Dong-Geun Yoo

Late Quaternary transgressive and highstand systems tracts in the northern East China Sea mid-shelf

Abstract Analysis of very high-resolution (3.5 kHz) seismic profiles and sediment data from the northern East China Sea mid-shelf region reveals that the sedimentary sequence above the acoustic basement consists of five late Quaternary sedimentary units comprising transgressive and highstand systems tracts formed after the last glacial maximum. The transgressive systems tract includes four units: (1) incised-valley fill (Unit I), (2) transgressive estuarine/deltaic complex (Unit II), (3) transgressive sand sheet (Unit III), and (4) tidal sand ridge (Unit IV). The lower two units (Units I and II), representing a paralic environment, consist of sediment preserved from shoreface erosion. The upper units (Units III and IV), corresponding to a marine environment, consist of sediments produced through shoreface erosion during the Holocene transgression. A ravinement surface separates the lower and the upper units. The highstand systems tract is composed of a distal mud patch (Unit V) derived from the Huanghe and Changjiang rivers in China. Dispersal and deposition of this modern mud are largely controlled by the regional circulation pattern and sediment supply.

High-Resolution Seismic Study as a Tool for Sequence Stratigraphic Evidence of High-Frequency Sea-Level Changes: Latest Pleistocene-Holocene Example from the Korea Strait

ABSTRACT Sequence stratigraphic analysis of high-resolution seismic reflection profiles and sediment data reveals that the latest Pleistocene-Holocene deposits in the Korea Strait shelf off the southeastern Korean Peninsula form a high-frequency sequence consisting of a set of lowstand, transgressive, and highstand systems tracts that corresponds to a fifth-order (20 kyr) sea-level cycle. Eight depositional systems, each with different seismic facies, constitute the systems tracts. The lowstand systems tract, consisting of sandy mud, forms a deltaic wedge that pinches out updip near or at the relict shelf edge. The transgressive systems tract, consisting mainly of sands, includes six depositional systems: (1) inner-shelf transgressive layer, (2) transgressive estuarine-deltaic complex, (3) transgressive sand ridge, (4) mid-shelf transgressive layer, (5) incised-channel fill, and (6) beach-shoreface complex. Although the transgressive systems tract is widely distributed, it is much thinner than the lowstand and highstand systems tracts. The highstand systems tract is composed of a prodelta-shelf complex consisting almost exclusively of Holocene muds. The distribution and geometry of the systems tracts in the latest Pleistocene-Holocene sequence in the Korea Strait shelf is different from that of Vails model in that: (1) the lowstand systems tract consists only of the lowstand deltaic wedge, which forms an elongated sediment body along the paleoshelf edge, and (2) the highstand systems tract is completely confined to the inner shelf and forms a nearshore belt parallel to the coastline. High-frequency (20 kyr), high-amplitude (magnitude of about 130 m) sea-level change, together with strong currents flowing northeastward along the southeastern coast of the Korean Peninsula and varying sediment supply, have resulted in this rather unusual stratigraphic architecture.

Transgressive sand ridges on the mid-shelf of the southern sea of Korea (Korea Strait): formation and development in high-energy environments

Abstract The mid-shelf of the southern sea of Korea (Korea Strait) is covered by a series of shore-parallel sand ridges up to 63 km long, 3–9 km wide, and 22 m thick. The ridges are a moribund type but show well-organized, dipping strata, indicating appreciable progradation. The submerged paleo-channel system (the paleo-Seomjin River) associated with the sand ridge field suggests that the ridges primarily originated from river-mouth sandy shoals, which subsequently developed into shore-parallel, linear sand bodies in high-energy environments. The combined effect of coastal (tidal and longshore) and offshore ocean currents was possibly responsible for shore-parallel sediment transport and growth of the ridges. 14C dates, core lithology, and sea-level data suggest that the sand ridges mainly formed and developed during the middle period of the postglacial transgression. They are postulated to have developed sequentially from offshore to onshore in response to sea-level rise during that time. Very slow sea-level rise and high sediment supply through the paleo-channel system probably provided favorable conditions for forming the sand ridges. The ridges became inactive during the late transgression period as a result of a substantial reduction of wave and current strength and remained on the mid-shelf.

Evolution and chronology of late pleistocene shelf-perched lowstand wedges in the Korea strait

Abstract Detailed interpretation of high-resolution seismic profiles from the Korea Strait has shown the existence of a complex of three stacked sedimentary sequences on the outer shelf. The individual sequences, consisting of prograding wedge, are interpreted, from internal structure and sediment data, as a ‘shelf-perched lowstand wedge system’, related to alternating episodes of successive regression and transgression. Although the entire sequences have not been fully dated, it is believed that the sequences were deposited during late Pleistocene sea-level lowstands. Repeated falls of sea level (isotopic stages 8, 6 and 2) resulted in the formation of lowstand sedimentary sequences. During subsequent episodes of sea-level rise (isotopic stages 8/7, 6/5, and 2/1), the sequences were reworked and draped by thin transgressive sands.

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

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.

Seismic Stratigraphy and Depositional History of Holocene Transgressive Deposits in the Southeastern Continental Shelf, Korea

Analysis of high-resolution seismic profiles from the southeastern continental shelf of Korea reveals that the Holocene transgressive deposits consist of five sedimentary units characterized by retrograding or backstepping depositional arrangements. Unit I, forming a linear sediment body along the shelf margin, is an ancient beach/shoreface deposit formed during the early stage of transgression. During the transgression, the paleo-channels were backfilled with fluvial or coastal-plain sediments, forming Unit II as an incised-channel fill deposit. The near-surface sediment was reworked and eroded by shoreface erosion, forming a thin lag of sands (Unit III) on the midshelf. During the middle stage of the transgression, the shoreline may have stabilized at around 70 - 80 m below the present sea level for some period of time to allow the formation of sand ridge systems (Unit IV). Unit V in the inner shelf was deposited in an estuarine environment during the middle to late stage of transgression. Such transgressive stratigraphic architecture is controlled by a function of lateral changes in the balance among rates of relative sea-level rise, sediment input and marine processes at any given time.

Reservoir characterization of gas hydrate in the Northwestern part of the Ulleung Basin, East Sea

ABSTRACT An analysis of 3D seismic data from the northwestern part of the Ulleung Basin, East Sea, revealed that the gas hydrate stability zone (GHSZ) consists of five seismic units separated by regional reflectors. An anticline is present that documents activity of many faults. The seismic indicators of gas hydrate occurrence included bottom simulating reflector (BSR) and acoustic blanking in the gas hydrate occurrence zone (GHOZ). By the analysis of the seismic characteristics and the gradient of the sedimentary strata, the GHOZ was divided into four classes: (1) dipping strata upon strong BSR, (2) dipping strata below strong BSR, (3) parallel strata with acoustic blanking, and (4) parallel strata below weak BSR. Seismic attributes such as reflection strength and instantaneous frequency were computed along the GHOZ. Low reflection strength and high instantaneous frequency were identified above the BSR, indicating the occurrence of gas hydrate. A remarkably high reflection strength and low instantaneous frequency indicated the presence of free gas below the BSR. Considering the distribution of the gas hydrate and free gas, two gas migration processes are suggested: (1) stratigraphic migration through the dipping, permeable strata and (2) structural migration from below the GHSZ along faults.

