Airborne Lidar Hydrographic Survey for Homeland Security

Sea Technology, Mar 2008 by Sinclair, Mark

Remote, Uncharted Areas in Torres Strait Present Unique Challenges to the Hydrographic Surveyor

In early 2007, an extensive airborne lidar hydrographie (ALH) survey was completed in northern Australian waters. During a period of three months, a total area of 5,800 square kilometers was surveyed in Torres Strait and northern Great Barrier Reef waters. The survey was conducted for the Australian Hydrographie Office (AHO) by Tenix LADS Corp. (TLC) using the laser airborne depth sounder (LADS) Mk II system. The purpose of the survey was to facilitate cafo navigation by high-speed customs, fisheries and surveillance vessels and to update nautical charts.

Torres Strait is a shallow body of water situated between the Coral and Arafura seas.

The survey of an area of 750 square kilometers in northern Torres Strait was of particular importance because the bathymetry within this area was unknown. No soundings or isobaths were displayed on the nautical chart. The area was particularly challenging due to its remoteness, shallow depths and complex tidal regime. The average water depth is only five meters below datum, the range of tide exceeds five meters and very strong tidal streams run through the area. The seabed consists of coarse sand, and powerful hydrodynamic processes have created large sand wave fields throughout the area, some of which dry at low water levels.

Lidar was selected as the preferred technology because multibeam echo sounders in a surface vessel would have been too slow and expensive in this extensive, shallow area. The area presented unique technical challenges due to environmental conditions, particularly the weather and water clarity.

The weather is dictated by the seasonal north-south movement of the Intertropical Convergence Zone. During the southern hemisphere's winter, or dry season, southeast trade winds blow continuously across the area and generate steep seas that stir up seabed sediment and increase turbidity. During the southern hemisphere's summer, or wet season, when cyclones occur, heavy rainfall increases sediment discharge from the Fly River and other tributaries on the southern coast of Papua New Guinea. A turbidity management plan was developed to control these factors.

LADS System

The survey was conducted using the LADS Mk II system, installed in a Dash 8-202 series aircraft. This system is distinct from the Royal Australian navy (RAN) LADS system, installed in the navy's Fokker F27 aircraft. The RAN system is being replaced by a new system of similar capability to LADS Mk II, which at the time of writing is about to be installed in the F27 aircraft.

The basic operating concept is a pulsed neodym i urn-doped yttrium aluminum garnet green laser which is scanned below the aircraft. At the sea surface, the footprint of each laser spot is approximately 2.5 meters in diameter, and as the beam passes through the water, it slowly diverges due to scattering. Reflected laser returns from the sea surface, water column and seabed are captured by a green receiver, digitized and logged for subsequent calculation of position and depth.

The most important characteristics of the system for operations in Torres Strait were the high-power laser, wide-aperture green receiver and high-gain photo-multiplier tube under automatic gain control. These fundamental system design features enabled the collection of high signalto-noise ratio waveforms, essential in the marginal conditions of Torres Strait.

The programmable scanning LADS system enables the spacing between each laser sounding to be maintained independently of the aircraft operating height. This is important when the survey altitude is varied.

For example, near high ground and at night, operations can be conducted above 2,000 feet for safety considerations, but in conditions of low clouds, operations can be conducted below the cloud base at 1,200 feet.

This feature was particularly important during survey operations in Torres Strait prior to the onset of the wet season, when low cloud bases are common.

A number of selectable sounding patterns are provided to satisfy different survey requirements. For example, a 2.5 by 2.5-meter laser spot spacing mode provides the capability to detect International Hydrographic Organisation (IHO) Order 1 size objects (two by two-meter cube) in a single pass in average water conditions (a signal-to-noise ratio of raw laser waveforms of greater than 24 decibels). The five by five-meter laser spot spacing mode of operation satisfies the Order 1 b requirements of the new IHO S-44 standard, which is currently awaiting ratification. Other sounding patterns are also provided.

In Torres Strait, mainline sounding was conducted using the five by five-meter mode, and critical shoals in the vicinity of navigable routes were examined using higher density modes.

The LADS Mk II system has been upgraded through continuous technical development. During the data collection phase of this project, TLC engineers set a new digital camera to work, investigated improvements to the near-land mode of operation, conducted laser control and acquisition computer network monitoring and tested a new infrared receiver and a new laser. This work did not interfere with data collection or the quality of the results. An improved digital imagery capture and processing system and algorithms to calculate the optical reflectivity of the seabed have subsequently been implemented into the LADS Mk II system.