Crustal structure of the continental shelf off northwest Britain from two‐ship seismic experiments

Geophysics ◽  
1984 ◽  
Vol 49 (10) ◽  
pp. 1605-1621 ◽  
Author(s):  
E. J. W. Jones ◽  
R. S. White ◽  
V. J. Hughes ◽  
D. H. Matthews ◽  
B. R. Clayton
Keyword(s):  
Continental Shelf ◽  
Crustal Structure ◽  
Velocity Structure ◽  
Mode Conversion ◽  
S Wave ◽  
Seismic Profiles ◽  
Low Velocity ◽  
Lateral Velocity ◽  
Air Gun ◽  
Velocity Changes

Two‐ship multichannel seismic profiles, using expanding spread and constant offset source‐receiver configurations, were shot with explosive charges and a 16.4 ℓ air gun to investigate the structure of the continental shelf off northwest Britain. A long‐range (69 km) expanding spread profile reveals that the crystalline basement off northern Scotland is covered by sedimentary sections up to 2.5 km thick, and is divisible into two seismic units with velocities of 6.1 km/s and 6.6 km/s. Prominent supercritical Moho reflections indicate a crustal thickness of 26.7 km. Strong sedimentary and basement refractions, together with oblique reflections from the vicinity of the Moho, have been profiled at a constant air gun‐receiver offset of 10 km across a sedimentary basin west of the Orkney Islands. On the Outer Hebridean shelf to the west of mainland Scotland, the metamorphic basement lies within 250 m of the sea floor. P- to S-wave conversion occurs at the basement surface; [Formula: see text] gives a Poisson’s ratio of 0.27–0.31 at depths of 300–1 000 m. Marked changes in mode conversion efficiency are observed on constant offset profiles and are attributed to variations in the velocity structure of an uppermost low‐velocity (4.9 km/s) layer of weathered basement. The deeper crustal velocity appears uniform with depth, although there is some evidence of significant lateral velocity changes (6.28–6.61 km/s). In contrast to the shelf north of Scotland, reflections from within the basement and from near the base of the crust are recorded only sporadically on constant‐offset profiles. A strong event at 10.4 s two‐way reflection time appears to have arisen from a seismic discontinuity within the upper mantle. The differences in seismic character of the basement on the two‐ship profiles suggest significant variations in crustal structure within the Caledonian foreland of northern Britain.

Mapping crustal structure of the Nechako–Chilcotin plateau using teleseismic receiver function analysis,

2014 ◽  
Vol 51 (4) ◽  
pp. 407-417 ◽  
Author(s):  
H.S. Kim ◽  
J.F. Cassidy ◽  
S.E. Dosso ◽  
H. Kao
Keyword(s):  
Wave Velocity ◽  
Crustal Structure ◽  
Velocity Structure ◽  
Receiver Function ◽  
Function Analysis ◽  
Crustal Thickness ◽  
Receiver Functions ◽  
S Wave ◽  
Low Velocity ◽  
Velocity Models

This paper presents results of a passive-source seismic mapping study in the Nechako–Chilcotin plateau of central British Columbia, with the ultimate goal of contributing to assessments of hydrocarbon and mineral potential of the region. For the present study, an array of nine seismic stations was deployed in 2006–2007 to sample a wide area of the Nechako–Chilcotin plateau. The specific goal was to map the thickness of the sediments and volcanic cover, and the overall crustal thickness and structural geometry beneath the study area. This study utilizes recordings of about 40 distant earthquakes from 2006 to 2008 to calculate receiver functions, and constructs S-wave velocity models for each station using the Neighbourhood Algorithm inversion. The surface sediments are found to range in thickness from about 0.8 to 2.7 km, and the underlying volcanic layer from 1.8 to 4.7 km. Both sediments and volcanic cover are thickest in the central portion of the study area. The crustal thickness ranges from 22 to 36 km, with an average crustal thickness of about 30–34 km. A consistent feature observed in this study is a low-velocity zone at the base of the crust. This study complements other recent studies in this area, including active-source seismic studies and magnetotelluric measurements, by providing site-specific images of the crustal structure down to the Moho and detailed constraints on the S-wave velocity structure.

Crustal structure across the Nain – Makkovik boundary on the continental shelf off Labrador from seismic refraction data

1996 ◽  
Vol 33 (3) ◽  
pp. 460-471 ◽  
Author(s):  
Ian Reid
Keyword(s):  
Continental Shelf ◽  
Wave Velocity ◽  
Crustal Structure ◽  
Lower Crust ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
P Wave ◽  
Air Gun ◽  
P Wave Velocity

A detailed seismic refraction profile was shot along the continental shelf off Labrador, across the boundary between the Archean Nain Province to the north and the Proterozoic Makkovik orogenic zone to the south. A large air-gun source was used, with five ocean-bottom seismometers as receivers. The data were analysed by forward modelling of traveltimes and amplitudes and provided a well-determined seismic velocity structure of the crust along the profile. Within the Nain province, thin postrift sediments are underlain by crust with a P-wave velocity of 6.1 km/s, which increases with depth and reaches 6.6 km/s at about 8 km. Moho is at around 28 km, and there is no evidence for a high-velocity (>7 km/s) lower crust. The P- and S-wave velocity structure is consistent with a gneissic composition for the Archean upper crust, and with granulites becoming gradually more mafic with depth for the intermediate and lower crust. In the Makkovik zone, the sediments are thicker, and a basement layer of P-wave velocity 5.5–5.7 km/s is present, probably due to reworking of the crust and the presence of Early Proterozoic volcanics and metasediments. Upper crustal velocities are lower than in the Nain Province. The crustal thickness, at 23 km, is less, possibly due in part to greater crustal stretching during the Mesozoic rifting of the Labrador Sea. The crustal structure across the Nain–Makkovik boundary is similar to that across the corresponding Archean–Ketilidian boundary off southwest Greenland.

Structure beneath Queen Charlotte Sound from seismic-refraction and gravity interpretations

1992 ◽  
Vol 29 (7) ◽  
pp. 1509-1529 ◽  
Author(s):  
Tianson Yuan ◽  
G. D. Spence ◽  
R. D. Hyndman
Keyword(s):  
Crustal Structure ◽  
Velocity Structure ◽  
Sedimentary Basin ◽  
Single Channel ◽  
Velocity Variation ◽  
Moho Depth ◽  
High Porosity ◽  
Upper Crust ◽  
Low Velocity ◽  
Data Set

A combined multichannel seismic reflection and refraction survey was carried out in July 1988 to study the Tertiary sedimentary basin architecture and formation and to define the crustal structure and associated plate interactions in the Queen Charlotte Islands region. Simultaneously with the collection of the multichannel reflection data, refractions and wide-angle reflections from the airgun array shots were recorded on single-channel seismographs distributed on land around Hecate Strait and Queen Charlotte Sound. For this paper a subset of the resulting data set was chosen to study the crustal structure in Queen Charlotte Sound and the nearby subduction zone.Two-dimensional ray tracing and synthetic seismogram modelling produced a velocity structure model in Queen Charlotte Sound. On a margin-parallel line, Moho depth was modelled at 27 km off southern Moresby Island but only 23 km north of Vancouver Island. Excluding the approximately 5 km of the Tertiary sediments, the crust in the latter area is only about 18 km thick, suggesting substantial crustal thinning in Queen Charlotte Sound. Such thinning of the crust supports an extensional mechanism for the origin of the sedimentary basin. Deep crustal layers with velocities of more than 7 km/s were interpreted in the southern portion of Queen Charlotte Sound and beneath the continental margin. They could represent high-velocity material emplaced in the crust from earlier subduction episodes or mafic intrusion associated with the Tertiary volcanics.Seismic velocities of both sediment and upper crust layers are lower in the southern part of Queen Charlotte Sound than in the region near Moresby Island. Well velocity logs indicate a similar velocity variation. Gravity modelling along the survey line parallel to the margin provides additional constraints on the structure. The data require lower densities in the sediment and upper crust of southern Queen Charlotte Sound. The low-velocity, low-density sediments in the south correspond to high-porosity marine sediments found in wells in that region and contrast with lower porosity nonmarine sediments in wells farther north.

