scholarly journals Crustal velocity structure of the Superior and Grenville provinces of the southeastern Canadian Shield

1997 ◽  
Vol 34 (8) ◽  
pp. 1167-1184 ◽  
Author(s):  
S. Winardhi ◽  
R. F. Mereu
Keyword(s):  
Greenstone Belt ◽  
Velocity Structure ◽  
Canadian Shield ◽  
Crustal Thickening ◽  
Surface Velocity ◽  
Data Set ◽  
Tectonic Zone ◽  
Near Surface ◽  
Abitibi Greenstone Belt ◽  
Sudbury Structure

The 1992 Lithoprobe Abitibi–Grenville Seismic Refraction Experiment was conducted using four profiles across the Grenville and Superior provinces of the southeastern Canadian Shield. Delay-time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another, with the largest velocity values occurring underneath the Central Gneiss and the Central Metasedimentary belts south of the Grenville Front. The Grenville Front Tectonic Zone is imaged as a southeast-dipping region of anomalous velocity gradients extending to the Moho. The velocity-anomaly maps suggest an Archean crust may extend, horizontally, 140 km beneath the northern Grenville Province. Near-surface velocity anomalies correlate well with the known geology. The most prominent of these is the Sudbury Structure, which is well mapped as a low-velocity basinal structure. The tomography images also suggest underthrusting of the Pontiac and Quetico subprovinces beneath the Abitibi Greenstone Belt. Wide-angle PmP signals, indicate that the Moho varies from a sharp discontinuity south of the Grenville Front to a rather diffuse and flat boundary under the Abitibi Greenstone Belt north of the Grenville Front. A significant crustal thinning near the Grenville Front may indicate post-Grenvillian rebound and (or) the extensional structure of the Ottawa–Bonnechere graben. Crustal thickening resulting from continental collision may explain the tomographic images showing the Moho is 4–5 km deeper south of the Grenville Front.

Crustal velocity structure from SAREX, the Southern Alberta Refraction Experiment

2002 ◽  
Vol 39 (3) ◽  
pp. 351-373 ◽  
Author(s):  
Ron M Clowes ◽  
Michael JA Burianyk ◽  
Andrew R Gorman ◽  
Ernest R Kanasewich
Keyword(s):  
Velocity Structure ◽  
Data Interpretation ◽  
Crustal Thickening ◽  
Tectonic Zone ◽  
Crustal Layer ◽  
Crustal Velocity ◽  
Crustal Velocity Structure ◽  
Southern Alberta ◽  
Magmatic Underplating ◽  
Lower Crustal

Lithoprobe's Southern Alberta Refraction Experiment, SAREX, extends 800 km from east-central Alberta to central Montana. It was designed to investigate crustal velocity structure of the Archean domains underlying the Western Canada Sedimentary Basin. From north to south, SAREX crosses the Loverna domain of the Hearne Province, the Vulcan structure, the Medicine Hat block (previously considered part of the Hearne Province), the Great Falls tectonic zone, and the northern Wyoming Province. Ten shot points along the profile in Canada were recorded on 521 seismographs deployed at 1 km intervals. To extend the line, an additional 140 seismographs were deployed at intervals of 1.25–2.50 km in Montana. Data interpretation used an iterative application of damped least-squares inversion of traveltime picks and forward modeling. Results show different velocity structures for the major blocks (Loverna, Medicine Hat, and Wyoming), indicating that each is distinct. Wavy undulations in the velocity structure of the Loverna block may be associated with internal crustal deformation. The most prominent feature of the model is a thick (10–25 km) lower crustal layer with high velocities (7.5–7.9 km/s) underlying the Medicine Hat and Wyoming blocks. Based on data from lower crustal xenoliths in the region, this layer is interpreted to be the result of Paleoproterozoic magmatic underplating. Crustal thickness varies from 40 km in the north to almost 60 km in the south, where the high-velocity layer is thickest. Uppermost mantle velocities range from 8.05 to 8.2 km/s, with the higher values below the thicker crust. Results from SAREX and other recent studies are synthesized to develop a schematic representation of Archean to Paleoproterozoic tectonic development for the region encompassing the profile. Tectonic processes associated with this development include collisions of continental blocks, subduction, crustal thickening, and magmatic underplating.

Refraction wavefield migration

Geophysics ◽  
2020 ◽  
Vol 85 (6) ◽  
pp. Q27-Q37
Author(s):  
Yang Shen ◽  
Jie Zhang
Keyword(s):  
Signal To Noise Ratio ◽  
Surface Velocity ◽  
Data Set ◽  
Near Surface ◽  
Depth Migration ◽  
Reflection Data ◽  
Imaging Tool ◽  
Imaging Results ◽  
Migration Method ◽  
Single Method

Refraction methods are often applied to model and image near-surface velocity structures. However, near-surface imaging is very challenging, and no single method can resolve all of the land seismic problems across the world. In addition, deep interfaces are difficult to image from land reflection data due to the associated low signal-to-noise ratio. Following previous research, we have developed a refraction wavefield migration method for imaging shallow and deep interfaces via interferometry. Our method includes two steps: converting refractions into virtual reflection gathers and then applying a prestack depth migration method to produce interface images from the virtual reflection gathers. With a regular recording offset of approximately 3 km, this approach produces an image of a shallow interface within the top 1 km. If the recording offset is very long, the refractions may follow a deep path, and the result may reveal a deep interface. We determine several factors that affect the imaging results using synthetics. We also apply the novel method to one data set with regular recording offsets and another with far offsets; both cases produce sharp images, which are further verified by conventional reflection imaging. This method can be applied as a promising imaging tool when handling practical cases involving data with excessively weak or missing reflections but available refractions.

Estimation of the near-surface velocity structure of the Yasur-Yenkahe volcanic complex, Vanuatu

2012 ◽  
Vol 227-228 ◽  
pp. 50-60 ◽  
Author(s):  
Laurence Perrier ◽  
Jean-Philippe Métaxian ◽  
Jean Battaglia ◽  
Esline Garaebiti
Keyword(s):  
Velocity Structure ◽  
Surface Velocity ◽  
Volcanic Complex ◽  
Near Surface

Turning‐ray tomography

Geophysics ◽  
1995 ◽  
Vol 60 (6) ◽  
pp. 1917-1929 ◽  
Author(s):  
Joseph P. Stefani
Keyword(s):  
Velocity Structure ◽  
Initial Point ◽  
Real Data ◽  
Point Sources ◽  
Surface Velocity ◽  
Inversion Algorithm ◽  
Low Velocity ◽  
Linear Inversion ◽  
Near Surface ◽  
Velocity Inversion

Turning‐ray tomography is useful for estimating near‐surface velocity structure in areas where conventional refraction statics techniques fail because of poor data or lack of smooth refractor/velocity structure. This paper explores the accuracy and inherent smoothing of turning‐ray tomography in its capacity to estimate absolute near‐surface velocity and the statics times derived from these velocities, and the fidelity with which wavefields collapse to point diffractors when migrated through these estimated velocities. The method comprises nonlinear iterations of forward ray tracing through triangular cells linear in slowness squared, coupled with the LSQR linear inversion algorithm. It is applied to two synthetic finite‐ difference data sets of types that usually foil conventional refraction statics techniques. These models represent a complex hard‐rock overthrust structure with a low‐velocity zone and pinchouts, and a contemporaneous near‐shore marine trench filled with low‐ velocity unconsolidated deposits exhibiting no seismically apparent internal structure. In both cases velocities are estimated accurately to a depth of one‐ fifth the maximum offset, as are the associated statics times. Of equal importance, the velocities are sufficiently accurate to correctly focus synthetic wavefields back to their initial point sources, so migration/datuming applications can also use these velocities. The method is applied to a real data example from the Timbalier Trench in the Gulf of Mexico, which exhibits the same essential features as the marine trench synthetic model. The Timbalier velocity inversion is geologically reasonable and yields long and short wavelength statics that improve the CMP gathers and stack and that correctly align reflections to known well markers. Turning‐ray tomography estimates near‐surface velocities accurately enough for the three purposes of lithology interpretation, statics calculations, and wavefield focusing for shallow migration and datuming.

Seismic Microzonation using 6C Measurements

2020 ◽  
Author(s):  
Sabrina Keil ◽  
Joachim Wassermann ◽  
Heiner Igel
Keyword(s):  
Velocity Structure ◽  
Building Design ◽  
Wave Dispersion ◽  
Velocity Profiles ◽  
Surface Velocity ◽  
Intelligent Building ◽  
Near Surface ◽  
Velocity Models ◽  
Single Station ◽  
Risk Areas

<p>Microzonation is one of the essential tools in seismology to mitigate earthquake damage by estimating the near surface velocity structure and developing land usage plans and intelligent building design. The number of microzonation studies increased in the last few years as induced seismicity becomes more relevant, even in low risk areas. While of vital importance, especially in densely populated cities, most of the traditional techniques suffer from different short comings. The microzonation technique presented here tries to reduce the existing ambiguity of the inversion results by the combination of single-station six-component (6C) measurements, including three translational and three rotational motions, and more traditional H/V techniques. By applying this new technique to a microzonation study in Munichs (Germany) inner city using an iXblue blueSeis-3A rotational motion sensor together with a Nanometrics Trillium Compact seismometer we were able to estimate Love and Rayleigh wave dispersion curves. These curves together with H/V spectral ratios are then inverted to obtain shear wave velocity profiles of the upper 100 m. The resulting 1D velocity profiles are used to estimate the local shaking characteristics in Munich. In addition, the comparison between the estimated velocity models and the borehole-derived lithology gives a positive correlation, indicating the applicability of our method.</p>

Land FWI in the Delaware Basin, west Texas: A case study

2020 ◽  
Vol 39 (5) ◽  
pp. 324-331
Author(s):  
Gary Murphy ◽  
Vanessa Brown ◽  
Denes Vigh
Keyword(s):  
Seismic Data ◽  
Waveform Inversion ◽  
Surface Velocity ◽  
High Signal ◽  
Data Sets ◽  
Data Set ◽  
Low Frequencies ◽  
Near Surface ◽  
Delaware Basin ◽  
West Texas

As part of a wide-reaching full-waveform inversion (FWI) research program, FWI is applied to an onshore seismic data set collected in the Delaware Basin, west Texas. FWI is routinely applied on typical marine data sets with high signal-to-noise ratio (S/N), relatively good low-frequency content, and reasonably long offsets. Land seismic data sets, in comparison, present significant challenges for FWI due to low S/N, a dearth of low frequencies, and limited offsets. Recent advancements in FWI overcome limitations due to poor S/N and low frequencies making land FWI feasible to use to update the shallow velocities. The chosen area has contrasting and variable near-surface conditions providing an excellent test data set on which to demonstrate the workflow and its challenges. An acoustic FWI workflow is used to update the near-surface velocity model in order to improve the deeper image and simultaneously help highlight potential shallow drilling hazards.

Orogen parallel and transverse shearing in the Opatica belt, Quebec: implications for the structure of the Abitibi Subprovince

1992 ◽  
Vol 29 (11) ◽  
pp. 2429-2444 ◽  
Author(s):  
Keith Benn ◽  
Edward W. Sawyer ◽  
Jean-Luc Bouchez
Keyword(s):  
Greenstone Belt ◽  
Regional Scale ◽  
Structural Evolution ◽  
Fault Zones ◽  
Crustal Thickening ◽  
Superior Province ◽  
Northern Margin ◽  
Abitibi Greenstone Belt ◽  
Ductile Shearing ◽  
Abitibi Subprovince

The late Archean Opatica granitoid-gneiss belt is situated within the northern Abitibi Subprovince, along the northern margin of the Abitibi greenstone belt. Approximately 200 km of structural section was mapped along three traverses within the previously unstudied Opatica belt. The earliest preserved structures are penetrative foliations and stretching and mineral lineations recording regional ductile shearing (D1). Late-D1 deformation was concentrated into kilometre-scale ductile fault zones, typically with L > S tectonite fabrics. Two families of lineations are associated with D1, indicating shearing both parallel and transverse to the east-northeast trend of the belt. Lineations trending east-northeast or northwest–southeast tend to be dominant within domains separated by major fault zones. In light of the abundant evidence for early north–south compression documented throughout southern Superior Province, including the Abitibi greenstone belt, D1 is interpreted in terms of mid-crustal thrusting, probably resulting in considerable crustal thickening. Movement-sense indicators suggest that thrusting was dominantly southward vergent. D2 deformation resulted in the development of vertical, regional-scale dextral and sinistral transcurrent fault zones and open to tight upright horizontal folds of D1 fabrics. In the context of late Archean orogenesis in southern Superior Province, the tectonic histories of the Abitibi and Opatica belts should not be considered separately. The Opatica belt may correlate with the present-day mid-crustal levels of the Abitibi greenstone belt, and to crystalline complexes within the Abitibi belt. It is suggested that the Abitibi Subprovince should be viewed, at the regional scale, as a dominantly southward-vergent orogenic belt. This work demonstrates that structural study of granitoid-gneiss belts adjacent to greenstone belts can shed considerable light on the regional structure and structural evolution of late Archean terranes.

A seismic-reflection-based regional cross section of the southern Abitibi greenstone belt

1995 ◽  
Vol 32 (2) ◽  
pp. 135-148 ◽  
Author(s):  
S. L. Jackson ◽  
A. R. Cruden ◽  
D. White ◽  
B. Milkereit
Keyword(s):  
Greenstone Belt ◽  
Seismic Reflection ◽  
Lower Layer ◽  
Middle Layer ◽  
Shear Zones ◽  
Low Pressure ◽  
Crustal Thickening ◽  
Upper Crust ◽  
Abitibi Greenstone Belt ◽  
Seismic Reflection Profiles

Seismic reflection profiles from the southern Abitibi greenstone belt reveal four first-order subdivisions: (1) Between 0 and ~4.5 s, the upper crust is weakly reflective, with prominent local to laterally extensive reflections. (2) Between ~4 and ~9 s, the crust is strongly and heterogeneously reflective with laterally continuous reflections. (3) From ~9 to ~13 s, the crust is more homogeneously reflective and displays downward decreasing reflectivity. (4) Below ~13 s (Moho?) the upper mantle is weakly reflective. The upper layer may correspond to subgreenschist–greenschist-facies supracrustal rocks cut by low-angle shear zones and intruded by regional tabular batholiths; the middle layer, to ductiley deformed amphibolite-facies gneisses, granitoids, and (or) metasediments; and the lower layer, to more homogeneously deformed granulite-facies rocks. North-dipping, low-angle reflections extending beneath both diverse supracrustal assemblages and regional batholiths may represent structural detachments upon which both the supracrustal assemblages and batholiths were imbricated and translated southward. However, the preservation of regional low-pressure metamorphic rocks and the common para-autochthonous relationships between assemblages suggest that thrust-related vertical separations and the magnitude of crustal thickening were not large. Steeply dipping regional shear zones within the greenstone belt appear to disrupt subhorizontal reflections down to ~15 km and may represent late-tectonic strains, which were progressively concentrated into linear zones during continued north–south shortening. The crustal-scale structure determined from the seismic reflection profiles, combined with surface geology, is compatible with post-2.70 Ga north–south shortening accommodated by south-directed(?) thrusting in a thermally softened mid crust and by upright folding in the upper crust. This scenario is comparable to recently proposed models for the Paleozoic, high-temperature, low-pressure Lachlan fold belt of Australia.

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