New constraints on the geometry of the Lac Bouchette gabbro – anorthosite from magnetotellurics and magnetics

1995 ◽  
Vol 32 (9) ◽  
pp. 1365-1377 ◽  
Author(s):  
R. L. Kellett
Keyword(s):  
High Frequency ◽  
Seismic Reflection ◽  
Fault Zones ◽  
The South ◽  
Grenville Province ◽  
Magnetotelluric Soundings ◽  
The North ◽  
Geophysical Study ◽  
Magnetic Modelling ◽  
Tectonic Boundaries

A cross section of the resistivity structure through the Lac Bouchette gabbro–anorthosite provides a new image of the thin-skinned geometry of an allochthonous terrane in the western Grenville Province of Canada. Two-dimensional inversion of high-frequency magnetotelluric soundings and magnetic modelling indicate that the gabbro–anorthosite is a 1.5 km thick slice bounded by conductive thrust faults. Graphite, which is present at the margins of the gabbro–anorthosite and in the metasedimentary Réservoir Cabonga terrane to the south, is the most likely source of the enhanced electrical conductivity in the fault zones. The southern margin of the gabbro–anorthosite dips at about 15° to the south beneath the Réservoir Cabonga terrane. The gabbro–anorthosite can be divided into a highly magnetic gabbroic body in the south, a less magnetic metagabbro in the north, and a thin anorthositic lense in the centre. The combination of closely spaced magnetotelluric soundings and magnetic modelling provides independent constraints for gravity and seismic reflection studies in progress. The geometry of the Lac Bouchette gabbro–anorthosite, revealed by this geophysical study, supports a hypothesis that some gabbro–anorthosites behave as competent blocks adjacent to the major tectonic boundaries of the Grenville Province.

scholarly journals Solar cycle, seasonal, and asymmetric dependencies of thermospheric mass density disturbances due to magnetospheric forcing

2019 ◽  
Vol 37 (5) ◽  
pp. 989-1003 ◽  
Author(s):  
Andres Calabia ◽  
Shuanggen Jin
Keyword(s):  
Solar Cycle ◽  
High Frequency ◽  
Polar Region ◽  
Rotation Axis ◽  
Mass Density ◽  
Principal Component ◽  
Geomagnetic Variation ◽  
The South ◽  
The North ◽  
Possible Cause

Abstract. Short-term upper atmosphere variations due to magnetospheric forcing are very complex, and neither well understood nor capably modeled due to limited observations. In this paper, mass density variations from 10 years of GRACE observations (2003–2013) are isolated via the parameterization of annual, local solar time (LST), and solar cycle fluctuations using a principal component analysis (PCA) technique. The resulting residual disturbances are investigated in terms of magnetospheric drivers. The magnitude of high-frequency (δ < 10 d) disturbances reveals unexpected dependencies on the solar cycle, seasonal, and an asymmetric behavior with smaller amplitudes in June in the south polar region (SPR). This seasonal modulation might be related to the Russell–McPherron (RM) effect. Meanwhile, we find a similar pattern, although less pronounced, in the northern and equatorial regions. A possible cause of this latitudinal asymmetry might be the irregular shape of the Earth's magnetic field (with the north dip pole close to Earth's rotation axis, and the south dip pole far from that axis). After accounting for the solar cycle and seasonal dependencies by regression analysis to the magnitude of the high-frequency perturbations, the parameterization in terms of the disturbance geomagnetic storm-time index Dst shows the best correlation, whereas the geomagnetic variation Am index and merging electric field Em are the best predictors in terms of time delay. We test several mass density models, including JB2008, NRLMSISE-00, and TIEGCM, and find that they are unable to completely reproduce the seasonal and solar cycle trends found in this study, and show a clear overestimation of about 100 % during low solar activity periods.

scholarly journals REMARKS ON THE KARSTIFICATION IN THE WIDER AREA OF THE UPPER MESSINIA CLOSED HYDROGEOLOGICAL BASIN (SW PELOPONNESUS, GREECE)

2017 ◽  
Vol 43 (4) ◽  
pp. 1785 ◽  
Author(s):  
I. Mariolakos ◽  
E. Spyridonos
Keyword(s):  
Water Flow ◽  
Upper Cretaceous ◽  
Fault Zones ◽  
Karstic Aquifer ◽  
The South ◽  
Quaternary Deposits ◽  
The North ◽  
Hydraulic Conditions ◽  
Underground Flow ◽  
Hydrogeological System

The Upper Messinia Basin is part of the Kalamata - Kyparissia graben structure, defined by two fault zones, one of E-W direction to the north and one of N-S direction to the east. The Tripolis and the Pindos alpidic units build up the basement of the region, and are covered by post-alpidic Pleistocene and quaternary deposits. The Basin is a closed hydrogeological system. In the upper cretaceous Pindos limestones, we observe two different karst types, according to drilling results. In the western part of the basin, where the limestones appear on the surface, the merokarst type is observed and water flow is limited in separated karstic conduits. On the contrary, in the central part of the basin, where the limestones underlie the clastic post-alpidic formations, the holokarst type is observed and a rich karstic aquifer is developed. This is explained by the closed hydraulic conditions. In the western part of the basin the karstic aquifer is unconfined, while in the central part it is confined, due to the overlying impermeable post-alpidic sediments and the overall closure of underground flow to the south. It appears that the confined conditions led to the development from merokarst to holokarst in this region.

3-D Geoelectrical Characterisation of the Central Volcanoes of São Miguel Island (Azores Archipelago, Portugal) using Broad-Band Magnetotelluric Data

2020 ◽  
Author(s):  
Duygu Kiyan ◽  
Colin Hogg ◽  
Volker Rath ◽  
Andreas Junge ◽  
Rita Carmo ◽  
...  
Keyword(s):  
Fracture Zone ◽  
Hydrothermal System ◽  
High Frequency ◽  
Broad Band ◽  
Hydrothermal Systems ◽  
Volcanic System ◽  
Magnetotelluric Soundings ◽  
Field Campaign ◽  
The North ◽  
Swarm Activity

&lt;p&gt;The Azores islands are located at the triple junction between the North American, Eurasian and African plates. The Mid-Atlantic Ridge separates the North America from Eurasia and African plates, while Azores-Gibraltar Fracture Zone is the boundary between Eurasia and African plates. S&amp;#227;o Miguel Island, situated at the southeastern part of the western segment of the Azores-Gibraltar Fracture Zone, has three active strato-volcanoes, Sete Cidades, Fogo (&amp;#193;gua de Pau), and Furnas. At Furnas and Fogo volcanoes, intense circulation of volcanic fluids at depth leads to high CO&lt;sub&gt;2&lt;/sub&gt; outgassing and flank destabilisation, whereas its neighbour Congro Fissural volcanic system, located between Fogo and Furnas volcanoes, experiences significant seismic swarm activity and poses considerable threat to the local population. Enhanced electrical conductivity values are typically associated with volcanic-hydrothermal systems and the modelled conductivity structures can provide constraints on these volcanic and hydrothermal processes.&lt;/p&gt;&lt;p&gt;Our previous work on Furnas volcano, which yielded a revised conceptual model developed from 39 high-frequency magnetotelluric soundings that imaged the hydrothermal system of the volcano to a depth of 1 km directly beneath the caldera, has now been expanded to include 35 additional broad-band magnetotelluric soundings from a recent field campaign conducted in late 2018, to image deeper and broader to gain new insights into the regional context of the Furnas volcanic system. The resistivity model of Furnas shallow hydrothermal system constructed from high-frequency dataset delineated two enhanced conductive zones, one at 100 m and another at 500 m depth, separated by a resistive layer. The shallow conductor has conductivity less than 1 S/m, which can be explained by clay mineral surface conduction with a mass fraction of at least 20% smectite. The deeper conductor extends across the majority of the survey area and is located at depths where smectite is generally not formed. We interpret this as the result of saline aqueous fluids near the boiling point, inferring temperatures of at least 240 &lt;sup&gt;o&lt;/sup&gt;C. The less conductive layer found between these conductors is interpreted to be steam-dominated and coincides within the mixed-clay zone found in many volcanic hydrothermal systems. 3-D inversions using the deep-probing data indicate continuation of a strong conductive zone towards the south, beneath the 1630 Dome, which represents the most recent phase of eruptive activity in the multi-caldera complex. During the 2018 field campaign, we have enlarged our study to include 50 broad-band soundings on the adjacent Fogo (&amp;#193;gua de Pau) volcano and Congro Fissural volcanic system. The Fogo-Congro region is subjected to seismic swarm activity and its relationship with the geoelectrical structure is being investigated.&lt;/p&gt;

Constraints on Reelfoot Rift Evolution from a Reflection Seismic Profile in the Northern Rift

1992 ◽  
Vol 63 (3) ◽  
pp. 233-241 ◽  
Author(s):  
M.B. Goldhaber ◽  
C.J. Potter ◽  
C.D. Taylor
Keyword(s):  
Seismic Data ◽  
Seismic Reflection ◽  
Seismic Profile ◽  
Fault System ◽  
Seismic Reflection Profile ◽  
The South ◽  
Dome A ◽  
Reflection Seismic ◽  
The North ◽  
Northwestern Margin

Abstract An 82.8 km segment of a northwest-southeast trending seismic-reflection profile across the northernmost part of the Reelfoot rift shows that the Cambrian rift geometry there is quite distinct from that of the main part of Reelfoot rift to the south, and that of the Rough Creek graben to the east. The profile is within the area of intersection of the Reelfoot rift and Rough Creek graben and shows a systematic southeastward thickening of the Cambrian synrift clastic sequence with as much as 1940 meters of section present against the Pennyrile fault system as compared to 970 meters near the Lusk Creek and Shawneetown fault systems, towards the northwestern margin of the rift. This contrasts with the more symmetric rift pattern in the seismically active zone to the south, where the maximum thickness of synrift sediments is along the rift axis, and with an opposite sense of rift asymmetry in the Rough Creek graben, where the synrift sequence thickens to the north against the Rough Creek - Shawneetown fault. Reflection patterns in the vicinity of Hicks dome, a “cryptovolcano”, are consistent with the hypothesis that the dome originated by explosive release of mantle-derived gases associated with alkali volcanism. The seismic data also reveal that the fluorine mineralization in the area is associated with faults that offset basement; this is further evidence that deeply-derived fluids are significant in the geologic evolution of the area.

Seismic response and prediction of Ordovician fractured vuggy carbonate reservoirs in the Tazhong Shunnan well A block

2017 ◽  
Vol 5 (1) ◽  
pp. SB45-SB55 ◽  
Author(s):  
Li Zhou ◽  
Hanming Gu ◽  
Lingwei Ma ◽  
Jiao Xue ◽  
Zongjie Li
Keyword(s):  
High Frequency ◽  
Seismic Reflection ◽  
Random Medium ◽  
Low Frequency ◽  
Fault Zones ◽  
Carbonate Reservoirs ◽  
Medium Theory ◽  
Significant Difference ◽  
Seismic Reflections ◽  
Reflection Characteristics

Ordovician fractured vuggy carbonate reservoirs, which are deeply buried in the Tazhong Shunnan area in China, are characterized by high heterogeneity. Meanwhile, there is no significant difference between the geophysical characteristics of the reservoirs and that of the surrounding rocks. We have introduced the multiscale stack random medium theory and built some theoretical seismic-geologic models for the fractured vuggy carbonate reservoirs. Furthermore, we obtained the seismic reflection characteristics corresponding to these models using finite-difference forward modeling. The small random vugs are characterized by weak and chaotic reflections with high frequency, and the large vugs are characterized by strong and chaotic reflections with low frequency. The amplitude of the seismic reflections increases with the increasing vug density, and it decreases with the increasing roughness factor. Combining the synthetic reflection characteristics corresponding to the fractured vuggy carbonate reservoirs and the actual seismic reflections from the drilled reservoirs, we summarized the recognition patterns of the carbonate reservoirs. The predicted results found that the potential fractured vuggy reservoirs at the top of Yijianfang Formation are located in the southwest and northeast, in the vicinity of fault zones. The reservoirs in Peng-Laiba Formation are distributed in the northwest of the block.

scholarly journals Seismic imaging across fault systems in the Abitibi greenstone belt – An analysis of pre- and post-stack migration approaches in the Chibougamau area, Quebec, Canada

2020 ◽  
Author(s):  
Saeid Cheraghi ◽  
Alireza Malehmir ◽  
Mostafa Naghizadeh ◽  
David Snyder ◽  
Lucie Mathieu ◽  
...  
Keyword(s):  
Seismic Reflection ◽  
Seismic Imaging ◽  
Massive Sulfide ◽  
Geological Structure ◽  
The South ◽  
Element Analysis ◽  
Fault Systems ◽  
The North ◽  
Relative Contribution ◽  
Abitibi Subprovince

Abstract. Two high-resolution seismic reflection profiles acquired north and south of Chibougamau, located in the northeast of the Abitibi subprovince of Canada help understand historic volcanic-hosted massive sulfide (VMS) deposits and hydrothermal Cu-Au mineralization found there. Major faults crossed by the profiles include the Barlow fault in the north and the Doda fault and the Guercheville fault in the south, all targets of this study that seeks to determine spatial relationships with known metal endowment in the area. Common-offset DMO corrections and common-offset pre-stack time migrations (PSTM) were considered. Irregularities of the trace midpoint distribution resulting from the crooked geometry of both profiles and their relative contribution to DMO and PSTM method and seismic illumination were assessed in the context of the complex subsurface architecture of the area. To scrutinize this contribution, seismic images were generated for offset ranges of 0–9 km using increments of 3 km. Migration of out-of-plane reflections used cross-dip element analysis to accurately estimate the fault dip. The seismic imaging shows the thickening of the upper crustal rocks near the fault zones along both profiles. In the north seismic reflection section, key geological structure identified include the Barlow fault and two diffraction sets imaged within the fault zone that represent potential targets for future exploration. The south seismic reflection section shows rather a complicated geometry of two fault systems. The Guercheville fault observed as a subhorizontal reflector connects to a steeply dipping reflector. The Doda fault dips subvertical in the shallow crust but as a steeply dipping reflection set at depth. Nearby gold showings suggest that these faults may help channel and concentrate mineralizing fluids.

SEISMIC STRATIGRAPHIC EVALUATION OF A NEOCOMIAN SUBMARINE FAN SYSTEM, BROWSE BASIN, NORTH WEST SHELF

1992 ◽  
Vol 32 (1) ◽  
pp. 171
Author(s):  
L.R. Miller ◽  
W.J. Stuart
Keyword(s):  
High Frequency ◽  
Seismic Reflection ◽  
Low Frequency ◽  
High Amplitude ◽  
Seismic Facies ◽  
Submarine Fan ◽  
North West ◽  
The North ◽  
Braided Channel ◽  
Low Amplitude

A possible submarine fan system of Valanginian age occurs in the south of the western half of Permit WA-212-P in the Browse Basin. Seismic mapping and interpretation have allowed the recognition of five seismic facies which are considered representative of this fan system.The five seismic facies are the upper-middle fan braided channel facies, the upper-middle fan braided interchannel facies, the lower fan channel facies, the lower fan sheet facies, and the lower fan lobe fringe facies. The reflections of the upper-middle braided channel fan facies are discontinuous, disrupted, convex up, low amplitude and high frequency. The interchannel facies has reflections that are concave up, continuous, low frequency and moderate to high amplitude. The lower fan channel facies are recognised by convex up, discontinuous, high frequency and low amplitude reflections. The lower fan sheet facies is noted by mounded configurations with continuous, moderate to high amplitude, moderate frequency reflections. The lower fan lobe fringe facies reflections are flat, often shingled reflections with moderate discontinuity, moderate to high amplitude and low to moderate frequency.Since no wells penetrate the submarine fan, the interpretation is based on seismic reflection configurations which are considered typical of submarine fan segments. The interpreted ancient submarine fan occurs on the basin floor adjacent to a probable ramp type margin, and manifests shape and setting consistent with known submarine fans, such as the Eocene Frigg Fan of the North Sea, and the Lower Cretaceous Barrow Group turbidites in the Carnarvon Basin.Seismic facies mapping, in conjunction with sequence stratigraphy concepts, is particularly useful in areas such as the Browse Basin where considerable marine shale sections exist with little structure, and sequences with reservoir potential continue to be a problem to locate. For instance, in Caswell-1, a well drilled in 1977 immediately north of the permit area, 200 barrels of oil flowed from thin sands within a shale sequence of Albian age. Results of this study indicate that local seismic reflection signatures may be indicative of potential sandstone reservoirs in the vicinity of the study area.

scholarly journals Seismic imaging across fault systems in the Abitibi greenstone belt – an analysis of pre- and post-stack migration approaches in the Chibougamau area, Quebec, Canada

Solid Earth ◽  
2021 ◽  
Vol 12 (5) ◽  
pp. 1143-1164
Author(s):  
Saeid Cheraghi ◽  
Alireza Malehmir ◽  
Mostafa Naghizadeh ◽  
David Snyder ◽  
Lucie Mathieu ◽  
...  
Keyword(s):  
Seismic Reflection ◽  
Seismic Imaging ◽  
Massive Sulfide ◽  
The South ◽  
Element Analysis ◽  
Fault Systems ◽  
The North ◽  
Relative Contribution ◽  
Abitibi Subprovince ◽  
Seismic Reflection Profiles

Abstract. Two high-resolution seismic reflection profiles acquired north and south of Chibougamau, located in the northeast of the Abitibi subprovince of Canada, help understand historic volcanically hosted massive sulfide (VMS) deposits and hydrothermal Cu–Au mineralization found there. Major faults crossed by the profiles include the Barlow fault in the north and the Doda fault and the Guercheville fault in the south, all targets of this study that seeks to determine spatial relationships with a known metal endowment in the area. Common-offset DMO corrections and common-offset pre-stack time migrations (PSTMs) were considered. Irregularities of the trace midpoint distribution resulting from the crooked geometry of both profiles and their relative contribution to the DMO and PSTM methods and seismic illumination were assessed in the context of the complex subsurface architecture of the area. To scrutinize this contribution, seismic images were generated for offset ranges of 0–9 km using increments of 3 km. Migration of out-of-plane reflections used cross-dip element analysis to accurately estimate the fault dip. The seismic imaging shows the thickening of the upper-crustal rocks near the fault zones along both profiles. In the northern seismic reflection section, the key geological structures identified include the Barlow fault and two diffraction sets imaged within the fault zone that represent potential targets for future exploration. The south seismic reflection section shows rather a complicated geometry of two fault systems. The Guercheville fault observed as a subhorizontal reflector connects to a steeply dipping reflector. The Doda fault dips subvertical in the shallow crust but as a steeply dipping reflection set at depth. Nearby gold showings suggest that these faults may help channel and concentrate mineralizing fluids.

scholarly journals Results from a geological reconnaissance around Svartenhuk Halvø, West Greenland

1973 ◽  
Vol 50 ◽  
pp. 1-26
Author(s):  
V Münther
Keyword(s):  
Fault Zones ◽  
The South ◽  
Large Area ◽  
The West ◽  
North East ◽  
The North ◽  
South West ◽  
Early Tertiary ◽  
Lateral Extent ◽  
Tectonic Movements

Svartenhuk Halvø is built up primarily of Tertiary basalts; these overlie Cretaceous and early Tertiary sediments, and overlap onto the Precambrian basement. The basalt series can be divided into a lower and an upper series; although displaced by faults, the boundary between these series can be followed across the peninsula. The thickness of the lower basalt series is estimated to be about 2-3 km in the south of the peninsula and barely 1 km in the north; the sub-aquatic basalt breccia is included in these thicknesses. FauIts causing repetitions of the lava succession have resulted in the series being preserved over a rather large area. The general dip of the lavas is 3-4 dregrees towards SW in the east and 8-10 degrees, also towards SW, in the west. Locally dips between 10 and 20 degrees or even steeper are seen; these are the resulf of drag along fault zones in Arfertuarssuk fjord and Kugssineq valley, and between Svartenhuk Halvø and Ubekendt Ejland. The youngest fault has a displacement of 500 m or more and has downthrown the basement area to the north-east in relation to the sediment-basalt breccia-basalt series to the south-west. The upper basalt series has by far the greater lateral extent and covers the gneiss and metasediment area to the north and north-east at least as far as the Inland lee. The dip of the flows in this part of the basalt series is considerably lower than in much of the lower basalt series, but faults repeating the succession are also frequently encountered within the upper basalts. The tectonic movements evidence a strong E-W (or NE-SW) tension, never a compression; the weak anticlinal and synclinal structures which are seen are interpreted as resulfing from differential sagging. The lower basalt series is thought to have arisen from fissure eruptions, with the main area of eruption in the east. The lavas are very rich in olivine (i. e. are pieritic). The upper basalt series probably arose from central eruptions and smaller fissure eruptions, and the area of eruption is thought to have shifted to the west. The upper lavas become poorer in olivine; andesitic lavas represent perhaps a closing phase, more local in its distribution and perhaps resulting from magmatic assimilation of pre-basaltic sediments. "Iron basalt" and intrabasaltic breccia have not been noted on Svartenhuk Halvø.

scholarly journals Application de la géophysique (méthode géoélectrique) à la reconnaissance du plateau de Meknès (Bassin de Saïss), Maroc [Application of geophysical prospecting (geoelectrical method) for the hydrogeological reconnaissance of the Meknes Plateau (Saiss basin), Morocco]

2001 ◽  
Vol 3 (1-2) ◽  
pp. 35-53
Author(s):  
Ali ESSAHLAOUI ◽  
Hassane SAHBI ◽  
Nacer EL YAMINE
Keyword(s):  
Global Structure ◽  
Mountain Range ◽  
Quantitative Interpretation ◽  
Geophysical Prospecting ◽  
The South ◽  
Borehole Data ◽  
The North ◽  
Geophysical Study ◽  
Electrical Soundings ◽  
Middle Atlas

A geophysical study by the geoelectrical method using electrical soundings, has been carried out in the south-western and southern parts of the Meknes plateau which belongs to the Saïss basin located between the Rif mountain range to the North and the Middle Atlas range to the South. Two significant aquifers are found in the Meknes Plateau: the first one is located in the shallow Plio-Quaternary formations while the second one is inside the deep carbonate beds of the Lias. The quantitative interpretation of electrical soundings from both surface and borehole data resulted in isoresistivity, isopach and longitudinal isoconductance maps that have both a quantitative and a qualitative interest for a better understanding of the global structure of the hydrogeological basin of Saiss.

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