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GEOG 2420 (3)
Chapter 12

Geog 2420 Chapter 12

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GEOG 2420
John Lindsay

Geog 2420 Chapter 12: Earth Imaging of Water Jenson - water covers approximately 74% of the earth’s surface Remote Sensing Surface Water Biophysical Characteristics - remote sensing can be used to inventory and monitor spatial extent, organic/inorganic constituents, depth and temperature of water Water Surface, Subsurface Volumetric and Bottom Radiance - the total radiance (Lt) recorded by the sensor onboard the aircraft or satellite is a function of the electromagnetic energy from the four sources: - Lp is the portion of radiance recovered by a remote sensing instrument resulting from the downwelling solar and sky radiation that never actually reaches the water surface - Ls is the radiance from the downwelling solar and sky radiation that reaches the air water interface (free surface layer) but only penetrates it a millimetre or so and then is essentially reflected from the water surface - Lv is the radiance that actually penetrates the air water interface, interacts with the water and organic/inorganic constituents and then exits the water column without encountering the bottom - Lb is that portion of the recorded radiance resulting from the downwelling and sky radiation that penetrates the air water interface, reaches the bottom of the water body, is propagated back through the water column and then exits the water column Spectral Response of Water as a Function of Wavelength - it is useful to understand how pure water selectively absorbs or scatters incident, doweling light in the water column - pure water is free from organic and inorganic matter - the most noticeable characteristics is that the least amount of absorption and scattering of incident light takes place in the blue wavelength region approx 400-500 nm, with the minimum located at approx 460-480nm - these wavelengths of violet to light blue light penetrate further than any other type of light into the water column - incident green and yellow light is absorbed very well as well - the molecular scattering of violet and blue light in water and significant absorption of green, yellow, orange and red wavelength light in the same water cause pure water to appear blue Monitoring the Surface Extent of Water Bodies - best wavelength region for discriminating land from pure water is the near infrared and middle infrared regions at wavelengths between 740-2500 nm - in the near/middle infrared regions, water bodies appear very dark,even black because they absorb almost all the incident radiant flux, especially when the water is pure land surfaces are typically composed of vegetation and bare soil that reflect significant amounts of infrared energy - this causes land surfaces to appear relatively bright when there are organic and inorganic constituents in the water, these materials will cause surface reflection and subsurface volumetric scattering to take place, radically increasing the near infrared radiant flux Spectral Response of Water as a Function of Organic and Organic Constituents - Monitoring Suspended Minerals, Chlorophyll and Dissolved Organic Matter - subsurface volumetric radiance, which is the radiance from the downwelling solar and sky radiation that actually penetrates the air water interface, interacts with the water and organic/inorganic materials and then exits the water column toward the sensor without encountering the bottom Suspended Minerals - minerals such as silicon, aluminium and iron oxides are found in suspension in most natural water bodies - the particles range from fine clay particles to silt to fine grain sand and coarse grain sand - the sediment comes from a variety of sources including upland agricultural cropland erosion, weathering mountainous terrain, shoreline erosion and volcanic eruptions - clear, deep ocean far from shore rarely contains suspended minerals - inland and near shore water bodies may carry a significant load of suspended sediment that can dramatically impact the spectral reflectance characteristics of the water bodies - monitoring the type, amount and spatial distribution of suspended minerals is very important - sediment also effects the water quality and its suitability for drinking, recreation and industrial purposes - it serves as a carrier and storage agent of pesticides, absorbed phosphorous, nitrogen and organic compounds and can be an indicator of pollution - it requires in situ measurements of suspended mineral concentrations and relating it the the remote sensor data to derive a quantitative relationship - the pectoral reflectance of suspended sediment in surface waters function of both the quantity and characteristics of the material in the water - sediment concentration is measured in situ using either a see chi disk or a nephelometric turbidity unit - as the suspended sediment concentration is increased, reflectance increases at all wavelengths for both clayey and silty soils - the peak of reflectance shifts toward longer wavelengths in the visible region as more suspended sediment are added - more green, red and near infrared radiant flux is reflected - a water body with suspended sediment will general appear brighter in imagery Chlorophyll - plankton is the generic term used to describe all the living organisms