Crustal structure from two-ship multichannel seismic profiles on the continental shelf off Northwest Scotland

1982 ◽  
Vol 90 (1-2) ◽  
pp. 167-178 ◽  
Author(s):  
R.S. White ◽  
E.J.W. Jones ◽  
V.J. Hughes ◽  
D.H. Matthews
Keyword(s):  
Continental Shelf ◽  
Crustal Structure ◽  
Seismic Profiles

Interpretation of three-dimensional seismic refraction data from western Hecate Strait, British Columbia: structure of the crust

1993 ◽  
Vol 30 (7) ◽  
pp. 1440-1452 ◽  
Author(s):  
J. A. Hole ◽  
R. M. Clowes ◽  
R. M. Ellis
Keyword(s):  
Velocity Structure ◽  
Seismic Refraction ◽  
Three Dimensional ◽  
Surface Expression ◽  
Wide Angle ◽  
Lateral Velocity ◽  
Near Surface ◽  
Velocity Anomaly ◽  
Air Gun ◽  
Refraction Data

As part of a multidisciplinary investigation of the structure and tectonics of the Queen Charlotte Basin and underlying crust, deep multichannel seismic reflection and coincident crustal refraction data were collected in 1988. Energy from the reflection air-gun array source was recorded at land sites at offsets appropriate to record crustal refraction and wide-angle reflection data. Refraction data recorded in a broadside geometry provide good three-dimensional coverage of western Hecate Strait. These data are modelled using tomographic inversion techniques to determine the three-dimensional velocity structure of the crust in this region. The one-dimensional average velocity increases rapidly with depth to 6.5 km/s at 7 km depth. Velocities from 7 to at least 12 km depth remain approximately constant and are associated with rocks of the Wrangellia terrane. Significant lateral velocity variations, including large differences in near-surface velocities attributable to surface features, relatively low velocities representing interbedded Tertiary sediments and volcanics, and a deep high-velocity anomaly that may represent the root of an igneous intrusion, are mapped. Wide-angle reflections from the Moho are used to determine the thickness of the crust. The Moho is at 29 km depth beneath the east coast of the Queen Charlotte Islands. This is deeper than the Moho observed below Queen Charlotte Sound and as deep as, or deeper than, that below Hecate Strait. Crustal thinning during Tertiary extension was thus greatest beneath the surface expression of the Queen Charlotte Basin, leaving the crust under the islands considerably thicker than under the basin. In an alternate or additional explanation, compression at the continental margin during the last 4 Ma may have been taken up by thickening or underplating of the continental crust beneath the islands. If the Pacific plate is subducting beneath the islands, the Moho observations constrain the slab to dip greater than 20–26°.

Seismic tomography studies of cover thickness and near-surface bedrock velocities

Geophysics ◽  
2006 ◽  
Vol 71 (6) ◽  
pp. U77-U84 ◽  
Author(s):  
B. Bergman ◽  
A. Tryggvason ◽  
C. Juhlin
Keyword(s):  
Nuclear Waste ◽  
Velocity Structure ◽  
Synthetic Data ◽  
General Situation ◽  
Seismic Profiles ◽  
Low Velocity ◽  
Near Surface ◽  
Reflection Seismic ◽  
Crystalline Bedrock ◽  
Cover Thickness

Reflection seismic imaging of the uppermost kilometer of crystalline bedrock is an important component in site surveys for locating potential storage sites for nuclear waste in Sweden. To obtain high-quality images, refraction statics are calculated using first-break traveltimes. These first-break picks may also be used to produce tomographic velocity images of the uppermost bedrock. In an earlier study, we presented a method applicable to data sets where the vast majority of shots are located in the bedrock below the glacial deposits, or cover, typical for northern latitudes. A by-product of this method was an estimate of the cover thickness from the receiver static that was introduced to sharpen the image. We now present a modified version of this method that is applicable for sources located in or on the cover, the general situation for nuclear waste site surveys. This modified methodalso solves for 3D velocity structure and static correctionssimultaneously in the inversion process. The static corrections can then be used to estimate the cover thickness. First, we test our tomography method on synthetic data withthe shot points in the bedrock below the cover. Next, we developa strategy for the case when the sources are within the cover. Themethod is then applied to field data from five crooked-line,high-resolution reflection seismic profiles ranging in lengthfrom 2 to [Formula: see text]. The crooked-line profiles make the study 2.5dimensional regarding bedrock velocities. The cover thicknessalong the profiles varies from 0 to [Formula: see text]. Estimated thickness ofthe cover agrees well with data from boreholes drilled near theprofiles. Low-velocity zones in the uppermost bedrock generallycorrelate with locations where reflections from the stackedsections project to the surface. Thus, the method is functional,both for imaging the uppermost bedrock velocities as well as for estimating the cover thickness.

