BALTIC GAS - ERANET BONUS-73 EU Funding


Verbundprojekt: BALTIC GAS - Methanemission in der Ostsee: Gasspeicherung und Auswirkungen des Klimawandels und der Eutrophierung, Vorhaben

​TP3: Seismische und akustische Charakterisierung flacher Gasvorkommen in der Ostsee

Joint Project: BALTIC GAS - Methane emission in the Baltic Sea: Gas storage and effects of climate change and eutrophication - WP3: Seismic and Acoustic Characterization of Shallow Gas in the Baltic Sea

​​Volkhard Spiess

Duration: 2009-01-01 to 2011-12-31, Funding: 82'700 € , FKZ 03F0488C

BMBF Final Report TP3


Partners

  • Center for Microbiology, University of Aarhus, Denmark - Prof. Bo Barker Jørgensen (Coordinator)
  • National Environmental Research Institute, Silkeborg, Denmark - Dr. Henrik Fossing
  • Max Planck Institute for Marine Microbiology, Bremen, Germany (MPI) - Dr. Timothy Ferdelman
  • Institute for Baltic Sea Research, Warnemünde, Germany (IOW) - Prof. Dr. Gregor Rehder
  • Alfred Wegener Institute for Polar and Marine Researach, Bremerhaven, Germany (AWI) - Prof. Dr. Michael Schlüter
  • Department of Geosciences, University of Bremen, Bremen, Germany (GeoB) - Prof. Dr. Volkhard Spieß
  • Geological Survey of Denmark, Copenhagen, Denmark (GEUS) - Dr. Jørn Bo Jensen
  • Intitute of Oceanology, Polish Academy of Science, Gdansk, Poland - Prof. Dr. Zygmunt Klusek
  • Department of Geology, Lund University, Sweden - Prof. Dr. Daniel Conley
  • Department of Geology and Geochemistry, Stockholm University, Sweden - Dr. Volker Brüchert
  • Department of Earth Sciences, Utrecht University, The Netherlands - Prof. Dr. Philippe van Cappellen
  • Winogradsky Institute of Microbiology, Russian Academy of Sciences, Moscow, Russia - Prof. Dr. Nicolai Pimenov


Cruises


Introduction

Most of the methane in shallow marine environments is of microbiological origin and is produced through the degradation of organic matter buried below the sulphate zone. The methane is produced by methane-producing microorganisms through a process called methanogenesis. As methane builds up in the sediment, it migrates towards the sediment surface either by molecular diffusion or as free gas bubbles and it penetrates the sulphate zone in the upper subsurface layers. Sulphate is the oxidant for methane, so at the sulphate-methane transition (SMT) zone most of the methane is converted to CO2. CO2 then diffuses up into the water column and is taken up by planktons.

On a global scale it is estimated that less than 10% of the methane produced in the sediment is released from the sea floor. This implies that by far most of the CH4 is effectively scavenged before it reaches the sediment surface. But methane also can accumulate beneath the SMT zone: these free gas bubbles develop at sediment depths where the methane concentration exceeds saturation at the ambient hydrostatic pressure. This pressure, and thus the saturating methane concentration, depends on the water depth. Occasionally, these free gas bubbles escape from the seafloor in a diffusive or an eruptive manner, it the latter case often creating a pockmark in the seafloor.

Methane is an effective greenhouse gas (with a thermal molecular absorption coefficient 25-fold higher than that of CO2) and although it still plays a smaller role, its concentration in the atmosphere has been increasing relatively faster than that of CO2 over the past decades. The ocean is today only a minor contributor to the atmospheric methane flux, yet special attention is needed because the flux has during the Pleistocene been positively correlated with global warming and might therefore cause a positive feed-back on the Earth’s heat budget.

Enhanced eutrophication by nutrients such as nitrate or phosphate leads to increased production of organic matter in marginal seas like the Baltic Sea. Climate change may increase the water temperature and reduce deep water ventilation, factors which will stimulate anaerobic degradation of organic matter buried in the sediment. As a consequence, more free methane gas will accumulate in the sediment and emission to the water column and into the atmosphere could be enhanced significantly. Accumulation of shallow gas in the seabed may pose hazards to seabed structures such as wind farms, pipelines, power or communications cables, and off-shore drilling operations by destabilizing the seafloor. Enhanced ebullition from hot-spots of shallow gas will also enhance the emission of H2S, which is toxic to fish and other marine life and is highly corrosive.

