Résumé

The Gulf of Gabes in Tunisia is unique among
Mediterranean coastal environments by shallow
water extension. The hydrodynamics of this area
are influenced by semi diurnal tides with maximum
amplitude of 1.8m and a minimum of 0.3m.
Kerkennah islands, located in this gulf, are
characterized by a -10 m isobath at few kilometers
away from the shoreline. This archipelago is
occupied by sebkhas at low elevation and extension
in the sea as well as “plans” only slightly higher
(the highest point of the archipelago is 13 m and
50% of the archipelago is at an average altitude of
less than 2.5 m). The coastline is characterized by
its bluffs, low rocky shores, beaches or tidal
marshes. The lithology composition is dominated
by smooth rocks (sandstone and mio-pliocene,
clay). These features, combined with a sea level
rise and an active subsidence, constitute major risk
factors. Islands vulnerability is increased by sebkha
(salted low lands) extension which accounts now
for 45% of the total.
Study relies on the 30 years archive of Landsat 5
TM sensor managed by GSFC/NASA with 30m
spatial resolution. The depth assessment has been
carried out by an empiric method based on TM1
channel which has the better water penetration
properties (up to 25 m). We focused on summer
period and selected images from July 1986, August
1987, June 2003 and July 2009. The same
preprocessing (atmospheric corrections,
radiometric and geometric) has been applied. All
bands were visualized and analyzed in order to
extract relevant information, on the coastline, water
land interface, and on the marine environment
around the Kerkennah islands. The first step has
been to apply a near infrared based mask in order to
minimize the effect of the continent brightness.
Then an unsupervised comparison of the same
sensor is computed on two key dates. Due to seabed
homogeneity the change attribute is used as the
local variation of luminance pixel obtained by
subtraction between the bands TM1 1986 and 2009
.An invariant image differencing indicative of
changes between the two scenes has been
generated. The resulting image with positive or
negative value is likely to characterize changes
while those with near zero values correspond to the
pixels unchanged.
This image emphasizes the changes in time and
space, of the submarine morphology of the
continental shelf around the islands. The
reflectance change is induced by changes in depth
especially after the removed components in
seawater and the influence of suspended sediment
transport and also minimized the effect of the tide
as the images used have been downloaded all
during the same local time.
In the graphical representation, highest
differences values appear in saturated red or blue.
The coastal part has a pronounced positive
change related to the withdrawal of the coastline.
Sea flooding affects more some parts of the
coast than other (south and northwest of Chargio
Island).
The negative change area (red colored) show a
seabed reflection decrease. This change, obviously
related to a water column rise, is mainly noticeable
around the channels, the saline area and around the
Gremdi Island.
THEMATIC MAPPER DATA PROCESSING FOR MULTI
TEMPORAL MAPPING OF UNDERWATER
CONTINENTAL SHELF AROUND KERKENNAH
ISLANDS (TUNISIA)
Rim KATLANE1, Jean-Claude BERGES2, Gérard BELTRANDO2, Fouad ZARGOUNI1
1 Université Tunis El Manar, Unité de Recherche de Géomatique, Géologie Structurale et appliquée.
Université El Manar Tunis Tunisie,
Katlanerim@yahoo.fr, fouadzargouni@yahoo.fr,
2 Université Diderot, UMR 8586 du CNRS (PRODIG), 75205 Paris Cedex 13 France
Jean-Claude.Berges@univ-paris1.fr, beltrando@univ-paris-diderot.fr.
Correspondance auteur : Gérard BELTRANDO, Tél 06 85 56 28 99
A comparison plot radiance values (mW.cm-2.
sr-1.μm-1) was extracted along a radial with mesh
30 m along an axis 10 km in order to show changes
over time between the summer of 1986, 2003 and
2009. The first pixel from 0 to 80 for the years 1986
and 2003 shows an occupied land, while in 2009
this same area becomes flooded.
The treatment of multitemporal and multisource
analysis is essentially based on the mapping of
recent and ancient marine dynamics around the
Kerkennah islands. The spatiotemporal changes in
underwater morphology show a regression of the
coastline by location, a submergence of small plots
of Sebkha, an emergence and expansion of some
submarine channels.

MOTS-CLÉS : Landsat TM, Kerkennah, plateau continental