Gravity probability tomography: a new tool for buried mass distribution imaging

Following the probability tomography principles previously introduced to image the sources of electric and electromagnetic anomalies, we demonstrate that a similar approach can be used to analyse gravity data. First, we give a coherent derivation of the Bouguer anomaly concept as a Newtonian-type integral for an arbitrary mass distribution buried below a non-flat topography. A discretized solution of this integral is then derived as a sum of elementary contributions, which are cross-correlated with the gravity data function in the expression for the total power associated with the Bouguer anomaly. To image the mass distribution underground we introduce a mass contrast occurrence probability function using the cross-correlation product of the observed Bouguer anomaly and the synthetic field due to an elementary mass contrast source. The tomographic procedure consists of scanning the subsurface with the elementary source and calculating the occurrence probability function at the nodes of a regular grid. The complete set of grid values is used to highlight the zones of highest probability of mass contrast concentrations. Some synthetic and field examples demonstrate the reliability and resolution of the new gravity tomographic approach.     

二维电阻率成像研究

电阻率成像中最关键的问题就是获得雅可比偏导数矩阵。本文从二维微分方程的积分解出发推导了一种新的电阻率成像的雅可比偏导数矩阵,同时形成了成像方程。用内外迭代相结合的高斯塞德儿迭代方法解成像方程可以得到电阻率的分布图像。数值模拟结果表明该方法是有效和可靠的,尤其值得注意的是积分法电阻率成像方法初始模型可以采用均匀模型,减小了对初始模型的依赖。对用其它方法难以获得好的成像结果的单一高阻体,积分法也得到了较好的成像结果。河南商丘某野外资料结果表明,成像结果和实际地质情况吻合较好。     

Electrical resistance tomography experiments at the Oregon Graduate Institute

Three controlled experiments were conducted at the Oregon Graduate Institute (OGI) with the purpose of evaluating electrical resistance tomography for imaging underground processes associated with in-situ site assessment and remediation. The OGI facilities are unique: a double-wall tank 10 m square and 5 m deep, filled with river bottom sediments and instrumented for geophysical and hydrological studies. At this facility, liquid contaminants could be released into the confined soil at a scale sufficiently large to represent real-world physical phenomena.In the first test, images of electrical resistivity were made before and during a controlled spill of gasoline into a sandy soil. The primary purpose was to determine if electrical resistivity images could detect the hydrocarbon in either the vadose or saturated zone. Definite changes in electrical resistivity were observed in both the vadose and saturated soils. The effects were an increase in resistivity of as much as 10% above pre-release values. A single resistive anomaly was imaged, directly below the release point, principally within the vadose zone but extending below the phreatic surface. The anomaly remained identifiable in tomograms taken two days after the release ended with clear indications of lateral spreading along the water table.The second test involved electrical resistance measurements before, during, and after air sparging in a saturated soil. The primary purpose was to determine if the electrical images could be used to detect and delineate the extent of the zone influenced by sparging. The images showed an increase of about 20% in resistivity over background values within the sparged zone and the extent of the imaged zone agreed with that inferred from other information.Electrical resistivity tomography measurements were made under a simulated oil storage tank in the third test. Comparison of images taken before and during separate releases of brine and water showed effects of changes induced by the water or brine. The simulated leak and its location were imaged as a conductive anomaly centered near the point of origin and were observed to spread with time during the release.     

Introduction to ground surface self-potential tomography

A new approach to self-potential (SP) data interpretation for the recognition of a buried causative SP source system is presented. The general model considered is characterized by the presence of primary electric sources or sinks, located within any complex resistivity structure with a flat air-earth boundary. First, using physical considerations of the nature of the electric potential generated by any arbitrary distribution of primary source charges and the related secondary induced charges over the buried resistivity discontinuity planes, a general formula is derived for the potential and the electric field component along any fixed direction on the ground surface. The total effect is written as a sum of elementary contributions, all of the same simple mathematical form. It is then demonstrated that the total electric power associated with the standing natural electric field component can be written in the space domain as a sum of cross-correlation integrals between the observed component of the total electric field and the component of the field due to each single constitutive elementary charge. By means of the cross-correlation bounding inequality, the concept of a scanning function is introduced as the key to the new interpretation procedure. In the space domain, the scanning function is the unit strength electric field component generated by an elementary positive charge. Next, the concept of charge occurrence probability is introduced as a suitable function for the tomographic imaging of the charge distribution geometry underground. This function is defined as the cross-correlation product of the total observed electric field component and the scanning function, divided by the square root of the product of the respective variances. Using this physical scheme, the tomographic procedure is described. It consists of scanning the section, through any SP survey profile, by the unit strength elementary charge, which is given a regular grid of space coordinates within the section, at each point of which the charge occurrence probability function is calculated. The complete set of calculated grid values can be used to draw contour lines in order to single out the zones of highest probability of concentrations of polarized, primary and secondary electric charges. An extension to the wavenumber domain and to three-dimensional tomography is also presented and discussed. A few simple synthetic examples are given to demonstrate the resolution power of the new SP inversion procedure.     

中强震前新沂台电阻率各向异性值（S）变化特征

对新沂地电阻率观测资料进行处理的基础上,研究了地震地电阻率各向异性的前兆异常特征,结果表明：1．在中强震前,震区一定范围内的地电阻率各向异性值（S）会出现上升型异常变化,这种变化表现为在原有平稳变化的背景上的上升变化或者是在原有上升趋势背景上的加速上升变化、亦或是在原有下降趋势背景上的上升变化;2．台站周围400km内5级以上地震发生前均存在明显的地电阻率各向异性值（S）前兆异常。     

渭河盆地隐伏断层的视电阻率特征

渭河盆地电测剖面显示，隐伏断层的破碎带上都呈现出视电阻率负异常．随着勘探深度的增加，异常的幅度也随之增大，破碎带宽度的轮廓也更加清楚．视电阻率异常点呈直线展布是确定隐伏断层走向和评价勘探结果准确度的标准．断层两侧视电阻率的差异反映了其两侧不同介质的电性差异，也是垂直断距的反应．断层破碎带上视电阻率负异常是由于丰富地下水所致     

井间电阻率层析成像研究新进展

由于井间电阻率层析成像方法具有施工方便、成本低、对钻孔无破坏作用且穿透深度也较大等优点，因而得到越来越广泛的应用，在过去几年内对该方法研究的重点在于提高计算效率和改善成像质量，本文从井间电阻率层析成像的基本原理出发，对该方法近年的新进展进行了比较和综述，还介绍了概率层析成像算法的原理，认为今后几年内在此领域内的理论研究重点将是探索有效的反演方法和提高成像质量。     

Imaging a 3D geological structure from HEM, airborne magnetic and ground ERT data in Kalat-e-Reshm area, Iran

A set of geophysical data collected in an area in Iran are analyzed to check the validity of a geological map that was prepared in connection to a mineral prospecting project and also to image the spatial electrical resistivity distribution. The data set includes helicopter electromagnetic (HEM), airborne magnetic and ground electrical resistivity measurement. Occam approach was used to invert the HEM data to model the resistivity using a layered earth model with fixed thicknesses. The algorithm is based on a nonlinear inverse problem in a least-squares sense.The algorithm was tested on a part of an HEM dataset acquired with a DIGHEM helicopter EM system at Kalat-e-Reshm, Semnan in Iran. The area contains a resistive porphyry andesite that is covered by Eocene sedimentary units. The results are shown as resistivity sections and maps confirming the existence of an arc like resistive structure in the survey area. The resistive andesite seems to be thicker than it is indicated in the geological maps. The results are compared with the reduced to the pole (RTP) airborne magnetic anomaly field data as well as with two ground resistivity profiles. We found reasonable correlations between the HEM 1D resistivity models and 2D models from electrical resistivity tomography (ERT) inversions. A 3D visualization of the 1D models along all flight lines provided a useful tool for the study of spatial variations of the resistivity structure in the investigation area.     

九寨沟M_S7.0地震地电阻率变化时空演化分析

本文应用归一化变化速率(NVRM)方法,研究了2017年九寨沟M_S7.0地震周边800km范围内25个地电阻率台站2012—2018年的连续观测数据,分析了震中区周围的地电阻率时、空演化过程.结果显示:(1)25个台站中共有14个台站在震前出现了地电阻率变化,结合本次地震震源机制等分析其时空变化,认为邻近地震破裂带的临夏、通渭、武都和宝鸡台地电阻率在震前出现了下降—折返回升型的变化,符合前人已总结的强地震前地电阻率的变化过程.(2)地震发生前震中区周围大区域内出现了地电阻率负异常变化,且以震中区为中心呈条带分布,异常区的长轴方向与地震断裂带走向或地震烈度分布的长轴方向几乎垂直,与地震主压应力轴方向吻合.本文认为九寨沟M_S7.0强震发生之前,震中区周围出现的区域性地电阻率异常空间丛集现象是很好的中期和短临前兆;地电阻率震前异常动态演化、各向异性等特征对认识本次强震发震断层活动以及震源区应力场分布有启示意义.     

地电阻率短临前兆异常的判定方法研究

本文用一阶差分和相干加权计算等方法对地电阻率观测资料进行处理，其结果较好地突出了短临前兆异常。进而研究了地电阻率短临前兆变化特征及其短临预报指标；采用归一化方法将电阻率各异常参量转换为地震前兆信息量，为多种前兆方法进行综合跟踪预报提供了必要的基础资料。     

电阻率层析成像非线性反演

本文通过概率成像法、α中心法、遗传算法的有机结合, 实现了2-D电阻率层析成像非线性反演.首先,根据概率成像结果指示出的2-D地质异常体的相对电性和大致分布范围,合理地确定出α中心的位置与强度系数的变化范围.其次,以α中心法作为2-D地电结构的正演手段,采用遗传算法反演出各α中心的强度系数.在此基础上, 应用α中心法重新构建出地下介质电阻率的分布.通过对均匀半空间中赋存2-D地质异常体模型的反演试验, 获得了令人满意的结果.     

The Buffon's needle problem and the design of a geophysical survey

A logical procedure for designing a geophysical survey when sampling an area with a regular grid can be summarized as follows: model the expected anomaly, estimate the expected noise level, estimate the area of the anomaly above the noise level, choose the spacing, in both the x - and y -directions, of the measurement grid. This last step can be approached according to two main strategies: either when applying the sampling theorem to the shortest dimension of the anomaly or when using a coarser grid, leaving a more complete definition of the anomaly to a later fitting. When following this second option, it can be constructive to estimate the probability of intercepting a given anomaly with a specific segment of profile and a given profile spacing. This latter procedure is analysed by considering a rectangle approximating the plane projection of the anomaly shape and taking into account various ratios between the grid spacing and the rectangle sides. The formulae for estimating the probability of intersecting a given anomaly with a given segment of a given profile spacing are calculated. To demonstrate the accuracy of the results, a Monte-Carlo simulation on a synthetic magnetic map was performed, obtaining, for different ratios between the sides, segment length and profile interval, an agreement better than 0.1% with the analytical formulae.     

电阻率层析成像和激电测深对栾川铅锌矿的联合勘探

在栾川铅锌矿的物探工作中，采用了电阻率层析成像和激电测深两种方法，电阻率层析成像法采用温纳装置或偶极装置，电极数量和极距视情况而定，主要调查工作区内隐伏构造情况，激电测深采用对称四极测深法，最小AB／2距为3M，最大AB／2距为500M，主要研究激电测深参数的变化规律，对于含矿构造而言，激电参数呈异常高值反映，视极化率在15－40％，两种方法的联合应用，具有快速而准确的特点，在探矿工作中取得了良好的效果。     

露天煤矿地下水电磁探测与类型判别

为准确确定露天煤矿地下水分布与类型,根据不同赋存条件电磁差异,基于磁场强度与电阻率参数进行异常划分与类型判别。以新疆某露天煤矿为例,采用高精度磁法通过磁异常强度确定烧变岩边界,并采用高密度电法和瞬变电磁法通过电阻率圈定低阻异常区,结合异常响应差异分别推断地下水分布范围与赋存类型。地面钻探验证探测成果的准确性。结果表明:露天煤矿烧变岩水与砂岩裂隙水电磁响应差异明显,基于磁场强度与电阻率的双参数综合电磁法能准确圈定其分布范围,并判别其赋存类型。     

A gap in the North American Central Plains conductivity anomaly

The palaeotectonic models of the North American continent have been deeply influenced by the concept of a North American Central Plains conductivity anomaly (NACPCA) which was postulated as a result of large area, wide-spaced, array studies, using the Magnetic Deep Sounding (MDS) method.

A recent closely spaced magnetotelluric (MT) survey across this proposed anomaly near the Canadian-U.S. boundary by Jones and Savage (1986) failed to detect the NACPCA at the location suggested by the MDS. However, this survey did verify a conductivity anomaly discovered by Rankin and Kao (1979) and a new anomaly 75 km east of NACPCA. Our present study verifies this latter anomaly and quantitative results are presented. The striking isotropy in apparent resistivity does not prevent the interpretation of anomalous resistivity distributions. This is discussed in the text.

The failure of the MDS array studies to detect our anomaly is readily explainable by the wide station spacing; however, the failure of the MT survey to detect the NACPCA leads us to the conclusion that it may not be a continuous feature.     

金属大棚对周口台地电阻率干扰定量分析

周口地电阻率观测2015年出现异常变化,因异常期间该台场地存在较多干扰源。为正确评价该次异常,在电测深结果的基础上建立三维数值模型,采用有限元数值计算,定量评估该台测区周围金属蔬菜大棚等干扰源对地电阻率观测的影响量。结果表明:金属大棚可能是周口台本次地电阻率异常的主要影响因素。     

Resistivity and IP modelling of an anisotropic body located in an isotropic environment

A solution based on Tabarovskii's coupled pair of surface integral equations is given for the potential of a direct current flowing in an electrically anisotropic body and within the enclosing isotropic surroundings. The sources of the secondary potential exterior and interior to the body are fictitious surface charge distributions. The equations are solved numerically using point matching with pulse functions as subsectional basis functions. The model used in the applications is a long prism, excited by long line current electrodes aligned parallel to the strike. The strike length is set at a length sufficient to guarantee 2D behaviour of the model.   Comparisons of computation results indicate that for the models, electrode arrays and numerical procedures applied, the solutions based on fictitious surface sources converge faster and behave more regularly than those based on real surface charges. When compared with previously published integral equation solutions, the present solution seems to be relatively efficient, even in the case of purely isotropic models. The model experiments also showed that at moderate resistivity contrasts, the anomaly shapes are strongly dependent on the directions of the principal axes of the body resistivity. However, when the external resistivity is more than 100 times that of the geometric mean of the principal resistivities in the body, with the principal resistivities differing from each other by at most one order of magnitude, the contribution of the anisotropy to the anomaly diminishes as a result of electrical saturation.     

地震电磁辐射前兆异常特征和异常指标关系研究

根据收集的地震电磁类资料, 总结分析了地震电磁异常特征及震例的异常指标间的关系。据统计, 一般对M≥ 3.0地震, 电磁异常平均出现在震前10～45天, 异常的平均传播距离为100～600 km, 震级越大, 异常提前出现的时间越早, 异常传播的距离越大; 一定范围内, 震级和异常提前出现的时间、 震级和异常传播的距离存在简单的线性关系。地震发生在异常出现后两个月之内的概率达93.5%。有88%的地震发生在异常结束后或异常结束当天, 有98%的地震发生在异常结束后20天以内。     

Study of the Ability of the Magnetotelluric Method to Monitor Earthquakes

Most earthquakes on the Chinese mainland have occurred within the depth range of 10-25 km,which is usually called the seismogenic layer.According to the rule of penetration depth,the precursor of the electric anomaly,which is transferred from the seismogenic layer to the Earth's surface,should be very weak,but the ability to monitor earthquakes can be greatly improved if the field precursor effect is taken into consideration because of expansion of the electric anomaly area.The results of one-and two-dimensional modeling computations are consistent with the observed fact that the apparent resistivity decreases before earthquakes.This provides good evidence for our inference.The corresponding physical mechanism is that the conductivity of the anomaly area may be increased by the rock dilatation and fluid permeation due to the stress effect during the developing process of earthquakes.     

苍山地震与新沂台前兆异常

根据新沂台前兆资料，运用加卸载响应比、差分法对地磁场Z分量进行分析；运用滑动平均对短水准资料进行分析；运用速率法对视电阻率进行分析；结果表明：在苍山地震前，新沂台前兆资料具有良好的中、短期异常，并有进入临震的异常标志。