Distribution and Acoustic Characteristics of Shallow

Gas in the Korea Strait Shelf Mud off SE Korea

DAE CHOUL KIMGWANG HOON LEEYOUNG KYO SEOGIL YOUNG KIM

Department of Environmental Exploration EngineeringPukyong National UniversityBusan, Korea

SEOK YUN KIMJEONG CHANG KIM

Department of OceanographyPukyong National UniversityBusan, Korea

SOO CHUL PARK

Department of OceanographyChungnam National UniversityDaejon, Korea

ROY WILKENS

Hawaii Institute of Geophysics and PlanetologyUniversity of HawaiiHonolulu, Hawaii, USA

Shallow gas in the Korea Strait shelf mud (KSSM) off SE Korea, revealed by high-resolution subbottom profiles, is associated with acoustic blanking, acousticturbidity, seepages with plumes in the water column, and seafloor depressions. Theacoustic blanking, characterized by strong, consistent top reflection and wipeoutbelow, is most dominant. The seaward edge of the acoustic blanking zone generallycoincides with the 100-m water-depth contour, suggesting that the water depth (the

Marine Georesources and Geotechnology, 22: 21–31, 2004

Copyright # Taylor & Francis Inc.

ISSN: 1064-119X print=1521-0618 online

DOI: 10.1080=10641190490466928

Received 17 September 2003; accepted 26 March 2004.This work was supported by the Korea Research Foundation (Grant No. KRF-2002-015-

CP0431). We would like to thank Jae-Sik Yoon, Sung-Wook Heo, and Byeong-Hoon Nam fordigitizing the data. Special thanks go to the crew of the R=V Tamyang of Pukyong NationalUniversity for their logistical help and professional manner during data collection.

Address correspondence to Gwang Hoon Lee, Department of Environmental Explora-tion Engineering, Pukyong National University, Busan 608-737, Korea. E-mail: gwanglee@pknu.ac.kr

21

pressure) may control the distribution of shallow gas. The acoustic turbidity,characterized by diffuse top reflection, is a dark smear, partially blanking the databelow. The seepages with plumes, characterized by vertical smearing and disturbedseafloor, are seen only along the shallowest, landward edge of the acoustic blankingzone. This may suggest that the decreased gas solubility at shallow water depths,caused by the lowered pressure, increases the volume of free gas in the sediments,facilitating the gas escape. The seafloor depressions, interpreted as pockmarks, areaccompanied by cone-shaped acoustic masking, which is probably the reflection froma narrow vent of gas. The gas-related acoustic anomalies appear to occur mostly inthe upper, recent mud of the KSSM. Neither permeable beds nor faults, which canact as vertical migration pathways for deep thermogenic gas, are evident in therecent mud. We interpret that the bacterial degradation of organic matter in situ isthe main source for the gas in the KSSM. The upwelling off SE Korea may be animportant source for the increased organic matter in the area.

Keywords shallow gas, acoustic blanking, acoustic turbidity, Korea Strait shelfmud

The most conspicuous sedimentary features in the inner shelf of Korea are themud belts that have formed during the last postglacial sea-level rise (ca. 15,000 yrsBP) (Figure 1) (Park et al. 1999). The Korea Strait shelf mud (KSSM)off the southeastern coast of Korea is particularly interesting because of theextensive shallow gas. Gas in shallow marine sediments has been documented in over100 locations worldwide (Fleischer et al. 2001). The presence of gas within sedimentssignificantly alters sediment physical properties, thereby reducing sediment strengthand sediment sound speed and attenuating and scattering acoustic energy (Andersonand Hampton 1980, Wheeler 1990). In high-resolution subbottom profiles, thegas-bearing sediments appear as acoustic turbidity, acoustic masking or blanking,and enhanced reflections (Judd and Hovland 1992). Gas emission also formspockmarks and domes on the seafloor and plumes in the water column (Field andJennings 1987, Judd and Hovland 1992). Gas venting from the gas-bearingsediments may be making a significant contribution to atmospheric methane, whichis one of the most important greenhouse gases (Fleischer et al. 2001, Hovland et al.1993).

Choi et al. (1997) first reported the acoustic turbidity in high-resolutionsubbottom profiles from the KSSM. Park et al. (1999) confirmed the extensivedistribution of the acoustic turbidity in the KSSM. Park et al. (1999) further sug-gested that high primary productivity in the coastal waters off the southeastern coastof Korea, due to coastal upwelling (Lee and Na 1985, Lee and Kim 2003, Lee et al.2003), has provided large amounts of organic matter to the KSSM and that thebacterial decomposition of the organic matter has produced gas. The acousticturbidity in the KSSM usually occurs at about 3–5m below the seafloor, locallyreaching the water-seabed interface (Park et al. 1999). Sediment cores that pene-trated the uppermost part of the acoustic turbidity in the KSSM have a strongmethane odor even after weeks in a refrigerated container (Jung J. H., personalcommunication 2002). The in situ sound velocities decrease abruptly when thesegas-charged sediments are encountered (Gorgas et al. 2003). X-radiographstaken on the cored sediments from the KSSM also show numerous microcracksthat are probably due to the expansion of gas bubbles during core retrieval(Gorgas et al. 2003).

22 D. C. Kim et al.

Despite these observations, however, the detailed distribution and acousticcharacteristics of the shallow gas in the KSSM remain poorly understood. In thisstudy, we analyzed high-resolution subbottom profiles from the KSSM: (1) todescribe the acoustic characteristics of the shallow gas, (2) to map its arealdistribution, and (3) to discuss its origin. The data used in this study consist of about

Figure 1. Location of the Korea Strait shelf mud (KSSM). The heavy lines in the main figurerepresent location of chirp profiles. In the regional map, KSSM, Korea Strait shelf mud;

CSSM, central South Sea mud; SEYSM, Southeast Yellow Sea mud (adapted from Park et al.1999). Contours are water depths in meters.

Shallow Gas in the Korea Strait Shelf Mud 23

400 km of Chirp (2–7 kHz) profiles (Figure 1), collected on the RV Tamyang ofPukyong National University in 1999 and 2000, using a Datasonics Chirp system(CAP 6000). A combination of GPS and radar systems was used for navigation.

Regional Setting

The study area includes the Korea Strait shelf off the southeastern coast of Korea(Figure 1). The Korea Strait shelf becomes wider to the south where the outer shelf isoccupied by the NE-SW trending trough. The outer shelf is largely dominated byrelict coarse-grained sediments, whereas the inner shelf (<70–100m water depth) isalmost completely covered by the KSSM. The principal source for the muddysediments of the KSSM is the Nakdong River, which is the only major river systemin the southeastern coast of Korea (Park et al. 1999). About two-thirds of the fine-grained sediments from the Nakdong River are carried northeastward by strongshelf currents and deposited along southeastern Korea, forming the KSSM (Parket al. 1999). The KSSM extends to the southwestern shelf of the East Sea (Sea ofJapan), covering over 2,400 km2 of the surface area (Park et al. 1999). It consists oftwo stratigraphic units: (1) the recent mud, deposited since the mid-Holocene (ca.5,000–6,000 yrs BP) when sea level reached nearly its present position, and (2) thetransgressive mud, deposited during the postglacial transgression (ca. 15,000–ca.6,000 yrs BP) (KIGAM 2000). A thin transgressive sand layer separates the recentand transgressive muds. This sand layer corresponds to a prominent reflector inhighresolution subbottom profiles, forming the acoustic basement.

The organic carbon contents of the recent mud of the KSSM range from 0.6-1.35% near the mouth of the Nakdong River and from 1.25–2.21% away fromthe river mouth (Park et al. 1999). The higher organic carbon contents away from theriver are probably due to high surface-water productivity, resulting from theupwelling off the southeastern coast of Korea (Park et al. 1999). The C=N ratios(8.0–30.0) of the organic matter in the cored sediments from the KSSM are betweenthose of marine (ca. 6.0) and terrestrial (>20.0) organic matter (Nakai et al. 1982),suggesting mixed marine and terrestrial sources (Park et al. 1999).

