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Applying Geographic Information Science (GIScience) to Coastal Bathymetry

How sonar is used in Marine Science

How Deep Is It? Echo-Sounding Technologies as a Tool for Marine Resource Managers

© Zack G. Covell - Oregon State University - GEO 565 - An Annotated Bibliography - 11/30/2007

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Hello, my name is Dungy! You can click on me to learn about a phenomenon called "hypoxia" where the creatures like me have trouble breathing and sometimes die. Hypoxic events often occur in benthic (bottom) habitats where I live and are associated with climatic change and ocean circulation patterns. To explore and map coastal bathymetry through GIS-based tools scientists may obtain more information about the benthic conditions and can gauge episodic (periodic) events and the effects on marine resources.

Zack asked me to tell you about Echosounding technology as it were. At first I objected saying, "why echosounding and why me?" I told him Echosounding has changed greatly since the 1980's and that Leadline, Single Beam, & Multi Beam sonar has advantages and disadvantages for coastal zone and oceanic mapping. Detecting the bottom profile, or bathymetry, uses sound waves that propagate through the water column.

"The speed of sound depends on the temperature of the water, its salinity, and the pressure (which is equivalent to depth below the sea surface).  The speed of sound ranges between 1400 and 1570 m/sec (4593 and 5151 ft/sec).  This is roughly 1.5 km/sec (just under 1 mile/sec) or about 4 times faster that sound travel through air." http://www.punaridge.org/doc/factoids/Sound/Default.htm

Lead lines sent overboard were dropped into the depths to determine a basic depth.

Then ship-based expeditionary research used Single Beam acoustic sonar pulses for years by using a transceiver (transducer/receiver) mounted to the hull of the boat. But the problem arose that depth data could only be collected directly underneath the ship's trackline.

Soon technology improved to Multi Beam sonar which could again me mounted to the ship or towed with the ship (towfish) by a swinging back and forth motion covering a "swath" of ocean features below and a much larger area at one time, in one pass.

"Historically, the mandate for soundings has come from the need to chart hazards to navigation, that is, bottom features that are so shallow that a ship could run aground on them at low tide. This naturally concentrates mapping efforts very close to shorelines. More recently, there has also been some interest in mapping exclusive economic zones (EEZs), which extend outward 200 nautical miles from shore. The distribution of soundings in the ocean is relatively dense in shallow coastal areas and EEZs, but very sparse in the open ocean." (Smith, W.H.F. & Sandwell, D., 2003)

http://www.punaridge.org/doc/factoids/Sound/Default.htm

Measuring water depth: Echosounding

Annotated Bibliography (for Marine Resource Managers who wish to learn about echosounding technology used in bathymetric mapping of the ocean floor)

Bartlett, J., Beaudoin, J., Hughes Clarke, J.E., S. Brucker (2006). ArcticNet: The Current and Future Vision of its Seabed Mapping Program. _________Proceedings, Canadian Hydrographic Conference 2006, CDROM.

An intriguing article, this work was focused on the Northwest Passage of the Canadian Arctic. Arctic ice sheets have been melting at an increased rate and this work, subject to the accessibility of the cold arctic region, re-commissioned an old ice-breaking vessel to conduct multibeam and acoustical surveys within the region. Topographical data is necessary because as the ice melts in the Northwest Passage increased shipping traffic uses alternate travel routes not previously navigable and this prompted the need for more accurate bathymetric information about coastal hazards. A great partnership was created to share the sonar information with mapping and chart-building organizations to aid in decreasing overhead costs and salvage of ships run aground or stuck in the ice. Initially the geological structure of the region was mapped using GIS visualization tools to better pinpoint underwater ridges and glacial sole marks, which are straight, parallel underwater ridges that have troughs and are oriented and moving in the same direction. Glacial sole marks may be caused by ice sheet movements or massive ice melt processes, but this knowledge is now yet known. Hydrographers used sea temperature and salinity information to instruct crew controlling the echosounding equipment on how to correct multibeam sonar data, vis-a-vis, adjusting the refraction of the sound frequencies used and correlating this with the rate-of-return in time so as to obtain highly accurate measurements. Collecting large amounts of data for eighty days annually was enticing for federal agency funding support opportunities and this hydrographic survey took full advantage of that fact. Eco-tourism opportunities also were enhanced by the GIS for the simple fact that more safe and established navigation routes allowed for increased tourism. Smaller ship traffic can also navigate through the passage as a larger collection of updated navigational charts and routes enable quicker access to geo-referenced data sets. In other words, more accurate oceanographic data can help mitigate the massive costs associated with rescue and salvage of shipwrecked and/or stranded parties in the frigid tundra.

 

Beyer, A., Schenke, H.W., Klenke, M., Niederjasper, F. (2003, July). High resolution bathymetry of the eastern slope of the Porcupine _________Seabight. Marine Geology,198, Issues 1-2, 27-54.

The Porcupine Seabight, off western Ireland, has a bathymetry region of deep canyons and giant carbonate mounds. Using multibeam, sidescan, and other sonar equipment this project needed to obtain a comprehensive outlook on the structure and morphological processes of the deep canyons and giant mounds to finish a complete bathymetric model of the region and learn about the seabight in general. Bathymetric data collected and evaluated from the GEBCO Digital Atlas 1997 (GDA 97) one arc-minute grid. A high level of accuracy was configured by a group called GEOMOUND through utilizing previous grid spacing data of a Digital Terrain Model set to 50m and an accuracy better than 1% of the water depth was achieved for 96% of the soundings, yielding a large-scale bathymetric map of the region. A 10% overlap in sounding coverage was the goal via GPS navigation during the cruises to achieve accurate characteristics of the canyons and carbonate mounds. Once obtained this information was forwarded to the appropriate scientific community researchers to perform more integral studies of the seabight from the comfort of their desktops combined with their expertise in geomorphology.

 

Bosman, C., Uken, R., Smith, A.M. (2005). The bathymetry of the Aliwal Shoal, Scottburgh, South Africa. South African Journal of __________Science, 101,5 & 6, May / Jun.

To promulgate additional information about the Aliwal Shoal, situated parallel to the southeastern boundary of South Africa, researchers created the first geo-referenced bathymetric map of this previously unmapped well-preserved reef. A new order for a Marine Protected Area (MPA) requires an assessment of biological activity through the use of a GIS-compatible map. The goal was to identify potential boundaries of the MPA by creating a basemap of attributes using echosounding and GPS data. This geographic region is also frequented by divers and used for many forms of recreation. Describing the seafloor via hydrographic echosounders and narrow-beam transducers from two ship cruises they utilized special Surfer 7 software and point kriging to make a model of contour (depth) lines and used this information to find new dive spots in previously unknown shallows. Digital terrain models assisted in creating 3-D models of the continental shelf in deeper areas and for learning about Aliwal Shoal reef morphology. The data collected was converted for accuracy by tidally correcting the points in two different ways. First, the ship navigation trackline adjusted the points using GPS capable of a single-second update rate and sub-meter accuracy. Second, all the points taken were changed to coincide with the corrected mean sea-level (MSL) datum. Eventually, the points of the data of the seafloor map were spaced 1.29 meters apart along each trackline. Since the region in question has had pollution problems MPA management officials were able to use bathymetric information to model environmental impacts from both effluent discharge and coastal pipeline outfalls.

 

Eden, H., Müller, V, Vorrath, D. (2000, September). Determining the nautical depth, the stratification and characteristics of suspensions and _________sediments by echo sounder technology, especially in muddy areas, reaches a new quality level by using DSLPâ – an innovative echo _________sounding technology. HANSA International Maritime Journal, 137, Jahrgang, Nr.9.

Sediments in the water column are often suspended by currents, wind, and weather. Taking into account the stratification of the water column this paper describes an echosounding technology method called Detection of Sediment-Layers and Properties (DSLP). Acoustical depth measurements are contingent upon consistent readings in time from sonar-return for obtaining high-quality data. This article comments on the inaccuracy of comparing differing acoustical frequencies and depth measurements in the water column. The authors point out a "detection problem" where sediment layers not well consolidated in the water column make using multibeam echosounding and other frequency-dependent technologies antiquated. In other words, correlating depth measurements using differing frequency-dependent equipment alone cannot effectively offset the "detection problem" because both solid and fluid materials in the water return frequencies at different intensities. Thus, to properly create a bathymetric survey map frequency-independent elements, like the level of backscatter, must be accounted for. DSLP, in this instance, was used to calibrate the methods for determining depth by measuring the return quality (intensity) and detection of specific frequencies adjusting for any inconsistencies in the two-way travel time of the frequencies back to the transducer on the hull of the vessel.

Determining a strict rule for depth measurement in benthic zones where irregular rock ridges, soft strata, suspended sediments, and fluid mud are present immediately becomes apparent when the data does not portray the actual location of features on the bottom. This is a result of the aforementioned "detection problem." DSLP was primarily used as a technology to test its effectiveness in-situ, but also to compare and compile DSLP with traditional leadline, sounding pole, and sensor results to differentiate between water densities and viscosity as it relates to cost determination parameters for dredging and navigation channel maintenance. This was another interesting portion of this article as it dealt with dredging operations and using DSLP, combined with bathymetric data sets in a GIS, to find out the stratification of the sediment layers...for informing dredge operations, volume, transport, and disposal of material. Knowing more about the stratification of sediments decreased both cost and time spent when assessing where and how to dispose of dredged materials, which sometimes may be contaminated.

 

Florence, L., Eittreim, Wong and Stephen L (2002, March). Continental shelf GIS for the Monterey Bay National Marine Sanctuary. Marine _________Geology,181, Issues 1-3, 317-321.

This publication was all about using United States Geological Survey (USGS) data previously collected in twenty percent of the areal extent of the Monterey Bay National Marine Sanctuary. The larger goal of the work was to identify earthquake fault lines underwater. The USGS bathymetric data was originally obtained using side-scan sonar equipment. Using ArcInfo (a GIS software package) the researchers gathered data in the Universal Transverse Mercator (UTM) Zone 10 coordinate system datum NAD83 (North American Datum collected in 1983). The USGS data was then used in gridded raster data layers and registered .tiff files in ArcInfo to convert data into a usable format for visualizing the continental shelf. Mentioned in the article were how point, line, and polygon attributes were viewed in ArcInfo "coverage" format (read/write coverages into binary data) and ArcView shapefiles (.shp). Testing and trusting that the accuracy of their data conversions were acceptable the final portion of the paper briefly addressed how within the GIS sediment samples taken corresponded to specific locations (potentially on fault lines). The analysis of the many related database tables in the GIS assisted seismologists in mapping the fault lines.

 

Goldfinger C., Romsos, C., Patton, J. (2007). Visualizing the Seafloor. Leica Geosystems Geospatial Imaging, LLC. Unpublished. For more _________information see: Active Tectonics Seafloor Mapping Lab.

This short article looks at submarine landslides from earthquakes. An episodic event such as an earthquake can lead to underwater landslides that flush massive quantities of sediment into the water column. This turbidity of the ocean (highly-stratified) eventually settles into a large deposit called a Turbidite. Following the Asian tsunamis an assessment of Sumatran and Indonesian Turbidites were studied and analyzed. The way in which the Turbidites were used to find out earthquake history in the region entailed collecting sediment core samples from the ocean seafloor. Again, multibeam swath sonar was used to map the bathymetry to identify the most suitable sites for Turbidite sampling. Using a geospatial tool called ERDAS IMAGINE this research collected navigational waypoints (coordinates on the Earth) to navigate a precise back and forth course to map all the areas of the seafloor. A high-tech system of sensors accounted for the exact trackline of the ship and referenced the data simultaneously returning from the sonar pulses to direct the best navigational direction for mapping what is below. National Science Foundation (NSF) computer software was used to compute the relative altitude of the boat from rolling, heaving, and yawning as depth measurements were being taken. Still in motion, waypoints are provided to the wheelhouse of the vessel for navigation instructions (direction and speed). Aided by software the motion of the ship itself is computed and the end-result is a Digital Elevation Model (DEM) of the seafloor. This DEM can be used to locate the best sites for core sampling of Turbidites as well as making the DEM data available to the public and fisheries managers who wish to study the bathymetry and how it relates to a specific species environment.

 

Hughes Clarke, J.E. (2006). Applications of Multibeam Water Column Imaging for Hydrographic Survey. The Hydrographic Journal, April _________Issue.

Multibeam sonar was originally produced for the benefit of the fishing community. Over time technological advances in computing software, like the ArcGIS platform, allowed for many other bathymetric and hydrographic surveys as data from these studies are not static and require larger amounts of storage space. An in-depth (no pun intended) evaluation of current hydrographic equipment and quality controls available to users is described in this publication. EM3002 (a multibeam system) data was collected and this technology could differentiate between acoustical frequencies produced by the R/V (research vessel) itself, 3rd-party sonar and sounds, bubbles, and rough ocean conditions. A major portion of this article addresses the importance of the thermocline (temperature) and halocline (salinity) in the ocean and its relationship to acoustical wave (sonar) signatures, or how the dynamic nature of sonar boundaries vary with depth in determining the density of the water column that the sonar is propogating through. Scattering of the acoustic waves occurs, especially near uneven bathymetric features and shipwrecks. An example given was how a broken mast on a shipwreck skews/scatters the sonar in such a way that the deck of the sunken ship below will not be properly mapped in terms of its depth and boundary points. To work to find the boundaries of the actual shipwreck below the mast a polar intensity plot was used changing the angles and algorithms of the sonar frequencies to basically bypass any midwater features and attempt to hit the seafloor and return. A final synopsis of this article is that the thermocline is what hydrographers using sonar in conjunction with a GIS most often deal with when mapping in the ocean and the halocline is often the major factor when gauging depth in estuarine environments. And lastly, impedance in acoustic energy used in echosounding is expected and this is why the more experienced the people working with the tools are the higher a likelihood the boundaries of underwater features will be most accurately determined.

 

Joseph, D., Hussong, D. (1998). Geospatial Management of Commercial Seafloor Data. Fugro Seafloor Surveys, Inc. Retrieved November 3, _________2007 from: http://gis.esri.com/library/userconf/proc03/p1161.pdf.

Laying transoceanic telecommunications cables has traditionally evolved through singlebeam echosounding techniques. This informative article highlights new sounding equipment and survey protocols used in creating 3-D visualizations of cable route planning. A major expense to consider is the type of cable to be laid in a distinct type of sediment or other substrate on the seafloor. Typically, any cables less than 2000 meters in depth are trenched and buried to avoid ship traffic and ocean turbulence. This company uses multi-million dollar ploughs to trench and bury the cables and strives to understand the composition of the ocean bottom by shifting from older Computer Aided Drafting (CAD) programs (often used by those in the engineering fields) to ArcGIS products. However, CAD-made charts of the ocean are still hard-copy and often used in MicroStation. Chart must first be digitized into a GIS with attribute information of the proper resolution, location, source, and edition to move away from static forms of bathymetric representation to GIS-capable models. A database was created for a more accurate model of the subsurface environment and this information was used to assess the ploughability of the substrate below to minimize costs and time in laying transoceanic cable. Separating data types and running macros in MircoStation, this work prepared a GIS database to run .mxd files before the surveys began to view charts digitally and update them as bathymetric data was collected. Hyperlinking documents and data as the surveys began the GIS component used raster calculator in Spatial Analyst to determine where slopes of greater than or equal to15 degrees were present and where rock versus soft strata were present within the imagery pixel values. The resulting output of strata present and its slope was used to plan cable placement location. Interpolation hints as to where to place cables and avoid potential earthquake locations that may equate to fixing a broken or severed cable, costing millions. In the end a geodatabase is rendered which produces data by attribute type or by UTM zone, depending upon the customer expectations and needs.

