断层含水层在非等温条件下的构造性渗流——一种震前热前兆异常产生的机理

本文用一个简单的解析模式来计算,在被断层带切断的含水层,由于形变产生的渗流引起的热量瞬时流动。在这个模式中,含水层流体的补给被假定只是通过弱隔水层垂向渗流。在这种条件下,能量平衡方程要计算由于含水层体积变化和形变产生的水力压头变化引起的流体水平向和垂直向流动的单位热量传递。数值模拟的结果表明在应力增加期,当渗流的地下水与含水层的地下水混合的时候,地温就表现为瞬时的升高。相应地,当这一渗流过程停止的时候,地温迅速回到平衡状态。利用此解析模型,可以重建观测记录中的地温前兆异常特征。按照模型的结果,地温升高达到稳定状态的时间和流体的补给率相关。热异常的幅度与弱隔水层的温度梯度有关,弱隔水层与含水层之间的渗透率也起到很重要的作用。模型的结果表明热前兆异常只是在离断层近的区域表现明显,由于在这个区域构造渗流的过程对含水层的水力压头有直接的影响。如果断层面对形变敏感,应力模式的改变会使得构造渗流率发生永久的改变,从而很容易使区域的温度场产生剧烈的、不可恢复的改变。在某些构造活跃区域的井孔地温记录中,一些短临异常变化的特征和按照“构造渗流”模型的预测结果非常相似。在土库曼斯坦的Firjusa、乔治亚州的Didiweli和日本的Izu peninsnla都有这种震前热前兆模型的例子。研究表明,与小规模构造形变相关的热前兆异常并不是罕见的。热前兆异常的监测要有合适的井孔和高精度的监测仪器,其观测时间至少要与区域地震复发周期相当。     

Microphysical characteristics of rainfall during different seasons over a coastal tropical station using disdrometer

Drop size distribution (DSD) over the tropical region exhibit pronounced variations during different monsoon seasons. Measurements from an impact type Joss–Waldovgel disdrometer is used for characterization of drop size distribution and its integral parameters over a tropical coastal station (Thiruvananthapuram, 8.31°N, 76.54°E, 20 m asl). Rain events were identified during the winter, premonsoon, summer monsoon and postmonsoon seasons from 8 years, computed rain duration (min) and accumulated rain water (mm). Rain intensity (mm h^?1), mean drop diameter (D_m, mm) and total number concentration of raindrops (N_T, m^?3) were calculated on each sampling interval and classified in to different bins. The different range bins of rain intensity and their relative contributions towards total rainfall are different for different seasons. Maximum events were reported on the R2 (heavy drizzle/light rain) type, but the contribution of rainfall (mm) is mainly registered on R4 (heavy rain) type. Similarly, the N_T and D_m are also showing different characteristics during different monsoon seasons. Frequency of occurrence of D_m is higher in D_m2 (1–2 mm) followed by D_m1 (D_m < 1 mm) and then D_m3 (2–3 mm) with difference in magnitudes for different seasons. On analysing relative rainfall contribution from different mean diameter bins, it can be observed that D_m2 and D_m3 (1–3 mm) are the major contributors to the total rainfall. In the case of N_T, both frequency and accumulated water are almost same or comparable for the different bins during all the seasons. The D_m and N_T are positively related with different intensity bins. The lower rainfall intensity bins show higher duration during the summer monsoon season and lower duration during the premonsoon season, the higher intensity range bins show lower duration for the premonsoon season and higher duration for the postmonsoon season.     

Temporal and depth distribution of microepiphytes on Posidonia oceanica (L.) Delile leaves in a meadow off Tunisia

A survey on the epiphytic microorganisms growing on Posidonia oceanica leaves was conducted along a depth transect along the coast of Eastern Tunisia (Mahdia). Samples were collected by SCUBA diving at depths of 3, 5, 10 and 12 m in July 2008 and January 2009. A total of 58 microepiphyte taxa were identified. Multivariate analyses revealed temporal and spatial variation of the abundance of epiphytic microalgae. Water motion, light availability, temperature and motility of species seem to be responsible of temporal and bathymetric variations of epiphytic microalgae. Unlike diatoms, dinoflagellates were more abundant in deep waters, suggesting that they are more vulnerable to hydrodynamics. The significant correlation between leaf area index and abundance of epiphytic species indicates that the phenological parameters of the host plant influence the abundance of the epiphytic microorganisms. Among the epiphytic dinoflagellates, our data showed a great number of potentially toxic species (Alexandrium minitum, Amphidinium carterae, Karenia selliformis, Coolia monatis, Karlodinium veneficum, Ostreopsis siamensis, Prorocentrum concavum, Prorocentrum minimum, Prorocentrum rathymum and Prorocentrum lima). These species were more abundant in the deep station under more sheltered conditions than found at inshore shallower stations, which are subjected to high water motion.     

On the resonance effect by dynamic soil–structure interaction: a revelation study

The present study makes an attempt to investigate the soil–structure resonance effects on a structure based on dynamic soil–structure interaction (SSI) methodology by direct method configuration using 2D finite element method (FEM). The investigation has been focused on the numerical application for the four soil–structure models particularly adjusted to be in resonance. These models have been established by single homogenous soil layers with alternating thicknesses of 0, 25, 50, 75 m and shear wave velocities of 300, 600, 900 m/s-a midrise reinforced concrete structure with a six-story and a three-bay that rests on the ground surface with the corresponding width of 1,400 m. The substructure has been modeled by plane strain. A common strong ground motion record, 1940 El Centro Earthquake, has been used as the dynamic excitation of time history analysis, and the amplitudes, shear forces and moments affecting on the structure have been computed under resonance. The applicability and accuracy of the FEM modeling to the fundamental period of soils have been confirmed by the site response analysis of SHAKE. The results indicate that the resonance effect on the structure becomes prominent by soil amplification with the increased soil layer thickness. Even though the soil layer has good engineering characteristics, the ground story of the structure under resonance is found to suffer from the larger soil layer thicknesses. The rate of increment in shear forces is more pronounced on midstory of the structure, which may contribute to the explanation of the heavily damage on the midrise buildings subjected to earthquake. Presumably, the estimated moment ratios could represent the factor of safeties that are excessively high due to the resonance condition. The findings obtained in this study clearly demonstrate the importance of the resonance effect of SSI on the structure and can be beneficial for gaining an insight into code provisions against resonance.     

Use of geothermal methods in outlining deep groundwater flow systems in Paleozoic interior basins of Brazil

Results of regional-scale geothermal studies are presented, providing new insights into the characteristics of deep groundwater flow systems in the Paleozoic sedimentary basins in the Amazon, Paraná and Parnaíba regions of Brazil. The study makes use mainly of bottom-hole temperature data sets for oil wells, the depths of which vary from 1,000 to 4,000 m. The techniques employed in data analysis have allowed identification of non-linear features in vertical distributions of temperature, produced by deep groundwater flows in the study area. According to the results obtained, vertical velocities of subsurface flows are found to fall in the range 10^?10 to 10^?9 m/s, while the horizontal velocities are significantly higher, of the order 10^?8 m/s. Identification of large-scale down flows has allowed inferences as to the existence of lateral movements of groundwater. The basins in the Amazon region are found to be characterized by widespread down flow of groundwater, implying the existence of distributed recharge systems operating on regional scales. There is a systematic decrease in horizontal velocities along the direction from west to east. This feature is considered indicative of gravity driven flows induced by episodes of uplift, since Miocene times, in the Andean region.     

Spatial scale variability in shoot density and epiphytic leaves of Posidonia oceanica on Kerkennah Island (Tunisia) in relation to current tide effects

A study was undertaken of the patterns of spatial variability, epiphytic biomass and distribution of epiphytic fauna and flora of Posidonia oceanica. Samples were taken at four stations located approximately 4 km apart, exposed to different current conditions. Stations A and B, situated near the Oued Mimoun tidal channel with its relatively strong bi‐directional flows, were affected by high current tide. The other two stations, North Oued Mimoun (L1) and South Oued Mimoun (L2), were located further from the channel, in low current tide conditions. Sampling was conducted in the Attaya area of Kerkennah Island (Tunisia) in August 2009 at depths between 2 and 3 m, with the results indicating differences among the stations. Shoot density decreased when exposed to high levels of hydrodynamic activity generated by current tides whereas the epiphytic biomass of P. oceanica leaves decreased at sheltered stations located far from the channel. Epiphytic algae such as Heterokontophyta, Rhodophyta and Chlorophyta, and epiphytic fauna represented by Bryozoa, Hydrozoa, Annelida, Porifera and Tunicata, dominated the epiphytic assemblages and were abundant at the station most exposed to high current tide hydrodynamics. Cyanobacteria, however, were dominant in stations exposed to low current tide.     

Characteristic Study of the Boundary Layer Parameters over the Arabian Sea and the Bay of Bengal Using the QuikSCAT Dataset

The marine atmospheric boundary layer （MABL） plays a vital role in the transport of momentum and heat from the surface of the ocean into the atmosphere. A detailed study on the MABL characteristics was carried out using high-resolution surface-wind data as measured by the QuikSCAT （Quick scatterometer） satellite. Spatial variations in the surface wind, frictional velocity, roughness parameter and drag coefficient for the different seasons were studied. The surface wind was strong during the southwest monsoon season due to the modulation induced by the Low Level Jetstream. The drag coefficient was larger during this season, due to the strong winds and was lower during the winter months. The spatial variations in the frictional velocity over the seas was small during the post-monsoon season （-0.2 m s^-1）. The maximum spatial variation in the frictional velocity was found over the south Arabian Sea （0.3 to 0.5 m s^-1） during the southwest monsoon period, followed by the pre-monsoon over the Bay of Bengal （0.1 to 0.25 m s^-1）. The mean wind-stress curl during the winter was positive over the equatorial region, with a maximum value of 1.5×10^-7 N m^-3, but on either side of the equatorial belt, a negative wind-stress curl dominated. The area average of the frictional velocity and drag coefficient over the Arabian Sea and Bay of Bengal were also studied. The values of frictional velocity shows a variability that is similar to the intraseasonal oscillation （ISO） and this was confirmed via wavelet analysis. In the case of the drag coefficient, the prominent oscillations were ISO and quasi-biweekly mode （QBM）. The interrelationship between the drag coefficient and the frictional velocity with wind speed in both the Arabian Sea and the Bay of Bengal was also studied.     

Oxygen isotopic fractionation between oxygen of different sites in hydroxyl-bearing silicate minerals

A method has been developed for extraction of hydroxyl oxygen from hydroxyl-bearing silicate minerals for oxygen isotopic analysis.The δO¹⁸ of oxygen of the OH groups is significantly different from that in the rest of the mineral structure. The isotopic fractionation between the two types of sites has the potential to be a sensitive geothermometer.Several δO¹⁸ values were obtained for oxygen of the OH attached to different silicate structures as well as for two muscovite samples with quantitatively estimated different temperatures of formation.The 1000 ln α (mineral-OH) values ranged from 5.2%. for muscovite to about 12.6%. for kaolinite and chlorite.     

Assessing groundwater storage in the Kairouan plain aquifer using a 3D lithology model (Central Tunisia)

The aquifer of the semi-arid Kairouan plain has been exploited for decades to supply the growing irrigated agriculture and the need of drinking water. In parallel, the major hydraulic works drastically changed the natural groundwater recharge processes. The continuous groundwater level drop observed since the 1970s naturally raises the question of groundwater storage sustainability. To date, hydrogeological studies focused on groundwater fluxes, but the total amount of groundwater stored in the aquifer system has never been fully estimated. This is the purpose of the present paper. A complete database of all available geological, hydrogeological and geophysical data was created to build a 3D lithology model. Then, the lithological units were combined with the hydraulic properties to estimate the groundwater storage. Over the 700 km² of the modelled area, the estimated storage in 2013 was around 18?×?10⁹ m³ (equivalent to 80 times the annual consumption of 2010) with a highly variable spatial distribution. In 45 years (1968–2013), 12% of the amount of groundwater stored in the aquifer has been depleted. According to these results, individual farms will face strong regional disparities for their access to groundwater in the near future.