Fall,classification, and exposure history of the Mifflin L5 chondrite     

Noriko T. Kita  Kees C. Welten  John W. Valley  Michael J. Spicuzza  Daisuke Nakashima  Travis J. Tenner  Takayuki Ushikubo  Glenn J. MacPherson  Linda Welzenbach  Philipp R. Heck  Andrew M. Davis  Matthias M. M. Meier  Rainer Wieler  Marc W. Caffee  Matthias Laubenstein  Kunihiko Nishiizumi 《Meteoritics & planetary science》2013,48(4):641-655

The Mifflin meteorite fell on the night of April 14, 2010, in southwestern Wisconsin. A bright fireball was observed throughout a wide area of the midwestern United States. The petrography, mineral compositions, and oxygen isotope ratios indicate that the meteorite is a L5 chondrite fragmental breccia with light/dark structure. The meteorite shows a low shock stage of S2, although some shock‐melted veins are present. The U,Th‐He age is 0.7 Ga, and the K‐Ar age is 1.8 Ga, indicating that Mifflin might have been heated at the time of the 470 Ma L‐chondrite parent body breakup and that U, Th‐He, and K‐Ar ages were partially reset. The cosmogenic radionuclide data indicate that Mifflin was exposed to cosmic rays while its radius was 30–65 cm. Assuming this exposure geometry, a cosmic‐ray exposure age of 25 ± 3 Ma is calculated from cosmogenic noble gas concentrations. The low ²²Ne/²¹Ne ratio may, however, indicate a two‐stage exposure with a longer first‐stage exposure at high shielding. Mifflin is unusual in having a low radiogenic gas content combined with a low shock stage and no evidence of late stage annealing; this inconsistency remains unexplained.  相似文献   

Unified multi‐depth‐level field decomposition          下载免费PDF全文

Niels Grobbe  Joost van der Neut  Evert Slob  Kees Wapenaar  Carlos Almagro Vidal  Guy Drijkoningen 《Geophysical Prospecting》2016,64(2):361-391

Wavefield decomposition forms an important ingredient of various geophysical methods. An example of wavefield decomposition is the decomposition into upgoing and downgoing wavefields and simultaneous decomposition into different wave/field types. The multi‐component field decomposition scheme makes use of the recordings of different field quantities (such as particle velocity and pressure). In practice, different recordings can be obscured by different sensor characteristics, requiring calibration with an unknown calibration factor. Not all field quantities required for multi‐component field decomposition might be available, or they can suffer from different noise levels. The multi‐depth‐level decomposition approach makes use of field quantities recorded at multiple depth levels, e.g., two horizontal boreholes closely separated from each other, a combination of a single receiver array combined with free‐surface boundary conditions, or acquisition geometries with a high‐density of vertical boreholes. We theoretically describe the multi‐depth‐level decomposition approach in a unified form, showing that it can be applied to different kinds of fields in dissipative, inhomogeneous, anisotropic media, e.g., acoustic, electromagnetic, elastodynamic, poroelastic, and seismoelectric fields. We express the one‐way fields at one depth level in terms of the observed fields at multiple depth levels, using extrapolation operators that are dependent on the medium parameters between the two depth levels. Lateral invariance at the depth level of decomposition allows us to carry out the multi‐depth‐level decomposition in the horizontal wavenumber–frequency domain. We illustrate the multi‐depth‐level decomposition scheme using two synthetic elastodynamic examples. The first example uses particle velocity recordings at two depth levels, whereas the second example combines recordings at one depth level with the Dirichlet free‐surface boundary condition of zero traction. Comparison with multi‐component decomposed fields shows a perfect match in both amplitude and phase for both cases. The multi‐depth‐level decomposition scheme is fully customizable to the desired acquisition geometry. The decomposition problem is in principle an inverse problem. Notches may occur at certain frequencies, causing the multi‐depth‐level composition matrix to become uninvertible, requiring additional notch filters. We can add multi‐depth‐level free‐surface boundary conditions as extra equations to the multi‐component composition matrix, thereby overdetermining this inverse problem. The combined multi‐component–multi‐depth‐level decomposition on a land data set clearly shows improvements in the decomposition results, compared with the performance of the multi‐component decomposition scheme.  相似文献   

Sources of dissolved oxygen in monitoring and pumping wells     

Matthijs Bonte  Bas Wols  Kees Maas  Pieter Stuyfzand 《Hydrogeology Journal》2017,25(1):55-66

Groundwater monitoring and pumping wells set in anoxic aquifers require attention to keep the groundwater free of dissolved oxygen (DO). In properly constructed monitoring or pumping wells, two processes can however still introduce oxygen to anoxic groundwater: (1) permeation of oxygen through polymer materials such as silicone, PVC, HDPE or Teflon, and (2) thermally driven convection, which can occur in all types of piezometers or wells, regardless of construction material, when the water table or pressure head is close (<10 m) to the land surface. Here, field measurements (temperature and DO well loggings) from a monitoring well in Bilthoven, the Netherlands, are combined with analytical and numerical modelling to investigate the role of both processes on oxygenation of anoxic groundwater in wells. The results of numerical and analytical modeling show that both permeation and convection can introduce oxygen into anoxic wells to near saturation concentrations. In the field data gathered, convection is primarily responsible for oxygen intrusion up to a depth of around 12 m. Oxygen intrusion through convection and permeation in monitoring and pumping wells may influence groundwater sampling and analyses, and may contribute to well clogging, depending on site conditions. The combination of field and modelling provides new insights into these processes, which can be used for both groundwater sampling and pumping well design.  相似文献   

Wavefield extrapolation and prestack depth migration in anelastic inhomogeneous media   总被引：6，自引：0，他引：6  

Jianfeng Zhang†  Kees Wapenaar 《Geophysical Prospecting》2002,50(6):629-643

Wavefield depth extrapolation and prestack depth migration in complex anelastic media are studied. Kjartansson's frequency‐independent Q law is used to describe the absorption of seismic energy. The macromodel used is analogous to the macromodel used for current migration schemes except that an additional frequency‐independent Q macromodel needs to be provided. Absorption in the forward one‐way propagator is introduced by assuming a complex phase velocity, and the inverse one‐way propagator is obtained using the reciprocity theorem for one‐way wavefields in dissipative media. The stability of the inverse propagator is achieved by limiting the angle of propagation of wavefields. A table‐driven explicit operator scheme for imaging complex 2D anelastic media is presented. High‐accuracy, short convolution operators are designed by the weighted least‐squares method, and two kinds of imaging conditions are proposed. Numerical examples of depth extrapolation in laterally varying media, the migration of a spatial impulse with dispersion as well as shot record depth migration demonstrate the potential of the proposed explicit forward operator, the explicit inverse operator and the prestack depth migration scheme, respectively.  相似文献   

Relations between reflection and transmission responses of three-dimensional inhomogeneous media   总被引：1，自引：0，他引：1  

Kees Wapenaar  Jan Thorbecke  Deyan Draganov 《Geophysical Journal International》2004,156(2):179-194

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Palaeomagnetic constraints on formation of the Mianwali reentrant, Trans-Indus and western Salt Range, Pakistan     

Chris T. Klootwijk  Russell Nazirullah  Kees A. de Jong 《Earth and Planetary Science Letters》1986,80(3-4):394-414

Successions of Lower to lower Middle Cambrian, Upper Permian to Upper Triassic and Lower Tertiary carbonates and arenites have been sampled in five sections, representative of the three main segments of the Mianwali reentrant in the (Trans-Indus) Salt Range (northern Pakistan), i.e.: the southern Khisor Range, the northern Surghar Range and the western Salt Range. Comparison of primary and secondary magnetization directions with the Indian APWP demonstrates the secondary origin of the Mianwali reentrant and shows a pattern of rotations which varies in sense and magnitude along the reentrant with the main structural trends. Data from the Trans-Indus and western Salt Range and published Early Cambrian, Early Permian and Late Tertiary palaeomagnetic results from the southern Salt Range and the Potwar Plateau show that the Hazara Arc underwent a 20–45° counterclockwise rotation relative to the Indian Shield. A contrasting clockwise rotation over about 45° has recently been established for thrust sheets in the opposing eastern limb of the Western Himalayan Syntaxis, i.e. for the Panjal Nappe [1] and the Riasi thrust sheet [2]. These palaeomagnetically established rotations conform with the about 75° azimuthal change in structural trend along the Syntaxis, and support Crawford's [3] suggestion that the Salt Range was originally in line with the northwestern Himalaya. The Salt Range front prograded and moved southwards as part of the Hazara Arc thrust sheet, detached from basement along the evaporitic Salt Range Formation. The Mianwali reentrant originated through obstruction of the southwards advancing thrust sheet by moulding around basement topography of the northwest oriented Sarghoda Ridge.  相似文献   

Transforming identity discourses to promote local interests during municipal amalgamations     

Kees Terlouw 《GeoJournal》2018,83(3):525-543

This paper discusses the different ways in which local identities are used in two Dutch municipalities. Like all local administrations these Dutch municipalities have to deal with external forces by plotting their own course between closing-off and opening-up. Local identities are used not only for resisting external threats like municipal amalgamations, but also to attract external resources. It proved useful to distinguish between primary identity discourses based on the widely recognised dominant characteristics of the local community, and secondary identity discourses based on how communities within a municipality have over time learned to deal with these different primary local identities. During an amalgamation this secondary identity discourse disappears with the old municipality. The disappearance of the protective shield of a secondary identity discourse can threaten the underlying primary local identities, and can bring local identities into the centre of the local political debate. A perceived external threat frequently changes the character of these local identities. They can become more inward oriented, focus more on their historical roots and their differences with others; they ‘thicken’ into resistance identity discourses. In other cases the secondary identity discourse of a municipality is too weak and indistinct to support the primary local identities of its communities. Municipal amalgamation can then help to promote a new more attractive secondary, ‘thin’ regional identity discourse based on a selection of characteristics used in established primary local identity discourses.  相似文献   

An evaluation of the distribution and scavenging habits of northern fulmars (Fulmarus glacialis) in the North Sea     

Camphuysen  Kees ; Garthe  Stefan 《ICES Journal of Marine Science》1997,54(4):654-683

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Continuous seismic and magnetic profiles over the Mid Atlantic Ridge between 12° and 18°N     

Bastiaan J. Collette  Kees Rutten  Hans Schouten  A. Peter Slootweg 《Marine Geophysical Researches》1974,2(2):133-141

Results are given of the Mid Atlantic Ridge section of a transoceanic survey carried out on board of freighters heading for South America and of ships of both the Royal Navy and the Royal Netherlands Navy. The tracks cross the Mid Atlantic Ridge between 12° and 18°N (GEBCO sheets 153/154).  相似文献   

The discrepancy in PAMINA between the European image of a cross-border region and cross-border behaviour     

Kees Terlouw 《GeoJournal》2008,73(2):103-116

Subsidizing cross-border regions is a method to close the gap between citizens and the European Union. This analysis of PAMINA, a cross-border region in the Rhine Valley near Karlsruhe, discusses some of the difficulties of this policy. There are structural mismatches between the scales of different cross-border relations. These vertical mismatches are linked to the differences in the horizontal logics of economic and administrative cross-border relations. Especially cross-border commuting, made possible by European economic integration, has improved the daily life of many inhabitants of this region. Paradoxically this regional success of European economic integration is disconnected from the EU funded cross-border region. They not only relate to different scales, but the same spatial asymmetry generating this cross-border behaviour hinders administrative cross-border cooperation in PAMINA.

Kees TerlouwEmail:

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