LBL Underwater Positioning

Long baseline (LBL) positioning has many applications both commercially and in research, from surveying a ship hull to positioning offshore platforms in deeper waters. The technique consistently provides accuracies in the order of decimetres over large areas, independent of depth. Recent advances include the integration with other acoustic techniques or with inertial navigation systems for greater levels of accuracy. Since navigation by GPS isbased on electromagnetic signals that cannot penetrate water, subsea positioning requires a different approach. As sounds waves travel through the sea they alternately compress and decompress the water molecules; these compressions/decompressions are detected as changes in pressure.... (read more)

2008-01-19 12:00:00

USGS Tethered ACP Platforms

New design means more safety and accuracy

The US Geological Survey has developed an innovative tethered platform that supports an Acoustic Current Profiler (ACP) in making stream-flow measurements (use of the term ACP in this article refers to a class of instruments and not a specific brand name or model). The tethered platform reduces the hazards involved in conventional methods of stream-flow measurement. The use of the platform reduces or eliminates time spent by personnel in streams and boats or on bridges and cable-way and stream-flow measurement accuracy is increased. One vital mission of the US Geological Survey (USGS) is to provide flow data for streams across... (read more)

2008-01-01 01:00:00

Multibeam Mapping of Continental Margin Morphology

Bathymetry in understanding oceanic & continental evolution

A jigsaw puzzle of lithospheric plates comprises the outer shell of the earth. Oceanic plates, formed by ascending magma at mid-ocean ridges and transported towards the continents by internal convective forces, collide with continental plates at convergent margins and are subducted beneath them. This is accompanied by violent geological processes such as earthquakes, volcanic eruptions, submarine landslides and tsunamis. The speed of drift of the plates is in the order of several centimetres per year, too slow to be observed directly. Still, these processes shape the surface of the ocean bottom and submarine morphology records these forces acting on the... (read more)

2008-01-01 01:00:00

USBL in Dam Survey by ROV

An USBL underwater positioning system is the ideal solution for dam survey by ROV, providing easy and fast set-up and accurate 3D measurement for traceability, but not all USBL systems can meet the demands of this environment. PLSM (France) has developed a light and polyvalent system for use in shallow water down to 300 metres. The author describes AQUA-METRE R300 and some typical results. In France, ten-yearly dam inspection is mandatory for medium to large dams. There are about three hundred dams between the Pyrenees and Brittany. The main goal is to survey the submerged concrete wall in order to... (read more)

2008-01-01 01:00:00

Surveying Deep-water Seeps at Convergent Continental Margins

On geological time scales, the floor of the ocean is young compared to the continents, as it is continuously recycled by internal convective forces. Oceanic lithosphere forms by ascending magma at mid-ocean ridges and is consequently transported towards the continents. At convergent margins it is subducted beneath them and ultimately transferred back into the Earth’s interior. During their lifetime the oceanic lithosphere becomes cooler and heavier, and increasingly more sediments are deposited on top of them. During subduction, the oceanic crust with its overlying sediments is brought to great depths and exposed to high pressure and temperature. As a result... (read more)

2008-01-01 01:00:00

BlueTEC Texel

A Floating Tidal Energy Platform Prototype

Traditional solutions for generating electricity from the tides, where tidal turbines are installed on the seabed, require high-end offshore installation vessels and do not allow for easy access for maintenance. The BlueTEC floating tidal energy platform has a different approach. It is a permanently moored platform which generates renewable electricity from tidal currents by holding a tidal turbine underneath. The BlueTEC Texel is the first prototype and serves as a demonstration platform and is being used for R&D testing purposes. The project was realised through a partnership between companies active in the maritime and offshore industry; Bluewater, Damen Shipyards, Van... (read more)

2016-08-25 12:03:44

Surveying the Florida Reef

The Florida Reef became a menace to shipping in the 1500s when the Spanish discovered that the safest way home for their New World gold and silver was north through the Florida Straits. With the Cay Sal and Bahama banks to starboard, the reef, arcing along the Florida Keys, to port, and the Gulf Stream on their stern, first the treasure fleets and later thousands of European ships made the passage through the ‘New Bahama Channel’. As the territory west of the Appalachians and then west of the Mississippi was settled, American ships began to fill the Straits as well,... (read more)

2012-07-26 03:14:24

The Order of Canada

Interview with Mike Eaton, CHS Hydrographer and Mariner (Rtd)

There were three major overlapping phases in Mike Eaton’s career: Arctic developments in the Canadian Hydrographic Service, positioning systems development while at the Bedford Institute and electronic chart development after his retirement. In the Arctic he found ways to acoustically measure the depth beneath the ice and developed a rather hairy towfish sounding method from a helicopter in open waters. At the Bedford Institute he worked with Paul Brunavs and Dave Gray on the propagation of the radio ground waves used by Decca, Loran-C etc. Later he became deeply involved in the initial specifications of the electronic chart, and was... (read more)

2008-01-01 01:00:00

Depth of Cable Cover Gets New Look

New Technology Reduces Cost

Burying utility cables under the seabed in shallow water environments to ensure cable safety is accelerating because of green power generation initiatives and the relative ease of securing permits to lay cable in rivers or estuaries compared to onshore right-of-ways. In locations with shifting sands and bottom profiles, verification that the utility line remains covered is often required. A new remote sensing method has been developed to accomplish depth-of-cover utility surveys at greatly reduced cost. Maintaining underwater utility infrastructure varies with water depth. ROVs are generally used in deep water. In less than 50 metres, such as nearshore, estuaries, bays,... (read more)

2011-07-15 04:13:41

Lidar Seafloor Classification

New Application for HawkEye II Airborne Lidar System

Airborne Lidar (Light Detecting and Ranging) is a well-established remote sensing technique which scans the terrain and seafloor with high-frequency laser pulses. Until now, bathymetric Lidar data has been processed to generate marine depth information and seafloor topography. However, Airborne Hydrography AB (AHAB) has developed a new method of processing data which allows information about the actual properties of the seafloor to be extracted. The accuracy and efficiency of reflectance imaging for seafloor classification is demonstrated here with data from coastal sites in both Massachusetts and Sweden. By Xin Liu, Andreas Axelsson and H. Michael Tulldahl, Sweden. With its advantage... (read more)

2011-05-17 04:02:56