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981.
Karl Thomas Hjelmervik Jan Kristian Jensen Petter ?stenstad Atle Ommundsen 《Ocean Dynamics》2012,62(2):253-264
Sonar performance modeling is crucial for submarine and anti–submarine operations. The validity of sonar performance models
is generally limited by environmental uncertainty, and particularly uncertainty in the vertical sound speed profile (SSP).
Rapid environmental assessment (REA) products, such as oceanographic surveys and ocean models may be used to reduce this uncertainty
prior to sonar operations. Empirical orthogonal functions (EOF) applied on the SSPs inherently take into account the vertical
gradients and therefore the acoustic properties. We present a method that employs EOFs and a grouping algorithm to divide
a large group of SSPs from an ocean model simulation into smaller groups with similar SSP characteristics. Such groups are
henceforth called acoustically stable groups. Each group represents a subset in space and time within the ocean model domain.
Regions with low acoustic variability contain large and geographically contiguous acoustically stable groups. In contrast,
small or fragmented acoustically stable groups are found in regions with high acoustic variability. The main output is a map
of the group distribution. This is a REA product in itself, but the map may also be used as a planning aid for REA survey
missions. 相似文献
982.
Coseismic slip model of the 2007 August Pisco earthquake (Peru) as constrained by Wide Swath radar observations 总被引:1,自引:0,他引:1
Mahdi Motagh Rongjiang Wang Thomas R. Walter Roland Bürgmann Eric Fielding Jan Anderssohn Jochen Zschau 《Geophysical Journal International》2008,174(3):842-848
The Pisco earthquake ( M w 8.0; 2007 August 15) occurred offshore of Peru's southern coast at the subduction interface between the Nazca and South American plates. It ruptured a previously identified seismic gap along the Peruvian margin. We use Wide Swath InSAR observations acquired by the Envisat satellite in descending and ascending orbits to constrain coseismic slip distribution of this subduction earthquake. The data show movement of the coastal regions by as much as 85 cm in the line-of-sight of the satellite. Distributed-slip model indicates that the coseismic slip reaches values of about 5.5 m at a depth of ∼18–20 km. The slip is confined to less than 40 km depth, with most of the moment release located on the shallow parts of the interface above 30 km depth. The region with maximum coseismic slip in the InSAR model is located offshore, close to the seismic moment centroid location. The geodetic estimate of seismic moment is 1.23 × 1021 Nm ( M w 8.06), consistent with seismic estimates. The slip model inferred from the InSAR observations suggests that the Pisco earthquake ruptured only a portion of the seismic gap zone in Peru between 13.5° S and 14.5° S, hence there is still a significant seismic gap to the south of the 2007 event that has not experienced a large earthquake since at least 1687. 相似文献
983.
新疆天山西部三个云杉上树线树轮最大密度年表的研制、相关性及其气候信号分析 总被引:13,自引:3,他引:10
针对新疆天山西部伊犁、博尔塔拉蒙古自治区州三大山体北坡云杉上树线树木年轮最大密度资料,采用区域曲线、负指数曲线、样条函数三种不同生长去趋势方法,研制树轮最大密度年表,分析不同采点和树轮去趋势方法对树轮最大密度年表在不同频域的相关性及气候信号的影响发现:①三个采点间的最大树轮密度变化,在高频变化方面最为相似,低频变化存在着较大差异;②天山西部山脉北坡上树线的树轮最大密度年表对年表研制的三种不同去趋势方法不敏感,而采点间的距离和是否位于同一气候区是影响上树线树轮最大密度年表间相关性的重要因子;③天山西部山脉北坡上树线的树轮最大密度年表对当年的温度具有显著的相关性,年降水量越多的山区,其上树线最大密度年表对温度的相关性越好,且显著相关平均温度涉及的月数也不断增多. 相似文献
984.
A central depository for meteor orbits obtained by photographic techniques, as a part of the IAU Meteor Data Center, was moved
to the Astronomical Institute of the Slovak Academy of Sciences in Bratislava in 2001. The current version of the catalogue
contains data on 4581 meteor orbits obtained by 17 different stations or groups from the period 1936 to 1996. Since 1996 a
few huge campaigns were organised including very successful Leonids and Perseids. That is why we would prepare a new more
complete version of the database. The main aim of this paper is a call to the observers of meteors having new or recalculated/remeasured
data on photographic meteors to send them to the MDC, where after a check and consultations with the observer, the orbits
will be included in the database. 相似文献
985.
Determining the amplitude and timing of streamflow discontinuities: A cross wavelet analysis approach 总被引:1,自引:0,他引:1 下载免费PDF全文
Records of natural processes, such as gradual streamflow fluctuations, are commonly interrupted by long or short disruptions from natural non‐linear responses to gradual changes, such as from river‐ice break‐ups, freezing as a result of annual solar cycles, or human causes, such as flow blocking by dams and other means, instrument calibrations and failure. The resulting abrupt or gradual shifts and missing data are considered to be discontinuities with respect to the normal signal. They differ from random noise as they do not follow any fixed distribution over time and, hence, cannot be eliminated by filtering. The multi‐scale resolution features of continuous wavelet analysis and cross wavelet analysis were used in this study to determine the amplitude and timing of such streamflow discontinuities for specific wavebands. The cross wavelet based method was able to detect the strength and timing of abrupt shifts to new streamflow levels, gaps in data records longer than the waveband of interest and a sinusoidal discontinuity curve following an underlying modeled annual signal at ±0.5 year uncertainty. Parameter testing of the time‐frequency resolution demonstrated that high temporal resolution using narrow analysis windows is favorable to high‐frequency resolution for detection of waveband‐related discontinuities. Discontinuity analysis on observed daily streamflow records from Canadian rivers showed the following: (i) that there is at least one discontinuity/year related to the annual spring flood in each record studied, and (ii) neighboring streamflows have similar discontinuity patterns. In addition, the discontinuity density of the Canadian streamflows studied in this paper exhibit 11‐year cycles that are inversely correlated with the solar intensity cycle. This suggests that more streamflow discontinuities, such as through fast freezing, snowmelt, or ice break‐up, may occur during years with slightly lowered solar insolation. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
986.
Patricia Larrea Jan R. Wijbrans Carlos Galé Teresa Ubide Marceliano Lago Zilda França Elisabeth Widom 《Bulletin of Volcanology》2014,76(2):1-15
Lava flows spanning the eruptive record of Graciosa Island (Azores archipelago) and a gabbro xenolith were dated by 40Ar/39Ar in order to constrain the Pleistocene and Holocene volcanic evolution of the island. The results range from 1.05 Ma to 3.9 ka, whereas prior published K–Ar and 14C ages range from 620 to 2 ka. The formation of the Serra das Fontes shield volcano started at minimum 1.05 Ma, and the magmatic system was active for ca. 600 ky, as suggested by the formation of the gabbro xenolith by magmatic differentiation. Evolved magmas making up the Serra das Fontes–Serra Branca composite volcano were generated at ca. 450 ka. After a period of ca. 110 ky of volcanic inactivity and erosion of volcanic edifices, volcanism was reactivated with the formation of the Vitória Unit NW platform. Later, the development of the Vulcão Central Unit started with the formation of monogenetic cones located to the south of the Serra das Fontes–Serra Branca–Vitória Unit. This volcanism became progressively more evolved and was concentrated in a main eruptive center, forming the Vulcão Central stratovolcano with an age older than 50 ka. The caldera related to this stratovolcano is older than 47 ka and was followed by effusion of basaltic magmas into the caldera, resulting in the formation of a lava lake, which ultimately spilled over the caldera rim at ca. 11 ka. The most recent eruptions on Graciosa formed two small pyroclastic cones within the caldera and the Pico do Timão cone within the Vitória Unit at ca 3.9 ka. 相似文献
987.
The Hronov-Po?í?í Fault Zone (HPFZ) is an active tectonic area with regularly occurring shallow earthquakes up to magnitude 5. For their exact locations, at least an average velocity model of the area is needed. A method of measuring local phase velocities of surface waves using the array of stations deployed permanently in the HPFZ is introduced. Seven regional and teleseismic events are selected to represent different backazimuths of propagation. Applicable range of periods is estimated for each event. The coherency of the waves reaching the array is constraining the short period range. The dimension of the array is a limiting factor for the long-periods. A dispersion curve of Rayleigh wave phase velocity measured at the vertical component and characterizing 1D properties of the target area is determined using the seven measurements for the interval from 1 to 40 s. An isometric method is used to invert the determined dispersion curve for shear and longitudinal velocity distribution from the surface to the depth of 65 km. 相似文献
988.
Göran Broström Kai H. Christensen Magnus Drivdal Jan Erik H. Weber 《Ocean Dynamics》2014,64(7):1039-1045
In this study, we consider the origin of the Coriolis-Stokes (CS) force in the wave-averaged momentum and energy equations and make a short analysis of possible energy input to the ocean circulation (i.e., Eulerian mean velocity) from the CS force. Essentially, we find that the CS force appears naturally when considering vertically integrated quantities and that the CS force will not provide any energy input into the system for this case. However, by including the “Hasselmann force”, we show some inconsistencies regarding the vertical structure of the CS force in the Eulerian framework and find that there is a distinct vertical structure of the energy input and that the net input strongly depends on whether the wave zone is included in the analysis or not. We therefore question the introduction of the “Hasselmann force” into the system of equations, as the CS force appears naturally in the vertically integrated equations or when Lagrangian vertical coordinates are used. 相似文献
989.
This study is an application of a Real Time Recurrent Neural Network (RTRN) in the detection of small natural seismic events in Poland. Most of the events studied are from the Podhale region with a magnitude of 0.4 to 2.5. The population distribution of the region required that seismic signals be recorded using temporary stations deployed in populated areas. As a consequence, the high level of seismic noise that cannot be removed by filtration made it impossible to detect small events by STA/LTA based algorithms. The presence of high noise requires an alternate method of seismic detection capable of recognizing small seismic events. We applied the RTRN, which potentially can detect seismic signals in the frequency domain as well as in the phase arrival times. Data results of small local seismic events showed that the RTRN has the ability to correctly detect most of the events with fewer false detections than STA/LTA methods. 相似文献
990.