DISCOVERY OF DIRECT-SHIPPING IRON ORE BY GEOPHYSICAL METHODS IN THE CENTRAL PART OF THE LABRADOR TROUGH*

The deposits of the central sector of the Labrador Trough are magnetite-rich iron ores subjected to enrichment by the process of leaching. In spite of the fact that some ore types are porous and low in density, most of the ore zones have undergone a net increase in density brought about by enrichment. In the course of their formation, the magnetic characteristics of the rocks have changed and a great deal of the magnetite was altered to relatively non-magnetic iron oxides and hydrous iron oxides. Consequently, induced as well as remanent magnetization decreased, but the ratio of the latter with respect to the former increased substantially. All drift-covered areas underlain with iron-formation are potential ore producing areas. From measurements of the first two physical properties and determination of the thickness of overburden by means of a combined gravity-magnetic-seismic or resistivity survey, it is possible to detect and distinguish the iron formations and potential ore zones from the country rocks. In spite of the fact that the iron formations are a complex assemblage of rock types, each having different physical and chemical properties, a qualitative and semi-quantitative correlation between the results of the surveys and the geology as well as the structure of the area could be established. Consequently, in order to increase the chances of success, first a mathematical correlation of the amplitude of the gravity anomalies with the specific gravity, the iron content, the silica content, the porosity factors and the dimensions of the underlying orebodies, and secondly, a quantitative correlation of the ground magnetic intensity relative to the reference lithologie unit with the percentage of Fe₃ O₄ recovery and the volume magnetic susceptibility were done for many deposits. A few discoveries were made with this method. Finally, a fully quantitative interpretation of the geophysical data using a multimodeling analysis led to the discovery of additional direct-shipping iron ore deposits in the Schefferville mining district of Canada.     

PROSPECTING FOR SUBSURFACE WATER IN THE CENTRAL VOLCANIC HIGHLAND OF ARMENIA BY GEOPHYSICAL METHODS*

Work carried out in several volcanic areas in Armenia has led to a combination of geophysical methods for hydrogeological problems. Prospecting of interlava and sublava water flows is done by electrical profiling and vertical electrical sounding. When mapping the relief of sublava water-resisting rocks, the electrical sounding (ES) method is widely used. This is based on the evaluation of lava resistivity. The method discussed allows to determine the most probable value of this parameter, by using statistical methods.     

APPLICATION OF GEOPHYSICAL METHODS FOR STUDYING THE TECHNICAL STATUS OF EARTH DAMS*

Earth dams are constructed on a wide scale for hydrotechnical and reclamative purposes. However, the great water permeability of soils and their bad gradation are frequently responsible for the development of mechanical suffusion and other phenomena resulting in dam destruction. Geophysical observations allow under certain conditions to forecast the development of adverse factors. They thus are, as practice shows, an important tool to observe the technical status of dams. The difference in propagation velocities of elastic waves in the zone of aeration and full saturation determines the applicability of the reflected wave method for determining the depth of seepage flow in the dam supporting mass. This can be also achieved by electrical sounding and induced polarization methods since the resistivity and polarizability of air-dry and water-saturated grounds differ considerably. Maps of equal potentials measured on the dam slope reflect the configuration of the contours of water table of the water flow passing through the dam. This makes it possible to obtain data on the direction and intensity of flow as well as on the presence of heterogeneous grounds filling the dam supporting mass. Besides, regime observations conducted according to the SP method allow to forecast the development of suffusion and observe the redeposition of grounds. If the SP anomalies do not increase with time, the suffusion processes can be regarded as fading out. Enhanced amplitude and increasing areas of anomalies show the intensification of these processes. Combined application of some of the above mentioned methods allowed to obtain many practical results on a number of dams in Transcaucasia and Central Asia. The true position of the depression surface determined in the dam of one of the high-mountain water reservoirs, for instance, considerably differed from the designed one. The configuration of the seepage flow surface and its fluctuation caused by the rise and drop of the water level in the reservoir have been studied in detail on another dam situated on a plain. The sites of the most intensive outwash of fine material have also been located and the processes of soil redeposition in the dam supporting mass characterized. The data of the geophysical investigations were used to plan hydroinsulation and repair work.     

GEOPHYSICAL CATEGORIZATION OF THE PRECAMBRIAN IN SOUTH INDIA*

The ground follow-up of a magnetic and radiometric air survey had to cope with the usual dilemma to check a great number of anomalies within a short time. A limited aggregate of magnetic anomalies, expected to correspond to magnetite quartzites was statistically selected for ground identification by this method: the ratios length/width times amplitude were listed for all coherent contours and the calculation of the standard deviation per unit area resulted in different key-numbers for a new lithological “Salem Unit” and for the charnockitic or gneissic environment. The ground work thus directed and reduced by 85% yielded a substantial potential of iron ore. This was supported by by abundant determinations of the magnetic susceptibilities, confirming the sources of anomalies and revealing the amenabilities of iron ores for the magnetic separation process. The lines of truncation of anomalies were found to represent a system of local and regional faults and shear zones, which segregated the area into different tectonic blocks. These sutures have also provided the ways of intrusion for alkaline and basic magmae in the style of a “Rift” structure, housing several carbonatites and impregnations of metal sulphides. The airborne radiometrics obtained many uranium indications by gamma ray spectrometry. However, they led only to disseminated uranium-silicates, associated with syenites, granites and pegmatites, each emanating a characteristic photon energy spectrum. But in general the radiation of thorium prevails, marking northern Madras as a “Thorium Province”.     

CORRELATION METHODS OF TRANSFORMATION AND INTERPRETATION OF GEOPHYSICAL ANOMALIES*

The methods of anomaly transformations considered are based on a system of combined analysis of the geophysical field and a priori) information on the structure of a geological object. The methods involve calculation of a transformative polynomial (describing geophysical noise) which makes it possible to separate the residual field component related to the geological characteristic under study in a correlatively optimal way. The structure of the transformative polynomial is determined by the nature of the geophysical noise that is eliminated by the field transformation. Various correlation methods of anomaly transformations arise, depending on the structure of the transformative polynomial chosen. By way of example, the correlation method employed for separating the geophysical anomalies is shown to be highly effective in investigating the local geological structure.     

THE POSSIBILITIES OF GEOPHYSICAL METHODS APPLIED FOR INVESTIGATING THE IMPACT OF MAN ON THE GEOLOGICAL MEDIUM*

Geophysical methods can be applied to investigate the harmful effect of man's activities on the environment: the study of specific electrical resistivities and natural electric fields of filtrational origin makes it possible to control the penetration of exogenic pollutants into soil as well as desalinization and secondary salinization of soils; electrometric and seismometric methods allow to observe the groundwater level near water reservoirs thus evaluating the harmful effect of backing up natural groundwater flow; contrasting properties of bedrocks and rocks in a landslide body permit application of seismic and electrical prospecting methods. Observation of the changes in specific electrical resistivities with time on the slopes of quarries is an effective method of assessing slope stability and predicting landslide hazard. Mining activities, groundwater pumping, and oil extraction are the main causes of endogenic pollution of geological medium; surface and borehole geophysical methods make it possible to assess vertical and horizontal displacements of the interfaces between salty and fresh subterranean waters resulting from an extensive exploitation of water supply sources; activation of geodynamic processes associated with mining activities is determined from the data of ‘regime’ electrometric, seismometric, gravimetric, and inclinometric observations. Geophysical methods are applied widely for studying the intensification of physico-geological processes under the impact of man. A specific object of electrometric investigations is degradation of permafrost and intensification of karst processes in soluble rocks. The principal advantage of geophysical investigations lies in the possibility of creating high spatial and temporal density of observations permitting an extensive employment of statistical methods in the assessment of the impact of man on the geological medium.     

INFLUENCE OF A CONDUCTING SHIELD IN THE ONE-LOOP VERSION OF THE TRANSIENT PULSE INDUCTION METHOD*

The transient response of a conductive shell-shell model in the one-loop version was obtained analytically. The results indicate that four zones, namely early, late early, intermediate, and late zone can be identified in the total transient characteristic of the model. In case the measurements are carried out in the late early zone, a conductive target appears as a resistive one. It is suggested that the optimum time of measurement should be so selected as to fall in the intermediate zone.     

INTERPROFILE CORRELATION AND SELF-SETTING FILTRATION METHODS OF ANALYSIS OF GEOPHYSICAL DATA *

Application of statistical methods of analysis of geophysical data is often helpful for the detection of weak anomalies against a noisy background. The processing and analysis of areal geophysical data by two such methods are presented. The advantage of these methods are illustrated with the help of two field examples.     

