Detecting Coral Reef Bleaching with High Resolution Satellite Imagery Products

Methodology

1. Image Aquisition - When a bleaching event is suspected due to anomalously high Sea Surface Temperature (SST), satellite imagery of the threatened reef is acquired. The "event" image is purchased along with an archived "reference" image of the same reef system. Ideally, the images have identical viewing angle geometries and cloud cover is sparse. Choppy seas are also better avoided.
2. Co-Registration and Subsetting - The images need to be the same size and co-registered to the extent possible. As these images are taken with the same instrument the spatial resolution is comparable. Using ENVI, the event image is "warped" to be maximally aligned with the reference image. Then both images are cropped to identical size.
3. Normalization - Neither the reference image nor the event image are calibrated to absolute radiance, which adds uncertainty to their inter-comparison. There are many factors contributing to the differences in brightness including:
   • Changes in vegetation and other physical differences
   • Different viewing geometries
   • Different solar zenith and azimuthal angles
   • Instrument fluctuations
   • Wave patterns
   An effort is made to normalize the brightness of the event image to the reference image. The assumption is that the darkest pixels of each image (typically the shadow of a cloud on the open water) should be approximately equal. And the brightest pixels of each image (typically sunlight reflected off of white beach sand) should also be approximately equal. From these two assumptions it is possible to derive a gain and offset (for each channel).

   The event image is scaled by the derived gain and offset. The resultant normalized image has similar values to the reference image in the very dark and very bright extremes. Presumably, this linear scaling is also valid for the mid-range of the values - allowing the images to be radiometrically compared.

4. Difference Calculation - By subtracting the reference image from the normalized event image, changes are accentuated. Areas with little change between the two images should show difference values near zero. A brightening caused by the event should appear as a positive number and a darkening would be negative. The difference image is typically displayed using an "equalization" enhancement. This equalization attempts to distribute the color range of the image equally between the 0-255 palate range of the display. The effect is to exaggerate subtle differences in the mid-range of the image.
5. Change Detection Analysis and Interpretation - During a coral bleaching event, coral looses the algae that accounts for most of its pigmentation. The coral, which is usually multi-colored, appears white.

   Healthy Coral	Bleached Coral
   Images from the NOAA Photo Library

   Remote sensing of a bleached coral reef should show significantly more reflectance in the green and blue bands. Since the penetration of light in the red and near-infrared (NIR) is smaller, detecting change would be unlikely. So a bleaching event should be characterized by a positive signal in IKONOS channels 1 and 2, and values near zero in channels 3 and 4. The characteristics of these channels and how they are mapped in the difference images are shown in this table:

   IKONOS Channel	Wave Band	Mapping
   1 (blue)	0.455-0.516 µm	R: Red
   2 (green)	0.506-0.595 µm	G: Green
   3 (red)	0.632-0.698 µm	B: Blue
   4 (near-infrared)	0.757-0.853 µm	Not Displayed

   The result is that a brightening in IKONOS channels 1 and 2 (and relatively no change in channel 3), as associated with bleaching, will appear gold in the equalized Difference image.