present in a water body that cannot resist the current - plankton may be subdivided further into algal plant organisms and animal organisms, bacteria and lower plant forms such a algal fungi - phytoplankton are single celled plants smaller than the size of a pinhead - chlorophyll a pigments in the plants absorb most of the incident blue light, causing the photosynthetic portion of each to appear dark - phytoplankton sink to the water floor when they die - zooplankton migrate to the surface to feed and then sink to greater depths during the day - they sink to the bottom when they die carrying carbon with them - phytoplankton use carbon dioxide and produce oxygen - in this way the water bodies and oceans act as a carbon sink, a place that disposes of global carbon which otherwise can accumulate in the atmosphere as carbon dioxide - other global sinks include land vegetation and soil - because different types of phytoplankton have different concentrations of chlorophyll, they appear as different colours to sensitive remote sensors - thus recording a particular colour of a water body allows us to estimate the amount and general type of phytoplankton in that area and tells us about the health and chemistry of the water - chlorophyll a introduced into pure water changes its spectral reflectance characteristics - strong chlorophyll absorbs blue light between 400 and 500 nm - strong chlorophyll a absorbs red light at 675 nm - reflectance maximum around 550 m caused relatively lower absorption of green light - chlorophyll concentration increases, there is a significant decrease in the relative amount of energy reflected in the blue and red wavelengths but an increase in the green light - Chlorophyll in Ocean Water - MODIS - 8 -16 bands for ocean colour, phytoplankton concentration and biogeochemistry - red colours reveal high concentrations of chlorophyll - yellows and greens = medium - blue = low - Chlorophyll in Coastal and Inland Water - often difficult to disentangle the information about phytoplankton pigments in the remote sensor data from the effects of suspended inorganic materials or dissolved organics Dissolved Organic Material - sunlight penetrates into the water column a certain photic depth - phytoplankton within the photic depth of the water column consume nutrients and covert them into organic matter via photosynthesis - bacterioplankton eat the decompose this organic material - all this conversion introduces dissolved organic matter - there may be sufficient dissolved organic matter in the water to reduce the penetration of light - the more productive the phytoplankton, the greater the release of dissolved matter - humic substances may be produced - these often have a yellow appearance and represent an important colorant agent in water column that needs to be taken into consideration - these dissolved humid substances are called tell substance and can impact the absorption and scattering of light in the water column and change the colour of water Water penetration and Bathymetry - bathymetric mapping can be performed using passive optical or active remote sensing systems - mostly SONAR (which reflects sound waves) or LIDAR (which reflects laser light) are used Bathymetric Mapping Using Passive Optical Remote - the optimum optical wavelengths to obtain bathymetric information are approx .44 to .54 um - requires the water to be almost free from organic and inorganic constituents such a chlorophyll and suspended sediments that would cause scattering or absorption to take place - it is important to take into account the fact that the light from sun is bent from its true course in both the atmosphere and in the water column, causing bathymetric information in imagery to not be in its proper planimetric position Bathymetric Mapping Using Active SONAR - Sound Navigation and Ranging - can be used to collect measurements on the sea floor - when it is used to measure the distance to the bottom it is known as echo sounding - echo sounds are used to measure water depth sending acoustic pulses via a transducer - the acoustic pulses are reflected by the sea floor and the reflected echo are received by the transducer - the elapsed time between the outgoing and incoming echo can be used to accurately determine depth - 3 main types of SONAR: single beam, multi beam and side scan - single beam SONAR use a transducer that emits a single sound pulse into the water column at specific intervals with a narrow acoustic foot print - is used along transect lines and typically can’t provide continuous coverage of the seafloor - the output resolution is determined by the acoustic footprint size, sampling interval, sampling speed and distance between transects - multi beam SONAR provides users with two kinds of data: bathymetric and acoustic backscatter - they can be used in extremely deep water - they emit multiple sound pulses that cover large overlapping swats of the sea floors, giving continuous coverage - back scatter signals can be used for habitat or feature mapping - Side Scan SONAR are usually towed behind a boat - they are used primarily for searching for and detecting objects on the sea floor, not for bathymetric mapping - the two fish SONAR points multiple beams at angles and covers large over lapping swatches enabling continuous coverage - most do not provide
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