Optical calibration, bathymetry, water column correction and bottom typing of shallow marine areas, using passive remote sensing imageries

Geraldton, Western Australia

work done nov-dec 2011


1 - NO NEED for field data, nor for atmospheric correction
2 - this is demonstrated in this website, using a variety of hyper/multi spectral data
Requirements are
1 - homogeneous water body and atmosphere
2 - some coverage of optically deep water
3 - some coverage of dry land
Problems are
1 - the precision on estimated depth is found wanting, because the noise-equivalent change in radiance  of accessible data is too high for shallow water column correction work 
2 - radiance data should be preprocessed by the provider at level 1 in order to improve S/N ratio
3 - exponential decay: the deeper/darker the bottom, the poorer the performances
I keep digging
until suitable data
become available
WV2 at Geraldton sorry: private pages
co-registered images and seatruth profiles courtesy of FUGRO-NPA

Only the above two TM images yield "reasonable" results.
They exhibit the same topographic trends and features.
I have managed so that they yield approximayely the same computed depth.
It shall be seen that the WV2 image
does not add any major novel feature to the description below.
A gently sloping rocky platform, 5-10 m deep,
followed by a deeper lagoon ~15 m deep,
and then by a series of rocky reefs,
in potent and complex hydrodynamic conditions.

This is how the litterature describes
the ~ 10 km wide shallow waters
along the coasts of WA.
The above two Landsat images yields results
which nicely fits with this description.

Add to it an ubiquitous cover
of sands, green seagrasses,
brown algae and very dark epiphyts.
Long sand dunes over a rocky platform
  • A bright bottomed shoal appears on profile_4A, centered  at ~9 km.
    • It is ~10 km long, ~1.5 km wide, and ~ 4.5 m high.
  • That's what a submerged sand dune resting on a gently sloping rocky platform would look like, in which case we can't expect it to be stable over time.
  • But its water column corrected reflectance only reaches 28% of the reflectance of the sand dunes on land: this calls for fairly dark terrigeneous sands and/or partial coverage with seagrass.
Long sand dunes over a rocky platform
Many such dunes are observed, oriented SW-NE.
  • They seem to evolve as a result of the complex hydrodynamic conditions determined by a deeper lagoon and a series of rocky reefs farther offshore.
  • They might as well be somehow connected to the sand spits and tombolos which characterize the intertidal domain.

"foul waters/atmosphere"