月表撞击坑形成过程数值模拟理论与方法

数值模拟是研究撞击坑形成过程的一种主要方法,尤其是认识撞击坑形成机制的重要手段。撞击坑形成过程数值模拟的基本原理是用离散方法描述物质在高速撞击作用下的运动及状态,在模拟中首先将物质与空间划分成离散的网格,在每一次迭代计算中逐步求解各个网格的形变、运动与状态改变的规律。牛顿运动定律、物质的连续体模型与热力学方程是撞击坑形成过程数值模拟的理论基础,牛顿运动定律以偏微分方程的形式贯穿在离散化的网格空间中,物质的连续体模型将物质的屈服强度与破裂、温度、孔隙、振动等联系起来,而热力学方程则通过其他热力学参数计算网格单元内物质的压强与物质所处的状态。     

基于全景相机数据的嫦娥四号着陆区次级坑统计分析

玉兔二号月球车在嫦娥四号着陆区发现了许多坑缘呈破碎石块状的小型撞击坑,其直径大多在亚米级,过去对于这种小尺度撞击坑的研究大多源于Apollo、Surveyor、Ranger系列任务,然而当时获取的影像质量较低,且并未覆盖月球背面.小尺度撞击坑在月球表面分布十分广泛,与月球最表层的演化密切相关,因此对其展开研究具有重要的意义.本文利用嫦娥四号任务获取的高分辨率全景影像,统计了8、9、11～15月昼影像中新鲜撞击坑的直径、频数、深度等信息,并根据这些撞击坑和周围石块的形态特征,推测这些坑应该形成于不同方向的二次撞击事件,这为揭示玉兔二号月球车行进途中观测物质的来源提供了重要信息.小型二次撞击坑从深径比中难以与一次撞击坑区分,且一般处于饱和状态,因此应避免使用小尺寸的撞击坑进行定年.     

月表典型区撞击坑形态分类及分布特征

月球表面环形构造主要有撞击坑、火山口和月海穹窿3种,其中撞击坑分布最广泛,是研究月表环形构造的主要内容。由于月表撞击坑数量大、种类多及其形成伴随着整个月球地质的演化过程,因此这种月表地形地貌比较完整地记录了月球表面地貌随时间的改造过程以及改造类型。文中通过研究撞击坑遥感影像及形貌特征,总结归纳为简单型、碗型、平底型、中央隆起型、同心环型、复杂型及月海残留型7种撞击坑类型,用来描述月表典型区域撞击坑的形态特征。从结构和物质两方面进行了月表典型区域撞击坑的形态地貌参数提取,综合利用嫦娥一号CCD 影像数据、LROC数据,得到了该区域撞击坑形态数据(坑底、坑唇、坑壁、坑缘、溅射物覆盖层、中央峰)和形态测量数据(直径、深度、地理位置)。研究发现,LQ 4地区的撞击坑分布可分为月陆区和月海区,月陆区的撞击坑多以中小型撞击坑为主,其分布密度极高,形成年代较早,月海区撞击坑多为年轻的撞击坑,分化程度较低,分布密度也较低。     

基于遥感蚀变信息提取巴林格撞击坑周边铁陨石

目前地球上已经得到确认的撞击坑有190余个,其中直径小于1 km的简单撞击坑绝大部分是由铁质撞击体撞击形成的。由铁质撞击体撞击而成的撞击坑周边存在大量的铁陨石物质,这些铁陨石物质的空间分布特征对研究撞击坑的撞击过程和机理具有重要意义。铁元素的异常富集也可作为探寻地球表面疑似撞击坑的重要信息。为了获取撞击坑周围的铁陨石残片,早期主要通过人工方式进行实地调查,但这种方法效率低下且需要投入大量人力物力。基于铁陨石独特的光谱特征,利用遥感蚀变信息提取手段可以很方便地获取撞击坑周边的铁陨石物质。根据铁陨石矿物的波谱特征,以美国亚利桑那州巴林格撞击坑(Barringer Meteor Crater)为研究对象,基于Landsat 8 OLI数据,采用目前提取蚀变信息的常用方法:波段比值(BandMath)—主成分分析法进行撞击坑周边铁陨石信息的提取。提取结果与前人实地调查获取的铁陨石分布情况契合程度较好。撞击坑东侧、东南侧、西南侧等处的铁陨石聚集区在提取结果图上均有较好的反映。表明利用波段比值-主成分分析方法提取巴林格撞击坑周边铁陨石信息是可行的,实验结果准确地获得了该撞击坑周围的铁陨石空间分布信息,为探寻地球表面撞击成因的环形构造提供了可行方案,同时为未来同类撞击坑信息提取提供了重要的方法参考。     

沉积数值模拟研究现状及实例

沉积数值模拟是开展沉积学定量研究的重要手段,在油气地质学、水利工程等领域中发挥着重要作用。本文基于大量国内外文献调研,结合实际研究工作,系统概述了沉积数值模拟的发展历程及模型分类,归纳了几何规则、水动力方程、扩散方程、元胞自动机和模糊逻辑等5种模拟方法,对比各种方法的基本原理、优缺点以及与其相对应的代表性模拟软件的特点;总结了沉积数值模拟在层序地层学,沉积过程及沉积机理,“源-汇”系统和油气地质学等4个研究领域中的应用现状及独特优势;并结合东营凹陷北部陡坡带断层控制背景下两种湖相重力流沉积数值模拟研究实例,进一步说明了沉积数值模拟在沉积学研究中的有效性与重要性;最后展望了探索不同沉积机制数值模拟新方法,多地质过程耦合数值模拟与地质几何建模一体化,基于人工智能驱动的沉积过程反演数值模拟以及在非常规油气资源勘探开发中深入应用是该领域未来的发展方向。     

地下水数值模拟不确定性分析研究进展

地下水数值模拟不确定性分析旨在提高研究区域地下水流的模拟精度。学者们将地下水数值模拟不确定性分析分为:模型的不确定性、参数的不确定性以及资料的不确定性三类,其中参数的不确定性分析在研究中是最为重要的。同时,对模型、参数、资料不确定性分析的研究进展和成果进行归纳总结,补充关于参数不确定性分析过程中的替代模型的一些研究成果以及模型不确定性分析的多模型分析;强调在地下水数值模拟的过程中,重视模型、资料的不确定性分析;展望未来,随着地下水数值模拟不确定性分析研究的深入,方法和应用会更加多样化。     

基于最优分割分级法的月球撞击坑分级及其演化分析

撞击坑是月球表面广泛分布的重要构造形态,占据了月球表面的大部分面积。撞击坑的直径差别很大,从几微米到数百千米,其退化程度与形成年代具有密切关系。为了研究不同地质年代形成的撞击坑直径大小及其演化规律,需采用量化分级方法对大小不同的撞击坑进行定量分级和统计分析。本文在月表撞击坑数据库LU60645GT和Lunar_Impact_Crater_Database(2011)的基础上,结合数据库中撞击坑的直径、深度和年代信息,利用最优分割分级法对撞击坑直径进行定量化分级,并根据分级结果,综合分析撞击坑几何形态特征及其演化规律。研究结果表明,撞击坑形态特征的演化与年代有密切的关系。在相同级别、相同地体下,撞击坑形成的年代越早,其形态特征的精细结构退化程度越明显,只保留了大体的几何形状;而在不同级别、相同地体、相同年代下的撞击坑形态特征则由简单逐渐变为复杂,坑物质也逐渐变得复杂。     

MODFLOW和FEFLOW在国内地下水数值模拟中的应用

数值模拟是研究地下水资源问题的主要手段。介绍MODFLOW和FEFLOW这两款地下水数值模型在国内地下水模拟中的应用,比较分析二者在基本原理、模型特点、以及应用领域等方面的差异,提出地下水数值模型的发展趋势。     

Apollo 11和嫦娥四号着陆区撞击坑退化对比分析

Apollo 11和嫦娥四号(Chang'E-4)是人类探月历史上的里程碑,它们的着陆区分别位于月球正面和背面.对两个着陆区内不同退化程度撞击坑的统计和对比分析有助于揭示研究区域的地质年龄和演化历史,对月球地质研究有着重要的意义.本文使用LRO NAC影像和DTM产品对两个着陆区附近1 km2范围内撞击坑的退化进行分析,通过目视解译识别撞击坑并根据形貌将其分为不同的类别,然后对各类撞击坑进行统计,最后根据统计结果分析区域的地质年龄和撞击坑退化速度.结果 表明Apollo 11和Chang'E-4着陆区附近直径在5～300 m的撞击坑累计大小频率分布规律基本相同,撞击坑累积数量随直径的减小呈指数关系增加;撞击坑最初退化速度较快,随着退化程度的增加,退化速度急剧降低;两个区域的地质年龄相近,撞击历史相似.     

