images that provide information about the average radar BSI from the various areas of the sea surface. Areas with low intensity are more distinct in the BSl images than in the raw sea echo radar image. The BSI images are further processed using first- and higher-order statistical methods to achieve a binary image indicating whether or not there is oil present in a certain area. If the binary oil spill images (OS images) indicate that oil is present on the sea surface, the system automatically starts to track the oil spill and derives location, extent and drift. The tracking of the oil spill is represented in trace images where the OS images from successive detections are shown relative to the current position of the vessel.

The BSI, OS and Trace images are parts of the Miros OSD tactical display, along with wind, current and wave information.

The Field Trials

The OSD system was developed using data collected by the Wavex system onboard the vessel Johan Hjort during the deep spill trials in 2000. The system has since then been subject to extensive testing in full-scale field trials. A prototype of the Miros OSD system was tested during field trials in 2004 arranged by Norwegian Clean Seas Association for Operating Companies (NOFO) as one of several stand-alone oil detection systems.

The October 2004 trials were dedicated to testing of oil spill detection equipment. Six releases totalling 1 cubed meter of crude oil were made. The main objective for the OSD system was to verify that the ship-based radar system continuously, and in near real time, can determine the extent and location of oil on the sea surface within a range of one nautical mile in sea states corresponding to Beaufort 2 to 6. The wind speed varied between light and moderate breeze (Beaufort 2 to 4) and the OSD system provided reliable results under these weather conditions. The lower operational wind speed limit of 2 meters per second for the system was verified. Several of the oil spills were released in order to test the operational advantage provided by the system during darkness. The OSD system provided good quality BSI images with clearly visible oil spills. However, no automatic detections were made with the prototype version of the system. The OSD system hardware and software performance was continuous and stable, both with respect to real-time use and with respect to continuous storage of data required for post-processing. The collected raw data were a valuable input in order to improve the automatic detection and tracking algorithms of the system after the trials. The successful trials, along with promising post-processing results with the improved detection algorithms encouraged NOFO to buy a system for test in the 2005 oil on water exercise.