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1.
Aspects of the interpretation of measured data on the gravity gradient tensor (GGT) are examined. The problem is posed in relation to the great progress achieved in recent years in the development of instrumentation and the method of GGT measurements on mobile carriers. In our opinion, the new methods of measurement and the new data obtained with their help require the development of new methods of interpretation of potential fields. The paper addresses two methods taking advantage of simultaneous measurements of all components of the GGT and anomalies of the gravitaty field V z. It is shown that the joint analysis of all GGT components can provide independent constraints on the noise level in various components. The method of tensor deconvolution proposed in the paper is a tensor analogue of the Euler method. The method is based on the calculation of invariants and is, therefore, stable with respect to the orientation uncertainties of the measuring system. The method provides means for estimating the structural index and, therefore, is particularly effective in the treatment of fields that contain isometric and/or elongated anomalies. The calculation of invariants and the tensor ratio can also be used for the development of procedures enabling automatic estimation of the axis strike azimuths of elongated anomaly-forming bodies. 相似文献
2.
A very fast and efficient approach for gravity data inversion based on the regularized conjugate gradient method has been developed. This approach simultaneously inverts for the depth ( z), and the amplitude coefficient ( A) of a buried anomalous body from the gravity data measured along a profile. The developed algorithm fits the observed data by a class of some geometrically simple anomalous bodies, including the semi-infinite vertical cylinder, infinitely long horizontal cylinder, and sphere models using the logarithms of the model parameters [log( z) and log(| A|)] rather than the parameters themselves in its iterative minimization scheme. The presented numerical experiments have shown that the original (non-logarithmed) minimization scheme, which uses the parameters themselves ( z and | A|) instead of their logarithms, encountered a variety of convergence problems. The aforementioned transformation of the objective functional subjected to minimization into the space of logarithms of z and | A| overcomes these convergence problems. The reliability and the applicability of the developed algorithm have been demonstrated on several synthetic data sets with and without noise. It is then successfully and carefully applied to seven real data examples with bodies buried in different complex geologic settings and at various depths inside the earth. The method is shown to be highly applicable for mineral exploration, and for both shallow and deep earth imaging, and is of particular value in cases where the observed gravity data is due to an isolated body embedded in the subsurface. 相似文献
3.
A simple and fast determination of the limiting depth to the sources may represent a significant help to the data interpretation. To this end we explore the possibility of determining those source parameters shared by all the classes of models fitting the data. One approach is to determine the maximum depth-to-source compatible with the measured data, by using for example the well-known Bott–Smith rules. These rules involve only the knowledge of the field and its horizontal gradient maxima, and are independent from the density contrast.Thanks to the direct relationship between structural index and depth to sources we work out a simple and fast strategy to obtain the maximum depth by using the semi-automated methods, such as Euler deconvolution or depth-from-extreme-points method (DEXP).The proposed method consists in estimating the maximum depth as the one obtained for the highest allowable value of the structural index ( Nmax). Nmax may be easily determined, since it depends only on the dimensionality of the problem (2D/3D) and on the nature of the analyzed field (e.g., gravity field or magnetic field). We tested our approach on synthetic models against the results obtained by the classical Bott–Smith formulas and the results are in fact very similar, confirming the validity of this method. However, while Bott–Smith formulas are restricted to the gravity field only, our method is applicable also to the magnetic field and to any derivative of the gravity and magnetic field. Our method yields a useful criterion to assess the source model based on the (∂ f/∂ x) max/ fmax ratio.The usefulness of the method in real cases is demonstrated for a salt wall in the Mississippi basin, where the estimation of the maximum depth agrees with the seismic information. 相似文献
4.
The application of semi‐automatic interpretation techniques to potential field data can be of significant assistance to a geophysicist. This paper generalizes the magnetic vertical contact model tilt‐depth method to gravity data using a vertical cylinder and buried sphere models. The method computes the ratio of the vertical to the total horizontal derivative of data and then identifies circular contours within it. Given the radius of the contour and the contour value itself, the depth to the source can be determined. The method is applied both to synthetic and gravity data from South Africa. The Matlab source code can be obtained from the author upon request. 相似文献
5.
