摘要:

ABSTRACTTwo particular sources of distortion, which may be encountered when applying tomographic imaging techniques to crosshole seismic data, have been investigated.Errors in survey locations of the shots and receivers can produce significant distortions in the images obtained. A simple method for solving simultaneously for the velocity field and shot and receiver location errors is presented and applied to synthetic and real data.Reflection and refraction of rays at velocity interfaces may produce poor density and angular coverage of the rays within the region of interest. It is shown that the effect of the velocity field on the ray coverage can significantly affect the resolution in the velocity image, even if ray bending is taken into account. One consequence of this effect is that, in some cases, little improvement in image quality is achieved by using curvi‐ray rather than straight‐ray inversion techniques, despite the occurrence of pronounced ray bend

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02160.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe estimation of velocity and depth is an important stage in seismic data processing and interpretation. We present a method for velocity‐depth model estimation from unstacked data. This method is formulated as an iterative algorithm producing a model which maximizes some measure of coherency computed along traveltimes generated by tracing rays through the model. In the model the interfaces are represented as cubic splines and it is assumed that the velocity in each layer is constant. The inversion includes the determination of the velocities in all the layers and the location of the spline knots.The process input consists of unstacked seismic data and an initial velocity‐depth model. This model is often based on nearby well information and an interpretation of the stacked section.Inversion is performed iteratively layer after layer; during each iteration synthetic travel‐time curves are calculated for the interface under consideration. A functional characterizing the main correlation properties of the wavefield is then formed along the synthetic arrival times. It is assumed that the functional reaches a maximum value when the synthetic arrival time curves match the arrival times of the events on the field gathers. The maximum value of the functional is obtained by an effective algorithm of non‐linear programming.The present inversion algorithm has the advantages that event picking on the unstacked data is not required and is not based on curve fitting of hyperbolic approximations of the arrival times. The method has been successfully applied to both synthetic and fie

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02161.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe detection and resolution of a thin layer closely situated above a high‐impedance basement are predominantly determined by both the frequency content of the incident seismic wavelet and the existence of the nearby high‐impedance bedrock.The separation of the thin layer and the basement arrivals is investigated depending on the low‐frequency content of the wavelet. The high‐frequency content of the wavelet is kept constant. The initial wavelet spectrum with low frequencies has a rectangular shape. All wavelets used have zero‐phase characteristics. Numerical and analogue seismic modelling techniques are used. The study is based on the geology of the Pachangchi Sandstone in West Taiwan.Firstly the resolution of a thin layer between two half‐spaces is examined by applying the Ricker and De Voogd‐Den Rooijen criteria. The lack of low‐frequency components of the incident seismic wavelet reduces the shortest true two‐way traveltime by about 20%. In addition, low‐frequency components of the wavelet diminish the deviation between true and apparent two‐way traveltime by about 65% for layer thicknesses in the transition from a thick to a thin layer.The second step deals with the influence of a high‐impedance basement just below a thin layer on the detection and resolution of that thin layer. Reflected signal energies and apparent two‐way traveltimes are considered. The reflected signal energy depends on the low‐frequency content of the incident wavelet, the layer's thickness and the distance between the basement and the layer. This applies only to layers with thicknesses less than or equal to one‐third of the mean wavelength in the layer, and a distance to basement in the range of one to one‐half of the mean wavelength in the rock material between layer and basement.The minimum thin‐layer thickness resolvable decreases with increasing distance to the basement; i.e. for a layer thickness of one‐third of the mean wavelength in the layer the relative error of the two‐way traveltime increases from 5% to 30%, if the distance is reduced from one to one‐half of the mean wavelength in the material between the basement and the thin layer.Finally, a combination of vertical seismic profiling and downward‐continuation techniques is presented as a preprocessing procedure to prepare realistic data for t

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02162.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe statistical treatment of deep seismic reflections from several different geological units has resulted in different reflectivity histograms. Reflectivity in old and cold shields differs significantly from that in younger, warmer areas. In the shields reflectivity is generally poor and concentrated in the upper crust whilst Caledonian and Variscan areas show strong reflecting lamellae in their lower crust. Also the length of the reflecting elements varies with age and heat flow. The lower crust in young areas is a zone of a strong viscosity minimum as derived from model studies with a temperature dependent rheology. The subhorizontal reflecting lamellae in the lower crust are considered to have been created in a large‐scale high‐temperature, low‐viscosity ordering process whose remnants are still preserved today. Local‐scale differences of reflectivity histograms define certain subprovinces which can be distinguished by their specific p

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02163.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTStatistical deconvolution, as it is usually applied on a routine basis, designs an operator from the trace autocorrelation to compress the wavelet which is convolved with the reflectivity sequence. Under the assumption of a white reflectivity sequence (and a minimum‐delay wavelet) this simple approach is valid. However, if the reflectivity is distinctly non‐white, then the deconvolution will confuse the contributions to the trace spectral shape of the wavelet and reflectivity.Given logs from a nearby well, a simple two‐parameter model may be used to describe the power spectral shape of the reflection coefficients derived from the broadband synthetic. This modelling is attractive in that structure in the smoothed spectrum which is consistent with random effects is not built into the model. The two parameters are used to compute simple inverse‐ and forward‐correcting filters, which can be applied before and after the design and implementation of the standard predictive deconvolution operators. For whitening deconvolution, application of the inverse filter prior to deconvolution is unnecessary, provided the minimum‐delay version of the forward filter is used.Application of the technique to seismic data shows the correction procedure to be fast and cheap and case histories display subtle, but important, differences between the conventionally deconvolved sections and those produced by incorporating the correction procedure into the processing sequence. It is concluded that, even with a moderate amount of non‐whiteness, the corrected section can show appreciably better resolution than the conventionally proc

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02164.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTAn in‐seam fan shoot was conducted in 1981 over a 300 m × 500 m panel of the Schwalbach seam at Ensdorf by a team from Prakla‐Seismos AG of Hannover under contract to Saarbergwerke AG, Saarbrücken. The object was to study SH‐mode propagation in the coal seam waveguide. The high quality dataset retrieved provides a general and valuable test bed with which to compare the performance of in‐seam seismic velocity analysers.Five different dispersion analysers are demonstrated using the Schwalbach data. They are all based on thea prioriassumption of coal seam homogeneity and isotropy. Space or time windows limit the resolution of the Fourier moving‐window analyser, the migration based phase‐velocity analyser, and the double Fourier transformer. The other two analysers, the maximum entropy moving‐window analyser and the phase‐moveout analyser, achieve noise‐limited super‐resolution by predicting the probable behaviour of the wavefield outside the window.The coal seam's characteristics conform to those predicted for a simplified model based on proposals by Elsen, Rüter and Schott of Westfälische Berggewerkschaftskasse, Bochum. The slight discrepancy between theoretical and actual dispersion characteristics could be reduced by increasing the model's complexity. However, there would be no material gain without testing the validity of the signal processing assumptions of seam i

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02165.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe WKB‐method is used for the derivation of both the complex dispersion relation and displacement functions for Love channel‐waves that propagate in a coal seam of varying thickness. The constantQ‐model is used to describe the anelastic friction. With numerical solutions of the absorption‐dispersion relation, the influence of thickness changes on the phase velocity and absorption coefficient of Love seam‐waves is analysed at various frequencies. It is shown that the changes in the seam thickness can be optimally detected around the average Airy‐phase frequency. An equivalence is pointed out between the wave guide structures: homogeneous with varying seam thickness and horizontally inhomogeneous with constant sea

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1988.tb02166.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY