Upper Mississippi embayment shallow seismic velocities measured in situ

Vertical seismic compressional- and shear-wave (P-and S-wave) profiles were collected from three shallow boreholes in sediment of the upper Mississippi embayment. The site of the 60-m hole at Shelby Forest, Tennessee, is on bluffs forming the eastern edge of the Mississippi alluvial plain. The bluffs are composed of Pleistocene loess, Pliocene-Pleistocene alluvial clay and sand deposits, and Tertiary deltaic-marine sediment. The 36-m hole at Marked Tree, Arkansas, and the 27-m hole at Risco, Missouri, are in Holocene Mississippi river floodplain sand, silt, and gravel deposits. At each site, impulsive P- and S-waves were generated by man-made sources at the surface while a three-component geophone was locked downhole at 0.91-m intervals.

Consistent with their very similar geology, the two floodplain locations have nearly identical S-wave velocity (V_S) profiles. The lowest V_S values are about 130 m s⁻¹, and the highest values are about 300 m s⁻¹ at these sites. The shear-wave velocity profile at Shelby Forest is very similar within the Pleistocene loess (12 m thick); in deeper, older material, V_S exceeds 400 m s⁻¹.

At Marked Tree, and at Risco, the compressional-wave velocity (V_P) values above the water table are as low as about 230 m s⁻¹, and rise to about 1.9 km s⁻¹ below the water table. At Shelby Forest, V_P values in the unsaturated loess are as low as 302 m s⁻¹. V_P values below the water table are about 1.8 km s⁻¹. For the two floodplain sites, the V_P/V_S ratio increases rapidly across the water table depth. For the Shelby Forest site, the largest increase in the V_P/V_S ratio occurs at 20-m depth, the boundary between the Pliocene-Pleistocene clay and sand deposits and the Eocene shallow-marine clay and silt deposits.

Until recently, seismic velocity data for the embayment basin came from eartquake studies, crustal-scale seismic refraction and reflection profiles, sonic logs, and from analysis of dispersed earthquake surface waves. Since 1991, seismic data for shallow sediment obtained from reflection, refraction, crosshole and downhole techniques have been obtained for sites at the northern end of the embayment basin. The present borehole data, however, are measured from sites representative of large areas in the Mississippi embayment. Therefore, they fill a gap in information needed for modeling the response of the embayment to destructive seismic shaking.     

内蒙古中部四子王旗大庙岩体时代及成因

华北北缘的内蒙古中部地区出露大量晚古生代-早中生代花岗岩类,在空间上构成一条巨大的东西向花岗岩带.四子王旗大庙岩体作为一个典型的代表,以花岗闪长岩为主,其内部普遍发育暗色微粒包体(MMEs),是认识花岗岩岩石成因和演化的关键.本文对包体及寄主岩进行了同位素测年、岩相学、矿物化学、全岩主量元素和微量元素分析.寄主岩石中的锆石LA-ICPMS U-Pb年龄平均为265±7Ma(2σ),包体中单颗粒黑云母Rb-Sr年龄为253±5Ma(MSWD=0.85),属晚二叠世-早三叠世岩浆活动的产物.包体具塑性外形及岩浆结构,存在多种不平衡矿物组合;MME中的斜长石An组分及黑云母斑晶中MgO成分呈多期震荡,同时总体上均显示出幔部高于核、边部的特征,暗示斑晶可能为围岩捕虏晶,这种相似的成分变化指示包体与寄主岩相互作用引起的结晶环境改变,标志着岩浆成分的变化,是岩浆混合的标志之一;主量和微量数据进一步证明岩体的岩浆混合成因.Rb/Sr-K/Rb变化关系反映包体非结晶分异或黑云母堆晶的产物,而Ce/Pb-Ce、Ba-δEu和P_2O_5-δEu图及其他微量元素比值图等均表明花岗闪长岩体发生了岩浆混合作用,这也得到岩浆物理化学条件的支持.岩浆底侵和岩浆混合作用是该区岩体形成的主要机制和方式.岩石地球化学特征表明该岩体不同于加厚地壳和俯冲洋壳熔融的TTG和埃达克质岩石,而黑云母矿物化学和岩石地球化学显示其构造背景很可能为同碰撞环境.     

Precise measurement of seismic travel times from an air gun source

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.     

Quantitative Distribution of Deep-water Benthos in Western Central Pacific

Altogether 10 surfaces and 8 cores were sampled quantitatively from the Western Central Pacific(4°00′S—5°00′N, 160°00′—165°00′E and 2°00′—10°00′S, 170°00′—173°20′E) in 1978 and 1979. The results of preliminary analysis show that the faunal composition was quite simlpe, only eight species of macrobenthic invertebrates, four species of Polychaete, two     

Numerical evaluation of the Hilbert transform by the Fast Fourier Transform (FFT) technique

Summary. Three Fast Fourier Transform numerical methods for computing the Hilbert transform have been evaluated for their accuracy by numerical examples. All three methods employ the property that the Hdbert transform is a convolution. The first method uses the result that the Fourier transform of 1/π x is — isgn(ω). The second method is based on a discrete Hilbert transform introduced by Saito. The third method, introduced in this research note, uses linear interpolation to transform the Hilbert transform integral into a discrete convolution. The last method is shown by numerical examples from fault dislocation models to be more accurate than the other two methods when the Hilbert transform integral has high-frequency components.     

大气气溶胶粒径分布特征与气象条件的相关性分析

通过温度、相对湿度和风速等气象因素与不同粒径大气气溶胶粒子数浓度和质量浓度的相关性,分析气象条件对大气气溶胶的影响和作用机制。结果表明：气象因素对0.2—0.6 μm的气溶胶影响最大。温度升高既有利于增强大气扩散作用也有利于二次气溶胶生成,因此温度与超细气溶胶(小于0.1μm)呈正相关性,而与粒径较大的气溶胶呈负相关。风速主要影响气溶胶的水平扩散,对超细气溶胶无显著影响,而与粗粒径气溶胶呈负相关。相对湿度会促进超细气溶胶的聚积,使之生成较大粒径气溶胶。因此相对湿度与超细气溶胶呈较强的负相关,而与较粗粒径气溶胶呈正相关。     

Temperature Dependence of Single-Crystal Spinel (MgAl2O4) Elastic Constants from 293 to 423°K Measured by Light-Sound Scattering in the Raman-Nath Region

Summary. The temperature dependence of single-crystal elastic constants of synthetic stoichiometric MgAl₂O₄ spinel has been measured by the light-sound scattering technique in the Raman-Nath region. The crystal is set into forced vibration by a single crystal LiNbO₃ transducer coupled to one crystal face. A He-Ne Laser beam is diffracted by the stress-induced birefringence inside the crystal. The diffraction angle is determined from the distance between two spots exposed on a photographic plate by the first order diffracted beams as measured by a microdensitometer. The sound wavelength inside the crystal is then inferred from the laser diffraction angle. Combining the sound wavelength with the measured transducer frequency, the velocity inside the crystal is determined typically to a precision of 0·05 per cent. In this method, the measurement of velocity is not dependent on either the determination of sample length or on phase shifts at sample-transducer interface. Velocities of four pure modes, L //[001], T //[001], L //[110], and T //[110]( P //[1 1 0] are measured in the temperature range between 293 and 423 °K. A linear temperature dependence is fit to the data by a least square method. Values obtained at 25 °C from this linear fit are
The temperature dependence of the adiabatic elastic constants and bulk and shear (VRH average) moduli is computed using the density and literature value of thermal expansion coefficient. Values obtained are:
A comparison with previous measurements by pulse superposition and ultrasonic interferometry methods is made. Disagreement, when present, is discussed in terms of the separate measuring techniques. Finally, the present method, with its possibility for further improvement, is evaluated as a new method to measure temperature and pressure dependence of elastic constants.