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.     

Depth determination from a non-stationary magnetic profile for scaling geology

The conventional spectral analysis method for interpretation of magnetic data assumes stationary spatial series and a white‐noise source distribution. However, long magnetic profiles may not be stationary in nature and source distributions are not white. Long non‐stationary magnetic profiles can be divided into stationary subprofiles following Wiener filter theory. A least‐squares inverse method is used to calculate the scaling exponents and depth values of magnetic interfaces from the power spectrum. The applicability of this approach is demonstrated on non‐stationary synthetic and field magnetic data collected along the Nagaur–Jhalawar transect, western India. The stationarity of the whole profile and the subprofiles of the synthetic and field data is tested. The variation of the mean and standard deviations of the subprofiles is significantly reduced compared with the whole profile. The depth values found from the synthetic model are in close agreement with the assumed depth values, whereas for the field data these are in close agreement with estimates from seismic, magnetotelluric and gravity data.     

山西阳高──河北容城剖面大地电磁资料的二维反演解释

介绍了一种快速二维大地电磁自动反演方法及其在阳高－容城剖面大地电磁资料解释中的应用．二维反演结果显示，研究区浅层电性结构与地表地质有较好的对应性；在中、下地壳范围内发育着断断续续的低阻带，并沿剖面大致分为三段；上地幔低阻带由北西向南东方向逐渐变浅，上地幔低阻带深度陡变带与巨型重力异常高梯度带对应．与一此较大型的断裂带对应的区段在电性上有较清楚的显示．     

Direct interpretation of 2D potential fields for deep structures by means of the quasi-singular points method

The founder of the Russian school of direct interpretation of potential fields (with minimal prior geological‐geophysical information) was V.M. Berezkin, who introduced the operator of total normalized gradient for the 2D interpretation of profile gravity data sets. This operator was successfully applied in searches of hydrocarbon reservoirs. The further development of this approach (the so‐called quasi‐singular points method) has allowed solution also to various structural problems, using mathematical criteria for the transition from extremes of total normalized gradient fields to coordinates of anomalous sources. The main numerical evaluation strategy is based on stabilized downward continuation of field derivatives and specific use of the filtration properties of Fourier series approximation. The characteristic properties of the quasi‐singular points method are: 1) presentation of a more general total normalized gradient function through additional parameters (derivative order m, form of smoothing function Q, number of Fourier coefficients N* with maximal N), optimum values being chosen during a peak‐spectrum analysis of the interpreted function; 2) calculation of the set of total normalized gradient fields for various values of N*/N, representing coordinate systems {x,N*/N} as an ‘axes tree’ of extrema, where each 2D total normalized gradient field is representationally compressed in a 1D line, permitting a) immediate overview of the positions of the axes in all variants of the calculated fields and b) reduction of the retained information, as required in subsequent interpretation; 3) development of two criteria for transition from extrema of total normalized gradient fields to the coordinates of anomaly sources. The quasi‐singular points method is intended for tracing limiting gently‐sloping boundaries, if their micro‐relief features are sources of the interpreted anomaly but sub‐vertical contacts may also be traced. The method has been tested in delineating various geological structures. One of the most challenging, successfully achieved, was tracing of the Moho discontinuity and study of the upper mantle, using only Bouguer anomaly data along interpretation profiles. This is attested in an example of two regional profiles intersecting the European part of Russia. The central part of one of them coincides with the results from a deep seismic profile.     

攀西地区重力场特征及地壳密度结构

攀西地区位于峨眉山大火成岩省中西部,构造和岩浆特征显著,地震活动强烈.通过对野外重力测量得到的云县—会东和普洱—七甸两条剖面的高精度重力观测数据进行处理和分析,构建了沿剖面的二维地壳密度结构,其中普洱—七甸剖面与孟连—马龙宽角地震剖面部分位置重合.同时结合区域重力异常特征及下地壳视密度填图结果,得到如下初步认识:红河断裂带是南北地震带南段地区重要的构造分界线,断裂带南北向密度结构和莫霍面分布形态存在较大差异,沿走向构造变化.云县—会东剖面上大姚—会东段下地壳底部存在密度较高的壳幔过渡层,结合研究区下地壳底部壳幔过渡层的密度分布特征,认为该过渡层不是攀西裂谷下的"裂谷垫",而是由岩浆底侵作用造成的.     

The North American Central Plains conductivity anomaly and its correlation with gravity, magnetic, seismic, and heat flow data in Saskatchewan, Canada

The North American Central Plains conductivity anomaly (NACP) lies, virtually in its entirety, within the Trans-Hudson Orogen. Accordingly, should these two features prove to be contemporaneous, then the geometrical relationship between these two is of foremost importance to any evolutionary tectonic model proposed to explain the collision of the Superior and Churchill Provinces in the Hudsonian, and a model that does not include a mechanism for the generation of this anomaly is obviously untenable.

