Laser Hydrography

Not being an expert in Laser Hydrography I asked the two companies that recently used this technology in New Zealand to submit explanations on the systems for our readers. - Gary

Articles

Laser Airborne Depth Sounder - LADS

By Commander Mark Sinclair RAN (Ret)

Already proven as a cost-effective survey technique, in 1998/99 airborne laser bathymetry consolidated its role in hydrographic survey with new generation technology commercially available for survey worldwide. The new systems include significant upgrades to meet industry needs in terms of speed, depth, accuracy and flexibility. Case studies of surveys flown late in 1998 in environmental conditions as diverse as Norway, United Kingdom, Australia and New Zealand confirm the success of this technology, and suggest that airborne laser bathymetry is coming soon to survey coastal waters near you.

Developed throughout the 1970's and 1980's in Australia, North America and Europe, airborne laser bathymetry was proven in the early 1990's as a cost-effective survey technique for complex coastal waters. Since 1993 airborne laser bathymetry has routinely surveyed at least 50% of the total area surveyed by the Royal Australian Navy Hydrographic Service in Australian waters each year.

Recent Developments

In 1998 new generation systems were commissioned for commercial survey, providing more efficient, faster and more accurate survey at greater depths. These new systems are now deployed worldwide in a wide range of applications including nautical charting and in support of oil and gas exploration and production, EEZ baseline delimitation, oceanography, fisheries management, coral reef and marine resource management.

In this article, the example of the second generation of Australia's Laser Airborne Depth Sounder (LADS MkII) is used to demonstrate the updated capability and efficiency of the airborne laser bathymetry. Recent surveys with differing purposes, executed in Norway, Australia and New Zealand, are used as case studies.

Practical Capability of Laser Airborne Bathymetry

Airborne laser bathymetry involves the collection of accurate, high density bathymetric data using laser light pulsed from a fixed wing or rotary wing aircraft. Three-dimensional digital depth data is captured by measuring the time difference between returns of light reflected from the sea surface and from the seabed.

These airborne systems can survey at up to 10 - 20 times the rate of conventional surface vessels, at less than 20 - 30% of the overall cost.

The Royal Australian Navy (RAN) has used the original MkI version of LADS for the past 6 years in routine survey. The RAN LADS was specifically developed to address Australia's EEZ survey backlog, estimated at up to 100 survey years using conventional ship-based acoustic equipment.

Since 1993 the RAN LADS has surveyed over 50,000 square kilometres in Australian waters, and it is estimated that use of the system in concert with the RAN surface fleet will reduce the survey backlog to around 15 years. The RAN uses LADS data to produce Australia's IHO compliant chart series.

LADS MKII - the New Generation

Mounted in a deHavilland Dash 8-202 fixed wing aircraft, the upgraded version of the RAN LADS, known as LADS MkII was brought into commercial service in September 1998. In its first months of service, LADS MkII completed commercial survey assignments in Norwegian, Australian and New Zealand waters.

LADS MkII incorporates improved laser, computer and navigation technology in a faster aircraft platform, enabling the system to collect higher density data at deeper depths and at higher productivity. It is therefore significantly more cost effective than the RAN LADS and conventional survey vessels.

Following is a summary of LADS MkII's performance characteristics:

  • Sounding Rate - 900 Hz (3.24 million soundings/hour)
  • Area coverage - 19 sq nm/hour (64 square kilometres/hour)
  • Sounding density - 5m x 5m (2m x 2m, 3m x 3m, 4m x 4m capability)
  • Swath width - 240m
  • Depth range - 0.5m to 70m
  • Depth accuracy - S44 IHO Standard for Hydrographic Surveys - Special Publication of 4th Edition 1998, Order 1
  • Position accuracy - 5m CEP 95%
  • Data processing to data collection ratio - Better than 1:1
  • Output - Fairsheet plots and digital data in ASCII format

This improves significantly on the RAN LADS which had a sounding rate of only 168 Hz creating a 10m x 10m grid pattern, and a 50m depth capability.

In the past year, European and North American manufacturers have also been upgrading their systems to varying standards and specifications from their original operational versions.

Performance in Diverse Environmental Conditions for Diverse Tasks

The world's operational airborne laser systems can be deployed to sites around the globe. Some systems are designed for installation in 'aircraft of opportunity'. Others are permanently installed in larger aircraft with the capacity to deploy worldwide and execute long survey sorties or transit long distances to remote survey sites.

