Lithospheric structure of the Trans-Hudson Orogen from seismic refraction - wide-angle reflection studies

2005 ◽  
Vol 42 (4) ◽  
pp. 435-456 ◽  
Author(s):  
Balázs Németh ◽  
Ron M Clowes ◽  
Zoltan Hajnal
Keyword(s):  
Seismic Refraction ◽  
Suture Zone ◽  
P Wave ◽  
Limited Area ◽  
Lithospheric Structure ◽  
Complex Deformation ◽  
Velocity Anisotropy ◽  
Metamorphic History ◽  
The North ◽  
Reflection Data

The Trans-Hudson Orogen (THO) is the world's largest Paleoproterozoic orogenic belt. Data from three refraction profiles are used to investigate its lithospheric structure in Saskatchewan and Manitoba. R1 crosses the orogen from the Hearne craton on the west to the Superior craton on the east; R2 and R3 are along the orogen. P-wave velocity structural models are generated using a ray-based technique. On line R1, higher crustal velocities in its eastern part coincide with rocks of the Flin Flon – Namew gneiss complex. Depth to Moho is in the 40–45 km range and equates to that from the reflection data, including a small crustal root below the Sask minicontinent. Along lines R2 and R3, depth to Moho varies from about 40 km up to 55 km at the north end of R2 and south end of R3. In general, variations in crustal velocity and depth to Moho do not correlate with the location and extent of geological domains; they appear to reflect the complex deformation and metamorphic history of the crustal rocks. Mantle velocities are high, ~8.2 km/s. However a limited area shows prominent velocity anisotropy, with values of 8.6 km/s along R2 and R3 and 8.1 km/s along R1. We speculate that the observed anisotropy represents an ~100-km-wide mantle suture zone resulting from the collision of Archean plates. The suture zone accommodated limited extensional deformation, associated with a counterclockwise rotation of the Superior plate, to generate the anisotropy. In this model, the lithospheric mantle of the THO internal domains and Sask craton are detached.

Anisotropic geometrical-spreading correction for wide-azimuth P-wave reflections

Geophysics ◽  
2006 ◽  
Vol 71 (5) ◽  
pp. D161-D170 ◽  
Author(s):  
Xiaoxia Xu ◽  
Ilya Tsvankin
Keyword(s):  
Transversely Isotropic ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Fractured Reservoir ◽  
Geometrical Spreading ◽  
Spreading Factor ◽  
Velocity Anisotropy ◽  
Reflection Data ◽  
Fitting Procedure ◽  
Avo Attributes

Compensation for geometrical spreading along a raypath is one of the key steps in AVO (amplitude-variation-with-offset) analysis, in particular, for wide-azimuth surveys. Here, we propose an efficient methodology to correct long-spread, wide-azimuth reflection data for geometrical spreading in stratified azimuthally anisotropic media. The P-wave geometrical-spreading factor is expressed through the reflection traveltime described by a nonhyperbolic moveout equation that has the same form as in VTI (transversely isotropic with a vertical symmetry axis) media. The adapted VTI equation is parameterized by the normal-moveout (NMO) ellipse and the azimuthally varying anellipticity parameter [Formula: see text]. To estimate the moveout parameters, we apply a 3D nonhyperbolic semblance algorithm of Vasconcelos and Tsvankin that operates simultaneously with traces at all offsets andazimuths. The estimated moveout parameters are used as the input in our geometrical-spreading computation. Numerical tests for models composed of orthorhombic layers with strong, depth-varying velocity anisotropy confirm the high accuracy of our travetime-fitting procedure and, therefore, of the geometrical-spreading correction. Because our algorithm is based entirely on the kinematics of reflection arrivals, it can be incorporated readily into the processing flow of azimuthal AVO analysis. In combination with the nonhyperbolic moveout inversion, we apply our method to wide-azimuth P-wave data collected at the Weyburn field in Canada. The geometrical-spreading factor for the reflection from the top of the fractured reservoir is clearly influenced by azimuthal anisotropy in the overburden, which should cause distortions in the azimuthal AVO attributes. This case study confirms that the azimuthal variation of the geometrical-spreading factor often is comparable to or exceeds that of the reflection coefficient.

Seismic refraction observations north of the Brownson Deep*

1952 ◽  
Vol 42 (4) ◽  
pp. 291-306
Author(s):  
J. B. Hersey ◽  
Charles B. Officer ◽  
H. R. Johnson ◽  
S. Bergstrom
Keyword(s):  
High Speed ◽  
Seismic Refraction ◽  
Low Frequency ◽  
Good Evidence ◽  
Constant Frequency ◽  
Reflected Waves ◽  
Surface Reflection ◽  
The North ◽  
Reflection Data ◽  
Compressional Velocity

Abstract Results of several refraction profiles made on the rise to the north of the Brownson Deep are presented. Good evidence for a high-speed layer with travel-time curves showing a compressional velocity of 7.94 km/sec. and an intercept of 8.4 sec. is presented, and the presence of an overlying lower-speed layer (6.64 km/sec., intercept 8.1 sec.) is demonstrated on less complete evidence. Neither layer correlates with existing reflection data in the area. Two sets of secondary low-frequency arrivals are tentatively interpreted as a refracted shear wave and a wave that has taken a bottom and surface reflection and then a basement refraction. Evidence is presented for a newly observed forerunner of the refracted-surface reflected waves of the permanent sound channel (RSR waves) consisting of a train of nearly constant frequency waves which appear to travel between surface and bottom via the RSR path and along the bottom at the speed of sound in water at the bottom.

scholarly journals The Effect of Aspect and Elevation on Critical Zone Architecture in the Reynolds Creek Critical Zone Observatory: A Seismic Refraction Study

2021 ◽  
Vol 3 ◽  
Author(s):  
Travis Nielson ◽  
John Bradford ◽  
W. Steven Holbrook ◽  
Mark Seyfried
Keyword(s):  
Seismic Refraction ◽  
Critical Zone ◽  
Snow Accumulation ◽  
P Wave ◽  
The North ◽  
Refraction Tomography ◽  
Critical Zone Observatory ◽  
Reynolds Creek ◽  
The Difference ◽  
Weathering Zone

In the northern hemisphere within snow-dominated mountainous watersheds north-facing slopes are commonly more deeply weathered than south-facing slopes. This has been attributed to a more persistent snowpack on the north facing aspects. A persistent snowpack releases its water into the subsurface in a single large pulse, which propagates the water deeper into the subsurface than the series of small pulses characteristic of the intermittent snowpack on south-facing slopes. Johnston Draw is an east-draining catchment in the Reynolds Creek Critical Zone Observatory, Idaho that spans a 300 m elevation gradient. The north-facing slope hosts a persistent snowpack that increases in volume up drainage, while the south-facing slope has intermittent snowpack throughout the drainage. We hypothesize that the largest difference in weathering depth between the two aspects will occur where the difference in snow accumulation between the aspects is also greatest. To test this hypothesis, we conducted four seismic refraction tomography surveys within Johnston Draw from inlet to outlet and perpendicular to drainage direction. From these measurements, we calculate the weathering zone thickness from the P-wave velocity profiles. We conclude that the maximum difference in weathering between aspects occurs ¾ of the way up the drainage from the outlet, where the difference in snow accumulation is highest. Above and below this point, the subsurface is more equally weathered and the snow accumulations are more similar. We also observed that the thickness of the weathering zone increased with decreasing elevation and interpret this to be related to the observed increase soil moisture at lower elevations. Our observations support the hypothesis that deeper snow accumulation leads to deeper weathering when all other variables are held equal. One caveat is the possibility that the denser vegetation contributes to deeper weathering on north-facing slopes via soil retention or higher rates of biological weathering.

