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1.
Satellite gradiometry is an observation technique providing data that allow for evaluation of Stokes’ (geopotential) coefficients.
This technique is capable of determining higher degrees/orders of the geopotential coefficients than can be achieved by traditional
dynamic satellite geodesy. The satellite gradiometry data include topographic and atmospheric effects. By removing those effects,
the satellite data becomes smoother and harmonic outside sea level and therefore more suitable for downward continuation to
the Earth’s surface. For example, in this way one may determine a set of spherical harmonics of the gravity field that is
harmonic in the exterior to sea level.
This article deals with the above effects on the satellite gravity gradients in the local north-oriented frame. The conventional
expressions of the gradients in this frame have a rather complicated form, depending on the first-and second-order derivatives
of the associated Legendre functions, which contain singular factors when approaching the poles. On the contrary, we express
the harmonic series of atmospheric and topographic effects as non-singular expressions. The theory is applied to the regions
of Fennoscandia and Iran, where maps of such effects and their statistics are presented and discussed. 相似文献
2.
Three-Dimensional Magnetotelluric Inversion: An Introductory Guide for Developers and Users 总被引:5,自引:1,他引:4
Weerachai Siripunvaraporn 《Surveys in Geophysics》2012,33(1):5-27
In the last few decades, the demand for three-dimensional (3-D) inversions for magnetotelluric data has significantly driven
the progress of 3-D codes. There are currently a lot of new 3-D inversion and forward modeling codes. Some, such as the WSINV3DMT
code of the author, are available to the academic community. The goal of this paper is to summarize all the important issues
involving 3-D inversions. It aims to show how inversion works and how to use it properly. In this paper, I start by describing
several good reasons for doing 3-D inversion instead of 2-D inversion. The main algorithms for 3-D inversion are reviewed
along with some comparisons of their advantages and disadvantages. These algorithms are the classical Occam’s inversion, the
data space Occam’s inversion, the Gauss–Newton method, the Gauss–Newton with the conjugate gradient method, the non-linear
conjugate gradient method, and the quasi-Newton method. Other variants are based on these main algorithms. Forward modeling,
sensitivity calculations, model covariance and its parallel implementation are all necessary components of inversions and
are reviewed here. Rules of thumb for performing 3-D inversion are proposed for the benefit of the 3-D inversion novice. Problems
regarding 3-D inversions are discussed along with suggested topics for future research for the developers of the next decades. 相似文献
3.
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5.
根据维纳滤波理论导出的位场向下延拓滤波器为最佳下延滤波器,但因其实现需要已知待求位场和噪声的功率谱而在实际应用中受到限制.针对该问题,本文首先提出一种基于位场径向平均功率谱的位场噪声水平估计方法,进而利用偏差准则求取正则化参数,实现位场正则化向下延拓;然后将位场正则化下延结果的功率谱作为待求位场功率谱的估计初值,采用带修正项的迭代维纳滤波方法来更新对待求位场功率谱的估计,最后提出本文的位场向下延拓改进迭代维纳滤波方法.基于理论重力模型数据及航磁实测数据进行了向下延拓对比试验,结果表明,改进迭代法具有较好的收敛性,且下延精度优于Tikhonov正则化法和递增型维纳滤波法. 相似文献
6.
A geoid solution for airborne gravity data 总被引:2,自引:0,他引:2
Airborne gravity data is usually attached with satellite positioning of data points, which allow for the direct determination
of the gravity disturbance at flight level. Assuming a suitable gridding of such data, Hotine’s modified integral formula
can be combined with an Earth Gravity Model for the computation of the disturbing potential (T) at flight level. Based on
T and the gravity disturbance data, we directly downward continue T to the geoid, and we present the final solution for the
geoid height, including topographic corrections. It can be proved that the Taylor expansion of T converges if the flight level
is at least twice the height of the topography, and the terrain potential will not contribute to the topographic correction.
Hence, the simple topographic bias of the Bouguer shell yields the only topographic correction. Some numerical results demonstrate
the technique used for downward continuation and topographic correction. 相似文献
7.
The eigenvalue decomposition technique is used for analysis of conditionality of two alternative solutions for a determination
of the geoid from local gravity data. The first solution is based on the standard two-step approach utilising the inverse
of the Abel-Poisson integral equation (downward continuation) and consequently the Stokes/Hotine integration (gravity inversion).
The second solution is based on a single integral that combines the downward continuation and the gravity inversion in one
integral equation. Extreme eigenvalues and corresponding condition numbers of matrix operators are investigated to compare
the stability of inverse problems of the above-mentioned computational models. To preserve a dominantly diagonal structure
of the matrices for inverse solutions, the horizontal positions of the parameterised solution on the geoid and of data points
are identical. The numerical experiments using real data reveal that the direct gravity inversion is numerically more stable
than the downward continuation procedure in the two-step approach. 相似文献
8.
V. V. Shcherbakova V. P. Shcherbakov Yu. S. Bretshtein G. V. Zhidkov 《Izvestiya Physics of the Solid Earth》2010,46(12):1035-1051
We present the results of analyzing a representative collection of the middle Miocene 12.4–10.0 Ma basalts that compose the
volcanic cover of the Shufan and Sovgavan plateaus, namely the Nikolo-L’vovsk (NL) and Sovetskaya Gavan (SG) volcanic fields.
Preliminary data are obtained about the relicts of some volcanic edifices within the West and East Sikhote-Alin volcanic belts,
namely the Shishlovskii, Malyshevo, and Truzhenik objects. It is established that the volcanic rocks from these localities
are characterized by similar petrologic and magnetic properties. Thermal cleaning of the samples is carried out, and the coordinates
of the paleomagnetic pole are determined as Λ = 190.2°E, Φ = 71.3°N for basalts of the Nokolo-L’vovsk area and Λ = 180.4°E,
Φ = 71.9°N for rocks from the Sovgavan locality. These values are consistent with the data for coeval volcanics from other
regions of Eurasia. Reliable determinations of the paleointensity H
pal for a representative collection of samples were obtained using the Thellier method. The corresponding values of the virtual
dipole moment (VDM) are almost half its present-day value. The analysis of the Miocene VDM values available from the world
database revealed a low average field 5.06 × 1022 Am2 characterized by high variance σ = 2.13 × 1022 Am2 at that time. The similarity of VDM values for the Miocene characterized by frequent inversions and for the Cretaceous Superchron
supports the hypothesis of the lack of a correlation between the VDM values and the frequency of geomagnetic inversions. 相似文献
9.
针对位场向下延拓的不适定性,我们将位场向下延拓视为向上延拓的反问题,提出以位场最小曲率作为约束条件来求解稳定的下延位场.我们将剖面位场向上延拓表达式用傅里叶矩阵的形式表示,以矩阵乘法形式给出延拓的表达式,同时向待反演的下延位场引入最小曲率约束,得到向下延拓的最小曲率解,并利用正交变换给出了更为简洁的频率域解.随后,利用Kronecker积将上述全部结果拓展至三维位场,给出了三维位场向下延拓的最小曲率解.此外,我们将位场数据的填充、扩充问题与向下延拓问题统筹考虑,提出一种新的向下延拓迭代格式,该算法面向实际资料处理需求、无须预扩充或填补数据.下延迭代时,对原始数据直接向下延拓,而空白部分利用上一次下延位场估计的上延值替代其空白值并对其向下延拓,直至获得最小曲率约束下稳定的向下延拓结果.同时,我们也讨论了利用改进L曲线和广义交叉验证(GCV)计算正则参数最优估计的问题.对理论模型和实际航空重力资料进行了向下延拓检验,处理结果表明位场向下延拓的最小曲率方法解能满足实际位场资料对向下延拓处理的需求,具有较高的下延精度. 相似文献
10.
重力场向下延拓能够突出局部和浅部的异常信息,分离叠加的异常特征.但是向下延拓通常具有过程不稳定、下延深度小、结果不准确等问题.针对向下延拓所存在的不足,本文利用重力场及其垂向一阶导数,基于辛普森(Simpson)求积公式,推导出重力场向下延拓米尔尼(Milne)公式.将本文向下延拓方法应用于模型数据,向下延拓模型结果及误差曲线表明,相对于向下延拓快速傅里叶变换(FFT)法和积分迭代法,向下延拓Milne法的深度更大,相对误差更小;相对模型值,向下延拓Milne法能够获得稳定且准确的结果.对加拿大乃查科(Nechako)盆地地区实测航空重力数据进行本文方法向下延拓验证,处理结果表明,相对于实测异常,本文方法向下延拓结果能够很好还原实测数据,并且在进一步向下延拓中反映原始异常的趋势,增强局部和细小异常信息. 相似文献
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12.
Cui Xingfu Li Hongbing Hu Ying Liang Hong Qi Li 《应用地球物理》2007,4(2):94-100
When topography and low velocity zone differences vary greatly, conventional vertical static time shifts will cause wavefield distortion and influence wave equation seismic imaging for seismic data acquired on a complex near surface. In this paper, we propose an approach to datum correction that combines a joint tomography inversion with wavefield continuation to solve the static problem for seismic data on rugged acquisition topography. First, the near surface model is obtained by refracted wave tomography inversion. Second, the wavefield of sources and receivers are continued downward and upward to accomplish datum correction starting from a flat surface and locating the datum above topography. Based on the reciprocal theorem, Huygens' and Fresnel principles, the location of sources and receivers, and regarding the recorded data on the surface as a secondary emission, the sources and receivers are upward-continued to the datum above topography respectively. Thus, the datum correction using joint tomography inversion and wavefield continuation with the condition of a complex near surface is accomplished. 相似文献
13.
为了实现曲面随机点位场数据的曲面延拓和转换,以磁异常位场数据为例,采用一组磁偶极子作为等效源,置于观测面下方的一个曲面上,把观测磁异常作为这组磁偶极子所产生磁异常的边界条件,通过求解线性方程组的方法反演磁偶极子磁矩的大小,再根据反演结果正演所要计算的磁异常数据,实现了曲面随机点磁异常位场数据的向上延拓、向下延拓、求导以及化极处理.在数据量较大时,为了提高反演计算的速度,把磁异常数据和磁偶极子分成若干小块,再利用各块磁异常数据分别反演该块数据下方磁偶极子的磁矩,并通过迭代计算来逐步取得更准确的反演结果.模型试验表明,磁异常位场数据向上延拓的均方根误差小于±2nT,向下延拓和化极也可以取得较高的精度,所提出的分块处理方法提高了延拓和转换的速度,实际资料处理给出了曲面随机点航磁异常数据向下延拓和化极的一个例子. 相似文献
14.
位场向下与向上延拓之间存在固有的内在联系,向上延拓解算具有稳定可靠的优良特性,本文据此提出了借助向上延拓信息实现航空重力向下延拓稳定解算的两种方法,分别建立了点对点向下解析延拓模型和最小二乘向下解析延拓模型.其核心思想是,依据泰勒级数展开模型,将位场向下延拓解算过程转换为向上延拓计算和垂向偏导数解算两个步骤,通过第一步的处理有效抑制数据观测噪声对解算结果的干扰,通过第二步的处理成功实现向下延拓反问题的稳定解算,较好地解决了向下延拓解算固有的不适定性问题.分析研究了两种解析延拓模型的计算精度及适用条件,利用超高阶位模型EGM2008建立的模拟标准场数据对两种模型解算结果的合理性和有效性进行了数值验证,证明本文新方法实用易行,具有较高的应用价值. 相似文献
15.
Jaymie R. Meliker Geoffrey M. Jacquez 《Stochastic Environmental Research and Risk Assessment (SERRA)》2007,21(5):625-634
Our research group recently developed Q-statistics for evaluating space–time clustering in case–control studies with residential histories. This technique relies
on time-dependent nearest-neighbor relationships to examine clustering at any moment in the life-course of the residential
histories of cases relative to that of controls. In addition, in place of the widely used null hypothesis of spatial randomness,
each individual’s probability of being a case is based instead on his/her risk factors and covariates. In this paper, we extend
this approach to illustrate how alternative temporal orientations (e.g., years prior to diagnosis/recruitment, participant’s
age, and calendar year) influence a spatial clustering pattern. These temporal orientations are valuable for shedding light
on the duration of time between clustering and subsequent disease development (known as the empirical induction period), and
for revealing age-specific susceptibility windows and calendar year-specific effects. An ongoing population-based bladder
cancer case–control study is used to demonstrate this approach. Data collection is currently incomplete and therefore no inferences
should be drawn; we analyze these data to demonstrate these novel methods. Maps of space–time clustering of bladder cancer
cases are presented using different temporal orientations while accounting for covariates and known risk factors. This systematic
approach for evaluating space–time clustering has the potential to generate novel hypotheses about environmental risk factors
and provides insights into empirical induction periods, age-specific susceptibility, and calendar year-specific effects. 相似文献
16.
Explicit formula for the geoid-quasigeoid separation 总被引:1,自引:0,他引:1
The explicit formula for the geoid-to-quasigeoid correction is derived in this paper. On comparing the geoidal height and
height anomaly, this correction is found to be a function of the mean value of gravity disturbance along the plumbline within
the topography. To evaluate the mean gravity disturbance, the gravity field of the Earth is decomposed into components generated
by masses within the geoid, topography and atmosphere. Newton’s integration is then used for the computation of topography-and
atmosphere-generated components of the mean gravity, while the combined solution for the downward continuation of gravity
anomalies and Stokes’ boundary-value problem is utilized in computing the component of mean gravity disturbance generated
by mass irregularities within the geoid. On application of this explicit formulism a theoretical accuracy of a few millimetres
can be achieved in evaluation of the geoid-to-quasigeoid correction. However, the real accuracy could be lower due to deficiencies
within the numerical methods and to errors within the input data (digital terrain and density models and gravity observations). 相似文献
17.
Downward continuation is a useful tool in the processing of potential field data, which can effectively enhance weak anomalies and identify overlap anomalies, but we all know that the computation of downward continuation is unstable, and easily distorts the true feature of potential field data. Because the computation of upward continuation and horizontal derivatives is stable, we proposed using the combination of upward continuation and horizontal derivative to accomplish the downward continuation of potential field data. The proposed method is demonstrated on synthetic potential field data, and the results show that the proposed method can finish the downward continuation of the data stably and precisely, and the precision of the proposed method is higher than the traditional method. We also apply it to real potential field data, and the results show that the proposed method accomplishes the downward continuation of the real data stably. 相似文献
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A new method for suppressing multiple reflections in seismograms is developed. It is based on a downward continuation procedure which uses the full acoustic wave equation (hyperbolic form) as a downward continuation operator. We demonstrate that the downward continuation of the recorded wave field maps a reflectivity function without multiply reflected events. The method is applied successfully to individual traces of plane-wave decomposed (slant-stacked) synthetic and field data. 相似文献
20.
2-D crustal Poisson’s ratio from seismic travel time inversion in Changbaishan Tianchi volcanic region 总被引:1,自引:0,他引:1
Based on the inversion method of 2D velocity structure and interface, the crustal velocity structures of P-wave and S-wave
along the profile L
1 are determined simultaneously with deep seismic sounding data in Changbaishan Tianchi volcanic region, and then its Poisson’s
ratio is obtained. Calculated results show that this technique overcomes some defects of traditional forward calculation method,
and it is also very effective to determine Poisson’s ratio distribution of deep seismic sounding profile, especially useful
for study on volcanic magma and crustal fault zone. Study result indicates that there is an abnormally high Poisson’s ratio
body that is about 30 km wide and 12 km high in the low velocity region under Tianchi crater. Its value of Poisson’s ratio
is 8% higher than that of surrounding medium and it should be the magma chamber formed from melted rock with high temperature.
There is a high Poisson’s ratio zone ranging from magma chamber to the top of crust, which may be the uprise passage of hot
substance. The lower part with high Poisson’s ratio, which stretches downward to Moho, is possibly the extrusion way of hot
substance from the uppermost mantle. The conclusions above are consistent with the study results of both tomographic determination
of 3D crustal structure and magnetotelluric survey in this region.
Foundation item: Key Project from China Earthquake Administration and the Project (95-11-02-01) from Ministry of Science and Technology (2001DIA10003).
Contribution No. RCEG200401, Geophysical Exploration Center, China Earthquake Administration. 相似文献