UMD AOSC Seminar
Shallow-water Remote Sensing: The Unsolved Problem of Atmospheric Correction
Dr. Curtis Mobley
Sequoia Scientific, Inc.
Both the military and ecosystem managers require high-spatial-resolution maps of bathymetry and bottom type in shallow coastal waters, estuaries, and rivers. The last decade has seen the development of spectrum-matching techniques for retrieval of this environmental information from airborne and satellite hyperspectral imagery. These techniques rely on both the magnitude and spectral shape of the remote-sensing reflectance (i.e., of the water-leaving radiance) and thus require accurate radiometric calibration of the sensor and accurate atmospheric correction of the measured at-sensor radiances.
Atmospheric correction as developed for open-ocean (Case 1 water) multispectral remote sensing relies on a “black pixel” assumption. That is to say, it is assumed that there is no waterleaving radiance (a black pixel) in the near IR. The Rayleigh contribution to the measured atsensor radiance can be computed and subtracted out. The residual radiances in the near IR are then assumed to be due solely to aerosol scattering. These radiances are used to determine a best-fit aerosol model, which is then used to compute and remove the aerosol path radiance at shorter wavelengths. The remaining signal is the desired water-leaving radiance. However, in very shallow water, there can be non-zero water-leaving radiance in the near IR because of bottom reflectance. Shallow waters often contain resuspended sediment that is strongly scattering and can give non-zero water-leaving radiance in the near IR (Case 2 water). In either case the black-pixel assumption is no longer valid, and classical atmospheric correction techniques fail.
Other atmospheric correction techniques have been developed for use with optically shallow or Case 2 water. These include empirical line fits and radiative transfer modeling. However, those techniques both require field measurements simultaneous with the image acquisition. Such measurements cannot be made in denied-access areas and are not practical for routine application. These techniques are thus limited in their applicability.
My talk will review the spectrum-matching techniques currently used for bathymetry and bottom classification retrievals, as well as the techniques used for atmospheric correction in optically shallow and Case 2 waters. Examples of successes and failures will be presented.
Thursday, May 3, 2012
AOSC494: Atmospheric and Oceanic Science Seminar
AOSC 818: Frontiers in Atmosphere, Ocean, Climate, and Synoptic Meteorology Research