On this background described above there is an urgent need to better understand the marine methane cycle in general and the methane balance in the Baltic Sea in particular. The international research project ‘Baltic Gas’ aimed to better understand how climate change and long-term eutrophication affect the accumulation of shallow gas and the emission of methane from the seabed to the water column and into the atmosphere. 

The project brought together a multidisciplinary team of scientists with the goal to:

  • quantify and map the distribution and flux of methane in the Baltic Sea,
  • analyze the controls on the relevant key biogeochemical processes,
  • integrate seismo-acoustic mapping with geochemical profiling,
  • model the dynamics of Baltic Sea methane in the past (Holocene period), present (transport-reaction models), and future (with predictive scenarios),
  • identify hot-spots of gas and potential future methane emission in a Baltic database available for national authorities and scientists.

 

University of Bremen as part of BALTIC GAS / Work Package 3

The occurrence of shallow gas in marine sediments can be easily detected by acoustic profiling even at quantities as little as 0.5% of the pore volume. At seismoacoustic frequencies, typically a few hundred Hz to a few tens of kHz, acoustic propagation in the sediment is severely affected by the presence of free gas and this causes visible gas signatures and characteristic seismic ‘textures’ on seismo-acoustic profiles.

The overarching goal of WP 3 was 1) to detect, map, and quantify shallow gas in key areas of the Baltic Sea, 2) to relate these gas occurrences to the geological and geophysical structures, 3) to investigate the properties of gas-bearing sediments, 4) to quantify the amount of gas by innovative solutions, and 5) to verify them by quantitative assessment of the gas density from sampling results. Based on the results of the gas distribution and geophysical and core logging data, zones of recent and future sediment weakness for gas escape were going to be identified.


BONUS+ (Baltic Gas) international PhD training course – Seismoacoustic imaging of Sedimentary and Gas-related Features in the Baltic Sea

A training BONUS-course Seismo-acoustic Imaging of Sedimentary and Gas-related Features in the Baltic Sea organized by University of Bremen, Germany, and University of Szczecin, Poland, took place in the Malkocin Conference Center of the University of Szczecin (Poland) and on board the Polish M/V Nawigator XXI during 15-27 July, 2010.

Altogether 20 students participated of which 6 students came from the University of Szczecin (Poland) and 14 students were active in the BONUS-projects: Baltic Gas, Inflow and Hyper comprising the „BONUS-institutions: Institute of Oceanology of the Polish Academy of Sciences (Poland), P.P. Shirshov Institute of Oceanology of the Russian Academy of Sciences (Kaliningrad, Russia), and University of Bremen (Germany).

During the three-day preparatory course the marine geology of the Baltic Sea was presented by an invited lecture (Jan Harff, IOW/US), and the relevant instruments and survey methods of acoustic surface and sub-surface imagery were introduced to both geophysicist and non-geophysicist participants. Discussions about cruise-planning strategies aimed to acquaint participants with considerations leading to flexibility and suc-cessful decisions in scientific cruise management.

The seagoing expedition on M/V Nawigator XXI was carried out in the Polish waters of the Baltic Sea. Seismic and side scan sonar data were collected in the Pomeranian Bay, eastern Bornholm Deep and offshore Wladyslawowo. During these days, participants gathered experience in equipment handling, data acquisition, processing of seismoacoustic data, and using preliminary interpretations to aid cruise planning. 

The expedition was followed by a two-day post-cruise workshop. Results were evaluated, put in scientific context, and collected in a preliminary cruise report. Cruise participants presented selected topics in short lectures, highlighting different aspects of new data from the perspective of regional geology. Main scientific results include indications of shallow gas found south of Bornholm (Fig.3.), and the mapping of a basement fault zone in the eastern study area. 

The course convinced us that a mixture of theory and practice taught in groups produces fruitful discussions between young scientists and enthusiasm as well as knowledge about the selected topic.


