摘要:

AbstractTheoretical seismograms for an explosive source in a multilayered elastic medium are constructed by Fourier synthesis and plane wave superposition. The calculation scheme which builds up a reflection matrix layer by layer in the frequency and wave number domain allows the inclusion of attenuation and a choice of the level of internal multiples in each layer.Comparative calculations of theoretical seismograms for an elastic model and in the acoustic approximation, neglecting shear, show that the main differences arise at large offsets. The inclusion of shear waves leads to lower reflected P wave amplitudes at the end of the spread but only small amounts of converted phases.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1979.tb00972.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractSeismic velocity anisotropy is predicted for cracked rocks containing either a naturally‐occurring or an artificially‐induced preferred orientation of cracks. Methods developed for the study of velocity anisotropy in the uppermost mantle have been applied to thein situmeasurement of P‐wave velocity anisotropy in fractured Carboniferous Limestone in Northwest England. At three different sites, first‐arrival travel‐time data has been obtained using conventional refraction equipment and a weight‐drop source. Velocity anisotropy of 15–29% is identified with directions of maximum and minimum velocities that can be broadly related to previously mapped orientati

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1979.tb00973.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractThe quantitative estimate of the anomalies in the seismic signal associated with a realistic subsurface hydrocarbon bearing anticline is discussed. This estimate is derived by the calculation of synthetic seismograms for a dissipative media. The reflection amplitude anomaly associated with gas deposit are as much as three times the background level. However, oil deposits give rise to much weaker anomalies. A decrease of the reflection amplitude by about fifty percent occurs for horizons lying below the gas bearing zone, causing a shadow zone. Time sag anomaly is dependent chiefly on the thickness of the gas bearing zone. The appearance of the anomaly on the time section is modified by the structure, and flat reflectors may be seen over the structure owing to the mutual cancellation of the structural rise and the time sag. Flat spots corresponding to the horizontal fluid interfaces may be expected at the edges of the pool only. The presence of hydrocarbon bearing layers causes a diagnostic character in the composite reflection signal which is very clearly seen in the spectra, the cross correlation functions, and the cross‐spectra in profiles crossing the hydrocarbon bearing structures. These are potential diagnostic tools for locating hydrocarbon deposits, particularly when entrapped in thin layer

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1979.tb00974.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractSeismic exploration techniques which have been developed for oil prospecting contribute a valuable means for surveying coal measures. Since the object is to detect minor faults within the first 1500 m, rather than structural features at great depth, the new technique requires much higher resolution in the early part of the traditional seismic cross‐section.Higher resolution means broader bandwidth, which must be obtained by extending the high frequency end of the spectrum. This is achieved (a) by scaling down the explosive charge size and using single geophones instead of groups, and (b) by reducing the sampling interval in space and time. Noise which does not scale down includes static anomalies and ground‐roll. The consideration of statics, ground‐roll, and the high‐cut filtering effect of the near surface layers forces the use of deep shot holes and, where possible, deep detectors. This approach is confirmed by experiments and has been implemented on a regular basis in production.It is demonstrated that the present technique will clearly resolve faults with a vertical throw of about 5 m at 800

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1979.tb00975.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractTwo different methods for the construction of an approximation to bicubic splines for interpolating irregularly spaced two‐dimensional data are described. These are referred to as the least squares line (LSL) and linear segment (LINSEG) construction procedures.A quantitative test is devised for investigating the absolute accuracy and efficiency of the two spline interpolation procedures. The test involves (i) laying of artificial flight lines on the analytically known field of a model, (ii) interpolation of field values along the flight lines and their subtraction from the original field values to compute the residuals. This test is applied on fields due to four models (three prism models and one dyke model) placed at different depths below the flight lines, and for each case the error estimates (the mean error, the maximum error and the standard deviation) are tabulated.An analysis of the error estimates shows in all cases the LSL interpolation to be more accurate than the LINSEG, although the latter is about 50% faster in computer time. The relative accuracy and efficiency of the LSL interpolation is also tested against a recent method based on harmonization procedure, which shows the latter to be more precise, though much slower in spee

