Vector Data Extraction from Forward-Looking Sonar Imagery for Hydrographic Survey and Hazard to Navigation Detection R. Glenn Wright Cheryl M. Zimmerman GMATEK, Inc., Annapolis, MD USA FarSounder, Inc. World Maritime University, Malmo, Sweden Warwick, RI USA [email protected] / [email protected] [email protected] Abstract— This paper describes research to determine the investigating its use as a means to navigate in uncharted and effectiveness of forward-looking sonar as a means to safely poorly charted areas are described along with their results navigate vessels in frontiers such as the Arctic and other regions illustrating the effectiveness of this technology and limitations that may be lacking recent or comprehensive hydrographic that may restrict its use. Conclusions regarding the viability of survey. Key elements of this investigation include the range at FL sonar to aid in vessel navigation are provided, along with which valid measurements may be taken, uncertainty in future directions in research likely to follow. measurement, confidence level of the measured value and resolution available to detect underwater hazards affixed to the II. CHARTING DEFICIENCIES bottom and suspended within the water column to provide time sufficient to enable the crew to take action to alter course and/or The deficiencies of hydrographic surveys in areas such as speed to avoid casualty. An additional factor involves examining the Arctic and the vast archipelagos of the Pacific Ocean have forward-looking sonar measurements as a means to survey been demonstrated through dramatic groundings of vessels. shallow sea bottom where hydrography data does not exist or is One example is the cruise chip M/V Clipper Adventurer that not accurate, potentially offering a valuable resource to ran aground on 27 August 2010 in Coronation Gulf, Nunavut supplement scarce national hydrographic office assets to in the Northwest Passage on an uncharted shelf in an area accomplish this task. An assessment of viability is also made where the depths of the waters were virtually unknown [1]. regarding compliance with the International Hydrographic Another is the United States Navy minesweeper USS Organization (IHO) standards for hydrographic surveys that form the basis for soundings that appear on navigation charts. Guardian that grounded on 17 January 2013 on Tubbataha Reef in the Sulu Sea near the Philippines due to a failure of Keywords— forward-looking sonar; navigation; nautical the crew to reconcile a discrepancy of the 4 to 8 mile distance charts; hazard to navigation; grounding; Arctic; reef between the charted and actual positions of the reef [2]. A most recent example is MSV Fennica where, on 3 July 2015 I. INTRODUCTION while enroute between Dutch Harbor Alaska and the Shell Oil Research is presently underway investigating the use of drilling field in the Chukchi Sea, the vessel was holed when it three-dimensional forward-looking (FL) sonar as a means to traveled near a previously uncharted rocky shoal [3]. enhance the safety of navigation to avoid vessel groundings in The significance of these charting discrepancies is greatly areas where hydrographic surveys may not exist, bottom heightened due to the increase in vessel traffic as a result of conditions have changed due to storm action, in natural energy exploration in the Arctic itself as well as transit traffic disaster and war zones where navigation infrastructure has seeking new trade routes between Europe and Asia. The U.S. been destroyed, where physical aids to navigation (AtoN) are Coast Pilot states that much of the Bering Sea area is “only not available due to ice movement, and amongst coral reefs partially surveyed, and the charts must not be relied upon too where sinkers to place AtoN cannot be used due to their closely, especially near shore. The currents are much adverse effect on the environment. The problem is defined in influenced by the winds and are difficult to predict; dead terms of navigation charts containing soundings that are reckoning is uncertain, and safety depends upon constant incomplete or have sparse coverage making them unreliable, vigilance” [4]. especially in the Arctic but also problematic worldwide. A review of IHO standards for hydrographic survey is provided, The problem being addressed is illustrated by Figure 1, identifying the “order of survey” to which such data products which represents a portion of official National Oceanographic must conform. A summary of the current state-of-the-art in and Atmospheric Administration (NOAA) marine navigation multibeam and multiple transducer sonar systems for chart: #16103 for Cape Beaufort, Alaska that is effectively hydrographic survey is provided, followed by a description of devoid of soundings information. This is typical of many current FL sonar technology with a comparison of their official nautical charts in the area where few, if any, soundings similarities and differences in capabilities. Experiments exist exemplifying the difficulties involved in ship navigation. 978-0-933957-43-5 ©2015 MTS Indeed, most soundings that do exist were taken during World This same concern also extends to the Chukchi Sea in the War 2 when the first (and probably last) of these surveys was Arctic Ocean within which Royal Dutch Shell is pushing performed. The area represented in this chart situated on the ahead with plans to explore for oil with two or three wells northern slope of Alaska is regular and featureless, sloping using as many as 30 or so vessels to support their activities. gradually from the shoreline with low and grassy inland Sparse hydrographic coverage in a highly travelled area can terrain. Assumptions may be made, with various degrees of lead to an unwarranted level of confidence in chart content accuracy, that the same land topography may also extend into that may result in an accident. Such a theory has already the sea resulting in a fairly regular bottom. This theory is gained support through the holing of MSV Fennica. The supported by other portions of this chart for which soundings official NOAA chart near Dutch Harbor Alaska, shown in do exist that depict such bottom topography. Figure 3, depicts water sufficient for navigation where an uncharted shoal exists and holing occurred. Fig. 1. Portion of NOAA Chart 16103, Cape Beaufort, Alaska. However, of much greater concern are areas that are only partially surveyed that show highly irregular bottoms. Such an Fig. 3. Portion of NOAA Chart 16530, Captains Bay, Unalaska Island. (soundings in fathoms) area is shown in Figure 2, with partial soundings where there is high potential for hazards to navigation to exist in the blank Given the inaccuracies and the unreliable nature of portions of the chart. navigation charts in these areas it is prudent for mariners to consider the use of three-dimensional FL sonar to display real time hydrographic imagery of the underwater environment directly ahead of the vessel that may help to avert a grounding, or worse: casualty that results in loss of life or a major oil or chemical spill. III. INTERNATIONAL HYDROGRAPHIC ORGANIZATION STANDARDS The IHO establishes international minimum standards for the performance of hydrographic surveys that are used as the source of soundings information on navigation charts by national hydrographic authorities. The currently established guidance is the February 2008, IHO Special Publication No. 44, IHO Standards for Hydrographic Surveys [5]. Hydrographic surveys are performed based upon the “orders of survey considered acceptable to allow hydrographic offices / organizations to produce navigation products that will allow the expected shipping to navigate safely across the areas surveyed” [6]. The most rigorous of the orders is the Special Fig. 2. Portion of NOAA Chart 16540, Shumagin Islands to Sanak Islands, Order survey intended for use where under-keel clearance is Alaska Peninsula. (soundings in fathoms) most critical, generally in waters less than 40 meters in depth. As the purpose of FL sonar is to detect hazards to navigation IV. HYDROGRAPHIC SURVEY TECHNOLOGY that may jeopardize safe navigation, one would presume that Modern hydrographic surveys are performed using data obtained should be in accordance with the requirements multibeam echo sounders that transmit hundreds of beams of for Special Order surveys. The next level of survey is Order sound through the water to acquire water depth information in 1a, “intended for those areas where the sea is sufficiently a survey area; capturing images of the seabed and determining shallow to allow natural or man-made features on the seabed least water depths over critical items such as wrecks, to be a concern…” The elements that comprise these standards obstructions, and dangers to navigation; and to detect objects relevant to this discussion are summarized in Table I and in general. Using this method a “swath” of soundings (i.e., include horizontal and vertical uncertainties to specified depths) is produced across an area of approximately two to confidence levels. four times, and up to twelve times the water depth depending Total Horizontal Uncertainty (THU) or position upon the instrument. uncertainty is determined based upon a reference to a datum, e.g., WGS84, consisting of the combined contributions of all Measurements can be taken using a signal generated at uncertainty sources using a statistical method to establish the the vessel in a fan-shaped pulse that propagates vertically position uncertainty at the 95% confidence level. This would down to the sea bed and is reflected back to the vessel. A also apply to hazards to navigation suspended within the water towed-array following a vessel from which measurements are column. taken may also be used. Measured values include depth along with heave, roll, pitch, yaw, positioning and timing to Likewise, Total Vertical Uncertainty (TVU) or depth precisely determine water depths and locations. uncertainty is the uncertainty of the reduced depths determined based upon reference to a vertical datum, There are many manufacturers of multibeam echo consisting of the combined contributions of the individual sounders used extensively on a variety of hydrographic survey uncertainties.