Multichannel Seismic Data Research Articles

Carbonate mounds in the Santaren Channel, Bahamas: A current-dominated periplatform depositional regime

New hydroacoustic data, high resolution multichannel seismic reflection data, CTD casts and sampling document the dominance of bottom currents and the occurrence of carbonate mounds in the Santaren Channel off Great Bahama Bank and Cay Sal Bank. Based on these data sets, three types of carbonate mounds have been identified in water depths between 285 and 685m. Their distribution is strongly related to the present current regime. Bank-driven sediments are caught up by the currents and redeposited mainly in the channel center, resulting in the accumulation of giant confined drifts flanked by moats. The moats at the banks toe-of-slope are formed by a southward and northward flowing water mass, respectively, mainly Antarctic Intermediate Water and Subtropical Underwater. The observed carbonate mounds are concentrated in these moats. Mound type 1 has an elongated shape parallel to the prevailing bottom current direction, a gently inclined flank facing the water flow and a steep flank in its current shadow. Its upstream flank is covered by a thin sediment wedge whereas the lee side is marked by a parabolic scour. Type 1 occurs at both bank margins but mainly off Great Bahama Bank, consisting of cold-water coral rubble floating in a sediment matrix and is, thus, interpreted as cold-water coral mound. By contrast, type 2 mounds are restricted to the Cay Sal margin and have a predominately conical outline. Bottom samples including living sessile crinoids show that its surface must be lithified and colonized by macroepibenthos, except corals. The mounds are situated in pockmark-like depressions that are underlain by fluid escape structures like pipes and chimneys. These fluid pathways are generated by a network of fractures. Mound type 2 is probably formed by abiotic carbonate precipitation; however, a chemoherm nature cannot be ruled out. Mound type 3 has a conical to polygonal shape and occurs off Cay Sal Bank and Great Bahama Bank. It is surrounded by a scour which is elongated in downcurrent and crescent in upcurrent direction. A sediment bulge formed at its front. This type is consistent with blocks and boulders colonized by macroepibenthos and related to gravity-controlled mass movements at the steep bank slopes.Principally, two habitats can be distinguished: the Great Bahama Bank periplatform deep-water province dominated by mound type 1 and the Cay Sal Bank slope to basin province occupied by mound type 2 that is associated with pockmark-like structures. Mound type 3 occurs in both habitats. Physical water mass properties for the existence of cold-water corals are limited to a depth range of ca. 430 to 685m, with seawater densities (σ-θ) of 26.93 to 27.29kg/m and temperatures below 13°C. Their dominance off Great Bahama Bank might be addressed to a more effective food supply induced by local downwelling.

Cenozoic magmatism in the northern continental margin of the South China Sea: evidence from seismic profiles

Igneous rocks in the northern margin of the South China Sea (SCS) have been identified via high resolution multi-channel seismic data in addition to other geophysical and drilling well data. This study identified intrusive and extrusive structures including seamounts and buried volcanoes, and their seismic characteristics. Intrusive features consist of piercement and implicit-piercement type structures, indicating different energy input associated with diapir formation. Extrusive structures are divided into flat-topped and conical-topped seamounts. Three main criteria (the overlying strata, the contact relationship and sills) were used to distinguish between intrusive rocks and buried volcanos. Three criteria are also used to estimate the timing of igneous rock formation: the contact relationship, the overlying sedimentary thickness and seismic reflection characteristics. These criteria are applied to recognize and distinguish between three periods of Cenozoic magmatism in the northern margin of the SCS: before seafloor spreading (Paleocene and Eocene), during seafloor spreading (Early Oligocene–Mid Miocene) and after cessation of seafloor spreading (Mid Miocene–Recent). Among them, greater attention is given to the extensive magmatism since 5.5 Ma, which is present throughout nearly all of the study area, making it a significant event in the SCS. Almost all of the Cenozoic igneous rocks were located below the 1500 m bathymetric contour. In contrast with the wide distribution of igneous rocks in the volcanic rifted margin, igneous rocks in the syn-rift stage of the northern margin of the SCS are extremely sporadic, and they could only be found in the southern Pearl River Mouth basin and NW sub-sea basin. The ocean–continent transition of the northern SCS exhibits high-angle listric faults, concentrated on the seaward side of the magmatic zone, and a sharply decreased crust, with little influence from a mantle plume. These observations provide further evidence to suggest that the northern margin of the SCS is a magma-poor rifted margin.