有限元数值模拟法基本原理及其在地质构造变形研究中的应用综述

在构造变形研究中,传统的构造解析法存在一定局限性,而数值模拟方法可模拟时空变化,成为地球科学研究的重要手段。有限元法是一种模拟线性、变形问题较为高效的数值模拟法。本文详细阐述了有限元法的基本原理,并从地质模型、力学模型、计算模型三个方面解释了数值模型的建立过程,以及模型建立所需要的边界条件的确定方法,重点总结了近些年来有限元法在国内外构造变形研究中的应用和发展状况,涉及盆地、造山带、断层、俯冲带以及褶皱数值模拟等几方面内容。同时,分析了有限元数值模拟法在我国大陆构造变形中的应用进展。最后,讨论了有限元方法尚存在的问题及其发展趋势。     

Particle size distribution and strain rate attenuation in hypervelocity impact and shock recovery experiments

Particle size distribution (PSD) is an often used parameter to describe and quantify fragmentation of deformed rock. Our analyses of shock deformed sandstone show that dynamic fragmentation influences the PSD, expressed as fractal dimension (D-value). Image analysis was used to derive fractal dimensions from a hypervelocity impact cratering experiment (2.5 mm steel sphere, 4.8 km/s) and a planar shock recovery experiment (2.5 GPa). The D-values in the cratering experiment decrease from 1.74 at the crater floor to 0.84 at a distance of 7.2 mm to the crater floor. The D-values found in this experiment are closely related to the microstructural features found at distinct distances from the crater floor. The obtained values are in good agreement with the D-values reported for fault zones, impact sites and deformation experiments. The D-value measured in the shock recovery experiment is 2.42. Such high D-values were usually attributed to abrasive processes related to high strain. Since the strain in our experiment is only ∼23% we suggest that at highly dynamic deformation very high d-values can be reached at small strain. To quantify this, numerical impact modelling has been used to estimate strain rates for the impact experiment. This is related to the activation of more inherent flaws and fracture bifurcation at very high strain rates ∼>10² s⁻¹.     

撞击过程和内太阳系撞击历史

在太阳系的形成和演化过程中,发生在天体物质间的撞击作用是最重要的地质过程之一.撞击构造是地外天体表面最常见的地貌单元,大部分天体的地貌演化主要受撞击作用控制.撞击过程产生的温度、压力和应变速率比岩石圈内的其他地质过程高多个数量级,形成广泛分布的撞击产物,如气化物、熔融物、冲击变质和变形等.虽然撞击过程转瞬即逝,撞击作用向天体注入能量并改变其内、外结构,对天体的圈层系统产生长远影响.持续撞击在天体表面累积了大量的撞击坑,撞击坑的空间分布反映了受外来撞击的历史.内太阳系在～3. 8 Ga前的撞击频率更高,但是大量撞击盆地是否灾变式的密集形成仍在持续争议;～3. 8 Ga以来的撞击频率趋于稳定,但是缺乏具有明确事件指代性的标定样品.在同一天体上,撞击坑的空间密度指示了相应地质单元的形成时间,因此撞击坑统计常被用于估算地外天体表面地质单元的相对年龄.基于月球软着陆探测任务返回的样品,前人已约束了不同直径的月球撞击坑的形成频率,进而建立了使用撞击坑统计估算月球表面地质单元的绝对模式年龄的方法.另外,内太阳系天体可能经历了相似的撞击历史,因此地-月系统的撞击频率已被缩放至其他类地行星.撞击坑统计是探索太阳系天体的撞击历史、遥估地外天体表面的相对和绝对年龄的主要方法,也是行星地质研究的基本工具.该方法的整体可靠性已得到大量实验的验证.同时,该方法在理论基础和技术细节上还存在大量的不确定性.修正该方法是完善太阳系撞击历史的重要研究内容,也是未来采样返回探测任务的重要科学目标.     

