Estimating the free gas content in Baltic Sea sediments using compressional wave velocity from marine seismic data

Tóth, Zsuzsanna; Spieß, V.; Mogollón, José M; Jensen, Jørn Bo

doi:10.1002/2014jb010989

Cited by 48 publications

(48 citation statements)

References 51 publications

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“…The gas content estimates derived from the amplitude losses are similar to gas concentrations measured in pressurized cores from the Eckernförde Bay, where at a water depth of ∼25 m, the average methane gas concentration at in situ pressure and temperature was approximately 0.02% in the muddy sediments [ Abegg and Anderson , ; Anderson et al , ; Martens et al , ]. Gas contents estimated from compressional wave velocities in another shallow gas patch in the southern part of the Bornholm Basin (Figure ) were between 0.03% and 0.07% [ Tóth et al , ]. The 4.3 kHz PS data inside that gas patch showed no reflections beneath the gas front and this complete acoustic blanking is consistent with the somewhat higher gas contents determined there.…”

Section: Discussionmentioning

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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Section: Discussionmentioning

confidence: 99%

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

2015

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“…) and interval velocities (e.g., Leighton and Robb ; Toth et al . , ; Vardy et al . ), P‐wave attenuation measurements (e.g., Morgan et al .…”

Section: Integrated Frameworkmentioning

confidence: 99%

“…; Morgan, Vanneste and Vardy ; Toth et al . ; Cevatoglu et al . ; Toth, Spiess and Kiell ; Vardy et al .…”

Section: Introductionunclassified

“…Ultimately, we present a number of ideas for future research activities in this field. published literature demonstrably derives more advanced properties, such as gas saturation (e.g., Morgan et al 2012;Morgan, Vanneste and Vardy 2014;Toth et al 2014;Cevatoglu et al 2015;Toth, Spiess and Kiell 2015;Vardy et al 2015), that are of broader interest to geologists and geotechnical engineers.This limited quantitative characterization and disparate terminology often leads to problems integrating geophysical data with the results from directly sampled geological and geotechnical data. Purely qualitative integration based on seismo-and lithostratigraphic correlation is often difficult because, while the geological/geotechnical data are recorded in depth below seafloor (SF), the geophysical data are acquired in two-way travel 19693-NSG17 August BOOK.indb 387 12/07/17 09:56 19693-NSG17 August BOOK.indb 388 12/07/17 09:56 Remote characterization of shallow marine sediments 389…”

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State‐of‐the‐art remote characterization of shallow marine sediments: the road to a fully integrated solution

Vardy

Vanneste

Henstock

et al. 2017

Near Surface Geophysics

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Current methods for characterizing near‐surface marine sediments rely on extensive coring/pene‐trometer testing and correlation to seismic facies. Little quantitative information is regularly derived from geophysical data beyond qualitative inferences of sediment characteristics based on seismic facies architecture. Even these fundamental seismostratigraphic interpretations can be difficult to correlate with lithostratigraphic data due to inaccuracies in the time‐to‐depth conversion of geophysical data and potential loss and/or compression of high‐porosity and under‐consolidated sea‐floor material during direct sampling. To complicate matters further, when quantitative information is derived from marine geophysical data, it often describes the sediments using terminology (e.g., acoustic impedance and seismic quality factor) that is impenetrable to geologists and engineers. In contrast, for hydrocarbon prospecting, reservoir characterization using quantitative inversion of geophysical data has developed enormously over the past 20 years or more. Impedance and amplitude‐versus‐angle inversion techniques are now commonplace, whereas computationally expensive waveform inversions are gaining traction, and there is a well‐developed interface between these geophysical and reservoir engineering fields via rock physics. In this paper, we collate and review the different published inversion methods for high‐resolution geophysical data. Using several case study examples spanning a broad range of depositional environments, we assess the current state of the art in remote characterization of shallow sediments from a multidisciplinary viewpoint, encompassing geophysical, geological, and geotechnical angles. By identifying the key parameters used to characterize the subsurface, a framework is developed whereby geological, geotechnical, and geophysical characterizations of the subsurface can be related in a less subjective manner. As part of this, we examine the sensitivity of commonly derived acoustic properties (e.g., acoustic impedance and seismic quality factor) to more fundamentally important soil properties (e.g., lithology, pore pressure, gas saturation, and undrained shear strength), thereby facilitating better integration between geological, geotechnical, and geophysical data for improved mapping of sediment properties. Ultimately, we present a number of ideas for future research activities in this field.

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Characterization of gas‐bearing sediments in the coastal environment using geophysical and geotechnical data

Dusart

Tarits

Fabre

et al. 2022

Near Surface Geophysics

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Seismic investigation in marine gas-bearing sediments fails to get information below the acoustic mask created by free gas. To circumvent this problem, we combined collocated multichannel ultra-high resolution seismic imaging, marine electrical resistivity tomography (MERT) and core sampling to study the physical properties of gas-bearing sediments in the Bay of Concarneau (France). We obtained sections of compression (P-) wave velocitvalues where free gas was identified in seismic data. We tested a joint processing workflow combining the 1D inversion of the MERT data with the 2D P-wave velocity through a structural coupling between resistivity and velocity. We obtained a series of 2D resistivity models fitting the data whilst in agreement with. The resulting models showed the continuity of the geological units below the acoustic gas fronts which is associated with paleo-valley sediment infilling. We were able to demonstrate relationships between resistivity and velocity differing from superficial to deeper sediments. We established these relationships at the geophysical scale then compared the results to data from core sampliand porosity). We inferred the porosity distribution from the MERT data. At the core locations, we observed a good agreement between this geophysical scale porosity and the core data both within and outside the gas-bearing sediments. This agreement demonstrated that resistivity could be used as a proxy for porosity where no was available below gas caps. In these regions, the observed low resistivity showed a high porosity (60-70%) down to about 10-20 m in depth in contrast with the surrounding medium with porosity less than 55%. These results support the hypothesis that failures inside the paleo-valley sediment could control the gas migration

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Estimating the free gas content in Baltic Sea sediments using compressional wave velocity from marine seismic data

Cited by 48 publications

References 51 publications

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

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

State‐of‐the‐art remote characterization of shallow marine sediments: the road to a fully integrated solution

Characterization of gas‐bearing sediments in the coastal environment using geophysical and geotechnical data

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