Seismic reflection data

In contrast to the use of marine seismic reflection techniques for reservoir-scale applications, where seismic inversion for quantitative sediment analysis is common, shallow-water, very-high-resolution seismic reflection data are seldom used for more than qualitative reflection interpretation. Here, a quantitative analysis of very-high-resolution marine seismic reflection profiles from a shallow- water (<50 m water depth) fjord in northern Norway is presented. Acquired using Sparker, Boomer, and Chirp sources, the failure plane of multiple local landslides correlates with a composite reflection that reverses polarity to the south. Using a genetic algorithm, a 1D post-stack acoustic impedance inversion of all three profiles is performed, calibrating against multi-sensor core logger (MSCL) data from cores. Using empirical relationships the resulting impedance profiles are related to remote sediment properties, including: P-wave velocity; density; mean grain size; and porosity. The composite reflector is consistently identified by all three data sources as a finer-grained (by one Phi), lower density (c. 0.2 g/cm3 less than background) thin bed, with an anomalous low velocity zone (at least 100 m/s lower than background) associated with the polarity reversal to the south. Such a velocity contrast is consistent with an accumulation of shallow free gas trapped within the finer- grain, less permeable layer. This study represents the first application of acoustic impedance inversion to very-high-resolution seismic reflection data and demonstrates the potential for directly relating seismic reflection data with sediment properties using a variety of commonly used shallow seismic profiling sources.

Submarine gravity flows are a key process for transporting large volumes of sediment from the continents to the deep sea. The location, volume, and character of the sediment bypassed by these flows dictates the areal extent and thickness of the associated deposits. Despite its importance, sediment bypass is poorly understood in terms of flow processes and the
associated stratigraphic expression. We first examine the relationships between the physical parameters that govern bypass in flows, before assessing the variable stratigraphic expression of bypass from modern seafloor, outcrop, and subsurface datasets. Theoretical and numerical approaches distinguish grain size, slope, flow size, and sediment concentration as parameters that exert major controls on flow bypass. From field data, a suite of criteria are established to recognize bypass in the geological record. We identify four bypass-dominated zones, each of which is associated with a set of diagnostic criteria: slope-channel bypass, slope-bypass from mass wasting events, base-of-slope bypass, and basin-floor bypass. As the expression of bypass varies spatially and is dependent on the scale of observation, a range of scale-dependent criteria are required for robust interpretation
of these zones in the field or subsurface. This synthesis of deep-water sediment bypass highlights the challenge in quantitatively
linking process with product. The establishment of criteria to recognize sediment bypass, qualitatively linked with flow
processes, is an important step towards improving our understanding of submarine flow dynamics and resultant stratigraphic architecture.

The integral solution of the wave equation has long been one of the most popular methods for imaging (Kirchhoff migration) and inverting (Kirchhoff inversion) seismic data. For efficiency, this process is commonly formulated as a time-domain operation on each trace, applying antialiasing through high-cut filtering of the operator or pre-/postmigration dip filtering. Migration in the time domain, however, does not allow for velocity dispersion; standard antialiasing methods assume a flat reflector and tend to overfilter the data. We have recast the Kirchhoff integral in the frequency domain, enabling robust antialias filtering through appropriate dip limiting of each frequency and implicit accommodation of true dispersion. Full frequency decomposition of the input seismogram can be approximated by band-pass filtering (or correlation with band-limited source sweeps for Chirp/Vibroseisdata) into a few narrow-band traces that cumulatively retain the full source bandwidth. From prior knowledge of the source waveform, we have defined suitable bandwidths to describe broadband (3.0 octaves) data using just six frequency bands. Kirchhoff migration of these narrow-band traces using coefficients determined at their central frequencies significantly improves the preservation of higher frequencies and cancellation of steeply dipping aliased energy over traditional time-domain antialiasing methods. If, however, two bands per octave cease to be a robust approach, our frequency-approximated approach provides the processor with ultimate control over the frequency decimation, balancing increased resolution afforded by more bands against computing cost, whereas the number of frequency bands is few enough to permit detailed control over frequency-dependent antialias filtering parameters.

