Reconstructing pre-erosion topography using spatial interpolation techniques:A validation-based approach

Understanding the topographic context preceding the development of erosive landforms is of major relevance in geomorphic research, as topography is an important factor on both water and mass movement-related erosion, and knowledge of the original surface is a condition for quantifying the volume of eroded material. Although any reconstruction implies assuming that the resulting surface reflects the original topography, past works have been dominated by linear interpolation methods, incapable of generating curved surfaces in areas with no data or values outside the range of variation of inputs. In spite of these limitations, impossibility of validation has led to the assumption of surface representativity never being challenged. In this paper, a validation-based method is applied in order to define the optimal interpolation technique for reconstructing pre-erosion topography in a given study area. In spite of the absence of the original surface, different techniques can be nonetheless evaluated by quantifying their capacity to reproduce known topography in unincised locations within the same geomorphic contexts of existing erosive landforms. A linear method (Triangulated Irregular Network, TIN) and 23 parameterizations of three distinct Spline interpolation techniques were compared using 50 test areas in a context of research on large gully dynamics in the South of Portugal. Results show that almost all Spline methods produced smaller errors than the TIN, and that the latter produced a mean absolute error 61.4% higher than the best Spline method, clearly establishing both the better adjustment of Splines to the geomorphic context considered and the limitations of linear approaches. The proposed method can easily be applied to different interpolation techniques and topographic contexts, enabling better calculations of eroded volumes and denudation rates as well as the investigation of controls by antecedent topographic form over erosive processes.     

海底广角地震剖面反演方法对比——以南海礼乐滩OBS剖面为例

广角地震测线（OBS973-2）位于南海南部陆缘,其地壳深部构造是研究南海共轭扩张及形成演化的直接证据之一.本文采用2D射线追踪技术,结合与之重合的多道地震测线（NH973-2）时深转换结果,对OBS973-2测线重新进行了正、反演研究,得到了礼乐滩及邻近海区的精细地壳结构.与前人结果相比,本文基于正反演速度模型,把测线分为陆壳区（0~200 km）、洋陆过渡区（200~280 km）和洋盆区（280~370 km）.地壳结构在不同区域差异明显,陆壳区沉积层厚度横向差异大,且速度横向不均匀,地壳整体厚度大（约20 km）,有横向速度差;洋陆过渡区速度和厚度横向均匀,地壳减薄（约8 km）;洋壳区地壳厚度减薄至6 km.与以往研究相比,新的认识集中在两个方面,（1）在方法上,综合广角地震和多道地震数据,借助正演和反演方法,能够得到更多更可靠的地壳结构信息.（2）在地壳结构上,结合广角地震与多道地震,得到洋陆过渡区莫霍面向海减薄的形态及其埋深（约12~18 km,海平面为0 km）;进一步验证礼乐滩区域在洋陆过渡区没有明显的高速层,为非火山型陆缘,其共轭扩张点为中沙地块;陆壳区上地壳强烈的拉张作用在速度模型表现出横向速度异常和低速区,在多道地震剖面上表现为大量10~20 km的正断层.     

三维地震与OBS联合勘探揭示的神狐海域含水合物地层声波速度特征

以三维高分辨地震与海底高频地震仪（OBS）联合勘探数据为基础,获得海底之下沉积层的地震反射成像剖面及多波信息,并以此确定研究区含天然气水合物沉积层的纵、横波速度的变化特征.根据走时反演获得的横波速度与纵波速度对比分析发现,研究区海底之下500 m深度范围内的某些沉积层具有较高的纵横波速度,这一纵波速度升高区域与水合物稳定带对应,而纵波速度下降并且横波速度变化较小的区域,可能与游离气的存在相关.游离气的可能存在与基于这一区域2007年钻探测井结果的普遍认识不完全相符.     

坡地坡向短波辐射参数化对不同天气过程的影响

为了改进太阳短波辐射参数化,运用考虑了坡地坡向短波辐射效应(SLOPE)的非静力中尺度模式GRAPES(全球/区域同化和预报系统),模拟和讨论了坡地坡向短波辐射效应在不同模式分辨率情况下对中国不同天气过程的影响.数值试验表明:(1)当水平分辨率较低且坡地平缓时,通常可忽略坡地辐射效应,但当水平分辨率较高且地形陡峭起伏时,则不能忽略;(2)在高分辨率(3 或6 km格距,下同)的情况下, 降水的增加和减少与坡地坡向引起的地表短波辐射(GSW)的减少和增加相对应,降水量的增减量大部分均为-5-5 mm;(3)在高分辨率的情况下,地表温度与地表热通量之间有较好的相关性,且大都发生在地形复杂的多山地区;(4)坡地坡向短波辐射效应引起的GSW、降水量、地表温度和地表热通量等要素的变化除了在地形复杂的多山地区比较明显之外,在其他地区也存在.这是由于考虑了坡度和坡向的作用后,影响了大气环流的变化,随着积分时间的增加,其他地区上述物理量也相应地存在一定的变化;(5)坡地坡向对短波辐射的作用,在降水量较小,云量较少的天气过程中,可以更真实地得到反映.因此,在地形复杂和水平分辨率较高情况下,需要考虑坡度坡向作用.     

变换坐标系下相移法起伏地表地震波场延拓

为解决地表起伏变化对地震波场的影响问题,本文提出将(x-z)域的曲网格映射成(ζ-η)域的矩形网格,推导出(ζ-η)域中的标量声波方程,根据推导出来的波动方程采用双平方根法将地表采集到的地震波场在(ζ-η)域中向下延拓,以解决地表起伏带来的负面影响.通过对模型的计算,该方法取得了一定的效果.     

