Multichannel Seismic Reflection Data Research Articles

Seabed morphology and gas venting features in the continental slope region of Krishna–Godavari basin, Bay of Bengal: Implications in gas-hydrate exploration

Increased oil and gas exploration activity has led to a detailed investigation of the continental shelf and adjacent slope regions of Mahanadi, Krishna–Godavari (KG) and Cauvery basins, which are promising petroliferous basins along the eastern continental margin of India. In this paper, we analyze the high resolution sparker, subbottom profiler and multibeam data in KG offshore basin to understand the shallow structures and shallow deposits for gas hydrate exploration. We identified and mapped prominent positive topographic features in the bathymetry data. These mounds show fluid/gas migration features such as acoustic voids, acoustic chimneys, and acoustic turbid layers. It is interesting to note that drilling/coring onboard JOIDES in the vicinity of the mounds show the presence of thick accumulation of subsurface gas hydrate. Further, geological and geochemical study of long sediment cores collected onboard Marion Dufresne in the vicinity of the mounds and sedimentary ridges shows the imprints of paleo-expulsion of methane and sulfidic fluid from the seafloor. To understand the origin of the mounds and their relationship with gas hydrate/cold seep formation, we analyzed the multi-channel seismic reflection (MCS) data close to the mounds. The MCS data show that the subsurface layers beneath the mounds are folded. Below the folded overburden, we observe zones of no coherent reflections probably originating from Miocene sequence. Since the study area is located in shale tectonics regime where Miocene sequences are known to be overpressured, we interpret the zones of no coherent reflections as Miocene shale diapirs. The upward movement of shale diapirs has folded the overburden layer and resulted in the formation of numerous faults/fractures. These faults act as permeable pathways for fluid/gas movement facilitating the formation of gas hydrate and cold seeps close to shale diapiric mound.

Tectonics and seismicity of the ApulianRidge south of Salento peninsula(Southern Italy)

Multichannel reflection seismic data were acquired south of the Salento peninsula, in an area where crustal seismicity has been recorded. Seismic profiles show the presence of small grabens bounded by extensional faults with NW-SE direction. These grabens are filled with Plio-Quaternary sediments and represent the prolongation of the grabens located onshore in the Salento peninsula. Outer arc extension due to flexuring of the Adriatic-Apulian lithosphere under the double load of the Hellenides and Apennines-Calabrian arc is thought to have originated these grabens. The Adriatic-Apulian continental lithosphere presents a very small radius of curvature and a decoupling between upper crust and mantle lithosphere is expected. Inner arc compression within the upper crust may be responsible for the seismicity recorded in the area.

Tectonics related to the North Anatolian Fault in the Sea of Marmara: evidence from seismic reflection data

The North Anatolian Fault crosses the Sea of Marmara from east to west. Tectonic features of the Sea of Marmara were studied using multi-channel deep seismic reflection data. The northern branch of the North Anatolian Fault is active as a right lateral strike-slip fault zone and indicates both negative and positive flower structures. The North Anatolian Fault splays into two faults at the Sea of Marmara as a northern branch and north segment of the southern branch. The northern branch named the Main Marmara Fault extends in a complicated manner from the north of the Kapidagi Peninsula to westward in the Sea of Marmara. The north segment of southern branch extends between the Gemlik and Bandirma gulfs in the south of the Sea of Marmara. In addition, uplift areas arose by compression and a push-up style in between the Kapidagi Peninsula and the Main Marmara Fault. The North Anatolian Fault is characterized by a negative flower structure in basins and push-up style in uplift areas in the Sea of Marmara. An uplift area arose between the north segment of the southern branch and the northern branch of the North Anatolian Fault. The north segment of the southern branch of the North Anatolian Fault is a strike-slip fault and displays a pull-apart style in the seismic reflection data.

Terrestrial-style slow-moving earthflow kinematics in a submarine landslide complex

Morphometric analysis of Simrad EM300 multibeam bathymetric DEMs reveals details of deformation patterns in a ~ 145 km 2 submarine landslide complex that are commonly associated with slow-moving earthflows in terrestrial settings. This mode of failure, where existing landslide debris is remobilised repeatedly along discrete shear boundaries and is progressively conveyed through the complex, has not previously been recognised in the submarine environment. The kinematics contrast with the more traditional models of submarine landslide complex development in which repeated catastrophic failures each mobilise new source material to form a composite stacked landslide deposit. In our study of the Tuaheni landslide complex on the Hikurangi Margin of New Zealand, remobilisation has formed boundary shear zones imaged at the seafloor surface in multibeam data, and at depth in multichannel seismic reflection data. A significant amount of internal deformation has occurred within the debris streams. Phases of deformation appear to be partitioned longitudinally as extensional and contractional zones rooted into a basal decollement, and laterally with strike–slip shears partitioning discrete debris streams. While slow-moving terrestrial earthflows are activated by fluctuating piezometric levels typically controlled by precipitation, different processes cause the equivalent mobility in a submarine earthflow. Elevated pore pressures in submarine earthflows are produced by processes such as earthquake-generated strong ground motion and/or gas/fluid release. Earthflow movement in submarine settings is prolonged by slow dissipation in pore pressure.

Geology of the Shelves surrounding the New Siberian Islands, Russian Arctic

Abstract. A total of 11 700 km of multichannel seismic reflection data were acquired during recent reconnaissance surveys of the wide, shallow shelves of the Laptev and western East Siberian Seas around the New Siberian Islands. To the north of the Laptev Sea, the Gakkel Ridge, an active mid-ocean ridge which separates the North American and Eurasian Plates, meets abruptly the steep slope of the continental shelf. Extension has affected the Laptev Shelf since at least the Early Tertiary and has resulted in the formation of three major, generally north-south trending rift basins: the Ust' Lena Rift, the Anisin Basin, and the New Siberian Basin. Our data indicate that the rift basins on the Laptev Shelf are not continuous with those on the East Siberian Shelf. The latter shelf can best be described as an epicontinental platform which has undergone continuous subsidence since the Late Cretaceous. The greatest subsidence occurred in the NE, manifested by a major depocentre filled with inferred (?)Late Cretaceous to Tertiary sediments up to 5 s (twt) thick. On the basis of deep reflection data we revise and adjust Mesozoic domain boundaries around the New Siberian Islands.

West Antarctic Ice Sheet evolution: New insights from a seismic tomographic 3D depth model in the Eastern Ross Sea (Antarctica)

This work presents the results of the research project WISE (West Antarctic Ice Sheet Evolution) the main aim of which is to provide new insights about WAIS (West Antarctic Ice Sheet) evolution in the Eastern Ross Sea (Antarctica) since the late Miocene times, from morphological and geophysical analysis of glacial sediments. A square grid of 1230 km high-resolution multi-channel seismic reflection data, multi-beam bathymetry data and nine Sound Velocity Profiles (SVP) were collected in January 2006 from the northwestern sector of the Glomar Challenger Basin (Eastern Ross Sea), and tied to Deep Sea Drilling Program site 271. This work provides a detailed seismic stratigraphic analysis of the Miocene–Quaternary Ross Sea Sequences (RSS) 5–8, previously mapped using regional lower-resolution seismic profiles. Eight seismic units are identified on WISE high-resolution seismic profiles. These units and their boundaries in most of the cases match with previously published stratigraphy, although three new horizons have been identified as reflecting lateral and time-transgressive facies changes. We applied travel time reflection tomography to obtain a detailed 3D model of the investigated area, defined by a depth–seismic velocity volume and by the geometry of 5 selected surfaces. The resulting model is used to reconstruct the geomorphological and depositional history of this sector of the outer continental shelf and to infer ice sheet dynamics during Miocene to Quaternary times. A high velocity anomaly observed in units below the Ross Sea Unconformity 2 (RSU2) of Pliocene age (3–3.7 Ma) is interpreted as indicative of glacial sediments over-compacted by large scale grounding ice sheet expansion. The low velocity of the unit above RSU1 (ca. 0.7 Ma), is associated with normally compacted sediments. Contour depth and isopach maps show migration of the sediment depocenter from RSS-6 to RSS-8, indicating a change in the direction of grounding ice advance over the continental shelf. Acoustic facies and seismic velocity changes occurred during deposition of RSS-8, possibly reflecting a major change in the WAIS dynamics in late Pliocene–Quaternary times.

Submarine structures in the Gulf of İzmit, based on multichannel seismic reflection and multibeam bathymetry

Multichannel seismic reflection and multi-beam bathymetry data were used to study the active tectonic and syn-tectonic stratigraphic setting of the Gulf of Izmit in the Marmara Sea (Turkey). The gulf and its near surroundings are deformed by the northern strand of the dextral North Anatolian Fault. Three connected basins of the gulf, the western (Darica), central (Karamursel) and eastern (Golcuk) basins are formed by active faults, as observed in the stacked and migrated seismic sections, as well as the bathymetry map. The main branch and its surrounding sedimentary strata are confined by normal faults to the north and south. These normal faults converge at depth towards the main fault, forming a negative flower structure in the gulf. The average maximum sedimentation rate is 0.4 mm/year according to the three most recent seismo-stratigraphic units that are located to the south of the main fault branch within the central basin. A 20° south-dipping major discontinuity along the northern shoreline of the gulf represents the top of Paleozoic basement.

Variability of the bottom-simulating reflector (BSR) and its association with tectonic structures in the Chilean margin between Arauco Gulf (37°S) and Valdivia (40°S)

Multichannel seismic reflection data recorded between Arauco Gulf (37°S) and Valdivia (40°S), on the Chilean continental margin, were processed and modeled to obtain seismic images and sub-surface models, in order to characterize the variability of the bottom-simulating reflector (BSR), which is a geophysical marker for the presence of gas hydrates. The BSR is discontinuous and interrupted by submarine valleys, canyons, as well as by faults or fractures. The BSR occurrence is more common south of Mocha Island due to moderate slopes and greater organic matter contribution by rivers in that area. Tectonic uplift and structural instability change the stability gas hydrate zone and consequently the BSR position, creating in some cases missing or double BSRs. Our modeling supports the presence of gas hydrate above the BSR and free gas below it. Higher BSR amplitudes support higher hydrate or free gas concentrations. In the study area, gas hydrate concentration is low (an average of 3.5%) suggesting disseminated gas hydrate distribution within the sediments. Also higher BSR amplitudes are associated with thrust faults in the accretionary prism, which serve as conduits for gas flow from deeper levels. This extra gas supply produces a wider thickness of gas hydrates or free gas.

Morphostructure and evolution of submarine canyons across an active margin: Cook Strait sector of the Hikurangi Margin, New Zealand

The Cook Strait sector of the Hikurangi Margin contains several canyons including New Zealand's largest canyon, the multi-branched shelf-indenting Cook Strait Canyon. The morphology of the canyons reflects the transition from subduction related thrust faulting to oblique collision and strike–slip faulting. High resolution EM300 multibeam and multichannel seismic reflection data reveal: i) widespread, deep-seated bedrock landsliding as a principal control on canyon enlargement; ii) a retrogressive entrenchment of tributary submarine catchments in response to a base-level perturbation which has rejuvenated canyons inferred to have been inactive as top to bottom sediment conduits since the last glaciation; and iii) the control of structurally generated bathymetric relief on both the routing of sediment pathways and the erosion of inter-canyon slopes. The models of canyon evolution demonstrated here have widespread implications for canyon development and evolution on other tectonically actively continental margins.

A distal earthquake cluster concurrent with the 2006 explosive eruption of Augustine Volcano, Alaska

Clustered earthquakes located 25 km northeast of Augustine Volcano began about 6 months before and ceased soon after the volcano's 2006 explosive eruption. This distal seismicity formed a dense cluster less than 5 km across, in map view, and located in depth between 11 km and 16 km. This seismicity was contemporaneous with sharply increased shallow earthquake activity directly below the volcano's vent. Focal mechanisms for five events within the distal cluster show strike-slip fault movement. Cluster seismicity best defines a plane when it is projected onto a northeast–southwest cross section, suggesting that the seismogenic fault strikes northwest. However, two major structural trends intersect near Augustine Volcano, making it difficult to put the seismogenic fault into a regional-geologic context. Specifically, interpretation of marine multichannel seismic-reflection (MCS) data shows reverse faults, directly above the seismicity cluster, that trend northeast, parallel to the regional geologic strike but perpendicular to the fault suggested by the clustered seismicity. The seismogenic fault could be a reactivated basement structure.

Seismic indicators of gas hydrate and associated gas in the Ulleung Basin, East Sea (Japan Sea) and implications of heat flows derived from depths of the bottom-simulating reflector

Analysis of multi-channel seismic reflection data from the Ulleung Basin, East Sea (Japan Sea), reveals various seismic indicators of gas hydrate and associated gas, including the bottom-simulating reflector (BSR), enhanced reflections below the BSR, and seismic chimneys. The recent recovery of massive gas hydrate by drilling, together with these seismic indicators, strongly suggests favorable conditions for the formation of gas hydrate in the area. The BSR is most common and of a wide range of amplitude and continuity. Seismic chimneys, characterized by columnar zones of amplitude reduction, terminate mostly within the gas hydrate stability zone (GHSZ), but a few extend to pockmarks or mounds on the seafloor. Seismic chimneys with seafloor expressions probably represent vertical vents for gas originating from below the GHSZ. The increase in the degree of amplitude reduction in seismic chimneys with increasing seismic frequency may be due to seismic attenuation by gas bubbles trapped in hydrate-filled/coated fractures in the GHSZ. BSR-derived heat flows range from about 65 to over 115 mW/m 2, comparable to those from direct measurements. Heat flows in the northern part of the study area are very high for the age of the basin, suggesting hotter than normal mantle temperature. The area of high (> 105 mW/m 2) heat flows also coincides largely with the distribution of the presumed incipient oceanic crust, formed during the earliest stage of seafloor spreading.

Model-Based Identification of the Base of the Gas Hydrate Stability Zone in Multichannel Reflection Seismic Data, Offshore Costa Rica

Along the pacific margin offshore Costa Rica the Bottom Simulating Reflector (BSR) shows a patchy occurrence in 2-D seismic reflection profiles. The reason for this can be either lack of free gas beneath parts of the gas hydrate stability zone (GHSZ) or poor seismic imaging. We compare far to near offset stacked common midpoint sections to reduce imaging ambiguity utilizing the amplitude variation with offset effect and thus successfully distinguish BSRs from regular sediment reflections. In combination with 1-D modeling of the base of the GHSZ we disqualify or qualify reflections in the predicted depth range as BSR. Additionally we calculate the heat flow and compare it with an analytical solution to detect thermal anomalies, for example, at the frontal prism. The higher confidence in BSR depths based on the far offset stacks and heat flow calculations allows further analyses on gas hydrate concentration estimates and tectonic evolution of the margin.

Tectonic history of northern New Caledonia Basin from deep offshore seismic reflection: Relation to late Eocene obduction in New Caledonia, southwest Pacific

New, high‐quality multichannel seismic reflection data from the western New Caledonia offshore domain allow for the first time the direct, continuous connection of seismic reflectors between the Deep Sea Drilling Project 208 drill hole on the Lord Howe Rise and the New Caledonia Basin. A novel seismic interpretation is hence proposed for the northern New Caledonia Basin stratigraphy, which places the Eocene/Oligocene unconformity deeper than previously thought and revisits the actual thickness of the pre‐Oligocene sequences. A causal link is proposed between the obduction of the South Loyalty Basin over New Caledonia (NC) and the tectonic history of the northern New Caledonia Basin. Here it is suggested that as the South Loyalty Basin was being obducted during early Oligocene times, the NC Basin subsided under the effect of the overloading and underthrusted to accommodate the compressional deformation, which resulted in (1) the uplift of the northern Fairway Ridge and (2) the sinking of the western flank of New Caledonia. This event also had repercussions farther west with the incipient subsidence of the Lord Howe Rise.

A refraction seismic transect from the Faroe Islands to the Hatton‐Rockall Basin

The crustal structure of the Faroe‐Rockall Plateau was studied by a 790‐km‐long refraction seismic transect consisting of two intersecting lines. The air gun shot spacing was 200 m, and the signals were recorded by 77 ocean bottom seismometers. A P wave velocity model was developed from forward and inverse modeling of the wide‐angle seismic data and incorporation of coincident multichannel reflection seismic data. Continental crust with velocities ranging from 5.6 to 6.8 km/s can be traced from the Faroe Islands, across the banks to the SW of the Faroes and into the Hatton‐Rockall Basin. The thickness of the subvolcanic crust is up to 25 km on the banks but is as little as 8 km in the channels between the banks. The thinning in the channels may be related to NW‐trending shear zones extending from major lineaments in NE Rockall Trough. Basalt layers are found along the entire transect with a total thickness of up to 4 km. Two layers with velocities of 4.9–5.2 and 5.3–5.6 km/s are thought to represent Paleogene flood basalts that can be correlated from the Faroe Islands to George Bligh Bank. Close to George Bligh Bank, an 80‐km‐wide and up to 9‐km‐thick body with velocities of 6.5 km/s is interpreted as intrusion. A 5‐km‐thick, high‐velocity lower crustal layer (7.3 km/s) extends from the area of the intrusion into the northern Hatton‐Rockall Basin. At the northern flank of Lousy Bank the transition zone to oceanic crust was encountered.

Lower slope morphology of the Sumatra trench system

ABSTRACTAt convergent margins, the structure of the subducting oceanic plate is one of the key factors controlling the morphology of the upper plate. We use high‐resolution seafloor mapping and multichannel seismic reflection data along the accretionary Sumatra trench system to investigate the morphotectonic response of the upper plate to the subduction of lower plate fabric. Upper plate segmentation is reflected in varying modes of mass transfer. The deformation front in the southern Enggano segment is characterized by neotectonic formation of a broad and shallow fold‐and‐thrust belt consistent with the resumption of frontal sediment accretion in the wake of oceanic relief subduction. Conversely, surface erosion increasingly shapes the morphology of the lower slope and accretionary prism towards the north where significant oceanic relief is subducted. Subduction of the Investigator Fracture Zone and the fossil Wharton spreading centre in the Siberut segment exemplifies this. Such features also correlate with an irregularly trending deformation front suggesting active frontal erosion of the upper plate. Lower plate fabric extensively modulates upper plate morphology and the large‐scale morphotectonic segmentation of the Sumatra trench system is linked to the subduction of reactivated fracture zones and aseismic ridges of the Wharton Basin. In general, increasing intensity of mass‐wasting processes, from south to north, correlates with the extent of oversteepening of the lower slope (lower slope angle of 3.8° in the south compared with 7.6° in the north), probably in response to alternating phases of frontal accretion and sediment underthrusting. Accretionary mechanics thus pose a second‐order factor in shaping upper plate morphology near the trench.

Seismic analysis and distribution of a bottom-simulating reflector (BSR) in the Chilean margin offshore of Valdivia (40° S)

Multi-channel seismic (MCS) reflection data recorded offshore from Valdivia (40° S), in the Chilean margin, were processed to obtain a seismic image to establish structural characteristics and relate them to the presence of the bottom-simulating reflector (BSR). Seismic structure velocity of the BSR was determined using 1-D forward modeling. Recorded seismograms for two representative common mid-point (CMP) gathers were compared with synthetics, using different physical parameters to fit the waveforms. Our results confirm the presence of gas hydrates above the BSR. The BSR spatial continuity appears to be either interrupted or irregular due to the presence of faults. Tectonic movements can change the gas hydrate stability zone and consequently the BSR disappears or becomes weaker. Structural and topographic factors, differences in concentration, vertical distribution characteristics and internal structure of gas hydrates can influence BSR amplitude behavior. Variability in the concentration, volume, and extra supply of free gas coming from faults could be the main factors in the change of BSR amplitudes. The inclusion of the attenuation factor in the modeling supports the existence of free gas below the BSR. It is possible that the free gas below the BSR is distributed in bubbles or “bags”.

Crustal structure of the SW Moroccan margin from wide-angle and reflection seismic data (the Dakhla experiment). Part B — The tectonic heritage

The Dakhla 2002 experiment on the Northwest African margin obtained deep penetration, multichannel reflection (MCS) and wide-angle seismic (OBS) data. These results were combined with total magnetic intensity data, kinematic reconstructions and geological field studies to decipher the early tectonic and sedimentary history of the South Moroccan margin, adjacent to the Reguibat Shield. The structural study of this margin shows that the crust thins, from about 28 km beneath the un-thinned continental domain, to less than 10 km over a lateral distance of about 120 km. The Dakhla margin differs from the structural scheme of the adjacent African margin segments. There, the Moho rise, generally observed just above the deepening of the top of the basement, is achieved with an offset of around 50 km (with respect to the latter). In Early Jurassic time, the break-up of continental crust in what is now the Central Atlantic led to the formation of two basins, one on either side of the Reguibat, which correspond to the older parts of segments of the Late Palaeozoic Hercynian orogen. The Dakhla segment appears as a Precambrian cratonic zone, squeezed between two orogenic segments, which have remained unaffected by break-up processes; and the lower crust of this particular domain has behaved differently from all other neighbouring Appalachian (North America) and Mauritanide domains. This observation points to the important role of tectonic inheritance in the structural development of passive continental margins.

Multidisciplinary investigations exploring indicators of gas hydrate occurrence in the Krishna–Godavari Basin offshore, east coast of India

We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in 2002–2003 in the Krishna–Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW–ESE-trending sedimentary ridge in the south. Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO4 2−) concentrations (28.7 to <4 mM), increasing downcore methane (CH4) concentrations (0–20 nM) and relatively high total organic carbon contents (1–2.5%), and microbial analyses a high abundance of microbes in top core sediments and a low abundance of sulphate-reducing bacteria in bottom core sediments. Methane-derived authigenic carbonates were identified in some cores. Combined with evidence of gas-escape features in association with bottom simulating reflections, the findings strongly suggest that the physicochemical conditions prevailing in the study area are highly conducive to methane generation and gas hydrate occurrence. Deep drilling from aboard the JOIDES Resolution during 2006 has indeed confirmed the presence of gas hydrate in the Krishna–Godavari Basin offshore.

Subsurface structure of the Myojin Knoll pumiceous volcano obtained from multichannel seismic reflection data

Abstract The Myojin Knoll is a submarine volcano that has a classically beautiful conical-shaped silicic caldera whose surface is covered by pumice. To determine the tectonic structure inside the caldera wall and beneath the caldera floor of this pumicious submarine volcano, we carried out a structural interpretation study using newly collected deep-penetrating multichannel seismic (MCS) reflection data. We also conducted a detailed velocity analysis of the MCS data, which facilitated the interpretation study. The results demonstrate that approximately 90% of the caldera wall is composed of pumiceous volcanic breccia. This finding supports those of previous researchers who, based on seafloor observations, single-channel seismic reflection, and gravity and geomagnetic data, concluded the Myojin Knoll is a knoll having a pumiceous caldera wall underlain by a pre-caldera rhyolitic stratovolcano edifice. We also determined a down-warping reflector approximately 800 m beneath the caldera floor. A seismic unit immediately above the reflector has a higher P-wave velocity than the pumice units and shows a chaotic seismic reflection pattern. We interpreted the reflector to be the bottom of a possible shallow magma chamber where the magma would undergo repeated expansion and contraction as a result of recurrent eruption activities.

Imaging meddy finestructure using multichannel seismic reflection data

This work illustrates the great potential of multichannel seismic reflection data to extract information from the finestructure of meddies with exceptional lateral resolution (10–15 m). We present seismic images of three meddies acquired in the Gulf of Cadiz (SW Iberian Peninsula), which consist of concentric reflectors forming oval shapes that sharply contrast with the background oceanic structure. The seismic images reveal the presence of different regions within the meddies that are consistent with those observed in historical temperature (T) and salinity (S) data. The core region, characterized by smooth T and S variations, is weakly reflective. The double‐diffusive upper and lower boundaries and the lateral‐interleaving outer edges, characterized by stronger T and S contrasts, display strong reflectivity bands. These new observations clearly show differences between layers developed at the upper and lower boundaries that can contribute to the knowledge of mixing processes and layering formation in oceans.