In order to map better the trend of the NACP in the Province of Saskatchewan, Canada, two magnetotelluric (MT) profiles were conducted over the NACP previously defined by magnetometer arrays and profiles. Data from these two profiles, along with an earlier MT profile just north of the U.S./Canadian border, suggest that the NACP is not a continuous feature, but rather that it exhibits a definite break at latitude 51° N. Other geophysical evidence examined herein is concordant with this characteristic. If a second MT anomaly mapped to the northwest of this break is, in fact, a manifestation of the same geological structure, then one possible interpretation is of a major NW-SE trending sinistral fault in the deep crust, previously undetected, with a movement of some 100–150 km along strike.

The MT data from the southernmost profile, after correction for static shift, are modelled in a 2D manner, and it is shown that the NACP is consistent with an arcuate structure in vertical section of high conductivity (> 2 S m⁻¹) beginning at a depth of 10 km centred on 103° W dipping down to the west reaching possibly the base of the crust. It is also shown that such an arcuate shape in section can explain an observed gravity high of 40 mgal. Such high conductivities cannot be explained in terms of connected fluids, as it would require implausibly high porosities (12–20%). A comprehensive interpretation of all the geophysical data would have to account for the observed characteristics of the anomalous region; which are, high electrical conductivity, positive density contrast, no magnetization, and high heat flow in the basement, in terms of a single causative body. One possible explanation is presented in terms of a zone of lithospheric weakness along which was emplaced low-density differentia from a mantle-derived body. An alternative explanation could be the phase transformation of obducted material into eclogite or serpentinite. However, difficulties exist with both of these explanations and further data are required.     

地质观测、地震剖面和重力测量的综合方法在前陆褶皱冲断带的应用：以天山北麓呼图壁河剖面为例

褶皱逆冲带的几何学研究是造山带研究的热点,但是无论是传统构造地质学方法还是地球物理学方法都在研究褶皱逆冲带几何特征时存在多解性.为了制约这种多解性,本文以天山北麓的呼图壁河剖面为列,介绍一种地质与地球物理相结合的研究方法.该方法首先沿呼图壁河剖面进行详细的地表观测,获取地表的构造地质数据形成初步的地质模型,其次结合地表构造和钻井分层数据,对收集到的石油地震剖面进行重新解释.然而,地震反射数据只分布在盆地内部,在盆山结合带缺失或者不清晰,因此对该剖面进行了重力测量并计算出布格重力异常.结合盆地各沉积地层和基底密度值,用重力正演方法模拟呼图壁河剖面的密度结构.研究结果显示沿呼图壁河剖面并不存在天山北缘断裂,盆地的沉积盖层可以从准噶尔盆地连续过度到天山内部并不整合覆盖在天山古生代基底之上.这一结果与西段金钩河剖面的天山基底逆冲到准噶尔盆地显然不同,说明了天山北缘盆山结合带构造的多样性.利用平衡剖面技术,恢复的呼图壁河平衡剖面缩短量约为4.8 km,对比前人研究,说明了天山北缘的缩短量沿东西方向存在显著的不均一性.本研究也说明这种构造地质与地震及非震地球物理相结合的方法可以广泛地被应用于褶皱逆冲带.     

Global Optimisation of Time Domain Electromagnetic Data Using Very Fast Simulated Annealing

--Efficacy of the global optimisation technique is demonstrated in the inversion of time domain electromagnetic data. Transient EM responses observed using a coincident loops system along several profiles and different time channels over a plate like conducting body are inverted, using very fast simulated annealing (VFSA) as an optimisation tool. Three time channels for each profile are considered in the inversion. Study reveals that only one run of global inversion considering a single multi-channel profile is not enough to resolve all the nine model parameters of a plate-like conducting body. However, the global inversion of a single multi-channel profile with several runs yields a mean model that is quite close to the true model. Considering many profiles and time channels together in the global inversion can yield reliable estimates of all the parameters. Computationally, this is not an efficient procedure. Analysis of the results shows, however, that two distant profiles are enough to yield all the model parameters reliably, even after one run of the global inversion. Both noise-free and noisy synthetic data are used in the inversion. Finally, field data are also inverted to study the performance of the global inversion.     

