Marine Seismic Reflection Data Research Articles

Along-strike segmentation of the northeastern Brazilian margins controlled by geodynamics and lithospheric inheritance

Passive continental margins depict a wide range of crustal architectures. Such observation mainly come from wide-angle seismic that has proven to be a valuable tool to constrain margins’ onshore and offshore crustal structures. Yet, it is still unclear what are the main drivers of such diversity and whether some more general processes do also control their formation. Here, by analyzing coincident marine seismic reflection and refraction data acquired parallel to the coast of northeast Brazil we shed a unique view on the along-strike segmentation of its passive margins. Our two P-wave velocity models add up to 10 other wide-angle velocity models build in the scope of the SALSA (Sergipe ALagoas Seismic Acquisition) project to provide a coherent picture of the crustal and upper mantle architecture of the offshore domains of the Jequitinhonha, Almada, Camamu, Jacuípe, Sergipe and Alagoas margins. In the Camamu-Jacuípe segment, both crustal necking and the continent to ocean transition were shown to be sharp. Along strike, our models show extremely thin crust above a thick anomalous velocity layer and a relatively low density mantle. This architecture is interpreted as made of collapsed continental blocks above either highly intruded lower continental crust and/or serpentinized mantle that is locally exhumed. It greatly differs from the crustal structure imaged to the south, within the Jequitinhonha and Almada segments, where our models support the existence of a vast and homogeneous domain of exhumed lower continental crust. It also considerably contrasts with the offshore architecture of the Sergipe-Alagoas segment to the north. There, the necking and transition to normal oceanic crust was shown to be narrow but gradual and dominated by middle to lower continental crust. On our along-strike models, it is characterized by a thicker but also highly intruded continental crust. Crustal architecture evolves brutally at the frontiers of these segments which coincides precisely with the Camamu Triple Junction and the offshore prolongation of the Vaza Barris fault zone. The former is one of the major geodynamic features in the development of the Central Segment of the South-Atlantic Ocean while the later marks the suture between the São-Francisco Craton and the northeastern Brazilian Orogenic Belt. Therefore, we clearly evidence the first-order control of both geodynamics and lithospheric inheritance on the along-strike segmentation of the northeastern Brazilian passive margins. Our results emphasize the crucial role of plate kinematics and lithospheric strength in shaping their architectures and the exhumation of middle/lower continental crust as a more general process of their formation.

Southern Ocean biogenic blooms freezing-in Oligocene colder climates

Crossing a key atmospheric CO2 threshold triggered a fundamental global climate reorganisation ~34 million years ago (Ma) establishing permanent Antarctic ice sheets. Curiously, a more dramatic CO2 decline (~800–400 ppm by the Early Oligocene(~27 Ma)), postdates initial ice sheet expansion but the mechanisms driving this later, rapid drop in atmospheric carbon during the early Oligocene remains elusive and controversial. Here we use marine seismic reflection and borehole data to reveal an unprecedented accumulation of early Oligocene strata (up to 2.2 km thick over 1500 × 500 km) with a major biogenic component in the Australian Southern Ocean. High-resolution ocean simulations demonstrate that a tectonically-driven, one-off reorganisation of ocean currents, caused a unique period where current instability coincided with high nutrient input from the Antarctic continent. This unrepeated and short-lived environment favoured extreme bioproductivity and enhanced sediment burial. The size and rapid accumulation of this sediment package potentially holds ~1.067 × 1015 kg of the ‘missing carbon’ sequestered during the decline from an Eocene high CO2-world to a mid-Oligocene medium CO2-world, highlighting the exceptional role of the Southern Ocean in modulating long-term climate.

