The difference of about 40% in the results of the lake water content between both earlier studies, necessitated further investigations with improved equipment such as the Multibeam Echosounder (MBES). (2006) intimating that the use of a single beam sonar for bathymetric surveys was inappropriate for lakes with steep walls, used a plumbline and handheld GPS.
(1987) used a monobeam sonar for the bathymetric survey of Lake Monoun and revealed a crater of 96 m depth with gentle to steep walls. Several methods with varying results have been applied by previous workers at Lake Monoun to decipher the underwater topography as well as to produce the depth volume variation curve (hypsographic curve). In view of the fact that the amount of gas in the different layers of the lake is a function of the corresponding volumes of water, an exact description of the topography of the lake floor is of paramount importance for effective monitoring of degassing (Halbwacks et al., 2006). Monitoring of the degassing ensures both an evaluation of the system as well as an appraisal of the amount of gas remaining inside the lake at any given time. This method which was developed and experimented at Lakes Nyos and Monoun though less accurate than the classical method, it is both easier and faster.
Thus, another method, which uses Sound Speed (SS) and Electrical Conductivity (EC) in carbonated water called SS-EC method was introduced to measure and estimate the total concentration of dissolved CO 2 (Sanemasa et al., 2015 Saiki et al., 2016). For example, more than 4 h is needed to collect water samples at different depths along a single vertical profile at Lake Nyos (210 m deep), followed by chemical analyses in the laboratory. The classical method of direct CO 2 measurement, which necessitates chemical analysis of water samples (Kusakabe et al., 2000, Kusakabe et al., 2008 Yoshida et al., 2010) is accurate, but requires time and effort. In order to evaluate the amount of gas remaining inside the lake at a given time, several methods of CO 2 measurement were tested. Detailed descriptions of these techniques can be found in (Halbwachs et al., 1993 2004, this volume Halbwachs and Sabroux, 2001). The degassing protocols were set up, artificial degassing pipes designed, constructed and installed in Lakes Monoun and Nyos. After this tragedy, politicians and scientists discussed and agreed on a way to remove the gas accumulated in the lakes so as to prevent future similar disasters. These gas explosions have been attributed to prior built-up of excessive magmatic CO 2 in the bottom layers of both lakes (Kling et al., 1987 Sigurdsson et al., 1987).
In 1984, a gas explosion from this lake killed 37 people and two years later, a similar event occurred at Lake Nyos (barely 100 km apart) killing 1746 people. The mean annual rainfall and temperature are 1540 mm and 26 ☌ respectively (Wirmvem et al., 2013). The climate is equatorial characterized by a long rainy season which runs from March to November and a dry season that stretches from December to February. The geology is essentially made up of Quaternary volcano sediments underlain by a Precambrian basement complex of Pan-African age (Kamtchueng et al., 2016). Lake Monoun which lies between longitude 10.57 0-10.6 0 E and latitude 5.575 0–5.585 0 N is the largest (surface area 0.62 km 2) and deepest (maximum depth 100 m) of the seven volcanic lakes located within the Noun plain of the West region of Cameroon (Fig.
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