The non-hydrostatic part of the second zonal harmonic term has been included in the Indian gravity anomaly. The interpretation of the potential, gravity and deflection of the vertical has been produced by a new method. The depth of the source is 580 km and the solution satisfies the temperature gradient of 1°K/km. The ratio Δ Vp/Δ p > 0 shows the thermal nature of the density distribution near 670 km. 相似文献
6.
A new method is introduced to determine the top and bottom depth of a vertical line using gravity anomalies. For this, gravity at a distance x from the origin and horizontal derivative at that point are utilized. A numerical value is obtained dividing the gravity at point x by horizontal derivative. Then a new equation is obtained dividing the theoretical gravity equation by the derivative equation. In that equation, assigning various values to the depth and length of vertical line, several new numerical values are obtained. Among these values, a curve is obtained for the one that is closest to the first value from attending the depth and length values. The intersection point of these curves obtained by repeating this procedure several times for different points x yield the real depth and length values of the line. The method is tested on two synthetics and field examples. Successful results are obtained in both applications. 相似文献
7.
In this paper the application of an edge detection technique to gravity data is described. The technique is based on the tilt
angle map (TAM) obtained from the first vertical gradient of a gravity anomaly. The zero contours of the tilt angle correspond
to the boundaries of geologic discontinuities and are used to detect the linear features in gravity data. I also present that
the distance between zero and
±p\mathord | / |
\vphantom p4 4 \pm {\pi \mathord{\left/ {\vphantom {\pi 4}} \right. \kern-\nulldelimiterspace} 4} pairs obtained from the TAM corresponds to the depth to the top of the vertical contact model. Alternatively, the half distance
between
- p\mathord | / |
\vphantom p4 4 - {\pi \mathord{\left/ {\vphantom {\pi 4}} \right. \kern-\nulldelimiterspace} 4} and
+ p\mathord | / |
\vphantom p4 4 + {\pi \mathord{\left/ {\vphantom {\pi 4}} \right. \kern-\nulldelimiterspace} 4} radians is equal to the depth to the same model. I illustrate the applicability of the present method by gravity data due
to buried vertical prisms, imaging the positions of the edges of the prisms. The results obtained from the theoretical data,
with and without random noise, have been discussed. The analysis of the TAM has been demonstrated on a field example from
the Kozaklı-Central Anatolian region, Turkey, and the location and depth of the edges of the structural uplifts of the Kozaklı graben are imaged. The results indicated
that depth values from these sources have ranged between 0.2 and 0.6 km. I have also compared the Euler deconvolution technique
with the TAM images obtained from the first vertical gradient of residual gravity anomaly. Both techniques have agreed closely
in detecting the horizontal location and depth of the uplift edges in the subsurface with good precision. 相似文献
8.
More than 3800 measurements of the 630.0, 557.7, and 427.8 nm emission intensities have been statistically manipulated, and the dependences of the I 630/ I 427.8 and I 557.7/ I 427.8 ratios on the I 427.8 nm emission intensity have been obtained. The I 630/ I 427.8 ratio decreases from 2 to 0.4 when the I 427.8 nm emission intensity increases from 0.1 to 3 kR. In the I 427.8 nm emission range 0.1–1.8 kR, the I 557.7/ I 427.8 ratio tends to increase and takes the values 4.2–6.4. The experimental results have been confirmed by theoretical calculations. The obtained I 557.7/ I 427.8 ratios suggest that the NO density at a maximum of its height profile is on the average 10 8 cm ?3 in typical nighttime auroras. 相似文献
9.
We have developed a least-squares approach to determine simultaneously the depth to both the top and base of a buried finite
vertical cylinder (vertical line element approximation) and a 2-D vertical thin sheet from moving average residual anomaly
profiles obtained from gravity data using filters of successive window lengths. The method involves using a relationship between
the depth to the top, and base of the source and a combination of windowed observations. The method is based on computing
the standard deviation of the depths to the top, determined from all moving average residual anomalies for each value of the
depth to the base. The standard deviation may generally be considered a criterion for determining the correct depth to the
top and base of the buried structure. When the correct depth to the base value is used, the standard deviation of the depths
to the top is less than the standard deviation using incorrect values of the depth to the base. This method can be applied
to residuals as well as to the observed gravity data. The method is applied to synthetic examples with and without random
errors and tested on two field examples from the USA and Canada. 相似文献
10.