Towards quantification of shallow free gas in Baltic Sea sediments (Toth et al., 2015)

The presence of free gas bubbles introduces fundamental differences in the properties of sediments and the response to seismic waves. While high frequency acoustic waves are strongly attenuated, lower frequency seismic waves are able to penetrate gas-charged sediment layers. The attempts to quantify the amount of gas based on the seismoacoustic data began with the investigation of seismic attributes that are sensitive to changes in gas content in the sediment. 

One seismic attribute indicating the presence of gas is compressional wave velocity and the multichannel seismic dataset (acquisition with several hydrophones/channels with increasing offsets) allows the determination of the velocity of seismic waves, which will be altered due to the effect of gas bubbles. The speed in gas-bearing sediment is significantly reduced due to lower wet bulk density and modification of other elastic and sediment physical properties. By carefully determining interval velocity from raw multichannel seismic data, based on a theoretical geoacoustic model, the amount of free gas in the sediment can be estimated. 

Velocity determination can be achieved in several ways. The Interactive Velocity Analysis module in the processing software VISTA, which is conventionally used in processing of shallow water seismic data, provides a rough, rather initial velocity field. We performed additional velocity analysis on pre-stack time migrated data, which, although time consuming and computationally intensive, allowed the determination of the velocity field over gassy areas more accurately and more extensively in space.

Depending on stratification (identifiable reflectors), the accuracy and resolution varies significantly, nevertheless, analysis showed that the sound speed in the gas-charged sediments is reduced significantly. In general, velocity drops from   ~1450 m/s in non-gassy fine-grained surficial sediments down to a few hundred m/s in the gas-charged zone. Beneath the gas patches, the post-glacial and glacial sediments reveal higher velocities (>1500 m/s) again.

To quantify the gas content based on the velocity field, we used Anderson & Hampton’s geoacoustic model (1980), which describes the relationship between compressional wave velocity and the physical properties of gas-bearing marine sediments. In the model, gas bubbles are assumed to be fully contained within the pore space, thus modifying its compressibility. Taking the interval velocity values between reflectors, free gas content in the pore volume can be estimated. Values of the free gas content at the test location in the Bornholm Basin range from 0.1 to 2%, where sensitivity becomes reduced. These numbers are basically in agreement with the geochemical modelling results.


Geoacoustic characterization of shallow gas content in Baltic Sea sediments (Toth et al., 2014a)

When excited, gas bubbles in the sediment resonate at a fundamental frequency, which is mainly determined by their size and the physical properties of the surrounding medium. As a result, acoustic behaviour will be different below, at and above resonance frequency, and attenuation due to the scattering effects will be strongest close to the resonance frequency. 

By imaging shallow gassy sediments at a broader frequency range, the gas bubbles in the sediment can be physically characterized in situ from their acoustic response. Using multi-frequency datasets and narrowing down the resonance frequency makes it possible to form an idea about the characteristics and amount of gas bubbles in the sediment. In the Bornholm Basin, gassy areas were surveyed with three frequencies of the Parasound sediment echosounder (4.2, 18.5 and 42.8 kHz) on board of R/V Maria S. Merian. High reflection amplitudes from and strong signal attenuation beneath the gas front occur at the lowest imaging frequency of Parasound (4.2 kHz), although natural attenuation normally increases with increasing frequency. Accordingly, this effect can be attributed to bubble resonance behaviour, which is not observed at the two higher frequencies. 

Based on the theoretical considerations of Anderson and Hampton (1980) and for typical sediment properties, bubble size distribution at the investigation site in the Bornholm Basin is likely to peak near ~2 mm (4.2 kHz) and smallest bubbles are larger than 0.2-0.4 mm (42.8 and 18.5 kHz, respectively).

The analysis of amplitudes of the multi-frequency Parasound data acquired on MSM 16/1 clearly indicates the frequency dependent attenuation and that attenuation near resonance frequency can be a measure of gas content. 

These two examples demonstrate a major progress in the attempt to quantify gas volumes in marine sediments from geophysical analyses. Using a diverse suite of seismic and acoustic equipment in parallel together with advanced methods of data processing and analysis, remote profiling measurements come in reach for gas quantification. While larger uncertainties still exist and basic physical concepts still have to be developed and tested, the acquired results for gas content and bubble sizes seem to be in good agreement with evidence from biogeochemical measurements and modelling.