Quantitative Seismic Geomorphology of Four Different Types of the Continental Slope Channel Complexes in the Canterbury Basin, New Zealand

Modern marine seismic reflection data have allowed for the study of the morphology of submarine channels in a 3D perspective. This study presents a quantitative analysis of continental slope channel complexes morphology within the Canterbury Basin (New Zealand). It aims to characterize the morphology and seismic facies of submarine channels to better understand their formative sedimentary processes, paleoenvironment, and petroleum prospectivity. Submarine channel morphometric parameters are measured at 150 m intervals perpendicular to the axis of channels complexes. Based on the morphology analysis of erosional surfaces and seismic facies of channel complexes filling deposits, four types of continental slope channel complexes are found in the study area. These are vertical migrating channels, lateral migrating channels, V-shaped channels, and U-shaped channels. Furthermore, our work shows that channel morphometry varies over distances of 0.1 to 14 km in this continental slope system. These changes indicate a combination of submarine gravity flow process and channel wall collapse responsible for the development of continental slope channel complexes. Regionally, the evolution of the channels indicated less significant regional plate movement. This quantitative seismic geomorphology approach of characterizing submarine channels system has broader applications for better interpretation of paleoenvironment and petroleum prospectivity within frontier basins.

Downward continuation of marine seismic reflection data: an undervalued tool to improve velocity models

SUMMARY The purpose of marine seismic experiments is to provide information of the structure and physical properties of the subsurface. The P-wave velocity distribution is the most commonly modelled property, usually by inversion of arrival times or waveform attributes. In wide-angle seismic reflection/refraction (WAS) experiments, arrival times of seismic phases identified in data recorded by ocean bottom seismometers (OBS) are used to image relatively deep structures. Most WAS experiments have relatively low redundancy and produce robust velocity models of limited resolution. The shallow subsurface is also commonly studied with multichannel seismic (MCS) data recorded by towed streamers, a technique that is highly complementary to WAS. In this case, the recording of refractions as first arrivals is limited primarily by the streamer length and by features like water depth and the velocity structure and, in general, most refractions are masked by reflections and noise. However, MCS data of variable quality are available in many regions where no other data exist and previous work has shown that these data can also be used to retrieve velocity models through traveltime and full waveform inversion provided that first arrival information is properly extracted from the record sections. The most widely used tool to extract refractions as first arrivals from MCS recordings is the so-called downward continuation technique, which is designed for redatuming streamer field data to the seafloor. In this new virtual configuration, the early refractions transform to seismic phases that are becoming visible as first arrivals from nearly zero offset, facilitating their identification and use in traveltime tomography. However, there is limited literature, let alone available codes, to be used with the available MCS data sets. This work presents a user-friendly open source HPC software for redatuming 2-D streamer field data to the sea bottom for any seafloor relief. The main ingredient is the acoustic wave equation used backwards in time, allowing first the redatuming of the receivers and then the redatuming of the sources. Assessment tools are provided to evaluate the information available after redatuming for specific data acquisition configurations. Also, we present a step-by-step analysis that defines the most important features that influence the quality of the virtual, redatumed recordings.

An Integrated Multiscale Method for the Characterisation of Active Faults in Offshore Areas. The Case of Sant’Eufemia Gulf (Offshore Calabria, Italy)

Diagnostic morphological features (e.g., rectilinear seafloor scarps) and lateral offsets of the Upper Quaternary deposits are used to infer active faults in offshore areas. Although they deform a significant seafloor region, the active faults are not necessarily capable of producing large earthquakes as they correspond to shallow structures formed in response to local stresses. We present a multiscale approach to reconstruct the structural pattern in offshore areas and distinguish between shallow, non-seismogenic, active faults, and deep blind faults, potentially associated with large seismic moment release. The approach is based on the interpretation of marine seismic reflection data and quantitative morphometric analysis of multibeam bathymetry, and tested on the Sant’Eufemia Gulf (southeastern Tyrrhenian Sea). Data highlights the occurrence of three major tectonic events since the Late Miocene. The first extensional or transtensional phase occurred during the Late Miocene. Since the Early Pliocene, a right-lateral transpressional tectonic event caused the positive inversion of deep (>3 km) tectonic features, and the formation of NE-SW faults in the central sector of the gulf. Also, NNE-SSW to NE-SW trending anticlines (e.g., Maida Ridge) developed in the eastern part of the area. Since the Early Pleistocene (Calabrian), shallow (<1.5 km) NNE-SSW oriented structures formed in a left-lateral transtensional regime. The new results integrated with previous literature indicates that the Late Miocene to Recent transpressional/transtensional structures developed in an ∼E-W oriented main displacement zone that extends from the Sant’Eufemia Gulf to the Squillace Basin (Ionian offshore), and likely represents the upper plate response to a tear fault of the lower plate. The quantitative morphometric analysis of the study area and the bathymetric analysis of the Angitola Canyon indicate that NNE-SSW to NE-SW trending anticlines were negatively reactivated during the last tectonic phase. We also suggest that the deep structure below the Maida Ridge may correspond to the seismogenic source of the large magnitude earthquake that struck the western Calabrian region in 1905. The multiscale approach contributes to understanding the tectonic imprint of active faults from different hierarchical orders and the geometry of seismogenic faults developed in a lithospheric strike-slip zone orthogonal to the Calabrian Arc.

Structural and seismographic characteristics of a sector of the Magdalena Shelf, western margin of Baja California, Mexico, from 2D seismic reflection profiles

The Magdalena Shelf, located on the Pacific margin of Baja California, Mexico, is crosscut by the Tosco-Abreojos transform fault system, while the Santa Margarita and San Lazaro faults are sub-parallel and located east of the shear zone that cuts the continental slope longitudinally. Processing and interpretation 2D multichannel marine reflection seismic data show evidence for the structural deformation and seismostratigraphic characteristics of a sector of the shelf. This study allowed mapping unknown faults, of which the two main ones are named Iray and Magdalena Faults. San Lazaro fault has a strike of N 15° W and length of 70 km, with an apparent dip of ~ 42° to the NE. The San Lazaro fault controls the western flank of the basin of the same name forming a semi-graben of 20 km wide and its sedimentary fill of ~ 4.5 km in thickness. The Iray-Magdalena basin has a depth of ~ 4.0 km. We identified six seismic stratigraphic sequences, and two main discordances, which are considered to be regional discordances. One of which is the Cretaceous-Paleogene discordance, and the other corresponds to the Miocene discordance, which has been considered an important geological marker of the tectonic evolution of region. Acoustic basement defines a structural high parallel to the continental slope and aligned with outcrops of the ophiolite complex. The zone of active deformation of the Tosco-Abreojos fault system is between 15 and 18 km wide. This region may contain previously unknown structures and can represent a geological/seismic risk, which can have not only scientific implications, but also social impact in the communities of the region.

Comparison of the results of the suppression of surface related multiple reflections by predictive deconvolution in pre- and post-stack in 2D marine seismic reflection data: a case study from the Sea of Marmara

Comparison of the results of the suppression of surface related multiple reflections by predictive deconvolution in pre- and post-stack in 2D marine seismic reflection data: a case study from the Sea of Marmara

The Margin‐Oblique Kumawa Strike‐Slip Fault in the Banda Forearc, East Indonesia: Structural Deformation, Tectonic Origin and Geohazard Implication

AbstractThe northeast Banda margin is affected by the arc‐continent collision between the Australian continental margin and the Banda arc. Unlike margin‐parallel strike‐slip faults in other subduction zones, the margin‐oblique Seram‐Kumawa Shear Zone (SKSZ) has greatly deformed the Banda forearc with 80 km displacement along plate boundary. It trends ∼ WNW‐ESE comprising the Kawa shear zone (KSZ, >200 km) on the Seram island and the Kumawa fault (KF, >200 km) offshore SE Seram. Using multibeam bathymetry, marine seismic reflection and earthquake data, we studied the active tectonics and driving mechanism of the SKSZ with focus on its offshore part (KF). Our results show: (1) the KF is a young (∼2 Ma), linear crustal structure with three segments from NW to SE: pull‐apart basin (dextral), diffuse shear zone (sinistral) and distinct fault trace (sinistral); (2) It propagates southeastward and cross‐cuts the entire upper plate with prominent seafloor expression, but without seismicity along it, similar to the Banda detachment caused by forearc extension; (3) By incorporating the oblique collision and regional tectonics with our results, we proposed that the KF is formed primarily by the upper plate extension induced by subduction rollback during the last 2 Ma, with minor contribution from oblique arc‐continent collision; (4) The slip rate of the KF has reduced from 40 mm/yr over 2 Ma to <14 mm/yr at present. These observations combined with the clear seafloor faulting, 80‐km displacement and occurrence of a Mw 5.2 earthquake on its horsetail fault suggest it is currently locked awaiting a large earthquake.