Acoustic Types of Shallow Gas and Gas Escapes

The acoustic types of shallow gas and gas escapes in the KSSM, classified on thebasis of their acoustic signatures and geometry, include: (1) acoustic blanking, (2)acoustic turbidity, (3) gas seepages with plumes in the water column, and (4) seafloordepressions with cone-shaped acoustic masking. The areal distribution of theseacoustic types is shown in Figure 2. The acoustic blanking (Figure 3A), which wasdescribed generally as acoustic turbidity in earlier studies in the area, is characterized

Figure 2. Distribution of shallow gas. AB, acoustic blanking; AT, acoustic turbidity; GSF,gas-seepage field; SD, seafloor depression. Acoustic blanking covers nearly 70% of the survey

area. The seaward edge of the acoustic blanking generally coincides with the 100-m water-depth contour. Acoustic turbidity is seen only in the southernmost part of the area. At leasttwo gas-seepage fields are identified along the landward edge of the acoustic blanking zone.Dotted lines represent location of chirp profiles. Heavy lines and respective numbers represent

locations for Chirp profiles shown in other figures. Contours are water depths in meters.

"

24 D. C. Kim et al.

Shallow Gas in the Korea Strait Shelf Mud 25

Figure 3. Chirp profiles showing various acoustic anomalies in the KSSM. (A) Acousticblanking is characterized by very consistent and strong top reflection and complete wipeout of

seismic data below. The sides of the acoustic blanking are generally steep, almost vertical, withrounded edges. (B) Acoustic turbidity looks like a dark smear, characterized by a variabledegree of disturbance and irregular or diffuse upper boundary. (C) Seepages with plumes inthe water column are characterized by vertical smearing and disturbed seafloor reflection. (D)

Seafloor depressions are accompanied by cone-shaped acoustic masking. Cone-shapedacoustic masking with no apparent seafloor depression is also seen.

26 D. C. Kim et al.

by very consistent and strong top reflection and complete wipeout of seismic databelow. It is most dominant, covering almost 70% of the survey area. The acousticblanking is comparable to the acoustic masking described by Hovland and Judd(1988) and to the acoustic blankets described by Taylor (1992) and Garcia-Gil et al.(2002). The seaward edge of the acoustic blanking generally coincides with the 100-mwater-depth contour (Figure 2). The depth to the top of the acoustic blanking fromthe sea surface (Figure 4), computed assuming a sediment sound velocity of

Figure 3. Continued

Shallow Gas in the Korea Strait Shelf Mud 27

Figure 4. Depth (in meters) of the top of the acoustic blanking from sea surface. The top of

the acoustic blanking deepens gradually seaward from about 50m to over 100m.

28 D. C. Kim et al.

1,550m=s (Kim et al. 1992), increases gradually with water depth from less than 50mto over 100m. The sides of the acoustic blanking are generally steep, almost vertical,with rounded edges. Dome-shaped acoustic blanking is also seen in the southern-most part of the study area.

The acoustic turbidity (Figure 3B), characterized by a variable degree of dis-turbance, looks like a dark smear. The upper boundary of the acoustic turbidity isgenerally irregular or diffuse and relatively weak compared to that of the acousticblanking. Reflectors can occasionally be recognized below the acoustic turbidityzones. The acoustic turbidity is seen only in the southernmost part of the study area.Here, a narrow acoustic blanking zone appears gradually to change to the acousticturbidity to the northeast (Figure 2).

The seepages (Figure 3C) are characterized by vertical smearing and disturbedseafloor reflection. Weak but distinct plume-like features are seen in the watercolumn immediately above the seepages. It is difficult to map the individual seepagesbecause of their diffuse character and often weak appearance. The gas seepagesgenerally occur in association with acoustic blanking, causing the upper boundary ofthe acoustic blanking to be diffuse and weak. At least two gas-seepage fields are seenalong the landward edge of the acoustic blanking zone.

Two seafloor depressions, accompanied by cone-shaped acoustic masking(Figure 3D), are observed in the northernmost part of the study area. The tip of thecone-shaped acoustic masking in one seafloor depression appears to extend into thewater column. Cone-shaped acoustic masking with no apparent seafloor depressionis also seen.

Discussion

Shallow gas in marine sediments originates from either biological (biogenic) orabiological (thermogenic) processes. Biogenic gas is produced by bacterial degra-dation of organic matter at low temperatures, whereas thermogenic gas is producedby high-temperature cracking of organic compounds at considerable burial depths(Missiaen et al. 2002). The recent discovery of a commercially viable gas accumu-lation at the subsurface depth of over 2,500m, approximately 40 km east of theKSSM, indicates the presence of deep thermogenic gas in the southwestern East Sea(Lee and Kim 2002). The migration of thermogenic gas into the shallow subsurfacedepths requires migration pathways such as permeable carrier beds, faults, andunconformities. A deep core (>50m) collected from the southern part of the KSSM(KIGAM 2000) penetrated about 30-m thick recent mud, consisting almost entirelyof mud. The gas-related acoustic anomalies in the KSSM appear to occur mostly inthis homogenous mud, which is not likely to contain vertical permeable beds thatwould facilitate the migration of deep thermogenic gas. The high-resolution sub-bottom profiles also do not suggest any offsets or faults in the recent mud wheresome reflectors are seen.

We interpret that the gas causing the acoustic anomalies in the KSSM has beengenerated mostly in situ by the bacterial degradation of organic matter. The organiccarbon contents (1.25–2.21%) of the cored sediments in the study area are about twotimes those (0.6–1.35%) in the proximal area of the Nakdong River dispersal system(Park et al. 1999). The C=N ratios (8.04–18.13) of the organic matter in the coredsediments in the study area also suggest that a large portion of the organic matterswas derived from marine plankton (Park et al. 1999). Thus, the upwelling off the

Shallow Gas in the Korea Strait Shelf Mud 29

southeastern coast of Korea is probably a very important source for the increasedorganic matter in the study area.

The depth of the acoustic blanking from the sea surface in the study areaincreases gradually seaward from about 50m to over 100m. The seaward edge of theacoustic blanking coincides largely with the 100-m water depth although the KSSMextends further seaward for about 10 km to over 200-m water depth. This maysuggest that the areal distribution of the shallow gas is limited by the water depthand thus the pressure, which together with the temperature is the main controllingfactor in the solubility of gas in pore water. The increase in pressure with depthincreases the solubility of gas in pore water, which in turn decreases the volume offree gas in sediments. The increase in pressure may limit the generation of free gas inthe sediments beyond the 100-m water depth in the study area. The seasonal fluc-tuation of the depth to the top of the acoustic turbidity zone, caused by temperature-dependent solubility of gas in pore water, has been documented in a few cases (e.g.Hagen and Vogt 1999, Wever and Fiedler 1995).

The acoustic turbidity, characterized by the diffuse and weak top reflection, isprobably due to the relatively low volume (<1%) of free gas in the sediments(Fannin 1980). Some reflectors can be followed beneath the acoustic turbidity,suggesting that the attenuation of the acoustic energy at the top of the shallow gas isnot so severe as that causing the acoustic blanking. The strong reflection associatedwith the top of the narrow acoustic blanking zone in the southernmost part of thestudy area appears to become weak and diffuse to the northeast, changing into anacoustic turbidity. This is probably due to the gradual decrease of the volume of freegas in the sediment.

The seafloor features with acoustic anomalies in the KSSM include the seepageswith plumes in the water column and the depressions with cone-shaped acousticmasking. The seepages are seen only along the shallowest, landward edge of theacoustic blanking zone. This may suggest that the decrease in pressure withthe decreasing water depth decreases the solubility of gas, causing an increase of thevolume of free gas in the sediments along the landward edge of the acoustic blankingzone. As a result, the gas is released to the overlying water column. The loweredpressure may also reduce the sealing effect of the sediments, further helping the gasvent.

The seafloor of the study area is very smooth and no significant topographicfeatures are recognizable from the seismic profiles except for the two depressionswith cone-shaped acoustic masking in the northernmost part of the area. Theseafloor depressions are most likely pockmarks, caused by the removal of seafloorsediments by escaping gas (Judd and Hovland 1992). The cone-shaped acousticmasking is probably the reflection from a point reflector, which represents a narrow,vertical vent of gas. The cone-shaped acoustic masking with no apparent seafloordepression is probably the reflection from an out-of-plane gas vent.

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