On the Hudson River Estuary Benthic Mapper, researchers and users of the Hudson River can view benthic (river bottom) data on maps, and can combine map views to "overlay" maps of different data sets. (Also available on Benthic Mapper are interpretive maps showing features of the river bottom revealed by expert analysis of the benthic data.)
This page discusses the techniques used to gather the Hudson River benthic data. Details links give specifics about the data sets.
Survey Area - The Hudson River Estuary is being surveyed from the Federal Dam in Troy, New York, to Verrazano Narrows Bridge in New York Harbor. Benthic Mapper presents the data from the first third of the estuary to be surveyed - the map shows the areas for which data is currently available. Additional data will become available in the future.
(right) The red boxes indicate areas of the Hudson River Estuary currently available on Benthic Mapper
Data - Benthic Mapper presents nine data sets:
To view one of these data sets in Benthic Mapper, locate it in the Layers menu at the lower right in the blue frame and place a check in the box.
Note: Certain browsers may cause performance conflicts with the Benthic Mapper application.
Benthic Mapper works with Netscape versions 4.5, 4.6 and 4.7 higher or Internet Explorer versions 4, 5 and 6. It does not work with Netscape 6.0 and higher.
The ship tracks shown in Benthic Mapper are the actual paths, up/down and across the river, of vessels that collected the sidescan sonar and subbottom profile data. These tracks are created by Benthic Mapping Project survey vessels' real time differential global positioning systems (GPS), which continuously record the ships' locations. When measurements or samples are taken, the precise time, horizontal position (latitude and longitude) and the vertical position (relative to sea level) of the data sensor relative to the ship GPS antenna are recorded. The positional or spatial data must be accurate to better than 10 centimeters, to make it possible to correlate the data with the locations where samples and measurements were taken.
Two systems are used in the benthic mapping project to produce these two-dimensional map images of the floor of the estuary: sidescan sonar and multibeam swath sonar. These ship-mounted systems take acoustic measurements as the ship passes over the bottom.
Sidescan sonar sends out a beam of sound on either side of the ship and measures the amplitude of sound reflected from different parts of the river bottom in a 200-meter wide swath on either side of the ship. These reflectivity measurements reveal detailed structure and relative hardness of the river bottom.
Multibeam swath sonar measures water depth as well as reflectivity. The depth measurement is accomplished by measuring the time required for sound to travel from the surface ship to the water bottom and then back to the ship. At the same time the survey vessel measures the speed of sound in the water column. With the speed of sound and the travel time, one can calculate the water depth (depth = velocity x time). Bathymetric data are rendered on Benthic Mapper in two types of "aerial" displays: as a bathymetric contour map of the estuary floor (contour lines) and color-coded bathymetry (color bathymetry).
Both sidescan and multibeam swath systems make individual measurements for each square meter of water bottom ensonified by the acoustic beam. The multibeam's beam width is proportional to water depth. For most water depths that occur in the Hudson River Estuary the multibeam system has a much narrower beam width than the sidescan system; for this reason, the utility of the multibeam scanner is limited in depths of less than about 5 meters.
The Benthic Mapping project has employed sonar imaging techniques and radar imaging techniques to generate subbottom profiles. Subbottom profiles are two-dimensional, vertical cross sections of the estuary floor. Unlike "aerial" maps of the estuary floor, subbottom profiles reveal the sediment structure beneath the estuary floor. This is accomplished by using wave energy to probe the layers of sand and mud that lie beneath the water bottom using acoustic reflection (sonar) profiling - uses sound to image through water to the river bottom and ground-penetrating radar profiling - uses electromagnetic radar waves to image the subsurface. These "vertical slices" of the river bottom, yielded through the combination of these two techniques, is similar to the cross-sections of the human body displayed by CAT scans. They are very useful in locating areas of sediment erosion and deposition.
These two techniques (sonar and radar) have been used to produce subbottom profiles because of their different, but complementary, strengths and weaknesses.
Acoustic methods (sonar) cannot image past hard bottoms or in areas where decomposing organic materials create gas pockets in the near surface sediments. Radar, on the other hand, can image through both of these conditions, however, it is limited by water depth: passing through water weakens, or attenuates, the signal. Water depths of more than 20 feet result in strongly attenuated data; depths of more than 30 feet result in very weak signals, which often show little or no detail.
The radar data were acquired with a Mala Geoscience system using 200 MHz unshielded antennas. The subbottom profiling sonar data were acquired by the X-Star topside data acquisition unit and for most operations, the SB 4_24 tow fish, both manufactured by Edge Tech.
Sediment Profile Imagery (SPI) is a computer based examination of photographic images of the river bottom.
SPI technology quantifies more than 20 physical, chemical and biological parameters including: sediment grain size; prism penetration depth (into the sediment); surface pelletal layer; sediment surface relief; mud clasts; redox area; redox contrast; current apparent redox boundary; relict redox boundaries; methane gas vesicles; apparent faunal dominants; voids; burrows; surface features (e.g., worm tubes, epifauna, shell); dredged material; microbial aggregations, and successional stage.
Grab samplers are easy to use and obtain relatively large volumes of sediment at a time. The large samples produced are especially useful for identifying recent inputs of pollutants.
Grab samplers commonly use a set of jaws that shut when lowered to the sediment. The Ekman and Ponar grabs have vented or hinged tops that allow water to flow freely through the device during descent, reducing sediment disturbance from a shock wave in front of the sampler. However, upon retrieval, fine surface particulates can be carried away by out-flowing water.
Core samplers provide a cross-sectional slice of sediment, showing layers and providing information about sediment deposition. Core samples are well suited for assessing long-term (historical) inputs to the sediment.
Core samplers penetrate the sediment more deeply than grab samplers. They commonly consist of a tube with a valve at the top, which is closed by messenger to create a vacuum, preventing the sediments from washing out. The sampler is allowed to free fall from a sufficient height (usually 3-5 meters) to penetrate firm sediments, while in soft organic sediments, the weight of the core sampler is sufficient to drive the tube down without having to let it to free fall.
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