An exact description of the topography of the lake floor is important for our study since the volumes of gas in the different layers of the lake that we are trying to evaluate are calculated according to the corresponding volumes of water.

The bathymetry of Lake Nyos had already been undertaken during the 1995 field trip using a wide beam sonar. We had already noticed that the underwater topography did not really correspond to the bathymetric chart drawn up by Hassert. (1912) with a ten metre depth resolution and Nojiri et al. (1993) using a 25 m resolution.

The use of sonar results in serious mistakes in the case of a lake with steep sides or indeed with rocky cliffs. We thus carried out an exact bathymetry using a plumbline and a GPS. The depth resolution is now 5m.

The reading of the depths of the lake using the plumblinewas done by the IRGM team (Gaspar Ayssi, Romaric Ntchantcho, Issa, Raphael Abega etc). The data processing (depth contour maps, 3D representation) was done by Patrick Richon (CEA-DAM) using the Surfer software.