Acoustic imaging of shallow gas in Baltic Sea sediments

Transcription

1 Acoustic imaging of shallow gas in Baltic Sea sediments Rudolf ENDLER 1, Jens WUNDERLICH 2, Jens SCHNEIDER von DEIMLING 1, Stefan ERDMANN 2 1 Leibnitz Institute for Baltic Sea Research, Warnemünde Seestrasse 15, D Rostock, Germany Tel.: +49 (381) , Fax: +49 (381) , rudolf.endler@io-warnemuende.de dl d d 2 Innomar Technologie GmbH Schutower Ringstraße 4, D Rostock, Germany Tel.: +49 (0) Fax: +49 (0) info@innomar.com HYDRO 2010, November, Rostock-Warnemünde, Germany Introduction, Background Content The ongoing BONUS Baltic Gas project Physical properties of gas bearing sediments Methods of shallow gas mapping Geological settings and shallow gas types in the Bli Baltic Sea Conclusion and outlook References 1

2 Shallow gas relevance Change of Sediment Physical Properties (Richardson&Davis 1998), Sediment Stability, Geohazards (landslides, tsunamis,...), Offshore Operations (Sills & Wheeler 1992) Gas Hydrates, Oil and Gas Exploration (Orange et al. 2005) Environment, Fishery (H 2 S, ) Climate Change, Greenhouse Gas (Best et al. 2006) Relevant previous studies and projects The Coastal Benthic Boundary Layer (CBBL) Special Research Program, a 5 year Office of Naval Research study Goal: Physical characterization and modeling of benthic boundary layer processes and the impact these processes have on seafloor structure, properties and behavior. Detailed studies: Eckernförde (Baltic Sea), West Florida Sand Sheet, southeast of Panama City, Florida. See RICHARDSON &BRYANT 1996 The EU METROL Project Goal: controls and mechanisms of methane production and breakdown in ocean European margin sediments, Duration: , Link: The German BMBF Geotechnology program; topic: Gas hydrates in the Geo System & Methane in the Geo / Bio System Duration: , 11 Projects Link: juelich.de/ptj/geotechnologien/gashydrate 2

3 The BONUS Baltic Gas project Frame: BONUS ERA NET ( org/about bonus) Title: Methane emission in the Baltic Sea: Gas storage and effects of climate change and eutrophication Duration: ua Web Page: ( Baltic Gas Project Goals Quantification and mapping of distribution and fluxes of methane in the Baltic Sea Investigation of the controls on the relevant key biogeochemical processes Integrated seismo acoustic mapping with geochemical profiling Modeling of the dynamics of Baltic Sea methane in the past (Holocene period), present (transport reaction reaction models), and future (with predictive scenarios) Identification of hot spots of gas and potential future methane emission in a Baltic database available for national authorities and scientists. 3

4 Baltic Gas List of Participants and Principal Scientists: Center for Geomicrobiology, Dept. of Biological Sciences, Aarhus University, Denmark Bo Barker Joergensen (Coordinator) National Environmental Research Institute, Aarhus University, Denmark Henrik Fossing Geological alsurvey of DenmarkandGreenlandand Greenland, Denmark, Bo BoJensenJørn Jørn Max Planck Institute for Marine Microbiology, Germany, Timothy Ferdelman Department of Geology, Lund University, Sweden, Daniel Conley Institute of Oceanology, Polish Academy of Science, Poland, Klusek Zygmunt Baltic Sea Research Institute Warnemünde, Germany Gregor Rehder Winogradsky Institute of Microbiology, Russian Academy of Sciences, Russia, Nikolay Pimenov Alfred Wegener Institute Instituteof Polar and Marine Research, Germany, Michael Schlüter Stockholm University, Sweden, Volker Brüchert Department of Earth Sciences, Utrecht University, The Netherlands, Philippe Van Cappellen Department of Earth Sciences, University of Bremen, Germany, Volkhard Spiess Shallow gas processes Methane (CH 4 ) is produced mostly in ocean margin sediments through the microbial degradation of organic matter buried below the sulphate zone. Methane may come also from deeper, hydrocarbon related sources. As CH 4 builds up in the sediment it migrates upwards towards the sediment surface either by molecular diffusion or as free gas bubbles. Most of the methane is continuously degraded subsurface by the process of anaerobic methane oxidation (AOM). The CH 4 is oxidized to CO 2 by microorganisms, probably by a metabolic co-operation between archaea and bacteria. The sulphate-methane transition (SMT) constitutes a, generally very effective, barrier against CH4 escape from deep sediment strata. On a global scale, more than 90% of all CH4 produced in marine sediments is retained at this methane barrier and thus does not reach the sea floor. The CH 4 occasionally escapes into the bottom water, thereby generating escape structures such as pockmarks Sulfate [mm] Methane [mm] JØRGENSEN&KASTEN

5 Physical Properties of Gassy Sediments 10 6 single gas bubble resonance in sediments cy [Hz] bubble resonance frequenc Eckernförde data bubble radius [m] Resonance frequency of a single gas bubble in water (green line), mud (black line) and silt (blue line), depth: 22 m Pwave-velocity and attenuation versus gas bubble resonance frequency in mud and silt (redrawn from Anderson & Hampton 1980, Fig. 15, 16) Sound velocities and wet bulk densities below and above bubble resonance Because hydrodynamic permeability is very low the Hamilton-Gassmann model can be used (change of the bulk modulus and bulk density of the pore water gas mixture), Shear wave velocities of 15m/s in mud and 250 m/s in silt are used for estimating the dynamic shear bulk modulus Water depth is 22 m, depth in sediment 2m (see Wilkens& Richardson dwb (green lines) [kg m-3] Vp [m/s]; sound velocity an wet bulk density in gassy in sediments wbd-silt-gassy 1200 wbd-mud-gassy Vp-gas free silt-gassy 200 mud-gassy water saturation 1998) Vp- Vp- 5

6 An example for geohazards caused by shallow gas : crater structures in the Namibia shelf mudbelt gassy gassy 660m crater gassy 5m Echosounder SEL96 (20 khz) record of gas-free and heavily gas-charged diatommud deposits and a large crater structure The crater is caused by uplift of extremly gas charged mud layers Strong methane and H2S discharge in water column and atmosphere Widespread deaths of benthic organisms like lobsters Upwards travelling gas bubbles (oblique stripes), water column HF SES96 station work in crater ROV video picture, mud surface inside a crater, holes and gas bubbles Observations of small temporary mud islands The standard procedure for acoustic mapping of shallow gas Visual interpretation of acoustic records and identification of gas charged sediments by a number of typical features like acoustic turbidity, layer enhancement etc. Picking of lateral extension and depth in sediment of the gas containing layer Data compiling in databases and GIS See e.g. Laier&Jensen

7 Preliminary map of shallow gas occurrences (purple areas and stars) in Baltic Sea bottom sediments, revealed by acoustic methods Purple areas: extended shallow gas regions Purple stars: small gassy patches Sources: Laier&Jensen 2007 IOW-data Problems of acoustic shallow gas mapping Data come from different acoustic devices and reflect gas bubbles in a different manner depending on the used device parameters (e.g. pulse type, frequency) Data were recorded during different cruises and times, but e.g. the shallow gas depth is changing during the seasons The lower border of the gas charged layer is hidden in the acoustic records Shallow gas bodies often appear as diffuse, gradually increasingbackscatterregions regions in the acousticrecords There are different types of free gas accumulations in sediments The quantitative estimation of the gas content from acoustic data is still an unsolved task 7

8 The multifrequency approach: a method to obtain quantitative gas data from acoustic records Simultaneous use of multiple narrow band pulses covering an extended frequency range Obtain bubble resonance frequency / bubble size from highest frequenc dependent backscatter Obtain gas content from frequency dependent sound velocity / layer depth SES2000-deep record of gassy mud over late glacial silt clay deposits in the northern Bornholm Basin; left LF 5 khz channal, rigth HF 37 khz channel The multifrequency approach: SES96 and MFE2000 Acoustic images of the same shallow gas structure in the Bornholm Basin (File: ). Red colours indicate high amplitudes, green-blue colours represent low amplitudes. Upper picture: SES96 record,,p pulse:10 khz, 0.2 ms. The surface of the bubbly layer is located about 1m below the sea-bottom (narrow high backscatter stripe). Deeper layers are hidden. Lower picture: Multi- Frequency Echosounder MFE2000 record, pulse: 28 khz 0.32 ms. The bubbly layer appears as a broader region, less distinct down to a depth in mud of more than 4m. Deeper layers are partly visible below the gassy Layer 80m 90m 100m 110m 120m For description of the SES96/2000 system see: Wunderlich et al free gas Littorina mud Baltic Ice Lake clay / silt deposits 8

9 Geological settings and shallow gas occurrences Acoustic transect across the Mecklenburg Bay showing different free gas structures NE SW 30m mud free gas from deeper sources free gas from the uppermost mud layer till Transect Mecklenburg Bay 40m Late / postglacial silt and clay layers 50m PAP; SES96 15 khz pulse length 0.2 ms, Length of line:32 nm Shallow gas in the Arkona Basin S more sandy deposits gas N gas gas gas PAP SES96 15 khz, 0.2ms, Length of line: 18.6 nm 9

10 Gas accumulation at the base of the mud layer. Stolpe fore delta mud gas bubbles silt / clay till SES96-profile pos , depth interval 2 m, horizontal distance interval 200m, sound velocity 1440 m/s. Modern channel systems incised in mud deposits with shallow gas; Stolpe fore delta mud gas till Recent channel in mud with indication of gas contained in the sediment (red plume below the pockmark. SES96-profile pos , depth interval 2 m, horizontal distance interval 100m, sound velocity 1440 m/s. 10

11 Small scale gas structures in the Gdansk Basin S N Holocene marine mud clay-silt, Yoldia, Ancylus, gas warved clay, BIL till Acoustic stratigraphy of the Gdansk Basin, southern part. Gas accumulations related to organic matter deposited in small depressions (channels?) incised into the Baltic Ice Lake clay sequences. (SES96 record) SES96 profile along the NE SW axis of the Gotland Basin Hydrodynamic active region, big lateral changes in modern sediment thickness and properties thick late glacial deposits over the whole basin (varved / homogeneous clays, BIL, ~ 30m) Virtually no free gas in the deposits, because of the high pressure? Litorina-recent mud Yoldia-Ancylus warved clay, BIL homogeneous clay, BIL 11

12 Conclusions and outlook There are several types of free gas occurences in Baltic Sea sediments allowing a classification based on their genesis. Freegas productionandaccumulation accumulation is controlled by thegeological settings and the environmental conditions. Using acoustic data from different devices for shallow gas mapping the measurement parameters like pulse frequency, bandwidth etc. have to be taken in to account The multifrequency approach, the simultaneous use of multiple narrow band pulses covering an extended frequency range, have proved to be a very usefull method for mapping free gas in sediments. The following work in the frame of Baltic Gas project will focus on the layer specific extraction of wave form parameters from the different frequency channels and their use in geoacoustic modeling to obtain quantitative gas data References ANDERSON, A.L., HAMPTON, L.D.: Acoustics of gas bearing sediments, JASA, 67(6), pp BEST, A.I, RICHARDSON, M.D., BOUDREAU, B.P.,et al.: Shallow seabed methane gas could pose coastal hazard. EOS, 87: , 2006 JACKSON, D. R., RICHARDSON, M.D.: High Frequency Seafloor Acoustics., Springer New York., pp 616, 2007 JØRGENSEN, B. B., KASTEN, S.: Sulfur cycling and methane oxidation, pp In H. D. Schulz and M. Zabel (eds.), Marine Geochemistry, 2nd ed. Springer, Berlin, 2006 LAIER, T., JENSEN, J. B.: Shallow gas depth contour map of the Skagerrak western Baltic Sea region, Geo Marine Letters 27: , 2007 Orange, D.L., Garcia Garcia, A., McConnell, D., et al.: High resolution surveys for geohazards and shallow gas: NW Adriatic (Italy) and Iskenderun Bay (Turkey)." Marine Geophysical Researches 26: , 2005 RICHARDSON, M. D., BRYANT, W.R.: Benthic boundary layer processes in coastal environments: An introduction. Geo Marine Letters 16: ,1996 RICHARDSON, M. D., DAVIS, A. M.,:Modeling methane rich sediments of Eckernförde Bay. Continental Shelf Research 18(14 15): , 1998 SILLS, G. C., WHEELER, S.J.: The significance of gas for offshore operations. Continal Shelf Research 12(10): , 1992 WUNDERLICH, J, WENDT, G., MÜLLER,S.: "High resolution echo sounding and detection of embedded archaeological objects with nonlinear sub bottom profilers." Marine Geophysical Researches 26: ,