摘要:

ABSTRACTLarge horizontal offset and large vertical offset approximations in the modified Cagniard method for computing synthetic seismograms in a horizontally layered model of the earth are derived. They apply to each of the generalized‐ray constituents into which the seismic wave motion is decomposed. For the results applying to large horizontal offset, which are known in the seismological literature, a simplified derivation is given. The results for large vertical offset, which are of particular interest to vertical seismic profiling, are new. The use of the large‐offset approximations, both horizontal and vertical, leads to a considerable reduction in computation time for synthetic seismograms compared with the full three‐dimensional version of the modified Cagniard m

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02174.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTRay theories are a class of methods often chosen to compute synthetic seismograms due to their efficiency and ability to deal with complex, three‐dimensional inhomogeneous media. To deal with the large number of rays needed to compute synthetic seismograms, a ray generation algorithm is given which is capable of generating a numerical code describing each ray. The code describes a subset of all possible rays by considering only pre‐critical reflections. In a horizontally plane‐layered medium the generation of rays and computation of amplitudes and traveltimes can be efficiently accomplished by grouping the rays into reflection order and dynamic analogue groups. Expressions summing all unconverted rays and rays with a single mode conversion are given for source and receiver located at arbitrary positions within the medium. Examples of zero‐offset synthetic VSPs obtained by this method ar

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02175.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTUsing synthetic data, it is demonstrated that the amplitude spectra of post‐critical plane‐wave components are stable and equal to the amplitude spectrum of the input wavelet (critical reflection theorem). Our analysis and physical explanation of the theorem are based only on amplitude versus offset arguments. The stability of the spectra in the post‐critical region is directly related to a high amplitude post‐critical reflection that dominates the trace in the slant‐stack domain. The validity of the theorem for both the acoustic and elastic cases, its assumptions and limitations, are also examined with emphasis on applications for processing seismic reflection data. Based on the theorem, a deterministic procedure is developed (assuming minimum‐phase properties) for wavelet estimation and subsequent deconvolution. We call this method Post‐critical Deconvolution, which emphasizes reliance on post‐critical reflection data. The performance of the method is shown with real data and the results are compared to those obtained with conventional deconvolu

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02176.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTA plasma gun source has been successfully used to obtain sub‐bottom profiles. The profiles show better penetration than with a 3‐5 kHz source and more resolution than with an air gun. The plasma gun source is compact, self‐contained, and requires no complex auxiliary equipment. The device was deployed from large research vessels and a small boat. The plasma gun produces sufficient acoustic energy and with its characteristically short pulse length and broad bandwidth it is an attractive sub‐bottom profiler, especially in shallow fresh or sal

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02177.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTElectromagnetic scale modelling for most conductors encountered in prospecting requires models of conductivity in the range 10‐1000 S/m. A polyester composite filled with aluminium fillers is used to design the required materials. The fillers are chosen with a high aspect ratio to achieve the desired conductivities without altering mechanical properties. The design process, consisting of a chemical reaction, is simple, repeatable and easy to realize in a geophysical laboratory.The conductivity of the designed material is estimated by an inductive method based upon the variation of the quality factor of a coil in which the samples are inserted. This method is much less influenced by the distribution and orientation of fillers in the resin matrix than a galvanic method.The conductivities obtained fill the major part of the interval 10‐1000 S/m without requiring unreasonable mixing ratios. The homogeneity of the material was tested and the anisotropy was low at high conductivities. The electrical properties do not vary with time in free air or when immersed in salt water.We propose standard curves giving conductivities of composites filled with aluminium fibres and flakes. A large cylindrical model of fixed resin‐to‐filler ratio was made to show the capability of the technique to produce models of useful size. Its measured conductivity agrees well with the expected conductivity from the established standard curves. A scale modelling experiment with the horizontal loop technique was carried out over a horizontal thin sheet made of the material, and again the conductivity obtained from the results agrees reasonably well with the expected value.The noted difference between the inductively and galvanically measured conductivities of the proposed material presently restricts the use of the technique to EM scale experiments for which the host rock is considered very re

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02178.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTIn the quantitative data interpretation for HLEM induction prospecting, a vertical half‐plane model in an insulating medium is widely employed. For this assumption to be valid, the steeply dipping massive sulphide dykes must have large strike lengths and depth extents, but small thickness.We report investigations, using the laboratory scale‐modelling method, on the response variation of large vertical conductors as the thickness is varied. We conclude that a steeply dipping large dyke can be approximated by a half‐plane model only if its thickness is less than half the skin depth. An inductively thick conductor produces larger amplitudes and relatively higher quadrature compared to a thin conductor, even if both have the same induction n

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02179.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe cross‐correlation technique for increasing the anomaly‐to‐noise ratio is applied to the interpretation of resistivity profiles.To verify the method theoretically, resistivity profiles above a body having the shape of a parallelepiped were simulated with correlated and uncorrelated noise. Seven different electrode geometries are discussed. As a practical test, we considered profiles of a geoelectrical survey to locate tombs at a site of archaeological interest.When the shape and dimensions of the anomaly can be foreseen, the cross‐correlation method can be applied and it gives a substantial improvement in the signal‐to‐

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02180.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY