Seismo-acoustic signatures of shallow free gas in the Bornholm Basin, Baltic Sea

Tóth, Zsuzsanna; Spieß, V.; Jensen, Jørn Bo

doi:10.1016/j.csr.2014.08.007

Cited by 40 publications

(32 citation statements)

References 34 publications

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“…Seismo‐acoustic imaging of free gas depends on the measurement frequency and the resonance frequency of gas bubbles in the sediment (Tóth, Spiess, and Jensen ). Theory predicts that the resonance of gas bubbles in sediments under small acoustic perturbations and its frequency is mainly related to bubble size, sediment physical properties, and the ambient hydrostatic pressure (Anderson and Hampton ).…”

Section: Introductionmentioning

confidence: 99%

“…Lower frequency seismic waves (0.1 kHz–1 kHz) are below the resonance frequency of gas bubbles; thus, free gas only affects the bulk properties of the sediment (compressional wave velocity and bulk density). In seismic data, the top of the gas‐charged sediment layer appears as reversed polarity reflections and enhanced reflections (Tóth, Spiess and Jensen ). Ideally, geophysical surveys investigating shallow gas utilize instruments with different frequencies to reliably detect gassy sediments and to image the underlying geological structures.…”

Section: Introductionmentioning

confidence: 99%

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

Visnovitz

Bodnár

Tóth

et al. 2015

Near Surface Geophysics

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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.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Visnovitz

Bodnár

Tóth

et al. 2015

Near Surface Geophysics

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“…The other site where gas contents were estimated from compressional wave velocities [Tóth et al, 2014b] is indicated with the blue line. Gray denotes areas with shallow gas (compiled from Jensen and Endler [2012] and Tóth et al [2014a]).…”

Section: Theory Of the Acoustic Behavior Of Gas-bearing Sedimentsmentioning

confidence: 99%

“…Gas bubbles create various features in seismoacoustic profiles, because their presence causes significant changes in the physical and acoustic properties of the sediment [Anderson and Bryant, 1990;Judd and Hovland, 1992]. The expressions of free gas and the penetration through the gassy layer are strongly dependent on the measurement frequency [Hart and Hamilton, 1993;Mathys et al, 2005;Schneider von Deimling et al, 2013;Tóth et al, 2014a]. High-frequency acoustic waves are strongly scattered by gas bubbles and this results in chaotic reflections and blank zones in acoustic profiles [Judd and Hovland, 1992].…”

Section: Introductionmentioning

confidence: 99%

Frequency dependence in seismoacoustic imaging of shallow free gas due to gas bubble resonance

2015

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Shallow free gas is investigated in seismoacoustic data in 10 frequency bands covering a frequency range between 0.2 and 43 kHz. At the edge of a gassy patch in the Bornholm Basin (Baltic Sea), compressional wave attenuation caused by free gas is estimated from reflection amplitudes beneath the gassy sediment layer. Imaging of shallow free gas is considerably influenced by gas bubble resonance, because in the resonance frequency range attenuation is significantly increased. At the resonance frequency of the largest bubbles between 3 and 5 kHz, high scattering causes complete acoustic blanking beneath the top of the gassy sediment layer. In the wider resonance frequency range between 3 and 15 kHz, the effect of smaller bubbles becomes dominant and the attenuation slightly decreases. This allows acoustic waves to be transmitted and reflections can be observed beneath the gassy sediment layer for higher frequencies. Above resonance beginning at ∼19 kHz, attenuation is low and the presence of free gas can be inferred from the decreased reflection amplitudes beneath the gassy layer. Below the resonance frequency range (<1 kHz), attenuation is generally very low and not dependent on frequency. Using the geoacoustic model of Anderson and Hampton, the observed frequency boundaries suggest gas bubble sizes between 1 and 4–6 mm, and gas volume fractions up to 0.02% in a ∼2 m thick sediment layer, whose upper boundary is the gas front. With the multifrequency acoustic approach and the Anderson and Hampton model, quantification of free gas in shallow marine environments is possible if the measurement frequency range allows the identification of the resonance frequency peak. The method presented is limited to places with only moderate attenuation, where the amplitudes of a reflection can be analyzed beneath the gassy sediment layer.

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“…As a result of the EU projects METROL (7th Framework Programme) and BALTIC GAS (EU BONUS Programme), new information has been published on the distribution of shallow free methane in Holocene sediments in the Bornholm Basin (Jensen & Endler ; Tóth et al . , b). However, a detailed description of the formation of the Bornholm Basin and its relation with the more recent sedimentation history is still lacking and is the subject of this paper.…”

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confidence: 99%

The Bornholm Basin, southern Scandinavia: a complex history from Late Cretaceous structural developments to recent sedimentation

Jensen

Moros

Endler

et al. 2016

Boreas

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The Bornholm Basin has experienced a complex history linked closely to the structural development of the Tornquist Zone strike‐slip fault system. The latest major tectonic development was late Cretaceous and early Tertiary Rønne Graben inversion caused by dextral transpressional strike‐slip movements focused around rigid blocks such as Bornholm. By applying the strike‐slip concept, it is possible to show that the Rønne Graben inversion push‐up structure was created by restraining left step‐over. The fault system continues northwest and north of Bornholm where the strike‐slip movement resulted in right step‐over and the formation of a pull‐apart basin: the northern part of the Bornholm Basin. Detailed seismo‐acoustic and sediment core studies show that, despite a rather uniform present bathymetry in the central part of the Bornholm Basin, the Quaternary deposits mirror the pull‐apart basin synsedimentary depositional pattern with depocentres close to the major normal faults, bounding the Christiansø Ridge. IODP Site M0065 penetrated a secondary fault zone and provided detailed information on neotectonic events and on environmental changes during the Holocene. The generally rather homogeneous sediments in the cores indicate a dominance of oxic conditions in the Bornholm Basin during the Holocene. However, strong internal seismic reflectors, indirectly reflecting variations in the sediment density and the reflection coefficient, were caused by a shift to stronger laminated, organic‐rich sediments that were deposited during the Medieval Warm Period (MwP). The variable depth positions of the MwP‐related strong reflectors indicate large differences in the spatial and temporal sedimentation pattern in the Bornholm Basin. These differences in the depositional environment may have been caused by local neotectonic events in the basin.

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Seismo-acoustic signatures of shallow free gas in the Bornholm Basin, Baltic Sea

Cited by 40 publications

References 34 publications

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

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

Frequency dependence in seismoacoustic imaging of shallow free gas due to gas bubble resonance

The Bornholm Basin, southern Scandinavia: a complex history from Late Cretaceous structural developments to recent sedimentation

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