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1.
A discussion is given of the requirements, the advantages and the methods to be considered in attempting the quantitative interpretation of gravity or magnetic fields from computed maps of the vertically derived field. The transform which is used here is the first vertical derivative (or vertical gradient) with or without downward continuation, but the computed maps are in fact obtained by a controllable Fourier method in which two kinds of operations can be simultaneously performed in complete independance: on one hand the separation of any part of the data by frequency cut-off, and on the other hand the transformation by vertical derivation or continuation of the part which is retained. Taking as raw data either actual surveys or artificially constructed maps, it is first shown how separated and transformed maps of this type can be efficiently obtained under quite flexible conditions, using a special computer program. It is further seen that for correctly controlled filterings the accuracy of the computed maps actually permits to take them as the basis for quantitative interpretation. To effect this, any one of the conventional methods which make use of equivalent model computations may in the first place be adapted to the interpretation of gradients, with the benefits, however, of an enhanced lateral separation of the anomalies and of a large attenuation of the regional effects. Particularly, the delineation of horizontal contours for even fairly complex models can often be made directly in a sufficiently safe way on the anomalies as they show on the vertical gradient maps. This greatly accelerates the process of determining equivalent model bodies. More special methods of interpretation can also be designed by taking into account first the fact that the vertical derivation of the field amounts to an operation of separation on the field's sources themselves, and in addition the availability of the frequency form of the information as a result of using a Fourier method of transform computation. Trial utilisations of various such interpretation processes of either the conventional or the less conventional type, are presented especially in connection with an interpretation study on transformed maps of parts of an offshore aeromagnetic survey (English Channel). 相似文献
2.
HENRI NAUDY ET HENRI DREYER 《Geophysical Prospecting》1968,16(2):171-178
Digital aeromagnetic profiles recorded with a high accuracy can be filtered with an electronic computer. Linear filtering is not very efficient, because the frequency spectrum of a given anomaly is very wide. But other methods are possible which, through a step by step analysis of the profile, leave some categories of anomalies unaltered while they remove the others completely. Our method uses as a criterium the width of the anomalies; it distinguishes and processes differently “bell-shaped” and “multi-legged” anomalies. Some examples of the use of the method are shown. 相似文献
3.
The gravity anomaly has been computed above isostatic, thermally-balanced speading centers that cool by conduction through their top surfaces. Isothermal, and therefore isodense, surfaces were treated as topographic boundaries between layers of different density, and Fourier transforms of power series of the topographic height were used to find the gravity. Convergence requires that the anomaly tend to zero with increasing distance from the ridge crest, and when this is obtained, a crestal positive anomaly is flanked by compensating negatives. Both the magnitude and the spatial width of the anomalies decrease with increasing spreading rate.The ～5 mgal gravity anomalies observed over fast-spreading ridges are matched well by the calculations, but slow-spreading ridges usually have a central rift valley in place of the smooth crest of the idealized isostatic thermal model. The mass deficiency of the valley cancels out the ～40 mgal positive peak that would otherwise occur. The short-wavelength anomaly amplitudes of such ridges are less than 25 mgal and follow the observed local rift valley and flanking ridge topography closely. Excess positive gravity and topography of the flanking ridges suggests compensation of the mass deficiency in the rift valley. However, long-wavelength gravity anomalies such as those observed in the northern Mid-Atlantic cannot be due to topography that is isostatically compensated at a shallow depth. These must be caused either by dynamic forces or by large-scale density differences compensated at much greater depths. 相似文献
4.
Numerous studies have already been carried out on the Fourier Transform and its geophysical applications. The utilisation of computers has brought with it the digitalisation of the major method, reflection shooting, and the different handling techniques for numerical data have given birth to numerous papers on the subject. Gravimetric surveying has always been a numerical method, but it is evident that for it, too, new possibilities have been opened. Consequently, gravimetric experts are becoming increasingly interested in the theory and applications based on the study of frequencies spectra using the Fourier Transform. The researches of C. A. Schwartz, W. Sokoloff and W. C. Dean have been followed by other interesting ideas concerning the data processing of gravity measurements, such as: calculation of the first vertical derivative (V. Baranov), interpretation of the anomalies with the aid of their spectra (M. E. Odegard and J. Berg Jr.), isopach reduction in gravimetric surveying (J. L. Bible), etc. All these ideas and techniques have the same purpose: to make the interpretation easier. But, although they have the same aim and tackle the same type of difficulty, it would seem that the gravimetric and the seismic experts have developed their own tools independently of each other, with no consideration for the fact that both geophysical methods, in particular the methods of treatment of raw measurements, are, if not identical, at least very close to one another and that, consequently, any improvement in one method may be useful to the other. The purpose of the present paper is to reconsider the philosophy of the seismic and gravimetric methods, starting with data recording, then dealing with the most important data processing systems and finally ending with the interpretation. The paper bases its approach on two points of view which are in fact complementary: – Although digital data processing is almost always effected in the functional sphere by convolutions, it is much easier to understand and to conceive these systems if one reasons alternately in the functional and frequency spheres; this is possible by using the Fourier Transform. – By considering the problem in frequencies, there is no fundamental difference between the seismic and gravimetric methods. A curve plotted in gravity units, as a function of the distance, and a seismic trace which represents the variations of the output of a galvanometer, as a function of the time, are identical from the point of view of the Fourier Transform. With these ideas in mind, the following problems are dealt with: – Seismic and gravity signals. – The sampling problem in gravimetry (data sampling rate non-constant). – Presentation and discussion of spectra of some synthetic and practical examples: ?Wave number filtering. ?Frequencies filtering. ?The problem of the frequency o (horizontal and vertical derivatives). ?Continuation = deconvolution. – Other applications of the Fourier Transform in gravimetry. 相似文献
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Mike Sandiford 《Earth and Planetary Science Letters》2002,201(2):309-320
The late Phanerozoic Alice Springs Orogen in central Australia is an archetypal intraplate orogen characterised by a dense, granulitic core exhumed from beneath a carapace comprising a highly radiogenic granitic mid-upper crust and sediments deposited in a shallow intracratonic basin. Exhumation occurred in large part along a crustal penetrative fault system, the Redbank Shear Zone, producing one of the largest gravity anomalies (∼150 mgal) known from the continental interiors. The lithospheric strength implied by the preservation of this anomaly for more than 300 Myr raises the intriguing conundrum of what localised the intraplate deformation in the first place. Available biostratigraphic and thermochronologic data imply bulk convergence rates of less than 1 mm/yr for the orogen as a whole, several orders of magnitude lower than typical of plate margin orogens. The thermal and mechanical evolution of intraplate orogens deformed at such low thermal Peclet numbers differs in fundamental ways from plate margin orogens. In particular, at such low thermal Peclet numbers the conductive response to exhumation of heat sources cools the mid to deep crust during progressive orogenic activity. This is consistent with the hypothesis that the density structure and associated gravity anomalies may have been locked-in by virtue of the strength acquired during the orogenic process provided that the lithospheric strength changes associated with a reduction in average crustal temperature of 20-30°C are of the same order as the forces that drive intraplate deformation. 相似文献
7.
P. Kearey 《Earth and Planetary Science Letters》1976,28(3):371-378
The Labrador Trough is a linear fold-belt marking the junction of the Superior and Churchill structural provinces in northern Quebec. Gravity profiles across the trough are characterized by gently decreasing anomalies over the Superior Province reaching a minimum beneath the trough and thence increasing abruptly over the Churchill Province to a level some 15 mgal higher than the Superior. Superimposed on this higher level are several broad maxima parallel to the trough, one of which corresponds to an extensive outcrop of migmatites.The gravity profiles may be interpreted in terms of a relatively elevated Conrad discontinuity beneath the Churchill Province isostatically compensated by a thickened lower crust. Such a model is consistent with basement reactivation following collision of the Superior and Churchill continental plates. Collision results in crustal thickening by ductile flow in the upper mantle and consequent partial melting in the lower crust leads to differentiation of the crust into a refractory lower part and potash-rich upper part separated by a zone of migmatites.The geological history of the Labrador Trough, and its present-day structure as deduced from gravity studies, is consistent with a sequence of events involving the gradual closure of a small ocean dividing the Superior and proto-Churchill crustal plates in Aphebian times. The closure was effected by subduction beneath the Churchill culminating in collision during the Hudsonian Orogeny and the formation of the present structural configuration after deep erosion. 相似文献
8.
CH. HEMON 《Geophysical Prospecting》1969,17(2):165-181
In the case of simple time series it is shown that prediction operators can be considered as deconvolution operators which are easily obtained. These operators possess the special feature of having a minimum phase, and their spectrum modulus represents, with a good dynamic range, the reciprocal of the square root of the modulus of the original autocorrelation spectrum. A generalization of the simple time series theory to the functions of two independent variables is possible in as much as, on a given section, the application of the multiple time series method enables the function of time and distance variables to have well-defined statistical properties; it is necessary, in particular, that the processes involved are stationary with respect to the two independent variables. In the case of multiple time series the application of the Prediction Theory permits greater uniformity of the traces because it enhances the events which show a good correlation between traces and, on the other hand, tends to minimize the random events which cannot be correlated between traces. The signal-to-noise ratio is thus increased to a great extent. 相似文献
9.
After a brief discussion of MT modeling methods, we expose a particular application of finite elements that may be assimilated to a finite-difference method. The resulting linear equations obtained are similar to the transmission network equations for two-dimensional media. The introduction of resistivity discontinuities is more rigorous than in the usual finite-difference development. The consequence is an improved accuracy. We also present some conclusions about various problems encountered in modeling, such as the choice of network boundary conditions, linear system resolution and the final derivation of apparent resistivity. Application of successive over-relaxation is discussed and we detail rules for mesh design that control result accuracy and iterative convergence. 相似文献
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跨断层形变测量异常特征分析 总被引：5，自引：0，他引：5
在统计近40年我国强震前跨断层形变异常的基础上, 对异常持续时间、 异常空间分布、 异常幅度特征及它们之间的关系进行了分析, 讨论了跨断层形变观测异常特征和监测不同震级地震跨断层场地的最大间距及观测周期。 结果表明: 异常主要以中短期和中期异常为主, 尤其在震前1年内异常最多; 异常持续时间与震级呈正比例的关系; 异常在空间分布上有随震级增大而扩大的现象; 异常幅度有随异常持续时间增大而增大的趋势; 震中距在160~350 km范围内异常幅度较大; 异常形态主要分为趋势型和突变型两种。 监测不同震级的地震跨断层形变场地间距可以不同, 监测7级地震形变场地间距应小于450 km; 6级应小于300 km; 5级应小于200 km; 4级应小于100 km。 跨断层形变观测周期应不大于3个月。 相似文献
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2005年3月28日苏门答腊发生8.6级地震,通过分析处在地磁活动平静期的震前15天的数据,发现3月20、22、23和28日在赤道区域多个参量上出现电离层扰动,如电场和等离子体参量,认为这些电离层扰动可能与本次强震有关.震前电子密度异常有两种形态,一种是在原来的峰值变化区电子密度单调增加,幅度超过1σ,如3月20日和28日的异常;另外一种是改变了原来在赤道区的单峰变化形态,而呈现了双峰值和赤道谷值的变化形态,如3月22日和23日的扰动.分析结果显示,这两种形态的电子密度扰动都分布在一个比较大的范围内,经度上跨越100°,纬度范围30°,显示了比较长的异常持续时间和全球变化特性.VLF频段电场频谱的数据分析表明双峰形态的扰动不是磁共轭效应.同时低频电场扰动相对电子密度而言集中在一个相对小范围内,当Ne单调上升时,电场扰动却只出现在离震中最近的轨道上.分析认为震源产生的电场信号直接传播进入电离层是非常困难的,但是它可以通过岩石层-大气层-电离层圈层耦合机制造成电离层扰动.结合离子成分及其变化形态的分析,震前氧离子变化形态与氢、氦离子的形态相反,这与其他地震前的电离层异常扰动形态是类似的.综合分析认为,对于与地震相关的电离层异常的识别需要结合多个观测参量的联合分析. 相似文献
14.
宁波台数字式温度计三次受强雷电干扰记录的异常特征 总被引：3，自引：0，他引：3
对宁波台数字化地热(水温)仪器3次受到强雷电干扰所记录的异常特征进行分析后认为：地震台站观测仪器数字化改造以后,强雷电干扰已成为影响仪器设备正常运行、观测资料连续稳定的重要因素之一。雷电越强,对地热(水温)观测资料的影响越大．影响的持续时间越长,反之越小,与雷电的持续时间无关。引起的测值异常量可以是正常波动值的一倍到数百倍．影响时间从1小时到数天。强雷电干扰引起的异常如果没有被及时发现,则较难与前兆异常区别。但只要避雷措施达到技术要求,一般的雷电天气是没有影响的。 相似文献
15.
