Journal of geomagnetism and geoelectricity最新文献

{"title":"Data Processing Procedures for Minami-Kayabe Magnetotelluric Soundings","authors":"K. Yamane, S. Takasugi","doi":"10.5636/JGG.49.1697","DOIUrl":"https://doi.org/10.5636/JGG.49.1697","url":null,"abstract":"MT and AMT surveys were conducted in the Minami-Kayabe area, southern Hokkaido, over a period of about two months in the fall of 1988. In this survey, 161 sites were arranged in a square lattice shape. FFT techniques were used in the initial data processing in 1998. Here we describe improved data processing using the cascade decimation technique. Data were processed efficiently by constructing a system using coherency and remote-reference. More than 10 sets of threshold values for the coherencies of (Ex, Hy) and (Ey, Hx) were then determined. Power spectra were sorted by these threshold values and then stacked. Satisfactory results were obtained for the frequency range from 3.4 Hz to 250 Hz. At frequencies below 3.4 Hz, however, the result of cascade decimation was not satisfactory because a sufficient number of stacks could not be obtained due to high local noise and limited measurement time.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"40 4","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133391883","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"On-Land Detecting a Motionally Induced Electric Field: Test Measurements in Northern Finland.","authors":"N. Palshin, P. Kaikkonen, L. Vanyan, J. Tiikkainen, V. H. Rukol","doi":"10.5636/JGG.49.1343","DOIUrl":"https://doi.org/10.5636/JGG.49.1343","url":null,"abstract":"Test measurements of the electric voltage in Kilpisjarvi, northern Finland during September 9-October 12, 1995 were made with the aim of studying the possibility of detecting a motionally induced electric field by means of a horizontal electric dipole 3800 meters in length located at a distance of about 50 km from the nearest fjord of the Norwegian Sea. In addition to the electric voltage we measured at Kilpisjarvi, the water level, magnetic field and meteorological data from Tromso, Norway, the meteorological data from Kilpisjarvi and the magnetic field data from Sodankyla, Finland were used in analysis. Raw data were preprocessed to eliminate non-stationarities (remove outliers) and to remove solar daily variations, and were then smoothed. No significant influence of the local temperature, humidity and rainfall on the smoothed voltages was found. Probably due to a source effect the voltage was not coherent with the magnetic field from Tromso and Sodankyla. The long-period anomaly with the period of about 16 days was observed in the smoothed voltage, water level and air pressure. More data are necessary to derive a definite conclusion about the nature of the observed phenomenon, which is probably associated with direct atmospheric forcing on the coastal zone of the Norwegian Sea.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"18 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125234506","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Present State of Art of Long Period Magnetotellurics in the Western Part of the Bohemian Massif","authors":"Václav Ceav, Josef Pex, O. Praus","doi":"10.5636/JGG.49.1559","DOIUrl":"https://doi.org/10.5636/JGG.49.1559","url":null,"abstract":"Research into the complex geological structure of the Middle European Variscides in the western part of the Bohemian Massif has largely intensified in connection with the KTB deep borehole project in the Oberpfalz, Germany. First results of long period magnetotelluric studies along an about 100 km long profile in SW Bohemia, close to the KTB drilling site, are presented here. They represent a partial step within a broader project of constructing a 3-D geoelectrical model of the western margin of the Bohemian Massif and resolving the relations of anomalous geoelectrical features to the geological structures of the region. The data roughly fit a composite model of a 2-D regional structure, striking E-W, overlaid by a layer of highly distorting near-surface inhomogeneities. The distorting layer displays large anisotropy with preferred conductivity in NW-SE to NNW-SSE direction. Attempt is made to estimate the absolute static shift of the MT curves. Several variants of 2-D models for the deep regional structure are discussed, including an anisotropic model which can qualitatively explain the discrepancy of the principal directions indicated by the MT and GDS data. The relation of the results obtained along the particular profile to the regional geoelectrical background is studied.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"6 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114697686","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A Look at Galvanic Distortion in the Tasman Sea and the Juan de Fuca Plate","authors":"S. N. White, A. Chave, J. Filloux","doi":"10.5636/JGG.49.1373","DOIUrl":"https://doi.org/10.5636/JGG.49.1373","url":null,"abstract":"We have examined magnetotelluric (MT) data from the Juan de Fuca plate and the Tasman Sea to understand the influence of galvanic distortion by local and regional topography. Galvanic (non-inductive) distortion, caused by the build up of charge along conductivity gradients, is present in both regions. Its effects can be removed by decomposition of the impedance tensor assuming that both electric and magnetic field distortion are present. Unlike for most land-based data, magnetic field distortion is necessary to explain the seafloor data. Electric field distortion parameters resulting from the full decomposition can be interpreted equivalently in terms of either 2-D or 3-D distorting bodies, but these interpretations cannot be differentiated using MT observations alone. The data are compared to the overall geology to determine whether 2-D or 3-D distortion is more probable. The electric field distortion parameters in both locations behave appoximately frequency-independently, as expected for galvanic distortion. When interpreted in terms of 2-D distorting bodies, the superficial strikes were found to parallel the trends of coastlines and large-scale submarine features (e.g. mid-ocean ridges). This suggests that distortion both in the Tasman Sea and on the Juan de Fuca plate is dominantly galvanic and appears to be caused by regional, large-scale, 2-D features rather than small-scale, 3-D distorting bodies.