Earth-strain observations made with the Cambridge laser strainmeter

Summary. A span of 74 days of data from the 55-m iodine stabilized laser strainmeter in Queensbury tunnel, Yorkshire, England has been analysed in detail. The strain noise level observed by the instrument is similar to that observed by American laser strainmeters at low (tidal) frequencies, but is much greater at higher (seismic) frequencies. This is explained in terms of ocean loading and microseismic noise. A residual time series was obtained by subtracting a best-fit tide from the data, and the larger residuals correlate well with storm surges in the North Sea; this is a further manifestation of ocean loading. The high noise level at seismic frequencies renders the Queensbury site rather unsuitable for normal mode studies, but free oscillations have been observed following an M_s 7.6 earthquake.     

Spatial Characteristics of Regional Wildfire Frequencies in Intermountain West Grass-Dominated Communities

Understanding the environmental factors that influence spatial patterns of rangeland wildfires facilitates both fire and land management decisions. I used discriminant analysis to examine ignition frequency of lightning-initiated fires in grass-dominated communities in the Intermountain West between 1980 and 1994, then mapped regional fire frequencies to illustrate spatial patterns. Two canonical discriminant functions effectively separate groups of high, medium, and low ignition frequencies, based on climate conditions and fuel characteristics. Regions of highest frequency tend to have large elevational differences, more mesic climates, and less annual grass cover. Spatial patterns of ignition frequencies tend to reflect local topography, with higher frequencies west of 119° W. Expansion of exotic annual grasses throughout the region may be reducing differences in ignition frequency between regions of highest and lowest frequency.     

Spatial Characteristics of Regional Wildfire Frequencies in Intermountain West Grass-Dominated Communities

Understanding the environmental factors that influence spatial patterns of rangeland wildfires facilitates both fire and land management decisions. I used discriminant analysis to examine ignition frequency of lightning-initiated fires in grass-dominated communities in the Intermountain West between 1980 and 1994, then mapped regional fire frequencies to illustrate spatial patterns. Two canonical discriminant functions effectively separate groups of high, medium, and low ignition frequencies, based on climate conditions and fuel characteristics. Regions of highest frequency tend to have large elevational differences, more mesic climates, and less annual grass cover. Spatial patterns of ignition frequencies tend to reflect local topography, with higher frequencies west of 119° W. Expansion of exotic annual grasses throughout the region may be reducing differences in ignition frequency between regions of highest and lowest frequency.     

岷江干旱河谷豆科植物多样性及其空间格局

为研究岷江干旱河谷的豆科植物物种多样性及其在纬度和海拔梯度上的分布格局,在从四川汶川到松潘的干旱河谷两岸选择了7个地点,通过204个样方 (2 m×2 m) 调查,分析了不同纬度和海拔豆科物种组成、高度、盖度与根系结瘤的空间分布特点.结果表明:(1) 在所有样方中共出现乡土豆科植物16属,38种,其中灌木7属19种;草本植物7属17种,1或2 a生草本 6种,多年生草本11种.仅刺槐 (Robinia pseudoacacia L.) 和葫芦巴 (Trigonellae foenum-graecum L.) 为栽培物种.灌木在该地区豆科植物中占较大优势,50%以上灌木的频度与盖度均＞10%,尤其白刺花 (Sophora davidii)、小马鞍羊蹄甲 (Bauhinia brachycarpa var.microphylla)、岷谷木蓝 (Indigofera lenticellata) 等灌木频度＞20%,而所有草本植物的频率及盖度都＜10%. (2) 豆科植物丰富度及生长具有较为明显的空间差异,干旱河谷核心地区的干热、贫瘠环境中,总物种丰富度较高,但是其盖度与高度较小.随着纬度升高,灌木丰富度和频度均减少,草本丰富度与和频度增加.随着海拔上升,灌木丰富度与总物种丰富度都减小,草本植物丰富度变化不明显;植物平均高度与盖度高度也无明显的垂直变化规律.(3) 植物根系结瘤能力很低,66%物种未结瘤,并且幼苗结瘤能力显著大于成株.这一结果可为豆科植物资源的保护和开发利用提供科学依据.     

Seismic scattering in the upper crystalline crust based on evidence from sonic logs

Telluric distortion occurs when electric charges accumulate along near-surface inhomogeneities. At low frequencies, the electric currents associated with these charges can be neglected compared to currents induced deeper in the Earth. At higher frequencies, the magnetic fields associated with these currents may be significant. Some parameters describing the distortion magnetic fields can be estimated from measured magneto-telluric impedance matrices. For regional magnetic fields aligned with regional strike directions, parameters associated with the distortion magnetic field component parallel to the regional magnetic field are undeterminable, whereas parameters associated with the distortion magnetic field component perpendicular to the regional magnetic field can be estimated. Optimal estimates are straightforward even for the realistic case of measurement errors that are correlated between elements of a measured impedance matrix. In a simple example of a 1-D anisotropic model with anisotropy direction varying with depth, the modelling of distortion magnetic fields results in regional impedance estimates corresponding more closely to the responses of uncoupled isotropic models, allowing sensible interpretation of an additional one and a half decades of data.     

新疆夜雨和昼雨的空间分布和长期变化

用新疆98个气象站1960—2001年的昼夜降水量资料,分析了冷季（10月到翌年3月）和暖季（4—9月）昼夜降水频率和强度的差异和空间分布。结果表明：新疆暖季分别有两个频率≥55％的夜雨区和昼雨区。夜雨区分别位于西天山北坡和西昆仑山北坡;昼雨区位于阿尔泰山南坡和天山南坡等地。而到了冷季,暖季夜雨和昼雨的条状带相间的格局消失。除了海拔较高的西天山山区和海拔较低的塔里木盆地西部、吐善托盆地分别有小范围的夜雨区和昼雨区外,新疆其他地区的夜雨和昼雨的比例均在45％~55％之间,基本趋于平衡,夜雨和昼雨之差较暖季已明显减弱。对于暖季的大降水事件（一个夜间或白天降水>15 mm）,伊犁河谷、中东天山及天山北坡,昆仑山北坡60％以上发生在夜间,阿尔泰山和塔尔巴哈台南坡、天山东南部盆地地区60％以上发生在白天。另外,还对暖季夜雨区夏季降水的长期变化进行了分析,结果显示,这两个以夜间降水为主的区域,其夜间降水的增加率略大于白天,并不是很明显大于白天降水的增加率。     

An analytical approach for the scattering of SH waves by a symmetrical V-shaped canyon: shallow case

Based on the application of the region-matching technique, an analytical approach is presented for the scattering of plane SH waves from a shallow symmetrical V-shaped canyon, and then a series solution is derived. The analysed region is divided into an enclosed and an open region by introducing a semi-circular auxiliary boundary. In each region, the displacement field can be expressed as infinite sum of appropriate wavefunctions satisfying partial boundary conditions, respectively. The unknown coefficients can be determined by enforcing the continuity conditions in connection with the Graf's addition formula. The frequency- and time-domain responses are both evaluated and displayed for several physical parameters. From graphical results, the effects of the canyon depth on surface ground motion are conspicuous. The proposed series solutions can serve as benchmark for numerical methods, in particular for those at much higher frequencies.     

Pseudo-toplap in seismic models of the Schlern-Raibl contact (Sella platform, northern Italy)

In the Sella platform of the Dolomite Alps, the horizontal beds of the Raibl Formation directly overlie the Upper Schlern Dolomite (Cassian Dolomite of some authors). The Schlern Dolomite comprises steep clinoforms, together with a few tens of metres of horizontal topset beds in the platform interior. Two-dimensional seismic modelling techniques were used to determine the seismic-response of this interesting situation. To perform this, two different lithological models were constructed based on outcrops of the Sella south face. The first model is that of a rapidly prograding platform with slow aggradation in the platform interior. The second model shows clinoforms toplapping against the topset beds of the platform interior. P-wave velocities and bulk densities were assigned to the lithostratigraphical units in accordance with values from a similar study, involving the same formations. The vertical-incidence method was used to construct perfectly migrated time sections and depth sections of reflectivity. These were convolved with zero-phase Ricker wavelets of different peak frequencies to produce the final synthetic seismic sections. Using conventional, low frequencies (e.g. 25 Hz), the seismic response of the two models is almost identical. The topset beds and the Schlern-Raibl contact appear as one event. In a real seismic survey, both sections would be interpreted as toplap of Schlern clinoforms against the Raibl Formation. At higher frequencies (75 Hz), however, differences are revealed. The angle of progradation in the progradation & aggradation model becomes visible, as opposed to the horizontal surface in the progradation & toplap model. Topset beds are resolved separate from the Raibl Formation, but still appear to form a single dipping pseudo-toplap surface. Another modelling technique, simulating unmigrated sections, shows few differences between the two models even at high frequencies. In addition, the clinoforms are disturbed by the refraction of rays. This study demonstrates that, even under ideal acquisition and processing conditions, the seismic tool can introduce a pseudo-toplap. This implies that toplap in a seismic section is not necessarily toplap in outcrop.     

Methodologies for estimating t*(f) from short-period body waves and regional variations of t*(f) in the United States

Summary. In this paper we discuss some aspects of estimating t * from short-period body waves and present some limits on t* (f) models for the central and south-western United States (CUS and SWUS). We find that for short-period data, with frequencies above 1 or 2 Hz, while the average spectral shape is stable, the smaller details of the spectra are not; thus, only an average t *, and not a frequency-dependent t *, can be derived from such information. Also, amplitudes are extremely variable for short-period data, and thus a great deal of data from many stations and azimuths must be used when amplitudes are included in attenuation studies.
The predictions of three pairs of models for t* (f) in the central and south-western United States are compared with time domain observations of amplitudes and waveforms and frequency domain observations of spectral slopes to put bounds on the attenuation under the different parts of the country. A model with the t * values of the CUS and SWUS converging at low frequencies and differing slightly at high frequencies matches the spectral domain characteristics, but not the time domain amplitudes and waveforms of short-period body waves. A model with t * curves converging at low frequencies, but diverging strongly at high frequencies, matches the time domain observations, but not the spectral shapes. A model with nearly-parallel t* (f) curves for the central and south-western United States satisfies both the time and frequency domain observations.
We conclude that use of both time and frequency domain information is essential in determining t* (f) models. For the central and south-western United States, a model with nearly-parallel t* (f) curves, where Δ t *∼ 0.2 s, satisfies both kinds of data in the 0.3–2 Hz frequency range.     

Crosshole seismic waveform tomography – I. Strategy for real data application

The frequency-domain version of waveform tomography enables the use of distinct frequency components to adequately reconstruct the subsurface velocity field, and thereby dramatically reduces the input data quantity required for the inversion process. It makes waveform tomography a computationally tractable problem for production uses, but its applicability to real seismic data particularly in the petroleum exploration and development scale needs to be examined. As real data are often band limited with missing low frequencies, a good starting model is necessary for waveform tomography, to fill in the gap of low frequencies before the inversion of available frequencies. In the inversion stage, a group of frequencies should be used simultaneously at each iteration, to suppress the effect of data noise in the frequency domain. Meanwhile, a smoothness constraint on the model must be used in the inversion, to cope the effect of data noise, the effect of non-linearity of the problem, and the effect of strong sensitivities of short wavelength model variations. In this paper we use frequency-domain waveform tomography to provide quantitative velocity images of a crosshole target between boreholes 300 m apart. Due to the complexity of the local geology the velocity variations were extreme (between 3000 and 5500 m s⁻¹), making the inversion problem highly non-linear. Nevertheless, the waveform tomography results correlate well with borehole logs, and provide realistic geological information that can be tracked between the boreholes with confidence.     

Effects of coal dust on species composition of mosses and lichens in an arid environment

A wind-borne plume of coal dust has been deposited on the soil and vegetation next to a coal storage area at a power generation plant in Oregon (U.S.A.). Correlated with the deposition of coal dust were significantly increased soil temperatures, decreased pH, and increased moisture-holding properties. Some heavy metal concentrations were also higher on the plume. These differences are possibly responsible for the lower frequencies and diversity of lichen species on the plume. Moss diversity, frequency and percent cover are similar on and off the plume with the exception of Ceratodon purpureus. The increased cover of C. purpureus may be attributed to heavy metal tolerance on the plume.     

桃花迟早年型的冬季气候特点及花期预测

对广西桂林市桃花迟早年型冬季气候特点的分析表明,桃花偏早年型具有冬季降雨量偏多,12月中下旬气温偏低,1月下旬至2月下旬气温偏高等主要特征.用模糊数学方法对桃花迟早年型进行预测,可为花卉开花期应用和旅游业提供气象服务.     

九华山游客地方依恋与游后行为研究

地方因与人的紧密结合而使得空间充满了价值和意义。地方依恋是人与地方相互作用产生的情感联结,是近几年国外游憩地理学和环境心理学研究的热点。通过对宗教旅游地九华山进行实证研究,结果显示,旅游者对九华山的地方依恋处于中等程度,认同与依赖程度一般。地方依恋影响因素复杂,旅游者的背景、游览次数、对九华山的熟悉度等都会影响地方依恋,依恋程度越高,重游及推荐意愿越强。     

Inferring the relative measure of principal stress components

The phase velocity and the attenuation coefficient of compressional seismic waves, propagating in poroelastic, fluid-saturated, laminated sediments, are computed analytically from first principles. The wavefield is found to be strongly affected by the medium heterogeneity. Impedance fluctuations lead to poroelastic scattering; variations of the layer compressibilities cause inter-layer flow (a 1-D macroscopic local flow). These effects result in significant attenuation and dispersion of the seismic wavefield, even in the surface seismic frequency range, 10–100 Hz. The various attenuation mechanisms are found to be approximately additive, dominated by inter-layer flow at very low frequencies. Elastic scattering is important over a broad frequency range from seismic to sonic frequencies. Biot's global flow (the relative displacement of solid frame and fluid) contributes mainly in the range of ultrasonic frequencies. From the seismic frequency range up to ultrasonic frequencies, attenuation due to heterogeneity is strongly enhanced compared to homogeneous Biot models. Simple analytical expressions for the P -wave phase velocity and attenuation coefficient are presented as functions of frequency and of statistical medium parameters (correlation lengths, variances). These results automatically include different asymptotic approximations, such as poroelastic Backus averaging in the quasi-static and the no-flow limits, geometrical optics, and intermediate frequency ranges.     

The influence of fluid-sensitive dispersion and attenuation on AVO analysis

Analysis of seismic data suggests that hydrocarbon deposits are often associated with higher than usual values of attenuation, but this is generally ignored during amplitude-versus-offset (AVO) analysis. The effect can be modelled with equivalent medium theory based on the squirt flow concept, but the excess attenuation is associated with strong velocity dispersion. Consequently, when we study reflections from the interface between such an equivalent medium and an elastic overburden we find that the reflection coefficient varies with frequency. The impact of this variation depends on the AVO behaviour at the interface; class I reflections tend to be shifted to higher frequency while class III reflections have their lower frequencies amplified. We calculate synthetic seismograms for typical models using the reflectivity method for materials with frequency dependent velocities and attenuations, and find that these effects are predicted to be detectable on stacked data. Two field data sets show frequency anomalies similar to those predicted by the analysis, and we suggest that our modelling provides a plausible explanation of the observations.     

On azimuthal eigenwavenumbers associated with Laplace's tidal equations

Laplace's tidal equations for the case of an ocean of constant depth bounded by meridians were considered by two authors at a specific frequency as an eigenvalue problem in the azimuthal wavenumber. A finite spectrum of eigenwavenumbers was found. That eigenvalue problem is re-examined by means of asymptotic techniques and numerical integration of the governing equation of the problem. At low frequencies a formula connecting the frequency and the number of eigensolutions is established. It is shown that at a given frequency the spectrum of eigenwavenumbers is wider than that reported, but (for this type of solution) the meridional boundary conditions are satisfied approximately only for the case of very low frequencies.     

The effect of data windows on the estimation of free oscillation parameters

Summary In seeking to estimate the frequencies, decay rates, amplitudes and phases of the free oscillations of the Earth using observed seismic spectra, data windows are required to reduce the effects of mode—mode interference. Formal expressions for the variances and covariances of these estimated parameters are derived here, for the case of a single isolated mode and an arbitrary data window. Explicit results are given for a Hann and Blackman—Harris minimum four-term window as well as for a boxcar. The optimum or minimum variance record length for measuring frequencies and decay rates using a Hann window is 1.1 Q -cycles, and that for determining amplitudes and phases is 0.5 Q -cycles.     

南京市居民网络购物行为特征——以书籍和衣服为例

随着信息通讯技术(ICT)的飞速发展,网络购物日益成为当今社会经济生活的重要组成部分,深刻影响了居民的购物消费行为和传统零售业的发展。不同国家、不同人群、不同商品类型的居民购物行为具有一定差异。本文在问卷调查的基础上,分析南京市居民网络购物行为特征,并区分不同商品类型的居民网购行为特征差异,对理解网络购物和实体购物相互作用关系及网购对于交通出行和城市零售业空间的影响具有重要意义。结果表明：居民网络购物行为是一个复杂的过程,不同类型商品、不同频率网购人群在社会经济属性、空间特征、网络使用及购物习惯、购物态度上有明显差异,其中不同频率网购衣服的人群在特征上的差异更为显著,体验型商品对网络购物这一新型购物方式的响应更为强烈。     

A variational principle for the subseismic wave equation

Summary. A variational principle is developed for the subseismic wave equation governing the normal modes of the outer liquid core with frequencies below seismic frequencies (>300 μHz). The calculation of these modes is important both in determining the core contribution to the Earth's dynamical response to tidal and other forces and because their detection at the surface could provide valuable new insight into the density structure of the core, critical to theories of the geomagnetic dynamo. Included as a special case is a variational principle for the Poincaré equation governing inertial oscillations studied in the laboratory by Aldridge and others. This opens up the possibility of 'tracking' laboratory results over to the real Earth.     

On natural oscillations of connected ocean basins

Summary. Natural barotropic modes are found for systems of two or three circular basins (each of uniform depth and Coriolis parameter) linked by narrow straits. They are related to the individual basin modes for various basin and strait dimensions and Coriolis parameter values. For two basins joined by a short strait, the mode frequencies intersperse one for one the combined sequence of individual basin mode frequencies. There is no comparable result for three basins. Possible effects of the North Atlantic's open boundaries on its natural oscillations and diurnal tides are considered, and also the lowest modes of Green Bay and Lake Michigan.