The southwest border of the Parecis Basin (central Brazil) presents several occurrences of gold, copper and zinc. Parallel to this border, there is an alignment of magnetic anomalies with varied size and polarities. In particular, five magnetic anomalies are referred to, in this study, as SJ1 to SJ5. The proximity of these anomalies to each other makes it hard to isolate the magnetic component associated with each source. Furthermore, these anomalies have different magnetization directions, which require the use of a technique which is slightly or not affected by the presence of a remanent magnetization, as the amplitude of the anomalous magnetic field. Considering that, in these intrusions no outcrops are observed, the enhanced horizontal derivative technique was used to estimate the edges location and the depth of these sources. The geological context, allied to the results from the magnetic techniques, allowed to establish binds to restrain the interpretation of the results of the 3D inversion. This procedure permitted to compose three hypothesis to explain the magnetic behavior of the region, from which can be conclusively determined with a borehole analysis. 相似文献
16.
Jacqueline Mammerickx 《Earth and Planetary Science Letters》1981,53(2):147-157
Previously unreported depth anomalies in the central and eastern Pacific are described. Some of these depth anomalies exist over hot spots and propagating spreading ridges; they are not limited to the area of active volcanism but extend beyond it, into the areas toward which the volcanism is propagating. These areas may be “precursor” topographic features, showing up areas of impending or potential mid-plate volcanism or spreading. A distinction can be made between active depth anomalies and fossil ones. Gravity anomalies and high heat flow values can be correlated with active depth anomalies in one area, supporting the arguments favoring a thinning of the lithosphere as their underlying cause. 相似文献
17.
The expanding role of gravity prospecting in mineral and hydrocarbon exploration as well as seismic and volcanic risk studies will be related to its ability to provide high-resolution anomalies. To achieve this goal it is necessary to consider the errors resulting from the topographic corrections, in particular near-station effects. Such errors are relevant not only for severe topographies but also for relatively flat surfaces involving microgravity applications and large-scale surveys.
Indeed, the errors introduced by low-resolution digital elevation models can be of the same order of magnitude as the anomalies of interest. This basic fact is demonstrated by tests on synthetic models. The results of this analysis are summarized in an intuitive graph that can be used to estimate what near-station topographic resolution is necessary for a specific survey. For the Vulcano Island test site (in Southern Italy), we also compare results obtained using three different representations of the topographic surface with different horizontal resolutions and vertical precisions: one from existing photogrammetric data, the second from a digitized map and the third from a very high-resolution laser scanning system. Among the three methods, laser scanning is shown to obtain the highest resolution topographic reconstructions in the shortest time. Some weaknesses of the laser scanning methodology are discussed and we suggest ways of overcoming them. 相似文献
Indeed, the errors introduced by low-resolution digital elevation models can be of the same order of magnitude as the anomalies of interest. This basic fact is demonstrated by tests on synthetic models. The results of this analysis are summarized in an intuitive graph that can be used to estimate what near-station topographic resolution is necessary for a specific survey. For the Vulcano Island test site (in Southern Italy), we also compare results obtained using three different representations of the topographic surface with different horizontal resolutions and vertical precisions: one from existing photogrammetric data, the second from a digitized map and the third from a very high-resolution laser scanning system. Among the three methods, laser scanning is shown to obtain the highest resolution topographic reconstructions in the shortest time. Some weaknesses of the laser scanning methodology are discussed and we suggest ways of overcoming them. 相似文献
18.
