ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb01818.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractFor the correct interpretation of data gathered in the seismic prospecting of complex heterogeneous structures, elastic effects must often be taken into consideration. The use of the elastic wave equations to model the seismic response of an hypothesized geological structure is a valuable tool for relating observed seismic data to the earth's inhomogeneities and verify an interpretation.Several methods may be used to integrate numerically the partial differential equations describing elastic wave propagation. Pseudospectral (Fourier) methods represent the leading numerical integration technique. Their main advantage is high accuracy and suitability to vector and parallel computer architectures, while their main drawback is high computational cost. However, for a given accuracy, the required grid size with pseudospectral methods is smaller than that required by finite‐difference schemes, thus balancing the computational cost. We describe a two‐dimensional pseudospectral elastic model implemented on the vector multiprocessor IBM 3090 VF. The algorithm has been suitably adapted to fully exploit the computer architecture and thereby maximize the performance.The elastic model has been validated in a variety of problems in geophysics and, in particular, in the amplitude‐versus‐offset analysis which has proved to be an effective technique to extract additional information from the recorded (prestack) data. With proper conditioning and processing of seismic data, and separating amplitude variations due to changes in reflectivity from variations due to other effects, the resulting offset signatures have been successfully used, for instance, to distinguish true bright spots due to gas‐bearing sands, from false ones associated with lithological changes. To interpret the observed amplitude‐versus‐offset signatures, it is necessary to know the reflection coefficients as a function of angle and frequency for planar interfaces, as well as for other structures of geological interest.The modelling is first validated by computing the reflection coefficients for planar interfaces, and then used to analyse the reflection signatures of thin beds, corrugated interfaces and multilayers. Their implications, as well as impact on amplitude‐versus‐offset analysis, are discussed. We conclude that elastic modelling is an effective and valuable tool to further our understanding of the amplitude anomalies obser

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb01819.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA borehole sparker seismic source enclosed in a semi‐flexible tube has been developed to produce a short compressional seismic pulse with a frequency content in the range 250 Hz to 3.5 kHz with a peak power at 570 Hz. The pulse shape and frequency content are shown to be a function of the input power, the diameter of the spark chamber, the salinity of the electrolyte, the material of the spark chamber and the electrode configuration. When in a borehole, the source produces a vertically polarized shear wave but, being similar to a small explosive charge does not allow phase reversal as a means of identifying the shear wave in the received pulse train. The source is shown to be ideal for tomographic imaging surveys because of its repetitive nature, high frequency content and reliability. Very high resolution seismic reflection surveys are also shown to be possible under favourable circumstance

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb01820.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA new approximate method to calculate the space‐time acoustic wave motion generated by an impulsive point source in a horizontally layered configuration is presented. The configuration consists of a stack of fluid layers between two acoustic half‐spaces where the source and the receiver are located in the upper half‐space. A distorted‐wave Born approximation is introduced; the important feature of the method is the assumption of a background medium with vertical varying root‐mean‐square acoustic wave speed. A closed‐form expression for the scattered field in space and time as a function of the contrast parameters is deduced. The result agrees closely with rigorously calculated synthetic seismograms. In the inverse scheme the wave speed and mass density can be reconstructed within a single trace. Results of the inversion scheme applied to synthetic d

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb01821.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractUnderground gravity observations in deep coal mines using the conventional gravity meters Worden (type Master) and LaCoste‐Romberg (model D), both of which have been adapted to the fire damp regulations, can be accurate to ± 10 and ± 3 μgal, respectively. For underground determination of the vertical gradient of gravity the LaCoste‐Romberg meter is used together with a specially designed measuring tower. Using this euipment an accuracy in tower gradient observations of ± 30 E was obtained.To apply the equipment to precision gravity observations in underground situations an additional correction, i.e. a gallery correction, is needed. High accuracy in correction is achieved by a new method of three‐dimensional modelling. The gravity effect is computed for bodies with a surface approximated by triangular elements, which are generated from corner points of the body. The combination of gallery correction with tower gradient data leads to a new method forin situdensity determination. It offers the possibility of horizontal instead of vertical density profiling.To demonstrate the effectiveness of the developments in underground observations the localization of a pump room is presented. Microgravity and tower gradient observations were carried out to detect the cavity. The horizontal gradient was also calculated to give a more reliable

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb01822.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA particular methodology adapted to crystalline formations with a thin weathered zone was developed for a village hydrological project, in Benin. A combination of electrical profiles, Schlumberger and square arrays, was able to locate the most fractured zones in the basement. We present some results obtained from theoretical models as well as from field data.The suggested methodology uses both measurements of resistivity and anisotropy. Strong anisotropy and low resistivity indicate the most productive hydrogeological areas.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb01823.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb01824.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb01825.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY