基于双二次插值的探地雷达有限元数值模拟

从探地雷达(GPR)满足的波动方程出发,详细介绍了二维GPR模型单元剖分、二次插值、数值积分和有限元刚度矩阵总体合成的GPR有限元求解过程.为解决数值模拟时截断边界处的超强反射,采用Clay Bout透射边界条件对雷达波进行衰减,进而压制了来自截断边界处的反射波.在满足时间步长与空间网格差分稳定性前提下,采用中心差分法对GPR有限元方程进行离散,并用不完全LU分解预处理的BICGSTAB算法求解系数方程组,然后编制了基于双二次插值的GPR有限元正演模拟matlab程序.运用该程序分别对矩形和"V"字形两个典型地电模型进行正演计算,得到了正演剖面图,将该正演剖面图与基于线性插值的FEM算法的正演剖面图做了对比分析.结果表明基于双二次插值FEM算法相比基于双线性插值FEM算法异常响应更明显,具有更高的模拟精度,更有利于指导雷达剖面的数据解译.     

探地雷达测定土壤含水量的研究进展

结合国内外关于探地雷达测定土壤含水量的最新研究成果,阐述了探地雷达测定土壤含水量的基本原理和计算方法,总结和评价了土壤含水量与土壤介电常数的不同计算模型,针对在实际应用中存在的不足,提出了相关改进措施,并展望发展方向.     

双相混合物等效介电常数探地雷达数值研究

等效介电常数是混合物微观结构的宏观表征,与介质的属性参数密切相关.在传统随机介质建模理论的基础上,通过增加量化约束限定条件,精准控制双相混合物各组成物质的体积占比,建立了 22个体积占比相同、孔隙粒径尺度不等和20个孔隙粒径尺度相等、体积占比不同的双相混合物介质模型,进行探地雷达数值模拟,研究超宽频带条件下双相混合物各组成物质体积占比和孔隙粒径大小对其等效介电常数及探地雷达波场特征的影响.结果表明:对于各组成物质体积占比相同、孔隙粒径尺度不等的双相混合物,其等效介电常数基本相同,说明混合物等效介电常数主要取决于其各组成物质的体积占比及其介电常数,但由于混合物内部孔隙粒径尺度不等,其电磁散射强度和能量衰减损失存在明显差异;对于孔隙粒径尺度相等、体积占比不同的双相混合物,其空气体积占比越小,等效介电常数越大,与Rayleigh模型、CRIM模型等混合物等效介电常数经验公式计算结果差值越小、吻合越好,同时,其散射波振幅强度和波形杂乱程度越弱,研究结果可为解译双相混合物介质属性参数提供参考和指导.     

沥青心墙压实度的探地雷达探测研究

沥青心墙是水利水电工程的一种重要的防渗形式,其质量直接关系到工程的安全.压实度是评价其质量的重要指标,常采用取芯后实验室来测定,虽然探测结果可靠,但破坏性大、周期性长以及连续性差.我们采用探地雷达方法来评价沥青心墙的压实度.为消除沥青心墙的侧壁和两侧的堆石对探测结果的影响,本文模拟了雷达波在沥青心墙中传播,得出没有侧壁和堆石反射波影响的有效探测深度.并利用实测数据提取沥青心墙的雷达波属性如振幅、主频以及瞬态谱等,结合采样获得的孔隙度和密度,得出其与属性参数间的关系.研究表明,这种方法可以直观、快速、连续的反映心墙压实度变化,为工程质量的检测提供很好的依据.     

探地雷达三维高阶时域有限差分法模拟研究

探地雷达数值模拟中,时域有限差分法在时间和空间上一般采用二阶精度的中心差分近似(FDTD(2,2)),其形式简单,但数值色散误差较大,在复杂模型模拟时不能很好地反映模型的精细变化.高阶时域有限差分法能很好地改善数值色散带来的误差,提高模拟精度.本文基于三维高阶时域有限差分法的基本原理实现了探地雷达正演模拟,采用单轴各向异性完全匹配层(UPML)作为吸收边界条件,可以有效地吸收外向传播的电磁波,在大大地提高计算效率的同时,也能很好地改善边界的吸收效果.分析对比正演模拟结果,通过三维高阶时域有限差分正演能获得目标体准确电磁响应信息,并能很好的提高模拟精度.     

机载探地雷达的进展以及数值模拟

机载探地雷达可能解决危险环境或广域条件下的近地表探测问题,用于解决环境、生态或军事方面的问题.然而由于种种原因,该技术的发展却显得比较慢.为了推进该技术的发展,本文介绍了目前世界范围内机载探地雷达的进展,并利用时间域有限差分法对一些典型模型进行数值模拟,并用特定的偏移成像方法对模拟结果进行成像.目前存在的机载探地雷达主要有三种类型:第一种为将常规探地雷达天线悬挂在直升飞机上,第二种为针对机载探地雷达开发的雷达系统,第三种为具有探地能力的合成孔径雷达.数值模拟结果表明,不管是水平地面的情况下,还是起伏界面的情况下,机载探地雷达都能清楚探测一定深度范围内的地下目标.可见,机载探地雷达是存满希望的一种方法.     

全极化探地雷达正演模拟

全极化探地雷达不但能获取当前探地雷达使用的共极化信息,而且能使用交叉极化信息,进而更好地分析目标属性.本文在时域有限差分法的基础上,构建了全极化探地雷达的正演模拟方法,通过水平正交双方向同时接收获取全极化信息.标准目标体金属板和角反射器的正演模拟与实验室物理模型测试一致的共极化和交叉极化响应信息,证实了该正演模拟方法的有效性.同时还模拟了典型目标(随机表面和斜裂缝)的响应.正演模拟和实验结果均反映出结合交叉极化响应和共极化响应信息有助于识别和判断目标的方向、结构组成等属性.     

中尺度模拟土壤油污染区探地雷达探测效果分析

为使物理模拟实验效果与实际探测情形更为接近,在室外自然条件下建立中尺度土壤石油污染实验模型,油污染区扩展深度超过1 m,采用实地探测中常用的500 MHz雷达天线进行长期定时探测.通过实测雷达图像特征、土壤含水量含油量分析,并对比前人开展的小尺度室内模拟试验结果,综合评价探地雷达对油污染区的探测效果.研究表明探地雷达探测图像异常特征与污染区扩散阶段密切相关:包气带内油污染区会引起振幅增强;毛细带的油污染区则表现为水位面反射轴附近清晰可辨的高幅异常区,且水位面反射轴呈下凹状;随扩散过程持续进行,异常区下移与水位面反射轴相交,并产生水平扩张.当污染土含油饱和度大于20%时,可通过雷达图像异常区圈定污染范围;当污染土含油饱和度大于15%时,可通过频谱图出现低频响应的位置圈定污染区水平范围.中尺度实验结果与室内小尺度模拟结果具有一致性,可作为油污染区雷达图像异常的解译依据.     

探地雷达衰减补偿逆时偏移成像方法

探地雷达信号在地下介质中传播时易受到电导率所产生的衰减影响,从而使得传统偏移成像结果在高衰减区域变得模糊.本文提出了衰减补偿的逆时偏移方法来消除电导率的影响.该方法基于麦克斯韦方程组实现电磁波的正演模拟和逆时传播.通过改变衰减项的正负号,保证了逆时传播的时间对称性,从而能够重构出原始波场,实现衰减补偿.数值实验比较了传统逆时偏移方法和衰减补偿逆时偏移方法在存在高导异常区域情况下的成像效果,结果证明了衰减补偿逆时偏移方法能够很好地恢复由电导率造成的信号衰减,从而提高探地雷达剖面的分辨率.     

探地雷达逆时偏移成像方法研究现状及进展

基于双程波动方程的逆时偏移相比于传统的单程波偏移,具有原理简单、成像精确且无倾角限制、适应任意复杂速度模型等诸多优点,在探地雷达(GPR)高精度成像领域得到广泛关注.本文首先回顾和总结了逆时偏移方法在GPR成像领域的研究现状及其存在的问题.然后,分别从多偏移距GPR数据的快速采集,复杂介质的GPR逆时偏移成像、成像条件的稳定性、成像效果的改善、计算效率的提高等方面介绍了GPR逆时偏移成像研究的最新进展及其具体的应用效果.最后对GPR逆时偏移成像的发展趋势进行了总结.     

