Multichannel Seismic Imaging of the Northern Andean subduction margin in Ecuador: preliminary seismic processing results from HIPER campaign.

2021 ◽  
Author(s):  
Laure Schenini ◽  
Alexandra Skrubej ◽  
Mireille Laigle ◽  
Alessandra Ribodetti ◽  
Laure Combe ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Seismic Refraction ◽  
Rupture Zone ◽  
The West ◽  
Seismic Processing ◽  
Depth Migration ◽  
Time Migration ◽  
Sea Bottom ◽  
Lateral Extent ◽  
Basement High

<p>Offshore 2D-Multichannel seismic (MCS)-reflection profiles were acquired in northern Ecuador during the HIPER survey (March/April 2020, R/V L’Atalante) together with one 2D-OBS-seismic-refraction profile (presented in a joint abstract by A. Skrubej). This project (presented in a joint abstract by A. Galve) aims at deciphering the role of lower plate structural heterogeneities and fluids on subduction zone seismogenesis processes within the 2016 Pedernales rupture segment, which is characterized by contrasting slip behaviors. We put a particular emphasis on the segment located at the northern termination of the subducting Carnegie Ridge which was devoid of previous seismic investigations. Three lines of 315-km-long in total, one North of the 2016 Pedernales rupture zone sampling an area experiencing aseismic slip and two lines parallel to the trench, were recorded using an airgun source of 4990 in<sup>3</sup> and a 6-km-long streamer. In this study, we present in detail the seismic processing workflow used to produce an enhanced imaging of the Ecuadorian margin, a prerequisite for tackling the project’s objectives.</p><p>We performed routine MCS data processing onboard to produce post-stack time migrated sections using Geovation<sup>® </sup>CGG’s software. The dip-line collected across the northern Atacames seamounts area provides a detailed image through the whole Nazca oceanic crust down to the Moho, showing a normal crust thickness, at least on the oceanward portion, up to 15 km to the west of the trench. At the trench, we image a horst-like basement topographic high, which outcrops at sea-bottom, offsets the deformation front arcwards, with the outcropping frontal decollement reflector topping this oceanic basement high. Its nature, fluid content potential and lateral extent need to be determined, but its observation at the shallow portion of the interplate megathrust contribute to expand the inventory of subducting rough structures possibly impacting the megathrust frictional slip behavior.</p><p>Further advanced processing include noise attenuation, 2D-SRME multiple attenuation, Kirchhoff pre-stack time migration and preserved amplitude pre-stack depth migration (PSDM) performed in the angle domain. The megathrust fault located at the top of the subducting oceanic crust is imaged down to 7 km depth at a distance of 28 km from the trench which will contribute to complement the high-resolution version of the slab’s top topography close to the trench. A joint analyze of this MCS line and the coincident 2D-OBS-refraction Vp model, reveal that variations in moho acoustic features at 15 km distance to the west of the trench correlates with a 30 km wide and >10-km-thick low Vp anomaly. Nearby previous experiment SISTEUR seismic lines are being reprocessed using the same workflow, in order to further investigate the deep crustal seismic structures over the Pedernales 2016 rupture zone.</p>

TEMPERATURE VARIABILITY AT SENUNU BAY, WEST SUMBAWA

2013 ◽  
Vol 5 (2) ◽  
Author(s):  
Syamsul Hidayat ◽  
Mulia Purba ◽  
Jorina Waworuntu
Keyword(s):  
Layer Thickness ◽  
Mixed Layer ◽  
Cross Correlation ◽  
Sea Surface ◽  
Thermocline Depth ◽  
Kelvin Wave ◽  
The West ◽  
Sea Bottom ◽  
Annual Period ◽  
Regional Processes

