An interpretation of the first-arrival data of the Lake Superior Experiment by the time-term method

1966 ◽  
Vol 56 (1) ◽  
pp. 141-171
Author(s):  
M. J. Berry ◽  
G. F. West
Keyword(s):  
Lake Superior ◽  
Igneous Rocks ◽  
Low Velocity ◽  
The West ◽  
Head Waves ◽  
First Arrival ◽  
Keweenaw Peninsula ◽  
Time Term ◽  
Similar Basis ◽  
Mohorovičić Discontinuity

abstract The first-arrival data of the Lake Superior Experiment of 1963 have been interpreted by the time-term method. The analysis has shown the method to be well suited to this type of survey, and the results appear to be consistent and meaningful. Approximately 500 first-arrivals from head waves generated at the Mohorovičić discontinuity, have been reduced to estimates of crustal time-terms at over 100 locations. A much shallower refracting surface (here called the Upper Refractor) furnished nearly 1,000 observations to yield upper crustal time-terms at the same locations. The analysis reveals the material beneath the UR and beneath the M to have velocities of 6.63 and 8.10 km/sec respectively. The surface of the Upper Refractor, on the basis of a simple interpretation of the time-terms, is revealed as undulating, coming close to the surface at the edges of the lake and reaching maximum depths of approxmately 15 km to the east and west of the Keweenaw Peninsula. On a similar basis the Mohorovičić discontinuity is revealed as an easterly dipping surface, having a depth of approximately 35 km at the west end of the lake and reaching a maximum depth of about 60 km in the region just west of the Keweenaw Peninsula. Eastwards, the time-term values fluctuate but do not increase or decrease systematically. The velocity of the material lying above the Upper Refractor is not well determined, but appears to be roughly 5.5 km/sec. A perusal of geological literature suggests that this low velocity material is mostly sedimentary, filling a well-known synclincal basin whose axis bends around the Keweenaw Peninsula. This mainly sedimentary section is known to be underlain by a great thickness of igneous rocks, which in all probability corresponds to the Upper Refractor mapped by the seismic studies.

THE CRUST OF THE EARTH UNDER HUDSON BAY

1967 ◽  
Vol 4 (5) ◽  
pp. 949-960 ◽  
Author(s):  
J. A. Hunter ◽  
R. F. Mereu
Keyword(s):  
Crustal Thickening ◽  
Hudson Bay ◽  
Near Surface ◽  
Crustal Velocity ◽  
Surface Data ◽  
First Arrival ◽  
Minimum Depth ◽  
Time Term ◽  
Surface Geology ◽  
Mohorovičić Discontinuity

The Hudson Bay crustal experiment of 1965 involved 41 shots placed on two lines, E–W and NW–SE, in the Bay. The first arrival data of eight land stations situated around the bay were utilized in the time–term analysis. The preferred crustal velocity was found to be 6.32 ± .06 km/s, and the velocity of the upper mantle to be 8.23 ± .03 km/s. Depth calculations from time–terms and employing Geological Survey of Canada near-surface data, show the Mohorovicic discontinuity to be undulatory in nature throughout the bay. An overall rise of this interface occurs from a depth at Churchill of approximately 41 km to a minimum depth of approximately 27 km towards Gilmour Island. Crustal thickening occurs again on the east side of the bay, with a depth of 41 km at Povungnituk. As well, the crust thins towards the NW from an approximate depth of 37 km in the center to a depth of 26 km near Chesterfield Inlet. The correlation between existing surface geology and the Mohorovicic discontinuity undulations is discussed.

A TIME-TERM ANALYSIS OF THE FIRST ARRIVAL DATA FROM THE SEISMIC EXPERIMENT IN HUDSON BAY, 1965

1967 ◽  
Vol 4 (5) ◽  
pp. 901-928 ◽  
Author(s):  
Alan Ruffman ◽  
M. J. Keen
Keyword(s):  
Wave Velocity ◽  
Three Dimensional ◽  
Compressional Wave ◽  
Hudson Bay ◽  
Compressional Wave Velocity ◽  
Seismic Experiment ◽  
First Arrival ◽  
Time Term ◽  
Mohorovičić Discontinuity ◽  
Term Analysis

A time-term analysis is made of the first arrival data from the 41 shots of the1965 Hudson Bay seismic experiment. An investigation of the water-wave data is made to determine which of three possible series of navigation is most consistent. A single-layered crust with a compressional wave velocity of 6.33 km/s and an upper mantle compressional wave velocity of 8.27 km/s are proposed for Hudson Bay. The Mohorovičić discontinuity is found to have considerable topography with depths ranging from 42.7 km to less than 26 km. The Churchill–Superior boundary is proposed to be a three-dimensional crustal feature and is extended offshore from Cape Smith and extended westward to the north of the Ottawa Islands through approximately 59° 40′ N and 82° 00′ W. The Mohorovičić discontinuity rises to depths of about 26 km beneath Chesterfield Inlet and Baker Lake. The mantle is about 40 km deep at Churchill, Manitoba and rises to about30 km some 130 km west of Gilmour Island, then drops to almost 42 km farther east. The sudden drop is related to the Churchill–Superior boundary.

Enumeration and Identification of Asbestos Fibres in Water

1974 ◽  
Vol 32 ◽  
pp. 526-527 ◽  
Author(s):  
O. Mudroch ◽  
J. R. Kramer
Keyword(s):  
Water Supply ◽  
Water Samples ◽  
Drainage Basin ◽  
Lake Superior ◽  
Research Work ◽  
List Type ◽  
The West ◽  
Asbestos Fibre ◽  
Amphibole Asbestos

Approximately 60,000 tons per day of waste from taconite mining, tailing, are added to the west arm of Lake Superior at Silver Bay. Tailings contain nearly the same amount of quartz and amphibole asbestos, cummingtonite and actinolite in fibrous form. Cummingtonite fibres from 0.01μm in length have been found in the water supply for Minnesota municipalities.The purpose of the research work was to develop a method for asbestos fibre counts and identification in water and apply it for the enumeration of fibres in water samples collected(a) at various stations in Lake Superior at two depth: lm and at the bottom.(b) from various rivers in Lake Superior Drainage Basin.

scholarly journals VII.—The Late Palæozoic Alkaline Igneous Rocks of the West of Scotland

1912 ◽  
Vol 9 (3) ◽  
pp. 120-131 ◽  
Author(s):  
G. W. Tyrrell
Keyword(s):  
Thin Section ◽  
Basic Type ◽  
Igneous Rocks ◽  
The West ◽  
Fine Grained ◽  
Late Palaeozoic

2. Essexite. (a) Carskeoch type.—A small boss of essexite, of rather basic type, pierces the agglomerate of the Carskeoch vent, near Patna, Ayrshire. Macroscopically it is light grey, compact, and fine-grained. In thin section it shows a plexus of fluxionally-arranged plagioclase laths of the composition Ab An, the interstices being filled with subhedral augite of a nearly colourless variety, and fresh olivine. Here and there minute angular interspaces are occupied with turbid isotropic matter, the form and arrangement of the particles of which suggest the former presence of nepheline. A few broad plates of pale augite and crystals of olivine interrupt the general trachytoid fabric. Ilmenite and biotite occur sparsely, and a little orthoclase may be detected on the margins of the plagioclases, extending irregularly into the interspaces. This rock has a distinct individuality, and resembles neither the essexites of Lochend and Craigleith in the Lothians, nor the Crawfordjohn type described below. It is poor in alkali-felspars and felspathoids, is devoid of purple augite, and has a well-developed trachytoid fabric.

Exact seismograms for a point force using generalized ray theory

Geophysics ◽  
1983 ◽  
Vol 48 (11) ◽  
pp. 1421-1427 ◽  
Author(s):  
E. R. Kanasewich ◽  
P. G. Kelamis ◽  
F. Abramovici
Keyword(s):  
Half Space ◽  
Near Field ◽  
Sedimentary Basins ◽  
Transverse Component ◽  
Point Force ◽  
Seismic Exploration ◽  
Head Wave ◽  
Vertical Force ◽  
Low Velocity ◽  
Head Waves

Exact synthetic seismograms are obtained for a simple layered elastic half‐space due to a buried point force and a point torque. Two models, similar to those encountered in seismic exploration of sedimentary basins, are examined in detail. The seismograms are complete to any specified time and make use of a Cagniard‐Pekeris method and a decomposition into generalized rays. The weathered layer is modeled as a thin low‐velocity layer over a half‐space. For a horizontal force in an arbitrary direction, the transverse component, in the near‐field, shows detectable first arrivals traveling with a compressional wave velocity. The radial and vertical components, at all distances, show a surface head wave (sP*) which is not generated when the source is compressive. A buried vertical force produces the same surface head wave prominently on the radial component. An example is given for a simple “Alberta” model as an aid to the interpretation of wide angle seismic reflections and head waves.

scholarly journals IV.—The Glacial Deposits of Northern Pembrokeshire

1906 ◽  
Vol 41 (1) ◽  
pp. 53-87
Author(s):  
T. J. Jehu
Keyword(s):  
Igneous Rocks ◽  
Earth’S Crust ◽  
The West ◽  
Great Abundance ◽  
Glacial Deposits ◽  
North East ◽  
The North ◽  
Earth's Crust

The area embraced in this paper consists of that part of Pembrokeshire which lies to the north and north-east of St Bride's Bay. Bounded on the west by St George's Channel and on the north by Cardigan Bay, it extends to the north-east as far as the mouth of the river Teifi, near Cardigan.That part of the country which lies in the immediate neighbourhood of St David's has, through the laborious researches of the late Dr Hicks and others, become well-known to geologists, and may now be regarded as classic ground. The solid geology of this promontory has given rise to much discussion, and has, perhaps, attracted more attention than that of any other part of the Principality. The reason for this great interest is to be sought in the facts that the rocks of this area are of a very great antiquity, and that the sedimentary series contain the remains of some of the earliest organic forms yet found in the earth's crust, whilst the igneous rocks are also displayed in great abundance and variety, and present us, in the words of Sir Archibald Geikie, with “the oldest well-preserved record of volcanic action in Britain.”

IV.—Notes on the Arenig Rocks near Aberdaron, Carnarvonshire

1902 ◽  
Vol 9 (3) ◽  
pp. 118-122 ◽  
Author(s):  
Charles A. Matley
Keyword(s):  
Sedimentary Rocks ◽  
Igneous Rocks ◽  
The West

The geology of the Lleyn peninsula has from time to time attracted the attention of geologists, and several workers have added much to our knowledge of this district; but their researches have been mainly confined to (1) the strip of ancient rocks (usually assigned to the pre-Cambrian) in the west between Porth Dinlleyn and Bardsey Island, and (2) the igneous rocks lying in the Ordovician ground which occupies the eastern and larger portion of the peninsula. Thus the Ordovician sedimentary rocks have largely escaped attention, a result probably also due to their monotony, to the rarity of fossiliferous localities, and to the great extent to which the beds are concealed beneath accumulations of drift.

The upper mantle beneath the South Atlantic Ocean, South America and Africa from waveform tomography with massive data sets

2020 ◽  
Vol 221 (1) ◽  
pp. 178-204 ◽  
Author(s):  
N L Celli ◽  
S Lebedev ◽  
A J Schaeffer ◽  
M Ravenna ◽  
C Gaina
Keyword(s):  
South America ◽  
Upper Mantle ◽  
High Velocity ◽  
South Atlantic ◽  
The South ◽  
Low Velocity ◽  
The West ◽  
Velocity Anomaly ◽  
Archean Crust ◽  
Velocity Anomalies

SUMMARY We present a tomographic model of the crust, upper mantle and transition zone beneath the South Atlantic, South America and Africa. Taking advantage of the recent growth in broadband data sampling, we compute the model using waveform fits of over 1.2 million vertical-component seismograms, obtained with the automated multimode inversion of surface, S and multiple S waves. Each waveform provides a set of linear equations constraining perturbations with respect to a 3-D reference model within an approximate sensitivity volume. We then combine all equations into a large linear system and solve it for a 3-D model of S- and P-wave speeds and azimuthal anisotropy within the crust, upper mantle and uppermost lower mantle. In South America and Africa, our new model SA2019 reveals detailed structure of the lithosphere, with structure of the cratons within the continents much more complex than seen previously. In South America, lower seismic velocities underneath the transbrasilian lineament (TBL) separate the high-velocity anomalies beneath the Amazon Craton from those beneath the São Francisco and Paraná Cratons. We image the buried portions of the Amazon Craton, the thick cratonic lithosphere of the Paraná and Parnaíba Basins and an apparently cratonic block wedged between western Guyana and the slab to the west of it, unexposed at the surface. Thick cratonic lithosphere is absent under the Archean crust of the São Luis, Luis Álves and Rio de La Plata Cratons, next to the continental margin. The Guyana Highlands are underlain by low velocities, indicating hot asthenosphere. In the transition zone, we map the subduction of the Nazca Plate and the Chile Rise under Patagonia. Cratonic lithosphere beneath Africa is more fragmented than seen previously, with separate cratonic units observed within the West African and Congo Cratons, and with cratonic lithosphere absent beneath large portions of Archean crust. We image the lateral extent of the Niassa Craton, hypothesized previously and identify a new unit, the Cubango Craton, near the southeast boundary of the grater Congo Craton, with both of these smaller cratons unexposed at the surface. In the South Atlantic, the model reveals the patterns of interaction between the Mid-Atlantic Ridge (MAR) and the nearby hotspots. Low-velocity anomalies beneath major hotspots extend substantially deeper than those beneath the MAR. The Vema Hotspot, in particular, displays a pronounced low-velocity anomaly under the thick, high-velocity lithosphere of the Cape Basin. A strong low velocity anomaly also underlies the Cameroon Volcanic Line and its offshore extension, between Africa and the MAR. Subtracting the global, age-dependent VS averages from those in the South Atlantic Basins, we observe areas where the cooling lithosphere is locally hotter than average, corresponding to the location of the Tristan da Cunha, Vema and Trindade hotspots. Beneath the anomalously deep Argentine Basin, we image unusually thick, high-velocity lithosphere, which suggests that its anomalously great depth can be explained, at least to a large extent, by isostatic, negative lithospheric buoyancy.

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