Outlier detection algorithms and their performance in GOCE gravity field processing

The satellite missions CHAMP, GRACE, and GOCE mark the beginning of a new era in gravity field determination and modeling. They provide unique models of the global stationary gravity field and its variation in time. Due to inevitable measurement errors, sophisticated pre-processing steps have to be applied before further use of the satellite measurements. In the framework of the GOCE mission, this includes outlier detection, absolute calibration and validation of the SGG (satellite gravity gradiometry) measurements, and removal of temporal effects. In general, outliers are defined as observations that appear to be inconsistent with the remainder of the data set. One goal is to evaluate the effect of additive, innovative and bulk outliers on the estimates of the spherical harmonic coefficients. It can be shown that even a small number of undetected outliers (<0.2 of all data points) can have an adverse effect on the coefficient estimates. Consequently, concepts for the identification and removal of outliers have to be developed. Novel outlier detection algorithms are derived and statistical methods are presented that may be used for this purpose. The methods aim at high outlier identification rates as well as small failure rates. A combined algorithm, based on wavelets and a statistical method, shows best performance with an identification rate of about 99%. To further reduce the influence of undetected outliers, an outlier detection algorithm is implemented inside the gravity field solver (the Quick-Look Gravity Field Analysis tool was used). This results in spherical harmonic coefficient estimates that are of similar quality to those obtained without outliers in the input data.     

Installation of a vertical slurry wall around an Italian quarry lake: complications arising and simulation of the effects on groundwater flow

Slurry walls are non-structural barriers that are constructed underground to impede groundwater flow or manage groundwater control problems. The study area is in the Piemonte plain (Italy), close to the River Po. Quarrying works carried out below the piezometric surface created two big quarry lakes. The local groundwater system is characterized by a lower semi-confined aquifer, which is overlain by a semi-permeable bed of clayey peat (aquitard) and an upper unconfined aquifer. Locally, the peat fades away and the granulometry of this horizon becomes silty sandy. A planned enlargement of the quarry will increase the size and depth of the quarry lakes. So the aquitard bed between the two aquifers will be damaged, creating a mixing rate of groundwater. Such a procedure would not be compatible with the presence of two municipal wells upstream from the quarries. Consequently, the installation of a vertical diaphragm (slurry wall) is recommended to separate the aquifers and to act as a filter for the groundwater flowing from the unconfined to the semi-confined aquifer. To predict the consequences caused by the installation of the vertical diaphragm separating the unconfined aquifer and the semi-confined one, a specifically adjusted finite-difference model was used. The model showed a maximum rising of the water table equal to 12 cm, just upstream of the diaphragm and for a distance of about 100 m, and a maximum lowering of 2 cm just downstream of the diaphragm. However, the slurry wall would not cause any change in the piezometric head in the area where there are municipal wells and, hence, will not have any negative effect on the functionality of the municipal wells. Moreover, the migration of water from the unconfined aquifer through the vertical diaphragm will stimulate a series of attenuation and auto-depuration processes of eventual contaminants. These processes are due to the higher crossing time that the groundwater flow takes to go through the vertical barrier (t _a = 96.5 days, whereas for the horizontal semi-permeable layer t _a = 9.6 days). So, the vertical diaphragm can be a resolutive element, representing a mediation and separation factor between the unconfined and the semi-confined aquifers along the border of the quarrying areas, and a protective barrier for the water quality of the quarry lake and the semi-confined aquifer.     

轻型定向秸秆板-榫卯连接木骨架剪力墙抗震性能试验研究

为了促进适合我国国情的轻型木结构发展,介绍了一种自主开发的轻型定向秸秆板-榫卯连接木骨架剪力墙,该剪力墙由墙体模块、顶梁板、底梁板三部分组成,墙体模块的木骨架间采用榫卯连接,面板采用定向秸秆板.通过12片分别由3个剪力墙模块组成的剪力墙的水平单向加载和低周反复加载试验研究,得到了剪力墙的破坏特征、抗剪强度、滞回性能、延性及耗能等性能指标,分析了洞口大小、洞口位置及竖向荷载对剪力墙抗震性能的影响以及剪力墙与单个墙体模块抗震性能的区别.结果表明:剪力墙的破坏主要是面板破坏及面板与钉的连接破坏,剪力墙的抗剪强度比墙体模块的要低约10％,滞回曲线呈明显的反S型并有明显滑移,等效粘滞阻尼系数约为0.13.     

多年冻土区L型挡墙水平冻胀效应计算方法与分析

针对多年冻土区L型挡墙设计中水平冻胀力计算方法不完善的问题,基于利夫金地基模型及挡墙?土体之间的协同变形原理,分别建立了墙后有无换填土两种情况下的L型挡墙水平冻胀效应计算模型,利用叠加原理及有限差分法对所提出的计算模型进行了求解,并依托MATLAB自行编制了水?热?力耦合分析软件。结合工程实例,应用提出的L型挡墙水平冻胀效应计算方法得到的水平冻胀力值分别与现场实测值、修正土压力值、规范经验值及水?热?力耦合软件模拟值进行对比分析,结果表明：提出L型挡墙水平冻胀效应计算方法得到的水平冻胀力值与现场实测值及数值模拟值在大体趋势上吻合较好,修正土压力值、规范经验值低估了水平冻胀力对挡墙的作用;相比规范经验值及现场实测值,提出的L型挡墙水平冻胀效应计算方法得到的水平冻胀力沿墙高呈抛物线和梯形两种分布模式,更具有普遍性;通过多场耦合分析可知,所提出L型挡墙水平冻胀效应计算方法与水?热?力耦合方法得到冻胀力趋势相似,表明了计算方法的可行性,可为多年冻土区L型挡墙设计提供一定的理论支持和指导。     

惯性测量系统中垂线偏差和重力异常的确定

本文对惯性测量各项系统误差及异常重力场进行了分析和研究,并在此基础上建立了两种用于惯性重力测量的估算模型;模拟计算的结果表明,所建模型在原理上是可靠的,结果是令人满意的;同时,本文较为详细地分析了各项系统误差对异常重力元素估计精度的影响,对卡尔曼滤波应用中的有关问题进行了讨论。     

Preprocessing of gravity gradients at the GOCE high-level processing facility

One of the products derived from the gravity field and steady-state ocean circulation explorer (GOCE) observations are the gravity gradients. These gravity gradients are provided in the gradiometer reference frame (GRF) and are calibrated in-flight using satellite shaking and star sensor data. To use these gravity gradients for application in Earth scienes and gravity field analysis, additional preprocessing needs to be done, including corrections for temporal gravity field signals to isolate the static gravity field part, screening for outliers, calibration by comparison with existing external gravity field information and error assessment. The temporal gravity gradient corrections consist of tidal and nontidal corrections. These are all generally below the gravity gradient error level, which is predicted to show a 1/f behaviour for low frequencies. In the outlier detection, the 1/f error is compensated for by subtracting a local median from the data, while the data error is assessed using the median absolute deviation. The local median acts as a high-pass filter and it is robust as is the median absolute deviation. Three different methods have been implemented for the calibration of the gravity gradients. All three methods use a high-pass filter to compensate for the 1/f gravity gradient error. The baseline method uses state-of-the-art global gravity field models and the most accurate results are obtained if star sensor misalignments are estimated along with the calibration parameters. A second calibration method uses GOCE GPS data to estimate a low-degree gravity field model as well as gravity gradient scale factors. Both methods allow to estimate gravity gradient scale factors down to the 10⁻³ level. The third calibration method uses high accurate terrestrial gravity data in selected regions to validate the gravity gradient scale factors, focussing on the measurement band. Gravity gradient scale factors may be estimated down to the 10⁻² level with this method.     

复杂地质条件下组合围堰施工

在复杂的地质条件下,根据现场地质情况选择合理的围堰成为深基坑开挖过程中支护与止水的关键。结合柳州双拥大桥22号墩主塔基础施工,介绍了采用锁口钢管桩与钢筋混凝土支护桩相结合的组合围堰来进行复杂地质条件下的围堰施工技术。     

钢筋混凝土短肢剪力墙抗震性能试验研究

通过6片1：2单层短肢墙试体的低周反复荷戴试验,研究了在低周反复荷载作用下钢筋混凝土短肢剪力墙的整体工作性能、破坏形态及滞回特性。结果表明：试件的最终破坏均是由连梁失效引起的,连梁是短肢剪力墙结构的薄弱环节,因此,连梁的混凝土强度不应低于墙肢的混凝土强度,以免连梁出现粘结破坏;有翼墙的短肢剪力墙试体其延性系数都达到了3．5以上．其耗能能力较无翼墙短肢墙好。     

土质场地重力式挡土墙地震土压力振动台实验研究

汶川震区路基挡土墙震害表明,地震动荷载作用下重力式挡墙的位移、破坏与基础场地形式有关,除岩质场地和土质场地挡墙所共有的外倾形式,土质地基挡土墙还表现有整体推移及下部向外推移的倾转变形等复杂模式,因此地震土压力大小及分布也将受到这种复杂土-结相互作用的影响。基于碎石土及风化花岗岩填料的土质场地重力式挡土墙大型振动台模型实验,对挡土墙地震土压力及变形模式开展了对比研究,发现在强震作用下,土质地基挡墙因基础约束较弱而产生位移,并伴随明显的墙—土分离现象,致使实测地震土压力较之抗震设计规范计算值偏小（0.4g峰值加速度下约小6%～15%）,但作用点高度变化不大。由实验结果与现行抗震规范计算值的安全系数对比,认为对土质场地挡墙的地震土压力计算,按现行国内抗震设计规范基本能满足实际工程抗震设计需要;对于地震区挡墙设计,在允许挡墙发生少量容许位移的前提下可采用内摩擦角较大、自稳能力更好的墙背填料以减少地震土压力。     

A hybrid discrete–finite element model for numerical simulation of geomaterials

A hybrid discrete–finite element model is introduced for simulation of mechanical behavior of geomaterials. The soil or rock is modeled as a system of discrete balls that interact through normal and shear springs. The balls can be bonded at the contact points to withstand the applied deviatoric stresses. The important feature of this model is that the confining walls that can be imagined for example as the surrounding membrane or the mold in a physical test are modeled by deformable finite elements. This allows simulation of laboratory test features more realistically compared to the situations where the surrounding walls are rigid. The relationships between micro- and macro-properties are investigated in this paper as well. These relationships and the corresponding curves are helpful tools in calibration of the numerical model for the macroscopic elastic properties.