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Seismic expressions of shallow gas in the lacustrine deposits of Lake Balaton, Hungary
Author(s) -
Visnovitz F.,
Bodnár T.,
Tóth Zs.,
Spiess V.,
Kudó I.,
Timár G.,
Horváth F.
Publication year - 2015
Publication title -
near surface geophysics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.639
H-Index - 39
eISSN - 1873-0604
pISSN - 1569-4445
DOI - 10.3997/1873-0604.2015026
Subject(s) - geology , methane , upwelling , peat , geochemistry , carbon dioxide , paleontology , oceanography , ecology , biology
Lake Balaton, a large shallow lake in Central Europe (Hungary), has been the site of extensive ultra‐high‐resolution acoustic and multichannel seismic profiling in the period of 1997–2013. These surveys showed the widespread occurrence of shallow gas in the lake sediments and their immediate substrata. We analyzed about 2000 km of two‐dimensional profiles and mapped the different gas occurrences in the uppermost 20 m. The anomalies caused by free gas were identified, classified, and assigned to upper, middle and lower levels based on gas signatures and stratigraphic position. Monitoring of the uppermost gas front has revealed temporal variations between surveys from different years and seasons that manifested in the changes of free gas content in the upper two levels. Free gas in the lower part of the lake sediments and at around the base of the mud indicated greater stability. The different nature of the three free gas levels can be explained by vertical changes in quantity, production rate, and solubility of methane and carbon dioxide gases. We suggest that methane was derived from the microbial decomposition of organic matter in the mud and Pleistocene peat at the base of the mud, whereasCO 2is transported to the lower mud layers by upwelling fluids.