The purposes of this study were to determine the variability of temperature and its relation to regional processes in the Senunu Bay. The result showed clear vertical stratifications i.e., mixed layer thickness about 39-119 m with isotherm of 27°C, thermocline layer thickness about 83-204 m with isotherm of 14–26°C, and  the deeper layer from the thermocline lower limit to the sea bottom with isotherm <13°C. Temperature and the thickness of each layers varied with season in which during the Northwest Monsoon the temperature was warmer and the mixed layer was thicker than those during Southeast Monsoon. During Southeast Monsoon, the thermocline layer rose  about 24 m. The 2001, 2006, and 2009 (weak La Nina years),  the Indonesia Throughflow (ITF) carried warmer water, deepening thermocline depth and reducing upwelling strength.  In 2003 and 2008 thickening of mixed layer occurred in transition season  was believed  associated with the  arrival of Kelvin Wave from the west. In 2002 and 2004 (weak El Nino period,) ITF carries colder water shallowing thermocline depth and enhancing upwelling strength. In 2007 was believed to be related with positive IODM where the sea surface temperature were decreasing due to intensification of southeast wind which induced strong upwelling. The temperature spectral density of mixed layer and thermocline was influenced by annual, semi-annual, intra-annual and inter-annual period fluctuations. The cross-correlation between wind and temperature showed significant value in the annual period.  Keywords: temperature, thermocline, variability, ENSO, IODM.

Separation and imaging of seismic diffractions using migrated dip-angle gathers

Geophysics ◽  
2012 ◽  
Vol 77 (6) ◽  
pp. S131-S143 ◽  
Author(s):  
Alexander Klokov ◽  
Sergey Fomel
Keyword(s):  
Radon Transform ◽  
Real Data ◽  
Separation Procedure ◽  
Shape Difference ◽  
Depth Migration ◽  
Time Migration ◽  
Reflection Angle ◽  
Radon Space ◽  
Dip Angle ◽  
Analytical Expressions

Common-reflection angle migration can produce migrated gathers either in the scattering-angle domain or in the dip-angle domain. The latter reveals a clear distinction between reflection and diffraction events. We derived analytical expressions for events in the dip-angle domain and found that the shape difference can be used for reflection/diffraction separation. We defined reflection and diffraction models in the Radon space. The Radon transform allowed us to isolate diffractions from reflections and noise. The separation procedure can be performed after either time migration or depth migration. Synthetic and real data examples confirmed the validity of this technique.

The Lower Jurassic phosphorites of southeastern British Columbia and terrane accretion to western North America

1989 ◽  
Vol 26 (8) ◽  
pp. 1612-1616 ◽  
Author(s):  
T. P. Poulton ◽  
J. D. Aitken
Keyword(s):  
British Columbia ◽  
North America ◽  
Oceanic Crust ◽  
West Coast ◽  
Cold Water ◽  
Lower Jurassic ◽  
Depositional Model ◽  
Western North America ◽  
The West ◽  
Terrane Accretion

Sinemurian phosphorites in southeastern British Columbia and southwestern Alberta conform with the "West Coast type" phosphorite depositional model. The model indicates that they were deposited on or near the Early Jurassic western cratonic margin, next to a sea or trough from which cold water upwelled. This suggests that the allochthonous terrane Quesnellia lay well offshore in Sinemurian time. The sea separating Quesnellia from North America was partly floored by oceanic crust ("Eastern Terrane") and partly by a thick sequence of rifted, continental terrace wedge rocks comprising the Purcell Supergroup and overlying Paleozoic sequence. This sequence must have been depressed sufficiently that access of upwelling deep currents to the phosphorite depositional area was not impeded.

Late Paleocene radiolarian fauna from Tibet and its geological implications

2012 ◽  
Vol 49 (11) ◽  
pp. 1364-1371 ◽  
Author(s):  
Yinping Liang ◽  
Kexin Zhang ◽  
Yadong Xu ◽  
Weihong He ◽  
Xianyin An ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Foreland Basin ◽  
The West ◽  
Southern Tibet ◽  
Terminal Stage ◽  
Oceanic Basin ◽  
Late Paleocene ◽  
Geological Implications ◽  
Yarlung Zangbo Suture Zone ◽  
Radiolarian Fauna

