摘要:

ABSTRACTSeismic reflection data always exhibit a progressive loss of high‐frequency energy with time. This effect is partly attributable to irreversible processes such as the conversion of elastic energy into heat (commonly known as absorption), and partly to reversible processes associated with interference between reflected waves arriving at different times. This paper looks only at reversible linear elastic effects at normal incidence and asks the following question: if there were no such absorption, would there still be a progressive loss of high‐frequency energy?Using normal incidence and a layered elastic earth model we prove the following results.1. The normal incidence response of a sequence of plane parallel elastic layers is non‐white.2. The pressure wave reflected by a layer that is thin compared with a wavelength is differentiated with respect to the incident wave.3. The transmission response of a thin layer is consequently low‐pass and the transmission response of a sequence containing many thin layers is very low‐pass.4. The well‐known effect of the transport of acoustic energy by peg‐leg multiples within thin layers is identical with this low‐pass transmission response.5. It follows that the high frequency energy is reflected back early in the seismogram.6. By comparison, very low‐frequencies are transmitted through the layered sequence easily and are reflected with difficulty. There is probably a lack of low‐frequency energy in the reflection seismogram, by comparison with the spectrum of the incident plane wave.It follows that any meaningful evaluation of frequency‐dependent absorption in seismic data cannot take place unless the frequency‐dependent linear elastic effects are t

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1986.tb00509.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThe acoustical impedance distribution of the substratum, or equivalently, the reflection coefficient sequence, is determined from VSP data. This nonlinear inverse problem is solved by a least‐squares method.As the wavelet is unknown, the impedance distribution and the Neumann boundary condition (which characterizes the excitation of the medium) are simultaneously identified.The inversion method is applied to synthetic and field VSP's; the result is satisfactory, even when strong noise corrupts the data, provided that a suitable constraint on the impedance distribution is introduced in order to ensure the stability of the inverse problem.The reliability of the inversion result in the case of field VSP, is confirmed. Some ways in which this result may be used are illustrated (calibration of the seismic surface data, multiple identification, prediction ahead of the bit

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1986.tb00510.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThe study of rock stresses and their changes is of great importance for safety in mines. To detect dangerous stress accumulations in coal mines an empirical method, Jahn's drilling test, is generally used. An experimental survey to solve the same problem by geophysical measurements was undertaken in a Hungarian coal mine. The basic idea was to determine the easily measurable seismic velocities instead of the more difficult to measure stresses in the rocks, since there is a monotonic relation between them.During the survey seismic transmission‐type measurements were carried out in the fore‐field of longwall faces between the top and tail roads. The seismic velocity data obtained were processed using an iterative algebraic reconstruction technique to determine the ‘velocity field’, i.e., the seismic velocity distribution, of the area covered by the ray paths. By periodically repeating the measurements in the same area, it was possible to follow the changes in the stress conditions caused by mining ope

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1986.tb00511.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTOne of the important properties of a series of primary reflection coefficients is its amplitude distribution. This paper examines the amplitude distribution of primary reflection coefficients generated from a number of block‐averaged well logs with block thicknesses corresponding to 1 ms (two‐way time). The distribution is always essentially symmetric, but has a sharper central peak and larger tails than a Gaussian distribution. Thus any attempt to estimate phase using the bi‐spectrum (third‐order spectrum) is unlikely to be successful, since the third‐order moment is almost identically zero. Complicated tri‐spectrum (fourth‐order spectrum) calculations are thus required. Minimum Entropy Deconvolution (MED) schemes should be able to exploit this form of non‐Gaussianity. However, both these methods assume a white reflectivity sequence; they would therefore mix up the contributions to the trace's spectral shape that are due to the wavelet and those that are due to non‐white reflectivity unless corrections are introduced.A mixture of two Laplace distributions provides a good fit to the empirical amplitude distributions. Such a mixture distribution fits nicely with sedimentological observations, namely that clear distinctions can be made betweensedimentary bedsandlithological unitsthat comprise one or more such beds with the same basic lithology, and that lithological units can be expected to display larger reflection coefficients at their boundaries than sedimentary beds. The geological processes that engender major lithological changes are not the same as those for truncation of bedding. Analyses of sub‐sequences of the reflection series are seen to support this idea. The variation of the mixing proportion parameter allows for scale and shape changes in different segments of the series, and hence provides a more flexible description of the series than the generalized Gaussian distribution which is shown to also provide a good fit to the series.Both the mixture of two Laplace distributions and the generalized Gaussian distribution can be expressed as scale mixtures of the ordinary Gaussian distribution. This result provides a link with the ordinary Gaussian distribution which might have been expected to be the distribution of a natural series such as reflection coefficients. It is also important in the consideration of the solution of MED‐type methods. It is shown that real (coloured) primary reflection series do not seem to be obtainable as the deconvolution result from MED‐typ

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1986.tb00512.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTA new underground geoelectric method is presented for the determination of small tectonic disturbances and barrenings in coal seams. The distribution of the apparent resistivity can be mapped from the measured apparent resistance data by using a recently developed geoelectric imaging method. The applicability of the methods are proved by in situ measurements and by a model experiment.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1986.tb00513.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTThe telluric method of geophysical investigation is re‐evaluated and several improvements are suggested. Besides the traditional Jacobian, several new parameters that bear information on the structure under investigation are proposed as sounding tools. These parameters are calculated for several two‐dimensional models and plotted as pseudo‐sections.The model results, calculated over a wide frequency band (10‐4‐10‐2Hz) suggest that these new parameters can give very detailed information about the subsurface structures.By way of example, we present experimental results obtained along two profiles across familiar structures. The interpretation of the results leads to two‐dimensional models of the subsurface that are in good agreement with known geophysical and geological features of these

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1986.tb00514.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTA new integral‐equation program for calculation of the E‐polarization response allows modeling of VLF for plane wave or line source input, e.g., magnetotellurics and Turam responses. The anomalous conducting body is modeled by a number of square cells, each of individual size and resistivity, and with arbitrary position in the host medium. This provides a high degree of flexibility and allows for simulation of rather complex conductivity structures. The computation time has been drastically reduced by using techniques such as digital filtering and fast Fourier transformation.The interpretation of a measured Turam profile and the influence of galvanically channelled currents in the conducting body is discus

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1986.tb00515.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

摘要:

ABSTRACTStatistical multivariate methods for the integrated processing of airborne geophysical data were tested. The data consisted of magnetic, electromagnetic and gamma radiation measurements, to which cluster analysis, principal components analysis and discriminant analysis were applied. Also, auxiliary variables were derived from the original ones and their value was tested. Although the frequency distributions of the data do not favour statistical analysis, the practical results are acceptable. Principal component analyses show geological and technical aspects that are difficult to obtain from the original observations. In cluster analyses, the sources of measured fields control the grouping of variables. Discriminant analysis was applied to the automatic identification of rocks by geophysical data. The rocks investigated are metasediments and metavolcanics, some magnetic and others conductive. When all available geophysical data were included, correct identifications were made in more than 60% of cases. In particular, gamma ray observations were found to improve the discrimination of non‐magnetic and non‐conductive rocks. The geophysical similarity of rocks studied by cluster analysis depends on electrical and magnetic properties as well as on their origin; the content of radioactive elements in turn is related to the ori

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1986.tb00516.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1986.tb00517.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1986.tb00518.x

出版商:Blackwell Publishing Ltd    

年代:1986

数据来源: WILEY