A STUDY ON THE ALGORITHM FOR EXTRACTING EARTHQUAKE THERMAL INFRARED ANOMALIES BASED ON THE YEARLY TREND OF LST 
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下载免费PDF全文 SONG Dong-mei ZANG Lin SHAN Xin-jian YUAN Yuan CUI Jian-yong SHAO Hong-mei SHEN Chen SHI Hong-tao 《地震地质》2016,38(3):680-695
There are thermal infrared anomalies(TIA)before earthquake, and TIA has become one of the important parameters for assessing regional earthquake risk. However, not all of the surface infrared anomalies are related to tectonic activities or earthquakes. How to eliminate the influence of non-structural factors and extract the weak signals from strong disturbances is the key and difficult point for tectonic activities studies based on the thermal infrared remote sensing techniques. Land surface temperature(LST)background field is the basis for thermal infrared anomalies extraction. However, the established background fields in previous researches cannot eliminate the influence of climate changes, so the accuracy of thermal anomaly extraction is limited. Now an improved method is proposed. Combined with the periodic character of LST time series, harmonic analysis is lead into the process of LST background field establishment. Specifically, the yearly trend of LST is fitted based on Fourier Approximation method. As a new background field, the yearly trend is dynamic, includes the local and the yearly information. Then, based on the rule of "kσ", the earthquake anomalies, calculated by RST with the yearly trend of LST, can be extracted. At last, the effectiveness of the algorithm can be tested by the quantitative analysis of anomalies with anomaly area statistics, anomaly intensity statistics and distance index statistics. The Wenchuan earthquake was discussed again based on the proposed algorithm with MODIS land temperature products in 2008. The results show that, there were obvious pre-earthquake thermal anomalies along the Longmen Mountains faults with a longer time; but there were no anomalies when the earthquake happened; and the post-earthquake thermal anomalies occurred with much smaller amplitudes and scopes. Compared with the results derived from the traditional RST which is based on the spatial average of LST values, the TIA extracted by the new RST, which is based on the yearly trend of LST, is more fit with the active faults, and the process of the anomalies occurring and removing can be described in more detail. Therefore, as the background field to extract earthquake anomalies, the yearly trend of LST is more reliable. 相似文献
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地形改正是重力和电法勘探中用于消除测点周围地形起伏对观测结果影响的技术.由于地形形成自然电位异常的微观机理较为复杂,解析公式难以得到,可以采用最为直观的方法,寻找两者之间的拟合公式.根据研究,在自然电位梯度测量中,地形引起的自然电位异常分为两种:一是异常与地形起伏是镜像关系,这种情况占多数;另一种情况是异常与地形起伏为反镜像关系.多数地形起伏与异常存在线性关系、二次关系和指数关系,以线性关系为主.具体采用哪种拟合关系,可选取典型的地形和自然电位曲线,通过其变化规律进行判断;也可以采取试算的办法,通过改正效果的比较选择合适的拟合公式.本文选择鄂尔多斯盆地作为研究地点,提出了适用于自然电位梯度测量的地形改正方法.通过三种拟合公式改正效果的比较,得出线性拟合公式能更好地描述该测区内相对高差与自然电位改正量之间的关系.地形改正有效地消除了相对高差与自然电位之间的相关性,改正后的自然电位曲线能够较好地反映地下地质信息,自然电位等值线图异常更加突出.自然电位梯度测量中,地形起伏可影响异常形态,特别是起伏地形的面积与目标地质体的面积大小相当时,两者异常有可能难以区分,因此地形改正的研究具有重要的实用性. 相似文献
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Summary The methods of computation of the gravity difference between two stations depend on the assumed behaviour of the zero drift. When this drift is taken as following a quadratic law with respect to time, then the various methods give virtually the same value for the gravity difference and the estimated errors in it are also the same. In adjusting a network of gravity connections it is proposed that, to form a system of weighting, the estimated errors in the connections should be used in the place of the probable errors, which are unobtainable as the number of independent observations is too small. The estimated errors have been based on the standard deviation of an observation which is probably a characteristic of one gravimeter and one observer. The standard deviation of a single observation with a Worden gravimeter was found to be ±0.014 mgal as a result of a special set of experiments under field conditions. Large random errors have been observed in a Worden gravimeter and the cause is attributed to changes in the filament of the bulb which produces the reading index line.
Zusammenfassung Die Methoden der Berechnung der Schweredifferenzen zwischen zwei Stationen hängt von dem vorausgesetzten Verhalten der Nullpunkts-Änderung (Drift) ab. Wenn angenommen wird, dass diese Drift ein quadratisches Gesetz rücksichtlich der Zeit befolgt, dann geben die verschiedenen Methoden im wesentlichen den gleichen Wert der Schweredifferenz und ihres geschätzten Fehlers. Für die Ausgleichung eines Netzes von Schwereverbindungen wird, um ein System von Gewichten zu bilden, vorgeschlagen, dass die geschätzten Fehler in den Verbindungen anstatt der wahrscheinlichen Fehler verwendet werden, welche nicht abgeleitet werden können, da die Zahl der unabhängigen Beobachtungen zu klein ist. Die geschätzten Fehler wurden auf die mittlere Abweichung einer Beobachtung gegründet, welche wahrscheinlich eine charakteristische Eigenschaft eines Schweremessers und eines Beobachters ist. Als Resultat eines speziellen Satzes von Feldexperimenten wurde als mittlerer Fehler einer einzelnen Beobachtungen mit einemWorden-Schweremesser ±0.014 mgal gefunden.相似文献