| Fishing Consortium Uses Sonar Imaging to Find Orange Roughy Habitat
By Kevin P. Corbley In the race to become America's most popular white fish, Orange Roughy is giving Halibut a swim for its money. In fact, so many Americans are landing Orange Roughy on their dinner tables to enjoy its delicate taste and low cholesterol content, some seafood wholesalers say it would be the number one white fish if demand had not driven its price out of sight.
Fishermen off the coast of New Zealand, where the fish is found, have engaged in a lucrative effort to bridge the gap between supply and demand. One commercial fishing consortium in the area is experimenting with an advanced oceanographic remote sensing technique to locate the Orange Roughy's preferred habitat.
Early last year, the New Zealand fishing consortium teamed with the Hawaii Mapping Research Group (HMRG) at the University of Hawaii to test a bathymetry and imaging sonar system developed specifically for seafloor mapping. In the 1995 fishing season, some of the consortium's fleet will set sail equipped with seafloor imagery atlases printed directly from the digital images acquired during last year's mapping expedition.
Aside from its potentially revolutionary impact on the commercial fishing business, use of the hardcopy image atlases by ship captains at sea represents a significant trend occurring in the geographic information industry. Image processing systems are not always available to display data, and hardcopy is still a primary means of presenting imagery. To create hardcopies, many users are avoiding the photo laboratory and are turning to high-end color printers that accurately represent the information produced by advanced image processing packages. Mapping the Seafloor
HMRG is a mapping organization within the University of Hawaii's School of Ocean and Earth Science & Technology. It utilizes sonar and other data collection techniques to make topographic maps and images of the seafloor, primarily in support of scientific applications.
"Much of our work involves studies of plate tectonics," said Karen Sender, HMRG's data manager and image processing technician. "We map mid-ocean ridges where new crust is formed and coastal zones where plates are colliding."
The research group enhances and processes the image data before providing it to scientists in hardcopy form. Most geophysicists interpret hardcopy imagery on a light table, either out of preference or because their computers can't handle the 300 megabyte data sets acquired by the undersea imaging device, said Sender.
HMRG can take credit for several recent advancements in ocean monitoring technology. The most significant is the HAWAII MR1 system, which it developed in 1991. MR1 drastically improved the efficiency of seafloor mapping expeditions by simultaneously acquiring bathymetric readings and imaging data from a sonar apparatus towed behind a research vessel.
"Earlier sonar devices acquired seafloor image data that was processed later on land," explained Sender. "Geophysicists reviewed the image data to decide where to dredge, take core samples, or run magnetic surveys and gravity readings during future expeditions."
The MR1 system collects image data and displays it in real time to scientists on the vessel conducting the survey. This allows them to make immediate decisions about collecting additional data or repeating a survey over an area of particular interest. The data acquisition system is linked in real time to the ship's GPS navigation so that exact coordinates of correlative data collection can be tied to the imagery.
The MR1 system is composed of two sets of equipment - a phased array sonar device and the data acquisition system. The sonar device, which resembles a 12-foot-long torpedo dragged behind the research vessel, functions in a manner similar to side-looking radar. Instead of emitting radar microwave signals, the sonar emits sound waves from either side perpendicular to the heading of the ship.
Two rows of listening arrays in the sonar log the time it takes for the sound to bounce back from the seafloor and the intensity of the return signal. The two arrays also measure the phase angle difference of the signals that bounce back.
Bathymetric, or depth, information is determined by the time lag between emission and return, explained Sender, and intensity of the return signal provides an indication of seafloor texture. A low intensity return is typical of a soft sediment bottom, while a hard rock seafloor returns a strong signal. The phase angle difference indicates the direction and angle of the returned signals.
"Put all that information together and you get an image depicting the shape and texture of the seafloor," said Sender.
The size of the image swath varies with the "ping" rate of the sound emissions. Generally, HMRG runs its surveys at a swath width of 15 kilometers. Fishing with the MR1
Orange Roughy is a deep sea perch found exclusively in the waters around the south island of New Zealand. These fish are known to congregate around undersea volcanoes called sea mounts, which commercial fishermen attempt to locate using a device called an echo sounder.
This simple sonar system bounces a sound wave off the seafloor measuring water depth directly below the ship. The fishermen know that if the echo sounder suddenly returns a much shallower depth measurement, it could be pinging off a sea mount sticking up from the ocean floor.
Unfortunately, the echo sounder is literally a hit-or-miss technique. Because it measures depth only directly below the ship, the trawler could pass just to the side of the sea mount and never find it. The MR1, therefore, offers a huge advantage because it maps a wide swath on either side of the ship, allowing the vessel to follow a precise grid to map a large area.
