Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
775
Classification of bottom composition and bathymetry of shallow
waters by passive remote sensing
D.Spitzer & R.W.J.Dirks
Netherlands Institute for Sea Research, Den Burg, Texel
ABSTRACT: Reflectance of solar radiation contains information about the composition of the watercolumn as
well as about the bottom in areas where the visible light penetration depth exceeds the bottom depth.
When the mapping .of the bottom depth and composition is pursued, specific algorithms must be developed
in order to remove the influence of the watercolumn on the upwelling optical signals. Calculations were
performed relating the reflectance spectra to the parameters of the watercolumn and of the diverse bottom
types. Measurements of the underwater reflection coefficient of sandy, mud and vegetation-type seabottom
were performed. Two-flow radiative transfer model was employed, where the spectral signatures of suspended
and dissolved materials and of the bottom were introduced as the input parameters. Several algorithms
are proposed with respect to the application of Landsat MSS, TM and SPOT HRV scanners. Bottom depth and
features appear to be observable down to 3-20 m, dependently on the water composition and bottom type.
RESUME: La reflexion diffuse du rayonnement solaire détient des informations sur la composition de la
colonne d'eau ainsi que sur le fond de la mer dans des zones où la profondeur de pénétration de la lumière
visible excède la profondeur du fond. Pour dresser la carte de la profondeur du fond et de sa composition
il nous faut des algorithmes capables d'éliminer l'influence de la colonne d'eau sur les signaux optiques
ascendents.
Des calculs ont été faits pour estimer le rapport entre les spectres de réflexion diffuse avec les
paramètres de la colonne d'eau et des catégories diverses du fond. Des mesures de la réflexion diffuse
dans l'eau ont été effectuées sur des fonds de catégorie sableuse, boueuse et végétative. Le modèle de
transfert radiatif des deux flux a été utilisé, dans lequel les signatures spectrales des substances
dissoutes, des matières en suspension et du fond ont été introduit. Plusieurs algorithmes ont été proposés
en rapport avec l'application de Landsat MSS, TM et SPOT HRV scanners.
La profondeur et composition du fond se trouve détectable jusqu'à 3-20 m, selon la composition de l'eau et
de la catégorie du fond.
1 INTRODUCTION
In shallow waters and tidal areas solar radiation
transmitted by the water layer and reflected by the
bottom can substantially contribute to the upwelling
-optical signals. When appropriate absorption and
backscattering signatures (coefficients) of both,
the waterlayer and of the bottom, are introduced
into a rigorous radiative transfer model, the
contribution of the bottom reflectance can be
evaluated and used for the remote determination
of the bottom depth and type.
Spectral despendence of all the optical coefficients
must be accounted with respect to the spectral
characteristics of the remote (satellite or air
borne) sensors. Recent investigations on the
optical properties of the watermasses and on the
radiative transfer in natural waters, generally
initiated by the development of modern remote
sensing techniques, allow calculation of the up
welling spectral radiance and hence establishment
of useful algor ithms for bottom depth and bottom
and watercolumn composition mapping.
Two-flow radiative transfer model in its
reflectance form is used in this paper, with as
boundary condition the (wavelength dependent)
bottom reflectance. Variable concentrations of
chlorophyllous and non-chlorophyllous particulate
and organic dissolved materials are accounted as
well as various types of the sea bottom. Reflectances
within the spectral bands of the Landsat Multi-
Spectral Scanner (MSS), Landsat Thematic Mapper
(TM), SPOT High Resolution Visible (HRV) and the
TIROS-N series Advanced Very High Resolution
Radiometer (AVHRR) were computed in order to
develop appropriate algorithms suitable for the
bottom depth and type mapping. Ground resolution
(20-80m) of the Landsat and SPOT instruments is
more suitable for the coastal mapping purposes
than the resolution of the Coastal Zone Color
Scaner (CZCS, 800 m) and of the AVHRR (1100 m).
However, the wide accessibility of the AVHRR
data makes also this sensor attractive when large
scale mapping is pursued. The orbital repeat
cycles of Landsat and SPOT (16-26 days) are
sufficient for observation of long term bottom
depth and composition variability.
Linearity and high sensitivity of the algorithms
for the quantities to be detected is desired along
with low sensitivity of the other parameters.
2. TWO-FLOW RADIATIVE TRANSFER MODEL
Upwelling radiance measured by the remote optical
sensors is proportional to the reflectance R.
The reflectance dependence on the depth z and
wavelength X can be described by the radiative
transfer equation
dR(z, - X j- = - b (X) + (a (z)+b (z)+a,(z)+b,(z)) R(z,X)
dz d u u d d
-b 2 (z)R 2 (z,X)
u
where a , a^ are the absorption coefficients for
the upwelling and downwelling irradiance res
pectively, and analogously b u , b^ are the back-
scattering coefficients. For shallow waters,
where the light penetration depth exceeds the
bottom depth, the boundary condition R(h,X) = r(X),
