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),