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Divya Basuti
Detection and Analysis of Internal Waves Using Satellite Imagery of the North Indian
Ocean
Institute of Science and Technology, JNTU HyderabadCenter for EnvironmentEnvironmental Geomatics
Supervisor: Dr. N. Srinivasa Rao, Scientist and In charge, SDAPD, INCOIS
The Ocean: The Final Frontier?
Introduction
Internal waves are gravity waves that oscillate within rather than on the surface of a fluid medium.
Fig: Internal waves near Strait of Gibraltar
For internal waves to exist, the fluid must be stratified.
Stratification : Density decreases continuously with height due to changes in temperature or salinity or both.
Fig: Layers of ocean according to temperature and density
The layered nature of the ocean causes the shallower water which is warmer, less salty, and lighter to overlay deeper water which is colder, saltier, and denser.
Internal waves forms at the interface between the layers
Fig: Internal wave formation at the interface of the two different densities
Internal waves can be initiatedand set in motion by the following:
•Tides
•Wind stress
•Boats and submarine movement
•Currents moving over underwater topography (ridges)
Fig: Tides
Fig: Boat and Submarine movement
Fig: Wind Stress
Fig: Underwater ridges
Fig: Dead water
Data used:
Level 1B data at 250 M from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua and Terra satellites.
Fig: MODIS onboard AQUA satellite
Methodology
The MODIS level 1B data product at 250M resolution can be downloaded from the MODIS website in HDF.
Fig: MODIS website
They are converted into TIFF using the MODIS Swath Reprojection Tool available for download at the NASA website
Fig: MODIS swath reprojection Tool
The TIFF images can now be used in ERDAS IMAGINE software after they are layer stacked and saved with an .img extension
Fig: Erdas Imagine software
Visualization:
Reflectance pattern of sunlight off the ocean surface (sunglint) to be used to identify high frequency Internal wave.
Fig: Sunglint image of Indian Ocean
In absence of sunglint image processing methods such as contrast stretching can also be used to visualize internal waves.
Fig: Before and after Contrast stretching
Image Enhancement:
Fig. False color composite
Fig. Histogram Stretching
Pre Image Enhancement Post Image Enhancement
Fig: Before and after Image Enhancement using Histogram stretching technique
Identification:
The waves manifest themselves as alternating bandsof light and dark quasilinear strips that remain coherentover tens to hundreds of kilometers lasting several hours.
Fig: Visualization of internal waves by observing the alternating dark and light strips
AQUA-MODIS13:35 Hrs
Fig: Shift in position of wave packet between TERRA MODIS pass and AQUA MODIS pass.
TERRA-MODIS10:30 Hrs
Fig: Wave propagate and persists for several hours, Establishing that they are internal waves
Why study Internal waves?
Play a role in:
•Ocean circulation•Heat distribution •Plankton distribution
Affect the Performance of:
•Underwater technology•Acoustic communication•Marine cabling •Submerged vessels and vehicles•Underwater sound propagation
Fig: ocean circulation
Fig: submarine communication through SONAR
Fig: plankton distribution
Advantage of using MODIS data instead of SAR images in the study
Although IWs can be observed easily in SAR, these images are usually very expensive.Therefore in this project, We use the satellite ocean colour images, at no cost and easily available to the public, to detect IWs.
Fig: SAR image Fig: MODIS True Color Image
Conclusion:
Image enhancement techniques such as contrast stretching have been used in this study, in conjunction with the sunglint effect to better visualize internal waves.
Limitations and Recommendations:
The obstruction of desired feature by clouds is a very common problem encounter during detection of internal waves, as cloud cover is seen on about 67 percent of the Earth's surface at any given time, and less than 10 percent of the skies over the ocean are clear. In further studies, this can be overcome by using the cloud mask algorithm.
Bibliography•seafriends.org.nz/oceans/waves.htm•earthobservatory.nasa.gov•incois.gov.in•chapter7.tulaneuniversity.com•taylorandfrancis.com•http://www.nasa.gov/centers/goddard•http://modis.gsfc.nasa.gov/
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