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Proceedings of the IEEE

Issue 2 • Date Feb. 1989

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Displaying Results 1 - 9 of 9
  • Calibration case study of natural source electromagnetic array data recorded over a well in Oregon

    Page(s): 334 - 337
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (315 KB)  

    Experimental results are described for a data acquisition method originally developed by F.X. Bostick (1986) that measures the electric field continuously along a profile. A three step sequence is used in the automated interpretation of electromagnetic (EM) array data. Following some preliminary steps in which an initial model is constructed, each interpretive refinement of the model consists of (1) forward computation of the array response and the numerically evaluated Jacobian, or matrix of first partial derivatives of the response with respect to model parameters; (2) data processing for static removal making use of the relatively short spatial wavelength behavior of the static effect in an analogous fashion to Bostick's method and tuning the data to the desired exploration objective; and (3) a linear correction is made to the model parameters. This three-step sequence is repeated, possibly varying the model definition or processing as interpretive judgement dictates, until the quality of fit of synthetic and field data is less than the typical data uncertainty. These data can be processed to remove shallow effects while maintaining the deep penetration characteristics of the natural electromagnetic (EM) field. Processing and modeling techniques for such data are demonstrated on both synthetic and field data. Very good agreement with actual borehole data is demonstrated.<> View full abstract»

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  • Petroleum exploration using controlled-source electromagnetic methods

    Page(s): 338 - 362
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2546 KB)  

    A tutorial overview is presented of controlled-source electromagnetic (CSEM) geophysical methods as they are applied in petroleum exploration. CSEM methods utilize man-made sources to investigate the variation of electrical conductivity in the Earth, typically in the depth range of hundreds of meters to several kilometers. Over the frequency range used in CSEM (typically 0.1 Hz to 10 KHz), displacement currents can usually be ignored so the EM fields are diffusive in nature. The basic theory for CSEM methods and some simple approximate solutions which provide insight into the behavior of quasistatic EM fields are given. An overview of data inversion methods is also given, as well as examples of typical applications. It is argued that CSEM methods offer a number of advantages compared to natural-source electromagnetic (NSEM) methods such as magnetotellurics (MT). These include relative insensitivity to regional effects and small near-surface inhomogeneities which adversely affect MT, and the ability to utilize different source types and orientations to improve resolution. NSEM methods can, however, probe deeper conductivity structures than CSEM methods.<> View full abstract»

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  • The pick operating system: A practical guide [Book Review]

    Page(s): 363
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    Freely Available from IEEE
  • Unix system programming [Book Review]

    Page(s): 363 - 364
    Save to Project icon | Request Permissions | PDF file iconPDF (256 KB)  
    Freely Available from IEEE
  • The computer and the mind - An introduction to cognitive science [Book Review]

    Page(s): 364
    Save to Project icon | Request Permissions | PDF file iconPDF (141 KB)  
    Freely Available from IEEE
  • Three-dimensional magnetotelluric modelling and inversion

    Page(s): 318 - 333
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1324 KB)  

    An outline is presented of the nature of the problem of making quantitative use of magnetotelluric data for geophysical exploration. The authors discuss their view and experiences in dealing with the modeling and interpretation problems. The key step is considered to be the modeling step, which is that of predicting the response of a given distribution of electrical properties within the Earth to the electromagnetic excitation. Decisions about modeling methods also consider the interpretation process that will be used. The electromagnetic problem is three-dimensional which makes it necessary to use numerical methods. Difference equations are used and the authors investigate spacing requirements and two approaches to solving the system of equations: a relaxation method and a direct solution method. The relaxation method is found to be much faster, but it is very difficult to eliminate all the errors in the resulting solution. The interpretation problem is approached using a scheme that minimizes the joint probability of fitting the observed data and adhering to an a priori conductivity model. A relaxation procedure is used to solve this problem and the behavior of the procedure is examined associated with the application of magnetotellurics to exploration problems. Some of the problems in signal resolution that arise when interpreting the data are noted View full abstract»

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  • Token-ring local-area networks and their performance

    Page(s): 238 - 256
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    The major technical concepts underlying token-ring technology are examined, and performance issues arising in the design of such local-area networks (LANs) are detailed. Following a survey of analytical queuing models to describe the basic token-ring operation, three topics are discussed in detail: (1) the IEEE 802.5 token ring and its performance; (2) the ANSI fiber distributed data interface (FDDI) token ring and its performance; and (3) architecture and performance issues arising in the interconnection of token-ring networks with regard to the various components of a multiring architecture demonstrating which congestion-control problems can arise in such a network and how they can be overcome. The author concludes with a number of open questions concerning the understanding of the quantitative behavior of token-ring-based LANs View full abstract»

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  • A tutorial on hidden Markov models and selected applications in speech recognition

    Page(s): 257 - 286
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2272 KB)  

    This tutorial provides an overview of the basic theory of hidden Markov models (HMMs) as originated by L.E. Baum and T. Petrie (1966) and gives practical details on methods of implementation of the theory along with a description of selected applications of the theory to distinct problems in speech recognition. Results from a number of original sources are combined to provide a single source of acquiring the background required to pursue further this area of research. The author first reviews the theory of discrete Markov chains and shows how the concept of hidden states, where the observation is a probabilistic function of the state, can be used effectively. The theory is illustrated with two simple examples, namely coin-tossing, and the classic balls-in-urns system. Three fundamental problems of HMMs are noted and several practical techniques for solving these problems are given. The various types of HMMs that have been studied, including ergodic as well as left-right models, are described View full abstract»

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  • Magnetotelluric exploration for hydrocarbons

    Page(s): 287 - 317
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    The magnetotelluric (MT) method utilizes naturally occurring low-frequency electromagnetic energy to determine the electrical resistivity of the Earth's subsurface. The resistivity distribution of the subsurface is then interpreted in terms of rock type and geologic structure. MT-derived resistivity data are usually integrated with other geophysical results, and with surface and subsurface geologic data if available, to arrive at an interpretation, The basic relationships that govern the application of MT to hydrocarbon exploration are reviewed along with the electrical properties of rocks, MT energy sources, and briefly, MT theory. Data acquisition and processing and field procedures are covered. It is shown how by measuring the complete vector electromagnetic field an impedance tensor may be calculated, which, when evaluated as a function of frequency, samples the electrical characteristics of the Earth as a function of depth through the skin-depth relationship. A discussion of current usage is included, concluding with representative case studies which illustrate the application of MT to hydrocarbon-exploration problems View full abstract»

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H. Joel Trussell
North Carolina State University