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Friday, January 13, 2017
Effective surveying tool for shallow-water zones
Eomap is patenting technology that can map the water depth and chlorophyll content of lakes in satellite photographs, providing quality control for environmental projects that clean algae from lakes.
Such software processing is challenging to describe in patents, but the company made the investment to protect their innovations from competitors.
From Hydro by Dr. Knut Hartmann, Dr. M. Wettle, Dr. Thomas Heege, EOMAP GmbH & Co.KG, Germany
A recent article provides an overview of satellite-derived bathymetry methods and how data can be integrated into survey campaigns, and showcases three use cases.
Bathymetric data in shallow-water zones is of increasing importance to support various applications such as safety of navigation, reconnaissance surveys, coastal zone management or hydrodynamic modelling.
A gap was identified between data demand, costs and the ability to map with ship and airborne sensors.
This has led to the rise of a new tool to map shallow-water bathymetry using multispectral satellite image data, widely known as satellite-derived bathymetry (SDB).
Figure 1: The diagram shows the relative amount of measured light energy that contains water depth information.
Strictly speaking, the methods to derive information on seafloor topography using reflected sunlight date back to the 1970s but it has required iterative improvements of algorithms, computational power, satellite sensors and processing workflows to provide the current state of the art tool.
Today, a range of different methods exist under the umbrella of the SDB term.
However, as with traditional survey methods, it is imperative to understand the advantages, disadvantages and overall feasibility in order to evaluate the suitability and fit-for-purpose of a given SDB application.
Bathymetric Data Production using Optical Satellite Imagery
Historically, empirical methods were used, which require known depth information over the study area.
By comparing these known depths with the satellite signal, a statistical relationship can be derived that adequately describes depth as a function of the signal.
Aside from requiring known depth data, these methods will only work for a given satellite image.
A subsequent satellite scene, even of the same location, may contain different atmospheric and in-water parameters, and thus the statistical relationship needs to be re-calculated.
Another aspect of these methods is that the statistical relationship is only valid for one water type and one seafloor type.
Therefore, if an area contains different types such as coral, sediment, algae and rubble, the statistical relationship needs to be calculated for each of these substrate types.
The correct formula then needs to be applied to each pixel in the image, i.e.
the algorithm needs to be informed a-priori which substrate type it is encountering in that image pixel.
This brings the problem full circle back to one of the fundamental challenges of satellite-derived bathymetry: how do you know whether a darker signal is due to deeper water, a darker substratum, or a bit of both?
These methods can still be useful as they are relatively straightforward to implement (see The IHO-IOC GEBCO Cook Book, 2016).
Physics-based methods on the other hand, do not require known depth information for the study area, and can therefore be applied independent of satellite data type and study area.
These methods rely on fully describing the physical relationship between the measured light signal and the water column depth.
Optical variability in the atmosphere and water column is accounted for within the algorithm inversion, and no 'tuning' to known depths is required.
Therefore, an area which is physically inaccessible and for which there is no previous information known can be targeted.
Not surprisingly, these physics-based methods require more sophisticated algorithms and powerful processing capacity.
The benefit is that they typically prove to be more accurate, especially in areas with varying substrate types, turbidity and/or atmospheric conditions.
This is of particular importance because only a small fraction of the sunlight recorded by the satellite’s sensor originates from the source that can be associated with water depth.
Depending on the wavelength channel, this fraction varies typically between less than one and up to a maximum of 20%, going from near-infrared to green/blue light energy.
It is critical to accurately account for the other sources of light energy in order to separate out the relevant water column depth contribution to the measured signal.
Data Integration
The integration of SDB data into daily use can be straightforward if the bathymetric data quality and delivery formats follow best practice.
Hence the file formats typically follow industry standards (OGC) and enable a direct use in current GIS or online visualisation tools through Web Mapping of Coverage (WMS, WCS) interfaces, hydrographic software or scripting tools.
ISO conform metadata including important information on tidal corrections, processing levels and date and time of satellite recording are essential for geodata and are mandatory for all SDB data.
Furthermore, it is important to understand the uncertainties in the data as well as the limitations of SDB for a given application in order to integrate the data appropriately.
Such information needs to be expressed in uncertainty layers which should ideally include quantitative information.
For some applications, such as safety of navigation, additional information such as the ability to identify obstructions of different sizes needs to be included as well.
Use Case: Safety of Navigation
Satellite-derived Bathymetric information supports safety of navigation by providing up-to-date and high-resolution grids of the shallow-water zone.
This is of particular importance in areas with outdated charts or dynamic seafloor.
In addition to the bathymetric information, of particular importance is the identification of obstructions which could be a risk to navigation.
Figure 2: Current ENC (March 2016, left) and overlaid by SDB data (right) showing shoals misplacement and low details of the ENC compared to the Satellite-derived Bathymetry-ENC.
Ideally the bathymetric data are provided in the form of digital nautical charts (ENCs) and ECDIS (Electronic Chart and Display System) as the main navigation device which represents the standard for the majority of vessels.
Satellite-derived Bathymetry data cannot immediately be used for navigation with ECDIS – however, it can serve as an additional data source when updating the bathymetric information of nautical charts (paper or digital).
ENC Bathymetry Plotter, a recently finished software product of SevenCs’ chart production suite, represents a powerful tool to create depth-related information objects for inclusion in ENCs which fulfill all relevant IHO quality standards.
SevenCs and EOMAP have teamed together to provide an innovative service, the combination of up-to-date shallow water bathymetry provided as a standard ENC.
This can therefore be used immediately on board vessels.
An update of official ENCs which include Satellite-derived Bathymetric data, is therefore possible at the commencement of a voyage, but also during the vessel’s journey - via satellite communication - and therefore allows for the planning of more efficient shipping routes, increased safety as well as an improved situational awareness to react to a forced change of the shipping route (e.g.
weather events or other threats).
It is obvious that the need for updating ENCs for safety of navigation is of importance for poorly mapped areas.
It should not be understood to replace recent, high-resolution and quality ENCs if available.
In 2016, bathymetric data was provided to Van Oord covering several atolls in The Maldives.
The data were used to enhance safe navigation by charting all shoals which might or might not be indicated on Electronic Navigation Charts.
This contributed to efficient planning of the project’s activities.
Data were provided within a few days of ordering covering an area of several hundred sq.
km, which showcases the flexibility of the technique.
Figure 3: Baseline data on seafloor information based on satellite images and physics-based algorithms.
Use Case: Reconnaissance Survey
Satellite-derived Bathymetry can play a role as a reconnaissance survey tool in applications ranging from shallow-water seismic surveys, coastal engineering to optimal planning of acoustic surveys.
Although different in usage, all of these applications have in common that they require bathymetric data which is :
- spatial
- high resolution
- rapidly available
- affordable within a typical planning phase budget.
Many examples for these kinds of applications have already been published and two showcases are summarised in the following paragraphs.
In 2013, EOMAP mapped the shallow-water bathymetry of the entire Great Barrier Reef, Australia, at 30m grid resolution.
This was the first depth map of its kind for the entire Great Barrier Reef, and also the largest optical SDB dataset ever made.
In 2014, Shell published a paper on the use of EOMAP’s Satellite-derived Bathymetry (delivered at 2m grid resolution) to support their shallow-water seismic campaign in northwest Qatar (Siermann et al.
2014).
Shell summarised the benefits of using the satellite techniques over more traditional methods by citing a 1 Million USD costs savings and very timely delivery of the data.
Figure 4: Example of the seamless multisource bathymetric grid for the Persian Gulf, including Satellite-derived bathymetric data (left) and the GEBCO dataset (right).
Use Case: Basis Data for Hydrodynamic Modelling
Hydrodynamic modelling exercises, such as generating tsunami forecast models, are typically not the type of applications with budgets that allow for purchasing bathymetric survey campaigns using more traditional methods.
Commonly, very coarse resolution bathymetric grids such as GEBCO are used instead, but this has limited validity in coastal areas.
By using Satellite-derived Bathymetry, shallow-water depth data can be derived at fit-for-purpose grid resolution to within a limited budget.
As a standalone dataset it does not fulfil the modellers requirements but when merged with up-to-date information on the coastline – (also derived from the satellite imagery), survey and chart information, a seamless shoreline-to-deep-water dataset can be created, which greatly improves on currently available datasets.
Such a dataset was created for the Gulf region, which now serves as bathymetric dataset for tsunami modelling in the area.
Future Perspectives
Over the intermediate term it is expected that satellite-derived mapping of the seafloor will continue to be increasingly accepted and integrated as a survey tool - as is now already the case for a number of innovative user groups.
Developments are still needed in areas such as how to best quantify uncertainties and small scale obstructions.
One likely development will be the mutlitemporal and sensor agnostic mapping approach, which can be oversimplified as: use all available image data to the best possible extent and quality.
With the advances of cloud computing, physics-based algorithms and an increasing selection of image data, this is would be a natural evolution for Satellite-derived Bathymetry.
Links :
- GeoGarage blog : Evolution of ocean exploration: mapping the seafloor with ... /Satellite images are source for first-of-its-kind charts of ... / Studying the use of satellite-derived bathymetry as a new ... / Keeping false pass true / Avoiding rock bottom: How Landsat aids nautical charting
- YouTube : 2013 LEAD Webinar: Satellite Derived Bathymetry Webinar
Thursday, January 12, 2017
Communicating under sea ice
The long-range under-ice sound communication system developed by WHOI engineer Lee Freitag and his colleagues.
In the Arctic Ocean, a cold water layer bounded above and below by warmer layers acts as a "sound duct" that channels sound waves over long distances.
Sound beacons suspended in the channel emit information-carrying sound signals that travel to other buoys and to autonomous underwater vehicles under the ice.
Data is relayed from the buoys to scientists via satellite.
(Illustration by Eric Taylor, WHOI Graphic Services)
Engineers use ocean channel to efficiently relay sound
anks Island is one of 36,563 ice-covered islands sprinkled in the Arctic Ocean north of Canada.
It is home to the world’s largest population of muskoxen—about 68,000—one tiny village with a population of slightly more than 100 people, and an airport, which during the spring and summer of 2014 bustled with researchers poised to jump into the vast white Arctic.
Peter Koski and John Kemp, two engineers at Woods Hole Oceanographic Institution (WHOI), waited in the isolated village through days of high winds and frozen fog.
Finally, on a Saturday in March, the weather cleared.
A pilot gave the OK.
Then pilot, co-pilot, mechanic, Koski, and Kemp took off and flew out over the frozen Beaufort Sea in a small red Twin Otter plane packed full of cables and buoys.
During two days of hectic, hopscotching flights, taking off and landing on patches of floating sea ice, they set up equipment at eight remote sites to carry out a long-awaited experiment.
Their goal was to establish a long-distance communications system that would transmit and receive signals under water and under ice.
Like the telegraph in the Old West, such a system could open up this previously inaccessible ocean to exploration, allowing fleets of autonomous underwater vehicles to navigate and collect data in ice-covered areas where ships and people cannot easily go.
Such data are essential for scientists and the Navy to gain better understanding of the Arctic, a critical region for both environmental and military reasons, that is rapidly changing.
The key to the experiment lay in taking advantage of a naturally occurring layer of water that forms in the Arctic and efficiently channels sound over long ranges—a sound duct within the ocean.
Scientists and the Navy had exploited similar sound ducts in other oceans to measure the water temperatures and find distant submarines.
Would it work in the Arctic Ocean, where the upper 3,280 feet (1,000 meters) of the ocean is completely different than anywhere else in the world?
A possible future integrated acoustic communications system in the Arctic.
Autonomous vehicles and gliders transmit data via sound signals to transponders suspended beneath the ice.
The transponders send the data to the buoys antennas, and from there via satellite to scientists in other locations.
Scientists can control vehicles movements by communicating via satellite to the buoys, which send sound signals to the vehicles.
(Painting by E. Paul Oberlander, Woods Hole Oceanographic Institution)
Sound pipelines within the ocean
Many transmission options available on land, such as light and radio waves, don’t work under water.But as whales know well, sound travels far under water, especially low-frequency sound.
Indeed, scientists with acoustic receivers can sometime hear the deep tones of whale songs or sound waves from earthquakes from thousands of miles away.
During World War II, two scientists, Maurice Ewing and J.
Lamar Worzel, conducted basic research at WHOI on sound wave propagation in the ocean—seeking any advantages that would help the Navy detect enemy submarines or mines, or help American subs avoid detection.
In a critical experiment, they detonated one pound of TNT under water near the Bahamas and detected the sound 2,000 miles away near West Africa.
The test confirmed Ewing’s theory that low-frequency sound waves were less easily scattered or absorbed by water and could travel very far.
The scientists discovered a layer of water, between 2,000 and 4,000 feet deep in the ocean, that acted like a pipeline to channel low-frequency sound and transmit it over long distances: the SOFAR (Sound Fixing and Ranging) channel.
The explanation for the SOFAR channel is that the ocean settles into either denser or more buoyant layers of water based on their salinity and temperature.
Sound energy travels in waves that speed up in waters near the surface, where temperatures are warmer, or near the bottom, where water pressure is higher.
In between lies the SOFAR channel, which is bounded top and bottom by water layers where sound velocities are high and sound dissipates quickly.
The boundaries act like a ceiling and floor.
When sound energy enters the channel from below, it slows down.
When it interacts with the ceiling, it is refracted back downward.
Eventually it reaches the bottom boundary of the channel, the high-pressure water near the seafloor, and is refracted back upward again.
In this way, sound is efficiently channeled horizontally with minimal loss of signal.
The Navy immediately saw the value of the SOFAR channel.
It deployed a network of underwater microphones, called hydrophones, to optimally exploit the SOFAR channel to listen for submarines.
More than six decades later, WHOI researchers explored whether something similar might work in the Arctic.
A WHOI engineering team led by Lee Freitag and including Keenan Ball, James Partan, Peter Koski, and Sandipa Singh developed a system to achieve long-distance sound communication under the ice, enabling the control of navigation of autonomous vehicles.
Koski and Kemp brought it to Banks Island to put it to the ultimate test.
On the U.S. Coast Guard icebreaker Healy during a 2016 follow-on experiment in the Arctic, WHOI research engineer Lee Freitag examines the electronics to a new sound-based communications and navigation system that he and his colleagues developed and used in the Arctic.
(Photo courtesy of Lee Freitag, Woods Hole Oceanographic Institution)
A multi-layered Arctic Ocean
The reasons to study the Arctic are compelling.It is the region of the globe that is warming fastest, causing rapid changes in air-ice-ocean dynamics that not only change the Arctic’s climate but also have cascading impacts on global climate.
Arctic sea ice is diminishing in summer, opening navigation routes and changing the naval theater of operations.
Barriers to studying the Arctic are numerous: 24-hour darkness in winter, severe weather and safety concerns, high expense, and few ships capable of moving through ice.
Autonomous underwater vehicles (AUVs) offer a way around these difficulties, since they could work under the ice without scientists or ships present.
The biggest obstacle has been communications and navigation.
Even in summer, ice makes it impossible for an AUV to come to the surface, take a GPS reading, transmit its data and position, and receive commands.
“We wanted to learn whether we could use acoustic communication in the Arctic to support autonomous vehicles and sensors,” Freitag said.
“We’re exploiting the propagation of sound in the ocean to build a navigation and communications system in the Arctic, so we can tell the vehicles where the ice boundary is, whether they should go north or south, east or west.”
The system is designed to take advantage of a unique combination of conditions that creates a sound channel in the Arctic Ocean.
At the top of the world, water enters the Arctic Ocean from both the Atlantic and Pacific.
Both incoming water masses are warmer than the water residing in the central Arctic Ocean.
“A deeper layer of warm water comes in on the Atlantic side through the Fram Strait,” Freitag said, “and circulates around the Arctic Ocean at about three hundred meters depth.
A different current of warm water comes in from the Pacific side, from the Bering Sea, in the summer, and it goes to about fifty to a hundred meters deep.”
These different incoming currents create a watery “layer cake” of different densities and temperatures in the Canada Basin, where Freitag’s team worked.
“You have very cold Arctic air above the surface, causing very cold water at the surface,” Freitag said.
“Then a warmer layer originally from the Pacific at fifty to a hundred meters.
Then a layer of colder central Arctic Ocean water below that, and finally at three hundred meters, there’s the layer of warmer Atlantic water.”
The two warm layers create top and bottom boundaries to a colder layer, which is the sound duct.
While narrower in depth than the SOFAR channel of the temperate ocean, the sound channel in this area north of Alaska and Canada acts similarly.
“Sound stays in this duct, bounded by these two warm layers,” Freitag said.
“Warmer water above and below results in a faster sound speed.
Sound bends away from the faster water and the sound in the duct travels farther.
Nothing magic, it’s just physics.”
A lone buoy sits atop Arctic ice in the Canada Basin.
In the water below the buoy, a sound beacon in the cold-water "sound duct"; sends out sound-wave signals to communicate with other buoys and autonomous underwater vehicles hundreds of miles away, part of a new long-range under-ice acoustic communication system.
(Photo courtesy of Peter Koski, Woods Hole Oceanographic Institution)
Hopscotching on sea ice
Back on Banks Island, Koski and Kemp waited to test the new acoustic communications system as poor weather canceled takeoffs and research teams stacked up waiting for flights.“It’s late into March, and we had to do it before the ice condition deteriorated,” Koski said.
Sea ice begins to melt more as 24-hour sunlight returns in summer.
“Every day you wake up, and the pilots decide if the day is good to fly,” he said.
“Everything’s ready—you pack up and go.”
“The pilots have done it before, and they know what they’re looking at,” Koski said.
“They land somewhere and walk the ice, putting out black trash bags filled with snow to mark a runway—in case they need to take off in bad weather or another plane needs to find the runway the next day.”
Twin Otters carry a 2,000-pound payload, including people, equipment, and fuel, Koski said, so five people made up about half the load.
Each flight to an ice location took two hops, with a stop to refuel on ice five to ten feet thick.
“Sometimes, when a team intended to overnight on the ice, they took a bear dog in the plane,” Koski said.
“They hired a trapper or hunter from town, and his dog, to go out with them.
The dog sleeps on a pallet outside the tents and will whine or bark if it smells a polar bear.”
Koski and Kemp—WHOI’s Moorings Operations Group leader—used an auger to drill holes through the ice and install pairs of small buoys at intervals from 24 to 240 miles away from Banks Island.
“We did the farthest point first, so we didn’t get stranded, and made hops on the way back where we could refuel,” Koski said.
“The pilots like to help out.
They’re interested, and everyone depends on each other.
If anything happens to you, you’re two days from medical help.”
Each buoy connnected to a long cable fed through the drill-hole.
Each cable carried a transducer suspended within the sound duct, 328 feet down.
Every four hours, the transducers sent out sound signals at a frequency of 900 hertz, about the top of a soprano’s singing range.
“The signal levels are kept as low as possible to conserve energy, and span less than one minute every four hours, minimizing potential environmental impact.
In addition, the sources are never operated in sensitive areas near the Alaska coast,” Freitag said.
“We’d land, get the auger set up, twenty minutes to drill the hole,” Koski said.
“Someone stretches out the equipment and cable, so the buoy is a hundred meters away from the hole.
Make final electrical connections at that point, then put the transducer into the water and turn it on.
When we hear that it’s working, we drag the buoy to the hole, which lowers the transponder as you walk, and we set the buoy onto the hole.”
“The ‘go, no-go’ point is if you can hear the sound signal with your ears,” he said.
“If it’s working, you can hear it.
If yes, then you get back on the plane and go.”
Warming above and below the ice
Freitag was watching on his laptop from the United States, and WHOI scientist Steve Jayne was on Banks Island, when the first signals from the ice buoys deployed by Kemp and Koski reached them.Signals transmitted via satellite from all eight buoys came through.
“In the course of a weekend, they had put eight buoys in, and the buoys were all able to talk to each other,” Freitag said.
“In a short time, we went from not being positive that it would work for more than a hundred kilometers, to ‘Wow, this works at a few hundred kilometers!’ We were all very, very pleased!”
That July, researchers from the University of Washington launched gliders from a boat out of Prudhoe Bay, Alaska, to test whether the gliders would communicate with the buoy system.
The gliders traveled up and down through the ocean gathering temperature data.
They detected and responded to signals from the WHOI buoy system—but only when they were within the boundaries of the 328- to 984-foot (100- to 300-meter) sound duct.
“We learned that you have to be able to synchronize the time when the transducer’s beacons transmit to the time when the gliders are up in that layer of water—otherwise, they don’t hear it,” Freitag said.
Ironically, the Arctic sound duct that researchers may now use to gain understanding of the region’s rapid climate change is being strengthened by that very same climate change.
“Data show that over the last thirty years this warmer layer has gotten warmer,” Freitag said.
“And so the strength of this sound propagation duct in this part of the Arctic has grown, enabling this sound propagation.”
“What happens in the future, that’s not clear,” he said.
“But regardless, the warm-layer sound channel took some time to form, and it’s not going to go away very soon—given that the temperature of water in the Bering Sea coming into the Arctic has gone up as well.”
“The change in Arctic temperature is absolutely what has enabled this Arctic acoustic network to actually work the way that it does,” Freitag said.
“But in the middle of the winter, there’s still going to be ice.
No matter how open the Arctic gets in the sumer due to melt-back, it’s still going to freeze in the winter.”
Robotic vehicles could be gathering data under that winter ice, navigating via an under-ice communications system that transmits the data back to scientists warm and snug in their labs.
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