Discoloured waters in the Great Bahamas Bank
cause very bad artifacts in 4SM

Can a (semi)-analytical approach cope?


On March 21rst 2015, discoloured waters are flushed
away from the platform into the Tongue Of The Ocean in the Great Bahamas Bank

I hypothesize that, 
under soft modern sedimentation, there is a giant karst system
and a fresh water lens which is subject to tidal pumping,
through a great number of dolines, sinkholes and fractures.
Black waters seep up here and there from below
and might even entertain some kind of transient blooms.
This is the cause of bad artifacts in 4SM results.

Yellow substances and discolored waters
see Mobley 2004

Decay of organic matter produces "yellow substances" which absorb photons in the Blue range.
This affects heavily Coastal and Blue bands,
by reducing their water volume reflectance Lw
and increasing the operative 2K over the whole visible range. 
This denotes a "Coastal" water ype of Jerlov.
Such discoloured waters should exhibit
the following optical properties:

  • Increased operative KCOASTAL    and KBLUE 
    • i.e. ratio KBLUE/KGREEN >=1   (instead of ~0.4-0.7)
  • Decreased LwCOASTAL   and LwBLUE
  • Such waters tend to form conspicuous local bodies of discoloured waters
    • But we must assume that "yellow substances" are everywhere, with variable concentration
  • They seem to appear/disapear at the same location though
    • which means that they do not wander around
    • which also points to some local cause of the production of yellow substances, like accumulation of dead seagrass leaves, or seeping from some underground source (tidal pumping)
  • This is observed in most images of this scene,
  • They must not be confused with stretches of dense bottom vegetation
  • Bottom features are still visible through the discoloured water though
  • 4SM assumes horizontal homogeneity of water optical properties
  • 4SM water column correction results usually become very bad over areas of discoloured waters:
    • retrieved depths tend to be under-estimated
    • water column corrected bottom reflectances are under-estimated accordingly
  • Of course, only a (semi-)analytical method -along with a access to a good spectral library of bottom types and water IOPs- is capable of resolving the horizontal variations of water optical properties
whitings are associated with much stronger remote sensing reflectance due to suspended calcium carbonate particles,
  • resulting in a bright milky turquoise colour; shallow bottom detection possibly prevented
        not whitings
in contrast, discoloured areas here exhibit very  low remote sensing reflectance, particularly in the Blue range,
  • resulting in a dark/dull greenish colour; shallow bottom detection still occurs though
  • might be consistent with tidal pumping from organic-ladden fresh underground water lens through blue-sinkholes
  • they do not seem to appear in winter : May to October is the rainy season

September 7th 2013

August 12th 2015

September 7th  2013
  March 21rst 2015
      December 15th 2014    January 29th 2014

March 21rst 2013

September 7th 2013
August 12th 2015

December 15th 2014
 January 29th 2014 I came to accept that
there is no way in 4SM to avoid
bad underestimation of retrieved depth
and water column corrected reflectances
under such discoloured waters,
where shallow bottom is frequently detected
through such local "coastal waters".
  • The cause is that yellow substances absorb photons in the blue range,
    • and operative diffuse attenuation coefficient are reduced over the whole visible range.
  • The result in 4SM is that, under such local discoloured waters,  the retrieved depth is under-estimated and the water column bottom reflectance is under-estimated accordingly


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