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"81 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125169273","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Seafloor Electromagnetic Measurements above Axial Seamount, Juan de Fuca Ridge","authors":"S. Constable, G. Heinson, G. Anderson, A. White","doi":"10.5636/JGG.49.1327","DOIUrl":"https://doi.org/10.5636/JGG.49.1327","url":null,"abstract":"Magnetotelluric (MT) data were collected at three sites around the eastern rim of the caldera of Axial Seamount, on the Juan de Fuca Ridge. The seamount has been observed to be volcanically and hydrothermally active over the last ten years, and is therefore an excellent target for electromagnetic induction studies on the seafloor. This paper follows an initial interpretation by Heinson et al. (1996) with a more complete analysis of the MT data, to investigate both oceanographic induction effects and the resistivity structure beneath the seamount. From time series analysis of electric field data using multitaper methods, coherences between electric field data from different sites are significant at the 95% confidence level at periods less than 1 day, and generally greater than 0.8 at solar and ocean tidal periods. Spectral peaks at 16.7 hours and 4 days are observed; the former is due to inertial currents in the area, and the latter is probably a ridge-trapped Rossby wave. Robust MT impedance tensors are derived using a remote-reference, and tensor decomposition shows that there is no galvanic distortion and almost isotropic responses at each site. The MT data are inverted for 1D structure, and more complex 3D forward models used to assess the lateral extent of the resistivity structure. 1D inversions show that the data are consistent with a crust with a very high electrical conductance of 1200 ± 200 S and an asthenosphere of 5-50 Ω·m at a depth of 40 km, connected by a low resistivity lithosphere of 50-100 Ω·m. The low resistivity lithosphere acts as a leakage path to the mantle for induced currents in the ocean. 3D forward modelling suggests that this region may be present only beneath Axial Seamount, surrounded by a resistive lithosphere of 500-50, 000 Ω·m. The tectonic implications from these models are that a small fraction of melt is presently migrating from the melt source in the mantle to a crustal magma chamber beneath Axial Seamount. Bulk estimates of melt fractions are 1-10% for the asthenosphere, and 1% between the asthenosphere and the crustal magma chamber, although melt may be concentrated in fractures or pipes.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"51 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115504704","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"2-D Interpretation of Minamikayabe MT Data Set by the Controlled Random Search Technique","authors":"V. Červ, J. Pek","doi":"10.5636/JGG.49.1717","DOIUrl":"https://doi.org/10.5636/JGG.49.1717","url":null,"abstract":"Within the MT-DIW3 project of the comparison of various MT interpretation techniques, the controlled random search procedure was applied to the 2-D interpretation of the MT data from the Minamikayabe MT polygon in Japan. Three NE-SW profiles together with one perpendicular profile were 2-D modelled, simultaneously for both polarizations of the MT field. Although the 2-D approximation seems to be rather far from the real resistivity conditions, we could obtain relatively consistent interpretation results particularly for data within the short period range, up to 1 s. Better fit was achieved for apparent resistivities than for phases. For all NE-SW profiles, a near-surface conductive body is indicated below the central part of the polygon. At both sides of this body two deeper conductive zones are obvious. As for the perpendicular profile kb*s06, a large conductive zone is required below the SE margin of the profile.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"03 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130565592","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"High-Resolution Spectroscopy of the A-X and B-X System of CH in Comet Austin (1990 V)","authors":"S. J. Kim, M. Brown, H. Spinrad","doi":"10.5636/JGG.49.1165","DOIUrl":"https://doi.org/10.5636/JGG.49.1165","url":null,"abstract":"We analyzed the A-X(0-0) band of CH, which appears in high-resolution spectra of comet Austin (1990 V), in order to understand fluorescence and collisional processes that influence the rotational structure of the A-X(0-0) band. Some of the weak lines of the A-X (0-0) band are clearly resolved, which have not been previously resolved with relatively low-resolution spectroscopy. We unambiguously confirmed the B-X (0-0) band lines around 3890 A, which had been suspected previously, but it had not been clearly identified because of strong adjacent CN and C 3 bands. In order to analyze the cometary spectra we have conducted two different fluorescence calculations: a single-cycle fluorescence and fluorescent equilibrium. The fluorescent equilibrium model includes infrared and ultraviolet