Verteilung,Eigenschaften und Entstehung der Auswurfsmassen des Impaktkraters Nördlinger Ries

Regarding the importance of the Ries impact structure in the field of planetary geology an attempt is made to synthesize the presently known results of geologic mapping in the Ries area and of general field and laboratory observations. The ballistic bulk ejecta of the crater (diameter about 23 km) form a continuous blanket of mixed breccia (“Bunte Trümmermassen”). Outside the crater more than 90% of its constituents are derived from unshocked fragments of the 750 m thick pre-Ries sedimentary rock strata. This blanket is overlain with a sharp discontinuity by suevite breccia which forms a continuous layer inside the crater and patch-like isolated occurrences outside the crater. Their pattern of distribution is believed to be primary. Suevite is mainly composed of fragments of all stages of shock metamorphism derived from the crystalline basement. The present asymmetrical distribution of éjecta outside the crater rim results from relatively young erosion during the Pliocene and Pleistocene periods. The erosional history indicates that the primary distribution of ejecta was symmetrical with respect to the center of the crater extending up to a distance of at least some 40 km from the impact center. A number of characteristics of the structure and composition of the Ries ejecta formations are discussed and verified quantitatively by new field data. It is shown that the structure and composition of the ejecta formations can be explained qualitatively by the physics of impact cratering and be duplicated by hypervelocity cratering experiments.     

撞击坑统计定年法及对月球虹湾地区的定年结果

撞击作用是行星形成和表面重塑的重要地质过程,记录和揭示了行星的演化历史.撞击作用形成的撞击坑可用于研究天体表面地质单元形成的时间.依据内太阳系天体表面的撞击历史,总结了通过对撞击坑的直径和频率分布进行统计,计算天体表面模式年龄的原理和方法.在此基础上,利用美国“月球勘测轨道器(LRO)”广角相机获得的图像,对月球虹湾地区的撞击坑进行了直径-频率分布统计研究,获得其3个主要地质单元的绝对模式年龄分别为3.33 Ga、3.21 Ga和2.60 Ga,有效限定了本区主要地质事件发生的时间.      

The marine Lockne impact structure, Jämtland, Sweden: a review

The Lockne impact structure in Jämtland (63°00'20"N, 14°49'30"E) formed in the Middle Ordovician at approximately 455 Ma. The structure is a concentric crater with a total diameter of 13.5 km. The impact took place in a marine environment. Seawater played an important role in the cratering process and in crater morphology and the amount of melt remaining in the structure. Seawater rushed back into the crater in a resurge, eroding and redepositing the ejecta among the resurge deposit. Seawater furthermore facilitated the hydrothermal system, which was driven by the residual heat in the structure. The Lockne structure hosts shocked quartz and an iridium anomaly. The rim wall round the crater collapsed in the modification stage of the crater and was annihilated by the resurge. The fractured basement and the impact breccia were initially rich in open cavities. These became partly filled with dominantly calcite. The filling contributed to a low-density contrast, generating a negative gravity anomaly of 22 gu. The gravity model indicates a central uplift and a NW-directed tilt of the structure. This tilt is also seen in the magnetic models. The apparent absence of any impact melt is probably real and related to the environment of impact.     

The Waqf as Suwwan crater,Eastern Desert of Jordan: aspects of the deep structure of an oblique impact from reflection seismic and gravity data

The deeply eroded Waqf as Suwwan ring structure was recently discovered to be a large impact, the first identified in the near east. Large-scale reflection seismic structure shows the impact situated high on the northeastern flank of the Jordan Uplift sloping into Wadi Sirhan Basin. If exhumation is linked to the Arabia–Eurasia collision, a likely time window for the impact event may be latest Eocene to Late Oligocene. Impact into a shallow sea seems an optional scenario. Old reflection seismic lines offer limited insight into the deep structure of the rim and part of the central uplift of the complex crater. An important structural clue is provided by a well-resolved seismic horizon of a yet tentative correlation with a Paleozoic black shale. The central gravity high is compatible with a mass surplus by the uplift of denser Paleozoic basement below the central uplift. The gravity model further indicates a ring of dense Paleozoic sediments rising from below into the ring syncline. Seismics show presumably radial synclines in the central uplift which are interpreted by centripetal constrictional flow during crater collapse. Beneath the final crater’s outer boundary, a shallow-dip normal fault zone, subtle seismic structure in uncollapsed footwall segments reveal an asymmetry of strain. The asymmetry is attributed to the cratering flow by an oblique impact directed toward NE. The finding provides independent support to an earlier suggestion of impact obliquity based on vergency of folds exposed on the central uplift.     

Impact cratering — fundamental process in geoscience and planetary science

Impact cratering is a geological process characterized by ultra-fast strain rates, which generates extreme shock pressure and shock temperature conditions on and just below planetary surfaces. Despite initial skepticism, this catastrophic process has now been widely accepted by geoscientists with respect to its importance in terrestrial — indeed, in planetary — evolution. About 170 impact structures have been discovered on Earth so far, and some more structures are considered to be of possible impact origin. One major extinction event, at the Cretaceous-Paleogene boundary, has been firmly linked with catastrophic impact, but whether other important extinction events in Earth history, including the so-called “Mother of All Mass Extinctions” at the Permian-Triassic boundary, were triggered by huge impact catastrophes is still hotly debated and a subject of ongoing research. There is a beneficial side to impact events as well, as some impact structures worldwide have been shown to contain significant (in some cases, world class) ore deposits, including the gold-uranium province of the Witwatersrand basin in South Africa, the enormous Ni and PGE deposits of the Sudbury structure in Canada, as well as important hydrocarbon resources, especially in North America. Impact cratering is not a process of the past, and it is mandatory to improve knowledge of the past-impact record on Earth to better constrain the probability of such events in the future. In addition, further improvement of our understanding of the physico-chemical and geological processes fundamental to the impact cratering process is required for reliable numerical modeling of the process, and also for the correlation of impact magnitude and environmental effects. Over the last few decades, impact cratering has steadily grown into an integrated discipline comprising most disciplines of the geosciences as well as planetary science, which has created positive spin-offs including the study of paleo-environments and paleo-climatology, or the important issue of life in extreme environments. And yet, in many parts of the world, the impact process is not yet part of the geoscience curriculum, and for this reason, it deserves to be actively promoted not only as a geoscientific discipline in its own right, but also as an important life-science discipline.     

Tooting crater: Geology and geomorphology of the archetype large,fresh, impact crater on Mars

The 27.2 km diameter Tooting crater is the best preserved young impact crater of its size on Mars. It offers an unprecedented opportunity to study impact-related phenomena as well the geology of the crust in the Amazonis Planitia region of Mars. For example, the nearly pristine condition enables the partial reconstruction of the sequence of events for crater formation, as well as facilitates a comparison to deposits seen at the Ries crater in Germany. High-resolution images taken by the High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) on the Mars Reconnaissance Orbiter spacecraft have revealed a wealth of information on the distribution of features within the crater and beyond the rim: a large central peak, pitted material on the floor and terrace blocks, lobate flows interpreted to be sediment flows, impact melt sheets, four discrete layers of ejecta, and an asymmetric secondary crater field. Topographic data derived from the Mars Orbiter Laser Altimeter (MOLA) and stereo HiRISE and CTX images show that the central peak is ～1100 m high, the lowest point of the crater floor is 1274 m below the highest part of the rim, and the crater rim has ～600 m of variability around its perimeter. Layering within the cavity walls indicates ～260 m of structural uplift of the target material, which constitutes ～35% of the total relief of the rim. Abundant evidence is found for water flowing down the cavity walls, and on the surface of the ejecta layers, both of which took place sometime after the impact event. Thickness measurements of the ejecta layers reveal that the continuous blanket is remarkably thin (～3–5 m) in some places, and that the distal ramparts may be ～60 m high. Crater counts made on the ejecta layers indicate a model age of <3 Ma for the formation of Tooting crater, and that the target rocks have a model age of ～240–375 Ma. It is therefore possible that this may be the source of certain basaltic shergottite meteorites ejected at ～2.8 Ma that have crystallization ages which are comparable to those of the basaltic lava flows that formed the target materials for this impact event. The geology and geomorphology of Tooting crater may help in the interpretation of older large impact craters on Mars, as well as the potential role of target volatiles in the impact cratering process.     

Evidence that Lake Cheko is not an impact crater

In a provocative paper Gasperini et al. (2007) suggest that Lake Cheko, a ～300‐m‐wide lake situated a few kilometres downrange from the assumed epicentre of the 1908 Tunguska event, is an impact crater. In this response, we present several lines of observational evidence that contradicts the impact hypothesis for the lake’s origin: un‐crater‐like aspects of the lake morphology, the lack of impactor material in and around the lake, and the presence of apparently unaffected mature trees close to the lake. We also show that a tensile strength of 10–40 MPa is required for an asteroid fragment to traverse the Earth’s atmosphere and reach the surface intact and with sufficient velocity to excavate a crater the size of Lake Cheko. Inferred tensile strengths of large stony meteorites during atmospheric disruption are 10–100 times lower. We therefore conclude that Lake Cheko is highly unlikely to be an impact crater.