The Black Sea is an oval basin with an area of 423.000 km2, a volume of 534.000 km3 and a maximum depth of 2206 m. It is connected to Atlantic Ocean by Mediterranean Sea-Aegean Sea-Turkish Straits System (The Sea of Marmara and the straits of Bosphorus and Dardanelles) and it is also connected to Sea of Azov by the Kerch strait. The mean sea level at the Black Sea is about 55 cm higher than the level at the Aegean Sea. The maximum development of the Black Sea shelf is at the west of Crimean peninsula, where it is more than 190 km wide. Along the Turkish coast, Russian coast and south of the Crimean peninsula, the shelf exceeds 20 km in width in only a few places. The widths of about 40 km are found off the Bulgarian coast and south of the Sea of Azov. The Black Sea opened as a back-arc and/or intra-arc basin to the north of Pontide magmatic belt. It is seperated by the mid-Black Sea Ridge (comprising Andrusov and Archangelsky ridges). Western Black Sea Basin (WBSB) and Eastearn Black Sea Basin (EBSB) have different time and mechanisms of opening. The study area is located on the southern margin of the WBSB, on the fragment called Istanbul Zone (which is delimited by the Intra-Pontid suture in the south and by the WBSB to the north) and its offshore area. WBSB opened as a back-arc basin behind the southwards-moving Istanbul Zone and collided with the Sakarya Zone along the Intra-Pontide Suture during the Eocene. Istanbul Zone rifted off from its original location around the present Odessa Shelf during the early Cretaceous, and started to move southwards along the West Black Sea and West Crimean transform faults. The emplacement of the Istanbul Zone into its present position was possibly during the late Albian. Geophysical data have become increasingly important for rapidly developed tectonic research since 1960s. Seismic reflection method is the most effectively used geophysical method for investigation of the deep structure and the geological features beneath the sea floor. From the middle of 1990's to present, in spite of well-rounded but non-utilizable data of commercial two dimensional (2D) and three dimensional (3D) seismic studies of petroleum companies, the number of scientific studies and publications for academic purposes, revealing the geological structure beneath Turkish sector of the WBSB using marine seismic reflection data, has been increasing. Especially, there is a considerable amount of scientific articles focused on various subjects (such as mass wasting, sedimentation, sea-level fluctuations, BSR, Messinian events etc.) presenting detailed seismic sections and providing a good definition of the structural properties beneath the shelf and the continental slope of the southern part of the basin along Turkish margin. In tectonics, researchers search for ways to predict the time and location of damaging tectonic events such as earthquakes and tectonically induced events. To be neotectonically useful, the predictions must be precise enough to alleviate the loss of life and property. This quest for prediction techniques also involves the investigation of active tectonics. The earthquake occurred on October 15, 2016 (Ml=5.0) re-attracted attention to the tectonic activity of WBSB. This earthquake took place at 11:18 local time and lasted 7-8 seconds. It was felt in the cities of Istanbul, Kocaeli, Duzce, Sakarya, Zonguldak and even in the Bulgarian cities Varna and Bourgas. The epicenter of the earthquake was at a point 195 km north-east of Istanbul and 124 km north-west of Zonguldak. This earthquake with a shallow epicenter (11.4 km below the surface) was one of the rare instrumentally recorded strong earthquakes occured at the southern part of the WBSB. The Bartın Earthquake of September 3, 1968 (MS=6.6) is the strongest instrumentally recorded earthquake along the Turkish margin of Black Sea. This earthquake was the first seismological evidence of active thrusting occuring at the southern margin of the Black Sea and its fault-plane solution indicated a thrust mechanism which emphasized a different behaviour from the right-lateral strike-slip North Anatolian Fault. Focal mechanism derived from the solution of the moment tensor for the October 15, 2016 Black Sea earthquake, revealed another clue for the active thrusting at the WBSB with a very similar focal mechanism solution of Bartin Earthquake. The central and the deepest parts of the Black Sea was believed to be aseismic but, with the recent studies, the researchers showed that there are a significant number of earthquakes in the Black Sea, mostly of magnitude Mw=4.0 or smaller. They also revealed that the seismicity increases towards the margins of the Black Sea, with the largest events at the margins. The researchers also emphasized that, in southern Black Sea and the middle part, especially offshore of Bartın and Samsun, the earthquakes which have the reverse faulting component dominated solutions, are related with the compressional tectonic regime and the strikes of these earthquakes generally lie parallel to the basin. These earthquakes show that the region is not aseismic and produces earthquakes from time to time, though not very frequently. The folds are commonly accompanied by reverse faulting and many of these reverse faults are low angle and are called thrust faults. Some of the thrust faults and high-angle reverse faults may break the surface, but many others remain hidden within the cores of anticlines and are termed buried reverse faults. Buried active faults present a significant earthquake hazard-that large damaging earthquakes can occur on faults located entirely beneath or within folded rocks. The tips of such faults may be buried at depths of several kilometers, and when they rupture during earthquakes, uplift and folding occur at the surface. In this study, the fault structures that are considered to be formed by the effect of compressional tectonic regime and the structures formed by the activities of these faults beneath the shelf and slope areas between the region offshore Akçakoca-Cide at the southern part of the WBSB, revealed by using marine seismic reflection data. The marine seismic reflection data used in this study were collected by the collaboration of General Directorate of Mineral Research and Exploration (MTA), Cambridge University and Istanbul Technical University (ITU) in September, 1998. R/V MTA Sismik-1 was used to collect data on 14 seismic lines with a total length of 460 km. All necessary and some optional processes of typical marine seismic data processing sequence were applied to raw data to obtain time-migrated seismic sections. From the time-migrated seismic sections, to give examples for thrust fault structures and folds formed by the activities of these faults became possible. These images were beneficial to clarify the presence of the compressional regime effecting the study area. Available onland and offshore geological exploratory well data of Akçakoca-1, Akçakoca-2, Ereğli-1, Filyos-1, Bartin-1, Ulus-1, Amasra-1, Cakraz-1 and Gegendere-1 are obtained from General Directorate of Petroleum Affairs (PIGM) and used to correlate with seismic sections. Seismic markers of the time-migrated seismic sections were dated by referencing to Akcakoca-1 and Akcakoca-2 wells. Also in this study, the land-offshore geological sections prepared by means of the geological sections given by previous geological studies and the information about the continuation of the geological features from land to offshore in the study area was presented. These geological sections also give quite beneficial new evidences for the presence of the compressional tectonic regime in the study area. The important findings of this study were to give another clue for the presence of the compressional tectonic regime effecting the basin by using the outcomes of seismic reflection studies and to reveal the continuation of the thrust related geological features from land to offshore. The results obtained, agrees with the opinion that Black Sea is being compressed in N-S direction.

This study is part of the preliminary and final planning of an integrated urban plan on a new site for many thousands housings on an area of more than 500 Ha, located in Kintélé, Brazzaville (Republic of Congo). Our phase of preliminary project required a multidisciplinary studies, characterized by the integration of geological, geotechnical, geophysical fieldworks, and remote sensing analyses. Beside to the geological works, the goal of geophysical surveys is to detect, evaluate and model the superficial geo-stratigraphical structure. Aims of surveys were: • Geological reconstruction and update the existing geological map; • Reconstruction of deep structures and terrain seismic characterization. The work started with an accurate photo-geological interpretation trough images acquired from satellite in the visible and near infrared sections of electromagnetic spectrum (space borne images database offer by Google and the archived scene acquired by French satellite Pleiades 1A). This interpretation was finalized to interpret the main geomorphologic structures of the area and to detect the main geological features. The results of this phase were verified and updated by means of a detailed geological fieldwork. Active seismic methods in-array and single station passive methods allowed us to reconstruct the relatively deep structure, through the followings techniques: 11 seismic refractions in P-waves (array of 48 geophones, 14 Hz, 10 m of interdistance), 25 MASW profiles (with 24 geophones, 4.5 Hz, 3 m of interdistance), 74 HVSR graphs obtained from seismic noise recorded by passive single station measurements (with a 128Hz sample frequencies for 20 minutes). Geophysical data were processed taking into account also the available results of the direct investigations (boreholes and pressiometric tests) assumed as stratigraphical constrains. The results of active and passive seismic surveys underline each other the presence of two seismostrates with a contact depth from 35 to 40m below the ground level. Geological fieldworks and boreholes logs show the presence of silty-sand layer for the thickness investigated by seismic surveys, so the increasing of acoustic impedance is related to a variability of layer compaction. This variation is also proved by results of the performed pressiometric tests, which show an increasing of elastic modulus corresponding to the growing of acoustic impedance. In summary, the results of this study showed the substantial feasibility of the urban plan designed for Kintélé, of course subject to the necessary structural adjustments.