 

McMullen, K.Y., Poppe, L.J., Twomey, E.R., Danforth, W.W., Haupt, T.A., and Crocker, J.M. (2007) Sidescan Sonar Imagery, Multibeam _________Bathymetry, and Surficial Geologic Interpretations of the Sea Floor in Rhode Island Sound, off Sakonnet Point, Rhode Island. U.S. _________Geological Survey Open File Report 2007-1150, DVD-ROM.

In Rhode Island Sound multibeam bathymetry and sidescanning sonar, among other things, were used look at bottom morphology using National Oceanic and Atmospheric Administration (NOAA) data collected in 2004. A towfish behind the vessel shot acoustic sound waves to the bottom and then the work compiled this data along with historic seismic-reflection points to determine a corrected backscatter and radiometric distortion from the movement of the vessel and inaccuracies in sonar reflection. Using XSonar software, made by USGS, the intention was to convert .tiff images for importing them into Adobe Photoshop CS2. Once in Photoshop the images were stretched to improve the range of brightness (dynamic range) in a particular image file. Course-grained sediments were differentiated between fine-grained sediments by stretching the images and the color ranges resulting, from lightest to darkest, hinted as to the acoustic variance in reflectivity of the water column. Singlebeam data were also pulled out and vertically oriented to multibeam data to apply it to the National Geophysical Data Center (NGDC) Coastal Relief Model basic bathymetry. From here a one meter resolution raster grid was overlaid from the multibeam data collection and the morphology could be studied more intensely studied via cataloged and more accessible GIS database means.

 

Wright, D. (1999). Getting to the Bottom of It: Tools, Techniques, and Discoveries of Deep Ocean Geography. The Professional Geographer, _________51(3), 426-439.

A more comprehensive geographers' look at deep ocean exploration technologies are highlighted in this publication, including, but not limited to: GIS, submersibles, satellites, acoustical sonar systems, spatial data management, remotely-operated vehicles, and the evolution of Earth processes in the world ocean. Specific to swath mapping via acoustical systems and side scanning sonar the author speaks of obtaining high resolution bathymetric data using varying equipment and methodology. For example, seeking metadata reminiscent of high-quality resolution and adding it to a GIS is akin to searching for a needle in a haystack because the ocean is vast, data is often sparse, and expense overshadows the means to gain knowledge to acquire a certain end of understanding about bathymetry. For this reason the author does a good job at pointing out limitations that popular multibeam equipment present and how to modify research procedures to better manage the data that is available. Coupling available GIS-based information with other technology, like satellite imagery, one can leverage the GIS to examine bathymetry at a higher resolution than with one technology alone. The almost insurmountable cost of collecting high resolution imagery is noted to be offset by developing data management tools to capture and organize spatial information and catalog it in such a way that the user can query the data for specific topics of interest, considering the data has often been collected by other organizations at different times.

 

 

 

 

 

Kind Regards,

Zack G. Covell
Oregon State University Graduate Student
Marine Resources Management
Cell: (541) 602-2852
Email: zcovell@coas.oregonstate.edu

Survey on Communicating Ocean Science: Click Here

Other References

Smith, W.H.F., Sandwell, D. (2003). Conventional Bathymetry, Bathymetry from Space, and Geodetic Altimetry [Electronic version]. The Oceanography Society, 17(1).

Detailed Information below about swath systems:

Worldwide Seafloor Swath-Mapping Systems (December 2005)