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1.
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   

2.
Summary During the past 10 years the Geoacoustics Group of NOAA's Wave Propagation Laboratory studied travelling low-frequency pressure variations related to thunderstorms and severe weather. Two general categories of waves were associated with severe weather conditions: 'subsonic' pressure disturbances and infrasonic waves with acoustic velocities. The low-frequency pressure variations were measured at the Earth's surface using microphone arrays located at times thousands of kilometres from the severe-weather disturbance. The radiated infra- sound was related to thunderstorms penetrating the tropopause and spectral analyses were performed on several signals. Possible practical applications to storm warning and classification are discussed for both infrasound and 'subsonic' pressure disturbances. Past measurements of these signals are reviewed.  相似文献   

3.
Summary Observations at Inuvik (70.4° dipole latitude) have shown that supersonic motions of auroral arcs that sweep across the zenith from south to north during poleward expansions of auroral substorms do not generate observable auroral infrasonic waves. This is in contrast to the fact that equator-ward supersonic motions of similar auroral arcs do produce large amplitude infrasonic bow waves. These results imply an asymmetry in the basic generation mechanism of infrasound within the auroral electrojet arcs.  相似文献   

4.
We present new methods for the interpretation of 3-D seismic wide-angle reflection and refraction data with application to data acquired during the experiments CELEBRATION, 2000 and ALP 2002 in the area of the Eastern Alps and their transition to the surrounding tectonic provinces (Bohemian Massif, Carpathians, Pannonian domain, Dinarides). Data was acquired on a net of arbitrarily oriented seismic lines by simultaneous recording on all lines of seismic waves from the shots, which allows 2-D and 3-D interpretations. Much (80%) of the data set consists of crossline traces. Low signal to noise (S/N) ratio in the area of the young orogens decreases the quality of travel time picks. In these seismically heterogeneous areas it is difficult to assign clearly defined arrivals to the seismic phases, in particular on crossline record sections.
In order to enhance the S/N ratio, signal detection and stacking techniques have been applied to enhance the Pg -, Pn - and PmP phases. Further, inversion methods have been developed for the interpretation of WAR/R-data, based on automated 1-D inversion ( Pg ) and the application of the delay time concept ( Pn ). The results include a 3-D velocity model of the crust based on Pg waves, time and depth maps of the Moho and a Pn -velocity map. The models based on stacked data are robust and provide a larger coverage, than models based on travel time picks from single-fold (unstacked) traces, but have relatively low resolution, especially near the surface. They were used as the basis for constructing models with improved resolution by the inversion of picks from single-fold data. The results correlate well with geological structures and show new prominent features in the Eastern Alps area and their surrounds. The velocity distribution in the crust has strong lateral variations and the Moho in the investigation area appears to be fragmented into three parts.  相似文献   

5.
Summary. Seismic travel times for extrema, zero-crossings, or entire body waves need to be determined precisely to one part in 103 or better in several varieties of seismic studies employing an impulsive artificial source. Examples are crosshole surveys which delineate rock crack distribution separating the holes and monitoring of crustal seismic travel times in earthquake precursor studies. A timing resolution of one part in 103 has been achieved previously using digitally recorded seismic data. These methods, however, do not use interpolation between digitized data points as a method to increase the timing resolution. We report travel-time determinations based on interpolation between digitized points which achieve a precision of two parts in 104, a five-fold improvement over the existing methods. In addition, the effects of seismic noise on travel-time measurement have been compared for the extremum location, the unnormalized correlation, and the normalized correlation method. The following conclusions are drawn from this comparison: (1) the normalized correlation method provides an 18–55 per cent improvement in the standard deviation of the mean over the extremum location method, and (2) results as accurate as those by the normalized correlation can be obtained by the unnormalized correlation if a complete up-and-down swing of the waveform is used as the master trace and if the master trace is close to being sinusoidal. The advantage of the unnormalized correlation over the normalized correlation is speed; the unnormalized correlation is faster by a factor of 28 in computing time.  相似文献   