The Dead Sea Transform (DST) is a prominent shear zone in the Middle East. It separates the Arabian plate from the Sinai microplate and stretches from the Red Sea rift in the south via the Dead Sea to the Taurus-Zagros collision zone in the north. Formed in the Miocene 17 Ma ago and related to the breakup of the Afro-Arabian continent, the DST accommodates the left-lateral movement between the two plates. The study area is located in the Arava Valley between the Dead Sea and the Red Sea, centered across the Arava Fault (AF), which constitutes the major branch of the transform in this region.
A set of seismic experiments comprising controlled sources, linear profiles across the fault, and specifically designed receiver arrays reveals the subsurface structure in the vicinity of the AF and of the fault zone itself down to about 3-4 km depth. A tomographically determined seismic P velocity model shows a pronounced velocity contrast near the fault with lower velocities on the western side than east of it. Additionally, S waves from local earthquakes provide an average P-to-S velocity ratio in the study area, and there are indications for a variations across the fault. High-resolution tomographic velocity sections and seismic reflection profiles confirm the surface trace of the AF, and observed features correlate well with fault-related geological observations.
Coincident electrical resistivity sections from magnetotelluric measurements across the AF show a conductive layer west of the fault, resistive regions east of it, and a marked contrast near the trace of the AF, which seems to act as an impermeable barrier for fluid flow. The correlation of seismic velocities and electrical resistivities lead to a characterisation of subsurface lithologies from their physical properties. Whereas the western side of the fault is characterised by a layered structure, the eastern side is rather uniform. The vertical boundary between the western and the eastern units seems to be offset to the east of the AF surface trace.
A modelling of fault-zone reflected waves indicates that the boundary between low and high velocities is possibly rather sharp but exhibits a rough surface on the length scale a few hundreds of metres. This gives rise to scattering of seismic waves at this boundary. The imaging (migration) method used is based on array beamforming and coherency analysis of P-to-P scattered seismic phases. Careful assessment of the resolution ensures reliable imaging results.
The western low velocities correspond to the young sedimentary fill in the Arava Valley, and the high velocities in the east reflect mainly Precambrian igneous rocks. A 7 km long subvertical scattering zone (reflector) is offset about 1 km east of the AF surface trace and can be imaged from 1 km to about 4 km depth. The reflector marks the boundary between two lithological blocks juxtaposed most probably by displacement along the DST. This interpretation as a lithological boundary is supported by the combined seismic and magnetotelluric analysis. The boundary may be a strand of the AF, which is offset from the current, recently active surface trace. The total slip of the DST may be distributed spatially and in time over these two strands and possibly other faults in the area.

This study presents the results of a marine geophysical survey performed in the Igaliku fjord in southern Greenland in order to understand the harbour setting of the former Norse settlement Garðar (modern Igaliku). The aims of the survey were (a) to reconstruct the former coastline during the first centuries of the Norse settlement period (c. 11/12th centuries) and (b) to search for archaeological remains on the seabed connected to maritime traffic and trade. In order to approach these goals, we used an integrated marine survey system consisting of a side-scan sonar and a reflection seismic system. The system was designed for lightweight transport, allowing measurements in areas that are logistically difficult to access. The side-scan sonar data revealed no remains of clear archaeological origin. Bathymetric data from seismic seabed reflection and additional Differential GPS height measurements yielded a high-resolution bathymetric map. Based on estimates of Holocene relative sea le...

In hydrocarbon prospection, the inversion of marine geophysical data for remote reservoir characterization has developed enormously over the past 20+ years. While some techniques (e.g., waveform inversion) are computationally expensive to permit widespread application across all targets, other less expensive variants (e.g., impedance and amplitude-versus-angle inversion) have become a standard component of most interpretation workflows. In contrast, there has been very little progress toward the remote classification of near-surface sediments through the inversion of high-resolution geophysical data, with both academia and industry relying on extensive coring and stratigraphic correlations.

However, accurate stratigraphic correlation can be difficult due to inaccuracies in the time-to-depth conversion of the geophysical data, and the potential for loss and/or compression of high porosity and poorly-consolidated seafloor material during the intrusive sampling process. Additionally, when dealing with near-surface sediments (top 10s m below seafloor), the sampling procedure inherently alters the nature of the sampled material, introducing uncertainties on key mechanical parameters (e.g., porosity and undrained shear strength) that are difficult to quantify.

Recently, a new inversion method was presented, which is based on the application of post-stack acoustic impedance inversion to high-resolution marine seismic reflection data using a genetic algorithm and a convolutional forward model. Here, we further test this approach using a synthetic example and a marine, near-surface geophysical case study from a fjordic environment in Norway. We characterize the robustness of the algorithm under a variety of noise conditions and utilize the stochastic optimization approach to derive 95% confidence limits as well as a statistically ‘best’ model. Furthermore, we show how, through the use of global empirical relationships, soil mechanical properties can be derived (including effective stress, over pressure, clay/sand fraction and moisture content), while, through the application of soil mechanical models (e.g., White’s model), it is possible to estimate gas saturation.

In both case studies (synthetic and field), the inversion results demonstrate excellent correlation with direct sampling, even in the presence of modest noise contamination. We can identify metre-scale stratigraphic changes, as well as subtle decimetre-scale structures, such as a 40 cm thick composite landslide glide plane. The high-fidelity remote derivation of such soil properties has significant applications, both within academia and the offshore services/exploration industry.

High-resolution mapping of the Norwegian shelf and upper slope as well as fjords reveal a myriad of smaller-scale slope failures in different environmental settings. In this manuscript, we focus on mass movement processes in coastal areas (Finneidfjord), and pristine open ocean settings in intermediate water depths (Vesterålen) and deep water (Lofoten). The landslides have variable geomorphologic expression. They may occur in clusters or isolated. These landslides with their slip planes at shallow depths below the seabed appear as recent features. However, failure likely happened repeatedly and possibly frequently. Hence, recurring failures may affect coastal communities or jeopardize offshore engineering applications. Preconditional factors and sedimentary processes of these landslides are not well constrained. Therefore, an integrated research initiative was launched to improve our understanding of these landslides, using a combination of geophysical, geological, geotechnical laboratory testing as well as in situ tests (CPTU). This paper discusses the philosophy behind the programme and preliminary results.

The 1996 Finneidfjord landslide, which took four human lives in northern Norway, initiated along a weak layer offshore before encroaching the shoreline. The integration of sediment cores, free-fall cone penetrometer tests and high-resolution 3D seismic data indicate that the slide-prone layer is a regional event bed likely sourced from clay-slide activity in the catchment of the fjord. Its main influence on slope stability is attributed to their physical and geotechnical properties as they are softer and more sensitive than the surrounding bioturbated, fjord-marine deposit. The geotechnical properties are also likely affected by biogenic gas as found in the stratified event beds. Similar, fine-grained, stratified beds with comparable origin and properties also occur in other Norwegian fjords and are likely present along coastlines of other, previously glaciated margins, where they could be an important factor contributing to mass movements.

