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Acoustic imaging of shallow gas in Baltic Sea sediments 

Rudolf ENDLER1, Jens WUNDERLICH2, Jens SCHNEIDER von DEIMLING1, Stefan ERDMANN2

1Leibnitz Institute for Baltic Sea Research, WarnemündeSeestrasse 15, D-18119 Rostock, GermanyTel.: +49 (381) 5197 – 370, Fax: +49 (381) 5197 – 440,

il d lf dl @i d d

HYDRO 2010, 02.-05. November, Rostock-Warnemünde, Germany

e-mail: rudolf.endler@io-warnemuende.de

2 Innomar Technologie GmbHSchutower Ringstraße 4, D-18069 Rostock, GermanyTel.: +49 (0)3 81 44 079 0 Fax: +49 (0)3 81 44 079 299 e-mail: info@innomar.com

Content

• Introduction, Background

• The ongoing BONUS Baltic Gas“ project• 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 B l i SBaltic Sea

• Conclusion and outlook

• References

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„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 (H2S,…)

• Climate Change, Greenhouse Gas (Best et al. 2006)

Relevant previous studies and projectsThe 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: 2002‐2005, 

• Link: www.metrol.org

The German BMBF‐ Geotechnology – program; topic: Gas hydrates in the  Geo‐System & Methane in the Geo‐/ Bio‐System

• Duration:2000‐2008,   11 Projects

• Link: http://www.fz‐juelich.de/ptj/geotechnologien/gashydrate

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The BONUS „Baltic Gas“ project

• Frame: BONUS ERA‐NET (http://www bonusportal org/about bonus)(http://www.bonusportal.org/about_bonus)

• Title: Methane emission in the Baltic Sea: Gas storage and effects of climate change and eutrophication

• Duration: 01.11.2008 – 31.10.2011u a o 0 008 3 0 0

• Web‐Page: (http://balticgas.au.dk/)

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 models) and(Holocene period), present (transport‐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. 

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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

• Geolo i al S r e of Denmark and Greenland Denmark Bo Jensen Jørn• Geological Survey of Denmark and Greenland, Denmark, ‐‐Bo Jensen 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 of Polar and Marine Research Germany Michael• Alfred‐Wegener‐Institute of 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 (CH4) 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 builds up in the sediment it migrates upwards

000 5 10 15 20 25

Sulfate [mM]

•As CH4 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 sub-surface by the process of anaerobic methane oxidation (AOM). The CH4 is oxidized to CO2 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

50

100

200

150

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 CH4 occasionally escapes into the bottom water, thereby generating escape structures such as pockmarks

http://www.metrol.org/JØRGENSEN&KASTEN 2006

2500 2 4 6

Methane [mM]

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Physical Properties of Gassy Sediments

105

106

cy [

Hz]

single gas bubble resonance in sediments

102

103

104

bu

bb

le r

eso

nan

ce f

req

uen

c

Eckernförde data

Pwave-velocity and attenuation versus gas bubble resonance frequency in mud and silt

(redrawn from Anderson & Hampton 1980, Fig. 15, 16)

10-4

10-3

10-2

10

bubble radius [m]

Resonance frequency of a single gas bubble in water (green line), mud (black line) and silt (blue line), depth: 22 m

Sound velocities and wet bulk densities below and above bubble resonance

2000sound velocity an wet bulk density in gassy in sediments•Because hydrodynamic

permeability is very low the Hamilton Gassmann model can

800

1000

1200

1400

1600

1800

dw

b (

gre

en l

ines

) [k

g m

-3]

Vp-gas free

wbd-silt-gassy

wbd-mud-gassy

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

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 10

200

400

600

water saturation

Vp

[m

/s];

mud-gassy

silt-gassy•Water depth is 22 m, depth in sediment 2m

•(see Wilkens& Richardson 1998) Vp-

Vp-

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An example for geohazards caused by „shallow gas“: crater structures in the Namibia shelf mudbelt

660m

5

•Echosounder SEL96 (20 kHz) record of gas-free and heavily gas-charged diatom-mud deposits and a large crater structure

gassycrater

5m crater structure

•The crater is caused by uplift of extremly gas charged mud layers

•Strong methane and H2S discharge in water column and atmosphere

gassy

gassy

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

•Widespread deaths of benthic organisms like lobsters

•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 2007

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Preliminary map of shallow gas occurrences (purple areas and stars) in Baltic Sea bottom sediments, 

revealed by acoustic methods 

Purple areas: extendedshallow gas regions

Purple stars: smallgassy patches

Sources:www.metrol.orgLaier&Jensen 2007IOW-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 increasing backscatter regions in the acoustic recordsincreasing backscatter regions in the acoustic records

• 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

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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 depthObtain 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

Upper picture: SES96 record, pulse:10 kHz, 0.2 ms.

Acoustic images of the same shallow gas structure in the Bornholm Basin (File:2008-0620-2204-2254). Red colours indicate high amplitudes, green-blue colours represent low amplitudes. 80m

90m

For description of the SES96/2000 system see: www.innomar.com, Wunderlich et al. 2005

, p ,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 d l 28

Littorina mud100m

110m

120m

free gas

Baltic Ice Lake clay / silt deposits

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

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Geological settings and shallow gas occurrences

Acoustic transect across the Mecklenburg Bay showing different free gas structures

Transect Mecklenburg Bay

free gas from free gas from deeper sourcesdeeper sources

free gas free gas from the from the uppermost uppermost mud layermud layer

mudmud

NE SW

30m till

21.01.2009 PAP; SES96 15 kHz pulse length 0.2 ms, 21.01.2009 PAP; SES96 15 kHz pulse length 0.2 ms, Length of line:32 nmLength of line:32 nm

Late / postglacial silt and clay layers

Transect Mecklenburg Bay

40m

50m

Shallow gas in the Arkona Basin

S

more sandy deposits

Ngas

gasgas

gas

30.05.2008 PAP SES96 15 kHz, 0.2ms, Length of line: 18.6 nm

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Gas accumulation at the base of the mud layer. Stolpe fore delta 

mud

silt / clay

gas bubbles

till

SES96-profile pos050613-1707, 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

till

gas

Recent channel in mud with indication of gas contained in the sediment (red plume below the pockmark. SES96-profile pos050613-1907, depth interval 2 m, horizontal distance interval 100m, sound velocity 1440 m/s.

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Small scale gas structures in the Gdansk Basin

S N

Holocene marine mud

warved clay, BIL

clay-silt, Yoldia, Ancylus,

gas

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•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

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Conclusions and outlook

• There are  several types of free gas occurences in Baltic  Sea sediments allowing a classification based on their genesis.

• Free gas production and accumulation is controlled by the geologicalFree gas production and accumulation is controlled by the geological 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 sedimentsvery 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), pp1865‐1903

•BEST, A.I, RICHARDSON, M.D. , BOUDREAU, B.P.,et al.: Shallow seabed methane gas could posecoastal hazard. EOS, 87: 213‐220., 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. 271‐309. In H. D. Schulz andM. 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: 127‐141, 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: 247–266., 2005

•RICHARDSON, M. D., BRYANT, W.R.: Benthic boundary layer processes in coastal environments: An introduction. Geo‐Marine Letters 16: 133‐139.,1996

•RICHARDSON, M. D., DAVIS, A. M.,:Modeling methane‐rich sediments of Eckernförde Bay. Continental Shelf Research 18(14‐15): 1671‐1688., 1998

•SILLS, G. C., WHEELER, S.J.: The significance  of gas for offshore operations. Continal Shelf Research 12(10): 1239‐1250., 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: 123‐133., 2005