Seismo-acoustic shallow gas signatures from the Bornholm Basin, Baltic Sea (Toth et al., 2014b)

Free methane gas in seafloor sediments creates a variety of seismic signatures on seismo-acoustic profiles. The most commonly cited evidences are acoustic turbidity (chaotic reflections) and acoustic blanking (the absence of reflections), mostly recognized on high frequency acoustic data. Other often recognized signatures include high amplitude (enhanced) or reversed polarity reflections and reverberation due to the difference in the acoustic impedance contrast compared to normal interfaces in the sediment.

Newly identified gas signatures were described from the Bornholm Basin, Baltic Sea. 

On the lowest Parasound frequency (4.2 kHz), the gas appears as a patch or layer of point scatterers, which have variable amplitudes and reflections of the sediment layering below are considerably weaker than in the gas-free areas or completely absent (acoustic blanking). On the two other Parasound frequencies (18.5 and 42.8 kHz), the point scatterers are not observed, the presence of shallow gas can be inferred from sudden high attenuation and associated acoustic blanking. Contrary to the behavior of acoustic waves in fully saturated marine sediments, the degree of attenuation caused by the scattering effect of gas bubbles is decreasing towards higher frequencies (Fig. 5).

In the Bornholm Basin, the accumulation of free gas bubbles in the sediment is very close to the seafloor, 0.5 - 4 m (Fig. 6). Since free gas in the sediment represents a big change in acoustic impedance, this gives rise to changes in the normal positive seafloor reflection on multichannel seismic data. The proximity of the gassy layer to the seafloor causes interference between the positive seafloor and negative gas layer reflection, since the distance between the two interfaces is smaller than the wavelength of the seismic wave. The shape of the seafloor wavelet is therefore changed, having a more complicated form, higher amplitudes and lower frequency (Fig. 8).


The proximity of the gas-charged layer to the seafloor in some places also causes the polarity of the seafloor reflection to change. Reversed polarity then clearly indicates a decrease in density and/or velocity in the seabed sediment. In places where interference or polarity reversal is observed at the seafloor, a strong multiple of this reflection appears as well, indicating increased reflectivity. This phenomenon is referred to as reverberation or acoustic ringing. 

A low frequency multibeam assessment: Spatial mapping of shallow gas  by enhanced penetration and angular response anomaly  (Schneider-von Deimling et al., 2013)

This study highlights the potential of using a low frequency multibeam echosounder for detection and visualization of shallow gas occurring several meters beneath the seafloor. The presence of shallow gas was verified in the Bornholm Basin, Baltic Sea, at 90 m water depth with standard geochemical core analysis and hydroacoustic subbottom profiling (Parasound). Successively, this area was surveyed with a 95 kHz and a 12 kHz multibeam echosounder (MBES). 

The bathymetric measurements with 12 kHz provided depth values systematically deeper by several meters compared to 95 kHz data. This observation was attributed to enhanced penetration of the low frequency signal energy into soft sediments. Consequently, the subbottom geoacoustic properties highly contributed to the measured backscattered signals. Those appeared up to 17dB higher inside the shallow gas area compared to reference measurements outside and could be clearly linked to the shallow gasfront depth down to 5 meters below seafloor. No elevated backscatter was visible in 95 kHz MBES data, which in turn highlights the superior potential of low frequency MBES to image shallow subseafloor  features.  Small gas pockets could be resolved even on the outer swath (up to 65°). 

Strongly elevated backscattering from gassy areas occurred at large incidence angles and a high gas sensitivity of the MBES is further supported by an angular response analysis. The study showed that MBES together with subbottom profiling can be used as a tool for spatial subbottom mapping in soft sediment environments.



One year of continuous measurements constraining methane emissions from the Baltic Sea to the atmosphere using a ship of opportunity (Gülzow et al., 2013)

Methane and carbon dioxide were measured with an autonomous and continuously running system on a ferry line crossing the Baltic Sea on a 2 - 3 day interval from the Mecklenburg Bight to the Gulf of Finland in 2010. Surface methane saturations show great seasonal differences in shallow regions like the Mecklenburg Bight (103 - 507 %) compared to deeper regions like the Gotland Basin (96 - 161 %). The influence of controlling parameters like temperature, wind, mixing depth and processes like upwelling, mixing of the water column and sedimentary methane emissions on methane oversaturation and emission to the atmosphere were investigated. 