Windermere is a glacially overdeepened lake located in the southeastern Lake District, UK. Using the three-dimensional (3D) Chirp subbottom profiler, we image mass movement deposits related to the Younger Dryas (YD) within a decimetre-resolution 3D seismic volume, documenting their internal structure and interaction with preexisting deposits in unprecedented detail. Three distinct flow events are identified and mapped throughout the 3D survey area. Package structures and seismic attributes classify them as: a small (total volume of c. 1500 m-cubed) debris flow containing deformed translated blocks; a large (inferred total volume of c. 500,000 m-cubed), homogeneous fine-grained mass flow deposit; and a debris flow (inferred total volume of c. 60,000 m-cubed) containing small (c. 8.0 × 2.0 m) deformed translated blocks. Geomorphological mapping of their distribution and interaction with preexisting sediments permit the reconstruction of a depositional history for the stratigraphic units identified in the seismic volume.

Campi Flegrei is an active, resurgent caldera that is located a few kilometres west of the city of Naples, a densely populated urban settlement in southern Italy. Identifying, locating at depth and better defining the geometry of the magma feeding system of the caldera is highly relevant for assessing and monitoring its volcanic hazard. Based on a high resolution seismic reflection dataset, we investigated the deep structure of the volcano. Here we show that seismic wave amplitude variations with distance from the radiating source provide clear evidence for large amplitude seismic reflections from the top of an extended gas- and/or brine-bearing rock formation at about 3000 m and of an about 7500 m deep, low velocity layer, which is associated with a mid-crust, partial melting zone beneath the caldera. The modeling of magma properties based on measured seismic velocities indicates a relatively high melt percentage (in the range 80-90%). These new data suggest that a large magmatic sill is present well within the basement formations, which is possibly linked to the surface through a system of deep fractures bordering the caldera. The lateral extension and similar depth of the melt zone observed beneath the nearby Mt. Vesuvius support the hypothesis of a single continuous magma reservoir feeding both of these volcanoes.

An acoustic impedance inversion (AI) analysis has been carried out with an inversion-based model to identify reservoir rock (sandstone) on the Sahul platform, precisely at the Flamingo height. The Plover Formation is a formation containing gas sand interbedded with thin shale. Mistral 1 well log analysis was used to determine the value of the acoustic impedance of the sandstone. The results of log analysis on the Plover formation show that the reservoir of low impedance values is interpreted as sandstone 31000-37500 (ft/s) * (g/cc) while high impedance is interpreted as shale > 37500 (ft/s) * (g/cc). Time slice inversion of acoustic impedance shows that the sandstones of the Plover Formation are concentrated to the west of the Mistral 1 well, precisely in the Flamingo High structure of the Bayu Undan gas field at Timor Sea.

The Indian Plate has been the focus of intensive research concerning the flood basalts of the Deccan Traps. Here we document a volcanostratigraphic analysis of the offshore segment of the western Indian volcanic large igneous province, between the shoreline and the first magnetic anomaly (An 28 ∼63 Ma). We have mapped the different crustal domains of the NW Indian Ocean from stretched continental crust through to oceanic crust, using seismic reflection and potential field data. Two volcanic structures, the Somnath Ridge and the Saurashtra High, are identified, extending ∼305 km NE–SW in length and 155 km NW–SE in width. These show the internal structures of buried shield volcanoes and hyaloclastic mounds, surrounded by mass-wasting deposits and volcanic sediments. The structures observed resemble seismic images from the North Atlantic and northwest Australia, as well as volcanic geometries described for Réunion and Hawaii. The geometry and internal seismic facies within the volcanic...

Structural pattern mapping and modeling for structural styles delineation and prospect identification in the complex Xinglongtai-Majuanzi structure, Liaohe depression, China is a concern in exploring for hydrocarbon. This is as a result of the occurrence of varied faulting pattern and sub-seismic faults. Analysis was on attributes computed from migrated 3D seismic data, with support from well logs from the area. Architectural assessment, structural delineation and prospect analysis was done on the portion of the massive sag structure proven to have originated in the Paleogene period. Combinations of structural and stratigraphic attributes were used in evaluating the nature of the fairly consolidated turbidite deposits of the hydrocarbon prolific Shahejie formation amidst numerous syndepositional faults. Results indicate that normal and reverse faults are quite common in the study area with dipping angle between 50 and 80 degrees. The deposits of interbeded sand shale sequences are c...

In the UK, a combination of outcrop mapping, satellite digital elevation models, high-resolution marine geophysical data and a range of dating techniques have constrained the maximum limit and overall retreat behaviour of the British and Irish Ice Sheet (BIIS). The changing styles of deglaciation have been most extensively studied in the west and north-western sectors of the BIIS, primarily using offshore geophysical surveys. The surviving record in the southern, terrestrial sector is fragmentary, permitting only large-scale (tens of kilometres) and longer timescale (c. 1 ka) reconstructions of ice-margin movement, with limited information on deglacial processes. Here we present a high- resolution study of the retreat behaviour for a section of the southern ice-margin from Windermere in the Lake District, using high-resolution two-dimensional multi-channel seismic data, processed using prestack depth migration. By combining the seismic stratigraphy with landform morphologies, extant cores and seismic velocity measurements, we are able to distinguish between: over-consolidated till; recessional moraines; De Geer moraines; flowed till/ice-front fan; supra-/en-glacial melt-out till; and subsequent glaciolacustrine/lacustrine sedimentation. The results reveal a complex and active valley glacier withdrawal from Windermere that changed character between basins and produced two small, localized areas of ice-stagnation and downwasting. This study indicates that similar active ice-margin retreats probably took place in other valleys of the Lake District during the Late Devensian deglaciation rather than the previously held view of rapid ice-stagnation and downwasting. When combined with the regional terrestrial record, this supports a model of early ice loss in terrestrial England compared with other parts of the UK.

The Indian Plate has been the focus of intensive research concerning the flood basalts of the Deccan Traps. Here we document a volcanostratigraphic analysis of the offshore segment of the western Indian volcanic large igneous province, between the shoreline and the first magnetic anomaly (An 28 ∼63 Ma). We have mapped the different crustal domains of the NW Indian Ocean from stretched continental crust through to oceanic crust, using seismic reflection and potential field data. Two volcanic structures, the Somnath Ridge and the Saurashtra High, are identified, extending ∼305 km NE–SW in length and 155 km NW–SE in width. These show the internal structures of buried shield volcanoes and hyaloclastic mounds, surrounded by mass-wasting deposits and volcanic sediments. The structures observed resemble seismic images from the North Atlantic and northwest Australia, as well as volcanic geometries described for Réunion and Hawaii. The geometry and internal seismic facies within the volcanic...

We present a method to estimate seismic velocity and density contrasts at a given interface in a 1-D layered model using PS-to-PP reflection amplitude ratios. The velocity structure above the reflector is constrained by traveltime modelling, and the amplitude ratios are determined using the same source-receiver pair for the measured PP and PS amplitudes (common-offset geometry). Thereby, source and receiver site effects are cancelled, and the remaining propagation effects are included in the ray theoretical forward modelling of theoretical PS-to-PP ratios. A minimization of the least-squares misfit between observed and modelled ratios provides the remaining elastic parameters below the reflector of interest (P velocity, P-to-S velocity ratio, density). 1-D examples and a 2-D synthetic case study with a dipping reflector and a laterally varying overburden demonstrate the possibilities and limitations of the method. An application of the method to a 0.6 km deep reflector below the Campi Flegrei caldera, Italy, reveals a strong contrast with a P-velocity increase from less than 2 to 3.5 kms-1 and a decrease of the P-to-S velocity ratio from 3.6 to 1.75. The proposed PS-to-PP amplitude ratio analysis is applicable for wide-angle seismic reflection data, especially when strong elastic parameter contrasts are expected and when source amplitudes or site effects are poorly known.

The Dead Sea Transform (DST) is a prominent shear zone in the Middle East. It separates the Arabian plate from the Sinai microplate and stretches from the Red Sea rift in the south via the Dead Sea to the Taurus-Zagros collision zone in the north. Formed in the Miocene 17 Ma ago and related to the breakup of the Afro-Arabian continent, the DST accommodates the left-lateral movement between the two plates. The study area is located in the Arava Valley between the Dead Sea and the Red Sea, centered across the Arava Fault (AF), which constitutes the major branch of the transform in this region.
A set of seismic experiments comprising controlled sources, linear profiles across the fault, and specifically designed receiver arrays reveals the subsurface structure in the vicinity of the AF and of the fault zone itself down to about 3-4 km depth. A tomographically determined seismic P velocity model shows a pronounced velocity contrast near the fault with lower velocities on the western side than east of it. Additionally, S waves from local earthquakes provide an average P-to-S velocity ratio in the study area, and there are indications for a variations across the fault. High-resolution tomographic velocity sections and seismic reflection profiles confirm the surface trace of the AF, and observed features correlate well with fault-related geological observations.
Coincident electrical resistivity sections from magnetotelluric measurements across the AF show a conductive layer west of the fault, resistive regions east of it, and a marked contrast near the trace of the AF, which seems to act as an impermeable barrier for fluid flow. The correlation of seismic velocities and electrical resistivities lead to a characterisation of subsurface lithologies from their physical properties. Whereas the western side of the fault is characterised by a layered structure, the eastern side is rather uniform. The vertical boundary between the western and the eastern units seems to be offset to the east of the AF surface trace.
A modelling of fault-zone reflected waves indicates that the boundary between low and high velocities is possibly rather sharp but exhibits a rough surface on the length scale a few hundreds of metres. This gives rise to scattering of seismic waves at this boundary. The imaging (migration) method used is based on array beamforming and coherency analysis of P-to-P scattered seismic phases. Careful assessment of the resolution ensures reliable imaging results.
The western low velocities correspond to the young sedimentary fill in the Arava Valley, and the high velocities in the east reflect mainly Precambrian igneous rocks. A 7 km long subvertical scattering zone (reflector) is offset about 1 km east of the AF surface trace and can be imaged from 1 km to about 4 km depth. The reflector marks the boundary between two lithological blocks juxtaposed most probably by displacement along the DST. This interpretation as a lithological boundary is supported by the combined seismic and magnetotelluric analysis. The boundary may be a strand of the AF, which is offset from the current, recently active surface trace. The total slip of the DST may be distributed spatially and in time over these two strands and possibly other faults in the area.

We present a method to estimate seismic velocity and density contrasts at a given interface in a 1-D layered model using PS-to-PP reflection amplitude ratios. The velocity structure above the reflector is constrained by travel time modelling, and the amplitude ratios are determined using using the same source-receiver pair for the measured PP and PS amplitudes (common-offset geometry). Thereby, source and receiver site effects are cancelled, and the remaining propagation effects are included in the ray-theoretical forward modelling of theoretical PS-to-PP ratios. A minimization of the least-squares misfit between observed and modelled ratios provides the remaining elastic parameters below the reflector of interest (P-velocity, P-to-S velocity ratio, density). 1-D examples and a 2-D synthetic case study with a dipping reflector and a laterally varying overburden demonstrate the possibilities and limitations of the method. An application of the method to a 0.6 km deep reflector below the Campi Flegrei caldera, Italy, reveals a strong contrast with a P-velocity increase from less than 2 to 3.5 km/s and a decrease of the P-to-S velocity ratio from 3.6 to 1.75. The proposed PS-to-PP amplitude ratio analysis is applicable for wide-angle seismic reflection data, especially when strong elastic parameter contrasts are expected and when source amplitudes or site effects are poorly known.