海域航空重力快速构建区域大地水准面

我国在海域开展了大规模的航空重力勘探,这些资料对构建高精度大地水准面具有重要价值.基于此,本文提出一种利用海域航空重力测量数据快速构建大地水准面的方法.该方法基于移去-恢复法思想,利用位场最小曲率方法对航空重力数据进行高精度向下延拓并获取相应的扰动位,实现航空重力测量快速构建海域大地水准面.与斯托克斯积分计算相比,采用了处理效率更高的频率域位场转换,解决了向下延拓及垂向积分时航空重力异常数据空白及扩边问题,具有较高的位场转换精度.本文应用EGM2008模拟航空重力数据进行模型验证,计算结果与其给出的水准面的精度相当;同时,也选取GRAV-D计划的航空重力数据进行实际验证,计算结果与xGEOID18B水准面模型精度基本一致.模型验证和实际应用验证了本方法的实用性.

     

Evaluation of gravity data by using artificial neural networks case study: Seferihisar geothermal area (Western Turkey)

Artificial neural networks (ANN) have been used in a variety of problems in the fields of science and engineering. Applications of ANN to the geophysical problems have increased within the last decade. In particular, it has been used to solve such inversion problems as seismic, electromagnetic, resistivity. There are also some other applications such as parameter estimation, prediction, and classification. In this study, multilayer perceptron neural networks (MLPNN) and radial basis function neural networks (RBFNN) were applied to synthetic gravity data and Seferihisar gravity data to investigate the applicability and performance of these networks for the method of gravity. Additionally performance of MLPNN and RBFNN were tested by adding random noise to the same synthetic test data. The structure parameters, such as the depths, the density contrasts, and the locations of the structures were obtained closely for different signal-to-noise ratios (S/N). Bouguer data of Seferihisar area were analyzed by MLPNN and RBFNN to estimate depth, density contrast, and location of the structure. The results of MLPNN, RBFNN, and classical inversion method were compared for real data obtained from Seferihisar Geothermal area and similar structure parameters were obtained. The experiments show that in general RBFNN not only increases the speed of the training stage enormously, but also provides slightly better performance.     

A theoretical study on terrestrial gravimetric data refinement by Earth gravity models

The idea of this paper is to present estimators for combining terrestrial gravity data with Earth gravity models and produce a high‐quality source of the Earth's gravity field data through all wavelengths. To do so, integral and point‐wise estimators are mathematically developed, based on the spectral combination theory, in such a way that they combine terrestrial data with one and/or two Earth gravity models. The integral estimators are developed so that they become biased or unbiased to a priori information. For testing the quality of the estimators, their global mean square errors are generated using an Earth gravity model08 model and one of the recent products of the gravity field and steady‐state ocean circulation explorer mission. Numerical results show that the integral estimators have smaller global root mean square errors than the point‐wise ones but they are not efficient practically. The integral estimator of the biased type is the most suited due to its smallest global root mean square error comparing to the rest of the estimators. Due largely to the omission errors of Earth gravity models the point‐wise estimators are not sensitive to the Earth gravity model commission error; therefore, the use of high‐degree Earth gravity models is very influential for reduction of their root mean square errors. Also it is shown that the use of the ocean circulation explorer Earth gravity model does not significantly reduce the root mean square errors of the presented estimators in the presence of Earth gravity model08. All estimators are applied in the region of Fennoscandia and a cap size of 2° for numerical integration and a maximum degree of 2500 for generation of band‐limited kernels are found suitable for the integral estimators.     

Application of the finite element method to gravity data case study: Western Turkey

Gravity anomalies always include the total effects (combination of the structures which have different densities and depths) of the study area and beyond. And the well-known non-uniqueness of potential field modelling may lead to very different interpretation results.The finite element method (FEM), which has been used in potential field interpretation for decades, makes complex problems to be solved easily and accurately. The first step of FEM is to identify the elements and then to decide on the boundary of the solution space. In this step, the solution space is divided into elements. After determination of the geometrical structure of the solution space, the most suitable elements should be chosen for this geometrical structure. The agreement between the geometry and the elements is quite important for the convergence to the best possible solution.In this work, the methods of trend analysis, filtering, analytical continuation and FEM were applied to a theoretical model and to gravity data from western Turkey to produce the regional and the residual anomalies. The results were compared and it was found that the FEM produced more accurate results than other methods did.     

A new ellipsoidal gravimetric, satellite altimetry and astronomic boundary value problem, a case study: The geoid of Iran

A new gravimetric, satellite altimetry, astronomical ellipsoidal boundary value problem for geoid computations has been developed and successfully tested. This boundary value problem has been constructed for gravity observables of the type (i) gravity potential, (ii) gravity intensity (i.e. modulus of gravity acceleration), (iii) astronomical longitude, (iv) astronomical latitude and (v) satellite altimetry observations. The ellipsoidal coordinates of the observation points have been considered as known quantities in the set-up of the problem in the light of availability of GPS coordinates. The developed boundary value problem is ellipsoidal by nature and as such takes advantage of high precision GPS observations in the set-up. The algorithmic steps of the solution of the boundary value problem are as follows:

- Application of the ellipsoidal harmonic expansion complete up to degree and order 360 and of the ellipsoidal centrifugal field for the removal of the effect of global gravity and the isostasy field from the gravity intensity and the astronomical observations at the surface of the Earth.

- Application of the ellipsoidal Newton integral on the multi-cylindrical equal-area map projection surface for the removal from the gravity intensity and the astronomical observations at the surface of the Earth the effect of the residual masses at the radius of up to 55 km from the computational point.

- Application of the ellipsoidal harmonic expansion complete up to degree and order 360 and ellipsoidal centrifugal field for the removal from the geoidal undulations derived from satellite altimetry the effect of the global gravity and isostasy on the geoidal undulations.

- Application of the ellipsoidal Newton integral on the multi-cylindrical equal-area map projection surface for the removal from the geoidal undulations derived from satellite altimetry the effect of the water masses outside the reference ellipsoid within a radius of 55 km around the computational point.

- Least squares solution of the observation equations of the incremental quantities derived from aforementioned steps in order to obtain the incremental gravity potential at the surface of the reference ellipsoid.

- The removed effects at the application points are restored on the surface of reference ellipsoid.

- Application of the ellipsoidal Bruns’ formula for converting the potential values on the surface of the reference ellipsoid into the geoidal heights with respect to the reference ellipsoid.

- Computation of the geoid of Iran has successfully tested this new methodology.

Interpretation of gravity data using eigenimage with Indian case study: A SVD approach

A new technique for delineation of subsurface features using gravity eigenimage is proposed to separate the gravitational anomalies from its background. The singular value decomposition and multifractal method has been combined and tested on several synthetic gravity anomalies and also applied to field gravity dataset of Vindhyan basin in central India which is a promising area for petroleum exploration. The eigenimage of gravity data helps to understand the relationship between the geological structure and source of anomaly. Fault structure is the major structure and could be the principal contributor that differentiates the regional and local geo-anomalies in the studied region.     

Testing Stokes-Helmert geoid model computation on a synthetic gravity field: experiences and shortcomings

We report on testing the UNB (University of New Brunswick) software suite for accurate regional geoid model determination by use of Stokes-Helmert’s method against an Australian Synthetic Field (ASF) as “ground truth”. This testing has taken several years and has led to discoveries of several significant errors (larger than 5mm in the resulting geoid models) both in the UNB software as well as the ASF. It was our hope that, after correcting the errors in UNB software, we would be able to come up with some definite numbers as far as the achievable accuracy for a geoid model computed by the UNB software. Unfortunately, it turned out that the ASF contained errors, some of as yet unknown origin, that will have to be removed before that ultimate goal can be reached. Regardless, the testing has taught us some valuable lessons, which we describe in this paper. As matters stand now, it seems that given errorless gravity data on 1′ by 1′ grid, a digital elevation model of a reasonable accuracy and no topographical density variations, the Stokes-Helmert approach as realised in the UNB software suite is capable of delivering an accuracy of the geoid model of no constant bias, standard deviation of about 25 mm and a maximum range of about 200 mm. We note that the UNB software suite does not use any corrective measures, such as biases and tilts or surface fitting, so the resulting errors reflect only the errors in modelling the geoid.     

Regional compensation of subducted lithosphere: effects on geoid,gravity and topography from a preliminary model

It is proposed that the anomalous mass of subducted lithospheric slabs is partially compensated by two mechanisms: (a) by stresses transmitted up the slab to the surface lithosphere, causing the trench, and (b) by stresses transmitted through the mantle wedge overlying the slab, which cause a broad shallow depression. Results calculated from a preliminary two-dimensional model show that it can give a first-order account of observed topography, gravity and geoid anomalies. In particular, the model can reconcile the large mass anomaly predicted for the slab by thermal conduction theory with the smaller than expected gravity and geoid anomalies. Preliminary results indicate substantial compensation by mechanism (a), implying rather large stresses at the top of the slab: these could be important for theories of earthquake mechanisms and plate driving mechanisms. Roughly 50% of the possible compensation by mechanism (b) is indicated: the resulting depression, the order of 500 m deep and over 500 km wide, could explain anomalously deep marginal basins and some marine transgressions of continents.     

A study of regional gravity field recovery from GOCE vertical gravity gradient data in the Auvergne test area using collocation

Gravity field and steady-state Ocean Circulation Explorer (GOCE) is the first satellite mission that observes gravity gradients from the space, to be primarily used for the determination of high precision global gravity field models. However, the GOCE gradients, having a dense data distribution, may potentially provide better predictions of the regional gravity field than those obtained using a spherical harmonic Earth Geopotential Model (EGM). This is investigated in Auvergne test area using Least Squares Collocation (LSC) with GOCE vertical gravity gradient anomalies (T_zz), removing the long wavelength part from EGM2008 and the short wavelength part by residual terrain modelling (RTM). The results show that terrain effects on the vertical gravity gradient are significant at satellite altitude, reaching a level of 0.11 E?tv?s unit (E.U.) in the mountainous areas. Removing the RTM effects from GOCE T_zz leads to significant improvements on the LSC predictions of surface gravity anomalies and quasigeoid heights. Comparison with ground truth data shows that using LSC surface free air gravity anomalies and quasi-geoid heights are recovered from GOCE T_zz with standard deviations of 11 mGal and 18 cm, which is better than those obtained by using GOCE EGMs, demonstrating that information beyond the maximal degree of the GOCE EGMs is present. Investigation of using covariance functions created separately from GOCE T_zz and terrestrial free air gravity anomalies, suggests that both covariance functions give almost identical predictions. However, using covariance function obtained from GOCE T_zz has the effect that the predicted formal average error estimates are considerably larger than the standard deviations of predicted minus observed gravity anomalies. Therefore, GOCE T_zz should be used with caution to determine the covariance functions in areas where surface gravity anomalies are not available, if error estimates are needed.     

Understanding hydrological processes with scarce data in a mountain environment

Performance of process‐based hydrological models is usually assessed through comparison between simulated and measured streamflow. Although necessary, this analysis is not sufficient to estimate the quality and realism of the modelling since streamflow integrates all processes of the water cycle, including intermediate production or redistribution processes such as snowmelt or groundwater flow. Assessing the performance of hydrological models in simulating accurately intermediate processes is often difficult and requires heavy experimental investments. In this study, conceptual hydrological modelling (using SWAT) of a semi‐arid mountainous watershed in the High Atlas in Morocco is attempted. Our objective is to analyse whether good intermediate processes simulation is reached when global‐satisfying streamflow simulation is possible. First, parameters presenting intercorrelation issues are identified: from the soil, the groundwater and, to a lesser extent, from the snow. Second, methodologies are developed to retrieve information from accessible intermediate hydrological processes. A geochemical method is used to quantify the contribution of a superficial and a deep reservoir to streamflow. It is shown that, for this specific process, the model formalism is not adapted to our study area and thus leads to poor simulation results. A remote‐sensing methodology is proposed to retrieve the snow surfaces. Comparison with the simulation shows that this process can be satisfyingly simulated by the model. The multidisciplinary approach adopted in this study, although supported by the hydrological community, is still uncommon. Copyright © 2007 John Wiley & Sons, Ltd.     

山地小城市避震疏散通道通行能力评估及选择研究 ——以昆明东川城区为例

安全快速的避震疏散通道是新时代背景下提高城市抗灾韧性水平的基础保障,灾时充分发挥避震疏散通道的可靠性和时效性将减轻城市人员伤亡.选取山地小城市昆明东川城区为研究区域,通过系统分析避震疏散通道影响因素,对灾后避震疏散通道的通行能力进行评估,基于ArcGIS建立人口集聚点与避震疏散场所(OD)的归属关系,并在此基础上通过遗...     

Use of EGM08 model and shuttle radar topography mission data for geoid computation in the State of Rio de Janeiro,Brazil: A case study with Voronoi/Delaunay discretisations

The EGM08 geopotential model complete to degree and order 2159 was used in a remove-compute-restore (RCR) method for the geoid computation in the State of Rio de Janeiro, Brazil. Terrain and indirect effect corrections were computed using a 6-arcsec resolution DTE, derived from the TOPODATA Project (Shuttle Radar Topography Mission data) raised by the National Institute for Space Research. INPE, Brazil. We applied Voronoi/Delaunay discretisations for discrete Stokes integration. In these schemes, target area is partitioned into polygons/triangles, respectively, and the computation is carried out by point-wise numerical integration and no gridding is mandatory. For both procedures, the cells were produced using either observed gravity data combined with gridded Bouguer derived information. Particularly in Delaunay scheme, as the gravity anomalies are interpolated into the triangular cells, and geoid undulations are computed for their vertices, Stokes function singularity was gone. Externally estimated errors resulting from a comparison with GPS/leveling data were presented for both the schemes and classical ones, as well as for the EGM08 undulations. They yielded RMS differences equal to 0.105 m, 0.110 m, 0.110 m, 0.115 m and 0.228 m, respectively, for Voronoi, Delaunay, Voronoi/Delaunay with gridded-data alone and EGM08, computed between 32 GPS/leveling points.     

A regional gravimetric Moho recovery under Tibet using gravitational potential data from a satellite global model

The Moho information under Tibet Plateau is important for a better understanding of the geodynamic processes associated with the continental collision of the Indian and Eurasian tectonic plates and subsequent formation of Himalayan and Tibetan orogens. However, under the central and western parts of Tibet, the existing Moho models are still relatively inaccurate due to a sparse and irregular distribution of seismic surveys. To overcome this problem, the gravimetric data could be used to interpolate the Moho information, where seismic data are missing. In this study, we apply the gravimetric method for a regional Moho recovery under Tibet. Compared to existing methods that use either the gravity or gravity-gradient data, the method presented here utilizes a more generic definition based on a functional relation between the Moho depth and the gravitational potential. Since the gravity and gravity-gradient data have more regional support than the potential field, a numerical test is conducted to find an optimal data area extension that is needed to solve a regional inversion problem in order to reduce errors caused by disregarding the far-zone contribution. Our analysis shows that for the potential field such extension should be at least 25°, while 5° for the gravity and only about 1° for the gravity gradient. The comparison of our gravimetric result with the CRUST1.0 seismic model shows differences at the level of expected accuracy of the gravimetric method of about 5 km and without the presence of significant bias.     

Error sources and data limitations for the prediction of surface gravity: a case study using benchmarks

Gravity-based heights require gravity values at levelled benchmarks (BMs), which sometimes have to be predicted from surrounding observations. We use the Earth Gravitational Model 2008 (EGM2008) and the Australian National Gravity Database (ANGD) as examples of model and terrestrial observed data respectively to predict gravity at Australian National Levelling Network (ANLN) BMs. The aim is to quantify errors that may propagate into the predicted BM gravity values and then into gravimetric height corrections (HCs). Our results indicate that an approximate ±1 arc-min horizontal position error of the BMs causes maximum errors in EGM2008 BM gravity of ~22 mGal (~55 mm in the HC at ~2200 m elevation) and ~18 mGal for ANGD BM gravity because the values are not computed at the true location of the BM. We use RTM (residual terrain modelling) techniques to show that ~50% of EGM2008 BM gravity error in a moderately mountainous region can be accounted for by signal omission. Non-representative sampling of ANGD gravity in this region may cause errors of up to 50 mGals (~120 mm for the Helmert orthometric correction at ~2200 m elevation). For modelled gravity at BMs to be viable, levelling networks need horizontal BM positions accurate to a few metres, while RTM techniques can be used to reduce signal omission error. Unrepresentative gravity sampling in mountains can be remedied by denser and more representative re-surveys, and/or gravity can be forward modelled into regions of sparser gravity.     

重力基准快速测定与分析:以广西重力基点网及格值标定场重建为例

重力基点网是我国开展地质调查、地震监测、测绘等测量工作的重要基础设施,面对原有基点网覆盖面不足且受到不同程度损害这一现状,目前已不能满足开展大比例尺重力调查以及高频率重复监测等工作的需求.本文以广西重力基点网重建为契机,利用A10-022绝对重力仪对全区42个重力基点与2个相对重力仪标定场的绝对重力值进行了测定.整个实施周期涵盖了计量参数校准、全球绝对重力仪国际比对、室内同点重复性以及全网部分基点独立安装检查等较为完整的测量流程,并通过连续运输与观测期间环境因素的记录与对比分析,总结了适合于该类型仪器室外条件下工作的技术手段与应对措施.观测结果表明：利用A10绝对重力仪能够高效率与高精度的完成广西重力基点网室外设计点位测定工作,国际比对及重复性验证确保了本次重建基点结果的可信度;A10采用双模式的氦氖激光频率存在漂移并受温度及气压等多种因素的综合影响,但短期内中心频率未出现较大波动,建议定期对其进行校准;各基点观测误差主要受到台站质量、人文活动及突发事件等环境扰动的影响,并与仪器内外温差存在较高的相关性.本文给出的A10绝对重力仪的参数设计、技术手段与影响因素的详细分析,能够为今后同类型仪器的观测实施提供重要参考,并有助于室外绝对重力观测的全国推广与应用.

     

Assessment of a lumped coupled flow‐isotope model in data scarce Boreal catchments

Quantifying streamflow sources within remote, data scarce, Boreal catchments remains a significant challenge because of limited accessibility and complex, flat topography. The coupled use of hydrometric and isotopic data has previously been shown to facilitate quantification of streamflow sources, but application has generally been limited to small basins and short time scales. A lumped flow‐isotope model was used to estimate contributing streamflow sources (soil, ground, and wetland water) over a four‐year period in two large nested headwater catchments (Sapochi and Odei Rivers) in northern Manitoba, Canada. On average, the primary streamflow source was estimated as soil water (60%) in the Sapochi River, and groundwater (54%) in the Odei River. A strong seasonal influence was observed: soil water was the primary streamflow source in summer, changing to groundwater and wetlands during the winter. Interannual variability in streamflow sources was strongly linked to the presence or absence of late summer rainfall. The greatest uncertainties in source quantification were identified during the spring freshets and high precipitation events, and hence, simulations may be improved through explicit representation of the soil freeze/thaw process and data collection during this period. Assessment of primary streamflow components and qualitative uncertainty estimation using coupled isotope‐flow modelling is an effective method for first‐order identification of streamflow sources in data sparse remote headwaters. Copyright © 2016 John Wiley & Sons, Ltd.     

The Sarikavak Tephra, Galatea, north central Turkey: a case study of a Miocene complex plinian eruption deposit

The Sarikavak Tephra from the central Galatean Volcanic Province (Turkey) represents the deposit of a complex multiple phase plinian eruption of Miocene age. The eruptive sequence is subdivided into the Lower-, Middle-, and Upper Sarikavak Tephra (LSKT, MSKT, USKT) which differ in type of deposits, lithology and eruptive mechanisms.The Lower Sarikavak Tephra is characterised by pumice fall deposits with minor interbedded fine-grained ash beds in the lower LSKT-A. Deposits are well stratified and enriched in lithic fragments up to >50 wt% in some layers. The upper LSKT-B is mainly reversely graded pumice fall with minor amounts of lithics. It represents the main plinian phase of the eruption. The LSKT-A and B units are separated from each other by a fine-grained ash fall deposit. The Middle Sarikavak Tephra is predominantly composed of cross-bedded ash-and-pumice surge deposits with minor pumice fall deposits in the lower MSKT-A and major pyroclastic flow deposits in the upper MSKT-B unit. The Upper Sarikavak Tephra shows subaerial laminated surge deposits in USKT-A and subaqueous tephra beds in USKT-B.Isopach maps of the LSKT pumice fall deposits as well as the fine ash at the LSKT-A/B boundary indicate NNE–SSW extending depositional fans with the source area in the western part of the Ovaçik caldera. The MSKT pyroclastic flow and surge deposits form a SW-extending main lobe related to paleotopography where the deposits are thickest.Internal bedding and lithic distribution of the LSKT-A result from intermittent activity due to significant vent wall instabilities. Reductions in eruption power from (partial) plugging of the vent produced fine ash deposits in near-vent locations and subsequent explosive expulsion of wall rock debris was responsible for the high lithic contents of the lapilli fall deposits. A period of vent closure promoted fine ash fall deposition at the end of LSKT-A. The subsequent main plinian phase of the LSKT-B evolved from stable vent conditions after some initial gravitational column collapses during the early ascent of the re-established eruption plume. The ash-and-pumice surges of the MSKT-A are interpreted as deposits from phreatomagmatic activity prior to the main pyroclastic flow formation of the MSKT-B.     

流域数据模型研究—以太湖流域西苕溪流域为例

对江汉平原江陵剖面沉积物中的总有机碳（TOC）、总氮（TN）、有机碳同位素δ13C值和粒度参数的分布特征与环境气候的关系进行了研究.结果表明：沉积物中有机碳、总氮较高,有机质的δ13C值偏负,沉积物的平均粒径较粗时,气候温暖湿润;反之,沉积物中有机碳、总氮较低,有机质的δ13C值偏正,沉积物的平均粒径较细时,气候温凉偏干.结合14C测年,判别江汉平原江陵地区的古气候演化过程：8900-6070 aBP,为温湿时期;6000-4600 aBP为相对冷干时期;4600-2500 aBP为温暖湿润时期;2500 aBP以来为温凉偏干时期.     

基于Slepian方法和地面重力观测确定时变重力场模型: 以2011—2013年华北地区数据为例

时变重力场是研究地球系统内部物质运动和时空演化过程的有效途径.目前广泛使用的GRACE时变重力场模型受限于其空间分辨率（约400 km）,难以探测较小空间尺度的重力变化.本文首次尝试利用Slepian局部谱分析方法和多期地面重力观测确定更高空间分辨率的时变重力场模型.Slepian方法通过构建研究区域内的正交基函数,将信号能量集中在研究区域内部,是构建球面局部重力场模型的理想方法.本文根据Slepian方法的特点给出了区域重力场建模及参数优化的步骤,以我国华北地区为例,基于2011—2013多期地面观测确定了区域时变重力场模型,并与同区域由Slepian方法和GRACE卫星数据确定的重力变化进行了对比分析.结果表明：（1）贝叶斯信息量准则可作为确定Slepian展开最佳截断数的有效手段;（2）基于研究区域内现有重复测点数据,能够恢复120阶时变重力场,空间分辨率（半波长）约150 km;（3）2011—2013年间研究区域内GRACE估计结果与120阶地面结果在时空分布的显著趋势上存在较好的对应,证明了本文利用Slepian方法和地面观测所得时变重力场模型的可靠性.本文研究结果可为区域重力场建模提供新的参考,也可为华北地区水资源变化监测、构造活动分析以及地震风险性评估等研究提供高分辨率的时变重力场模型支撑.