Acquisition, processing and interpretation of high-resolution seismic data using a small-scale multi-channel system: an example from the Korea Strait inner shelf, south-east Korea

A high-resolution seismic survey was designed to improve the quality of seismic data and study the evolution of inner shelf deposits off the southern coast of Korea. A 0.5 L air gun and 6- or 8-channel streamer cable with a 5 m group interval were used as a seismic source and receiver system, respectively. Data recording was digitally performed at a shot interval of 2 s and a sample interval of 0.1 ms using a personal computer (PC)-based recording system with an analogue to digital (A/D) converter. In the data processing, deconvolution and static corrections were very effective for improving the data resolution. The data resolution and signal-to-noise (S/N) ratio were improved by using multi-channel digital seismic systems as opposed to a single-channel analogue streamer. The results show that a small-scale multi-channel seismic system is an effective way to investigate late Quaternary deposits. Seismic stratigraphic analysis of the high-resolution seismic profiles revealed that inner shelf deposits, up to 30 m thick, consist of three sedimentary units, which comprise transgressive and highstand systems tracts deposited after the last glacial period. The transgressive systems tract includes two units (I and II) separated by a ravinement surface (RS). The lower Unit I, lying below the RS, consists of estuarine sediments left behind by shoreface erosion during transgression, and represents a paralic component. The upper Unit II, including two sub-units (IIa and IIb), lies above the RS and consists of sand produced by shoreface erosion during landward transgression, which corresponds to a marine component. The uppermost Unit III lies above the maximum flooding surface and consists of thick mud clinoforms of the modern Nakdong and Seomjin subaqueous deltas. This unit likely formed during the recent sea level highstand and represents the highstand systems tract. The results of the present study show that a small-scale multi-channel seismic system is an effective way to improve the quality of seismic records and to document the evolution of shallow sediment deposits. The shelf deposits in the Korea Strait inner shelf consist of three distinct seismic units formed during the late Quaternary.

SEISMIC REFLECTION BLANK ZONES IN THE ULLEUNG BASIN, OFFSHORE KOREA, ASSOCIATED WITH HIGH CONCENTRATIONS OF GAS HYDRATE

It has recently been recognized that abundant gas hydrates occur in localized zones of upwelling fluids, with concentrations much higher than in regional distributions associated with bottomsimulating reflectors (BSRs). We report a study of multi-channel seismic reflection data across such structures in the Ulleung Basin, East Sea backarc offshore Korea, an area with few BSRs. The structures are commonly up to several km across and a few hundred meters in depth extent, and are characterized by reduced reflectivity and bowed-up sediment reflectors on time-migrated sections. The seismic pull-up mainly results from higher velocities, although physical deformation due to folding and faulting is not ruled out. Some of the features extend upward close to the seafloor and others only partway through the gas hydrate stability zone. The base of gas hydrate stability zone (BGHSZ), calculated assuming a regional average constant heat flow of 110 mW/m, is confirmed by the presence of gas inferred from reduced instantaneous frequencies and high instantaneous amplitudes, and from a decrease in seismic velocities. The ∗ Corresponding author: Phone: +1 250 721 6188 Fax +1 250 472 4620 E-mail: istoian@uvic.ca Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.

Estimation of Q from Zero-offset VSP Data in Gas Hydrate-bearing Zone

ABSTRACT Q-factor (or Q) that quantifies the attenuation, the intrinsic property of the material, is a very important required factor in extracting useful subsurface material properties such as lithological information, porosity, permeability, viscosity, and the degree of the saturation from the seismic data. When seismic energy propagates through the gas hydrate-bearing zone and a free gas layer below it, the considerable attenuation occurs and affects the amplitude and wavelet shape of recorded seismic data. Thus Q- factor extracted from seismic data can be used to locate the gas hydrate bearing zone and estimate its reserves. The spectral-ratio method has been widely used in computing the frequency-independent Qs from the zero-offset VSP data because of its ease and fastness. We developed a module of the spectral ratio method, and applied it to the synthetic zero-offset VSP data set and field zero-offset VSP data set. The field data were acquired at Mallik 3L-38 gas hydrate research well in Canada. The Q-factors calculated from the synthetic zero-offset VSP data using the spectral ratio method approached closer to the true values for the medium with low Q-factor than high Q-factor. The changes in the Q-factors extracted from the Mallik zero-offset data using the spectral ratio method agreed well with the boundaries of the layers, including gas hydrate zone, depicted in a reflection image.