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 Monoun had already been undertaken during the 1992 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 Sigurdsson et al. (1987).

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.

Comparison of a profile obtained from the bathymetric chart of Sigurdsson and the profile obtained with the plumbline.

The results are given below : in fact, the depth contour chart is very different to that published by Sigurdsson. The volume of water between 20 m and the bottom is estimated to be 8.1 Mm3 rather than the previous estimate of 5.9 Mm3, that is to say a difference of almost 40%. The total volume of dissolved gas is 17.25 Mm3 TPN instead of the 9.4 Mm3 previously estimated, which is to say a difference of more than 80%.


The reading of the depths of the lake using the hand held probe was done by the IRGM team (Gaspar Ayssi). The data processing (depth contour maps, 3D representation) was done by Patrick Richon (CEA-DAM) using Surfer software.