近地表密度估计的重力贝叶斯分析方法及在云南地区的应用

基于布格重力异常相对于地形起伏光滑分布的约束条件,从一维自由空气重力异常数据出发,采用贝叶斯方法估算近地表岩石密度,同时采用三次B样条函数拟合布格重力异常,获取光滑分布的布格重力异常.数据拟合和光滑约束之间的权重采用Akaike贝叶斯准则(ABIC准则)自动确定.均匀剖分模型和不均匀剖分模型数据试验都验证了该方法的有效性.相关参数评价表明,足够多的样条系数可以提高估计结果的准确性,样条系数的个数接近测点数时可获得较稳定的估计结果.增大异常的噪声水平时,ABIC准则可有效地自动增大先验光滑约束的权重.云南地区两条重力剖面应用结果表明,剖面沿线的近地表密度值起伏变化明显(达2.45~2.8g·cm^-3),前寒武纪和古生代地层密度相对较高(主要为2.53~2.75g·cm^-3),而中生代密度较低(2.45~2.73g·cm^-3);本文估计的近地表密度结果与区域物性资料及地表地质特征较吻合;估计的剖面布格重力异常具有光滑性;红河断裂两侧近地表密度差异较大,可达0.4g·cm^-3.本文获得的两条剖面近地表密度结构和布格重力异常为该区深部结构与构造研究提供更可靠的重力基础数据.     

大别造山带东段重力异常多尺度分解及其构造意义

文中通过多源数据融合、模型构建、数据试验、二维离散小波变换和功率谱分析等方法获取了大别造山带东段深、浅部场源布格异常及其场源似深度,并结合地壳结构、地质构造、岩石圈有效弹性厚度和地震活动等资料,讨论了地壳深、浅部的结构特征及地震活动构造背景。结果表明,低频布格异常显示大别造山带东段与华北地块间深部构造缝合带在东部应位于青山-晓天断裂前缘,在落儿岭-土地岭断裂和商城-麻城断裂之间向N偏移至梅山-龙河口断裂之下,造山带南侧与扬子地块间深部构造缝合带位于襄樊-广济断裂以北约20km,造山带东侧与扬子地块间的深部构造转换带位于郯庐断裂带之下,造山带东段腹地显著的低频布格异常低值表明对应部位的莫霍面存在明显下凹,造山带内部的布格异常高梯度带表明其深部结构不完整;高频布格异常揭示肥中断裂、六安-合肥断裂、肥西-韩摆渡断裂和郯庐断裂带等主要断裂对地壳中上部密度结构的影响明显,落儿岭-土地岭断裂对地壳中上部密度结构的影响范围向N延伸至肥西-韩摆渡断裂前缘。结合地震活动资料进一步分析认为,大别造山带东段与华北地块在青山-晓天断裂前缘附近接触和相互作用,且大别造山带东段地壳深、浅部结构均不完整,不利于应力积累,趋向于在断裂交错的脆弱部位频繁释放应力,是霍山地区小地震活动频繁的主要原因。     

A REGIONAL GRAVITY STUDY OF THE NORTHERN NORTH SEA (56–62° N)*

A Bouguer gravity anomaly map is presented of the North Sea and adjacent land areas in Norway and Denmark, covering an area situated between 56° and 62°N, 1°W and 10°E. The gravity data from the UK sector of the North Sea, the land and offshore areas of Denmark, and the land areas of Norway have been published before. However, the gravity data from the Norwegian sector of the North Sea are new. A large number (about 60) of individual gravity features can be defined in the mapped area. Most of those situated in the UK sector of the North Sea and on land in Norway have been discussed earlier; however, most of the anomalies found elsewhere which are qualitatively interpreted here have not been discussed before. An interpreted Bouguer anomaly map is presented which identifies all these features. The majority of the gravity anomalies encountered in the mapped area can be shown to be associated with one of the following geological features: (i) basement highs, (ii) large bodies of heavy basic or ultrabasic rock in the crystalline basement, (iii) large igneous intrusions within the sedimentary column and thick accumulations of volcanic rocks or their associated eruption centers, (iv) major basement faults. Large-scale geological structures such as the Central, Viking and Sogn Grabens and the East Shetland, Stord, Forth Approaches and Norwegian-Danish Basins are essentially in isostatic equilibrium and are only locally marked by relatively weak gravity minima. A residual gravity anomaly map has been produced by subtracting from the observed Bouguer anomalies the estimated gravity effect of an assumed thinned crust. This residual gravity anomaly map shows a number of features of the Bouguer anomaly field with greater clarity.     

Gravimetric study of a potential mineral deposit in the Itapororoca region, Brazil

A semi-detailed gravity survey was carried out over an area of 650 km² localized in the Eo-Neoproterozoic coastal zone of Paraiba State where 548 new gravity stations were added to the existing database. Gravity measurements were made with a LaCoste and Romberg model G meter with a precision of 0.04 mGal. The altitude was determined by barometric levelling with a fixed base achieving a 1.2 m measure of uncertainty, corresponding to an overall accuracy of 0.24 mGal for the Bouguer anomaly. The residual Bouguer map for a 7th degree regional polynomial showed a circumscribed negative anomaly coincident with a localized aero-magnetic anomaly and with hydro-thermally altered outcrops, near the city of Itapororoca. The 3D gravity modelling, constrained by geologic mapping was interpreted as a low density, fractured and/or altered material with a most probable volume of approximately 23 km³, extending to about 8,500 m depth. This result is in accordance with a volcanic body associated with hydrothermal processes accompanied by surface mineralization evidence, which may be of interest to the mining industry.     

Location of secondary faults from cross-correlation of the second vertical derivative of gravity anomalies1

A method is described to locate secondary faults, which can be difficult to identify on the Bouguer gravity map. The method is based on cross-correlation between the theoretical anomaly due to a vertical step and the second vertical derivative of the Bouguer anomaly. Faults are located from the closed maxima and minima on the cross-correlation contour map calculated for two perpendicular directions. One-dimensional model computations show that the magnitude of the extremum of the cross-correlation is related to the depth to the top of the hanging wall and the throw of the fault. Application of the method to the Bouguer gravity map of the former mouth of the Yellow River in the Shengli Oilfield area near the Bo Hai Sea shows the effectiveness of the method.     

Basement relief map of kalabsha and seiyal areas, northwest of Aswan lake, Egypt

The Bouguer anomaly map of the Kalabsha and Seiyal areas was analysed on the basis of the available data about the depth to the basement from three boreholes. The density contrast between the basement complex and the Nubia formation was estimated from the gravity values and basement depths. Applying the estimated value of the density contrast, a residual map representing the basement effect and a basement relief map were constructed. The basement depth in Kalabsha and Seiyal areas range from 150 to 500 m below mean sea level. Comparing the general characteristics of the Bouguer anomaly of the Kalabsha and Seiyal areas with the clusters of earthquakes in the area since 1982 till present reveals a gravity low coinciding with a deep focal area. The gravity values increase toward a shallow focal area.     

据断层附近布格重力异常分布反推地质时期断层运动的探讨

本文应用变形介质位移场产生的附加重力场的积分表达式,对麦克斯威尔介质半空间中断层长期活动引起的永久变形与现今布格重力异常分布的关系作了定量分析,并提出了由断层附近布格异常分布特征反推地质时期断层运动趋势的一般方法。其主要应用于平原地区断层运动(特别是扭动方式)的判别。文中以滦南断裂为实例,通过数值积分,计算了几种断层运动可能产生的地表重力异常。经理论计算与实测重力资料相对比,该断层曾发生过下盘上升的正倾滑运动,并具有顺扭走滑运动分量     

青藏高原东南缘多尺度重力场及构造含义

本文基于EGM2008重力场模型研究了青藏高原东南缘均衡重力异常和多尺度的布格重力异常特征,以鲁甸和景谷地震为例,认识其深部构造环境和动力学过程,为该区域的构造运动和地震孕育环境研究提供依据.结果表明,研究区布格重力异常和均衡重力异常与地质构造格局相关性较好,川滇地块剧烈的区域布格重力异常和非均衡状态与其强烈的地壳变形...     

分层重力图像方法及其应用

分层重力图像方法是依据三维地震波速的层析成像结果，通过速度与密度相关关系，转换得到分层的密度结构模型，计算并给出各分层在地表的重力异常分布图像．以中国大陆为例，给出其不同深度的7个分层的重力异常图像和不同分层叠加的综合（或总合）重力异常分布图像．经过与实测的中国布格重力异常资料对比分析，结果表明两者的重力场特征相当一致；分层重力图像的结果能较好地反映地下不同深度的物质分布．     

RESULTS OF GRAVITY SURVEY OVER RANIGANJ COALFIELD,INDIA*

Results of a gravity survey conducted over Raniganj coalfield, one of the Gondwana basins of Damodar Valley in north-eastern part of India, are presented. The gravity field was separated into regional and residual components. The residual Bouguer anomaly map shows that the coalfield is characterized by a gravity low of about—32 mGal associated with Gondwana sediments. The deepest part of basin is found to be located near Asansole (23° 40’N, 86° 55’E), where the maximum thickness of sediments is estimated to be about 1.3 miles (2.08 km). The faults along the northern as well as the southern boundaries are found to be normal. The Gondwana sediments appear to continue eastward beneath alluvium and laterite of Bengal basin as far as 87° 25’E.     

中国海及邻域重力场特征及其构造解释

利用中国海及邻域 2’× 2’由Seaset,Geosat,ERS 1及TOPEX/POSEIDON等卫星测高资料解算的自由空气重力异常 ,结合ETOPO5全球海底地形资料计算了中国海及邻域布格重力异常 ,并反演了得到了中国海及邻域地壳厚度 .经对重力异常特征进行分析 ,讨论了重力异常与大地构造及其活动的相关性 .     

喜玛拉雅“东构造结”地区特异重力场的探讨

跨越中、印、缅三国交界的青藏高原东南的喜玛拉雅“东构造结”地区（92°E~97°E，26°N~30°N）一半以上的面积尚没有重力测点，是重力数据空白区,故无法直接研究其重力场特征与深部地壳结构（构造）.本文分析了卫星重力异常的特性，提出应用卫星重力异常作为近似空间重力异常，并作布格改正后，得到的布格重力异常具有与该地区地形高程呈镜像相关的特征，可用以研究深部地壳结构.据三条重力剖面计算得到该地区三个地壳深部结构剖面的结果，给出青藏高原地壳厚度>70 km；喜马拉雅造山带为55 km左右；布拉马普特拉河谷盆地为33~35 km；那加山山脉地区为40~45 km，显示出三者为三个不同的构造单元.同时给出布拉马普特拉构造单元为相对高密度的刚性物质构成，随着印度洋板块向北运移，在碰撞、挤压下，插入青藏高原东南缘一带.导致该地带的强烈构造运动，和频发大、小地震.最后提出了几点认识和建议.     

MAPPING OF CRUSTAL DISCONTINUITIES BY WAVELENGTH FILTERING OF THE GRAVITY FIELD1

A statistical technique, based on the concept of a 1D energy density spectrum of the observed gravity field, has been used to compute ensemble average depths to various horizons containing causative sources of random geometric shape, size, density, etc. The plot of the logarithm of the energy of the observed Bouguer anomaly versus the angular frequency can be approximated, over a certain frequency band, by a linear segment whose slope is related to an average ensemble depth around which a random distribution of numerous anomalous sources exists. Suitable matched filters, based on the computed values of intercepts and slopes of several linear segments approximating the spectrum, have been used to deconvolve the gravity effects associated with the causative sources, occurring around their respective mean depths. The individual deconvolved gravity effects thus separated out have been modelled using the sin x/x method by assuming a fluctuating interface between two formations. The applicability of the present method has been assessed using two observed Bouguer anomaly profiles: one from Ujjain to Mahan, and the other from Jhansi to Mandla where Deep Seismic Sounding (DSS) results are available. The proposed geological crustal models along these two profiles exhibit reasonably good agreement with those obtained from DSS results. A geologically plausible model of the crust in a virgin region has been presented along a Bouguer anomaly profile from Jaipur to Raipur. The following main conclusions have been drawn from the present analysis: (1) The depths to the Moho and Archaean basement interfaces fluctuate between 33.2 and 36.8 km and between 4.6 and 7.0 km respectively. (2) The Narmada-Son Lineament (NSL) does not coincide exactly with the Moho upwarp beneath it. However, this offset is greater in the eastern part of the NSL rather than in the western part. (3) The development of the Satpura horst structure is due to a rise in the Moho interface in a compressional regime. (4) The intrabasement feature (depth from 5 to 12 km) represents a hybrid massif possibly formed due to an admixture of sialic and simatic crust under a tensional regime in the Ujjain-Mahan section.     

东北地区构造分区与深断裂研究——基于重力场小波多尺度分解

基于东北地区的布格重力异常资料,通过小波多尺度分解,将重力异常分解为四阶.提取主要反映深断裂的重力异常2、3阶小波细节,利用不同方向的水平梯度,计算并强化深断裂的线性特征,解译出12条深断裂,同时,结合功率谱法、东北地区地震分布、震源机制解和地质信息推断断裂的下切深度和性质,结果表明:12条深断裂分别对应拼贴地块的缝合带和对东北地区盆地群的分布与演化有着明显控制作用的岩石圈断裂两类;深断裂体系主要方向为北东、北北东向且大多为走滑断层;提取主要反映深层块体异常的3、4阶小波细节并结合深断裂体系将研究区划分为五个重力场一级分区,并首次将额尔古纳块体和兴安块体分开.五个大区分别为额尔古纳块体、兴安块体、松嫩块体、佳木斯块体和华北板块;其中兴安块体又分为二连盆地、大兴安岭两个二级分区;松嫩块体又分为小兴安岭、松辽盆地、张广才岭三个二级分区;佳木斯块体又分为那丹哈达地体、三江盆地、桦南—穆棱三个二级分区,华北板块又分为辽东半岛、华北板块、北部断裂带三个二级分区,结果表明了研究区的基本构造格局.     

渭河盆地及邻区重力异常小波多尺度分解与解释

基于EGM2008重力场模型计算获得了渭河盆地及邻区布格重力异常。采用小波多尺度分解方法对布格重力异常进行了4阶小波逼近和小波细节分解,同时基于平均径向对数功率谱方法定量化地计算出1~4阶小波细节和小波逼近所对应的场源平均埋深。结合区域地质和地震资料,对获得的重力场结果进行分析,得到如下结论:①鄂尔多斯地块、渭河盆地、秦岭造山带3个一级构造单元的布格重力异常之间存在明显差异;构造区内部重力异常也存在横向的显著差异。布格重力异常的走向、规模、分布特征与二级构造区及主要的断裂具有一定的对应关系。②渭河盆地及邻区布格重力异常1~4阶细节对应4~23 km不同深度的场源信息,鄂尔多斯地块南缘东、西部的地壳结构存在明显的差异;渭河盆地凹陷、凸起构造区边界清晰,断裂边界与重力异常边界具有较好的一致性;秦岭造山带重力异常连贯性不好,东、西部重力异常变化特征表现出明显的差异。③渭河盆地及邻区布格重力异常分布与莫霍面埋深具有非常明显的镜像关系。渭河盆地及邻区地震主要分布在六盘山—陇县—宝鸡断裂带、渭河断裂与渭南塬前断裂交汇处、韩城断裂与双泉—临猗断裂交汇处。渭河盆地及邻区重力异常主要由中上地壳剩余密度体所影响,这可能是该区地震以浅源地震为主的主要原因。     

新疆精河6.6级地震周边地区密度构造、均衡异常以及岩石圈挠曲机理

依据EIGEN-6C4重力模型和ETOPO1高程模型数据,围绕新疆精河6.6级地震展开岩石圈均衡与挠曲机理研究,得到如下结论：（1）震中附近的布格与自由空气重力异常分别为-221和-92mGal(10~(-5 )m·s~(-2)）,震中位于重力异常高梯度带上;（2）震中周边地区地壳厚度约为50km,密度结构总体变化平缓,东西方向地壳厚度变化较小,但自南向北地壳厚度逐渐变薄,精河6.6级地震初始破裂发生在上中地壳分界面附近;（3）震中附近岩石圈承载的垂向构造应力为20MPa左右,震中位于岩石圈垂向构造应力极大值附近的高梯度带上;（4）地震周边地区岩石圈有效弹性厚度最优解为26km,加载比最优解为F_1=1,F_2=F_3=0,表明该区域岩石圈相对坚硬,且导致岩石圈变形的初始加载全部来自地表．     

龙门山断裂带地壳结构的三维建模

为了实现龙门山区域地质信息的科学管理与共享,推动该区域地球物理资料的统一管理,本文在整理现有二维地球物理探测数据的基础上构造了龙门山断裂带区域的三维地壳结构模型,并结合实测的布格重力异常数据对模型进行了分析。结果表明,地层模型正演的整体重力场与实测布格重力异常基本吻合,初步证明该模型的正确性。局部重力场显示:在该模型的尺度下,沉积层对整体重力场的变化贡献较小;中上地壳有不同程度的隆起和坳陷,与前人研究所揭示的低速异常体吻合;地幔表现为规则的自西北至东南单调上升的重力异常梯度带,与前人反演结果的趋势一致,从而证明了模型的可靠性。最后在重力资料约束下对模型进行了反演,进一步修正了模型。      

遗传有限元法反演西安地区的地层密度变化

结合1992—1994年的重力变化资料和1988—1991年的水准资料,运用遗传有限元方法计算西安地区的重力变化和对应的地层密度变化,然后计算重力变化梯度场进一步分析西安地区重力变化的原因。结果表明,过量抽取承压地下水引起承压水位下降、黏土层释水压密和两者共同作用导致的地面沉降是西安地区的重力变化的主要原因。大部分地区承压含水层系统中黏土层的压密产生正的剩余地层密度,而在如小寨、铁炉庙区的局部区域,抽取潜水导致潜水位下降、水体流失是产生负的地层密度变化的主要原因,但不排除其与地裂缝活动相关的可能性     

关于地壳厚度计算方法的修正

利用前苏联学者（１９５８．）计算地壳厚度公式计算了兰州－天水－武都地区的布格重力异常．发现其结果与人工地震爆破得出的结果相差很大。造成差值大的原因可能是地壳界面边界效应的作用。为使计算结果尽量接近实际地壳厚度。所以要从计算地壳厚度公式中减去每一测点的效应值，依此法推算出任意点的地壳厚度。     

南北地震带中段深部构造与地震

南北地震带中段位于中国大腹部，是青藏块体、华北块体和扬子块体的中间枢纽部位。根据该区布格重力异常图，采用Ｐａｒｋｅｒ’ｓ三维位场方法进行莫氏面反演并计算该区不同窗口的剩余场推断和解释了条岩石圈断裂，进而研究它们与地震的关系。