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BALTIC GAS - ERANET BONUS-73 EU Funding

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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.