In February 2010, an event of elevated methane concentrations in the surface water and water column of the Arkona Basin was observed, which could be linked to a wind-derived water level change as a potential triggering mechanism. To investigate free gas accumulations in subsurface sediments as a possible source of this anomaly, seismoacoustic data collected in the Arkona Basin during student cruises of the University of Bremen were analyzed.

Based on the acoustic data collected throughout the Arkona Basin, free gas accumulations in the Holocene mud are known to be widespread (Mathys et al., 2005; Thießen et al., 2006). While these gas layers originate from organic matter degradation within the mud where gas bubbles are efficiently trapped, only one seabed feature has been found in the region so far, which may indicate focused gas escape from the subsurface. This feature (Fig. 10.) is a subtle 1 m deep depression in the seafloor of slightly elongated shape with a maximum diameter of ~120 m, which is associated with a strong, but reversed polarity seafloor return and structural disturbance beneath. This indicates the actual presence of free gas at the seabed, as the phase reversal is a result of the decrease of bulk density and/or seismic velocity in the gas-charged sediment. The pockmark is surrounded with high amplitude seafloor reflections on both sides forming a few hundred meter wide ring. The deeper one of the double reflector near the seafloor represents the top of the gas-charged sediment layer. A further distinct reflector with reversed polarity appears at 23 ms TWT bsf and at 44 ms TWT bsf, a strong reflection marks a density/velocity increase from glacial clay to compacted till or Cretaceous basement. Evident is the structural disturbance in the vicinity of the pockmark, as well as a positive relief of a sedimentary unit between 78 and 88 ms TWT. Together with a potential fault system at 1100 m offset, we see typical indications for deeper gas migration (absence of reflections, circular anomaly, anomalous sedimentary unit), which distinctly differs from the typical appearance of shallow gas elsewhere in the Baltic Sea.

Highly elevated methane concentrations in the water column were detected only 6.5 nautical miles apart from this feature and in combination with the recorded sea level fluctuation, a pressure-induced seepage event is speculated. Such an event could have caused extraordinary high methane values in the entire water column. It can be assumed that the oscillation of the sea level and the resulting sudden pressure drop lead to the abrupt transition of dissolved (pore water) methane into free gas followed by ebullition of free gas to the water column, or by a pressure-induced (amplified by wind and waves) pumping impulse on the pore water of the sediment and thus, seepage of methane-enriched water to the water column.


Sulfate and methane fluxes and organic matter mineralization across a  Holocene mud layer of increasing thickness in Aarhus Bay (W Baltic Sea)  (Flury et al., 2016)

In many areas of the Baltic Sea, the free gas appears to be linked to the thickness of an organic-rich Holocene mud layer (HML) deposited since the end of the last glacial period approximately 10 kyr BP. For example, in Eckernförde Bay the minimum HML thickness required to generate gas appears to be 8 m (Whiticar, 2002), while it is only 4–6 m in Aarhus Bay and Arkona Basin (Thiessen et al., 2006; Jensen and Bennike, 2009). The reasons for these differences are presently unclear, and may be due to differences in organic matter content and reactivity. A firm understanding of the processes leading to dissolved and gaseous CH4 accumulation thus underpins the basis on which accurate predictions of CH4 release from the sediments can be made.

The overall goal of this study was to understand the interplay between CH4 cycling in Holocene sediments and the thickness of the mud layer where biogenic CHis produced and consumed. Porewater and particulate samples were analysed from 12 closely spaced stations on a transect in Aarhus Bay (Denmark) that extends from gas–free to gas–bearing sediment and where the HML thickness gradually increases. To obtain the base of the Holocene mud beneath the gas-charged sediment, multichannel seismic data were collected along the coring transect. While high frequency chirp signals are strongly attenuated in the gas-charged sediment layer, lower frequency airgun seismic is able to penetrate and provides information of the geological structures from below. For the analysis, the seismic data were pre-stack time migrated to improve the initial velocity field through an additional Migration Image Velocity Analysis in order to correct properly for the velocity decrease in and beneath the gas-charged sediment layer. 

