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Evidence for Gassy Sediments on the Inner Shelf of SE Korea from Geoacoustic Properties

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Evidence for Gassy Sediments on the Inner Shelf of SE Korea from Geoacoustic Properties Continental Shelf Research 23 (2003) 821–834 Evidence for gassy sediments on the inner shelf of SE Korea from geoacoustic properties Thomas J. Gorgasa,*, Gil Y. Kimb, Soo C. Parkc, Roy H. Wilkensd, Dae C. Kime, Gwang H. Leef, Young K. Seoe a Hawaii Natural Energy Institute, University of Hawaii, P.O.S.T. Bldg. 109, 1680 East-West Rd., Honolulu, HI-96822, USA b Naval Research Laboratory, Code 7431, Stennis, Space Center, MS 39529, USA c Department of Oceanography, Chungnam National University, Taejon 305-764, South Korea d Hawaii Institute of Geophysics and Planetology, University of Hawaii, P.O.S.T. Bldg. 821, 1680 East-West Road, Honolulu, HI-96822, USA e Department of Environmental Exploration Engineering, Pukyong National University, Busan 608-737, South Korea f Department of Oceanography, Kunsan National University, Kunsan 573-701, South Korea Received 20 February 2002; accepted 20 January 2003 Abstract The inner shelf of SE Korea is characterized by an up to 40 m thick blanket of soft sediments often characterized by acoustic turbidity (AT). This AT is caused by a layer of sub-surface gas, which prohibits the identification of geological structures below that gas layer. Sound speeds were measured directly in these sediments using the Acoustic Lance (AL) in both mid- and late-September 1999. In situsoundspeeds obtained in mid-September varied between 1400 and 1550 m/s, and thus did not confirm the presence of gas within the top 3.5 m of the seafloor. However, signal waveforms suggested that a gassy layer might have been just below the depth penetrated by the Lance. In late-September, on the other hand, two sites showed an abrupt decrease in signal amplitudes and in sound speed (less than 800 m/s) at depths as shallow as 2 m below seafloor, indicating the presence of free gas bubbles. Piston-cored sediments were retrieved at the same sites in February 1999. X-radiographs of some of the cores revealed numerous microcracks caused by the expansion of gas bubbles during core retrieval. In contrast to in situ acoustic data, ultrasonic sound speeds acquired in the laboratory in May 1999 on those cores did not differentiate convincingly between gas-bearing and gas-free sediments. Our measurements on the SE Korean shelf with the AL provide new data on the in situ acoustic behavior of gassy sediments and the sediments that overlie them in zones of AT. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Acoustic properties; Attenuation; Bubbles; Core analysis; Gassy sediments; SE Korea shelf *Corresponding author. Tel.: 808-956-5711; fax: 808-956- 1. Introduction 2335. E-mail addresses: [email protected](T.J. Gorgas), The understanding of acoustic wave propaga- [email protected] (G.Y. Kim), scpark@cnu. ac.kr (S.C. Park), [email protected](R.H. Wilkens), tion through the sediment–water interface and in [email protected] (D.C. Kim), [email protected] shallow sub-surface layers of the seafloor is of (G.H. Lee), [email protected] (Y.K. Seo). considerable interest in underwater acoustics, 0278-4343/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0278-4343(03)00026-8 822 T.J. Gorgas et al. / Continental Shelf Research 23 (2003) 821–834 geophysics, ocean-engineering, and naval applica- shallow seas, including the Baltic Sea (Judd and tions (Richardson, 1986; Stoll, 1989; Kibblewhite, Hovland, 1992; Wilkens and Richardson, 1998) 1989). The sediment–water interface and and the inner shelf of SE Korea (Park et al., 1999) upper several meters of seafloor are characterized (Fig. 1). Methane in shallow marine sediments is by large gradients in physical and acoustic proper- formed mostly from biogenic processes, when ties (Hamilton, 1979; Richardson, 1986). The bacteria reduce organic matter within the upper reliable measurement of acoustic parameters at 10s or 100s of meters of the sub-seafloor (Flood- the seafloor, such as sound speed and attenuation, gate and Judd, 1992; Hagen and Vogt, 1999). has attracted considerable attention as a means of The propagation of acoustic waves and their defining geological processes that control acoustic attenuation in gassy sediments is not well under- and seismic wave propagation within the shallow stood, despite the frequent occurrences of acoustic seafloor. turbidity (AT) (acoustic blanking of sub-surface Methane-bearing sediments are widely distrib- structures) caused by the absorption and scattering uted throughout the world’s ocean (Fleischer et al., of acoustic energy by gas bubbles in the seabed 2001). They concentrate in organic-rich, muddy (Wilkens and Richardson, 1998). Sound speed is sediments of coastal waters, estuaries and adjacent typically much lower at acoustic frequencies in Fig. 1. Map of the study area and sites where