A diverse, abundant, and well-preserved radiolarian fauna in Jiazhu, Zhongba County of Tibet, in the western sector of Yarlung Zangbo Suture Zone, is assigned to a late Paleocene radiolarian zone, the Buryella pentadica interval zone, spanning 59–56.5 Ma. Regionally, a late Paleocene basalt block in the bathyal–abyssal siliceous mudstone and graywacke yielded an age of 59.1 Ma (zircon SHRIMP U–Pb). The late Paleocene radiolarian fauna, the tectonic attribution of the radiolarian cherts and the basalt block indicate that oceanic crust persisted in the Zhongba area until the late Paleocene and the initial collision between the Indian and Eurasian plates post-dates the late Paleocene. It is inferred that the Neo-Tethys transformed into a remnant oceanic basin in the late Paleocene, at the terminal stage of the oceanic crust subduction, and the closure of the remnant oceanic basin in the studied region took place after the late Paleocene. In contrast to the previous investigations, we suggest that there was a remnant oceanic basin to the west of the Saga area and a foreland basin to the east of Saga in southern Tibet during the late Paleocene. We argue that the closure of the Neo-Tethys progressed from east to west.

Crustal structure of the southeast flank of Kilauea Volcano, Hawaii, from seismic refraction measurements

1980 ◽  
Vol 70 (4) ◽  
pp. 1149-1159
Author(s):  
John J. Zucca ◽  
David P. Hill
Keyword(s):  
Oceanic Crust ◽  
Crustal Structure ◽  
Seismic Refraction ◽  
Kilauea Volcano ◽  
Geological Survey ◽  
Pn Velocity ◽  
Kīlauea Volcano ◽  
Volcanic Pile ◽  
Rift Zones ◽  
The U.S

abstract In November 1976, the U.S. Geological Survey, in conjunction with the Hawaii Institute of Geophysics, established a 100-km-long seismic refraction line normal to the southeast coast of Hawaii across the submarine flank of Kilauea Volcano. Interpretation of the data suggests that the oceanic crust dips about 2° toward the island underneath the volcanic pile. The unreversed Pn velocity is 7.9 km/ sec with crustal velocities varying strongly along the profile. Profiles across the rift zones of Kilauea suggest that the velocity in the rifts is higher than the velocity in the surrounding extrusive rocks and that the velocity in the southwest rift (∼6.5 km/sec) is lower than the velocity in the east rift (∼7.0 km/sec). The rift boundaries seem to dip away from the rift such that a large part of the volcanic pile is composed of the higher velocity core of riftzone rock.

Viscoacoustic reverse time migration of joint primaries and different-order multiples

Geophysics ◽  
2020 ◽  
Vol 85 (2) ◽  
pp. S71-S87
Author(s):  
Yingming Qu ◽  
Jinli Li ◽  
Zhe Guan ◽  
Zhenchun Li
Keyword(s):  
Vertical Direction ◽  
Seismic Attenuation ◽  
Frequency Noise ◽  
Reverse Time ◽  
Reverse Time Migration ◽  
Time Migration ◽  
Sea Bottom ◽  
Multiple Imaging ◽  
Propagation Paths ◽  
Different Order

Compared to primary arrivals, multiples have longer propagation paths and smaller reflection angles, leading to a wider illumination area in the horizontal direction and higher resolution in the vertical direction. Hence, it is better to make full use of the multiples rather than suppressing them. However, seismic attenuation exists widely in the subsurface medium, especially directly below the deep sea bottom. Therefore, to compensate for the attenuation effect during multiple imaging, we have developed a viscoacoustic reverse time migration (RTM) method of different-order multiples. Following the multiple propagation paths, we compensate for the attenuation during source wavefield forward propagation and receiver backward propagation, and we introduce a regularization operator to automatically eliminate the exponential high-frequency noise during the attenuation compensation process. Taking advantage of the full wavefield information, we jointly use the different-order multiples and primaries when implementing viscoacoustic RTM. In numerical examples, we validate the viscoacoustic RTM of different-order multiples in a three-layer attenuation model and an attenuating Sigsbee2B model. Our results suggest that our method can image the models using different-order multiples separately, which suppresses crosstalk artifacts, balances energy, raises resolution, and improves subsalt images dramatically.

Seismic investigation of the Coast Plutonic Complex – Insular Belt boundary beneath the Strait of Georgia

1984 ◽  
Vol 21 (9) ◽  
pp. 1033-1049 ◽  
Author(s):  
Donald J. White ◽  
Ron M. Clowes
Keyword(s):  
Crustal Structure ◽  
Seismic Refraction ◽  
Unconsolidated Sediments ◽  
The West ◽  
Strait Of Georgia ◽  
Seismic Properties ◽  
Air Gun ◽  
Plutonic Complex ◽  
Major Fault ◽  
Coast Plutonic Complex

The Strait of Georgia, a topographic depression between Vancouver Island and the mainland of British Columbia, is considered to be the boundary between two tectonic provinces: the Coast Plutonic Complex on the east and the Insular Belt to the west. The allochthonous nature of the Insular Belt has been established, mainly on the basis of paleomagnetic measurements. Various tectonic models to explain the geological differences between the two provinces have been proposed. One of these suggests that the boundary is an old transform fault zone and is represented currently by a thrust fault along the eastern side of the Strait of Georgia. Other models propose that the Coast Plutonic Complex is a feature superimposed by tectonic and metamorphic events after the accretion of the Insular Belt. Such models do not require a major crustal discontinuity along the Strait of Georgia.In May 1982, a seismic refraction survey using a 32 L air gun and a radio telemetering sonobuoy system was carried out in the Strait of Georgia with the objective of investigating the nature of this boundary and determining the upper crustal structure. Three reversed profiles across the strait were shot; these are supplemented by several high-resolution reflection profiles from previous experiments. Two-dimensional models of the crustal structure across the strait have been constructed using a forward modelling ray trace and synthetic seismogram algorithm to match the travel times and amplitude characteristics of the data.Three basic layers or strata form the models, for which the maximum depth of reliability is 3 km. The first layer consists of unconsolidated sediments and Pleistocene glacial deposits, and the second represents Late Cretaceous – early Tertiary basin fill sediments that form the Nanaimo Group, the Burrard–Kitsilano formations, and the Chuckanut Formation. The third layer is likely the extension of the Coast Plutonic Complex beneath the strait, but the westerly limit of this unit is undetermined because of seismic properties similar to those of the Insular Belt volcanics. A local fault is located ~15 km northeast of Galiano Island on the west side of the strait. However, our study shows no evidence for a major fault along the strait. Thus those aspects of tectonic models that require the existence of a major transform or transcurrent fault boundary along the Strait of Georgia. may have to be revised.

Depth migration by the Gaussian beam summation method

Geophysics ◽  
2010 ◽  
Vol 75 (2) ◽  
pp. S81-S93 ◽  
Author(s):  
Mikhail M. Popov ◽  
Nikolay M. Semtchenok ◽  
Peter M. Popov ◽  
Arie R. Verdel
Keyword(s):  
Wave Equation ◽  
Model Building ◽  
Velocity Structure ◽  
Velocity Model ◽  
Summation Method ◽  
Reverse Time ◽  
Reverse Time Migration ◽  
Depth Migration ◽  
Time Migration ◽  
Ray Methods

Seismic depth migration aims to produce an image of seismic reflection interfaces. Ray methods are suitable for subsurface target-oriented imaging and are less costly compared to two-way wave-equation-based migration, but break down in cases when a complex velocity structure gives rise to the appearance of caustics. Ray methods also have difficulties in correctly handling the different branches of the wavefront that result from wave propagation through a caustic. On the other hand, migration methods based on the two-way wave equation, referred to as reverse-time migration, are known to be capable of dealing with these problems. However, they are very expensive, especially in the 3D case. It can be prohibitive if many iterations are needed, such as for velocity-model building. Our method relies on the calculation of the Green functions for the classical wave equation by per-forming a summation of Gaussian beams for the direct and back-propagated wavefields. The subsurface image is obtained by cal-culating the coherence between the direct and backpropagated wavefields. To a large extent, our method combines the advantages of the high computational speed of ray-based migration with the high accuracy of reverse-time wave-equation migration because it can overcome problems with caustics, handle all arrivals, yield good images of steep flanks, and is readily extendible to target-oriented implementation. We have demonstrated the quality of our method with several state-of-the-art benchmark subsurface models, which have velocity variations up to a high degree of complexity. Our algorithm is especially suited for efficient imaging of selected subsurface subdomains, which is a large advantage particularly for 3D imaging and velocity-model refinement applications such as subsalt velocity-model improvement. Because our method is also capable of providing highly accurate migration results in structurally complex subsurface settings, we have also included the concept of true-amplitude imaging in our migration technique.

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