HMRG presented the Orange Roughy habitat mapping idea to the New Zealand fishing consortium, which agreed to fund the expedition and provide the vessel to tow the sonar, said Margo Edwards, HMRG's acting group director. Two cruises were conducted in the spring of 1994, covering both the east and west coasts of New Zealand's south island.
"The cruises were very successful," said Edwards. "The sea mounts provided very distinct returns because of their height above the seafloor and hard rock composition."
Equipping every fishing vessel in the fleet with an MR1 device is unnecessary because the system finds the fish habitat, which needs to be located only once. Real-time trolling with an MR1 is also impractical because technology has not evolved to make it economically feasible to put an image processing system in every ship.
For those reasons, HMRG is creating hardcopy atlases containing image maps of the New Zealand seafloor that will be carried in the fishing ships this year. Preserving Image Information
HMRG is producing the image atlases directly from digital data with an IRIS color inkjet printer. The mapping group had used color plotters, laser printers and local photo laboratories to generate output with considerable frustration before finally opting for a high-end printer that could be operated in house.
The color inkjet printer costs more than the other alternatives, Sender said, but it provides a substantial savings in something HMRG considers even more valuable - information.
In projects such as the New Zealand seafloor mapping expedition, sonar image data is processed and enhanced in much the same way as satellite imagery. Very subtle variations in seafloor texture and bottom features are graphically represented on the image processing system by slight differences in colors or gray tones. Each color shade carries a significant amount of information.
"With the IRIS printer, what you see on the [image processing] screen is what you get on paper," said Sender.
The continuous flow color inkjet process, developed originally for the graphic arts industry, has proved to be the only output technique capable of rendering on paper the full range of colors utilized by image processing workstations. These printers combine up to 124 microscopic droplets of dye to achieve just the right color in a single pixel, which is placed on the paper with exacting precision.
Remote sensing and GIS businesses have become major users of continuous flow inkjet technology, confirmed Peter Alpers, director of marketing communications at IRIS Graphics in Bedford, Mass. He said the printer's recent popularity is a natural extension of the image processing revolution in the 1980s. Image processing systems had to be improved substantially to effectively exploit data collected from sophisticated image acquisition systems.
"The next step has been to close the gap between image processing and image output because a significant amount of information was being lost during production of hardcopy images," said Alpers. "If the color on the paper doesn't precisely correspond to the image on the screen, information has been lost."
Color range is extremely important to HMRG in the New Zealand project because ocean depths will be printed in various colors on clear sheets that will overlay the black & white seafloor image maps in the atlases. Perhaps even more important, though, is the ability to reproduce those colors and gray scales in every atlas.
HMRG is creating four sets of 32"x24" atlases and 12 sets of 15"x11.5" atlases for the New Zealand consortium. Each atlas will contain 18 seafloor image maps covering 0.5 degrees by 1.0 degree of ocean bottom. Every image sheet and bathymetry overlay in each atlas will be a first generation print directly from the printer.
"Repeatability is the most important feature of high-end digital output printing," said Sender. "If I print 10 copies of the same image today or six months from now, I know that the pixel colors and pixel locations will be exactly the same on each print."
The mapping group uses a data processing and image enhancement software package developed in-house for use on a Sun workstation to mosaic the sonar swaths. Geographical Mapping Tool software is used for further image display and other GIS functions. Color keys, navigation information, text and contour lines are added to the map sheets and bathymetry overlays before the data is ported to the printer for output.
The printer requires about five minutes to ingest the data ported from the Sun. The actual plotting of one of the larger atlas sheets takes 45 minutes to an hour. During the plotting, the printer outputs up to 4 million droplets of dye per second.
HMRG produced the map atlases in less than four weeks and delivered them to New Zealand in November 1994. The four larger atlases will be kept at the fishing consortium's headquarters to plan the fishing expeditions. The smaller atlases will be taken aboard ship by the captains to lead them directly to the underwater volcanoes where the Orange Roughy are found.
The next major expedition for the MR1 will involve oceanographic mapping for the British Antarctic Survey. The U.S. military has recently expressed interested in using the device in cleaning up undersea debris left behind by offshore military operations. HMRG also expects to take the device on more "fishing trips" in the near future. Several fishing consortiums will examine the atlases this year and consider use of the MR1 in finding other fish species that school around prominent undersea features. About the Author:
Kevin P. Corbley is the principal in Corbley Communications of Denver, Colo., which provides public relations and marketing services to remote sensing, GIS and GPS firms. Imagery courtesy of The Hawaii Institute of Geophysics, Hawaii Mapping Research Group. Back |