6.
Chen  Fan  Cao  Anye  Liang  Zhengzhao  Liu  Yaoqi 《Natural Resources Research》2021,30(6):4515-4532

Mining-induced tremors are indispensable events that gestate and trigger coal bursts. The radiated energy is usually considered a key index to assess coal burst risk of seismic events. This paper presents a model to assess coal burst risk of seismic events based on multiple seismic source parameters. By considering the distribution and relation laws of the seismic source parameters of coal bursts, the model aims to identify dangerous seismic events that more closely match the characteristics of multiple seismic source parameters of coal bursts. The new coal burst risk index T is proposed. It consists of the similarity index SI (representing the similarity degree of relations between seismic events and coal burst events based on seismic source parameters) and the strength index ST (representing the burst strength of seismic events). We studied 79 coal burst events that occurred during extraction in LW250105 of the Huating coal mine in Gansu Province, China. We obtained the distribution and relation laws of multiple seismic source parameters of coal burst events to establish SI and ST. Two groups of seismic events with different energy distributions were examined to compare the assessment results based on the new model and energy criteria. The results show that 80% and 89% of seismic events with strong coal burst risk in Groups A and B, respectively, were coincident, and the seismic events with medium coal burst risk were slightly less compared to those based on radiated energy. The results indicate that the assessment based on the T value is a modification and optimization of that based on radiated energy. This model is conducive to improving the efficiency of monitoring and early warning of coal burst risk.

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7.
Summary. Numerical modelling is one of the most efficient methods for an investigation of the relationship between structural features and peculiarities of observed wavefields. It is practically the only method for 2-D and 3-D inhomogeneous media.
An algorithm based on ray theory has been developed for calculations of travel times and amplitudes of seismic waves in 3-D inhomogeneous media with curved interfaces. It was applied for numerical modelling of kinematic and dynamic characteristics of seismic waves propagating in laterally inhomogeneous media.
Travel-time and amplitude patterns were studied in the 2-D and 3-D models of a geosyncline, in which velocity distribution was given by an analytical function of the coordinates. For a more complicated model representing a subducting high-velocity lithospheric plate in a transition zone between oceanic and continental upper mantle, the velocity distribution was given by discrete values on a 2-D non-rectangular grid. It was shown that when a source was placed above the lithospheric plate, a shadow zone appeared along a strike of the structure, i.e. in the direction which is perpendicular to a strong lateral velocity gradient. Travel-time residuals were calculated along the seismological profile for a 3-D velocity distribution in the upper mantle beneath Central Asia, obtained as a result of inversion of travel times by the Backus-Gilbert method. They were found to be in a good agreement with the observed data.  相似文献   

8.
Summary. We investigate one-dimensional waves in a standard linear solid for geophysically relevant ranges of the parameters. The critical parameters are shown to be T*= tu/Qm where t u is the travel time and Qm the quality factor in the absorption band, and τ−1 m , the high-frequency cut-off of the relaxation spectrum. The visual onset time, rise time, peak time, and peak amplitude are studied as functions of T* and τ m. For very small τ m , this model is shown to be very similar to previously proposed attenuation models. As τ m grows past a critical value which depends on T* , the character of the attenuated pulse changes. Seismological implications of this model may be inferred by comparing body wave travel times with a'one second'earth model derived from long-period observations and corrected for attenuation effects assuming a frequency independent Q over the seismic band. From such a comparison we speculate that there may be a gap in the relaxation spectrum of the Earth's mantle for relaxation times shorter than about one second. However, observational constraints from the attenuation of body waves suggest that such a gap might in fact occur at higher frequencies. Such a hypothesis would imply a frequency dependence of Q in the Earth's mantle for short-period body waves.  相似文献   

9.
Summary Microphones and seismographs were co-located in arrays on Skidaway Island, Georgia, for the launchings of Apollo 13 and 14, 374 km to the south. Simultaneous acoustic and seismic waves were recorded for both events at times appropriate to the arrival of the acoustic waves from the source. Significant comparisons of the true signals are (1) the acoustic signal is relatively broadband compared to the nearly monochromatic seismic signal; (2) the seismic signal is much more continuous than the more pulse-like acoustic signal; (3) ground loading from the pressure variations of the acoustic waves is shown to be too small to account for the seismic waves; (4) the measured phase velocities of both acoustic and seismic waves across the local instrument arrays differ by less than 6 per cent and possibly 3 per cent if experimental error is included. It is concluded that the seismic waves are generated by resonant coupling to the acoustic waves along some 10 km of path on Skidaway Island. The thickness of unconsolidated sediment on the island is appropriate to a resonant ground wave frequency of 3.5 to 4 Hz, as observed. Under appropriate conditions, ground wave observations may prove more effective means of detecting certain aspects of acoustic signals in view of the filtering of wind noise and amplification through resonance.  相似文献   

10.
Summary. Based on accurately located 23 very shallow earthquakes ( h = 1–14 km) in northern and central Greece by portable networks of seismic stations and by the joint epicentre method, the travel times of the Pn -waves from the foci of these earthquakes to the sites of 54 permanent stations in the Balkan region have been determined. The travel times of Pn -waves in the central and eastern part of the area (eastern Greece, south-eastern Yugoslavia, the Aegean Sea, Bulgaria, southern Romania, western Turkey) fit a straight line very well with the Pn velocity equal to 7.9 ± 0.1 km s-1. On the contrary, the travel times of Pn -waves to stations in the western part of the area (Albania, western Greece) do not fit this curve because the Pn -waves travelling to these stations are delayed by more than 1 s due to the thicker crust under the Dinarides–Hellenides mountain range. Time delays for Pn -waves have been calculated for each permanent station in the Balkan area with respect to the mean travel-time curve of these waves in the central and eastern part of the area. Corrections of the travel times for these delays contribute very much to the improvement of the accuracy in the location of the shallow earthquakes in the Aegean and surrounding area.  相似文献   

11.
Summary. Refraction seismic profiles in the Alpine area occasionally show seismic 'shadows' which cannot be explained by a horizontal low-velocity channel. An example is demonstrated where the low P -velocity zone (Flysch) dips below a wedge of high P -velocity (the Calcareous Alps) which is a typical Alpine structure. This feature has been studied by means of a seismic model experiment. The corner of the wedge works as a source of diffracted P -, S - and Rayleigh-waves, when met by seismic waves. The diffracted waves can be distinguished from conventional waves by their amplitude–distance relation when the extinction coefficient is well known by an independent experiment.  相似文献   

12.
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13.
Summary. Peculiarities of propagation in the upper mantle of western Europe are documented by profiles of stations recording body waves generated by explosions. Azimuthal variations of travel times and amplitudes of P waves and possible birefringence of S waves may be associated with an anisotropic layer at depths where the lithosphere-asthenosphere transition is supposed to be.  相似文献   

14.
Summary A theoretical analysis is given for the acoustical behaviour of the pipe-microbarograph systems used to detect acoustic gravity waves and other modes of infrasound. It is shown how to compute the response of the microbarograph to a fluctuating pressure at any one inlet port of the pipe and how the results of such computations may be used to calculate the response to a plane sound wave traversing the system.
The analysis is illustrated by numerical examples obtained by means of a computer program. These examples confirm that the tapered tube modelled after Daniels' line microphone has very good characteristics, but that good results may also be obtained using pipes of uniform bore. The work leans heavily on Benade's calculations of sound propagation in a circular conduit.  相似文献   

15.
What can be learned from rotational motions excited by earthquakes?   总被引：1，自引：0，他引：1  
One answer to the question posed in the title is that we will have more accurate data for arrival times of SH waves, because the rotational component around the vertical axis is sensitive to SH waves although not to P-SV waves. Importantly, there is another answer related to seismic sources, which will be discussed in this paper.
Generally, not only dislocations commonly used in earthquake models but also other kind of defects could contribute to producing seismic waves. In particular, rotational strains at earthquake sources directly generate rotational components in seismic waves. Employing the geometrical theory of defects, we obtain a general expression for the rotational motion of seismic waves as a function of the parameters of source defects.
Using this expression, together with one for translational motion, we can estimate the rotational strain tensor and the spatial variation of slip velocity in the source area of earthquakes. These quantities will be large at the edges of a fault plane due to spatially rapid changes of slip on the fault and/or a formation of tensile fractures.  相似文献   

16.
徐彤  徐彬  吴健  胡艳莉  许正文 《极地研究》2014,26(3):316-323
"极区电急流天线"辐射依赖于低电离层D/E区背景电急流,而高电离层F区极低频调制加热,可产生抗磁性电流,形成极低频波辐射源。利用电离层F区一维时变加热数值模型,采用全波解算法研究高纬Troms(69.59°N,19.23°E)地区电离层F区极低频调制加热。模拟结果表明,极区高电离层激发的极低频波与极区低电离层激发的结果不同。加热泵波的有效辐射功率(effective radiated power,ERP)、调制频率及电离层背景对极低频波强度有着重要影响。  相似文献   

17.
Summary The displacement response of an elastic half space to a plane pressure wave is examined in order to establish the conditions under which sources of this type can contribute significantly to the long-period seismic noise field. The study is restricted to pressure waves which propagate at velocities well below the seismic wave velocities characteristic of the half space. The numerical studies indicate that pressure waves with amplitudes of 100 μbar or more can contribute significantly to the long-period vertical background noise observed at the surface, provided that the detectors are located on sections of alluvial fill or poorly to moderately indurated sandstones and shales whose thicknesses are greater than about a kilometre. These same waves can also create significant tilt noise on long-period horizontal seismographs located at or near the surface, regardless of the rock type. The seismic disturbances created by pressure waves decay rapidly away from the surface. Therefore, it appears that it may be possible to eliminate the effects of atmospherically generated noise by placing the detectors at moderate depths.  相似文献   

18.
Summary. The response of a stratified elastic medium can be conveniently characterized by the Green's tensor for the medium. For coupled seismic wave propagation ( P—SV or fully anisotropic), the Green's tensor may be constructed directly from two matrices of linearly independent displacement solutions. Rather simple forms for the Green's tensor can be found if each displacement matrix satisfies one of the boundary conditions on the seismic field. This approach relates directly to 'reflection matrix' representations of the seismic field.
For a stratified elastic half space the Green's tensor is used to give a spectral representation for coupled seismic waves. By means of a contour integration a general completeness relation is obtained for the 'body wave' and 'surface wave' parts of the seismic field. This relation is appropriate for SH and P–SV waves in an isotropic medium and also for full anisotropy.  相似文献   

19.
To calculate the minimum cost of travel to each location within a geographical area from a specified set of locations, most geographic information systems represent that area as a rectangular grid of discrete cells, each indicating the cost of traversing that cell's particular location. These increments of cost are then accumulated by proceeding from cell to adjacent cell in a manner that resembles the propagation of waves. Because this propagation is limited to the eight directions associated with each cell's eight neighbors, however, it is often misdirected, and travel costs are therefore often overestimated. This article discusses the context, precedent, design, implementation, performance, and implications of a new algorithm that eliminates such problems in a straightforward manner. It does so by retaining the octangular propagation mechanism of earlier algorithms while keeping track of the particular locations at which propagated waves of accumulating travel cost either refract or diffract. The approach also holds promise for significant improvement in areas ranging from dispersion modeling and shape analysis to interpolation and the delineation of cost-minimizing paths.  相似文献   

20.
The diffraction of P, S and Rayleigh waves by 3-D topographies in an elastic half-space is studied using a simplified indirect boundary element method (IBEM). This technique is based on the integral representation of the diffracted elastic fields in terms of single-layer boundary sources. It can be seen as a numerical realization of Huygens principle because diffracted waves are constructed at the boundaries from where they are radiated by means of boundary sources. A Fredholm integral equation of the second kind for such sources is obtained from the stress-free boundary conditions. A simplified discretization scheme for the numerical and analytical integration of the exact Green's functions, which employs circles of various sizes to cover most of the boundary surface, is used.
The incidence of elastic waves on 3-D topographical profiles is studied. We analyse the displacement amplitudes in the frequency, space and time domains. The results show that the vertical walls of a cylindrical cavity are strong diffractors producing emission of energy in all directions. In the case of a mountain and incident P, SV and SH waves the results show a great variability of the surface ground motion. These spatial variations are due to the interference between locally generated diffracted waves. A polarization analysis of the surface displacement at different locations shows that the diffracted waves are mostly surface and creeping waves.  相似文献   

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