Physical and Acoustic Properties of Gas-bearing Sediments in Jinhae Bay, the South Sea of Korea

Abstract

High-resolution seismic survey and sediment core sampling were conducted to investigate acoustic characteristics of gas-bearing sediments in Jinhae Bay, the southeast of Korea. The sediment in Jinhae Bay is mostly homogenous mud deposited after the Holocene transgression. Along with the 410 km of chirp seismic profiling, five piston core samples were collected on the track lines.Gassy sediments are common and occur widely in the bay. Core samples were analyzed for sediment texture, physical properties (porosity, water content, bulk density, and grain density), acoustic properties (compressional wave velocity and attenuation), and electrical resistivity. X-radiograph image analysis was also performed to observe the shape of degassing cracks. There is no significant downcore variation on physical and sediment textures regardless of existence of gas bubbles. However, compressional wave velocity dramatically decreases from average 1480 to 1380$739 m=s for the cores that penetrate the gas-bearing zones. This is probably due to degassying cracks that developed by escaping gases and free gas bubbles that are still trapped in the cores. Electrical resistivity is the only geotechnical property that increases in the gas-bearing zone where compressional wave velocity abruptly decreases. This indicates the possibility of using both electrical resistivity as an index variable as well as to compressional wave velocity to identify gassy sediment microstructure because there are little changes in texture and composition of sediment.

Characterization of gas‐bearing sediments in the coastal environment using geophysical and geotechnical data

“…In free-gas areas, the resistivity models show the lowest resistivity values (∼0.5 Ω⋅m; regions 1 to 4 in Figures 2 and 3). This observation seems in contradiction with what is usually observed in other studies on similar gas-bearing marine environments, where an increase in resistivity values compared to the surrounding background resistivity is correlated to the presence of free gas (Lee et al (2009) and Qian et al (2018) in shallow contexts; Goswami et al (2016) for a water depth of several hundreds of metres). The presence of gas (an electrical insulator) may tend to increase the resistivity of the bulk matrix because it may block or isolate the conductive water in the pores (Ren et al, 2010).…”

“…In core KS33, located in gas region 2 (see Figure 2), we observe a drop in (~1250 m/s) at 2.1 m BSF (Figure 6a). This is typical of gas-bearing marine sediment and has also been observed on the geophysical data (Figure 2b) and at similar depths in other studies (e.g., Lee et al 2009).…”

Sound Velocity in a Thin Shallowly Submerged Terrestrial-Marine Quaternary Succession (Northern Adriatic Sea)

“…The lowest sound velocity in our dataset was calculated for Borehole A-III-7/88, which penetrates a hyperbola-dense zone clearly visible on the corresponding sub-bottom sonar profile ( Table 2 and Figure 3c). Since even minor gas concentrations (1-2%) dramatically reduce sound velocity in sediments [83,84,[91][92][93], we attribute this significantly lower velocity value to gas in the Quaternary sediment. In the northern Adriatic seabed, gas seeps are commonly observed and are attributed to both deep and shallow sources [94][95][96][97][98].…”

“…Terrestrial-marine Quaternary successions often contain significant amounts of degrading organic matter; consequently, locally present gas further influences sound velocity in these settings (Section 4.1.3). Different gas indicators can easily be recognized from high-resolution geophysical data [85][86][87][88][89][90][91][92][93], facilitating the mapping of low-velocity areas. As the velocity decrease associated with the presence of gas is often quite variable e.g., [93], we suggest avoiding detailed velocity analysis in gas-rich areas.…”

Stratigraphy of late Quaternary deposits using high resolution seismic profile in the southeastern Yellow Sea

“…The top surface is relatively regular and round, but in some places, a partially uneven and erosional surface occurs. Within unit SY4, acoustic turbidities that are interpreted to be due to the entrapped gas bubbles (Kim et al, 2008;Lee et al, 2009) present a few meters below the seafloor (Figs. 3e5).…”