2D magnetic resonance tomography applied to karstic conduit imaging

Karstic conduits play a crucial role for water supply in many parts of the world. However, the imaging of such targets is generally a difficult task for most geophysical methods. Magnetic Resonance Sounding (MRS) is a geophysical method designed for imaging of water bearing structures. Initially, MRS was developed for characterizing horizontally stratified aquifers. However, when applying a 1D MRS measuring setup to the imaging of 2D–3D targets, the size of which may be much smaller than the loop, the accuracy and the lateral resolution may not be sufficient. We have studied the possibility of simultaneously processing several MRS aligned along a profile to perform a Magnetic Resonance Tomography (MRT). This work emphasizes the gain of resolution for 2D–3D imagery of MRT versus the interpolation of 1D inversion results of MRS along the same profile. Numerical modelling results show that the MRT response is sensitive to the size and location of the 2D target in the subsurface. Sensitivity studies reveal that by using the coincident transmitting/receiving (TX/RX) setup and shifting the loop around the anomaly area, the depth, section and position of a single karstic conduit with a size smaller than the MRS loop size can be resolved. The accuracy of the results depends on the noise level and signal level, the latter parameter being linked to the depth and volume of the karstic conduit and the water content in the limestone matrix. It was shown that when applying MRT to the localization of 2D anomalies such as karstic conduits, the inclination of the geomagnetic field, the orientation of the MRT profile and the angle of crossover of the conduit by the MRT profile must be taken into account. Otherwise additional errors in interpretation should be expected. A 2D inversion scheme was developed and tested. Both numerical and experimental results confirm the efficiency of the developed approach.     

Crust and Upper Mantle Structure in the Caribbean Region by Group Velocity Tomography and Regionalization

An overview of the crust and upper mantle structure of Central America and the Caribbean region is presented as a result of the processing of more than 200 seismograms recorded by digital broadband stations from SSSN and GSN seismic networks. Group velocity dispersion curves are obtained in the period range from 10s to 40s by FTAN analysis of the fundamental mode of the Rayleigh waves; the error of these measurements varies from 0.06 and 0.09 km/s. From the dispersion curve, seven tomographic maps at different periods and with average spatial resolution of 500 km are obtained. Using the logical combinatorial classification techniques, eight main groups of dispersion curves are determined from the tomographic maps and eleven main regions, each one characterized by one kind of dispersion curves, are identified. The average dispersion curves obtained for each region are extended to 150s by adding data from a larger-scale tomographic study (Vdovin et al., 1999) and inverted using a nonlinear procedure. A set of models of the S-wave velocity vs. depth in the crust and upper mantle is found as a result of the inversion process. In six regions we identify a typically oceanic crust and upper mantle structure, while in the other two the models are consistent with the presence of a continental structure. Two regions, located over the major geological zones of the accretionary crust of the Caribbean region, are characterized by a peculiar crust and upper mantle structure, indicating the presence of lithospheric roots reaching, at least, about 200 km of depth.     

Simulation of Atmospheric Boundary Layer Characteristics during Indian Summer Monsoon using Observations from Monsoon Trough Boundary Layer Experiment at Jodhpur, India

The diurnal structure of the boundary layer during Indian summer monsoon period is studied using a one-dimensional meteorological boundary layer model and the observations collected from the Monsoon Trough Boundary Layer Experiment conducted in 1990 at Jodhpur, India. The model was initialized with the observed temperature profiles at 0530 LST on 17 July, 1990 at Jodhpur and was run for 26 hours. The study is carried out with a geostrophic wind speed of 9.5 m s⁻¹ corresponding to the strong wind simulation. The mean thermodynamic and wind structure simulated by the model are in good agreement with those observed from 30 m tower. The computed surface layer characteristics such as the surface fluxes, TKE and standard deviations of velocity components are found to be reasonably in good agreement with those based on turbulence measurements. The shear and buoyancy budget computed from the model are also compared with the turbulence measurements. The integrated cooling budget in the nocturnal boundary layer is examined.     

Source mechanism and source parameters of May 28, 1998 earthquake,Egypt

On May 28, 1998, a moderate size earthquake of mb 5.5 occurred offshore the northwestern part of Egypt (latitude 31.45°N and longitude 27.64°E). It was widely felt in the northern part of Egypt. Being the largest well-recorded event in the area for which seismic data from the global digital network are available, it provides an excellent opportunity to study the tectonic process and present day stress field occurring along the offshore Egyptian coast. The source parameters of this event are determined using three different techniques: modeling of surface wave spectral amplitudes, regional waveform inversion, and teleseismic body waveform inversion. The results show a high-angle reverse fault mechanism generally trending NNW–SSE. The P-axis trends ENE–WSW consistently with the prevailed compression stress along the southeastern Hellenic arc and southwestern part of the Cyprean arc. This unexpected mechanism is most probably related to a positive inversion of the NW trending offshore normal faults and confirms an extension of the back thrusting effects towards the African margin. The estimated focal depth ranges from 22 to 25 km, indicating a lower crustal origin earthquake owing to deep-seated tectonics. The source time function indicates a single source with rise time and total rupture duration of 2 and 5 s, respectively. The seismic moment (M _o) and the moment magnitude (M _w) determined by the three techniques are 1.03 × 10¹⁷ Nm, 5.28; 1.24 × 10¹⁷ Nm, 5.33; and 1.68 × 10¹⁷ Nm, 5.42; respectively. The calculated fault radius, stress drop, and the average dislocation assuming a circular fault model are 7.2 km, 0.63 Mpa, and 0.11 m, respectively.     

中国北方干旱区地表湿润状况的趋势分析

利用 1 95 1～ 1 997年中国 1 6 0站月降水和平均气温资料 ,通过计算的地表湿润指数 Hi =PPe(P为观测的月降水总量 ,Pe为月最大潜在蒸发 ) ,对比分析了中国华北、西北两个典型干旱区区域平均地表湿润指数的年代年际变化特征及季节性差异 ,并讨论了它与降水和气温的联系。最后 ,给出了地表湿润指数年及各季节变化趋势的地理分布。研究表明 :西北西部和华北地区的年际及年代际变化趋势基本相反 ,前者地表为变湿趋势 ,后者为变干趋势。华北地区的干化趋势主要发生在夏秋季节 ,而西北除东部的秋季和西部的夏季外 ,其它季节均存在变湿趋势     

广州中尺度模式局地要素预报性能分析

该文简要介绍了华南中尺度模式的地表要素的预报诊断方案, 站点要素时间序列预报情况.用适合站点要素时间序列预报的检验方法, 检验并分析了各要素时间序列预报的性能.表明该中尺度模式的地表要素时间序列预报方案是有效的, 预报具有较高的准确性和应用价值.     

An Analysis Of Secondary Circulations And Their Effects Caused By Small-Scale Surface Inhomogeneities Using Large-Eddy Simulation

A parallelized large-eddy simulation model has been used to investigate the effects of two-dimensional, discontinuous, small-scale surface heterogeneities on the turbulence structure of the convective boundary layer.Heterogeneities had a typical size of about the boundary-layer heightz_i. They were produced by a surface sensible heat flux pattern ofchessboard-type and of strong amplitude as typical, e.g., for the marginalice zone. The major objectives of this study were to determinethe effects of such strong amplitude heat flux variations and to specify theinfluence of different speeds and directions of the background wind.Special emphasis has been given to investigate the secondary circulations induced by the heterogeneities by means of three-dimensional phase averages.Compared with earlier studies of continuous inhomogeneities, the same sizeddiscontinuous inhomogeneities in this study show similar but stronger effects.Significant changes compared with uniform surface heating are only observedwhen the scale of the inhomogeneities is increased to z_i. Especially the vertical energy transport is much more vigorous and even the mean emperature profile shows a positive lapse rate within the whole mixed layer. However, the effects are not directly caused by the different shape of the inhomogeneities but can mainly be attributed to the large amplitude of the imposed heat flux,as it is typical for the partially ice covered sea during cold air outbreaks.The structure of the secondary flow is found to be very sensitive to the wavelength and shape of the inhomogeneities as well as to the heatflux amplitude, wind speed and wind direction. The main controlling parameter is the near-surface temperature distribution and the related horizontal pressure gradient perpendicular to the main flow direction. The secondary flow varies from a direct circulation with updraughts mainly above the centre of the heated regions to a more indirect circulation with updraughts beneath the centre and downdraughts above it. For background winds larger than 2.5 m s^–1 a roll-like circulation pattern is observed.From previous findings it has often been stated that moderate backgroundwinds of 5 m s^–1 eliminate all impacts of surface inhomogeneitiesthat could potentially be produced in realistic landscapes. However, this studyshows that the effects caused by increasing the wind speed stronglydepend on the wind direction relative to the orientation of theinhomogeneities. Secondary circulations remain strong, even for abackground wind of 7.5 m s^–1, when the wind direction is orientatedalong one of the two diagonals of the chessboard pattern. On the otherhand, the effects of inhomogeneities are considerably reduced, even undera modest background wind of 2.5 m s^–1, if the wind direction isturned by 45°. Mechanisms for the different flow regimesare discussed.     

斯堪的那维亚臭氧亏损和地面加热

对ＮＡＳＡ的ＴＯＭＳ臭氧资料进行分析后指出：冬季在北极的斯堪的那维亚地区存在一个明显的臭氧亏损区，亏损区的中心值达－５０ ＤＵ，相当于该区域平均值的 １５％。对臭氧亏损和北大西洋海温的东西向差异作相关分析得到：其季节变化的相关系数为－０．９６，逐月（ １６８个月）变化的相关系数为－０．７０。同样对臭氧亏损与地面加热进行相关分析后指出：斯堪的那维亚地区的臭氧亏损和该地区地面的热通量关系极其密切，其相关系数均在－０．８７以上。由此我们认为：北大西洋暖流向北输送能量，引起斯堪的纳维亚地区地面加热，由此造成了冬季该地区的臭氧亏损。     

南海海平面高度年循环的特征

根据 ＴＯＰＥＸ／ ＰＯＳＥＩＤＯＮ－ＥＲＳ高度计提供的海平面高度异常资料和并行海洋气候模式（ＰＯＣＭ）模拟海平面高度资料，分析了南海海平面高度年循环特征。结果表明：ｌ月，３月和５月海平面高度的异常值分别与７月，９月，１１月的异常值相反。ｌ月（７月），深水海区与吕宋海峡的海平面高度为负（正）异常，在大部分陆架区和南海的西和南部，海平面高度为正（负）异常。在３月（９月），除海平面高度异常的量级已减少，且较小的ＳＳＨ正异常（负异常）出现在南海的中部以外，海平面高度异常的分布型与 １月（７月）类似； ＳＳＨ的年循环的最大振幅出现在吕宋岛的西北海域；风的季节变化是南海ＳＳＨ季节变化的主要原因。