摘要:

ABSTRACTThe possibilities for reconstructing seismic velocity distributions containing low‐velocity anomalies by iterative tomographic methods are examined studying numerical and analogue 2D model data. The geometrical conditions of the model series were designed to generalize the geometrical characteristics of a typical cross‐hole tomographic field case. Models with high (30%) and low (8%) velocity contrasts were realized. Traveltimes of 2D ultrasonic P‐waves, determined for a dense net of raypaths across each model, form the analogue data set. The numerical data consists of traveltimes calculated along straight raypaths. Additionally, a set of curved‐ray traveltimes was calculated for a smoothed version of the high‐contrast model.The Simultaneous Iterative Reconstruction Technique (SIRT) was chosen from the various tomographic inversion methods. The abilities of this standard procedure are studied using the low‐contrast model data. The investigations concentrate on the resolving power concerning geometry and velocity, and on the effects caused by erroneous data due to noise or a finite time precision. The grid spacing and the source and receiver patterns are modified. Smoothing and slowness constraints were tested. The inversion of high‐contrast analogue model data shows that curved raypaths have to be considered. Hence, a ray‐tracing algorithm using velocity gradients was developed, based on the grid structure of the tomographic inversion. This algorithm is included in the SIRT‐process and the improvements concerning anomaly localization, resolution and velocity reconstruction are demonstrated. Since curved‐ray tomography is time‐consuming compared with straight‐ray SIRT, it is necessary to consider the effects of grid spacing, ray density, slow

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02231.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTSeismic anisotropy is an unfamiliar concept to many geophysicists and the use of misleading and ambiguous terminology has made it more difficult to understand. I suggest here a consistent terminology in which simple expressions have specific meanings similar to their colloquial meanings. It is hoped that use of such language will help to make the increasing number of papers reporting seismic anisotropy more readily comprehensible to the non‐specialist. This not a manual of anisotropy, and it is not intended for theoreticians. It is a list of terms which may make anisotropy a little easier to understand for those more familiar with wave propagation in isotropic solid

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02232.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTResidual migration can correct for migration done with the wrong velocity. The initial migration can be done with a less accurate, fast time‐migration algorithm followed by a residual depth‐migration operator. This combines the speed and accuracy of different migration schemes.The residual depth‐migration equation can be obtained by an appropriate transformation of a typical finite‐difference, depth‐migration equation. Since the cheaper, initial time‐migration has already partially migrated the data, the residual depth‐migration has less work to do than if the depth‐migration were done in a single step. As a result, the overall process has increased the speed and reduced the dip limitations associated with typical depth‐migration operators. Results on a steeply dipping model demonstrate the validity and usefulness of residua

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02233.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTA method for transforming Normal Moveout corrected CMP‐gathers is proposed. The method is based upon the availability of a model of the CMP‐gather. However, the transformation can be performed with any degree of accuracy in the model. Ideally the employed model should be a synthesis of all availablea prioriinformation about the particular data set.Mathematically the transformation is performed as follows. The CMP‐gather is considered to be a matrix. This matrix is first decomposed into a set of submatrices of the same dimensions. Each submatrix consists of non‐zero elements or samples with the same relative amount of noise. By reducing each of these submatrices to a vector (a trace) we get a new set of traces. This set then represents the transformed CMP‐gather.The purpose of the transformation is to organize the CMP data in a form which makes it easier both to analyse the noise distribution and to take the necessary steps to improve the signal‐to‐noise ratio at the stacking stage. In principle the method incorporates the exploitation of multichannel recordings with the use of models. Several examples of transformed gathers and their applications to the improvement of real seismic d

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02234.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe techniques of downward continuation and imaging invented for seismic waves can be applied to other types of waves. We show how they can be applied to electromagnetic surveys conducted with Synthetic Aperture Radar (SAR). The algorithms used closely follow those used for seismic waves. Differences are induced by alternate wavelengths, wave velocities, distances between sources and reflectors, etc. We analyse in detail a survey carried out using a satellite; difficulties arise from because the orbit of the satellite cannot be approximated by a simple straight line if the spatial resolution of the survey is high. We determine appropriate techniques for the correction of the distortion induced by the latter and we delimit the resolution of the observed data, as seen from a satellite. Finally we show examples of the application of the technique of seismic migration to satellite data that were irradiated to Earth during the short but productive life of Seasat.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02235.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractMedia containing aligned cracks or ellipsoidal inclusions as well as media consisting of sequences of isotropic layers show transverse isotropy with respect to elastic wave propagation. However, the transversely isotropic media which are equivalent to media containing aligned inclusions do not necessarily have to be representable by sequences of stable isotropic layers. These transversely isotropic media can be modelled by such sequences if ‐ and only if ‐ several stability conditions are satisfied. Important parameters determining whether these conditions are satisfied are the aspect ratio of the inclusions and the material filling the inclusions, the‘fluid’. An analytical expression describing the range of aspect ratios for which the constraints are satisfied can be derived. This expression (which is a good approximation for several crack models) and numerical calculations show that media containing water‐filled inclusions can be represented by sequences of stable isotropic layers if the inclusions have aspect ratios less than 0.1. The limiting aspect ratio decreases for a decreasing ratio of the bulk modulus of the fluid to the shear modulus of the matrix material. Finally, media containing dry inclusions of any aspect ratio cannot be modelled by thin isotropic layering. These results depend only weakly on the crack density and on the matrix material. The representation of crack‐induced anisotropy by layer‐induced anisotropy can be used to classify crack‐induced anisotropy and might be useful in the separation of the cause of anisotropy and the determination of the natu

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02236.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe quality of Vibroseis survey data can be improved by continuously monitoring the vibrator's baseplate and reaction mass accelerations. Equipment failures can be detected as they occur, rather than relying on similarity trials at the beginning and end of the day's production. Equipment faults can then be corrected as they happen and thus would not have a detrimental effect on the quality of the survey data. Source efficiency can be optimized by monitoring the amount of harmonic distortion generated by the vibrator at different drive levels on the different surfaces which may be encountered during a survey. Phase problems introduced by poor coupling of the baseplate to the ground can also be identified and addressed in the field.Rapid analysis of vibrator signals is required if continuous monitoring is to be useful. Frequency‐time (f‐t) analyses of vibrator signals are often used in processing centres, but are slow and require a large storage capacity which makes the technique unsuitable for a field analysis system.The two methods proposed to analyse vibrator signals entail the use of hodograms and time‐varying notch filters. Hodograms provide a qualitative analysis of harmonic distortion and vibrator performance. A fast, time‐varying notch filter gives quantitative and qualitative information about the harmonic distortion present in the signal and can be used to identify problems with vibrator behaviour. Both the hodogram and fast, time‐varying notch filter methods can analyse the vibrator's reaction mass and baseplate accelerations as it progresses through its sweep and can present automatically interpreted results to the operator before moving to the next vibra

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1989.tb02237.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY