Seismic Reflection Analysis Research Articles

Northeastern Brazilian margin: Regional tectonic evolution based on integrated analysis of seismic reflection and potential field data and modelling

Integration of regional seismic reflection and potential field data along the northeastern Brazilian margin, complemented by crustal-scale gravity modelling, is used to reveal and illustrate onshore–offshore crustal structure correlation, the character of the continent–ocean boundary, and the relationship of crustal structure to regional variation of potential field anomalies. The study reveals distinct along-margin structural and magmatic changes that are spatially related to a number of conjugate Brazil–West Africa transfer systems, governing the margin segmentation and evolution. Several conceptual tectonic models are invoked to explain the structural evolution of the different margin segments in a conjugate margin context. Furthermore, the constructed transects, the observed and modelled Moho relief, and the potential field anomalies indicate that the Recôncavo, Tucano and Jatobá rift system may reflect a polyphase deformation rifting-mode associated with a complex time-dependent thermal structure of the lithosphere. The constructed transects and available seismic reflection profiles, indicate that the northern part of the study area lacks major breakup-related magmatic activity, suggesting a rifted non-volcanic margin affinity. In contrast, the southern part of the study area is characterized by abrupt crustal thinning and evidence for breakup magmatic activity, suggesting that this region evolved, partially, with a rifted volcanic margin affinity and character.

Seismic reflectors beneath the central cones of Aso Volcano, Kyushu, Japan

The subsurface structure of Aso Volcano, central Kyushu, Japan, has been imaged down to 10 km below sea level and notable features of subsurface structure in the central cones were revealed by a 3-D seismic reflection analysis. The 3-D reflection analysis included reflection enhancements and 3-D migration. The reflection enhancements and the NMO correction were applied on waveform data from the seismic experiment ASO98. The 3-D migration with a Kirchhoff integral was processed for 1397 NMO corrected traces to calculate the reflectivity at about 60 000 image points in an approximately 7 km by 7.5 km square region. The results of the 3-D migration show that the subsurface structure beneath the central cones can be divided into three parts, the western part, the eastern part, and the northwestern part. Bright reflectors appear in most of them and a significant discontinuity is inferred. The reflector horizon at 2 km below sea level spreads beneath the western and the eastern part and is probably the top surface of the Pre-Aso volcanic rocks or the basement rocks. Reflection voids appear in the western part and just beneath the active crater. The reflector void in the western part implies a hot region and is most probably the known seismic anomalies, such as low velocity region or attenuation region. Another reflector void beneath the active crater can also be associated with a hot region, apparently surrounding the conduit system of the current volcanic craters. The major discontinuity is located beneath the northwest flank of the central cones. This discontinuity divides the northwest region with weak reflectors from other parts. The discontinuity may be the trace of the Oita–Kumamoto Tectonic Line, which is the major tectonic line in central Kyushu, crossing the Aso caldera.

Effects of biogenic silica on sediment compaction and slope stability on the Pacific margin of the Antarctic Peninsula

AbstractAnalysis of physical properties measured on cores and on discrete samples collected by the Ocean Drilling Programme (ODP) Leg 178 on the Pacific margin of the Antarctic Peninsula reveals anomalous down‐hole curves of porosity, density, water content, and P‐wave velocity. These indicate an overall trend of increasing porosity with depth and suggest that the drifts are mostly undercompacted. In one of the two boreholes analysed, a sharp decrease in porosity, matching increasing bulk sediment density and increasing compressional velocity occurs towards the base of the hole, which corresponds to a bottom‐simulating reflector in the seismic section. Analysis of seismic reflection, down‐hole logging, geotechnical and mineralogical data from two drilling sites indicates that the observed anomalous consolidation trends are a consequence of the presence of biogenic silica (diatom and radiolarian skeletons) even with a small to moderate amount. Above the bottom‐simulating reflector, intergranular contacts among whole or broken siliceous microfossils prevent normal sediment consolidation. Diagenetic alteration of biogenic opal‐A to opal‐CT causes a dramatic reduction of intra‐ and interskeletal porosity allowing sediments to consolidate at depth. This results in overpressuring and a decrease in the effective stress. Excess fluids are expelled towards the sediment surface through near vertical, small throw normal faults extending from the diagenetic front to the seafloor and affecting the stability of the submarine slope in the form of gravitational creep along a weakened surface.This work shows how physical properties of shallow fine‐grained marine sediments can be analysed as basin‐wide indicators of biogenic silica abundance. The diagenetic alteration of siliceous microfossils is a possible cause of slope instability along world continental margins where bottom‐simulating reflectors related to silica diagenesis are present at a regional scale.

Seismic prospecting of a slope affected by deep-seated gravitational slope deformation: the Lago Sackung, Calabria, Italy

In this study seismic reflection and refraction were used to study the Sackung of Lago (Calabria, Italy). The seismic track is roughly 1200 m long. Seismic-reflection analysis has seldom been used to study mass movements; structurally landslides tend to be extremely complex, and the landslide body is not a good medium for seismic waves, because of dispersion and noise. However, through the use of recent sophisticated software, it has become possible for seismic reflection to obtain good results. In this case study, a package called Seistrex 3 was used. By means of static and topographic corrections, enhanced by filtering and migration, a seismic profile with good resolution of the geometry of layers and faults to depths of 100–400 m was obtained. This study shows that seismic reflection is more useful than seismic refraction for examining the internal structure of a landslide.

Seismic reflection analysis of the Manson Impact Structure, Iowa

Our combined interpretation of new, high‐resolution seismic reflection data and reprocessed, but previously published, industrial Vibroseis data indicates that the Manson Impact Structure, Iowa, has an apparent crater diameter of 35 km, an annular trough diameter of around 21 km, a shallow floor (0.6–0.7 km), and a central uplift that has a minimum diameter of 7.5 km. The two reflection lines are coincidentally located along an east‐west radial transect and are constrained by shallow drill information. Results from the two data sets are correlative; both data sets were instrumental to the final interpretation due to the trade‐off between resolution and depth of energy penetration. Based on the combined interpretation, structural uplift of the central peak is estimated to be around 2.8 km. Onlapping seismic sequences are present at the eastern edge of the central uplift. These seismic packages, observed only in the high‐resolution line, are interpreted to represent impact breccia or debris material that was shed from the central peak or dynamically transported from outside of the crater.

Formation of oceanic crust at slow spreading rates: New constraints from an extinct spreading center in the Labrador Sea

We present a joint analysis of seismic reflection and refraction profiles across an extinct spreading center in the Labrador Sea. The refraction data are used to constrain the crustal and subcrustal velocity structure, and the deep multichannel reflection data are used to determine the nature of the tectonic fabric within the crust. The resulting crustal model shows that major reductions in crustal thickness and velocity across the rift are associated with a broad zone of rotated fault blocks. Enhanced faulting persists over a width of 100 km, creating a deep rift valley and adjacent elevated ridges. The location of crustal-scale faults beneath the sides of the rift valley bound a region of reduced mantle velocity, consistent with its formation by 5%–10% serpentinization of mantle peridotite. The variation in crustal thickness across the rift indicates a 30%–40% reduction in the crustal thickness for full spreading rates <20 km/m.y. However, our results suggest that most of the observed crustal thinning is produced by postaccretionary tectonic extension of the crust, during an extended interval when the rate of melt supply did not keep pace with the rate of plate separation. Such a discontinuous process may not be properly represented by theoretical models of crustal formation that assume steady-state viscous flow.

Applications of land-based high resolution seismic reflection analysis to Quaternary and geomorphic research

The availability, in the last few years, of low cost microcomputers and multichannel seismographs has made possible the application of high resolution, digital, seismic reflection profiling to Quaternary problems. Seismic analysis of the subsurface can provide continuous, two dimensional sections containing detailed structural and stratigraphic information. A review of the general principles of shallow seismic reflection methods includes the nature of seismic sources (the use of shotgun shells), the application of the ‘optimum offset’ method, data processing and the issue of site dependency. A brief overview is provided of seismic and sequence stratigraphy. Four case studies illustrate the application of reflection seismic profiling: (1) Champlain Sea sediments (Late Wisconsinan) on the flank of the Gatineau Hills near Quyon, Québec; (2) the definition of bedrock valleys beneath Pleistocene deposits near Shawville, Québec; (3) the fill of a linear depositional basin — Okanagan Valley, British Columbia; (4) the seismic signatures of topset and foreset beds, slope failures and a distributary channel of the Fraser River delta, British Columbia.

Shallow Structure and Recent Evolution of the Aegean Sea Deduced from the Seismic Reflection Analysis: ABSTRACT

Together with the Tyrrhenian Sea, the Aegean Sea represents one of two marine basins still developing as a consequence of the subduction of the African lithosphere beneath Europe. Despite many geophysical similarities with the Tyrrhenian Sea, the Aegean displays a specific structural segmentation characterized by two distinct domains separated by the central Aegean. To the north of the basin, the so-called North Aegean trough likely represents the western marine extension of the transtensive Anatolian transform fault zone. The northern margin of this area contains a series of disconnected, often thickly sedimented small basins that probably initiated during the late Miocene as a consequence of a dominantly north-south extension; typical uppermost Miocene (Messinian) formations can be observed on seismic grounds. To the south, the Cretan Sea shows clear evidence of important distensive events occurring during two main episodes and following two main trends; a dominantly north-south-directed extension is responsibile for most of the structural features detected along both the Cretan and southern Cyclades margins.

Late Palaeozoic cold-climate aeolianites, southern Cooper Basin, South Australia

Summary Core facies analysis of the Permo-Carboniferous Gidgealpa Group of the southern Cooper Basin, South Australia has revealed sandstones of aeolian origin within the glacigenic Merrimelia Formation. The aeolianites comprise exclusively porous, poorly cemented, medium- and fine-grained sandstones dominated by parallel lamination. Sharp, planar truncation surfaces divide the suite into units composed mainly of translatent windripple lamination and minor amounts of sandflow bedding with primary depositional dip (post-compaction) varying between 0° and 25°. Distinctive compressional, tensional and dissipation structures are preserved in deformed lee-slope deposits and the inclusion of low-angle cross-strata and wedge-bedded units locally indicate deposition on the basal aprons of dunes or in contiguous sand sheets. Wet interdune sediments with adhesion ripples are also exhibited in the cores. The aeolianites, up to 232 m thick, have been traced subsurface by integrating core facies, wireline logs and seismic reflection analysis. A low-interval transit time on sonic logs is characteristic of the porous aeolianite sand and has facilitated mapping of the dune field using seismic data. The sandstone body trends SE-NW, is 6 to 8 km wide and passes laterally into outwash fan and fluvioglacial, braidplain deposits. Potential hydrocarbon traps may occur at the top of the aeolianites, or within them beneath intraformational seals formed by possible muddy interdune facies. The overall high-latitude, cold-climate nature of the Merrimelia Formation, deposited during the extensive Gondwanaland glaciation, is well known but this discovery not only records the first Gondwanan aeolianite suite but may also detail the first pre-Quaternary example of paraglacial aeolianites.

IASPEI Workshop: Seismic Modeling of Laterally Varying Structures

During the past 10 years, significant progress has been made in the methods of collection and analysis of seismic reflection and refraction data. This progress has led to the development of new models for the structure and composition of the earth's crust, based on sophisticated analysis of numerous profiles in many areas of geologic importance. The third triannual meeting of the IASPEI (International Association of Seismology and Physics of the Earth's Interior) Commission of Controlled Source Seismology was convened in Park City, Utah, on August 11–17, 1980, to bring together seismologists and geologists to explore and assess the progress of controlled source techniques (controlled sources include explosions, air guns, and Vibroseis‐type sources), and to evaluate its significance in terms of current models of the seismic velocity structure and composition of the crust and upper mantle. Particular attention was paid to the progress and problems in the modeling of two‐ and three‐dimensional structures.