Full Tensor Gravity Gradiometry (FTG) data are routinely used in exploration programmes to evaluate and explore geological complexities hosting hydrocarbon and mineral resources. FTG data are typically used to map a host structure and locate target responses of interest using a myriad of imaging techniques. Identified anomalies of interest are then examined using 2D and 3D forward and inverse modelling methods for depth estimation. However, such methods tend to be time consuming and reliant on an independent constraint for clarification. This paper presents a semi‐automatic method to interpret FTG data using an adaptive tilt angle approach. The present method uses only the three vertical tensor components of the FTG data ( Tzx, Tzy and Tzz) with a scale value that is related to the nature of the source (point anomaly or linear anomaly). With this adaptation, it is possible to estimate the location and depth of simple buried gravity sources such as point masses, line masses and vertical and horizontal thin sheets, provided that these sources exist in isolation and that the FTG data have been sufficiently filtered to minimize the influence of noise. Computation times are fast, producing plausible results of single solution depth estimates t hat relate directly to anomalies. For thick sheets, the method can resolve the thickness of these layers assuming the depth to the top is known from drilling or other independent geophysical data. We demonstrate the practical utility of the method using examples of FTG data acquired over the Vinton Salt Dome, Louisiana, USA and basalt flows in the Faeroe‐Shetland Basin, UK. A major benefit of the method is the ability to quickly construct depth maps. Such results are used to produce best estimate initial depth to source maps that can act as initial models for any detailed quantitative modelling exercises using 2D/3D forward/inverse modelling techniques. 相似文献
11.
In situ stress state becomes more and more significant with in-depth research on geodynamics and energy development.However,there has not been an economic and effective method developed to determine deep three-dimensional in situ stress.The Anelastic Strain Recovery(ASR)method is a newly developed technique that can determine three-dimensional in situ stresses.After the 12 May 2008 Ms8.0 Wenchuan earthquake,the ASR method was used for the first time in mainland China to measure the in situ stresses in the WFSD scientific boreholes in Sichuan Province,China.In this paper,the basic procedure of the ASR method is introduced in detail and the compliances of ASR for boring cores are investigated.The results show that the maximum principal stress direction was NW64°at a measured depth(MD)of 1173 m(vertical depth 1151 m)in WFSD-1.The ratio of shear mode to the volume mode compliance of ASR was 2.9.And the three principal stresses at 1173 m MD in WFSD-1are 43,28 and 25 MPa.Combined with stress measurement results determined using other in situ measurement methods along the Longmenshan fault zone,the directions of the maximum horizontal principal stress changes from E-W to NEE-SWW to NWW-SEE when moving from NE to SW along the Longmenshan fault zone.This change is in agreement with the stress regime of the Longmenshan fault zone of the Wenchuan Earthquake,which supports a stress regime consisting predominantly of thrusts in the southwest and strike-slip in the northeast. 相似文献
12.
—We have developed a least-squares minimization approach to depth determination from magnetic data. By defining the anomaly value T(0) at the origin and the anomaly value T( N) at any other distance ( N) on the profile, the problem of depth determination from magnetic data has been transformed into finding a solution to a nonlinear equation of the form f( z)=0. Formulas have been derived for a sphere, horizontal cylinder, dike, and for a geologic contact. Procedures are also formulated to estimate the effective magnetization intensity and the effective magnetization inclination. A scheme for analyzing the magnetic data has been formulated for determining the model parameters of the causative sources. The method is applied to synthetic data with and without random errors. Finally, the method is applied to two field examples from Canada and Arizona. In all cases examined, the estimated depths are found to be in goodagreement with actual values. 相似文献
13.
Summary The magnitudes M and maximum intensities I
0 of 66 earthquakes which occurred in the period from 1901 to 1973 in the eastern alpine area are compared with those of 62 earthquakes occurred in Friuli, northern Italy between 6 May and 15 September 1976. The average focal depth of the Friuli events is about 10 km. The existing empirical relation between M and I
0 for this area is improved by many new data. 相似文献
14.
The results of solving the inverse problem of forced nutations and free oscillations of the Earth by decomposing the Q-factor and small depth variations in density in a system of orthogonal functions are considered. These functions are determined by orthogonalization of the functional derivatives of the observed parameters with respect to the depth distributions of the sought parameters (assuming there are no distributions of the velocities of body seismic waves V p and V S with depth and unchanged total mass M and inertia moments I of the Earth). The examples are presented to illustrate the numerical solution of the inverse problem on finding the density distributions in the mantle and core of the Earth using orthogonalization of the integral constraints for the probable depth distributions of density describing the conditions of unchanged M and I, as well as the constraints posed by the data on the periods of the free low-order oscillations of the Earth. 相似文献
15.
Using the data of the ionospheric vertical sounding in Almaty, the response of various parameters of the nighttime F layer to the passage of an atmospheric gravity wave, generated during the large magnetic storm on July 24–25, 2004, is studied. The analysis of the phase relations between the variations in the electron density at the F layer maximum ( NmF), the layer maximum height ( hmF), and the layer half-thickness showed that they are determined by the slope of the wave phase front. It is shown that the half-thickness of the layer changes in anti-phase with the variations in NmF2. The known fact that the amplitudes of variations in the critical frequencies of the F 2 layer are smaller than the amplitudes of electron density variations at fixed heights is explained. 相似文献
16.
The ‘depth from extreme points’ method is an important tool to estimate the depth of sources of gravity and magnetic data. In order to interpret gravity gradient tensor data conveniently, formulas for the tensor data form regarding depth from the extreme points method were calculated in this paper. Then, all of the gradient tensor components were directly used to interpret the causative source. Beyond the gzz component, also the gxx and gyy components can be used to obtain depth information. In addition, the total horizontal derivative of the depth from extreme points of the gradient tensor can be used to describe the edge information of geologic sources. In this paper, we investigated the consistency of the homogeneity degree calculated by using the different components, which leads to the calculated depth being confirmed. Therefore, a more integrated interpretation can be obtained by using the gradient tensor components. Different synthetic models were used with and without noise to test the new approach, showing stability, accuracy and speed. The proposed method proved to be a useful tool for gradient tensor data interpretation. Finally, the proposed method was applied to full tensor gradient data acquired over the Vinton Salt Dome, Louisiana, USA, and the results are in agreement with those obtained in previous research studies. 相似文献
17.
Ambiguity in the inverse problem of retrieval of the mechanical parameters of the Earth’s shell and core from the set of data on the velocities V p and V S , of longitudinal and transverse seismic body waves, the frequencies f i and quality factors Q i , of free oscillations, and the amplitudes and phases of forced nutation is considered. The numerical experiments show that the inverse problem of simultaneous retrieval of the density profile ρ in the mantle-liquid core system and the mechanical quality factor Q μ of the mantle (if the total mass M and the total mean moment of inertia I of the Earth, and V p and V S are constant at all depths) has most unstable solutions. An example of depth distributions of ρ and Q μ which are alternative to the well-known PREM model is given. In these distributions, the values of M and I and the velocities V p and V S at all depths for the period of oscillations T = 1 s exactly coincide with their counterparts yielded by PREM model ( T = 1 s); however, the maximum deviations of the ρ and Q μ profiles from those in the PREM model are about 3% and 40%, respectively; the mass and the moment of inertia of the liquid core are smaller than those for the PREM model by 0.75% and 0.63%, respectively. In this model, the root mean square (rms) deviations of all the measured values of f i and Q i from their values predicted by theory are half to third the corresponding values in the PREM model; the values of Δ for natural frequencies of the fundamental tone and overtones of radial oscillations, the fundamental tones of torsional oscillations, and the fundamental tones of spheroidal oscillations, which are measured with the highest relative accuracy, are smaller by a factor of 30, 6.6, and 2 than those in the PREM model, respectively. Such a large ambiguity in the solution of the inverse problem indicates that the current models of the depth distribution of density have relatively low accuracy, and the models of the depth distribution of the mechanical Q in the mantle are extremely unreliable. It is shown that the ambiguity in the models of depth distribution of density considerably decreases after the new data on the amplitudes and phases of the forced nutation of the Earth are taken into account. Using the same data, one may also refine by several times the recent estimates of the creep function for the lower mantle within a wide interval of periods ranging from a second to a day. 相似文献
18.
