Range resolution dependence of VHF radar returns from clear-air turbulence and precipitation

With employing 1.5 h of the data observed by the Chung-Li VHF radar, the range resolution dependences of the VHF backscatter from refractivity fluctuation and precipitation are investigated in this article. It indicates that the atmospheric layer structure of refractivity seems to play a role in governing the range resolution dependence of clear-air turbulent echoes. Observations shows that the VHF clear-air echo power ratios for 4 to 2 μs pulse lengths are close to 3 dB in the middle or bottom side of the layer, while the ratios are significantly greater than 3 dB in the top side of the layer. The analysis of the precipitation echo power ratio for 4 to 2 ms pulse lengths shows that basically the ratios above 3.0 km are close to 3 dB, but enormously smaller than 3 dB below 3.0 km. The feature of extraordinarily small echo power ratios below 3.0 km is also observed for the radar returns from refractivity turbulence. The radar recovery effect is thought to be a primary factor responsible for the severe diminution of the echo power ratios at the lower altitudes. In addition, statistical analysis reveals that the range resolution effect on the first and second moments of the Doppler spectra for the radar echoes from clear-air turbulence and precipitation is insignificant and negligible. The dependences of the coefficient A and power B in the power-law approximation V_t=AP^B_r to the terminal velocity V_t and range-corrected echo power P_r are examined theoretically and experimentally. The results show that the coefficient A (powers B) is inversely (positively) proportional to the range resolution, in a good agreement with the observations. Because of the strong dependence of coefficient A and power B on the radar pulse width, it suggests that great caution should be taken in comparing the power-law expressions V_t=AP^B_r established from the radar returns obtained with different range resolutions.     

On the statistical distribution of seismic velocities in Earth's deep mantle

The existence of uncoupled shear (S) and compression (P) wave velocity variations in Earth's mantle is a characteristic that might only be explained by the presence of significant chemical and/or phase heterogeneity, with important implications for the dynamics and evolution of Earth's interior. While making a one-to-one comparison between tomographic models for P and S velocity (V_P and V_S) variations for a particular geographic region is ill-posed, their global statistical distributions reveal several robust characteristics indicative of the nature of uncoupled V_P and V_S in the deep mantle. We find that all of the V_P and V_S model distributions at a given depth are Gaussian-like throughout the lowermost mantle. However, a distinct low velocity feature is present in V_S distributions below ≈ 2200 km depth that is not present or is relatively weak in V_P models. The presence of anomalously low V_S material cannot be explained as an artifact, nor can the absence of a similarly strong feature in P models be ascribed to under-resolution. We propose that this feature can be partly explained by laterally variable occurrences of post-perovskite (pPv) lenses in the D″ layer, however, the persistence of significantly slow V_S regions at heights up to ≈ 700 km or more above the core–mantle boundary is likely to be incompatible with a pPv origin and might only be explained by the presence of a laterally discontinuous layer of chemically distinct material and/or some other kind of phase heterogeneity. There also exist significant discrepancies between tomographic models with respect to the width of the distributions as well as differences between the modeled peak values. We propose a scheme for comparison between different seismic models in which the widths of the dominant features in their statistical distributions is exploited.     

Density profile and modern crustal and uppermost mantle geotherm of the Voronezh crystalline massif

A density profile and a modern temperature distribution in the lithosphere of the Voronezh crystalline massif (VCM) are derived through the use of the V_P(z), V_S(z) seismic velocity models, petrological data, measurements of V_P, V_S, density (ρ) and mean atomic weight ($\text{m}$) for several groups of rocks and minerals of different composition and genesis, as well as from pressure and temperature derivatives for different thermodynamic regimes.     

Conditions for the occurence of unphysical negative values of the anelastic SH plane wave energy-based transmission coefficient

Consider a plane homogeneous harmonic SH wave incident upon an interface between two anelastic half-spaces. Computing the plane wave displacement and energy-flux-based reflection and transmission coefficients correctly requires determining the proper signs of the vertical slowness components of all the reflected and transmitted waves, i.e., determining which of the two values of the square root for a given vertical slowness should be chosen. For anelastic media, this can be problematic, as unphysical results can arise. Previous research has led to a specific recommendation on how to choose the signs. However, when this recommendation is employed, it is found via numerical experimentation that for certain values of the medium parameters, the energy-flux-based transmission coefficient T can be negative for certain supercritical values of the incidence angle, whereas physical reasoning suggests it should be zero. To investigate this seemingly unphysical result, an analytical determination of the mathematical conditions under which it occurs would be useful. Such a determination is performed in this article. Letting V ₁ and V ₂ be the wave speeds of homogeneous SH plane waves in the incidence and transmission media respectively, and Q ₁ and Q ₂ be the corresponding quality factors, with Q ₁, Q ₂ ? 1, it is found that if V ₁ < V ₂ and Q ₁ < Q ₂ (a common situation in the Earth), then T will be negative for part of the supercritical incidence angle range if 1 < Q ₂/Q ₁ ?? 2 ? (V ₁/V ₂)² and for all of it if 2 (V ₁/V ₂)² < Q ₂/Q ₁.     

Crustal Heterogeneity and Seismotectonics of the National Capital Region, Delhi, India

To better understand the relationship between crustal heterogeneity and seismotectonics in the National Capital Region (NCR), Delhi, India, we carried out local P- and S-velocity tomography beneath the NCR. First arrival times of the first P- and S-wave from 275 crustal earthquakes recorded by the Seismic Telemetry Network in and around Delhi of the India Meteorological Department, India, are inverted to obtain crustal P-, S-velocity and V _P /V _S variations in the region. Our tomographic images of the upper crust reflect well the surface geological and tectonic features. The Delhi fold belt is identified as low V _P, high V _S and low V _P/V _S. The Sohna hot spring region is appearing as low V _P, low V _S and high V _P /V _S correlating with the possible presence of fluid-filled rocks. The crustal seismicity is distributed in both the high- and low-velocity zones, but most distinctly in the low V _P /V _S region.     

Solar wind structure sources and periodicities of auroral electron power over three solar cycles

We assess the contributions of various types of solar wind structures (transients, coronal hole high-speed streams (HSS), and slow-speed wind) to hourly average auroral electron power (P_e). The time variation of the solar wind velocity (V_sw) and P_e are determined by HSS, which contribute ～47% to P_e and V_sw. Transients contribute ～42% of P_e in solar maxima, and ～6% in solar minimum. Cross-correlations of P_e with V_sw|B| for negative B_z are significant. P_e exhibits solar rotational periodicities similar to those for V_sw, with strong 7- and 9-day periodicities in 2005–2008 and equinox semiannual periodicities in 1995–1999.     

Shear wave velocity-based evaluation and design of stone column improved ground for liquefaction mitigation

The evaluation and design of stone column improvement ground for liquefaction mitigation is a challenging issue for the state of practice. In this paper, a shear wave velocity-based approach is proposed based on the well-defined correlations of liquefaction resistance (CRR)-shear wave velocity (V _s)-void ratio (e) of sandy soils, and the values of parameters in this approach are recommended for preliminary design purpose when site specific values are not available. The detailed procedures of pre- and post-improvement liquefaction evaluations and stone column design are given. According to this approach, the required level of ground improvement will be met once the target V _s of soil is raised high enough (i.e., no less than the critical velocity) to resist the given earthquake loading according to the CRR-V _s relationship, and then this requirement is transferred to the control of target void ratio (i.e., the critical e) according to the V _s-e relationship. As this approach relies on the densification of the surrounding soil instead of the whole improved ground and is conservative by nature, specific considerations of the densification mechanism and effect are given, and the effects of drainage and reinforcement of stone columns are also discussed. A case study of a thermal power plant in Indonesia is introduced, where the effectiveness of stone column improved ground was evaluated by the proposed V _s-based method and compared with the SPT-based evaluation. This improved ground performed well and experienced no liquefaction during subsequent strong earthquakes.     

四川、甘肃地区VS30经验估计研究

目前我国建筑工程抗震设计规范中对于工程场地条件的判断依据主要是地表以下20m深度范围内土层的等效剪切波速,简称VS20。相比之下,国外应用较广的是地表以下30m深度范围内的等效剪切波速,简称VS30。这种差别导致国内科研工作者在应用国外的地震工程、工程抗震模型时经常遇到对场地条件描述不准确的困难。为了解决这个问题,本文根据147个四川、甘肃地区国家强震动台站20m左右深度的钻孔剪切波速数据,利用延拓方法、场地分类统计方法以及基于地形特征的VS30估计方法研究各台站VS30与VS20的经验关系,对比发现基于速度梯度延拓的结果最为可取。参考国际上通用的Geomatrix Classification场地分类标准,最终得到四川、甘肃地区各类场地的平均VS30,此结果可以为缺乏钻孔数据的工程场地的VS30估计提供参考。     

Migration of seismic activity during the 1998 volcanic unrest at Iwate volcano,northeastern Japan,with reference to P and S wave velocity anomaly and crustal deformation

We describe the seismicity at Iwate volcano, northeastern Japan, during the volcanic unrest of 1998 with reference to a three-dimensional P and S wave velocity model from tomographic analysis. The abnormal seismic activity beneath Iwate volcano started under the caldera in February, 1998 and migrated westward in the period February to August, 1998. Previous geodetic modeling [Sato and Hamaguchi, Chikyu Monthly 21 (1999) 312–317] suggested the growth of a dike in the time of the seismic activity. Comparing the seismicity and dike extension with the tomographic images of the P and S wave velocity structure, we find that the trace of the growing dike coincides with the region of the high V_p and high V_p/V_s ratio beneath the volcano. The seismic and geodetic data are consistent with an intrusion of magma or other fluid under the caldera in 1998. Another pressure source causing the predominant crustal deformation at Iwate volcano was detected from geodetic data, which was located in the region with high V_p/V_s ratio under the western end of the volcano through the period from February to August. It is suggested that the activation of the point pressure source probably associated with the inflation of a hot fluid reservoir relate to a geothermal region adjacent to the western edge of the volcano.     

Seismic evidence for stepwise thickening of the crust across the NE Tibetan plateau

We performed a receiver function analysis on teleseismic data recorded along two 550 km-long profiles crossing the northeastern Tibetan plateau. Results from time to depth migration, grid-search V_p/V_s determination and simulated annealing inversion of waveforms, reveal that the crust thickens from ∼50 km near the northern edge of the plateau to ∼80 km south of the Jinsha suture in the Qiang Tang block. Crustal thickening occurs in staircase fashion with steps located beneath the main, reactivated sutures. The V_p/V_s ratio, close to the global continental average does not suggest widespread partial melting but rather a more usual separation between an upper felsic and a lower mafic part within the northeastern Tibetan crust.     

Effects of chemical,mineralogical, and temperature variations on the density and bulk sound velocity of the bottom lower mantle

In this study, we have modeled the density(ρ) and bulk sound velocity(VΦ) profiles of the bottom lower mantle using the experimental thermal equation of state(EoS) parameters of lower-mantle minerals, including bridgmanite, ferropericlase,CaSiO3-perovskite, and post-perovskite. We re-evaluated the literature pressure-volume-temperature relationships of these minerals using a self-consistent pressure scale in order to avoid the long-standing pressure scale problem and to provide more reliable constraints on the thermal EoS parameters. With the obtained thermal EoS parameters, we have constructed the ρ and VΦ profiles of the bottom lower mantle in different composition, mineralogy, and temperature models. Our modelling results show that the variations of chemistry, mineralogy, and temperature have different seismic signatures from each other. The Fe and Al enrichment at the bottom lower mantle can cause an increase in ρ but greatly lower VΦ. A change in mineralogy needs to be considered with the lateral variation in temperature. The cold slabs will be shown as denser regions compared to the normal mantle because of the combined effect of a lower temperature and the presence of a denser post-perovskite at a shallower depth,whereas the hot regions will have a 1–2% lower ρ than the normal mantle. VΦ of both cold slabs and hot regions will be lower than the normal mantle when bridgmanite is the dominant phase in the normal mantle, yet they will be greater once bridgmanite transforms into post-perovskite in the normal mantle. Our modeling also shows that the presence of a(Fe, Al)-enriched bridgmanite thermal pile above the core-mantle boundary will exhibit a seismic signature of enhanced ρ and VΦ, but a reduced VS,which is consistent with the observed seismic anomalies in the large-low-shear-velocity-provinces(LLSVPs). The existence of such a(Fe, Al)-enriched bridgmanite thermal pile thus can help to understand the origin of the LLSVPs. These results provide new insights for the chemical and structure of the deepest lower mantle.     

Global geomagnetic and auroral response to variations in the solar wind dynamic pressure on April 1, 1997

The geomagnetic and auroral response to the variations in the solar wind dynamic pressure (Pd) are investigated in the periods of positive values of the IMF B _z component. It is shown that the growth of Pd results in the intensification of luminosity along the auroral oval and in the poleward expansion of the poleward boundary of luminosity (PBL) in the nightside part of the oval by ～7° in latitude at a velocity of ～0.5 km/s and is accompanied by an enhancement of the DP2-type current system. A decrease in Pd, accompanied by an abrupt reversal of the IMF B _y polarity from positive to negative, results in an enhancement of the westward electrojet and in a poleward shift of PBL and electrojet center. The conclusion has been made that the available three types of auroral response to Pd variations differ in the azimuthal velocity of the luminosity region or particle precipitation along the auroral oval: V ₁ ～ 30–40 km/s, V ₂ ～ 10, and V ₃ ～ 1 km/s.     

Solar flare effect on the geomagnetic field and ionosphere

This paper studies the ionospheric and geomagnetic response to an X6.2 solar flare recorded at 14:30 UT on December 13, 2001, in quiet geomagnetic conditions which allow the variations in the geomagnetic field and ionosphere measurements to be easily related to the solar flare radiation.By using measurements from the global positioning system (GPS) and geomagnetic observatories, the temporal evolution of ionospheric total electron content variation, vTEC_V, and geomagnetic field variations, δB, as well as their rates of variation, were obtained around the subsolar point at different solar zenith angles. The enhancement of both parameters was recorded one to three minutes later than the Geostationary Operational Environmental Satellite (GOES) programme recording; such delay tends to depend on the latitude, longitude, and solar zenith angle of the observatory's observations.The vTEC_V is related to the local time and the δB to the intensity and position of the ionospheric currents.The vTEC_V′s maximum value is always recorded later than the maximum values reached by δB and the X-ray intensity. The maximum δB is larger in the local morning than in the afternoon.The rates of vTEC_V and δB have two maximum values at the same time as the maximum values recorded by Hα (for each ribbon).This work shows the quantitative and qualitative relations between a solar flare and the ionospheric and geomagnetic variations that it produces.     

Magnetic field dynamics based on SOHO/MDI data in the region of flares related to halo coronal mass ejections

Variations in the photospheric magnetic field in the region of solar flares, related to halo coronal mass ejections (HCMEs) with velocities V > 1500, 1000 < V < 1500, and V < 650 km/s, have been studied based on SOHO/MDI data. Using data with a time resolution of 96 min, it has been indicated that on average the ??B _L?? and ??|B _L|?? field characteristics increase nonmonotonically during 1?C1.5 days before a flare and decrease during 0.5?C1 days after a flare for groups of ejections with V > 1000 km/s for all considered HCME groups. Angle brackets designate averaging of the measured B _L magnetic field component and its magnitude |B _L| within an area with specified dimensions and the center coincident with the projection onto the region where the flare center field is measured. It has been established that a solar flare related to an HCME originates when the ??B _L?? and ??|B _L|?? values are larger than the boundary values in the flare region. Based on 1-min data, it has been found for several HCMEs with V > 1500 km/s that the beginning of powerful flares related to ejections is accompanied by rapid impulsive or stepped variations in ??B _L?? and ??|B _L|?? near the center of a flare with a size of approximately 4.5°. It has been established that the HCME velocity positively correlates with the |??B _L??| value at the flare onset.     

Adaptive neuro fuzzy inference system for compressional wave velocity prediction in a carbonate reservoir

Compressional-wave (V_p) data are key information for estimation of rock physical properties and formation evaluation in hydrocarbon reservoirs. However, the absence of V_p will significantly delay the application of specific risk-assessment approaches for reservoir exploration and development procedures. Since V_p is affected by several factors such as lithology, porosity, density, and etc., it is difficult to model their non-linear relationships using conventional approaches. In addition, currently available techniques are not efficient for V_p prediction, especially in carbonates. There is a growing interest in incorporating advanced technologies for an accurate prediction of lacking data in wells. The objectives of this study, therefore, are to analyze and predict V_p as a function of some conventional well logs by two approaches; Adaptive Neuro-Fuzzy Inference System (ANFIS) and Multiple Linear Regression (MLR). Also, the significant impact of selected input parameters on response variable will be investigated. A total of 2156 data points from a giant Middle Eastern carbonate reservoir, derived from conventional well logs and Dipole Sonic Imager (DSI) log were utilized in this study. The quality of the prediction was quantified in terms of the mean squared error (MSE), correlation coefficient (R-square), and prediction efficiency error (PEE). Results show that the ANFIS outperforms MLR with MSE of 0.0552, R-square of 0.964, and PEE of 2%. It is posited that porosity has a significant impact in predicting V_p in the investigated carbonate reservoir.     

Characterisation of ground motion recording stations in the Groningen gas field

The seismic hazard and risk analysis for the onshore Groningen gas field requires information about local soil properties, in particular shear-wave velocity (V_S). A fieldwork campaign was conducted at 18 surface accelerograph stations of the monitoring network. The subsurface in the region consists of unconsolidated sediments and is heterogeneous in composition and properties. A range of different methods was applied to acquire in situ V_S values to a target depth of at least 30 m. The techniques include seismic cone penetration tests (SCPT) with varying source offsets, multichannel analysis of surface waves (MASW) on Rayleigh waves with different processing approaches, microtremor array, cross-hole tomography and suspension P-S logging. The offset SCPT, cross-hole tomography and common midpoint cross-correlation (CMPcc) processing of MASW data all revealed lateral variations on length scales of several to tens of metres in this geological setting. SCPTs resulted in very detailed V_S profiles with depth, but represent point measurements in a heterogeneous environment. The MASW results represent V_S information on a larger spatial scale and smooth some of the heterogeneity encountered at the sites. The combination of MASW and SCPT proved to be a powerful and cost-effective approach in determining representative V_S profiles at the accelerograph station sites. The measured V_S profiles correspond well with the modelled profiles and they significantly enhance the ground motion model derivation. The similarity between the theoretical transfer function from the V_S profile and the observed amplification from vertical array stations is also excellent.     

Models of density and mechanical Q-factor distributions according to new data on the nutations and free oscillations of the Earth: 1. Ambiguity in the inverse problem

Ambiguity in the inverse problem of retrieval of the mechanical parameters of the Earth’s shell and core from the set of data on the velocities V _p and V _S , of longitudinal and transverse seismic body waves, the frequencies f _i and quality factors Q _i , of free oscillations, and the amplitudes and phases of forced nutation is considered. The numerical experiments show that the inverse problem of simultaneous retrieval of the density profile ρ in the mantle-liquid core system and the mechanical quality factor Q _μ of the mantle (if the total mass M and the total mean moment of inertia I of the Earth, and V _p and V _S are constant at all depths) has most unstable solutions. An example of depth distributions of ρ and Q _μ which are alternative to the well-known PREM model is given. In these distributions, the values of M and I and the velocities V _p and V _S at all depths for the period of oscillations T = 1 s exactly coincide with their counterparts yielded by PREM model (T = 1 s); however, the maximum deviations of the ρ and Q _μ profiles from those in the PREM model are about 3% and 40%, respectively; the mass and the moment of inertia of the liquid core are smaller than those for the PREM model by 0.75% and 0.63%, respectively. In this model, the root mean square (rms) deviations of all the measured values of f _i and Q _i from their values predicted by theory are half to third the corresponding values in the PREM model; the values of Δ for natural frequencies of the fundamental tone and overtones of radial oscillations, the fundamental tones of torsional oscillations, and the fundamental tones of spheroidal oscillations, which are measured with the highest relative accuracy, are smaller by a factor of 30, 6.6, and 2 than those in the PREM model, respectively. Such a large ambiguity in the solution of the inverse problem indicates that the current models of the depth distribution of density have relatively low accuracy, and the models of the depth distribution of the mechanical Q in the mantle are extremely unreliable. It is shown that the ambiguity in the models of depth distribution of density considerably decreases after the new data on the amplitudes and phases of the forced nutation of the Earth are taken into account. Using the same data, one may also refine by several times the recent estimates of the creep function for the lower mantle within a wide interval of periods ranging from a second to a day.     

GOCE, Satellite Gravimetry and Antarctic Mass Transports

In 2009 the European Space Agency satellite mission GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) was launched. Its objectives are the precise and detailed determination of the Earth’s gravity field and geoid. Its core instrument, a three axis gravitational gradiometer, measures the gravity gradient components V _xx , V _yy , V _zz and V _xz (second-order derivatives of the gravity potential V) with high precision and V _xy , V _yz with low precision, all in the instrument reference frame. The long wavelength gravity field is recovered from the orbit, measured by GPS (Global Positioning System). Characteristic elements of the mission are precise star tracking, a Sun-synchronous and very low (260 km) orbit, angular control by magnetic torquing and an extremely stiff and thermally stable instrument environment. GOCE is complementary to GRACE (Gravity Recovery and Climate Experiment), another satellite gravity mission, launched in 2002. While GRACE is designed to measure temporal gravity variations, albeit with limited spatial resolution, GOCE is aiming at maximum spatial resolution, at the expense of accuracy at large spatial scales. Thus, GOCE will not provide temporal variations but is tailored to the recovery of the fine scales of the stationary field. GRACE is very successful in delivering time series of large-scale mass changes of the Antarctic ice sheet, among other things. Currently, emphasis of respective GRACE analyses is on regional refinement and on changes of temporal trends. One of the challenges is the separation of ice mass changes from glacial isostatic adjustment. Already from a few months of GOCE data, detailed gravity gradients can be recovered. They are presented here for the area of Antarctica. As one application, GOCE gravity gradients are an important addition to the sparse gravity data of Antarctica. They will help studies of the crustal and lithospheric field. A second area of application is ocean circulation. The geoid surface from the gravity field model GOCO01S allows us now to generate rather detailed maps of the mean dynamic ocean topography and of geostrophic flow velocities in the region of the Antarctic Circumpolar Current.     

Three-Dimensional V P and V P /V S Structure in the Longtan Reservoir Area by Local Earthquake Tomography

High-resolution three-dimensional V _P and V _P /V _S images in the Longtan reservoir area were obtained from local earthquake data by using 3,178 events with total 24,153 P-wave and 23,987 S-wave arrivals collected from 23 seismic stations. The tomographic images show that significant V _P heterogeneities can be seen at layers of different depth in the Longtan reservoir area. Low-V _P anomalies both beneath and around the main rivers in the reservoir area may be related to the composition of rocks which are mainly deposit carbonate and arenaceous shale, which contributes to water saturation. We deduced that the high porosity rocks beneath the main rivers may be fully saturated with water. The phenomenon that V _P is relatively high in the area which is 10–20?km away from the rivers indicates that horizontal saturation of water is limited within a small range of area that is about 10–20?km from the main rivers. The characteristic is significant that seismicity in the Longtan reservoir area is coincident with the distribution of the low-V _P area. V _P /V _S tomographic images show that V _P /V _S ranges from 1.8 to 2.05 in shallow layers above 4?km depth beneath the Longtan reservoir, suggesting the properties of the rocks are limestone and shale. At the depth of 7?km, the distribution of V _P /V _S image varies quite remarkably, especially in the dam area. This demonstrates that the range of influence by the saturation of water in the media below the reservoir surface can reach 4–7?km depth in the dam area.