The first three commercial deployments of LADS MkII in 1998 show the versatility of airborne laser bathymetry across varied environmental conditions and for diverse survey tasks. In particular, these deployments show how the survey services can be tailored to meet specific needs, including:

  • Flexibility to survey at higher resolution sounding densities as required in complex Norwegian waters.
  • Efficiency to provide validated bathymetric scouting survey data to seismic vessels within 24 hours of acquisition, enabling uninterrupted seismic operations in Western Australia.
  • Fast mobilisation to site in the United Kingdom within 1-2 days of receipt of survey instructions.
Survey Operations - New Zealand

On Sunday 14 February 1999 LADS Mk II deployed to Invercargill New Zealand to conduct surveys for nautical charting for LINZ; LADS operated as a sub contractor to the RNZN. The area surveyed totalled over 1000 sq km. The survey areas included shallow sedimentary areas, rocky areas of coastline and deeper offshore areas strewn with hazards. Maximum depths of up to 55 metres were measured by the system.

Worldwide Coverage

In addition to Norwegian and Australian waters, reconnaissances for LADS Mk II surveys demonstrate that airborne laser bathymetry will be effective in many other parts of the world, including South East Asia, Middle East, Oceania, North America, South America and Western Europe.

The application of airborne laser bathymetry is more diverse than navigational charting and oil and gas exploration. In 1998/99 projects have been developed using laser airborne bathymetry to address significant marine management issues, including:

  • Baseline turning point and EEZ delimitation.
  • Fisheries management and coral reef mapping.
  • Coastal and marine resource management.
  • Environmental management and environmental impact assessment.
  • Strategic defence.
  • Port approaches.
  • Submarine pipeline planning and construction.
Cost-Effectiveness of the New Technology

As a LADS service provider we have found that our customers consider the LADS Mk II contract survey service, which is a fixed price service, to be very cost effective, particularly when areas are large or remote, complex or hazardous, or when the survey requirement is urgent.

As an owner of a LADS system, the RAN has experienced long term cost reductions (20 - 30 % of conventional survey costs) by using LADS Mk I to survey such areas.

Commander Mark Sinclair joined the Royal Australian Navy in 1977 and completed a Bachelor of Science Degree in Physical Oceanography at the University of New South Wales. He served 20 years as a seaman officer and as a hydrographic surveyor, during which time he commanded two RAN survey ships. With a graduate diploma in land data management at the Royal Melbourne Institute of Technology, Sinclair then served as officer-in-charge of the Royal Australian Hydrographic School followed by OIC of the RAN LADS system. He resigned from the RAN Hydrographic Service in December 1996 to join LADS Corporation and now holds the position of Survey Director.

Also refer to http://www.vsl.com.au

Hydrolink Carries Out High-Tech Hydrographic Survey

Hydrolink

New Zealand hydrographic surveying is being carried out using an advanced airborne surveying method never before deployed outside the American continent.

Recently established hydrographic company Hydrolink Limited has been surveying the Snares and Solander Islands using the airborne surveying method SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey). The SHOALS system - in its first application outside the American continent - is being operated from a Canadian Twin Otter aircraft, in conjunction with survey vessels which survey areas of deeper water.

Hydrolink Operations Manager Kevin Smith says that airborne surveying is extremely safe, fast and efficient.

"The Snares and Solander Islands are pretty rugged. So SHOALS is ideal for surveying the shallower areas close to shore, far safer than using a ship. It's also much faster than traditional vessel-based surveying, and meets the International Hydrographic Organisation's standards."

The airborne section of the survey has now been completed and the deeper water areas are being surveyed by boat.

Unlike most marine surveying methods - which use echo sounders to chart the seabed - SHOALS uses a laser which can penetrate water up to 60 metres in depth. The infrared beam from the laser is reflected off the water's surface, while the blue-green component of the laser beam penetrates the water and reflects off the sea floor. A computer calculates the depth from the time interval between the two readings.

SHOALS is also equipped with a geo-referenced and distance-calibrated video camera which records an image of the area as it is scanned by the laser.

After the airborne collection of data is complete, the raw lidar data is loaded for processing via a high density magnetic tape into the SHOALS Data Processing System, on a SUN SPARC computer workstation.

Hydrolink partner Fugro Survey Limited is using the Fugro-owned company John E Chance and the US Army of Engineers to operate SHOALS.

Hydrolink, which is jointly owned by Terralink NZ Limited and Hydrographic Sciences Australia, with partners Fugro Survey Limited and Seaworks Limited, was awarded three major contracts from Land Information New Zealand (LINZ) in 1998. The other contracts are the national contract for maintenance and distribution of New Zealand's hydrographic charts and a contract for hydrographic chart compilation and revision.

The Snares and Solander Islands are groups of islands west and south-west of Stewart Island, and form part of the base of New Zealand's 12 mile exclusive economic zone.

Information from Hydrolink Limited ksmith@hydrolink.co.nz
Also refer to http://shoals.sam.usace.army.mil


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