Deformation Pattern of the Northern Sector of the Malta Escarpment (Offshore SE Sicily, Italy): Fault Dimension, Slip Prediction, and Seismotectonic Implications

Marine seismic reflection data coupled with on-land structural measurements improve our knowledge about the active deformation pattern of the northern sector of the Malta Escarpment, a bathymetric and structural discontinuity in the near-offshore of Eastern Sicily. As favourably oriented to be reactivated within the Neogene Africa–Europe convergence, it is believed that the Malta Escarpment has a significant role in the recent seismotectonic framework of the Western Ionian Basin and the Hyblean foreland domain of SE Sicily, where some of the largest and most destructive Mediterranean earthquakes are located according to available historical catalogs. Offshore seismic data along with bathymetric grids illuminate the shallow subseafloor setting and allow more accurate mapping of the seafloor expression of previously identified faults in the area. The seismic interpretation and the near-fault sediment pattern analysis provide constraints on fault 3D geometries as well as on their through-time tectonic activity, suggesting also that part of the observed deformation may have been caused by nontectonic processes. Identified faults form currently an E-dipping, roughly N–S trending, and 60 km-long extensional belt deforming the seafloor with a significant displacement amount in the Ionian offshore between Catania and Siracusa. 3-dimensional parameters of faults were then used to derive expected magnitudes and their reactivation propensity. Empirical scaling relationships and forward methods point to a high seismic potential for the detected fault as well as predict the fault slip behavior according to the field-derived differential stress. This combined analysis along with faults displacement measurements pointed out how the longest and most continuous fault could be capable of generating M > 7 seismic events, putting forward strong seismotectonic implications for the adjacent and densely populated Hyblean Plateau. The expected magnitude and the estimated recurrence time interval are compatible with those inferred for large historical earthquakes in the area even if other offshore seismic sources cannot be ruled out.

Extracting Q Anomalies From Marine Reflection Seismic Data Using Deep Learning

Anelasticity of the earth subsurface medium, which is quantified by the quality factor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> , causes the dissipation of seismic energy. Strong attenuation effect resulting from geology such as gas clouds (gas-filled sandstone) is a challenging problem for high-resolution imaging. To compensate the attenuation effect, first we need to accurately estimate the attenuation parameter. However, it is difficult to directly derive a heterogeneous attenuation Q model. This research letter proposes a method to derive a Q model corresponding to strong attenuative media from marine reflection seismic data using convolutional neural network (CNN), a popular deep learning framework. We treat Q anomaly detection problem as a semantic segmentation task and train a network to perform a pixel-by-pixel prediction to invert a pixel group that belongs to the strong level of attenuation probability. The proposed method uses a volume of marine 3-D reflection seismic data for network training and validation, which needs only a small part of real data as the training set due to the feature of U-Net. In the final stage, to evaluate the attenuation model, we validate the predicted heterogeneous Q model using deabsorption prestack depth migration (Q-PSDM), a high-resolution imaging result in depth domain with appropriate compensation is obtained.