Following a brief overview of the history and the development of the Surface Wave Method—with a focus on techniques for processing and inverting field data—a Simplified Inversion Method (SIM) is described, which constitutes an improvement of the Satoh et al. (1991) [1] method. The SIM is a direct inversion method of surface wave dispersion data, making use of a penetration depth coefficient, aR, whose value is a function of Poisson's ratio and the overall shape of the dispersion curve. In the present study the coefficient aR has been evaluated using data from (a) an extensive database compiled from the technical literature and containing results of inverted surface wave measurements and nearby cross-hole/down-hole measurements, (b) results of side by side surface wave and cross-hole measurements, performed at five sites in the course of this study, (c) finite element analyses simulating the performance of surface wave measurements and thus providing “virtual” data, and (d) applying a current advanced inversion code, available on the Web. Based on all the above data, optimum values of aR (and of the corresponding uncertainty of the derived Vso vs. depth profile) have been estimated. These values were found to be independent of depth from ground surface. The results of all analyses and comparisons indicate that for the majority of realistic soil profiles (including cases of normal and inverse dispersion conditions) the proposed SIM provides very reliable Vso vs. depth profiles when a value of aR=0.63–0.67 is used in the inversion process. It is concluded that the SIM can be used with confidence as a direct inversion method of surface wave dispersion data. 相似文献
19.
利用北天山地区2016～2019年观测的4期流动重力观测资料,分析研究一年尺度的重力场动态变化特征,并利用小波分析方法,将不同场源深度的重力异常进行分离。通过功率谱分析,获取各阶小波重力细节对应的场源深度。研究结果表明,2017年8月9日精河 MS6.6地震前,震中位于负值集中区,四阶小波重力细节显示震中附近出现明显的四象限分布;2020年1月16日库车 MS5.6地震前,震中位于负值区,小波重力细节整体量值较小;功率谱估算的场源近似深度与2次地震的震源深度相近。 相似文献
20.
We investigate the Tibet–Quinghai plateau and the Tarim basin in terms of spatial variations of the elastic thickness ( Te) in the frame of the thin plate flexure model. The method of investigation makes use of a convolutive method, which allows high spatial resolution of the flexure properties and overcomes some of the problems tied to the spectral admittance/coherence methodologies. We study the relation between the topographic and subsurface loads and the observed crust–mantle interface (CMI) undulations, the latter having been obtained from gravity inversion. The gravity data used for the inversion are a unique set of high quality data available over the Chinese part of the plateau, and constitute the highest resolution grid today available in this impervious area. The gravity inversion is constrained by results from the study of the propagation of seismic waves. The two extensive sedimentary basins, the Tarim and the Qaidam basins, are modeled by forward gravity modeling. The oscillations of the CMI obtained from the gravity inversion agree well with those expected by loading the thin plate model of spatially variably elastic thickness with the surface and subsurface loads. It is found that the modeling of the sedimentary basins is essential in the flexure analysis. The spatial variations of elastic thickness correlate with the extensions of the different terrains that constitute the plateau. Most of the Tibet plateau has low Te, varying in the bounds 10–30 km, with lower values in the Qiangtang terrain, where the Te reaches 8 km. The Tarim and the Qaidam basins, Precambrian platforms overlain by sediments, are rigid and have a Te of up to 110 km and 70 km, respectively. The flexural analysis distinctly discerns the Tibet plateau, with thick crust, part of which is molten, from the cratonic areas, the Tarim and Qaidam basins, which though of thinner crust, act as undeformable rigid blocks. 相似文献
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