松辽盆地北部深层综合物探方法先导性研究

首先对岩石的速度、密度、磁化率、电阻率等物性资料进行统计分析 ,寻找它们之间的内在联系 ,在此基础上综合各种有用信息建立统一的物理 -地质模型。在统一的物理 -地质模型之上 ,进行重、磁、电、震单一方法的反演 ,分析各种物探方法反演结果的相容性与相背性 ,重新修改模型并进行重、磁、电、震的联合反演 ,最终确定深部地层的地质属性     

PROBLEMS IN THE USE OF GEOPHYSICAL INFORMATION WHEN INTERPRETING THE DATA OF SPACECRAFT OBSERVATIONS*

Observations made from space have become almost a standard method of geologic investigation. However, interpretation of space photographs is not always unambiguous. Particularly, the nature and spatial location of various objects photographed from space remain obscure. It is especially difficult to investigate territories where basement rocks are overlain by sedimentary cover. From the example of investigations carried out in the central part of the Russian Platform it is shown how separate lineaments and ovoids reflect the peculiar features of geologic, hydrogeologic and geophysical structure of the region. It has been established that—within the sedimentary cover—they correspond usually to zones of increased fissuredness and are associated with low-amplitude tectonic dislocations. The authors concentrate on the elucidation of the transfer of deep geologic information to the surface. The sedimentary cover is regarded as a communication channel which makes it possible to employ the concepts of information theory for assessing its “transparency”. The contrast due to heterogeneities of the sedimentary cover in space photographs and, consequently, the degree of their manifestation in geologic and physical fields, depends on the water content of the rocks composing the section. Along with hydrogeologic characteristics, geo-electric parameters, such as specific electric resistivity, longitudinal conductivity and transverse resistivity, can serve as criteria of the degree of rock moistening. When the location of lineaments does not coincide with the plane axes of geophysical anomalies, an inclination of the fault plane can be supposed. The angle of inclination is characterized by the deviation of the lineament from the axis of the anomaly. The data resulting from the investigation of selected standard structures were used to develop a classification based on the degree of their expression in hydrodynamic, geoelectric, gravitational, and magnetic fields and depending on the spatial position and peculiarities of geologic arrangement. For geophysical investigations of unknown structures detected by space observations, this classification can be used as the basis for solving inverse problems by the methods of mathematical statistics.     

Pn波的形成及其衰减特征——对加拿大地盾的研究

尽管P_n波品质因子（P_nQ）对上地幔流变性质研究及核试验监测有着深刻含义,迄今它仍然是一个难以准确测定的地震参量.这是因为除了众所周知的影响所有短周期Q的干扰因素外,在所有主要的区域震相中,地震工作者对P_n波的传播模式及几何扩散函数G=d^-k中的扩散率指数k了解最少.不同的研究者任意地假定不同的常数k,因此常算出非常不同的P_nQ值（例如文献[1]、[2]）.实际上,理论研究已表明,k随频率而变,变化的形式取决于上地幔的速度结构.本文介绍用一推广的逆向双台法同时测定加拿大东部的P_nQ和k值.我们的结果表明:1.加拿大东部的P_n波是由一系列的干涉潜波叠加而形成;2.视P_nQ（散射和固有非弹性的综合效应）随频率而增加,在15Hz处接近2000.     

青藏高原地体划分的地球物理标志研究

基于青藏高原巨厚的地壳结构和复杂的地球物理场特征,提出依据地震活动与波场标志、岩石层结构与速度场标志、古地磁标志、位场标志、温度场标志、地质与构造标志作为进行青藏高原地体划分的原则．据此,由北向南将青藏高原及其相邻地带划分为７个地体,即柴达木地体、昆仑地体、可可西里－巴颜喀拉地体、羌塘地体、拉萨－冈底斯地体、喜马拉雅地体和恒河平原地体,它们的分布格局与特征对青藏高原的形成、演化和板块运动及动力机制的研究起着重要的作用．     

Pn波的形成及其衰减特征——对加拿大地盾的研究

尽管Pn波品质因子（PnQ）对上地幔流变性质研究及核试验监测有着深刻含义,迄今它仍然是一个难以准确测定的地震参量.这是因为除了众所周知的影响所有短周期Q的干扰因素外,在所有主要的区域震相中,地震工作者对Pn波的传播模式及几何扩散函数G=d-k中的扩散率指数k了解最少.不同的研究者任意地假定不同的常数k,因此常算出非常不同的PnQ值（例如文献[1]、[2]）.实际上,理论研究已表明,k随频率而变,变化的形式取决于上地幔的速度结构.本文介绍用一推广的逆向双台法同时测定加拿大东部的PnQ和k值.我们的结果表明:1.加拿大东部的Pn波是由一系列的干涉潜波叠加而形成;2.视PnQ（散射和固有非弹性的综合效应）随频率而增加,在15Hz处接近2000.     

探测大城市活断层的地球物理方法

简要地介绍了开展大城市地震活断层探测的意义及世界各国开展活断层探测的基本情况。城市及邻近地区地震活断层引起的直下型地震 ,将会给大城市造成巨大的地震灾害。准确地探测出大城市的活断层分布 ,采取有效措施 ,可大大降低地震时的损失。文中重点介绍了开展地震活断层探测的地球物理方法的类型和主要特点 ,在区域探查与初步鉴定阶段、详细探测与精确定位阶段、孕震构造探测阶段可能涉及到的各种地球物理探测方法、适用范围及所能解决的主要问题     

青藏高原地体划分的地球物理标志研究

基于青藏高原巨厚的地壳结构和复杂的地球物理场特征，提出依据地震活动与波场标志、岩石层结构与速度场标志、古地磁标志、位场标志、温度场标志、地质与构造标志作为进行青藏高原地体划分的原则．据此，由北向南将青藏高原及其相邻地带划分为７个地体，即柴达木地体、昆仑地体、可可西里－巴颜喀拉地体、羌塘地体、拉萨－冈底斯地体、喜马拉雅地体和恒河平原地体，它们的分布格局与特征对青藏高原的形成、演化和板块运动及动力机制的研究起着重要的作用．     

SIGNIFICANCE OF INFORMATION UNCERTAINTY IN SETTING UP THE INTERPRETATIVE PATTERNS OF GEOPHYSICAL DATA*

Geophysical interpretation consists of the identification of geological parameters carried out in the state of uncertainty. From this follows that the identificative model must correspond to the degree of uncertainty. As uncertainty measure of the surveyed material the informative distance between different interpretive models is introduced. The influence of the measurement noise and the accuracy of measurement devices as well as the number of measurement sets worked out simultaneously for this survey is analyzed. Numerical examples indicate that the material considered to be not interpretable from the point of view of a certain accurate model may be interpreted by the use of a less accurate model.     

GEOPHYSICAL METHODS USED IN SOLVING SOME GEOLOGICAL PROBLEMS ENCOUNTERED IN THE CONSTRUCTION OF THE TREBIšNJIGA WATER POWER PLANT (in YUGOSLAVIA) *

The design and construction of hydroelectric works, and more particularly of storage reservoirs and dams, in karst areas pose extremely varied and complex engineering, geological and hydrogeological problems, for the solution of which no widely applied techniques have yet been developed. In addition to conventional geological and hydrogeological methods for the study of karst phenomena, the application of more efficient methods of a primarily geophysical nature is called for. These methods, which provide the best means for solving the complex problems encountered in karst, are steadily gaining acceptance. The Miru?e storage reservoir and the Gran?arevo dam are the key structures in the Trebi?njica hydroelectric complex and are located in an area of typical Dinaric karst. Many of the engineering, geological and hydrogeological problems connected with their design and construction were solved with the aid of electrical geophysical prospecting methods (especially the resistivity method). The investigations provided data on the depth of karstification (the relief of the base level of karst erosion), the position of intensively karstified and tectonically broken zones, as well as on the general orientation of the process of karstification. All these data were helpful in providing a more accurate insight into the impermeability of the rocks that formed the storage reservoir (in relation to the proposed impounding level); they also enabled the consolidation structures (impervious curtains), aimed at preventing excessive losses due to seepage, to be designed more economically.     

中国地球物理学会1992年学术活动概况

中国地球物理学会1992年主持召开了5次国内学术会议。5次会议共收到845篇论文,出席会议的代表共780人次。本文分别介绍了各次会议的基本情况、主要内容及学术水平等。     

ON THE ELECTRICAL BEHAVIOUR OF A POLARIZABLE SAMPLE STUDIED BY THE NORMALIZED TIME-INTEGRAL AND BALLISTIC METHODS*

In this paper the experimental data obtained studying the decay of a sample of pisolitic bauxite both by a ballistic method and the normalized time-integral procedure are compared. This comparison allowed me to note two peculiarities. First, the apparent capacitance of the sample, as well as its normalized time-integral, seems to show a characteristic behaviour within the same interval of the charging time. Secondly, while the apparent capacitance plotted versus time (measured from the energization interruption instant) appears to assume values which seem to tend towards asymptotic value of the apparent capacitance, it is to be noted instead that in these circumstances the relative normalized time integral shows no tendency to reach any limiting value.     

UNITS IN GEOPHYSICAL PROSPECTING*

A system of geophysical units is proposed which is based on the International System of Units, using the meter, the kilogram, the second and the ampere as fundamental units. A historical introduction is given and the reasons for preferring the use of the International System of Units in exploration geophysics, are explained. At the meeting of the European Association of Exploration Geophysicists at Stockholm a resolution will be proposed recommending the use in exploration geophysics of the International System of Units.