Measurement of near surface soil water content using a capacitance probe

The portable surface capacitance insertion probe was used to measure the in situ water content of near surface soil layers. The probe readings were calibrated against gravimetric samples collected over a wide range of water contents, and were found to be very closely correlated. The capacitance probe was used repeatedly in time and space at a field site where ridges and furrows ensured a regular pattern of soil water variations. The observed spatial variations of the water content were related to topography and land mangement. The degree of variability was dependent on the average water content.     

Characterization of pyroclastic fall and flow deposits from the 1815 eruption of Tambora volcano,Indonesia using ground-penetrating radar

Ground-penetrating radar (GPR) is used to image and characterize fall and pyroclastic flow deposits from the 1815 eruption of Tambora volcano in Indonesia. Analysis of GPR common-mid-point (CMP) data indicate that the velocity of radar in the sub-surface is 0.1 m/ns, and this is used to establish a preliminary traveltime to-depth conversion for common-offset reflection profiles. Common-offset radar profiles were collected along the edge of an erosional gully that exposed approximately 1–2 m of volcanic stratigraphy. Additional trenching at select locations in the gully exposed the contact between the pre-1815 eruption surface and overlying pyroclastic deposit from the 1815 eruption. The deepest continuous, prominent reflection is shown to correspond to the interface between pre-eruption clay-rich soil and pyroclastics that reach a maximum thickness of 4 m along our profiles. This soil surface is distinctly terraced and is interpreted as the ground surface augmented for agriculture and buildings by people from the kingdom of Tambora. The correlation of volcanic stratigraphy and radar data at this location indicates that reflections are produced by the soil-pyroclastic deposit interface and the interface between pyroclastic flows (including pyroclastic surge) and the pumice-rich fall deposits. In the thickest deposits an additional reflection marks the interface between two pyroclastic flow units.     

Assessing regional‐scale spatio‐temporal patterns of groundwater–surface water interactions using a coupled SWAT‐MODFLOW model

Interaction between groundwater and surface water in watersheds has significant impacts on water management and water rights, nutrient loading from aquifers to streams, and in‐stream flow requirements for aquatic species. Of particular importance are the spatial patterns of these interactions. This study explores the spatio‐temporal patterns of groundwater discharge to a river system in a semi‐arid region, with methods applied to the Sprague River Watershed (4100 km²) within the Upper Klamath Basin in Oregon, USA. Patterns of groundwater–surface water interaction are explored throughout the watershed during the 1970–2003 time period using a coupled SWAT‐MODFLOW model tested against streamflow, groundwater level and field‐estimated reach‐specific groundwater discharge rates. Daily time steps and coupling are used, with groundwater discharge rates calculated for each model computational point along the stream. Model results also are averaged by month and by year to determine seasonal and decadal trends in groundwater discharge rates. Results show high spatial variability in groundwater discharge, with several locations showing no groundwater/surface water interaction. Average annual groundwater discharge is 20.5 m³/s, with maximum and minimum rates occurring in September–October and March–April, respectively. Annual average rates increase by approximately 0.02 m³/s per year over the 34‐year period, negligible compared with the average annual rate, although 70% of the stream network experiences an increase in groundwater discharge rate between 1970 and 2003. Results can assist with water management, identifying potential locations of heavy nutrient mass loading from the aquifer to streams and ecological assessment and planning focused on locations of high groundwater discharge. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of multilayered groundwater mounds on water dynamics beneath a recharge basin: Numerical simulation and assessment of surface injection

Interactions between groundwater mounds caused by a geologic layer contrast affect the efficiency of managed aquifer recharge in arid areas. However, research has rarely examined the roles of groundwater mounding size variations on soil water dynamics in a stratified vadose zone in response to a sustained infiltration source. Numerical experiments were conducted on a two-dimensional vertical-section domain using HYDRUS software to simulate the behaviours of two adjacent (upper and lower) groundwater mounds underlying an infiltration basin subjected to clay loam and sandy alternately-layered soil profiles. The model successfully predicted the volume and extent of perched water and approximated vertical travel times during events generating downward fluxes from the surface injection. The response time of the mounding width (lateral extension) to the surface injection was delayed as compared to that of the mounding height (vertical extension), especially for the lower water mound. The mounding heights and widths show a strongly positive correlation with the infiltration rates of both high- and low-permeability layers where the injected water mounded, while the water storage amounts in the high- and low-permeability layers were governed by the mounding height and width, respectively. Exploratory simulations were then employed to assess the dependence of groundwater mounding behaviours and recharge performances on surface injection strategies. Results suggest that, by reducing injection rate or shortening injection duration, the near-term fraction of the surface injection converted to deep recharge is likely to be increased due to the narrowed groundwater mounding size, which would be limited by the water-retarding effect of layer contrasts. This study has important implications for predicting and understanding multilayered groundwater mounding behaviours and associated water mass balance under the geologic stratification, and is expected to aid in optimizing the infiltration basin operation for aquifer recharge.     

Quantification by numerical simulation of the impact of gullies on the water budget of a basement area,southeastern Brazil

ABSTRACT

Many regions of the world are affected by high density of large gullies that can impact on water resources. As these impacts have not been properly evaluated, this study presents a quantification model by numerical simulation of the effect of gullying on the water budget. An active gully of a very eroded region (Bação Complex) was selected and systematically monitored during a hydrological year. The simulation of the gully area by finite element method enabled the quantification of impacts, such as baseflow reduction and groundwater drawdown in the vicinity of this erosional feature. The baseflow reduction, when extrapolated to the whole complex, showed a significant baseflow loss during the dry seasons and an increase of stream flows during the rainy seasons, favouring floods. The numerical simulation results indicate that these modifications are relevant and must be considered when managing aquifers in intensely gullied areas with problems related to water availability.     

Characterization of volcanic deposits with ground-penetrating radar

 Field-based studies of surficial volcanic deposits are commonly complicated by a combination of poor exposure and rapid lateral variations controlled by unknown paleotopography. The potential of ground-penetrating radar (GPR) as an aid to volcanological studies is shown using data collected from traverses over four well-exposed, Recent volcanic deposits in western Canada. The deposits comprise a pumice airfall deposit (3–4 m thick), a basalt lava flow (3–6 m thick), a pyroclastic flow deposit (15 m thick), and an internally stratified pumice talus deposit (60 m thick). Results show that GPR is effective in delineating major stratigraphic contacts and hence can be used to map unexposed deposits. Different volcanic deposits also exhibit different radar stratigraphic character, suggesting that deposit type may be determined from radar images. In addition, large blocks within the pyroclastic deposits are detected as distinctive point diffractor patterns in the profiles, showing that the technique has potential for providing important grain-size information in coarse poorly sorted deposits. Laboratory measurements of dielectric constant (K') are reported for samples of the main rock types and are compared with values of K' for the bulk deposit as inferred from the field data. The laboratory values differ significantly from the "field" values of K'; these results suggest that the effectiveness of GPR at any site can be substantially improved by initial calibration of well-exposed locations. Received: 10 May 1996 / Accepted: 27 December 1996     

用瞬态多道瑞利波法研究夏垫隐伏断裂附近的浅层速度结构变化

用瞬态多道瑞利波法,对夏垫隐伏断裂附近的浅层速度结构进行了调查研究,利用f-k域分析方法提取瑞利波频散曲线,分别用遗传算法和半波长方法反演,得到断裂附近的横波速度结构和瑞利波相速度分布剖面。反演结果与钻孔资料的对比表明,瞬态瑞利波法对于探测上断点埋深较浅的隐伏断裂是有效和可靠的。     

From streambed temperature measurements to spatial‐temporal flux quantification: using the LPML method to study groundwater–surface water interaction

Knowledge on groundwater–surface water interaction and especially on exchange fluxes between streams and aquifers is an important prerequisite for the study of transport and fate of contaminants and nutrients in the hyporheic zone. One possibility to quantify groundwater–surface water exchange fluxes is by using heat as an environmlental tracer. Modern field equipment including multilevel temperature sticks and the novel open‐source analysis tool LPML make this technique ever more attractive. The recently developed LPML method solves the one‐dimensional fluid flow and heat transport equation by combining a local polynomial method with a maximum likelihood estimator. In this study, we apply the LPML method on field data to quantify the spatial and temporal variability of vertical fluxes and their uncertainties from temperature–time series measured in a Belgian lowland stream. Over several months, temperature data were collected with multilevel temperature sticks at the streambed top and at six depths for a small stream section. Long‐term estimates show a range from gaining fluxes of ?291 mm day^?1 to loosing fluxes of 12 mm day^?1; average seasonal fluxes ranged from ?138 mm day^?1 in winter to ?16 mm day^?1 in summer. With our analyses, we could determine a high spatial and temporal variability of vertical exchange fluxes for the investigated stream section. Such spatial and temporal variability should be taken into account in biogeochemical cycling of carbon, nutrients and metals and in fate analysis of contaminant plumes. In general, the stream section was gaining during most of the observation period. Two short‐term high stream stage events, seemingly caused by blockage of the stream outlet, led to a change in flow direction from gaining to losing conditions. We also found more discharge occurring at the outer stream bank than at the inner one indicating a local flow‐through system. With the conducted analyses, we were able to advance our understanding of the regional groundwater flow system. Copyright © 2015 John Wiley & Sons, Ltd.     

An englacial image and water pathways of the Fourcade glacier on King George Island, Antarctic Peninsula, inferred from ground-penetrating radar

The distribution of small fractures and water content of the Fourcade glacier on King George Island, Antarctica, was investigated in November 2006 and December 2007 by two ground-based (470-and 490-m-long profiles) and one helicopter-borne (470-m-long profile) ground-penetrating radar (GPR) surveys using 50-, 100-, and 500-MHz antennas. Radar images in the pre-migrated GPR sections are characterized by a smooth ice surface and irregular bed topography, numerous diffraction hyperbolas in the ice and at the g...     

A conceptual–numerical model to simulate hydraulic head in aquifers that are hydraulically connected to surface water bodies

In this paper, we present a conceptual‐numerical model that can be deduced from a calibrated finite difference groundwater‐flow model, which provides a parsimonious approach to simulate and analyze hydraulic heads and surface water body–aquifer interaction for linear aquifers (linear response of head to stresses). The solution of linear groundwater‐flow problems using eigenvalue techniques can be formulated with a simple explicit state equation whose structure shows that the surface water body–aquifer interaction phenomenon can be approached as the drainage of a number of independent linear reservoirs. The hydraulic head field could be also approached by the summation of the head fields, estimated for those reservoirs, defined over the same domain set by the aquifer limits, where the hydraulic head field in each reservoir is proportional to a specific surface (an eigenfunction of an eigenproblem, or an eigenvector in discrete cases). All the parameters and initial conditions of each linear reservoir can be mathematically defined in a univocal way from the calibrated finite difference model, preserving its characteristics (geometry, boundary conditions, hydrodynamic parameters (heterogeneity), and spatial distribution of the stresses). We also demonstrated that, in practical cases, an accurate solution can be obtained with a reduced number of linear reservoirs. The reduced computational cost of these solutions can help to integrate the groundwater component within conjunctive use management models. Conceptual approximation also facilitates understanding of the physical phenomenon and analysis of the factors that influence it. A simple synthetic aquifer has been employed to show how the conceptual model can be built for different spatial discretizations, the parameters required, and their influence on the simulation of hydraulic head fields and stream–aquifer flow exchange variables. A real‐world case was also solved to test the accuracy of the proposed approaches, by comparing its solution with that obtained using finite‐difference MODFLOW code. Copyright © 2011 John Wiley & Sons, Ltd.