During geomagnetic disturbances, electric fields induced in the Earth and in power systems, pipelines and submarine cables can interfere with the operation of these systems. Calculations for submarine cables are compl...During geomagnetic disturbances, electric fields induced in the Earth and in power systems, pipelines and submarine cables can interfere with the operation of these systems. Calculations for submarine cables are complicated by the need to consider not just the induction directly into the cable but also the earth potentials produced at the coast at each end of the cable. To determine the coast potentials, we present a new model of the ocean and earth conductivity structure that spans the whole length of a cable from one coast to another. Calculations are based on the generalised thin sheet approach introduced by Ranganayaki and Madden but converted to a transmission line model that can be solved using standard circuit theory techniques. It is shown how the transmission line model can be used to calculate the earth potential profile from one side of an ocean or sea to the other. Example calculations are presented for a shallow sea, a shallow ocean, and a deep ocean that are simplified approximations to the North Sea, Tasman Sea and Pacific Ocean and show that the peak potentials occur at the coast. An examination is also made of how the width of a shallow sea and the width of the continental shelf affect these coast potentials. The modelling technique and example results provide a guide for more detailed modelling of geomagnetic induction along the routes of specific submarine cables.展开更多
Quasi-integrity of continental crust between Mid-Archaean and Ediacaran times is demonstrated by conformity of palaeomagnetic poles to near-static positions between -2.7-2.2 Ga, -1.5-1.2 Ga and -0.75-0.6 Ga. Interveni...Quasi-integrity of continental crust between Mid-Archaean and Ediacaran times is demonstrated by conformity of palaeomagnetic poles to near-static positions between -2.7-2.2 Ga, -1.5-1.2 Ga and -0.75-0.6 Ga. Intervening data accord to coherent APW loops turning at ＂hairpins＂ focused near a continental-centric location. Although peripheral adjustments occurred during Early Proterozoic （-2.2 Ga） and Grenville （- 1.1 Ga） times, the crust retained a low order symmetrical crescent-shaped form constrained to a single global hemisphere until break-up in Ediacaran times. Conformity of palaeomagnetic data to specific Eulerian parameters enables definition of a master Precambrian APW path used to estimate the root mean square velocity （VRMS） of continental crust between 2.8 and 0.6 Ga. A long interval of little polar movement between -2.7 and 2.2 Ga correlates with global magmatic shutdown between -2.45 and 2.2 Ga, whilst this interval and later slowdown at -0.75-0.6 Ga to velocities of 〈2 cm/year correlate with episodes of widespread glaciation implying that these prolonged climatic anomalies had an internal origin; the reduced input of volcanically-derived atmospheric greenhouse gases is inferred to have permitted freeze-over conditions with active ice sheets extending into equatorial latitudes as established by low magnetic inclinations in glaciogenic deposits. VRMS vari- ations through Precambrian times correspond to the distribution of U-Pb ages in orogenic granitoids and detrital zircons and demonstrate that mobility of continental crust has been closely related to crustal tectonism and incrementation. Both periods of near-stillstand were followed by rapid VRMS recording massive heat release from beneath the continental lid at -2.2 and 0.6 Ga. The first coincided with the Lomagundi-Jatuli isotopic event and led to prolonged orogenesis accompanied by continental flooding and reconfiguration of the crust on the Earth＇s surface; the second led to continental break-up and instigated the comprehensive Plate Tectonics that has characterised Phanerozoic times. The Meso- proterozoic interval characterised by anorogenic magmatism correlates with low VRMS between - 1.5 and 1.1 Ga. Insulation of the sub-continental mantle evidently permitted high temperature melting and weakening of the crustal lid to enable buoyant emplacement of large plutons at high crustal levels during this magmatic event unique to Mesoproterozoic and early Neoproterozoic times.展开更多
Although Plate Tectonics cannot be effectively tested by palaeomagnetism in the Precambrian aeon due to the paucity of high precision poles spanning such a long time period,the possibility of Lid Tectonics is eminentl...Although Plate Tectonics cannot be effectively tested by palaeomagnetism in the Precambrian aeon due to the paucity of high precision poles spanning such a long time period,the possibility of Lid Tectonics is eminently testable because it seeks accordance of the wider dataset over prolonged intervals of time;deficiencies and complexities in the data merely contribute to dispersion.Accordance of palaeomagnetic poles across a quasi-integral continental crust for time periods of up to thousands of millions of years,together with recognition of very long intervals characterised by minimal polar motions(～2.6-2.0,～1.5-1.25 and～0.75-0.6 Ga)has been used to demonstrate that Lid Tectonics dominated this aeon.The new PALEOMAGIA database is used to refine a model for the Precambrian lid incorporating a large quasiintegral crescentric core running from South-Central Africa through Laurentia to Siberia with peripheral cratons subject to reorganisation at～2.1,～1.6 and～1.1 Ga.The model explains low levels of tidal friction,reduced heat balance,unique petrologic and isotopic signatures,and the prolonged crustal stability of Earth's"Middle Age",whilst density concentrations of the palaeomagnetic poles show that the centre of the continental lid was persistently focussed near Earth's rotation axis from～2.8 to 0.6 Ga.The exception was the～2.7-2.2 Ga interval defined by～90°polar movements which translated the periphery of the lid to the rotation pole for this quasi-static period,a time characterised by glaciation and low levels of magmatic activity;the～2.7 Ga shift correlates with key interval of mid-Archaean crustal growth to some 60-70%of the present volume and REE signatures whilst the～2.2 Ga shift correlates with the Lomagundiδ～(13)C and Great Oxygenation events.The palaeomagnetic signature of breakup of the lid at～0.6 Ga is recorded by the world-wide Ediacaran development of passive margins and associated environmental signatures of new ocean basins.This event defined the end of a dominant Lid Tectonic phase in the history of Earth's continental lithosphere recorded by the quasi-integral Precambrian supercontinent Palaeopangaea and the beginning of the comprehensive Plate Tectonics which has characterised the Phanerozoic aeon.Peripheral modifications to the lid achieved a symmetrical and hemispheric shape in Neoproterozoic times comparable to the familiar short-lived supercontinent(Neo)Pangaea(～350-150 Ma)and this appears to be the sole supercontinent cycle recorded by the palaeomagnetic record.Prolonged integrity of a large continental nucleus accompanied by periodic readjustments of peripheral shields can reconcile divergent tectonic analyses of Precambrian times which on the one hand propose multiple Wilson Cycles to explain some signatures of Plate Tectonics,and alternative interpretations which consider that Plate Tectonics did not commence until the end of the Neoproterozoic.展开更多
Occurrence of mafic dykes in Himalaya has been intriguing and debated since long because of its difficulty to ascent and emplacement through a thickened crust.Mafic dykes in Kumaun Lesser Himalaya(KLH)of central Indian
Broad-band and long-period magnetotelluric(MT)data were acquired along an east-west trending traverse of nearly 200 km across the Kachchh,Cambay rift basins,and Aravalli-Delhi fold belt(ADFB),western India.The regiona...Broad-band and long-period magnetotelluric(MT)data were acquired along an east-west trending traverse of nearly 200 km across the Kachchh,Cambay rift basins,and Aravalli-Delhi fold belt(ADFB),western India.The regional strike analysis of MT data indicated an approximate N59°E geoelectric strike direction under the traverse and it is in fair agreement with the predominant geological strike in the study area.The decomposed transverse electric(TE)-and transverse magnetic(TM)-data modes were inverted using a nonlinear conjugate gradient algorithm to image the electrical lithospheric structure across the Cambay rift basin and its surrounding regions.These studies show a thick(~1-5 km)layer of conductive Tertiary-Mesozoic sediments beneath the Kachchh and Cambay rift basins.The resistive blocks indicate presence of basic/ultrabasic volcanic intrusives,depleted mantle lithosphere,and different Precambrian structural units.The crustal conductor delineated within the ADFB indicates the presence of fluids within the fault zones,sulfide mineralization within polyphase metamorphic rocks,and/or Aravalli-Delhi sediments/metasediments.The observed conductive anomalies beneath the Cambay rift basin indicate the presence of basaltic underplating,volatile(CO2,H2 O)enriched melts and channelization of melt fractions/fluids into crustal depths that occurred due to plume-lithosphere interactions.The variations in electrical resistivity observed across the profile indicate that the impact of Reunion plume on lithospheric structures of the Cambay rift basin is more dominant at western continental margin of India(WCMI)and thus support the hypothesis proposed by Campbell&Griffiths about the plume-lithosphere interactions.展开更多
One of the modern applications of geomagnetism is determining the effect of geomagnetic disturbances on critical infrastructure such as power systems and pipelines. Assessing the geomagnetic hazard to such systems req...One of the modern applications of geomagnetism is determining the effect of geomagnetic disturbances on critical infrastructure such as power systems and pipelines. Assessing the geomagnetic hazard to such systems requires calculation of the geoelectric fields produced during geomagnetic disturbances. Such geoelectric fields can then be used as input to system models to calculate the impact on the system. This paper describes what is involved in calculating the geoelectric fields produced during real geomagnetic disturbances. The theory of geomagnetic induction is presented and used to derive the Earth transfer function relating the geoelectric and geomagnetic field variations at the Earth’s surface. It is then shown how this can be used to make practical calculations of the geoelectric fields and how the calculation process can be verified by comparison with analytic solutions obtained with synthetic geomagnetic variation data. The accuracy of the calculated geoelectric fields for geomagnetic risk assessments is limited, not by the accuracy of the calculation methods, but by the availability of geomagnetic field measurements and Earth conductivity information over the whole extent of the affected infrastructure.展开更多
The Singhbhum craton of the eastern Indian shield consists of two major crustal provinces viz.,Chotanagpur Gneissic Complex(CGC)and Singhbhum Granite Complex;separated by a Singhbhum Mobile Belt.There
Doppler velocities observed by the Rankin Inlet （RKN） PolarDARN radar are assessed with a focus on data in the beams oriented roughly along the magnetic meridian. Hourly scatter plots for every month are built. They...Doppler velocities observed by the Rankin Inlet （RKN） PolarDARN radar are assessed with a focus on data in the beams oriented roughly along the magnetic meridian. Hourly scatter plots for every month are built. They are shown to vary widely, with median values showing very clear magnetic local time variation with maximum magnitude during pre-noon and pre-midnight hours. The histograms contain a significant amount of very small velocity data that dominates at farther ranges and during the daytime. Near noon data show generally antisunward flows but at large ranges/magnetic latitudes and very close to noon, sunward flows occur for periods of positive IMF Bz. The reverse flows are stronger during spring equinox. The velocity magnitude was found to depend linearly on the IMF Be and interplanetary electric field. Velocities are often found to be smaller than those expected from the statistical convection model of Ruohoniemi and Greenwald -1996.展开更多
The variety of definitions of Fourier transforms can create confusion for practical applications. This paper examines the choice of formulas for Fourier transforms and determines the appropriate choices for geoscience...The variety of definitions of Fourier transforms can create confusion for practical applications. This paper examines the choice of formulas for Fourier transforms and determines the appropriate choices for geoscience applications. One set of Discrete Fourier Transforms can be defined that approximate Fourier integrals and provide transforms between sampled continuous functions in both domains. For applications involving transforms between a continuous function and a discrete function a second set of Discrete Fourier Transforms is needed with different scaling factors. Two classes of application are presented: those where either form of transforms can be used and those where it is necessary to use a particular transform to obtain the correct results.展开更多
One of the most important parameters in meteorology is the mean wind profile in the tropical cyclone boundary layer.The vertical profile of wind speed and wind direction were measured during the period of the Nisarga cy...One of the most important parameters in meteorology is the mean wind profile in the tropical cyclone boundary layer.The vertical profile of wind speed and wind direction were measured during the period of the Nisarga cyclone from May 31st,2020,to June 5th,2020,using the newly installed Phased Array Doppler Sodar system at the Center for Space and Atmospheric Science(CSAS),Sanjay Ghodawat University,Kolhapur(16.74◦N,74.37◦E;near India's western coast).Our analysis revealed that the maximum mean wind speed was 17 m/s on June 3,2020,at 10:00 IST.It also shows the change in wind direction from southwest to southeast on June 2 and 3,2020.Daily high-resolution reanalysis data in the domain,0–25◦N,65–110◦E,during the period from May 31st to June 5th,2020,revealed the variation of the atmospheric pressure of the Nisarga cyclone from 1000 to 1008 hPa,sea surface temperature(SST)between 30◦C and 31◦C,outgoing longwave radiation(OLR)between 100 and 240 Wm-2,wind speed between 3 and 15 m/s,and low values of vertical wind shear(VWS)were observed to the north of Nisarga track.These observations may provide more insights for the study of boundary layer turbulence during cyclonic activities.展开更多
Two Red Clay profiles near Xi' an and Xifeng were investigated in an attempt to determine magne-tostratigraphic and palaeoclimatic records. The results show that aeolian dust accumulation and the related East Asia...Two Red Clay profiles near Xi' an and Xifeng were investigated in an attempt to determine magne-tostratigraphic and palaeoclimatic records. The results show that aeolian dust accumulation and the related East Asia palaeomonsoon system had begun by 6.5 Ma, and it is deduced that the Tibetan Plateau had reached a significant elevation at that time. The late Tertiary palaeoclimatic history of the Red Clay as reflected by magnetic susceptibility is reconstructed during the period of 6.5-2.5 Ma. Stepwise increase in susceptibility of aeolian dust accumulation appears to have a close correlation to the uplift processes of the Tibetan Plateau. The remarkable increase of aeolian dust accumulation at 3.2 Ma appears to be due to the influence of global ice volume on the East Asia monsoon. Palaeomonsoon variation during the late Tertiary as recorded in the Red Clay sequences from the Chinese Loess Plateau can be regarded as the product of a number of interacting factors, such as uplift of the Tibetan Plateau, solar radiation, global ice volume, etc.展开更多
The present winter climate of continental China is associated with cold air activity.The cold air forming Siberian high pressure comes from the Northern Barents Sea.Northwestern Kara Sea and Northern Atlantic Ocean (f...The present winter climate of continental China is associated with cold air activity.The cold air forming Siberian high pressure comes from the Northern Barents Sea.Northwestern Kara Sea and Northern Atlantic Ocean (fig.1). in other words, mainly fromthe cold northern sea surface. west and east to Novaya Zemlya and the sea surface south展开更多
Rock-magnetic and palaeomagnetic studies have been carried out on the interval of famous fossil-bearing sedimentary rocks and its overlying basalts and underlying basalts at the Sihetun section, West Liaoning Province...Rock-magnetic and palaeomagnetic studies have been carried out on the interval of famous fossil-bearing sedimentary rocks and its overlying basalts and underlying basalts at the Sihetun section, West Liaoning Province. Normal polarity was obtained for the sedimentary interval and the underlying basalts, while reversed polarity was found in the overlying basalts. Taking account of the new 40Ar/39Ar ages (Swisher et al., 1999), we classified the fossil-bearing sedimentary interval into the Barremian M3n zone (Early Cretaceous age). Several abnormal horizons on magnetic properties, probably corresponding to the tuffs, were observed in the fossil-bearing sedimentary interval. This implies that the massive bio-extinction may link to dramatic environmental changes that were caused by volcanic eruptions.展开更多
Coupled paleomagnetic and geochronologic data derived from mafic dykes provide valuable records of continental movement.To reconstruct the Proterozoic paleogeographic history of Peninsular India,we report paleomagneti...Coupled paleomagnetic and geochronologic data derived from mafic dykes provide valuable records of continental movement.To reconstruct the Proterozoic paleogeographic history of Peninsular India,we report paleomagnetic directions and U-Pb zircon ages from twenty-nine mafic dykes in the Eastern Dharwar Craton near Hyderabad.Paleomagnetic analysis yielded clusters of directional data that correspond to dyke swarms at 2.37 Ga,2.22 Ga,2.08 Ga,1.89-1.86 Ga,1.79 Ga,and a previously undated dual polarity magnetization.We report new positive baked contact tests for the 2.08 Ga swarm and the 1.89-1.86 Ga swarm(s),and a new inverse baked contact test for the 2.08 Ga swarm.Our results promote the 2.08 Ga Dharwar Craton paleomagnetic pole(43.1°N,184.5°E;A95=4.3°)to a reliability score of R=7 and suggest a position for the Dharwar Craton at 1.79 Ga based on a virtual geomagnetic pole(VGP)at 33.0°N,347.5°E(a95=16.9°,k=221,N=2).The new VGP for the Dharwar Craton provides support for the union of the Dharwar,Singhbhum,and Bastar Cratons in the Southern India Block by at least 1.79 Ga.Combined new and published northeast-southwest moderate-steep dual polarity directions from Dharwar Craton dykes define a new paleomagnetic pole at 20.6°N,233.1°E(A95=9.2°,N=18;R=5).Two dykes from this group yielded 1.05-1.01 Ga 207Pb/206Pb zircon ages and this range is taken as the age of the new paleomagnetic pole.A comparison of the previously published poles with our new 1.05-1.01 Ga pole shows India shifting from equatorial to higher(southerly)latitudes from 1.08 Ga to 1.01 Ga as a component of Rodinia.展开更多
A backscatter model was developed for measur-ing wind field with the far-field approximation method.The theoretical computation and computer simulations with one spatial dimension show that this model can realisticall...A backscatter model was developed for measur-ing wind field with the far-field approximation method.The theoretical computation and computer simulations with one spatial dimension show that this model can realistically describe the physical meaning and process of the three methods in wind measurements including the spaced antenna(SA)method,Doppler beam swing(DBS)method,and spaced interferometry(SI).The computational difficulties of the traditional theoretical model cannot only be smoothed away,but common characteristics and differences of the three methods can be compared deeply.The comparison of the numerical results between the Wuhan medium frequency(MF)radar(30°N,114°E)observation and the computer simulation of the full correlation analysis(FCA)of the SA method indicates that the two results agree very well and this model has practical application.展开更多
文摘During geomagnetic disturbances, electric fields induced in the Earth and in power systems, pipelines and submarine cables can interfere with the operation of these systems. Calculations for submarine cables are complicated by the need to consider not just the induction directly into the cable but also the earth potentials produced at the coast at each end of the cable. To determine the coast potentials, we present a new model of the ocean and earth conductivity structure that spans the whole length of a cable from one coast to another. Calculations are based on the generalised thin sheet approach introduced by Ranganayaki and Madden but converted to a transmission line model that can be solved using standard circuit theory techniques. It is shown how the transmission line model can be used to calculate the earth potential profile from one side of an ocean or sea to the other. Example calculations are presented for a shallow sea, a shallow ocean, and a deep ocean that are simplified approximations to the North Sea, Tasman Sea and Pacific Ocean and show that the peak potentials occur at the coast. An examination is also made of how the width of a shallow sea and the width of the continental shelf affect these coast potentials. The modelling technique and example results provide a guide for more detailed modelling of geomagnetic induction along the routes of specific submarine cables.
文摘Quasi-integrity of continental crust between Mid-Archaean and Ediacaran times is demonstrated by conformity of palaeomagnetic poles to near-static positions between -2.7-2.2 Ga, -1.5-1.2 Ga and -0.75-0.6 Ga. Intervening data accord to coherent APW loops turning at ＂hairpins＂ focused near a continental-centric location. Although peripheral adjustments occurred during Early Proterozoic （-2.2 Ga） and Grenville （- 1.1 Ga） times, the crust retained a low order symmetrical crescent-shaped form constrained to a single global hemisphere until break-up in Ediacaran times. Conformity of palaeomagnetic data to specific Eulerian parameters enables definition of a master Precambrian APW path used to estimate the root mean square velocity （VRMS） of continental crust between 2.8 and 0.6 Ga. A long interval of little polar movement between -2.7 and 2.2 Ga correlates with global magmatic shutdown between -2.45 and 2.2 Ga, whilst this interval and later slowdown at -0.75-0.6 Ga to velocities of 〈2 cm/year correlate with episodes of widespread glaciation implying that these prolonged climatic anomalies had an internal origin; the reduced input of volcanically-derived atmospheric greenhouse gases is inferred to have permitted freeze-over conditions with active ice sheets extending into equatorial latitudes as established by low magnetic inclinations in glaciogenic deposits. VRMS vari- ations through Precambrian times correspond to the distribution of U-Pb ages in orogenic granitoids and detrital zircons and demonstrate that mobility of continental crust has been closely related to crustal tectonism and incrementation. Both periods of near-stillstand were followed by rapid VRMS recording massive heat release from beneath the continental lid at -2.2 and 0.6 Ga. The first coincided with the Lomagundi-Jatuli isotopic event and led to prolonged orogenesis accompanied by continental flooding and reconfiguration of the crust on the Earth＇s surface; the second led to continental break-up and instigated the comprehensive Plate Tectonics that has characterised Phanerozoic times. The Meso- proterozoic interval characterised by anorogenic magmatism correlates with low VRMS between - 1.5 and 1.1 Ga. Insulation of the sub-continental mantle evidently permitted high temperature melting and weakening of the crustal lid to enable buoyant emplacement of large plutons at high crustal levels during this magmatic event unique to Mesoproterozoic and early Neoproterozoic times.
文摘Although Plate Tectonics cannot be effectively tested by palaeomagnetism in the Precambrian aeon due to the paucity of high precision poles spanning such a long time period,the possibility of Lid Tectonics is eminently testable because it seeks accordance of the wider dataset over prolonged intervals of time;deficiencies and complexities in the data merely contribute to dispersion.Accordance of palaeomagnetic poles across a quasi-integral continental crust for time periods of up to thousands of millions of years,together with recognition of very long intervals characterised by minimal polar motions(～2.6-2.0,～1.5-1.25 and～0.75-0.6 Ga)has been used to demonstrate that Lid Tectonics dominated this aeon.The new PALEOMAGIA database is used to refine a model for the Precambrian lid incorporating a large quasiintegral crescentric core running from South-Central Africa through Laurentia to Siberia with peripheral cratons subject to reorganisation at～2.1,～1.6 and～1.1 Ga.The model explains low levels of tidal friction,reduced heat balance,unique petrologic and isotopic signatures,and the prolonged crustal stability of Earth's"Middle Age",whilst density concentrations of the palaeomagnetic poles show that the centre of the continental lid was persistently focussed near Earth's rotation axis from～2.8 to 0.6 Ga.The exception was the～2.7-2.2 Ga interval defined by～90°polar movements which translated the periphery of the lid to the rotation pole for this quasi-static period,a time characterised by glaciation and low levels of magmatic activity;the～2.7 Ga shift correlates with key interval of mid-Archaean crustal growth to some 60-70%of the present volume and REE signatures whilst the～2.2 Ga shift correlates with the Lomagundiδ～(13)C and Great Oxygenation events.The palaeomagnetic signature of breakup of the lid at～0.6 Ga is recorded by the world-wide Ediacaran development of passive margins and associated environmental signatures of new ocean basins.This event defined the end of a dominant Lid Tectonic phase in the history of Earth's continental lithosphere recorded by the quasi-integral Precambrian supercontinent Palaeopangaea and the beginning of the comprehensive Plate Tectonics which has characterised the Phanerozoic aeon.Peripheral modifications to the lid achieved a symmetrical and hemispheric shape in Neoproterozoic times comparable to the familiar short-lived supercontinent(Neo)Pangaea(～350-150 Ma)and this appears to be the sole supercontinent cycle recorded by the palaeomagnetic record.Prolonged integrity of a large continental nucleus accompanied by periodic readjustments of peripheral shields can reconcile divergent tectonic analyses of Precambrian times which on the one hand propose multiple Wilson Cycles to explain some signatures of Plate Tectonics,and alternative interpretations which consider that Plate Tectonics did not commence until the end of the Neoproterozoic.
基金supported under a DST (ESS/16/290/2006) grant sanctioned to Santosh Kumar
文摘Occurrence of mafic dykes in Himalaya has been intriguing and debated since long because of its difficulty to ascent and emplacement through a thickened crust.Mafic dykes in Kumaun Lesser Himalaya(KLH)of central Indian
基金the MoES,Govt.of India for funds received through the project(MoES/P.O(Seismo)/1(130)/2011)。
文摘Broad-band and long-period magnetotelluric(MT)data were acquired along an east-west trending traverse of nearly 200 km across the Kachchh,Cambay rift basins,and Aravalli-Delhi fold belt(ADFB),western India.The regional strike analysis of MT data indicated an approximate N59°E geoelectric strike direction under the traverse and it is in fair agreement with the predominant geological strike in the study area.The decomposed transverse electric(TE)-and transverse magnetic(TM)-data modes were inverted using a nonlinear conjugate gradient algorithm to image the electrical lithospheric structure across the Cambay rift basin and its surrounding regions.These studies show a thick(~1-5 km)layer of conductive Tertiary-Mesozoic sediments beneath the Kachchh and Cambay rift basins.The resistive blocks indicate presence of basic/ultrabasic volcanic intrusives,depleted mantle lithosphere,and different Precambrian structural units.The crustal conductor delineated within the ADFB indicates the presence of fluids within the fault zones,sulfide mineralization within polyphase metamorphic rocks,and/or Aravalli-Delhi sediments/metasediments.The observed conductive anomalies beneath the Cambay rift basin indicate the presence of basaltic underplating,volatile(CO2,H2 O)enriched melts and channelization of melt fractions/fluids into crustal depths that occurred due to plume-lithosphere interactions.The variations in electrical resistivity observed across the profile indicate that the impact of Reunion plume on lithospheric structures of the Cambay rift basin is more dominant at western continental margin of India(WCMI)and thus support the hypothesis proposed by Campbell&Griffiths about the plume-lithosphere interactions.
文摘One of the modern applications of geomagnetism is determining the effect of geomagnetic disturbances on critical infrastructure such as power systems and pipelines. Assessing the geomagnetic hazard to such systems requires calculation of the geoelectric fields produced during geomagnetic disturbances. Such geoelectric fields can then be used as input to system models to calculate the impact on the system. This paper describes what is involved in calculating the geoelectric fields produced during real geomagnetic disturbances. The theory of geomagnetic induction is presented and used to derive the Earth transfer function relating the geoelectric and geomagnetic field variations at the Earth’s surface. It is then shown how this can be used to make practical calculations of the geoelectric fields and how the calculation process can be verified by comparison with analytic solutions obtained with synthetic geomagnetic variation data. The accuracy of the calculated geoelectric fields for geomagnetic risk assessments is limited, not by the accuracy of the calculation methods, but by the availability of geomagnetic field measurements and Earth conductivity information over the whole extent of the affected infrastructure.
文摘The Singhbhum craton of the eastern Indian shield consists of two major crustal provinces viz.,Chotanagpur Gneissic Complex(CGC)and Singhbhum Granite Complex;separated by a Singhbhum Mobile Belt.There
基金supported by NSERC Discovery grant to AVK and the University of Saskatchewan graduate stipend to MG
文摘Doppler velocities observed by the Rankin Inlet （RKN） PolarDARN radar are assessed with a focus on data in the beams oriented roughly along the magnetic meridian. Hourly scatter plots for every month are built. They are shown to vary widely, with median values showing very clear magnetic local time variation with maximum magnitude during pre-noon and pre-midnight hours. The histograms contain a significant amount of very small velocity data that dominates at farther ranges and during the daytime. Near noon data show generally antisunward flows but at large ranges/magnetic latitudes and very close to noon, sunward flows occur for periods of positive IMF Bz. The reverse flows are stronger during spring equinox. The velocity magnitude was found to depend linearly on the IMF Be and interplanetary electric field. Velocities are often found to be smaller than those expected from the statistical convection model of Ruohoniemi and Greenwald -1996.
文摘The variety of definitions of Fourier transforms can create confusion for practical applications. This paper examines the choice of formulas for Fourier transforms and determines the appropriate choices for geoscience applications. One set of Discrete Fourier Transforms can be defined that approximate Fourier integrals and provide transforms between sampled continuous functions in both domains. For applications involving transforms between a continuous function and a discrete function a second set of Discrete Fourier Transforms is needed with different scaling factors. Two classes of application are presented: those where either form of transforms can be used and those where it is necessary to use a particular transform to obtain the correct results.
文摘One of the most important parameters in meteorology is the mean wind profile in the tropical cyclone boundary layer.The vertical profile of wind speed and wind direction were measured during the period of the Nisarga cyclone from May 31st,2020,to June 5th,2020,using the newly installed Phased Array Doppler Sodar system at the Center for Space and Atmospheric Science(CSAS),Sanjay Ghodawat University,Kolhapur(16.74◦N,74.37◦E;near India's western coast).Our analysis revealed that the maximum mean wind speed was 17 m/s on June 3,2020,at 10:00 IST.It also shows the change in wind direction from southwest to southeast on June 2 and 3,2020.Daily high-resolution reanalysis data in the domain,0–25◦N,65–110◦E,during the period from May 31st to June 5th,2020,revealed the variation of the atmospheric pressure of the Nisarga cyclone from 1000 to 1008 hPa,sea surface temperature(SST)between 30◦C and 31◦C,outgoing longwave radiation(OLR)between 100 and 240 Wm-2,wind speed between 3 and 15 m/s,and low values of vertical wind shear(VWS)were observed to the north of Nisarga track.These observations may provide more insights for the study of boundary layer turbulence during cyclonic activities.
基金Project supported by the National Natural Science Foundation of China and the Foundation of Xi'an Laboratory of Loess and Quatemary Geology,Chinese Academy of Sciences
文摘Two Red Clay profiles near Xi' an and Xifeng were investigated in an attempt to determine magne-tostratigraphic and palaeoclimatic records. The results show that aeolian dust accumulation and the related East Asia palaeomonsoon system had begun by 6.5 Ma, and it is deduced that the Tibetan Plateau had reached a significant elevation at that time. The late Tertiary palaeoclimatic history of the Red Clay as reflected by magnetic susceptibility is reconstructed during the period of 6.5-2.5 Ma. Stepwise increase in susceptibility of aeolian dust accumulation appears to have a close correlation to the uplift processes of the Tibetan Plateau. The remarkable increase of aeolian dust accumulation at 3.2 Ma appears to be due to the influence of global ice volume on the East Asia monsoon. Palaeomonsoon variation during the late Tertiary as recorded in the Red Clay sequences from the Chinese Loess Plateau can be regarded as the product of a number of interacting factors, such as uplift of the Tibetan Plateau, solar radiation, global ice volume, etc.
基金Project supported by the National Natural Science Foundation of China.
文摘The present winter climate of continental China is associated with cold air activity.The cold air forming Siberian high pressure comes from the Northern Barents Sea.Northwestern Kara Sea and Northern Atlantic Ocean (fig.1). in other words, mainly fromthe cold northern sea surface. west and east to Novaya Zemlya and the sea surface south
基金the National Natural Science Foundation of China (Grant No. 49834001) and the National Key Basic Research Program (Grant No. G1999075509) and the CAS (Grant No. KZ951-A1-401).
文摘Rock-magnetic and palaeomagnetic studies have been carried out on the interval of famous fossil-bearing sedimentary rocks and its overlying basalts and underlying basalts at the Sihetun section, West Liaoning Province. Normal polarity was obtained for the sedimentary interval and the underlying basalts, while reversed polarity was found in the overlying basalts. Taking account of the new 40Ar/39Ar ages (Swisher et al., 1999), we classified the fossil-bearing sedimentary interval into the Barremian M3n zone (Early Cretaceous age). Several abnormal horizons on magnetic properties, probably corresponding to the tuffs, were observed in the fossil-bearing sedimentary interval. This implies that the massive bio-extinction may link to dramatic environmental changes that were caused by volcanic eruptions.
基金This work was supported by the United States of America National Science Foundation grants EAR13-47942 and 602 EAR18-50693 to JGM。
文摘Coupled paleomagnetic and geochronologic data derived from mafic dykes provide valuable records of continental movement.To reconstruct the Proterozoic paleogeographic history of Peninsular India,we report paleomagnetic directions and U-Pb zircon ages from twenty-nine mafic dykes in the Eastern Dharwar Craton near Hyderabad.Paleomagnetic analysis yielded clusters of directional data that correspond to dyke swarms at 2.37 Ga,2.22 Ga,2.08 Ga,1.89-1.86 Ga,1.79 Ga,and a previously undated dual polarity magnetization.We report new positive baked contact tests for the 2.08 Ga swarm and the 1.89-1.86 Ga swarm(s),and a new inverse baked contact test for the 2.08 Ga swarm.Our results promote the 2.08 Ga Dharwar Craton paleomagnetic pole(43.1°N,184.5°E;A95=4.3°)to a reliability score of R=7 and suggest a position for the Dharwar Craton at 1.79 Ga based on a virtual geomagnetic pole(VGP)at 33.0°N,347.5°E(a95=16.9°,k=221,N=2).The new VGP for the Dharwar Craton provides support for the union of the Dharwar,Singhbhum,and Bastar Cratons in the Southern India Block by at least 1.79 Ga.Combined new and published northeast-southwest moderate-steep dual polarity directions from Dharwar Craton dykes define a new paleomagnetic pole at 20.6°N,233.1°E(A95=9.2°,N=18;R=5).Two dykes from this group yielded 1.05-1.01 Ga 207Pb/206Pb zircon ages and this range is taken as the age of the new paleomagnetic pole.A comparison of the previously published poles with our new 1.05-1.01 Ga pole shows India shifting from equatorial to higher(southerly)latitudes from 1.08 Ga to 1.01 Ga as a component of Rodinia.
文摘A backscatter model was developed for measur-ing wind field with the far-field approximation method.The theoretical computation and computer simulations with one spatial dimension show that this model can realistically describe the physical meaning and process of the three methods in wind measurements including the spaced antenna(SA)method,Doppler beam swing(DBS)method,and spaced interferometry(SI).The computational difficulties of the traditional theoretical model cannot only be smoothed away,but common characteristics and differences of the three methods can be compared deeply.The comparison of the numerical results between the Wuhan medium frequency(MF)radar(30°N,114°E)observation and the computer simulation of the full correlation analysis(FCA)of the SA method indicates that the two results agree very well and this model has practical application.