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1979.tb00976.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractCorrelations between longitudinal velocities and rock mechanic parameters such as fracture frequencies and “Rock Quality Designation” (RQD) values have been studied, based upon velocity data from various rock types and different geographical locations.The dispersion of values at different sites studied is on average ± 0.8 cracks per meter and for the RQD values ± 3.5%. Within sites the dispersion of individual values relative to the average for the site is ± 1.0 – 2.0 cracks per meter and ± 2 – 6% for the RQD values.The deviations are rather moderate, especially when considering the variation of rock type involved in the studies: amphibolite, granite, gneiss, meta‐anorthosite, pegmatite, porphyry, quartzite, and mylonite. The studies thus confirmed earlier assumptions that there is a strong correlation between longitudinal velocity and fracturing and that the velocities can be used to give rather accurate predictions of the quality of rock masses for construction purposes. The accuracy of the predictions increases if the velocity level of the more competent rock is taken into account.The correlation between velocity and fracturing is related to jointed but unweathered igneous and metamorphic rock and cannot be applied without introducing serious errors to a site where the rocks present a higher degree of alteration and weathering.Comparisons between rock permeability and longitudinal velocity proved that a more reliable general correlation is not likely to be found.By comparing the elastic moduliEdyn,μ, andkwithø,Vp/V8, andk/μ, indications have been obtained where the optimum rock conditions for a certain site are to be encountered. This has been verified by a similar comparison where the elastic moduli have been replaced by fracturing values.The value of the longitudinal velocity as a means to evaluate rock quality increases if the position of the velocity in the range of the Poisson's ratio has been established.The average relationships between longitudinal velocities and the corresponding elastic moduli proved to be:The values from each site differ from the average values with about ± 2 GPa forEdynand about ± 1 GPa for μ andk.It was confirmed that in igneous and metamorphic rocks longitudinal velocities ≤ 4000 m/s generally indicate rock masses where heavier tunnel support will be needed. This velocity limit corresponds to an average fracture frequency of about 10 cracks per meter and a RQD value of about 65 %. The prediction of the tunnel reinforcements needed at a particular site will, however, be improved if the general velocity level of the more compete

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1979.tb00977.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractThe reliable determination of the root‐mean‐square seismic velocity from multiple coverage seismic reflection data requires coherent horizontal reflections equally spaced in time. Observational error in therms‐velocity data produces significantly greater error in the instantaneous or interval velocity.A cubic spline approximation technique developed by Powell (1967) is designed to minimize the rapid velocity variation due to observational error and retain the significant variation in therms‐velocity data. The approximation algorithm requires the difference between the observed and approximatedrms‐velocity to be statistically random, and seeks to follow the “real” trends in therms‐velocity measurements and reject observational error. Provided the trends of therms‐velocity are reproduced, the instantaneous velocity derived from the spline approximation torms‐velocity data with significant scatter is more reliable than the interval velocity determined from the inaccuraterms‐velocity values using the Dix (1955) approximation. This interval velocity is also a more reliable indicator of variations of lithology.The utility of the spline approximation ofrms‐velocity is illustrated for a special common depth point (CDP) expanding reflection spread test recorded in Hardeman County, Texas by the Consortium for Continental Reflection Profiling (COCORP). In this wave test reflections are observed from several CDPs which are closely spaced. It is desired to combine the inaccurate observations from a collection of nearby CDPs. The approximation algorithm for therms‐velocity determination is applied to both therms‐velocity data for the individual CDPs and the collection ofrms‐veloci

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1979.tb00978.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractThe Minimum Entropy Deconvolution (MED) technique of Wiggins (1977) represents a breakthrough in deconvolution and will undoubtedly find wide application in many fields. MED does not require any phase assumptions about the disturbing function and seeks a deconvolved output which consists of the smallest number of large spikes consistent with the input data.The efficiency of MED is much improved when an exponential transformation is incorporated into the algorithm. This is particularly true when the input traces contain additive noise. In this case the noise suppression characteristics of MED are considerably enhanced by the transformation and the identification of smaller spikes is improved. This paper also presents a kurtosis criterion of simplicity rather than the varimax norm introduced by Wiggins. It appears that for a multiple trace input the kurtosis measure leads to improved results.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1979.tb00979.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

AbstractThe blind zone problem is examined with a view to provide a simple means of solving the problem in the general case of arbitrary number of refraction layers constituting the overburden to the masked layers. Considering the first arrival refraction interval corresponding to the masked layer which reduces to zero under blind zone conditions, a method to solve the problem is presented. Derivation of the critical distance expression is also included and convenient solutions to compute the thicknesses of the blind zone and its immediate overlayer are worked out. Based on test calculations on some known instances, the efficacy of the method is illustrated. A Fortran program for use when large numbers of refraction layers are involved under the overburden or when more than one masked layer is encountered is available from the authors.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1979.tb00980.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY