Seismic expressions of shallow gas in the lacustrine deposits of Lake Balaton, Hungary

Abstract

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, whereas CO2 is transported to the lower mud layers by upwelling fluids.

Sound Velocity in a Thin Shallowly Submerged Terrestrial-Marine Quaternary Succession (Northern Adriatic Sea)

“…They could be produced by reflections from gravel horizons; however, their occurrence does not correlate with gravel layers determined in the boreholes (Figures 3 and 4). The diffraction hyperbolas more likely indicate low concentrations of gas in the sedimentary column, commonly encountered in high-resolution geophysical profiles [85][86][87]. Reflective features in the water column (Figure 3b,c and Figure 4c; Section 3.2), which commonly result from gas-bubble plumes emitting from the seafloor [88][89][90], further indicate gas occurrence.…”

“…Terrestrial-marine Quaternary successions often contain significant amounts of degrading organic matter; consequently, locally present gas further influences sound velocity in these settings (Section 4.1.3). Different gas indicators can easily be recognized from high-resolution geophysical data [85][86][87][88][89][90][91][92][93], facilitating the mapping of low-velocity areas. As the velocity decrease associated with the presence of gas is often quite variable e.g., [93], we suggest avoiding detailed velocity analysis in gas-rich areas.…”

Sediment contributing invasive dreissenid species in a calcareous shallow lake – Possible implications for shortening life span of lakes by filling

“…There are numerous publications available, ranging from early works in the last century to more recent ones (Lőrenthey, 1911;Herodek et al, 1988a). Among these, studies of lake limnogeology (Cserny and Nagy-Bodor, 2000;Cserny, 2002;Jordan et al, 2005;Van Dessel et al, 2008) and seismics (Novák et al, 2010;Tóth et al, 2010;Zlinszky et al, 2010;Visnovitz et al, 2015) are also numerous.…”

Shallow seismic characteristics and distribution of gas in lacustrine sediments at Lake Erçek, Eastern Anatolia, Turkey, from high-resolution seismic data

“…In shallow water environments, such as lakes, rivers, estuary, and continental shelves, that are often characterized by rapidly accumulating, organic-rich, and ne-grained sediments (Visnovitz et al, 2015), seismic re ection pro ling is considered to be useful for the distributional pattern of shallow gas. Seismo-acoustic pro ling methods are non-invasive, relatively low cost and are based on continuous measurement over the area of interest, which provides a fast and detailed image of shallow gas occurrences and their spatial distribution (e.g., Orange et al 2005;Visnovitz et al, 2015;Jaśniewicz et al, 2019).…”

“…In shallow water environments, such as lakes, rivers, estuary, and continental shelves, that are often characterized by rapidly accumulating, organic-rich, and ne-grained sediments (Visnovitz et al, 2015), seismic re ection pro ling is considered to be useful for the distributional pattern of shallow gas. Seismo-acoustic pro ling methods are non-invasive, relatively low cost and are based on continuous measurement over the area of interest, which provides a fast and detailed image of shallow gas occurrences and their spatial distribution (e.g., Orange et al 2005;Visnovitz et al, 2015;Jaśniewicz et al, 2019). This is mainly due to the very distinctive acoustic properties of shallow gas in comparison with the surrounding water and sediments (Anderson and Hampton 1980;Wilkens and Richardson 1998;Jaśniewicz et al, 2019) and also, the strong attenuation of high-frequency signals that allows the detection of very low amounts of free gas in sediments (e.g., Duarte et al 2007;Visnovitz et al, 2015).…”