PENGOLAHAN DATA SEISMIK 2D MARINE MENGGUNAKAN ProMAX DI AREA TENGGARA PULAU SIMEULUE

PENGOLAHAN DATA SEISMIK 2D MARINE MENGGUNAKAN ProMAX DI AREA TENGGARA PULAU SIMEULUE

Utilization of airgun source signatures to improve seismic imaging of ocean water columns

Seismic oceanography is a new interdiscipline which uses “legacy” marine seismic reflection data collected by the oil industry to image ocean water columns like eddies and internal waves. Unlike seafloor reflections, ocean water-column reflections are typically weak due to the low acoustic impedance contrast, which challenges seismic oceanography. Utilization of the acoustic characteristics of low-frequency, broadband airgun source can enhance water-column signals for better ocean seismic imaging. Acoustics characteristics of received direct-path, water-column, and seafloor signals are analyzed and applied to design adaptive filtering process to enhance water-column signals. The processing increases the signal-to-noise ratio of water-column signal and attenuates the direct waves without changing the spectral content of the airgun source. The results can be useful to improve seismic oceanography by providing better imaging of ocean water columns.

The Romanche fracture zone influences the segmentation of the equatorial margin of Brazil

This study addresses the crustal segmentation of the Brazilian equatorial margin (BEM) controlled by the Romanche fracture zone (RFZ). It uses a combination of satellite and marine gravity anomalies and seismic reflection data to constrain the continent-ocean crust transition. We propose a simplified evolution model of the BEM that involves dextral strike-slip plate motion along the RFZ in an oblique margin transform setting, which caused differential extension of a 31 km thick continental crust and the subsequent generation of a 5.0–10 km thick oceanic crust in the early Cretaceous. As a result, the trend of the coastline is controlled by two main directions: NW-SE and E-W. Two NW-SE-oriented segments, which are related to the oblique spreading centers, formed during the initial generation of oceanic crust adjacent to the margin. The western segment exhibits an ~160-km-wide hyperextended crust orthogonal to the shoreline, with a constant thickness of 10 km. In contrast, the eastern segment exhibits a 90-km-wide continental crust and a 13 km-thick oceanic crust. The central E-W-oriented segment is related to the RFZ and presents a sharp and steep boundary between the continental and oceanic crusts. The breakup geometry and shoreline orientations and the subsequent oceanic spreading centers are oblique (~40°) to the transform. In the Santonian (~83.5 ± 8 Ma), when the transform margin reached its passive stage, isochron 34 became orthogonal to the transform. At present, the RFZ forms well-defined ridges in the equatorial Atlantic that rise to ~2.5 km above the surrounding basins. These ridges and a series of NW-SE-oriented seamounts acted as a topographic barrier and prevented the deposition of a significant volume of sediment sourced from the continent. In addition, neotectonic reactivation of the fracture zone with a major normal component along the continental-oceanic lithospheric boundary (COB) indicates that the RFZ is a long-lived fault.

Attenuation of receiver ghosts in variable-depth streamer high-resolution seismic reflection data

Receiver ghosts attenuate marine seismic reflection data at harmonic frequencies that depend on the propagation angle and the streamer depth below the sea surface. The resulting loss of bandwidth is one of the major factors hampering seismic resolution. In near-surface and legacy multi-channel data, receivers depth is often unknown and may vary significantly both along the streamer length and during the survey, making frequency–slowness deghosting techniques unsuitable. In this work, we present a method for the attenuation of receiver ghost reflections in data with an arbitrary streamer depth profile varying during the survey. For each trace, a different deghosting operator is estimated and applied at different two-way-time windows, in order to account for depth-dependent changes in reflection angles. The ghost null frequencies are picked on the time-varying power spectrum via an automatic algorithm, guided by a user-dependent a priori function, and optimised to respect the harmonics’ periodicity. The power of the inverse filter is adjusted by adaptively damping abnormal amplitudes in the deghosted spectra. The algorithm is applied to high-resolution (GI-gun, 20–400 Hz) and ultra-high-resolution (Sparker, 0.2–3.0 kHz) multi-channel datasets, yielding an excellent bandwidth recovery and gain in resolution in the final stacks. Limited computing time and straightforward application make the method widely applicable and cost-effective.

Time-lapse imaging using 3D ultra-high-frequency marine seismic reflection data

Time-lapse (4D) seismic imaging is now widely used as a tool to map and interpret changes in deep reservoirs as well as investigate dynamic, shallow hydrological processes in the near surface. However, there are very few examples of time-lapse analysis using ultra-high-frequency (UHF; kHz range) marine seismic reflection data. Exacting requirements for navigation can be prohibitive for acquiring coherent, true-3D volumes. Variable environmental noise can also lead to poor amplitude repeatability and make it difficult to identify differences that are related to real physical changes. Overcoming these challenges opens up a range of potential applications for monitoring the subsurface at decimetric resolution, including geohazards, geologic structures, as well as the bed-level and subsurface response to anthropogenic activities. Navigation postprocessing was incorporated to improve the acquisition and processing workflow for the 3D Chirp subbottom profiler and provide stable, centimeter-level absolute positioning, resulting in well-matched 3D data and mitigating 4D noise for data stacked into [Formula: see text] common-midpoint bins. Within an example 4D data set acquired on the south coast of the UK, interpretable differences are recorded within a shallow gas blanket. Reflections from the top and bottom of a gas pocket are imaged at low tide, whereas at high tide only the upper reflection is imaged. This case study demonstrates the viability of time-lapse UHF 3D seismic reflection for quantitative mapping of decimeter-scale changes within the shallow marine subsurface.

Wrapround Multiple Reflectors over the Seafloor in the Gas Hydrate Area: A Possible Indicator of Methane Plume

Seismic wraparound multiples which are produced by the previous shot and trapped between the sea surface and flat seabed in the two-dimensional marine seismic reflection data have the potential to provide information about oceanographic phenomena because their repetition in the water column could enhance the detectability of the disturbance of seismic wave propagation. By focusing on the disturbance of wraparound multiples, we inferred the possible presence of methane gas bubbles emitting from the seafloor over several kilometers within a submarine canyon with a flat seabed. We observed the significant disturbance of wraparound multiples in both pre-stacked gathers and common mid-point stacked sections, which may be caused by methane bubbles. The existence of a bottom simulating reflection, a fault, and Calyptogena colonies provides indirect evidence of methane gas bubble seeps, while the existence of a cold seep is direct evidence of them in the study area. Finally, we conducted preliminary numerical simulations of acoustic wave propagation to investigate the cause of disturbances of wraparound multiples. Our preliminary numerical simulations imply that the disturbance of wraparound multiples might be associated with the internal scattering caused by an aggregation of gas bubble seeps in the water column.

Using Data‐Regrouping Methods to Attenuate Shot‐to‐Shot Coherent Interference Noise in Marine Seismic Data

AbstractsUnwanted interference noise often causes problems during seismic reflection data acquisition and processing. In this paper we present two novel data‐regrouping methods that enable improved shot‐to‐shot coherent interference noise attenuation in marine seismic data. The two approaches are based on line mixing and/or shot skipping: (1) for a 3‐D marine seismic reflection data acquisition which is typically carried out by shooting closely spaced parallel lines, we propose to mix the shot gathers from two adjacent sail lines to break up shot‐to‐shot coherent noise. This approach takes advantage of the fact that adjacent sail lines are usually acquired with a significant time interval, resulting in different types of noise; (2) for a 2‐D/3‐D acquisition, we propose an algorithm to skip and reorder the original shots to decrease the continuity of the shot‐to‐shot coherent noise. This approach could also supply more aperture data for the first or last shot from both sides, which is beneficial for the coherent noise removal at the edge area. After regrouping the original data, the coherent noise can then be further randomized or isolated by sorting and transforming to other domains, typically the common p‐shot domain where a random noise attenuation tool could be applied to identify and attenuate such noise. Field data examples confirm the effectiveness of the methods.

Decimetric-resolution stochastic inversion of shallow marine seismic reflection data: dedicated strategy and application to a geohazard case study

Characterisation of the top 10-50 m of the subseabed is key for landslide hazard assessment, offshore structure engineering design and underground gas-storage monitoring. In this paper, we present a methodology for the stochastic inversion of ultra-high-frequency (UHF, 0.2-4.0 kHz) pre-stack seismic reflection waveforms, designed to obtain a decimetric-resolution re- mote elastic characterisation of the shallow sediments with minimal pre-processing and lit- tle a-priori information. We use a genetic algorithm in which the space of possible solutions is sampled by explicitly decoupling the short and long wavelengths of the P-wave velocity model. This approach, combined with an objective function robust to cycle skipping, outperforms a conventional model parametrisation when the ground-truth is offset from the centre of the search domain. The robust P-wave velocity model is used to precondition the width of the search range of the multi-parameter elastic inversion, thereby improving the efficiency in high dimensional parametrizations. Multiple independent runs provide a set of independent results from which the reproducibility of the solution can be estimated. In a real dataset acquired in Finneidfjord, Norway, we also demonstrate the sensitivity of UHF seismic inversion to shallow subseabed anomalies that play a role in submarine slope stability. Thus, the methodology has the potential to become an important practical tool for marine ground model building in spatially heterogeneous areas, reducing the reliance on expensive and time-consuming coring campaigns for geohazard mitigation in marine areas.

Strain partitioning in Southeastern Alaska: Is the Chatham Strait Fault active?

A 1200 km-long transform plate boundary passes through southeastern Alaska and northwestern British Columbia and represents one of the most seismically active, but poorly understood continental margins of North America. Although most of the plate motion is accommodated by the right-lateral Queen Charlotte–Fairweather Fault (QCFF) System, which has produced at least six M>7 earthquakes since 1920, seismic hazard assessments also include the Chatham Strait Fault (CSF) as a potentially active, 400 km-long strike slip fault that cuts northward through southeastern Alaska, connecting with the Eastern Denali Fault. Nearly the entire length of the CSF is submerged beneath Chatham Strait and Lynn Canal and has never been systematically imaged using high-resolution marine geophysical approaches. In this study we present an integrated analysis of new marine seismic reflection data acquired across Lynn Canal and tectonic block modeling constrained by data from continuous and campaign GPS sites. Seismic profiles cross the CSF at twelve locations spanning ∼50 km of fault length; they reveal thick (up to 300 m) packages of glaciomarine sedimentary facies emplaced on an unconformity surface that formed during the Last Glacial Maximum (LGM). Localized warping of post-LGM stratigraphy (∼13.9 kyr B.P. to present) appears to correlate with sediment drape on basement topography and current-controlled deposition. There is no evidence for an active fault along the axis of Lynn Canal in the seismic reflection data. Crustal block models constrained by GPS data allow, but do not require, a maximum slip rate of 2–3 mm/yr along the CSF; higher slip rates on the CSF result in significant misfit to GPS data in the surrounding region. Based on the combined marine geophysical and GPS observations, it is plausible that the CSF has not generated resolvable coseismic deformation in the last ∼13 ka and that the modern slip-rate is <1 mm/yr. We propose that models for strain transfer between the QCFF and the Denali Fault, and seismic hazard maps in general, may need to be reevaluated.

Seismic Attenuation for Characterization of Gas Hydrate Reservoir in Krishna-Godavari Basin, Eastern Indian Margin

ABSTRACT Gas hydrates have received global attention as a possible alternative non-conventional energy resource. Hence, the detection, characterization and quantification of gas hydrates are very important for evaluating the resource potential. Presence of gas hydrates in sediments above the bottom simulating reflector or BSR is associated with low attenuation or high quality factor (Q), whereas, free gas bearing sediments below the BSR exhibit high attenuation or low seismic Q. Here the logarithm spectral ratio (LSR) method is applied to marine seismic reflection data along two cross lines (18 and 46) in the Krishna-Godavari (KG) basin in eastern Indian margin, where gas hydrates have already been established by drilling/coring. The interval Qs is calculated for three sedimentary layers (A, B, and C) bounded by the seafloor, BSR, one reflector above and another reflector below the BSR at some common depth points (CDPs) to study the attenuation characteristics of sediments across the BSR. The estimated average interval Q (160) for the hydrate bearing sediments (layer B) is much higher than the average interval Q (80) for both the loose clayey sediments (Layer A) and underlying free gas saturated sediments (layer C). This demonstrates that estimation of seismic quality factor Q can be used for characterization of gas hydrate reservoir.