fluorescence processes as well as electron and neutral collisional effects, and therefore the model is a function of cometocentic distance. We found that single-cycle fluorescence models with a Boltzmann distribution in the X state fit the observed spectra better than the fluorescent equilibrium models. However, single-cycle fluorescence models with two different temperatures (130 K for Fl state and 250 K for F2 state) in the X state fit the Austin spectra significantly better than the single-cycle fluorescence model with the same temperature (150 K) for Fl and F2 states. This suggests that we are observing two different Boltzmann distributions of nascent, short-life CH radicals right after they were produced by photodissociations of parent molecules. We presented g-factors of the A-X (0-0) and B-X (0-0) bands as a function of heliocentric velocity based on single-cycle fluorescence models with a 150 K distribution in the X state. We have calculated the expected intensity of the fundamental band (v = 1 - 0) of CH and discussed the detectability of this band near 2730 cm -1 . We also discussed possible parent molecules of CH and long lifetimes of the parent molecules, which may explain extensive emissions of CH up to 10 5 km from the nucleus despite its short lifetime.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"6 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-10-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123768726","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Earth Induction Effect for Pc 5 Pulsations Observed by Unmanned Magnetometer Network Near Syowa Station, Antarctica.","authors":"O. Saka, M. Shimoizumi, N. Sato","doi":"10.5636/JGG.49.1197","DOIUrl":"https://doi.org/10.5636/JGG.49.1197","url":null,"abstract":"Unmanned magnetometer network observation was carried out near Syowa Station, Antarctica from early spring to summer seasons of 1988. The network consisted of three remote stations, H100 (80 km south of Syowa Station), Skarvsnes (50 km south-west of Syowa Station), and site F22 (20 km east of Syowa Station). From a cross-correlation analysis of Pc 5 events observed during the magnetometer operation, we found an unusual phase characteristics in vertical component of F22-Syowa pair, which can not be interpreted by a Pc 5 incidence for the ground conductivity with horizontally uniform distribution. We carried out a numerical simulation of Pc 5 incidence for the ground conductivity with a local conductivity anomaly, and based on that model calculation, we can infer a possible effect of local earth currents that may appear in the cross-correlation analysis for such a short separation.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"20 12","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-10-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114034877","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Gauss' H-Variometer at the Helsinki Magnetic Observatory 1844-1912","authors":"H. Nevanlinna","doi":"10.5636/JGG.49.1209","DOIUrl":"https://doi.org/10.5636/JGG.49.1209","url":null,"abstract":"During the early decades of the 19th century variations of the geomagnetic field at geomagnetic observatories were observed visually only. The most widely used method to observe changes of variometer magnets was Poggendorff's mirror-and-scale system aided with a telescope. Readings were made typically 1 ... 12 times per hour. For observations of the horizontal field, Gauss' bifilar H-variometer was in use at geomagnetic observatories. At the Helsinki observatory, founded in 1838, Gauss' bifilar instrument was continuously in operation for more than 60 years. During that time about 1, 000, 000 readings of H were accumulated. Most of the data are still unreduced and are available in original observational notebooks only. The quality of the H-data, at least in the period 1844-1848 studied here, seems to be reliable. Comparisons between Helsinki H-results and the present day Nurmijarvi observatory data, reveal similar daily variation curves at both observatories. In long-term changes, with varying number of annual sunspots, the daily range of H follows the same linear relationship as H at Nurmijarvi. We believe that the Helsinki H-data series is reliable and homogeneous and thus suitable for detailed studies of changes of the geomagnetic horizontal field component in the mid 19th century.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"47 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-10-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"122688370","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Low-Temperature Oxidation and Subsequent Downcore Dissolution of Magnetite in Deep-Sea Sediments, ODP Leg 161 (Western Mediterranean)","authors":"M. Torii","doi":"10.5636/JGG.49.1233","DOIUrl":"https://doi.org/10.5636/JGG.49.1233","url":null,"abstract":"Low-temperature experiments document changes in magnetic mineralogy in Pleistocene hemipelagic sediments at ODP Holes 976D and 977A. The Verwey transition was observed only for samples below ∼1.3 mbsf, which suggests depth-limited appearance of stoichiometric magnetite in the sediment column. Primary magnetite is interpreted to be covered with a maghemite skin as a result of in situ low-temperature oxidation on the sea floor. The oxidized maghemite skin gradually dissolves with depth, and the Verwey transition is observed below ∼1.3 mbsf. This depth matches the iron redox boundary inferred on the basis of a sediment color change from tan to green.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"272 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-10-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124753319","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}