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71.
Robert S. White John R. Smallwood Moritz M. Fliedner † Brian Boslaugh Jenny Maresh Juergen Fruehn ‡ 《Geophysical Prospecting》2003,51(3):215-231
We demonstrate that the use of long-offset seismic data allows wide-angle reflections and diving waves to be recorded, and that these can be used in conjunction with prestack depth migrations to constrain and to image the base of the basalt flows and the underlying structure in the Faeroe-Shetland Basin. Crustal velocity models are built first by inverting the traveltimes of the recorded reflections and diving waves using ray-tracing methods. Finer details of the velocity structure can then be refined by analysis of the amplitudes and waveforms of the arrivals. We show that prestack depth migration of selected wide-angle arrivals of known origin, such as the base-basalt reflection, using the crustal velocity model, allows us to build a composite image of the structure down to the pre-rift basement. This has the advantage that the wide-angle first-arriving energy must be primary, and not from one of the many multiples or mode-converted phases that plague near-offset seismic data. This allows us to ‘tag’ these primary arrivals with confidence and then to identify the same arrivals on higher-resolution prestack migrations that include data from all offsets. Examples are drawn from the Faeroe-Shetland Basin, with a series of regional maps of the entire area showing the basalt depths and the thickness of the basalt flows and underlying sediment down to the top of the pre-rift basement. The maps show how the basalts thin to the southeast away from their presumed source west of the present Faeroe Islands, and also show the extent to which the structure of the pre-rift basement controls the considerable variations in sediment thickness between the basement and the cap formed by the overlying basalt flows. 相似文献
72.
73.
74.
The features of seismic activity on Stromboli are discussed and compared in terms of their relationship with the main changes of volcanic activity from 1990 to 1993.We considered a statistical approach for our data analysis. Cluster analysis was used to seek out classes of spectra which might characterize the condition of the volcanic system. The classes we have found provide insights into a scenario which evolves through different phases of volcanic activity, from paroxysms to low activity. We show that episodes of lava effusion and lava fountaining are heralded by variations in the spectral features of tremor after a preparation time. This result highlights the importance of tremor, and reveals that long-term observations are key to examine slow modifications in a volcanic system such as Stromboli, characterized by open conduits, and persistent explosive activity. 相似文献
75.
76.
Susanne Flux Niranjan D. Chatterjee Klaus Langer 《Contributions to Mineralogy and Petrology》1984,86(3):294-297
Pure, synthetic 2M1-muscovites (Ms) and 2M1-paragonites (Pg) prepared at 7, 13.5, and 18 kbar
have been subjected to careful X-ray powder diffractometry and IR powder spectrometry. The results of X-ray studies show
that the lattice constants of these micas are independent of the pressure employed in synthesizing them. Similarly, the IR
powder spectra of Ms and Pg remain unaffected by the synthesis pressure. Neither the Al[4]-O-Al[4] vibrations decrease in relative intensity nor do the Si-O-Al[4] vibrations increase in intensity with increasing pressure of synthesis, as they should, if pressure-induced (Al, Si)[4] ordering had been operative. Finally, the v
OH bands in Ms and Pg are also found to retain their fine structural details, regardless of the pressures employed in synthesizing
these phases. In particular, the half widths of v
OH in these (Al,Si)[4]-disordered micas remain constant at around 60 cm−1, compared to ∼ 10 cm−1 in (Al,Si)[4]-ordered mica margarite.
In the light of this study, shortening of the b lattice parameter and progressive (Al,Si)[4] ordering with increasing pressure of synthesis of muscovites reported by Velde (1980) would appear to be anomalous. Consequently,
any discussion of the effect of (Al,Si)[4] ordering on the stability of muscovite is a moot point at present. 相似文献
77.
Summary Single crystals of paragenetic garnet (gt), omphacite (cpx) and kyanite (ky) were isolated from an eclogite xenolith from the Zagadochnaya kimberlite, Yakutia (grospydite Z13) and studied by polarized FTIR-microspectrometry in the OH valence vibrational region and by microprobe analyses. The coexisting minerals are homogeneous with respect to major and minor elements and have compositions near gross49pyr26alm20uvar4 (gt), jad45dio47hed6kos2 (cpx) and ky>97. Single crystal spectra show one OH-band for gt at 3630 cm–1 (halfwidth ca. 100 cm–1) which is very likely caused by vibrations of tetrahedral (OH)4-4– clusters replacing SiO4
4– tetrahedra. Cpx shows one strong, but weakly pleochroic band at 3464 cm–1 (halfwidth ca. 160 cm–1) and a weak satellite band centered at 3620 cm–1) with a distinct pleochroism. Ky OH spectra exhibit two sets of weak sharp pleochroic bands, a triplet, characteristic for high pressure ky, at 3439, 3410, 3387 cm–1) and a doublet at 3279, 3264 cm–1) (halfwidths ca. 10 cm–1) From integral and linear absorbances in the unpolarized spectra defect-hydroxyl contents in the three coexisting minerals were estimated and found near O.OX wt% H2O. The distribution scheme of hydrogen in the paragenetic minerals was evaluated to be cky < ccpx < cgt.
Dedicated to Prof. Dr. J. Zemann on the occasion of his 70th birthday
With 3 Figures 相似文献
Einkristall Infrarotspektren im Bereich der OH Grundschwingungen einer Granat-Omphacit-Kyanit Paragenese in einem eklogitischen Mantelxenolith
Zusammenfassung Aus einer Granat (Gt)-Omphacit (Cpx)-Kyanit (Ky) Paragenese eines Eklogit Xenoliths aus dem Zagadochnaya Kimberlit, Jakutien (Grospydit Z13), wurden Einkristalle isoliert und mit polarisierter FTIR-Mikrospektrometrie im OH Valenzschwingungsbereich und mit der Elektronenstrahl-Mikrosonde untersucht. Die koexistierenden Minerale sind hinsichtlich ihrer Haupt- und Nebenelemente homogen und haben Zusammen setzungen von etwa Gross49Pyr26Alm20Uvar4 (Gt), Jad45Dio47Hed6Kos2 (Cpx) and Ky>97 Einkristallspektren von Gt zeigen eine OH-Bande bei 3630 cm–1) (Halbwerts breite ca. 100 cm–1) die wahrscheinlich von Schwingungen tetraedrischer (OH)4 4– Gruppen, die SiO4 4– Tetraeder ersetzen, herrührt. Cpx zeigt eine starke, aber schwach pleochroitische Bande bei 3464 cm–1 (Halbwertsbreite ca. 160 cm–1) and eine schwache, deutlich pleochroitische Satellitenbande bei 3620 cm–1) Ky OH Spektren zeigen zwei Gruppen von schwachen, scharfen pleochroitischen Banden, ein fur Hochdruck Ky charakteristisches Bandentriplett bei 3439, 3410, 3387 cm–1) and ein Bandendublett bei 3279, 3264 cm–1) (Halbwertsbreiten ca. 10 cm–1) Aus den integralen and linearen Extinktionen der nicht-polarisierten Spektren wurde der Defekt-Hydroxyl Gehalt der drei koexistierenden Minerale abgeleitet and mit O.OX Gew% H2O festgelegt. Das Verteilungsschema des Wasserstoffs kann in der Mineralparagenese mit cKy < cCpx cGt angegeben werden.
Dedicated to Prof. Dr. J. Zemann on the occasion of his 70th birthday
With 3 Figures 相似文献
78.
V. M. Khomenko K. Langer A. Beran M. Koch-Müller T. Fehr 《Physics and Chemistry of Minerals》1994,20(7):483-488
A series of Ti-substituted pyrope crystals was synthesized in the system MgO-(Na2O)-Al2O3-TiO2-SiO2-H2O at PH20 = Ptot between 25 and 30 kbars and 975 and 1000° C, using graphite heated piston-cylinder devices. The crystals, ranging up to 500 m in diameter, were studied by X-ray, electron-microprobe and FTIR-microscope spectrometric techniques. The pyrope crystals were colourless when hem/mt or mt/wu buffers were used during the synthesis, and pale blue with the wu/iron buffer and in unbuffered runs. Sodium was not found in the synthetic crystals, titanium was always near 0.06 Ti atoms pfu, independent on the Ti-excess used in the starting material. A substitution Al2+[6]+Si4+[4]+4O2-= Ti4+[6]+[4]+[(OH)3O^5-, providing charge balance for octahedral Ti4+-substitution is found to be compatible with all properties (number, widths, position, integrated intensity) of the stretching vibrations of defect hydroxyls, which have energies 3684, 3568, 3525 cm-1.This work was made possible through a research fellowship, generously provided by the Alexander von Humboldt-Stiftung, Bonn, Bad Godesberg, to V.M.K. 相似文献
79.
Peter Marchev Hillary Downes Matthew F. Thirlwall Robert Moritz 《Mineralium Deposita》2002,37(6-7):669-677
80.
In recent years, numerous induced seismic events have occurred in the proximity of the natural gas field in Northern Germany. To monitor the seismicity and to asses the seismic hazard potential, a local monitoring network was installed in the area. Focusing on the seismicity hazard assessment, a major challenge is the characterisation of potential site effects due to local soil characteristics. This is quantitatively performed by estimating the shear-wave velocity (V s) variation with subsurface layer thickness. Such local effects can only be covered with a coarse spatial resolution due to the limited number of monitoring stations. Profiles were determined at three test sites (Langwedel, Walle and Bomlitz) by using a combined approach of small aperture 2D array ambient noise and small-scale active 1D measurements. The high-resolution frequency-wavenumber (HRFK), spatial autocorrelation (SPAC) and multichannel analysis of surface waves (MASW) methods were applied to the recorded ambient noise and active seismic data using various array sizes supplemented by the active measurements. This jointly allowed obtaining phase velocity dispersion curves covering a wide frequency range from 2 up to 32 Hz. The inversion of the obtained dispersion curves results in average S-wave velocity profiles down to depths of 70 m, identifying thin near-surface layers of a few meters as well as thicker layers of tens of meters in greater depth. A comparison with available borehole data shows a good correlation with the layering. Additionally, to asses the impact of a seismic event at the test sites, PGV estimations for various seismic events were performed. The final results of the test surveys demonstrate that the combined approach represents a suitable tool for near-surface characterisation, which can be used to improve the seismic hazard assessment in the area of the natural gas fields in Northern Germany. 相似文献