Context and objectives
Karst collapses are important natural hazards for roads, houses, industries and water
tables. More than 150 sink holes had been listed until today in the Tournaisis.
Generally, they are cylindrical collapses with a maximum diameter of twenty meters
and more than ten meters depth. The occurrence of those holes seems to be
fortuitous. However, prone karst collapse areas are spatially well correlated with the
location of joints in the Tournaisian limestone. It does not exist any signs to foresee
the danger. Indeed, the quaternary deposits hide completely the inner soil
movements, which will become sink holes.
The objective of this feasibility study consists in an attempt of demonstration that it is
possible to explain some parts of the topsoil moisture distribution extracted from ERS
SAR data by the natural fracturing of the Tournaisian limestone. If the correlation
exist, it will be possible to derive hazards and risks maps for decision planner
makers. Indeed, we need to solve a regional problem at reasonable cost. Microwave
remote sensing is of interest for that scale. However, due to the small size of
collapses, we need to find indirect information detectable with satellite radiometers
and obviously related to our problem. We assume that topsoil moisture is the data
which link up the remote sensing technique and the karst collapses.
Expected scientific results
After discussions with the members of the team, we think that the basic ideas are
relatively well founded for the parts where tertiary and quaternary deposits are
relatively thin. Some problems had been encountered with ERS data. We think that
the methodology should given accurate results with airborne SAR. Indeed, in that
way, some parameters could be more controlled such as the date of the fly in order
1) To free from the bad weather conditions and the roughness of the surface (control
the size of the vegetation over the test area);
2) To reduce the size of the pixels;
3) To increase the performance in the georeferencing step.
The cartography of the moisture depends on the roughness parameter. After the bad
experience of that feasibility, it seems to be clear that a strong knowledge of the
efficiency in the methodology of acquisition is the first factor to master. The prone
karst collapse area detection is really worrying in the Tournaisis part because of
increasing of pumping per year. At present, there is no state control about the relation
between the occurrence of sink holes and the pumping of the water table. Only few
scientists say about the problem or media when collapses had damaged
infrastructures. The high complexity to forecast such a phenomenon should imply
more research than presently.
|Project leader(s):||ULg - Laboratoire de Géomorphologie et Télédétection|