Detection and analysis of naturally fractured gas reservoirs: Multiazimuth seismic surveys in the Wind River basin, Wyoming

Geophysics ◽  
1999 ◽  
Vol 64 (4) ◽  
pp. 1277-1292 ◽  
Author(s):  
Robert E. Grimm ◽  
Heloise B. Lynn ◽  
C. R. Bates ◽  
D. R. Phillips ◽  
K. M. Simon ◽  
...  
Keyword(s):  
P Wave ◽  
Apparent Velocity ◽  
Fracture Density ◽  
S Wave ◽  
High Fracture ◽  
Velocity Anisotropy ◽  
Wind River Basin ◽  
Reflection Data ◽  
Parallel Volume ◽  
Wave Methods

Multiazimuth binning of 3-D P-wave reflection data is a relatively simple but robust way of characterizing the spatial distribution of gas‐producing natural fractures. In our survey, data were divided into two volumes by ray azimuth (approximately perpendicular and parallel (±45° to the dominant fracture strike) and separately processed. Azimuthal differences or ratios of attributes provided a rough measure of anisotropy. Improved imaging was also attained in the more coherent fracture‐parallel volume. A neural network using azimuthally dependent velocity, reflectivity, and frequency attributes identified commercial gas wells with greater than 85% success. Furthermore, we were able to interpret the physical mechanisms of most of these correlations and so better generalize the approach. The apparent velocity anisotropy was compared to that derived from other P- and S-wave methods in an inset three‐component survey. Prestack determination of the azimuthal moveout ellipse will best quantify velocity anisotropy, but simple two‐ or four‐azimuth poststack analysis can adequately identify regions of high fracture density and gas yield.

Constraining helicopter electromagnetic models of the Okavango Delta with seismic-refraction and seismic-reflection data

Geophysics ◽  
2014 ◽  
Vol 79 (3) ◽  
pp. B123-B134 ◽  
Author(s):  
Fabienne Reiser ◽  
Joel E. Podgorski ◽  
Cedric Schmelzbach ◽  
Heinrich Horstmeyer ◽  
Alan G. Green ◽  
...  
Keyword(s):  
Seismic Reflection ◽  
Lower Layer ◽  
Seismic Refraction ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
Saturated Sand ◽  
Resistive Layer ◽  
Wave Velocities ◽  
Reflection Data ◽  
Water Saturated

Electrical resistivity models derived from exceptionally high-quality helicopter transient electromagnetic data recorded across the Okavango Delta in Botswana, one of the world’s great inland deltas or megafans, include three principal layers: (1) an upper heterogeneous layer of dry and water-saturated sand, (2) an intermediate electrically conductive layer that likely comprises saline-water-saturated sand and clay, and (3) a lower fan-shaped electrically resistive layer of freshwater-saturated sand/gravel and/or crystalline basement. If part of the lower layer comprises a freshwater aquifer, it would be evidence for a recently proposed Paleo Okavango Megafan and a major new source of freshwater. In an attempt to constrain the interpretation of the lower layer, we acquired two high-resolution seismic refraction and reflection data sets at each of two investigation sites: one near the center of the delta and one along its western edge. The interface between unconsolidated sediments and basement near the center of the delta is well defined by an [Formula: see text] to [Formula: see text] increase in P-wave velocities, a change in seismic reflection facies, and a strong continuous reflection. This interface is about 45 m deeper than the top of the lower resistive layer, thus providing support for the Paleo Okavango Megafan hypothesis. Subhorizontal seismic reflectors are additional evidence for a sedimentary origin of the upper part of the lower resistive layer. In contrast to the observations at the delta’s center, the interface between unconsolidated sediments and basement along its western edge, which is also defined by a [Formula: see text] to [Formula: see text] increase in P-wave velocities and a continuous reflection, coincides with the top of the resistive layer.

scholarly journals Seismic wide-angle constrains on the structure of the northern Sicily margin and Vavilov Basin: implications for the opening of the Tyrrhenian back-arc basin

2020 ◽  
Author(s):  
Ingo Grevemeyer ◽  
Cesar Ranero ◽  
Nevio Zitellini ◽  
Valenit Sallares ◽  
Manel Prada
Keyword(s):  
Continental Crust ◽  
Seismic Refraction ◽  
Seismic Profile ◽  
Passive Margin ◽  
P Wave ◽  
Tyrrhenian Sea ◽  
Wide Angle ◽  
Central Mediterranean ◽  
The North ◽  
Back Arc

<p>The Tyrrhenian Sea in the central Mediterranean Sea was form by Neogene slab roll-back of the retreating Ionian slab about 6 to 2 Myr ago. Yet, little is known about the structure of its southern margin off Sicily as well as back-arc extension and spreading in the southern Tyrrhenian Sea to the north of Sicily. The Sicilian margin is generally classified as a passive margin bounding a young back-arc basin. However, focal mechanisms from regional earthquakes suggest that the margins suffers presently from compressional tectonics. New seismic refraction and wide-angle data were collected along seismic profile WAS4 during the CHIANTI survey of the Spanish research vessel Sarmiento de Gamboa in 2015. The profile runs from the centre of the Tyrrhenian Sea – the Vavilov Basin – across the margin of Sicily, approaching the Gulf of Castellammare to the northwest of Sicily. Reanalyzed multi-channel seismic data supports compressional tectonics across a small basin paralleling the coastline of Sicily, revealing recent inversion of the Tyrrhenian Basin. Offshore of Sicily WAS4 indicates a roughly 120-140 km wide domain showing seismic P-wave velocities characteristic for continental crust (Vp ~4-6.7 km/s) and a base of crust defined by a wide-angle Moho reflection. Continental crust reaches a maximum thickness of 22 km to the north of the Gulf of Castellammare and is thinning to ~9 km to the north of the Ustica Ridge. The compressional belt occurs in continental crust to the south of Ustica Ridge. In the Vavilov Basin, a lithosphere was sample where seismic P-wave velocity increases from approx. 3-4 km/s to 7.5 km/s. This velocity depth-distribution clearly shows profound similarities to serpentinized mantle and hence un-roofed mantle. Thus, seismic constrains support results from Ocean Drilling Program (ODP) hole 651A, which sample serpentinized peridotites in the Vavilov Basin. The transition between serpentinized mantle and continental crust is rather abrupt. Thus, within a ~10 km wide transitional domain, continental crust with a thickness of~ 9 km is juxtaposed against un-roofed mantle. All available data from the Tyrrhenian Sea support wide-spread mantle exhumation in the Vavilov Basin. Therefore, the Tyrrhenian Sea provides a rather different structure when compared to marginal basins in the Western Pacific and hence may not have supported a mid-ocean ridge-type spreading system opening the basin.</p>

Integration of Multi-geophysical Approaches to Identify Potential Pathways of Heavy Metals Contamination - A Case Study in Zeida, Morocco

2020 ◽  
Vol 25 (3) ◽  
pp. 415-423
Author(s):  
Ahmed Lachhab ◽  
El Mehdi Benyassine ◽  
Mohamed Rouai ◽  
Abdelilah Dekayir ◽  
Jean C. Parisot ◽  
...  
Keyword(s):  
Heavy Metals ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
Geophysical Methods ◽  
Low Frequency ◽  
Electrical Current ◽  
P Wave ◽  
Low Resistivity ◽  
Refraction Tomography ◽  
Geophysical Surveys

The tailings of Zeida's abandoned mine are found near the city of Midelt, in the middle of the high Moulouya watershed between the Middle and the High Atlas of Morocco. The tailings occupy an area of about 100 ha and are stored either in large mining pit lakes with clay-marl substratum or directly on a heavily fractured granite bedrock. The high contents of lead and arsenic in these tailings have transformed them into sources of pollution that disperse by wind, runoff, and seepage to the aquifer through faults and fractures. In this work, the main goal is to identify the pathways of contaminated water with heavy metals and arsenic to the local aquifers, water ponds, and Moulouya River. For this reason, geophysical surveys including electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and very low-frequency electromagnetic (VLF-EM) methods were carried out over the tailings, and directly on the substratum outside the tailings. The result obtained from combining these methods has shown that pollutants were funneled through fractures, faults, and subsurface paleochannels and contaminated the hydrological system connecting groundwater, ponds, and the river. The ERT profiles have successfully shown the location of fractures, some of which extend throughout the upper formation to depths reaching the granite. The ERT was not successful in identifying fractures directly beneath the tailings due to their low resistivity which inhibits electrical current from propagating deeper. The seismic refraction surveys have provided valuable details on the local geology, and clearly identified the thickness of the tailings and explicitly marked the boundary between the Triassic formation and the granite. It also aided in the identification of paleochannels. The tailings materials were easily identified by both their low resistivity and low P-wave velocity values. Also, both resistivity and seismic velocity values rapidly increased beneath the tailings due to the compaction of the material and lack of moisture and have proven to be effective in identifying the upper limit of the granite. Faults were found to lie along the bottom of paleochannels, which suggest that the locations of these channels were caused by these same faults. The VLF-EM surveys have shown tilt angle anomalies over fractured areas which were also evinced by low resistivity area in ERT profiles. Finally, this study showed that the three geophysical methods were complementary and in good agreement in revealing the pathways of contamination from the tailings to the local aquifer, nearby ponds and Moulouya River.

scholarly journals Dynamical Downscaling of ERA5 Data on the North-Western Mediterranean Sea: From Atmosphere to High-Resolution Coastal Wave Climate

2021 ◽  
Vol 9 (2) ◽  
pp. 208
Author(s):  
Valentina Vannucchi ◽  
Stefano Taddei ◽  
Valerio Capecchi ◽  
Michele Bendoni ◽  
Carlo Brandini
Keyword(s):  
Mediterranean Sea ◽  
Wind Wave ◽  
Dynamical Downscaling ◽  
Wave Climate ◽  
Western Mediterranean ◽  
Limited Area ◽  
Computational Domain ◽  
Wave Hindcast ◽  
The North ◽  
Western Mediterranean Sea

A 29-year wind/wave hindcast is produced over the Mediterranean Sea for the period 1990–2018. The dataset is obtained by downscaling the ERA5 global atmospheric reanalyses, which provide the initial and boundary conditions for a numerical chain based on limited-area weather and wave models: the BOLAM, MOLOCH and WaveWatch III (WW3) models. In the WW3 computational domain, an unstructured mesh is used. The variable resolutions reach up to 500 m along the coasts of the Ligurian and Tyrrhenian seas (Italy), the main objects of the study. The wind/wave hindcast is validated using observations from coastal weather stations and buoys. The wind validation provides velocity correlations between 0.45 and 0.76, while significant wave height correlations are much higher—between 0.89 and 0.96. The results are also compared to the original low-resolution ERA5 dataset, based on assimilated models. The comparison shows that the downscaling improves the hindcast reliability, particularly in the coastal regions, and especially with regard to wind and wave directions.

Thermal regime of the lithosphere in the Canadian ShieldThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.

2010 ◽  
Vol 47 (4) ◽  
pp. 389-408 ◽  
Author(s):  
Claire Perry ◽  
Carmen Rosieanu ◽  
Jean-Claude Mareschal ◽  
Claude Jaupart
Keyword(s):  
Heat Flow ◽  
Heat Generation ◽  
Seismic Refraction ◽  
Surface Heat ◽  
Canadian Shield ◽  
P Wave ◽  
Seismic Velocities ◽  
Flow Measurements ◽  
Surface Heat Flow ◽  
Mantle Heat Flow

Geothermal studies were conducted within the framework of Lithoprobe to systematically document variations of heat flow and surface heat production in the major geological provinces of the Canadian Shield. One of the main conclusions is that in the Shield the variations in surface heat flow are dominated by the crustal heat generation. Horizontal variations in mantle heat flow are too small to be resolved by heat flow measurements. Different methods constrain the mantle heat flow to be in the range of 12–18 mW·m–2. Most of the heat flow anomalies (high and low) are due to variations in crustal composition and structure. The vertical distribution of radioelements is characterized by a differentiation index (DI) that measures the ratio of the surface to the average crustal heat generation in a province. Determination of mantle temperatures requires the knowledge of both the surface heat flow and DI. Mantle temperatures increase with an increase in surface heat flow but decrease with an increase in DI. Stabilization of the crust is achieved by crustal differentiation that results in decreasing temperatures in the lower crust. Present mantle temperatures inferred from xenolith studies and variations in mantle seismic P-wave velocity (Pn) from seismic refraction surveys are consistent with geotherms calculated from heat flow. These results emphasize that deep lithospheric temperatures do not always increase with an increase in the surface heat flow. The dense data coverage that has been achieved in the Canadian Shield allows some discrimination between temperature and composition effects on seismic velocities in the lithospheric mantle.

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