首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Summary Recordings from a crustal seismic experiment, which was conducted in the Yellowknife area in 1966, were used for calibration of the Yellow-knife seismic array. In the immediate vicinity of the array the crust is found to be very uniform. A superficial layer with an intercept time of 0–172 ± 0–012s and unknown velocity is underlain by a crust with a P wave velocity of 6.04 ± 0–01 km s-1 near the top: assuming this velocity constant throughout the second layer, the total thickness of the crust is about 34 ± 2 km. The Mohorovicic discontinuity is horizontal under the array within the resolution of this experiment and the apparent Pn velocity is 8.15 km s-1. At a distance of a few tens of kilometres the crustal uniformity breaks down. The distances are such that, for most teleseismic signals, the effect of these in homogeneities should be negligible.  相似文献   

2.
Summary Infrasound with frequencies of 1–16 Hz, detected by an array of four thermistor flow-meter microphones in Sterling Forest, New York, was observed to have a continuous background with peak energy distributed near 16 Hz in frequency, with amplitudes of about 1 dyn cm-2, and arriving from the south-west and south-east at slightly above the speed of sound in air at ground level. The same array of microphones detected 5 dyn cm-2 signals from the Apollo 14. The earlier part of the 10-min signal arrived from the first stage re-entry, the later from the launch site vicinity. It is shown that aircraft beyond the visible and audible range can be detected and tracked by monitoring the infrasound emitted throughout most of the 1–16 Hz frequency band.  相似文献   

3.
Summary. Factors influencing the seismic response characteristics of marine profiling systems are reviewed. The single frequency case is used to illustrate the influence of different frequencies on the response, as well as the towing depths of the source and receiver, and the geometry of a linear receiving array. The more realistic case of band-limited source waveforms is considered, using frequency spectra calculated from theoretically derived airgun signals. The results show that the number and shape of sidelobes of the profiling system response, as well as the filtering characteristics for reflections arising from reflectors in the vertical plane perpendicular to the axis of the receiver array are determined by the depths of the source and receiver and the relative amplitudes of the frequencies in the source waveform. These factors, along with the configuration of the hydrophone elements in the receiver array, determine the frequency and amplitude attenuation of reflections in the vertical plane containing the receiver array.
The filtering characteristics of the system both in and out of the vertical plane containing the receiver array are discussed, with implications for discriminating between off-axis and in-plane reflections. A plan view of the response of the system is constructed in the time domain for various profiling configurations and sources of different frequency content at a given time. This example shows how useful the resulting pictures are for optimizing acquisition parameters in profiling experiments.  相似文献   

4.
Summary. Over 80 earthquakes, exclusively from the Hindukush focal region, which were recorded at the Gauribidanur seismic array (GBA) have been used in this study. These events have similar epicentral distances and a narrow azimuthal range from GBA but varying focal depths from 10 to 240 km. A fault plane dipping steeply (75°) in the north-west direction and striking N 66° E has been investigated on the basis of the spatial distribution of earthquakes in two vertical planes through 68° E and 32° N. Short period P -wave recordings up to 30 s were processed using the adaptive cross-correlation filtering technique. Slowness and azimuthal anomalies were obtained for first arrivals. These anomalies show positive as well as negative bias and are attributed to a steep velocity gradient in the upper mantle between the 400–700 km depth range where the seismic rays have their maximum penetration. Relative time residuals between the stations of GBA owe their origin very near to the surface beneath the array. A search of the signals across the array revealed that most of the events occurring at shallower depths had complex signatures as compared to the deeper events. The structure near the source region, complicated source functions and the scattering confined to the crust—upper mantle near source are mainly responsible for the complexity of the Hindukush earthquakes as the transmission zone of the ray tubes from turning point to the recording station is practically the same.  相似文献   

5.
Previous studies have suggested the presence of large-scale flow structures in gravel-bed rivers. These structures are pictured as intermittent high-speed wedges separated by regions of lower velocity. However, the characteristics of these structures have not been examined in detail through either visualisation techniques or detailed field measurements. This paper confirms the presence of large-scale flow structures in gravel bed rivers, pictures their sequence and patterns and characterises their mean and individual properties. The analysis relies on a new technique for displaying velocity fluctuations in a space–time matrix that allows one to see the structures as they pass an array of current meters. Streamwise and vertical velocities were measured simultaneously with an array of three electromagnetic current meters. The sampling frequency was 20 Hz. Five velocity profiles of up to 13 1-min series of measurements and one profile of three 20-min measurements were sampled. These data suggest the presence of large wedges of faster fluid joined by regions of slower fluid. Space–time correlation analysis confirmed the presence of vertical coherence of the flow. The average angle of the front of the wedges is 36°. Although individual structures are variable in size and shape, a new detection technique using all three velocity signals simultaneously showed that their average frequency is nine events per minute and their duration is more than 2 s. The high-speed wedges display a complex organisation and do not show a preferred sequence of events as was postulated by previous studies. Because of their duration and size, these high-speed wedges are likely to play a major role in bedload sediment transport.  相似文献   

6.
We propose a vertical array analysis method that decomposes complex seismograms into body and surface wave time histories by using a velocity structure at the vertical array site. We assume that the vertical array records are the sum of vertically incident plane P and S waves, and laterally incident Love and Rayleigh waves. Each phase at the surface is related to that at a certain depth by the transfer function in the frequency domain; the transfer function is obtained by Haskell's matrix method, assuming a 1-D velocity structure. Decomposed P , S and surface waves at the surface are estimated from the vertical array records and the transfer functions by using a least-squares method in the frequency domain; their time histories are obtained by the inverse Fourier transform. We carried out numerical tests of this method based on synthetic vertical array records consisting of vertically incident plane P and S waves and laterally incident plane Love and Rayleigh waves. Perfect results of the decomposed P , S , Love and Rayleigh waves were obtained for synthetic records without noise. A test of the synthetic records in which a small amount of white noise was added yielded a reasonable result for the decomposed P , S and surface waves. We applied this method to real vertical array records from the Ashigara valley, a moderate-sized sedimentary valley. The array records from two earthquakes occurring at depths of 123 and 148 km near the array (epicentral distance of about 31 km) exhibited long-duration later phases. The analysis showed that duration of the decomposed S waves was a few seconds and that the decomposed surface waves appeared a few seconds after the direct S -wave arrival and had very long duration. This result indicated that the long-duration later phases were generated not by multireflected S waves, but by basin-induced surface waves.  相似文献   

7.
The deployment of temporary arrays of broadband seismological stations over dedicated targets is common practice. Measurement of surface wave phase velocity across a small array and its depth-inversion gives us information about the structure below the array which is complementary to the information obtained from body-wave analysis. The question is however: what do we actually measure when the array is much smaller than the wave length, and how does the measured phase velocity relates to the real structure below the array? We quantify this relationship by performing a series of numerical simulations of surface wave propagation in 3-D structures and by measuring the apparent phase velocity across the array on the synthetics. A principal conclusion is that heterogeneities located outside the array can map in a complex way onto the phase velocities measured by the array. In order to minimize this effect, it is necessary to have a large number of events and to average measurements from events well-distributed in backazimuth. A second observation is that the period of the wave has a remarkably small influence on the lateral resolution of the measurement, which is dominantly controlled by the size of the array. We analyse if the artefacts created by heterogeneities can be mistaken for azimuthal variations caused by anisotropy. We also show that if the amplitude of the surface waves can be measured precisely enough, phase velocities can be corrected and the artefacts which occur due to reflections and diffractions in 3-D structures greatly reduced.  相似文献   

8.
Sampling efficiency of vertical array aeolian sand traps   总被引：1，自引：0，他引：1  
Z. S. Li  J. R. Ni 《Geomorphology》2003,52(3-4):243-252
Previous investigations have indicated that the sampling efficiency of aeolian sand traps is low and varies greatly in the near-bed region. Outside this region, the efficiency tends to be consistently higher for all types of trap. An evaluation was carried out to compare the sampling efficiency of different types of aeolian sand trap based on the comparison of the “actual” and the measured sand mass flux profiles, with emphasis on the single-tube vertical array trap, conventional array trap, and step-like array trap. A simple formula is proposed to express the actual vertical profile of sand mass flux, which has been validated with the unique data obtained with an isokinetic trap by [Sedimentology 45 (1998) 789]. Using the experimental data collected by the present authors and those by other investigators, sampling efficiencies of three types of trap are examined in terms of the frequency distribution of all the samples. For the single-tube traps, the sampling efficiency varies from 65% to 95%, with a mode at 75%. For both the conventional array and step-like array traps, sampling efficiencies range from 15% to 85%, with the modal frequencies at 35% and 75%, respectively. This review seems to suggest that the peak frequency with higher sampling efficiency coincides with the maximum sand-grain Reynolds number.  相似文献   

9.
Telemetric network observations of pulse-like geoelectric charge signals using a vertical dipole buried under the ground were undertaken at various observation sites over a wide area in Japan from April 1996. From continuous records of the signals during the six months following that, we attempted to select anomalous signals, possibly related to seismic electric activity. Special attention was paid to the elimination of noise resulting from industrial and meteorological electric activity, comparison with other electromagnetic signals in the VLF band and the relation between the precursor period and the distance from the eventual main shock that occurred in Japan. Four candidate precursor electric signals, which were not contaminated by industrial and meteorological electric activity, were then selected, of which the second appeared before the Akita-ken Nairiku-nanbu earthquake swarm, for which the maximum M = 5.9 occurred on 1996 August 11, and the third and fourth before the Chiba-ken Toho-oki earthquake, M = 6.6, on 1996 September 11. A tentative qualitative model explaining why the candidate precursory signal is related to stress building up before an earthquake is presented in terms of the electrification of gases released from fracturing rocks immediately prior to the main shock.  相似文献   

10.
基于GPS反射信号的岸基海冰探测的研究   总被引：1，自引：0，他引：1       下载免费PDF全文
本文论述了全球导航卫星系统反射信号测量（GNSS-R）在海冰介电常数和海冰密集程度方面的潜在的研究价值,对欧洲空间局在格林兰岛迪斯科海岸采集的海冰数据进行分析,处理了两种反射信号的幅度的量化关系,同时跟踪多颗GNSS卫星并且保证了数据处理时间的连续性和有效性。根据双极化反射信号的幅度极化比和模拟海冰表面复介电常数得出结论, 幅度极化比和海冰表面的介电常数变化有联系。  相似文献   

11.
Array techniques are particularly well‐suited for detecting and quantifying the complex seismic wavefields associated with volcanic activity such as volcanic tremor and long‐period events. The methods based on the analysis of the signal in the frequency domain, or spectral methods, have the main advantages of both resolving closely spaced sources and reducing the necessary computer time, but may severely fail in the analysis of monochromatic, non‐stationary signals. Conversely, the time‐domain methods, based on the maximization of a multichannel coherence estimate, can be applied even for short‐duration pulses. However, for both the time and the frequency domain approaches, an exhaustive definition of the errors associated with the slowness vector estimate is not yet available. Such a definition become crucial once the slowness vector estimates are used to infer source location and extent. In this work we develop a method based on a probabilistic formalism, which allows for a complete definition of the uncertainties associated with the estimate of frequency–slowness power spectra from measurement of the zero‐lag cross‐correlation. The method is based on the estimate of the theoretical frequency–slowness power spectrum, which is expressed as the convolution of the true signal slowness with the array response pattern. Using a Bayesian formalism, the a posteriori probability density function for signal slowness is expressed as the difference, in the least‐squares sense, between the model spectrum and that derived from application of the zero‐lag cross‐correlation technique. The method is tested using synthetic waveforms resembling the quasi‐monochromatic signals often associated with the volcanic activity. Examples of application to data from Stromboli volcano, Italy, allow for the estimate of source location and extent of the explosive activity.  相似文献   

12.
Electromagnetic investigations are usually intended to examine regional structures where induction takes place at a given period range. However, the regional information is often distorted by galvanic effects at local conductivity boundaries. Bahr (1985) and Groom & Bailey (1989) developed a physical distortion model for decomposing the MT impedance tensor, based upon local galvanic distortion of a regional 2-D electromagnetic field. We have extended their method to predict the magnetic variation fields created at an array of sites. The magnetic response functions at periods around 1000 s may be distorted by large-scale inhomogeneities in the upper or middle crust. In this period range, the data measured by a magnetometer array contain common information that can be extracted if the data set is treated as a unit, for example by using hypothetical event analysis. With this technique it is always possible to recover the regional strike direction from distorted data, even if a strong, spatially varying regional vertical field component is present in the data set. The determination of the regional impedance phases, on the other hand, is far more sensitive to deviations from the physical distortion model.
The approach has been used to investigate the Iapetus data set. For the array, which covers an area of 200  km × 300  km in northern England/southern Scotland, the technique revealed a common regional strike azimuth of ca . N125° E in the period range 500–2000  s. This direction differs from the strike indicated by the induction arrows, which seem influenced mainly by local current concentrations along the east–west-striking Northumberland Trough and a NE–SW-striking mid-crustal conductor. Both impedance phases are positive and differ by ca . 10°, which supports the assumptions of distortion fields in the data set and that the regional structure is 2-D.  相似文献   

13.
b
This paper describes a method in which vertical resistivity sections are generated tomographically from measurements on a linear array of equally spaced electrodes inserted at the ground surface. The array is multiplexed to a resistivity meter which gathers one set of all possible independent apparent resistivity measurements and the geophysical section is then reconstructed by backprojecting these weighted data, along equipotentials, into the subsurface. The technique has been evaluated numerically and in field trials over shallow archaeological structures at Fountains Abbey.  相似文献   

14.
FIR filter effects and nucleation phases   总被引：1，自引：0，他引：1  
The symmetric impulse response of linear phase Finite Impulse Response (FIR) filters most commonly used in modern seismic recording systems produces precursory signals to impulsive arrivals. These acausal filter-generated artefacts may result in misinterpretations of various onset properties. Prior to any onset interpretation, these effects have to be removed from the seismic record. This can be achieved without loss of bandwidth by post-filtration of the digital seismograms if the filter coefficients and the decimation ratios are known. We have analysed numerous signals from different instruments and sampling rates for precursory phases and found that—in contrast to commonly held beliefs—FIR-filter-related precursory signals are not always easy to recognize visually from their waveform signature. Furthermore, they can exhibit surprisingly similar properties to those reported for nucleation phases, although the majority of nucleation phases reported in the past have been obtained on instruments with a causal response. We demonstrate examples of filter-related precursory signals for events scanning nine orders of moment, from 1010 N m to 1019 N m. Surprisingly, the lower bound of the artefact durations as a function of seismic moment scales close to the cube root of the seismic moment. We interpret this as being caused by the fact that above a certain seismic moment, the attenuated source signal acts as a causal lowpass filter of a smaller bandwidth than the FIR filter. Assuming an ω-2 source model, constant stress drop and an empirical relationship between the maximum artefact duration and the cut-off frequency of the FIR filter, the artefact durations are expected to scale proportional to the 1/2.5 power of the seismic moment, in comparison to 1/3 as proposed for nucleation phases.  相似文献   

15.
Continuous GPS (CGPS) coordinate time-series are known to experience repeating deformation signals with seasonal and other periods. It is unlikely that these signals represent perfect sinusoids with temporally constant amplitude. We develop an analysis method that accommodates temporal variations in the amplitudes of sinusoidal signals. We apply the method to simulated coordinate time-series to numerically explore the potential consequences of neglecting decadal variation in amplitude of annual motions on the residual-error spectra of CGPS measurements, as well as potential bias in estimates for secular site velocity. We find that secular velocity bias can be appreciable for shorter time-series, and that residual-error time-series of longer duration may contain significant power in a broad band centred on semi-annual frequency if temporal variation in the amplitude of annual motions is not accounted for in the model used to reduce the observations to residuals. It may be difficult to differentiate the bandpass filtered signature of mismodelled loading signals from power-law noise, using residual-error spectra for shorter time-series. We provide an example application to a ∼9-yr coordinate time-series for a CGPS station located in southern California at Carbon Creek Control Structure (CCCS), which is known to experience large amplitude seasonal motions associated with the Santa Ana aquifer system.  相似文献   

16.
Summary. Teleseismic Rayleigh waves, MS > 7.0, in the period range 14 to 28 s, are well recorded by the short-period Benioff array within southern California. Multiple arrivals that hinder the determination of local phase velocity curves are detected by narrow band-pass filtering. The records are then windowed on distinct, coherent peaks that move uniformly across the array. Four to seven stations are included in the determination of both the phase velocity across the array and the incidence azimuth. For earthquakes in the western Pacific, the derived incidence azimuths are systematically rotated counterclockwise by 2–16°. Most of this rotation results from refraction at the continental shelf. Phase velocity data for both the southern Mojave—central Transverse Ranges and the Peninsular Ranges are inverted to obtain regional 5-wave velocity models. The starting models are constructed from travel-time studies of local sources, both natural and artificial. Poisson's ratio as a function of depth is calculated for these two regions. The comparison of Poisson's ratio with laboratory ultrasonic studies requires a quartz-rich crust within the southern Mojave—central Transverse Ranges and a mafic crust within the Peninsular Ranges.  相似文献   

17.
We present a new tool for efficient incoherent noise reduction for array data employing complex trace analysis. An amplitude-unbiased coherency measure is designed based on the instantaneous phase, which is used to weight the samples of an ordinary, linear stack. The result is called the phase-weighted stack (PWS) and is cleaned from incoherent noise. PWS thus permits detection of weak but coherent arrivals. The method presented can easily be extended to phase-weighted cross-correlations or be applied in the τ p domain. We illustrate and discuss the advantages and disadvantages of PWS in comparison with other coherency measures and present examples. We further show that our non-linear stacking technique enables us to detect a weak lower-mantle P -to- S conversion from a depth of approximately 840 km on array data. Hints of an 840 km discontinuity have been reported; however, such a discontinuity is not yet established due to the lack of further evidence.  相似文献   

18.
The zero-lag cross-correlation technique, used for array analysis in the hypothesis of plane waves, has been modified to allow the wave front to be circular. Synthetic tests have been performed to check the capability of the method, which returns the input test data when the source–array distances are not greater than two or three times the array aperture. For this distance range the method furnishes an estimate of the apparent velocity and the epicentral coordinates of the source. For more distant sources the method becomes equivalent to that based on the planar-wave approximation, which gives an estimate of the backazimuth to the source and the apparent velocity. The method has been applied to seismic data recorded at the active volcano located at Deception Island, Antarctica. 35 volcanic long-period events occurring in a small swarm were selected. Results show that the epicentres are close to the array (between 0.4 and 2 km) and aligned in a SW direction, in agreement with one of the main directions of the fracture system of Deception volcano.  相似文献   

19.
A massive vapour cloud explosion occurred at the Buncefield fuel depot near Hemel Hempstead, UK, in the morning of 2005 December 11. The explosion was the result of an overflow from one of the storage tanks with the release of over 300 tons of petrol and generating a vapour cloud that spread over an area of 80 000 m2, before being ignited. Considerable damage was caused in the vicinity of the explosion and a total of 43 people were injured. The explosion was detected by seismograph stations in the UK and the Netherlands and by infrasound arrays in the Netherlands. We analysed the seismic recordings to determine the origin time of 06:01:31.45 ±0.5 s (UTC) from P -wave arrival times. Uncertainties in determination of origin time from acoustic arrival times alone were less than 10 s. Amplitudes of P -, Lg and primary acoustic waves were measured to derive decay relationships as function of distance. From the seismic amplitudes we estimated a yield of 2–10 tons equivalent to a buried explosion. Most seismic stations recorded primary and secondary acoustic waves. We used atmospheric ray tracing to identify the various travel paths, which depend on temperature and wind speed as function of altitude, leading to directional variation. Refracted waves were observed from the troposphere, stratosphere and thermosphere with a good match between observed and calculated traveltimes. The various wave types were also identified through array processing, which provides backazimuth and slowness, of recordings from an infrasound array in the Netherlands. The amplitude of stratospheric refracted acoustic waves recorded by the array microbarometers was used to estimate a yield of 21.6 (±5) tons TNT equivalent. We have demonstrated through joint seismo-acoustic analysis of the explosion that both the seismic velocity model and the atmospheric model are sufficient to explain the observed traveltimes.  相似文献   

20.
Summary. A tripartite ocean-bottom seismograph array at the junction of the East Pacific Rise and Rivera Fracture Zone recorded an eathquake sequence, consisting of three main shocks ( m B= 4.3, 4.3 and 4.8) and numerous aftershocks from the fracture zone, in the distance range 35–50 km. Delineation of the rupture zones by aftershocks indicates that the first two main shocks took place on overlapping fault areas, while the third occurred over a fault area separated from the first by several kilometres. Both rupture zones were about 4 km long. Surface wave spectra indicate a shallow (about 3 km below the sea floor) source, as does OBS array phase velocity data. The seismic moments, obtained from teleseismic surface wave data, of 1.3, 2.1 and 2.8 × 1023 dyn cm, with the fault areas as delineated by aftershocks, imply a stress drop of about 8 bars for the main shocks. Aftershock sequences of each of the main shocks are similar, with a b -value of about 0.65. Teleseismic P travel times are similar to those from near-surface sources in Nevada.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号