Estimation of the mass density contrasts and the 3D geometrical shape of the source bodies in the Yilgarn area, Eastern Goldfields, Western Australia

We invert 2D surface gravity data constrained both by geological and seismic information. We use a number of pre-processing tools in order to reduce the general multi-body inversion into several single-body inversions, whereby we can reduce the overall complexity of the inversion task. This is done with as few assumptions as possible. Furthermore, for a single-body inversion we uncouple the determination of the shape of the causative sources from the determination of their mass density contrast to the surroundings. The inversion for the geometrical shape of the source body is done in steps. Firstly, a rough 3D shape of the source is modelled—a model consisting of the vertical mass columns of equal height. The horizontal extension is implied by the surface gravity signal. Subsequently, the shape of each source body is modified to obtain a better fit to the surface gravity data. In each modification step, the overall change of the shape of the source body is followed by an update of the mass density contrast to the surroundings. The technique was applied to a set of gravity data from the Eastern Goldfield area in Western Australia. The area has been widely studied in the past. In 1999, two seismic profiles that cross-sect the area were measured. Furthermore, an extensive geological modelling for the area has been conducted. The practical goal of this work was to verify the geological interpretation using the potential field data (mainly the gravity data although magnetic data were also available) and only weakly constrained by the seismic information. The result was the reconstruction of the ‘rough’ 3D geometry of the source bodies and the estimation of a constant mass density contrast to the surroundings. A possible extension of this technique for detailed studies of the geological model is briefly discussed.     

昆明地区重力异常数据处理

为了有效地划分出重力异常,利用小波多尺度分析和奇异值分解重建两种方法对昆明地区布格重力异常数据进行处理,实现了位场的分离.分离结果反映了不同深度和尺度地质体密度的不均匀性,从而较好地圈定了盆地,并揭示了几条主要断裂的空间延展特征.运用这两种方法对重力数据的处理都具有可行性.     

辽吉古裂谷部分地区深部地质结构特征

在4条实测高精度重、磁剖面及MT剖面探测的基础上,结合区域地质资料及野外地质研究,对辽吉古元古代裂谷（简称辽吉裂谷）边界、内部地质结构和基底组成等深部地质问题进行了论述。 新获取的非震地球物理剖面为揭示辽吉裂谷的深部地质结构提供了重要信息,在MT剖面上发现沿连山关—赛马一线存在一个明显的南倾构造界面,该界面为控制辽吉裂谷北部边界的伸展断裂构造面,以往认为在本溪市—本溪县一线辽河群分布的北部边界应为辽吉裂谷的古地理边界;证实南北辽河群岩相的差异主要是由相变造成的;MT剖面揭示出辽吉裂谷内部具有复式褶皱结构特征,基底埋深可达10km以上;区域地质研究及重磁电联合反演结果表明辽吉裂谷的基底主要发育2种类型：一是太古代表壳岩花岗岩型;二是太古代花岗岩-古元古代花岗岩型。     

综合物探技术识别地震反射异常体属性

埋藏在6000m以下的地震反射异常体是礁体还是火成岩体？或者是海底滑塌体？在高质量重磁电资采集的基础上，通过对研究区域内的地层岩石物性细致研究，利用CEMP资料界面约束二维反演、重磁力异常剥离技术和LCT重磁力地震联合反演解释技术细致研究了异常体的物性参数，从密度、磁化率、电阻率、速度等多种特征推测判断了深层异常体地质属性，为钻井部署提供了间接依据。     

Results of airborne magnetometer profile from offshore Mangalore to offshore Madras (India) along the 13th degree parallel

Summary A study is made of an airborne magnetometer profile from 130 km offshore Mangalore to 60 km offshore Madras approximately along the 13th degree parallel obtained in May 1967 by the National Geophysical Research Institute. The total length of the profile is about 770 km. A Bouguer gravity anomaly profile along same traverse over the land section has been also studied along with the magnetic profile.Eight major anomalies on the magnetic profile were chosen for depth calculations. Depths of the magnetic bodies were determined by elementary approximation method ofSmellie. It is found that in most of the cases the sources of the magnetic anomalies lie between 5 to 20 km (approximately). Only one anomaly yielded a depth of 2.7 km for its source. These anomalies are accounted for in terms of possible basic intrusions, faults, zones of weakness and ridge-like structures in deeper Crustal levels.N.G.R.I. Contribution Number 131.     

Generalized Solutions of the Inverse Problem and New Technologies for the Quantitative Interpretation of Gravity Anomalies

The approach, fundamentally different from the known ones, to estimating the spatial location of the domain filled with the disturbing masses based on the gravity field measurement data is suggested. The main idea of the approach is, using the set of the probable variants of the interpretation, to construct the distribution of a certain parameter associated with the estimate of probability of detecting the sources of the field in any point of the studied geological medium and then to apply this distribution to each domain eligible for being the true carrier of the anomalous masses. These constructions yield the generalized admissible solutions of the inverse problem with ranking the separate fragments of the model carrier in terms of the probability of detecting anomalous masses in them.     

