Structure of the earth's crust in Jordan from potential field data

Gravity and magnetic data were collected and used to study the crustal structure of Jordan. Three new geophysical maps of Jordan were created: a Moho discontinuity map, a crystalline basement surface map, and a map showing the lowest limit of magnetic blocks. Depths of the Curie Isotherm were also calculated. Results indicate that the depth to the Moho discontinuity in Jordan varies from 32 to 33 km in the northwest to 38 km in the southeast. The basement complex rocks outcrop on the surface in the southwest but lie at about 8 km in the northeast. The Curie Isotherm (585 °C) lies at a depth of about 10 km in the area east of the Dead Sea and dips southeastward towards the Al-Sirhan (Wadi Sirhan), southeast Jordan, where it is located at 35 km depth. Local isostasy of rock masses (blocks) in Jordan does not occur. Nevertheless, this does not rule out the possible existence of isostasy in a regional scale at greater depths within the mantle.     

Curvelet变换在位场数据去噪中的应用

为了解Curvelet变换在位场数据去噪处理中的效果,从Curvelet变换的原理着手,通过对多矩形棱柱体模型进行Curvelet变换去噪和普遍应用的小波阈值去噪试验对比,验证了Curvelet变换去噪的有效性问题。将其应用于实测位场数据的处理中,取得了较好的效果。理论和实测数据的处理结果都表明,Curvelet变换在位场数据的去噪处理中是可行有效的。     

Test of AMS 14C dating of pollen concentrates using tephrochronology

Previous attempts to radiocarbon date sediments >10 kyr from the high rainfall region of Westland, New Zealand, a critical location for investigation of interhemispheric patterns of climate change, have been problematic. This study, building on recent work by Vandergoes and Prior ( 2003 ), shows that AMS ¹⁴C dating of pollen concentrates has potential to provide more reliable ages than other sediment constituents, including plant macrofossils. The method was applied to sediments from three sites containing the 22.6k ¹⁴C yr Kawakawa Tephra, which provided an independent test of the ¹⁴C ages. Although some minor laboratory contamination was detected in tests on background standards, the modelled relationship between sample mass and measured ¹⁴C content permitted an appropriate correction to be determined. Improved pollen concentrations derived by density separation between 1.4 and 1.2 specific gravity and sieving in the range 10–50 μm provided either older ages than other fractions of the same sample or, where in situ contamination was not evident, equivalent ages. Differences in degree of in situ contamination between depositional environments indicated that, in Westland, lake sites may be less susceptible to contamination by younger carbon than peat sites, where this process may be facilitated by root penetration into underlying sediments. Copyright © 2006 John Wiley & Sons, Ltd.     

液化与非液化场地加速度反应谱对比

通过对2011年新西兰发生的6.3级地震中获取的硬土场地、软土场地和液化场地强震记录实测资料进行分析,研究包括3种类型场地的地震动特征及其相互关系。分析的强震记录为震中距小于50 km,且峰值大于0.05g的23个场地上的加速度记录,其中含3个硬场地、11个软场地和9个液化场地。3种类型场地上的放大系数谱对比分析表明,硬土场地、软土场地和液化场地上的地震动特征具有明显区别,三者层次清晰,差别显著,从地震动表现上液化场地已构成一个与常规场地并列的独立单元;液化场地减少地震动高频分量但同时对低频分量显著放大,与非液化场地相比,液化场地可使其上短周期结构反应减小一半,但同时可使其上长周期结构反应放大2.5～5.0倍;土层的液化对此次地震中克莱斯特彻奇市中心CTV大楼的破坏应有很大影响,大楼自振周期约为0.7 s,地震中土层液化使场地加速度反应谱卓越周期由0.1～0.3 s增到0.5～1.0 s,与大楼的自振周期趋于吻合,加重了大楼震害。以此为鉴,按现有规范中地震动的设计方法,如遇液化场地将对长周期结构给出明显危险的结果,因此从振动角度今后进行结构抗震设计时,可液化场地上的地震动应给予特殊考虑。     

基于小波变换的阈值降噪方法在地震资料处理中的应用

这里主要讨论了小波变换阈值降噪方法的基本原理,并且将广泛使用的通用阈值方法与Birge-Massart阈值方法进行了比较,展现了Birge-Massart阈值选取方法的优越性。通过对模拟数据和实际地震资料的处理结果表明,基于小波变换的Birge-Massart阈值降噪方法对地震信号降噪具有很好的效果。     

波浪及波流边界层泥沙起动规律

基于波浪边界层理论及单向流泥沙起动Shields曲线,推证出波浪泥沙起动Shields曲线;基于波流边界层理论,提出表述波流边界层动力特征的波流比因子X及非线性作用因子Y,并建立了Y与X的相关关系;在此基础之上,结合单向流及波浪泥沙起动Shields曲线,推证出波流共同作用下泥沙起动Shields曲线。结果表明:波浪泥沙起动Shields曲线在层流区与单向流光滑紊流区曲线保持一致,粗糙紊流区与单向流粗糙紊流区曲线保持一致,过渡区线型为折线,由层流区及粗糙紊流区曲线延长交汇获得;X及Y能够合理地表征波流边界层动力对比特征及非线性作用特征;波流泥沙起动Shields曲线介于波浪及单向流泥沙起动Shields曲线之间,随着波流比因子X的不同,依据非线性作用因子Y,自动在波浪及单向流泥沙起动Shields曲线之间非线性过渡。建立的波流泥沙起动Shields曲线与试验结果吻合较好,且能够概括单向流、波浪及波流等不同动力及细沙、粗沙等不同粒径的泥沙起动条件。     

应用MODIS影像估测太湖水体悬浮物浓度

以太湖为研究区域,同步获取悬浮物浓度实测数据、水体反射光谱数据和MODIS卫星影像数据,构建基于中分辨率成像光谱仪(MODIS)的悬浮物遥感估测模型.为了削弱大气效应,对MODIS影像了进行了粗略大气纠正.通过悬浮物特征光谱分析,将MODIS各敏感波段及波段组合与悬浮物浓度实测值进行相关分析,并应用实测光谱数据进行验证.在此基础上,运用回归分析建立半经验反演模型,并对模型进行了评价和应用.研究结果表明,MODIS影像可以很好地对大型内陆湖泊的悬浮物浓度进行遥感估测.250 m波段2 500 m波段4与1 000 m波段14是探测悬浮物的敏感波段.波段组合上,500 m组合因子r4/r3、r4-r3估测悬浮物含量的精度很高,适于构建反演模型;1 000 m波段8、11、131、4的多元组合也是构建模型的较好选择(R2均不低于0.85).     

Entrainment threshold of cohesionless sediment grains under steady flow of air and water

Existing formulations for bed sediment entrainment under steady flow are incapable of explaining two well-documented observational facts: (i) water flow requires considerably higher dimensionless shear stresses to move the bed grains than air flow; and (ii) under open channel flow, steep granular beds are more stable than beds with milder slopes. These two facts, together with recent direct measurements of forces acting on bed grains giving time-mean negative drags ( Schmeeckle et al. , 2007 ), question the conventional models of forces used so far. Here, fluid forces acting on bed particles are treated in a new way in order to take into consideration the fundamental interference effects, thus obtaining appropriate magnitude estimates that exhibit good agreement with direct force measurements by Schmeeckle et al. (2007) . Impulsive pressure fluctuations generated by turbulence are shown to be capable of dislodging the bed grains by saltation under air flow, whereas they can only produce a rocking effect under water flow, thus explaining the first anomaly. On the other hand, previous work by the authors allows a direct estimate of space averaged time-mean drag and lift forces exerted on bed grains. Both components have the same order of magnitude but, contrary to the common belief, the mean lift is downward, which provides an explanation for the second anomaly. Finally, spatial disturbances of pressure, both positive and negative, appear to generate maximum, persistent, local forces considerably greater than mean forces, thus allowing an explanation for the observed negative time-mean drag. A new formula for predicting incipient motion of sediment under open channel flow is derived, which incorporates all dynamically significant effects and gives very good agreement with observation for the entire range of bed slopes.     

Automated statistical matching of multiple tephra records exemplified using five long maar sequences younger than 75 ka,Auckland, New Zealand

Detailed tephrochronologies are built to underpin probabilistic volcanic hazard forecasting, and to understand the dynamics and history of diverse geomorphic, climatic, soil-forming and environmental processes. Complicating factors include highly variable tephra distribution over time; difficulty in correlating tephras from site to site based on physical and chemical properties; and uncertain age determinations. Multiple sites permit construction of more accurate composite tephra records, but correctly merging individual site records by recognizing common events and site-specific gaps is complex. We present an automated procedure for matching tephra sequences between multiple deposition sites using stochastic local optimization techniques. If individual tephra age determinations are not significantly different between sites, they are matched and a more precise age is assigned. Known stratigraphy and mineralogical or geochemical compositions are used to constrain tephra matches. We apply this method to match tephra records from five long sediment cores (≤ 75 cal ka BP) in Auckland, New Zealand. Sediments at these sites preserve basaltic tephras from local eruptions of the Auckland Volcanic Field as well as distal rhyolitic and andesitic tephras from Okataina, Taupo, Egmont, Tongariro, and Tuhua (Mayor Island) volcanic centers. The new correlated record compiled is statistically more likely than previously published arrangements from this area.     

Analysis and mapping of geochemical anomalies using logratio-transformed stream sediment data with censored values

There is lack of research and documentation of actual (as opposed to theoretical) benefits (e.g., mineral deposit discovery) of developments in compositional data analysis and imputation of censored values to mineral exploration geochemistry. In the present study, analyses of logratio- and ln-transformed stream sediment geochemical data containing ca. 30% of samples with censored values of a pathfinder element for the mineral deposit-type of interest yielded the following findings. Exclusion of those samples supports interpretation of multi-element anomalies reflecting the presence of mineralization. However, the multi-element anomaly maps obtained by exclusion of those samples are barely better than the multi-element anomaly maps derived by inclusion of those samples after replacing the censored values with 1/2 of detection limit or with imputed values. Logratio (i.e., alr, clr, or ilr) transformation, compared to ln-transformation, of stream sediment geochemical data does not improve mapping of pathfinder element anomalies reflecting the presence of mineralization. However, stream sediment geochemical data, excluding or including censored values (replaced with 1/2 of detection limit or with imputed values), should be clr- or ilr-transformed to enhance recognition of anomalous multi-element associations reflecting the presence of mineralization. The anomaly maps of multi-element associations derived from ilr-transformed data are better, albeit slightly, than the anomaly maps of multi-element associations derived from clr-transformed data. In the present study, the main benefit of either clr- or ilr-transformation, compared to either ln- or alr-transformation, of stream sediment geochemical data is the enhancement of anomalous multi-element associations reflecting the presence of mineralization. This is an important benefit because variations in trace element concentrations in regional-scale stream sediment geochemical data are mostly due to lithology and other factors (or processes) unrelated to mineralization. Further investigations of various exploration geochemical data are needed to demonstrate and document the actual (as opposed to theoretical) benefits of developments in compositional data analysis and imputation of censored values to mineral exploration.     

Distribution and assessment of sediment toxicity in Tamaki Estuary,Auckland, New Zealand

Heavy metal levels in surface sediments from Tamaki Estuary demonstrate significant up estuary increases in Cu, Pb, Zn, Cd and mud concentrations. Increased metal levels towards the head of the estuary are linked to local catchment sources reflecting the historical development, industrialisation and urbanisation of catchment areas surrounding the upper estuary. The relatively narrow constriction in the middle estuary (Panmure area), makes it susceptible to accumulation of upper estuary pollutants, since the constriction reduces circulation and extends the time required for fine waterborne sediments in the upper estuary to exchange with fresh coastal water. As a result fine fraction sediments trapped in the upper estuary facilitate capture and retention of pollutants at the head of the estuary. The increase in sandy mud poor sediments towards the mouth of the estuary is associated with generally low metal concentrations. The estuary’s geomorphic shape with a mid estuary constriction, sediment texture and mineralogy and catchment history are significant factors in understanding the overall spatial distribution of contaminants in the estuary. Bulk concentration values for Cu, Pb, Zn, and Cd in all the studied surface samples occur below ANZECC ISQG-H toxicity values. Cd and Cu concentrations are also below the ISQG-L toxicity levels for these elements. However, Pb and Zn concentrations do exceed the ISQG-L values in some of the surface bulk samples in the upper estuary proximal to long established sources of catchment pollution.     

小波变换在位场资料去噪和位场分离中的应用

根据小波变换的原理,研究了小波阙值去噪方法和位场分离方法在某实测布格重力异常的应用及效果。多矩形棱柱体模型试验表明:选取合适的小波基和小波参数,可以更好地提取出重力异常中的有用信息,为下一步在实测重力数据中的应用提供理论基础。实测布格重力资料处理结果表明:基于小波变换的去噪方法和位场分离方法,能够较好地去除随机噪声,更好地区别出局部异常和区域异常,提高数据处理与解释中有用信号识别的精度。     

The effect of crustal anisotropy on reflector depth and velocity determination from wide-angle seismic data: a synthetic example based on South Island, New Zealand

When deriving velocity models by forward modelling or inverting travel time arrivals from seismic refraction data, a heterogeneous but isotropic earth is usually assumed. In regions where the earth is not isotropic at the scale at which it is being sampled, the assumption of isotropy can lead to significant errors in the velocities determined for the crust and the depths calculated to reflecting boundaries. Laboratory velocity measurements on rocks collected from the Haast Schist terrane of South Island, New Zealand, show significant (up to 20%) compressional (P) wave velocity anisotropy. Field data collected parallel and perpendicular to the foliation of the Haast Schist exhibit as much as 11% P-wave velocity anisotropy. We demonstrate, using finite-difference full-wavefield modelling, the types of errors and problems that might be encountered if isotropic methods are used to create velocity models from data collected in anisotropic regions. These reflector depth errors could be as much as 10–15% for a 10-km thick layer with significant (20%) P-wave velocity anisotropy. The implications for South Island, New Zealand, where the problem is compounded by extreme orientations of highly anisotropic rocks (foliation which varies from horizontal to near vertical), are considered. Finally, we discuss how the presence of a significant subsurface anisotropic body might manifest itself in wide-angle reflection/refraction and passive seismic datasets, and suggest ways in which such datasets may be used to determine the presence and extent of such anisotropic bodies.     

A unified criterion for initiation of sediment motion and inception of sheet flow under water waves

A unified criterion is developed for initiation of non‐cohesive sediment motion and inception of sheet flow under water waves over a horizontal bed of sediment based on presently available experimental data. The unified threshold criterion is of the single form, U_o = 2πC[1 + 5(T_R/T)²]^?1/4, where U_o is the onset velocity of sediment motion or sheet flow, T is wave period, and C and T_R are the coefficients. It is found that for a given sediment, U_o initially increases sharply with wave period, then gradually approaches the maximum onset velocity U_o = 2πC and becomes independent of T when T is larger. The unified criterion can also be extended to define sediment initial motion and sheet flow under irregular waves provided the significant wave orbital velocity and period of irregular waves are introduced in this unified criterion.     

Sedimentary processes in the Selvage sediment-wave field, NE Atlantic: new insights into the formation of sediment waves by turbidity currents

An integrated geophysical and sedimentological investigation of the Selvage sediment-wave field has revealed that the sediment waves are formed beneath unconfined turbidity currents. The sediment waves occur on the lower continental rise and display wavelengths of up to 1 km and wave heights of up to 6 m. Wave sediments consist of interbedded turbidites and pelagic/hemipelagic marls and oozes. Nannofossil-based dating of the sediments indicates a bulk sedimentation rate of 2·4 cm 1000 years^–1, and the waves are migrating upslope at a rate of 0·28 m 1000 years^–1. Sediment provenance studies reveal that the turbidity currents maintaining the waves are largely sourced from volcanic islands to the south. Investigation of existing models for sediment-wave formation leads to the conclusion that the Selvage sediment waves form as giant antidunes. Simple numerical modelling reveals that turbidity currents crossing the wave field have internal Froude numbers of 0·5–1·9, which is very close to the antidune existence limits. Depositional flow velocities range from <6 to 125 cm^–1. There is a rapid increase in wavelength and flow thickness in the upper 10 km of the wave field, which is unexpected, as the slope angle remains relatively constant. This anomaly is possibly linked to a topographic obstacle just upslope of the sediment waves. Flows passing over the obstacle may undergo a hydraulic jump at its boundary, leading to an increase in flow thickness. In the lower 15 km of the wave field, flow thickness decreases downslope by 60%, which is comparable with results obtained for other unconfined turbidity currents undergoing flow expansion.     

Delineation of a landfill leachate plume and flow channels in coastal sands near Christchurch, New Zealand, using a shallow electromagnetic survey method

The Burwood landfill, which serves the city of Christchurch, New Zealand, is situated on coastal sands underlain by a sequence of aquifers and aquitards. Groundwater flow is toward the coast, located approximately 700 m from the landfill boundary. Shortly after completion of the first phase of the landfill, an array of wells was installed to detect any contaminant from the landfill. Leachate was detected in the wells closest to the landfill. A shallow electromagnetic (EM31) survey was carried out between the landfill and the coast, in order to delineate any leachate plume that may be present. On the basis of the geophysical results, a contaminant plume and buried channels connected to the coast were identified. Leachate flow initially occurs in what is probably a channel or pair of channels. Downgradient, the plume spreads out to the north and south as it moves eastward toward the coast. Using the geophysical results as a guide, a new set of wells was installed to confirm the presence of high leachate concentrations. Pore-water sampling confirms the presence of a leachate plume. Received, June 1998 /Revised, March 1999, January 2000/Accepted, January 2000     

Re-envisioning the structure of last glacial vegetation in New Zealand using beetle fossils

A series of 18 fossil beetle assemblages are used to reconstruct the paleoenvironment of the northwest West Coast, New Zealand, over the period of the last interstadial-stadial transition (ca. 37,000-21,300 cal yr BP). The samples were recovered from an in-filled hollow within a dune field ca. 9 km south west of Westport (41°47′S, 171°30′E). This fossil beetle reconstruction is compared to an existing palynological reconstruction from the same site. The beetle assemblages indicate an environment of marshy shrubland interspersed with closed canopy coastal vegetation prior to glacial onset, and a mosaic of closed canopy patches and open tussock grassland during full glacial conditions. These interpretations, contrast with the palynologically based interpretation which indicates subalpine shrubland prior to glacial onset and widespread grassland with little woody vegetation during the period of maximum glacial cooling. This study is consistent with other non-pollen studies in New Zealand and indicates that the palynological interpretation of the paleoenvironment of the Westport region downplays the importance of closed canopy vegetation in the area during the transition from interstadial to full glacial (stadial) conditions. It challenges the interpretation of open vegetation at low elevations during glacial periods from pollen studies.     

Cyclone track prediction using INSAT data

INSAT visible and infrared imageries of three cyclones in the Bay of Bengal during the period 1984–1987 were analysed with a view to improve the cyclone track prediction in this region. It was observed that the rotation in the major structural cloud features (as seen from the cloud-top temperature maps) associated with these cyclones in the Bay of Bengal is followed with a change in direction of their movement. This method is seen to be particularly effective when the cyclone is severe and when the major cloud features persist for a reasonably longer time. In the present study, only the direction of movement is forecast assuming a uniform speed of the cyclone.     

缺资料地区泥石流预警雨量阈值研究

合理的雨量阈值指标是保障泥石流预警报准确性的关键,对于研究泥石流形成机制、分析预测未来活动特点以及指导防治工程设计等方面均具有重要意义。由于国内山区大多数泥石流沟均无降雨和灾害资料,目前国内外通行的实证法和频率计算法不能满足其泥石流预警报的需要。通过分析泥石流预警区的降雨条件、水文特征及下垫面条件,提出了基于水力类泥石流起动机制来计算泥石流预警雨量阈值的方法。主要根据流域地形及松散固体物质等特征,计算该流域泥石流起动的临界水深,并结合流域产汇流机制、特征降雨量随海拔变化规律、暴雨雨型特征,进而计算该流域泥石流预警雨量阈值。研究成果在四川省凉山州宁南县城后山史家沟流域进行了应用和验证,结果表明该方法具有合理性和可行性。该方法解决了缺资料地区泥石流预警报的难题,为山区泥石流预警报提供了一种新的思路。