The seismic Moho structure of Shatsky Rise oceanic plateau, northwest Pacific Ocean

Abstract Oceanic plateaus are large igneous provinces formed by extraordinary eruptions that create thick oceanic crust, whose structure is poorly known owing to the lack of deep-penetration seismic data. Multichannel seismic (MCS) reflection and wide-angle refraction data allow us to show Moho structure beneath a large part of the Shatsky Rise oceanic plateau in the northwest Pacific Ocean. Moho reflectors in the two data sets can be connected to trace the interface from the adjacent abyssal plain across much of the interior. The reflectors display varied character in continuity, shape, and amplitude, similar to characteristics reported in other locations. Beneath normal crust, the Moho is observed at ∼13 km depth (∼7 km below the seafloor) in MCS data and disappears at ∼20 km depth (∼17 km below the seafloor) beneath the high plateau. Moho at the distal flanks dips downward towards the center with slopes of ∼0.5°–1°, increasing to 3°–5° at the middle flanks. Seismic Moho topography is consistent with Airy isostasy, confirming this widely-applied assumption. Data from this study show that crustal thickness between the massifs in the interior of the plateau is nearly twice normal crustal thickness, despite the fact that this crust records apparently normal seafloor spreading magnetic lineations. The Moho model allows improved estimates of plateau area ( 5.33 × 10 5 km 2 ) and volume ( 6.90 × 10 6 km 3 ), the latter assuming that the entire crust was formed by Shatsky Rise volcanism because the massifs formed at spreading ridges. This study is unique in showing Moho depth and structure over an extraordinarily large area beneath an oceanic plateau, giving insight to plateau structure and formation.

Submarine sedimentation processes in the southeastern Terceira Rift/São Miguel region (Azores)

Most case studies dealing with the evolution of volcanic islands concentrate on the onshore domain. In contrast, the interplay of constructional and destructive processes in the offshore domain is often paid less attention, although they commonly represent most part of the edifice. Combining bathymetric and multi-channel seismic data, this study focuses on the impact of volcanism, erosion, tectonics, time-variant sediment supply and climatic/oceanographic conditions on the submarine sedimentation processes in the southeastern Terceira Rift (Azores Plateau). Where the major onshore volcanic systems at São Miguel Island (the largest volcanic edifice in the southeastern Terceira Rift) evolved between the last and the previous glacio-eustatic sea level minimum, an average depth value of 142m of the corresponding shelf segments indicates a formation during the Last Glacial Maximum and a long-term subsidence of approximately 0.6mm/a. Below the shelf edge, the western flank is characterized by submarine volcanism, while the northern slope is mostly affected by strong turbidity events favored by the north coast's exposure to North-Atlantic storm swell. These turbidity currents pass the shelf break by retrogressive canyons/gullies and follow a widespread system of channels towards the deep sea. Where gullies are not connected to channel systems or where turbidity currents overspill the channel walls, stacked lobe deposits are formed. At the southern slope, a more constant downslope sediment transport leads to the formation of cyclic steps, chutes-and-pools and, further distal, turbidity lobes. Turbidity currents follow morphological features and enter the basins via structural bottlenecks. In the basins, bottom currents are controlled/deviated by volcanic highs or fault scarps, where they cause the deposition of drift bodies. High local accumulations of sediments caused by a channelized sediment flux, bottom currents or onshore volcanism were partly remobilized resulting in slumping/sliding and a volcanic ridge partly collapsed due to tectonic stress and/or gravity spreading.

High-resolution seismic velocity analysis as a tool for exploring gas hydrate systems: An example from New Zealand’s southern Hikurangi margin

The existence of free gas and gas hydrate in the pore spaces of marine sediments causes changes in acoustic velocities that overprint the background lithological velocities of the sediments themselves. Much previous work has determined that such velocity overprinting, if sufficiently pronounced, can be resolved with conventional velocity analysis from long-offset, multichannel seismic data. We used 2D seismic data from a gas hydrate province at the southern end of New Zealand’s Hikurangi subduction margin to describe a workflow for high-resolution velocity analysis that delivered detailed velocity models of shallow marine sediments and their coincident gas hydrate systems. The results showed examples of pronounced low-velocity zones caused by free gas ponding beneath the hydrate layer, as well as high-velocity zones related to gas hydrate deposits. For the seismic interpreter of a gas hydrate system, the velocity results represent an extra “layer” for interpretation that provides important information about the distribution of free gas and gas hydrate. By combining the velocity information from the seismic transect with geologic samples of the seafloor and an understanding of sedimentary processes, we have determined that high gas hydrate concentrations preferentially form within coarse-grained sediments at the proximal end of the Hikurangi Channel. Finer grained sediments expected elsewhere along the seismic transect might preclude the deposition of similarly high gas hydrate concentrations away from the channel.

Synthetic Modeling for an Acoustic Exploration System for Physical Oceanography

AbstractMarine multichannel seismic (MCS) data, used to obtain structural reflection images of the earth’s subsurface, can also be used in physical oceanography exploration. This method provides vertical and lateral resolutions of O(10–100) m, covering the existing observational gap in oceanic exploration. All MCS data used so far in physical oceanography studies have been acquired using conventional seismic instrumentation originally designed for geological exploration. This work presents the proof of concept of an alternative MCS system that is better adapted to physical oceanography and has two goals: 1) to have an environmentally low-impact acoustic source to minimize any potential disturbance to marine life and 2) to be light and portable, thus being installed on midsize oceanographic vessels. The synthetic experiments simulate the main variables of the source, shooting, and streamer involved in the MCS technique. The proposed system utilizes a 5-s-long exponential chirp source of 208 dB relative to 1 μPa at 1 m with a frequency content of 20–100 Hz and a relatively short 500-m-long streamer with 100 channels. This study exemplifies through numerical simulations that the 5-s-long chirp source can reduce the peak of the pressure signal by 26 dB with respect to equivalent air gun–based sources by spreading the energy in time, greatly reducing the impact to marine life. Additionally, the proposed system could be transported and installed in midsize oceanographic vessels, opening new horizons in acoustic oceanography research.

Seismic Time–Frequency Analysis via Empirical Wavelet Transform

Time–frequency analysis is able to reveal the useful information hidden in the seismic data. The high resolution of the time–frequency representation is of great importance to depict geological structures. In this letter, we propose a novel seismic time–frequency analysis approach using the newly developed empirical wavelet transform (EWT). It is the first time that EWT is applied in analyzing multichannel seismic data for the purpose of seismic exploration. EWT is a fully adaptive signal-analysis approach, which is similar to the empirical mode decomposition but has a consolidated mathematical background. EWT first estimates the frequency components presented in the seismic signal, then computes the boundaries, and extracts oscillatory components based on the boundaries computed. Synthetic, 2-D, and 3-D real seismic data are used to comprehensively demonstrate the effectiveness of the proposed seismic time–frequency analysis approach. Results show that the EWT can provide a much higher resolution than the traditional continuous wavelet transform and offers the potential in precisely highlighting geological and stratigraphic information.

Seismic sequence stratigraphic analysis of the carbonate platform, north offshore Taiping Island, Dangerous Grounds, South China Sea

Taiping Island, also known as Itu Aba, is the largest natural terrestrial landmass in the South China Sea and is centrally located. Using bathymetry and marine multi-channel seismic data, we explored the seismic stratigraphic features of the offshore and isolated carbonate platform north of Taiping Island. The western flank of the carbonate platform is characterized by an intercalation between high-amplitude and low-amplitude reflections, showing the landward and seaward migration of the platform foreslope deposits. In addition, there are two offshore carbonate build-ups that are underlain by normal faults. Six sequence boundaries and five depositional sequences caused by eustatic sea level cycles are identified and correlated with the eustatic sea level change chart. Although the evolution of the seismic sequences is partly controlled by local tectonics, the overall stacking pattern of the sedimentary strata in our study area reveals five third-order cycles and one second-order cycle, which is in accordance with the eustatic sea level chart. Additionally, the formations of the Western Taiping Seamount Group and the Zhenghe-Daoming Trough are preliminarily analyzed based on seismic data.

Downgoing plate topography stopped rupture in the A.D. 2005 Sumatra earthquake

Earthquakes in subduction zones rupture the plate boundary fault in discrete segments. One factor that may control this segmentation is topography on the downgoing plate, although it is controversial whether this is by weakening or strengthening of the fault. We use multichannel seismic and gravity data to map the top of the downgoing oceanic crust offshore central Sumatra, Indonesia. Our survey spans a complex segment boundary zone between the southern termination of the M w = 8.7, A.D. 2005 Simeulue-Nias earthquake, and the northern termination of a major 1797 earthquake that was partly filled by an M w = 7.7 event in 1935. We identify an isolated 3 km basement high at the northern edge of this zone, close to the 2005 slip termination. The high probably originated at the Wharton fossil ridge, and is almost aseismic in both local and global data sets, suggesting that while the region around it may be weakened by fracturing and fluids, the basement high locally strengthens the plate boundary, stopping rupture propagation.

Morphologies, classification and genesis of pockmarks, mud volcanoes and associated fluid escape features in the northern Zhongjiannan Basin, South China Sea

Based on new high-resolution multi-beam bathymetry and multichannel seismic reflection data, two new groups of numerous pockmarks and mud volcanoes were discovered in the northern Zhongjiannan Basin at water depths between 600 and 1400m. Individual pockmarks are circular, elliptical, crescent-shaped or elongated, with diameters ranging from several hundreds to thousands of meters and tens or hundreds of meters in depth, and they often form groups or strings. Crescent pockmarks, approximately 500–1500m wide in cross-section and 50–150m deep, occur widely in the southern study area, both as individual features and in groups or curvilinear chains, and they are more widespread and unique in this area than anywhere else in the world. Conical mud volcanoes, mostly with kilometer-wide diameters and ca. 100m high, mainly develop in the northern study area as individual features or in groups. Seismic data show that the observed pockmarks are associated with different kinds of fluid escape structures and conduits, such as gas chimneys, diapirs, zones of acoustic blanking, acoustic turbidity and enhanced reflections, inclined faults, small fractures and polygonal faults. The mapped mud volcanoes appear to be fed from deep diapirs along two main conduit types: the conventional conduits with downward tapering cones and another other conduit type with a narrow conduit in the lower half and emanative leakage passages in the upper half. Various types of pockmarks are found and a comprehensive pockmark classification scheme is proposed, according to: (a) their shape in plan view, which includes circular, elliptical, crescent, comet-shape, elongated and irregular; (b) their magnitude, which includes small, normal, giant and mega-pockmarks; and (c) their composite pattern, which includes composite pockmarks, pockmark strings and pockmark groups. For the genesis of the crescent pockmark (strings), a 5-stage speculative formation model is proposed, implying possible controlling factors of gravity sliding/slumping, fluid escape activity and sandbodies intrusion. Seismic data suggest that the mud volcanoes have likely undergone two episodes of evolution, which include a widespread drastic formation of mud volcanoes with dome-like structures, a wide range of seepage occurrences and the formation of complex sediments in the first stage, and the appearance of pockmarks, conical mud volcanoes (groups), and the formation of deformed, filled or broken buried complex sediments by continuous fluid flow processes during stage 2, in the Pliocene and the Quaternary. Pockmark gullies are extensively found along the slope, due to the interaction of slope failure and fluid escape processes. The Cenozoic sedimentary cover is thin in the Northern Zhongjiannan basin, which evolved from a Late Cretaceous or Palaeogene–Oligocene rift to the Neogene–Quaternary post-rift thermal subsidence, and therefore intense deep thermogenic fluid flow can easily affect the soft Cenozoic sediments, produce complex sediment deformation, and form numerous fluid flow structures at the seafloor, in particular pockmarks and mud volcanoes that dominantly emerged during the period of post-rift thermal subsidence and neotectonic movement since ca. 5.5Ma.

Geophysical evidence for widespread Cenozoic bottom current activity from the continental margin of Nova Scotia, Canada

Two-dimensional and three-dimensional multichannel seismic reflection data are utilized to examine previously unrecognized contourite depositional systems along the continental margin of Nova Scotia, Canada. Prior to this study these features were thought to be of limited extent in the study area. The new data show that contourite drifts are widespread with greatest drift development during the Late Miocene to Pliocene. Giant sediment waves form stacked, aggrading sequences of bedforms and their development is linked to pre-existing seafloor morphology. Small sediment drifts developed locally throughout the late Paleogene and Neogene, either southwest and down-current of seafloor obstacles or form channel fills. Major erosional pulses form regional seismic markers; first along the continental rise in the Early Oligocene, then along the continental slope during the Late Miocene and Pliocene. Three-dimensional seismic data show that erosion surfaces preserve along-slope amplitude anomalies, two-dimensional, low-relief sediment waves, and barchanoid bedforms. The geophysical evidence implies that a northeast-to-southwest, along-slope bottom current influenced Cenozoic depositional patterns throughout the study area.

Spreading dynamics and sedimentary process of the Southwest Sub-basin, South China Sea: Constraints from multi-channel seismic data and IODP Expedition 349

Neotectonic and sedimentary processes in the South China Sea abyssal basin are still debated because of the lack of drilling evidence to test competing models. In this study, we interpreted four multi-channel seismic profiles across the Southwest Sub-basin (SWSB) and achieved stratigraphic correlation with new drilling data from Integrated Ocean Discovery Program (IODP) Expedition 349. Neogene sediments are divided into four stratigraphic units, each with distinctive seismic character. Sedimentation rate and lithology variations suggest climate-controlled sedimentation. In the late Miocene winter monsoon strength and increased aridity in the limited accumulation rates in the SWSB. Since the Pliocene summer monsoons and a variable glacial–interglacial climate since have enhanced accumulation rates. Terrigeneous sediments in the SWSB are most likely derived from the southwest.Three basement domains are classified with different sedimentary architectures and basement structures, including hyper-stretched crust, exhumed subcontinental mantle, and steady state oceanic crust. The SWSB has an asymmetric geometry and experienced detachment faulting in the final stage of continental rifting and exhumation of continental mantle lithosphere. Mantle lithospheric breakup post-dates crustal separation, delaying the establishment of oceanic spreading and steady state crust production.

Structure and tectonics of the central Chilean margin (31°–33°S): implications for subduction erosion and shallow crustal seismicity

The pre- and current collision of the Juan Fernandez Ridge with the central Chilean margin at 31°–33°S is characterized by large-scale crustal thinning and long-term subsidence of the submarine forearc caused by subduction erosion processes. Here, we study the structure of the central Chilean margin in the ridge–trench collision zone by using wide-angle and multichannel seismic data. The transition from the upper to middle continental slope is defined by a trenchward dipping normal scarp with variable offsets of 500–2000 m height. Beneath the scarp, the 2-D velocity–depth models show a prominent lateral velocity contrast of >1 s−1 that propagates deep into the continental crust defining a major lateral seismic discontinuity. The discontinuity is interpreted as the lithological contact between the subsided/collapsed outermost forearc (composed of eroded and highly fractured volcanic rocks) and the seaward part of the uplifted Coastal Cordillera (made of less fractured metamorphic/igneous rocks). Extensional faults are abundant in the collapsed outermost forearc, however, landward of the continental slope scarp, both extensional and compressional structures are observed along the uplifted continental shelf that forms part of the Coastal Cordillera. Particularly, at the landward flank of the Valparaiso Forearc Basin (32°–33.5°S), shallow crustal seismicity has been recorded in 2008–2009 forming a dense cluster of thrust events of Mw 4–5. The estimated hypocentres spatially correlate with the location of the fault scarp, and they highlight the upper part of the seismic crustal discontinuity.

What 2‐D multichannel seismic and multibeam bathymetric data tell us about the North Sumatra wedge structure and coseismic response

AbstractRecent large earthquakes have prompted studies to reevaluate seismicity and rupture propagation behavior along the world's major subduction margins. Our study area covers the entire fore arc from northwest of northern Sumatra to west of Simeulue Island, the southern portion of the 2004 tsunamigenic earthquake rupture zone. The accretionary prism width is up to ~180 km, water depths between ~4.5 km near the Sunda Trench and <1 km on fore‐arc high. The wedge consists of a steep outer slope (5–12°), a plateau ~100–120 km wide with anticlinal folds spaced 2–15 km apart, and a steep inner slope adjacent to the Aceh Basin. Analysis of seismic profiles and bathymetry reveal three main structural zones consistent along‐strike, from the trench landward: (1) predominantly landward vergent folds, (2) mixed vergent folds, and (3) predominantly seaward vergent folds. This paper uses those zones to propose a geometry of an underlying rigid backstop. This backstop is seaward dipping and extends from under the Aceh Basin to beneath the mixed vergence zone. A dynamic backstop possibly exists seaward of the rigid backstop and is responsible for the steep slope of the outer prism. Indurated accreted sediments form the landward vergence zone. Along with the possible dynamic backstop beneath the outer wedge, and the rigid backstop in the inner wedge, all behave as a solid block coseismically. This block allows great earthquake rupture to propagate farther seaward toward the Sunda Trench, with resultant hazardous tsunamigenic potential.

Submarine explosive volcanism in the southeastern Terceira Rift/São Miguel region (Azores)

Morphologic studies with sonar data and in situ observations of modern eruptions have revealed some information suggesting how submarine volcanic cones develop, but the information only addresses the modern surfaces of these features. Here, we describe a study combining morphological data with high-resolution seismic reflection data collected over cones within the southeastern Terceira Rift — a succession of deep basins, volcanic bathymetric highs and islands (e.g. São Miguel) representing the westernmost part of the Eurasian–Nubian plate boundary.The cones (252) are distributed in depths down to 3200m and exhibit an average diameter of 743m, an average slope of 20° and heights mainly between 50 and 200m. The cones are here classified into three different categories by physiographic or tectonic setting (we find no particular morphometric differences in cone shapes between these areas). First, numerous cones located at the submarine flanks of São Miguel's Sete Cidades and Fogo Volcano are considered to be parasitic structures. Second, in the southeast of the island, they form a superstructure possibly reflecting an early submarine stadium of a posterior subaerial stratovolcano. Third, some cones are controlled by faults, mostly in a graben system southwest of the island. High-resolution multichannel seismic data indicates that the graben cones evolved synchronously with the graben formation. Bottom currents then probably removed the surficial fine grain-size fraction, leaving rough surface textures of the cones, which backscatter sonar signals strongly in the data recorded here. However, a young cone investigated in detail is characterized by a smooth surface, a marked increase of internal stratification with increasing distance from the summit and upwards concave flanks. Others exhibit central craters, suggesting an explosive than an effusive evolution of these structures. The morphological characteristics of these submarine cones show that they have similar sizes and shapes to cinder cones onshore São Miguel.

Evolution of the Kivu Rift, East Africa: interplay among tectonics, sedimentation and magmatism

AbstractThe seismically and volcanically active Kivu Rift, in the western branch of the East African Rift System, is a type locale for studies of high‐elevation, humid‐climate rift basins, as well as magmatic basin development. Interpretations of offshore multi‐channel seismic (MCS) reflection data, terrestrial radar topography, lake bathymetry and seismicity data recorded on a temporary array provide new insights into the structure, stratigraphy and evolution of the Kivu rift. The Kivu rift is an asymmetric graben controlled on its west side by a ca. 110 km‐long, N‐S striking border fault. The southern basins of the lake and the upper Rusizi river basin are an accommodation zone effectively linking 1470 m‐high Lake Kivu to 770 m‐high Lake Tanganyika. MCS data in the eastern Kivu lake basin reveal a west‐dipping half graben with at least 1.5 km of sedimentary section; most of the ca. 2 km of extension in this sub‐basin is accommodated by the east‐dipping Iwawa normal fault, which bounds an intrabasinal horst. Lake Kivu experienced at least three periods of near desiccation. The two most recent of these approximately correlate to the African Megadrought and Last Glacial Maximum. There was a rapid lake level transgression of at least 400 m in the early Holocene. The line load of the Virunga volcanic chain enhances the fault‐controlled basin subsidence; simple elastic plate models suggest that the line load of the Virunga volcanic chain depresses the basin by more than 1 km, reduces flank uplift locally and broadens the depocentre. Not only do the voluminous magmatism and degassing to the lake pose a hazard to the riparian population, but our studies demonstrate that magmatism has important implications for short‐term processes such as lake levels, inflow and outlets, as well as long term modification of classic half‐graben basin morphology.

High-speed download of seismographs using private cloud technology and a proportional integral derivative controller

ABSTRACTSeismic exploration is widely used in mining projects. A cable-free self-positioning seismograph has been developed by College of Instrumentation & Electrical Engineering of Jilin University that may collect and store seismic data wirelessly or by Ethernet. When large data sets are produced, data download may affect the schedule of a project. To improve the rate of transfer, a system that allows parallel download of multichannel seismic data is reported here. The system is based on private cloud technology and proportional integral derivative control. A private cloud was employed to provide suitable performance and availability. A proportional integral derivative control algorithm was developed for scheduling to maintain the load balance of the system. The system was employed in a seismic exploration project in Liaoning, China. The average download time of six lines was approximately 1.1 hours, demonstrating that the system reduced download time and enhanced exploration efficiency.

BSRs, estimated heat flow, hydrate-related gas volume and their implications for methane seepage and gas hydrate in the Dongsha region, northern South China Sea

To investigate the relationship among methane seepage, subsurface gas hydrate, and heat flow and estimate hydrate-related gas amount in the Dongsha region, northeastern South China Sea, in this study we first identified Bottom Simulating Reflector (BSR) using multi-channel seismic data collected in 2004. Subsequently, the thickness of gas hydrate stability zones (GHSZ) was calculated from the observed BSR using time–depth conversion, ranging from 60 to 300 m in seawater depths of 600–1200 m. Additionally, a seismic feature of double BSRs is locally observed in this region, which is possibly caused by either the presence of mixed gas source (thermogenic and biogenic) or migration of a pre-existing base of GHSZ. Geothermal gradient, heat flow and hydrate-related gas volume are calculated based on the thickness of GHSZ. The estimated geothermal gradient and heat flow varies from 20 to 100 °C/km with an average of 48 °C/km, and from 20 to 120 mW/m2 with an average of 54 mW/m2, respectively. Both estimated geothermal gradient and heat flow are generally consistent with those from measurements. High geothermal gradient (up to 60 °C/km) and heat flow anomaly (up to 80 mW/m2) are present in one methane seepage site, where BSR is clear and reliable. High heat flow could result in thin GHSZ in the continental slope. In this case, gas hydrate could form in the shallow sediments, making dissolved gas from hydrate and/or free deep-seated gas easy to migrate upwards to seafloor through faults, and thus resulting in the occurrence of methane venting. The hydrate-related gas volume is estimated as ∼6 × 1011 m3 over an area of ∼400 km2 with possible occurrence of BSR. This finding would shed lights on understanding potential amount of subsurface methane in the Dongsha region and place useful constraints for modeling GHSZ from measured heat flow data, especially in regions where no BSRs are present.

SEISMIC FACIES ANALYSIS AND STRUCTURAL INTERPRETATION OF DEEPWATER NW SABAH

The deepwaters of NW Sabah has been an interesting site for deepwater hydrocarbon exploration in Malaysia. Up to now, the exploration in this is mainly focused to the Late Miocene until the Pliocene siliciclastic sediment reservoirs distribution at the shelf edge. This paper shows a gross seismic facies mapping analysis and structural interpretation of regional deepwater NW Sabah especially at Sabah Trough. To convert depth, all seismic lines were picked and tied with selected wells. The results of the interpretation were then summarized and presented with relation to regional tectonic events. Eight seismic stratigraphic units, six seismic facies together with five sequence boundaries were recognized. Multichannel reflection 2D seismic data, gamma ray logs and biostratigraphy description from the three wells at deepwater fold-thrust belt and published tectono-stratigraphic scheme from Dangerous Grounds (Sabah Platform) in South China Sea were selected in this study. The propose of this study is to document the relevance of regional tectonic event between Dangerous Ground and Sabah Trough.

The continent‐ocean transition on the northwestern South China Sea

AbstractRifted margins are created as a result of stretching and breakup of continental lithosphere that eventually leads to oceanic spreading and formation of a new oceanic basin. A cornerstone for understanding what processes control the final transition to seafloor spreading is the nature of the continent‐ocean transition (COT). We reprocessed multichannel seismic profiles and use available gravity data to study the structure and variability of the COT along the Northwest subbasin (NWSB) of the South China Sea. We have interpreted the seismic images to discern continental from oceanic domains. The continental‐crust domain is characterized by tilted fault blocks generally overlain by thick syn‐rift sedimentary units, and underlain by fairly continuous Moho reflections typically at 8–10 s twtt. The thickness of the continental crust changes greatly across the basin, from ~20 to 25 km under the shelf and uppermost slope, to ~9–6 km under the lower slope. The oceanic‐crust domain is characterized by a highly reflective top of basement, little faulting, no syntectonic strata and fairly constant thickness (over tens to hundreds of km) of typically 6 km, but ranging from 4 to 8 km. The COT is imaged as a ~5–10 km wide zone where oceanic‐type features directly abut or lap on continental‐type structures. The South China margin continental crust is cut by abundant normal faults. Seismic profiles show an along‐strike variation in the tectonic structure of the continental margin. The NE‐most lines display ~20–40 km wide segments of intense faulting under the slope and associated continental‐crust thinning, giving way to a narrow COT and oceanic crust. Towards the SW, faulting and thinning of the continental crust occurs across a ~100–110 km wide segment with a narrow COT and abutting oceanic crust. We interpret this 3D structural variability and the narrow COT as a consequence of the abrupt termination of continental rifting tectonics by the NE to SW propagation of a spreading centre. We suggest that breakup occurred abruptly by spreading centre propagation rather than by thinning during continental rifting. We propose a kinematic evolution for the oceanic domain of the NWSB consisting of a southward spreading centre propagation followed by a first narrow ridge jump to the north, and then a younger larger jump to the SE, to abandon the NWSB and create the East subbasin of the South China Sea.