Microearthquake S-wave observations from 0 to 1 km in the Varian well at Parkfield, California

1995 ◽  
Vol 85 (6) ◽  
pp. 1805-1820
Author(s):  
Denis Jongmans ◽  
Peter E. Malin
Keyword(s):  
Fault Zone ◽  
Guided Waves ◽  
Velocity Structure ◽  
San Andreas Fault ◽  
High Gain ◽  
S Wave ◽  
Lateral Velocity ◽  
Wave Amplification ◽  
S Waves ◽  
San Andreas

Abstract High-gain three-component seismometers from 0- to 1-km deep along the Varian A-1 well at Parkfield, California, were used to record the waveforms of nearby microearthquakes. Despite being in the thick Tertiary sediments of the Parkfield Syncline, the S-wave amplification at this site is only about a factor of 3. The spectral content and spectral ratios of S waves along the well show that the average Qs in the top 1 km at this site is 37, with the Qs in different subintervals varying between 8 and 65. Based on initial S-wave polarizations, a complex S-wave velocity structure must exist at and below the Varian site. This structure appears to include position-dependent anisotropy as well as steep lateral velocity gradients. At a depth of 1 km, S-wave splitting parallel and normal to the San Andreas fault zone is consistently observed. This splitting scales at roughly 0.01 sec/km. Subsequent to the split S waves, the particle motion seems to be controlled by event focal mechanism. Above 1 km, the upgoing S waves attenuate and change directions of polarization, with a new splitting rate of 0.1 sec/km. Uniquely, for some events on the San Andreas fault immediately below the Varian site, large, post-S-wave signals with normal dispersion are present. We propose that these phases are fault-zone guided waves channeled from the San Andreas fault to the Varian site along the Gold Hill fault.

Observation of Higher-Mode Surface Waves from an Active Source in the Hutubi Basin, Xinjiang, China

2021 ◽  
Author(s):  
Zhanbo Ji ◽  
Baoshan Wang ◽  
Wei Yang ◽  
Weitao Wang ◽  
Jinbo Su ◽  
...  
Keyword(s):  
Surface Waves ◽  
Wave Velocity ◽  
Seismic Waves ◽  
Velocity Structure ◽  
Group Velocity Dispersion ◽  
S Wave ◽  
Contributing Factors ◽  
Low Velocity ◽  
Time Frequency ◽  
Wave Modes

ABSTRACT Basins with thick sediments can amplify and prolong the incoming seismic waves, which may cause serious damage to surface facilities. The amplification of seismic energy depends on the shear-wave velocity of the uppermost layers, which is generally estimated through surface wave analysis. Surface waves may propagate in different modes, and the mechanism of the mode development is not well understood. Exploiting a recently deployed permanent airgun source in the Hutubi basin, Xinjiang, northwest China, we conducted a field experiment to investigate the development of multimode surface waves. We observed surface waves at the frequency of 0.3–5.0 Hz with apparent group velocities of 200–900  m/s, and identified five modes of surface waves (three Rayleigh-wave modes and two Love-wave modes) through time–frequency and particle-motion analyses. We then measured 125 group velocity dispersion curves of the fundamental- and higher-mode surface waves, and further inverted the 1D S-wave velocity structure of the Hutubi basin. The S-wave velocity increases abruptly from 238  m/s at the surface to 643  m/s at 300 m depth. Synthetic seismograms with the inverted velocity structure capture the main features of the surface waves of the different modes. Synthetic tests suggest that the low velocity, high velocity gradient, and shallow source depth are likely the dominant contributing factors in the development of higher-mode surface waves.

The structure of Archean–Ketilidian crust along the continental shelf of southwestern Greenland from a seismic refraction profile

1992 ◽  
Vol 29 (2) ◽  
pp. 301-313 ◽  
Author(s):  
Deping Chian ◽  
Keith Louden
Keyword(s):  
Lower Crust ◽  
Velocity Structure ◽  
Seismic Refraction ◽  
Mobile Belt ◽  
Ocean Bottom ◽  
Upper Crust ◽  
S Wave ◽  
Seismic Line ◽  
Wave Velocities ◽  
Air Gun

The velocity structure of the continental crust on the outer shelf of southwestern Greenland is determined from dense wide-angle reflection–refraction data obtained with large air-gun sources and ocean bottom seismometers along a 230 km seismic line. This line crosses the geological boundary between the Archean block and the Ketilidian mobile belt. Although the data have high noise levels, P- and S-wave arrivals from within the upper, intermediate, and lower crust, and at the Moho boundary, can be consistently identified and correlated with one-dimensional WKBJ synthetic seismograms. In the Archean, P- and S-wave velocities in the upper crust are 6.0 and 3.4 km/s, while in the intermediate crust they are 6.4 and 3.6 km/s. These velocities match for the upper crust a quartz–feldspar gneiss composition and for the intermediate crust an amphibolitized pyroxene granulite. In the Ketilidian mobile belt, P- and S-wave velocities are 5.6 and 3.3 km/s for the upper crust and 6.3 and 3.6 km/s for the intermediate crust. These velocities may represent quartz granite in the upper crust and granite and granitic gneiss in the intermediate crust. The upper crust is ~5 km thick in the Archean block and the Ketilidian mobile belt, and thickens to ~9 km in the southern part of the Archean. This velocity structure supports a Precambrian collisional mechanism between the Archean block and Ketilidian mobile belt. The lower crust has a small vertical velocity gradient from 6.6 km/s at 15 km depth to 6.9 km/s at 30 km depth (Moho) along the refraction line, with a nearly constant S-wave velocity around 3.8 km/s. These velocities likely represent a gabbroic and hornblende granulite composition for the lower crust. This typical (but somewhat thin) Precambrian crustal velocity structure in southwestern Greenland shows no evidence for a high-velocity, lower crustal, underplated layer caused by the Mesozoic opening of the Labrador Sea.

scholarly journals Lowermost mantle shear-velocity structure from hierarchical trans-dimensional Bayesian tomography

2021 ◽  
Author(s):  
Sima Mousavi ◽  
Hrvoje Tkalčić ◽  
Rhys Hawkins ◽  
Malcolm Sambridge
Keyword(s):  
Velocity Structure ◽  
Spatial Scales ◽  
Model Space ◽  
Shear Velocity ◽  
Outer Core ◽  
Travel Times ◽  
S Wave ◽  
Low Velocity ◽  
Spatial Coverage ◽  
Lowermost Mantle

The core-mantle boundary (CMB) is the most extreme boundary within the Earth where the liquid, iron-rich outer core interacts with the rocky, silicate mantle. The nature of the lowermost mantle atop the CMB, and its role in mantle dynamics, is not completely understood. Various regional studies have documented significant heterogeneities at different spatial scales. While there is a consensus on the long scale-length structure of the inferred S-wave speed tomograms, there are also notable differences stemming from different imaging methods and datasets. Here we aim to overcome over-smoothing and avoid over-fitting data for the case where the spatial coverage is sparse and the inverse problem ill-posed. Here we present an S-wave tomography model at global scale for the Lowermost Mantle (LM) using the Hierarchical Trans-dimensional Bayesian Inversion (HTDBI) framework, LM-HTDBI. Our HTDBI analysis of ScS-S travel times includes uncertainty, and the complexity of the model is deduced from the data itself through an implicit parameterization of the model space. Our comprehensive resolution estimates indicate that short-scale anomalies are significant and resolvable features of the lowermost mantle regardless of the chosen mantle-model reference to correct the travel times above the D’’ layer. The recovered morphology of the Large-Low-Shear-wave Velocity Provinces (LLSVPs) is complex, featuring small high-velocity patches among low-velocity domains. Instead of two large, unified, and smooth LLSVPs, the newly obtained images suggest that their margins are not uniformly flat.

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