Seismic profile GeoB12-035 (Fig. 11.) shows the sedimentary strata and structures along the transect. The gas-charged layer in the mud appears as a high amplitude reflector with reversed polarity. Below the shallow free gas, the base of the marine units is marked by a sharp erosional boundary (Jensen and Bennike, 2009), traceable along the transect. The interference from several reflectors at the transition from non-gassy to gassy sediment (top and base of the gas-charged sediment layer, the base of units Marine 1 and 2) obscure reflector imaging, but with the use of e.g. instantaneous phase, the base of the HML could be clearly mapped below the free gas, showing the termination of unit Marine 1 near the transition and almost constant thickness of unit Marine 2 (8,5±1 m) in the gassy part.

The depth of the SMTZ was shallower at the gassy sites compared to the stations without a free gas phase and, consequently, SO42- and CH4 fluxes were also higher at the gassy sites. A suite of evidence including organic carbon oxidation states, stoichiometry of organic matter mineralization, and measured SRR all suggest that the degradability of organic matter is essentially the same at all stations. For this reason, we conclude that the main driver for the decreasing SMTZ depth across the transect is a positive feedback on methanogenesis rates as the SMTZ moves upwards and exposes more sediment to decay by methanogenesis. The initial trigger for the upward shift in the SMTZ is likely the gradually increasing thickness of the Holocene mud layer. This positive feedback may be a widespread feature of costal marine sediments and could enhance the potential formation and escape of gas from these settings.

 

 


Publications

Flury, Sabine, Roy H., Dale AW., Fossing H., Toth Z., Spiess V., Jensen J.B., Jörgensen B.B (2016)  Controls on subsurface methane fluxes and shallow gas formation in Baltic Sea sediment (Aarhus Bay, Denmark). Geochimica et Cosmochimica Acta. 188, 297-309, https://doi.org/10.1016/j.gca.2016.05.037.

Toth, Zsuzsanna, Spiess V., Keil H. (2015) Frequency dependence in seismoacoustic imaging of shallow free gas due to gas bubble resonance. Journal of Geophysical Research B: Solid Earth. 120(12), 8056-8072, https://doi.org/10.1002/2015JB012523.

Visnovitz, Feri, Bodnar T., Tóth Z., Spiess V., Kudo I., Timar G., Horvath F. (2015) Seismic expressions of shallow gas in the lacustrine deposits of Lake Balaton, Hungary. Near Surface Geophysics 13 (5), 433-447,  https://doi.org/10.3997/1873-0604.2015026.

Toth Zsuzsanna, Spiess V., Mogollon JM., Jensen JB. (2014b) Estimating the free gas content in Baltic Sea sediments using compressional wave velocity from marine seismic data. Journal of Geophysical Research B: Solid Earth. 119(12), 8577-8593, https://doi.org/10.1002/2014JB010989.

Toth Zsuzsanna, Spiess V., Jensen JB. (2014a) Seismo-acoustic signatures of shallow free gas in the Bornholm Basin, Baltic Sea. Continental Shelf Research. 88, 228-239, https://doi.org/10.1016/j.csr.2014.08.007.

lzow Wanda, Rehder G., Schneider v. Deimling J., Seifert T., Toth Zs. (2013) One year of continuous measurements constraining methane emissions from the Baltic Sea to the atmosphere using a ship of opportunity. Biogeosciences, 10, 81-99. https://doi.org/10.5194/bg-10-81-2013.

Toth, Zuszsanna (2013) Seismo-acoustic investigations of shallow free gas in the sediments of the Baltic Sea. Dissertation, xx. pp.. https://media.suub.uni-bremen.de/bitstream/elib/598/1/00103568-1.pdf.

Schneider von Deimling, Jens, Weinrebe W., Toth Z., Fossing H., Endler HR., Rehder G., Spiess V. (2013) A low frequency multibeam assessment: Spatial mapping of shallow gas by enhanced penetration and angular response anomaly. Marine and Petroleum Geology, 44, 217–222, https://doi.org/10.1016/j.marpetgeo.2013.02.013.


Master Theses

Carlos Ramos (2015) Acoustic velocity determination of shallow gas-bearing sediments from Aarhus Bay using travel-time tomographic inversion of 2D multichannel seismic data

Allroggen, Niklas (2011) Application of Pre-Stack Migration on Shallow Marine Seismic Data

Nach oben