汶川M_S8.0地震河谷地形对汉源县城高烈度异常的影响

汉源县城位于汶川M_S8.0特大地震六度区内唯一的八度异常区,为典型且罕见的远震高烈度异常区.汉源县城处于流沙河的河流阶地之上,河谷地形对地震震害有显著的影响.为定量分析河谷地形对汶川大地震汉源县高烈度异常的影响,基于汉源县城震害科学考察和场地勘察获取的资料,根据震害分布特征和流沙河谷地形的特点,选取1条实测得到的横切汉源县城并垂直流沙河河谷方向的典型剖面作为计算模型,以脉冲作为基底输入,采用有限差分方法研究了该剖面的场地放大效应,分析了地形对高烈度异常的影响.计算结果表明:汉源县城场地对地震动放大效应的显著频段是1.0~10Hz,且这一频段老县城场地的放大效应比新县城场地显著;汉源老县城场地对汶川M_S8.0地震主震的地震动有显著的放大效应,地表峰值加速度大大超过了抗震设计规范的规定值;汉源场地地形放大效应显著频段与汶川M_S8.0地震的能量集中频段基本吻合,汉源老县城建筑物的自振频率恰恰位于该频段,产生共振效应,从而造成更显著的放大效应,这也是汉源震害异常的主要原因之一.由此可见,河谷地形对地表地震动有重要的影响,在工程选址和抗震设计时应考虑其影响.     

广东庙湾岛风化坑的地貌特征及成因

风化坑是常见的岩石坑穴地形,但容易被误解或与其他成因坑穴地形混淆。通过对庙湾岛风化坑地貌特征及形态参数进行详细测量和分析,采集风化坑内残积碎屑物、坑外岩石和当地风化壳样品进行粉晶X射线荧光光谱化学主量分析（XRF）和X射线衍射矿物分析（XRD）,对风化坑发育的成因机制进行了探讨,并得出以下结论：1）风化坑的发育与地形坡度有关,通常出现在平坦裸露的岩石面上。2）风化坑的形态特征表现为宽且浅,口缘有出水口,底部平坦,侧壁最凹处与出水口高度一致,其口径发育的速度远大于深度,长轴发育速度远大于短轴。3）同一岩石面上的风化坑具有不同的大小和深度,显示了其发育程度不一,其最大、最小深度对比反映了风化坑的发育过程。4）风化坑碎屑物与表岩之间的元素迁移和化学蚀变指数（CIA值）的差异,反映了风化坑的化学风化成因。5）风化坑碎屑物的英长比大于表岩,证明了化学风化导致的矿物差异风化作用。研究表明：庙湾岛风化坑形成和发育的主要作用为坑内积水的化学风化。     

金山三岛滩涂水下地形演变浅析

文章采用GPS-RTK技术进行2016年金山三岛滩涂水下地形测量,绘制金山三岛滩涂水下地形云图,并与2014年和2015年滩涂水下地形测量结果进行对比,得出以下结论:金山三岛连线左右1km之内的地形起伏变化大,小金山岛南北侧各有一条深坑,大小金山岛之间有一条贯穿东西向的深槽,它的形成与涨落潮流不一致以及人为活动有关,大金山岛与浮山岛之间有一类似山脊样的地形,浮山岛南侧也有一深坑存在。近3年金山三岛滩涂水下地形变化不大,局部小范围内有淤积和冲刷。定期对三岛滩涂水下地形进行测量可以掌握其变化规律,为管理部门提供管理基础资料。     

The variation of turbulent diapycnal mixing at 18°N in the South China Sea stirred by wind stress

The spatial and temporal variations of turbulent diapycnal mixing along 18°N in the South China Sea(SCS) are estimated by a fine-scale parameterization method based on strain, which is obtained from CTD measurements in yearly September from 2004 to 2010. The section mean diffusivity can reach ~10~(–4)m~2/s, which is an order of magnitude larger than the value in the open ocean. Both internal tides and wind-generated near-inertial internal waves play an important role in furnishing the diapycnal mixing here. The former dominates the diapycnal mixing in the deep ocean and makes nonnegligible contribution in the upper ocean, leading to enhanced diapycnal mixing throughout the water column over rough topography. In contrast, the influence of the wind-induced nearinertial internal wave is mainly confined to the upper ocean. Over both flat and rough bathymetries, the diapycnal diffusivity has a growth trend from 2005 to 2010 in the upper 700 m, which results from the increase of wind work on the near-inertial motions.     

Gravity currents over a rigid and emergent vegetated slope

Lock-exchange experiments are conducted to investigate the effects of emergent vegetation on gravity currents flowing down a slope. Rigid and emergent cylinders are used to represent vegetation such as reeds in aquatic environments. The results show that the head of gravity currents without cylinders forms a semi-elliptic shape, similar to the flat bed case. For the slope-induced gravity, however, the head of gravity currents gradually grows and accelerates. A steeper slope without cylinders causes the evident entrainment and subsequent energy dissipation of gravity currents with ambient fluid. When the cylinder population becomes denser, developments of the head become slower, and the semi-elliptic head can be only seen at the very front end. As the density reaches 6.9%, the interface between the saltwater and freshwater performs an inclined straight line, corresponding to the pervious theoretical derivation. The gravity current event could go through two different routes under the similar toe speeds. The current head without cylinders grows along the downslope path, and the enrollment and mixing with ambient fluid could occur at the edge of the head; while the currents within dense cylinders (∼6.9%) has a nearly constant thickness and a smaller and more streamlined head at the very front end. As the cylinder density increases, the front speed of gravity currents transforms from acceleration to deceleration phases. The experimental results reveal that the head would accelerate over the downslope course if the cylinder density is less than 2%.