the AL was deployed in mid- and late-September 1999. Cored sediments were retrieved in February 1999 (ultrasonic sound speeds measured in May 1999). Grain sizes within the DMD vary from 6f in the south (Sites 7–9) to 8f in the North (Sites 19–21) between 50 and 130 m of water depth. Note the location of the Nakdong River, a significant source of organic matter to the DMD, and the direction of the Tsushima Current in northerly directions. Lines A–C represent chirp sounder profiles, and reveal some of the sub-seafloor structure and possible gas horizons (displayed with permission of Park et al., 1999). SF— seafloor, AT—acoustic turbidity (note that not all core and AL sites are located on available chirp profile images). T.J. Gorgas et al. / Continental Shelf Research 23 (2003) 821–834 823 gas-bearing sediments than in gas-free sediments, isobath over a wide portion of the outer shelf whereas sound speeds measured at ultrasonic (Park et al., 1999). Shell fragments characterize frequencies may not indicate the presence of gas these sandy sediments with grain sizes up to 2 mm (Wilkens and Richardson, 1998). Attenuation (Cho et al., 1999). During summer floods, a large across all frequencies increases significantly even volume of fine-grained sediments and organic in the presence of minute amounts of gas matter is discharged from the Nakdong River in (Anderson and Hampton, 1980a, b; Yuan et al., the south, and then dispersed by tidal and coastal 1992; Richardson, 1994; Fuet al., 1996b ). currents in northerly directions, eventually reach- In spite of evidence for gassy sediments on the ing the outer shelf (Kim and Lee, 1980; Kim et al., shallow SE Korean shelf in high-resolution chirp 1986; Kim et al., 1999). High sediment accumula- sounder profiles (e.g., Park et al., 1999), direct tion rates (2.5 mm/yr) and water column produc- acoustic measurements in these gas-bearing sedi- tivity increase the organic content in the sediments ments had not been made. This study contributes across the DMD, and ultimately lead to the new acoustic and physical property profiles formation of dissolved methane caused by a recorded in the area. Results presented in this biochemical degradation of the organic matter paper are compared with those previously ob- within the sediment column (Park et al., 1999). tained with the Acoustic Lance (AL) in Eckernf- Once methane production exceeds saturation orde. Bay, Germany (Western Baltic Sea) (Fuet al., levels, free methane bubbles form in the sediment 1996b; Wilkens and Richardson, 1998). (Martens et al., 1998). Similar to other shallow water seas, bottom currents on the inner shelf of SE Korea are probably too weak to cause 2. Regional setting significant erosion of surficial sediments (Nittrouer et al., 1998). The lack of erosion further enhances The inner and mid-shelf of SE Korea is the production and preservation of free gas within dominated by an up to 40 m thick accumulation shallow sub-seafloor depths. of Holocene mud (4–16 mm) in water depths High-resolution chirp reflection profiles show a shallower than 100 m (Park et al., 1999)(Fig. 1). 900 km2 AT zone underlying much of the DMD This mud forms a distinct belt, called the Distal (Park et al., 1999). Free gas bubbles in the muddy Mud Deposit (DMD) that extends approximately sediments absorb and scatter acoustic energy, from Busan to north of Pohang in the shelf- masking deeper sedimentary horizons. The turbid- parallel direction. A fraction of these muddy ity zones occur at approximately 3–5 m below the sediments is transported by the northward flowing seafloor (mbsf), but may reach the water–seabed Tsushima Current from the Nakdong River interface in some places (Park et al., 1999). One of through the Korea Strait and along the southeast these AT zones extends from Busan to Pohang coast of the Korean peninsula (Kim et al., 1986; along the southeastern coast, covering nearly Park and Choi, 1986; Kim et al., 1996; Kim et al., 40 km (Fig. 1). Park et al. (1999) identified a 1999; Park et al., 1999). The Tsushima Current is a distinct reflector in chirp sounder profiles that were branch of the Kuroshio Current in the East Sea recorded in May 1995 without AT. This reflector (Sea of Japan) and its current speeds reach close to was interpreted as the Holocene transgression with 1.0 m/s during the summer (Cho et al., 1999; Park changing sedimentary properties from mud to et al., 1999). sand (see: Fig. 3 in Park et al., 1999; Fig. 1 of the Over the last 6000 years, a modern, 0–40 m thick present paper). layer of mud has been accumulated over relict transgressive sands, which were deposited during the last glacial lowstand of sea level. Muddy clay- 3. Methods silt fractions are spilled regularly onto the inner shelf from the coastal watershed, whereas the relict The AL measures in situ interval sound speed sandy sediments are found seaward of the 80 m and effective attenuation profiles using full-wave- 824 T.J.
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