Electrical Struture of the Crust in Manas Earthquake Area,Xinjiang,China

Magnetotelluric sounding data obtalned recently in Manas earthquake area were processed. Inthe result, curves of apparent resistivity, impedance Phase, skewness and optimum rotationangle versus period and the real magnetic induction vectors were obtained. Then the data ofall measuring points were interpreted by 2D automatic inversion. The result indicates thatalong the sounding profile the shallow crust can be divided into 5 segments and the deep crustcan be divided into 3 segments, with faults or deep-seated fault zones as the contactboundaries between them. The sedimentary cover along the profile extents down to depthabout 12 km in maximum and a low-resistivity body exists in the crust in southern section ofthe profile. The interpretation results are well consistent with geological and othergeophysical data. The Manas M7. 7 earthquake occurred near a contact zone where theelectrical structure of the crust sharply changes.     

基于HHT提取昆明、下关重力固体潮的地震前兆信息

在重力固体潮地震前兆分析中引入HHT时频分析新方法.结合HHT的优越性、固体潮的特点和地震的非平稳过程特性,设计重力固体潮地震前兆分析的瞬时频率特征参数;以相应理论计算值作为参照背景,研究固体潮的震前变化特征.昆明、下关的震例分析表明, 的确存在瞬时频率特征参数的震前变化,且具短期、同步正异常特征;瞬时频率特征参数具有明确的物理意义,其震前变化反映了地震非平稳过程对理论重力固体潮的影响.     

CSAMT和重力方法在狮子湖温泉深部地球物理勘查中的应用

作为一种新型开发的绿色能源,地热资源被越来越多的人所重视.开采埋藏较深的地热资源风险大,因此开发前的地质和地球物理勘查是十分必要的.采用单一的地球物理方法勘探具有很大的风险,因此采用多种方法进行综合调查可以降低单一方法的风险,取得较好的效果.本文以狮子湖温泉为例,研究了地球物理方法在温泉勘探的应用.本次勘探采用可控源音频大地电磁法(CSAMT-Controlled Source Audio-frequency Magnetotellurics)和微重力测量,CSAMT是针对大地电磁测深法场源的随机性和信号微弱,提出的一种采用可以控制的人工场源改进方案.仪器采用美国Zonge公司生产的GDP-32Ⅱ.该方法由人工向地下供入音频谐变电流建立电磁场,通过仪器在地面接收从地下反馈来的信息,根据不同时代、岩性地层电性特征达到勘查目的.为此我们在测区做了两条剖面,从CSAMT反演图推断,自上而下可分为3个电阻率层,该剖面视电阻率具有很好的层理特征,反映了厚大的中新生代地层（Edn、K）覆盖.剖面西部有明显的泥盆系地层（D）存在.根据视电阻率的变化特点,可以推断这条剖面的5条断层.在剖面中部距地表400～800米深处存在一明显低阻区,推测应为含水破碎带或低阻泥岩.对数据进行二维反演,可以清楚的看到利于储水的盆地构造.CSAMT方法受静态效应影响很大.静态效应位移可能是由地形和电阻率的浅部的横向变化引起的,既是不可避免的,也是不可预测的.因此对数据进行预处理是十分必要的.减少静态效应的影响办法有以下三种：（1）、对效应进行理论计算;（2）、采用空间滤波和相位积分等处理方法;（3）、使用独立的、无静态效应的测量方法.计算静态效应理论值在理论上是简捷的,但在实际的野外条件下,由于无法预测引起静态效应的物体的几何尺寸和电性参数,因此这种方法无法得到可靠的校正值.空间滤波处理是目前广泛采用的一类方法,Bostick（1986）提出了消除MT数据中静态效应的电磁列阵剖面法（EMAP）.EMAP法由于采用连续的剖面测量,可采用窗口可变的自适应空间滤波器-汉宁窗（Hanning window）或叫余弦钟形滤波器消除静态效应.但是这种方法提供了静态效应的要求的数据密度,这就增大了大量的额外测量,提高了获得数据的代价.因此,我们做重力勘探与CSAMT相对比.微重力测量使用美国产LCR-D型重力仪.实测的微重力异常是地下由浅到深各类地质体的物性差异在地面综合叠加的效应,其中包括界面起伏、岩性不均匀、地壳与壳下物质的厚度变化等诸多地质因素在内.实测的重力异常值经过固体潮改正、零点漂移改正、布格改正、正常场改正之后,得到改正后的重力异常值.从微重力反演结果看来,自西往东重力异常逐渐减小,程台阶下降趋势,并趋于平缓,验证了CSAMT的异常结果.从而弥补了CSAMT法的不足.通过这两种方法,我们大致查明了新生代红层盆地的产出形态.该红层盆地西侧边缘位于青山小学一带,自西往东变深;并反演计算出新生代红层与晚古生代泥盆系地层的分界面.深部地球物理勘探的方法有很多,各有各自的优缺点,我们不能从单一的一种方法得到的结论来判断地下地质构造.CSAMT法勘探深度大,但由于本身的物理特性,导致静态效应、近场效应等影响甚大.严重影响我们对地下地质目标体深度的判断,重力和CSAMT法的相互验证,很好的说明了多种地球物理方法综合测定的优势.     

Structure of the Lower Crust Revealed by One- and Two-dimensional Modeling of Wide-angle Reflections — West Bengal Basin, India

--The crustal structure in the West Bengal basin, India has been investigated by means of wide-angle reflection data recorded along (i) Bishnupur-Palashi-Kandi, 227-km long profile in the north-south direction and (ii) Taki-Arambagh, 120-km long profile in the east-west direction. The data were acquired using multichannel digital seismic instruments with close station spacing. The crustal model, initially derived by 1-D forward modeling of the wide-angle reflection data, has been iteratively refined by 2-D ray tracing and modeling of travel-time observations and the corresponding synthetic seismograms computation. The structural contour map of the Moho prepared from the present data set, indicates the crustal thickness of about 37 km in the western margin of the basin, thinning to about 28 km in the east with an upwarp in the Moho boundary. The upwarp in the Moho and the inferred structural features may be indicative of crustal rifting. The well-known gravity anomaly in the West Bengal basin, 'Calcutta gravity high,' appears to have resulted from the Moho upwarp in combination with the huge thickness of sediments deposited east of the steep flexure of the crystalline basement representing the 'Hinze zone.'     

The distribution of deep source rocks in the GS Sag: joint MT–gravity modeling and constrained inversion

The coal-bearing strata of the deep Upper Paleozoic in the GS Sag have high hydrocarbon potential. Because of the absence of seismic data, we use electromagnetic (MT) and gravity data jointly to delineate the distribution of deep targets based on well logging and geological data. First, a preliminary geological model is established by using three-dimensional (3D) MT inversion results. Second, using the formation density and gravity anomalies, the preliminary geological model is modified by interactive inversion of the gravity data. Then, we conduct MT-constrained inversion based on the modified model to obtain an optimal geological model until the deviations at all stations are minimized. Finally, the geological model and a seismic profile in the middle of the sag is analysed. We determine that the deep reflections of the seismic profile correspond to the Upper Paleozoic that reaches thickness up to 800 m. The processing of field data suggests that the joint MT–gravity modeling and constrained inversion can reduce the multiple solutions for single geophysical data and thus improve the recognition of deep formations. The MT-constrained inversion is consistent with the geological features in the seismic section. This suggests that the joint MT and gravity modeling and constrained inversion can be used to delineate deep targets in similar basins.     

NVESTIGATION OF TECTONICS BY GRAVITY DETAILING*

Mapping of fault patterns is an important part of geophysical exploration. A computerized digital template analysis method is described which tests gravity maps for the effects of faults by comparing measured gravity data with calculated master curves. The interpreted gravity data are incorporated in a tectonic map using geological symbols and units, for ready use by the geologists. Tectonics can be investigated by gravity detailing if the smallest undulations on the Bouguer map are taken into consideration. Up to now, residual and derivative gravity maps have explained gravity effects by assuming spherical bodies which are almost unknown in geology. The method discussed here uses tectonic elements, such as fault-blocks and dikes as a basis of interpretation instead of the spherical bodies of the conventional interpretation methods. Gravity data can be easily and relatively cheaply obtained in the early phases of exploration by area wide spot coverage following lines of easy access such as roads, etc. Seismic studies, by contrast, have the disadvantage of being executed along predetermined profile lines. Only after sufficient detailing do seismic profiles permit-if at all -an areal mapping of faults. Thus a tectonic map is obtained only at the end of a geophysical survey instead of being available prior to the planning of costly seismic profiles. The use of gravimetric data and their interpretation by the suggested method provides tectonic detail maps in the early phases of geophysical studies. In addition, this article discusses a general geophysical interpretation method, using the investigation of faults by gravity as an example. Applications of this method for different gravity and magnetics problems as well as for combined interpretations are outlined. Detailed case histories will be published in later articles.