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- Volume 39, Issue 3, 1991
Geophysical Prospecting - Volume 39, Issue 3, 1991
Volume 39, Issue 3, 1991
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ADAPTIVE PICKING OF REFRACTED FIRST ARRIVALS1
More LessAbstractStatic correction computations require knowledge of the refracted traveltimes. Zero‐phase wavelet sources cannot be picked reliably when incoherent picking techniques are used.
Assuming a complex convolutional model for Vibroseis, a coherent picking technique based on the matched filter is described. In order to match the filter to the first arrival wavelet an adaptive algorithm is used. This allows the filter to change both with shot and offset so that all the properties of matched filtering such as improvement of S/N and resolution can be exploited. Incoherent picking is used before coherent picking to improve the convergence of adaptive picking.
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PRESTACK INVERSION OF GROUP‐FILTERED SEISMIC DATA1
By JAN HELGESENAbstractThree methods for least‐squares inversion of receiver array‐filtered seismic data are investigated: (1) point receiver inversion where array effects are neglected; (2) preprocessing of the data with an inverse array filter, followed by point receiver inversion; (3) array inversion, where the array effects are included in the forward modelling.
The methods are tested on synthetic data generated using the acoustic wave equation and a horizontally stratified earth model. It is assumed that the group length and the group interval are identical. For arrays that are shorter than the minimum wavelength of the emitted wavefield, and when the data are appropriately muted, point receiver inversion (first method) gives satisfactory results. For longer arrays, array inversion (third method) should be used.
The failure of the inverse array filter (second method) is due to aliasing problems in the data.
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EXTENSIVE‐DILATANCY ANISOTROPY: RELATIVE INFORMATION IN VSPs AND REFLECTION SURVEYS1
Authors GARETH S. YARDLEY and STUART CRAMPINAbstractShear‐waves have complicated interactions with the free surface, particularly in the presence of low‐velocity surface layers, topographic irregularities, and the expected near‐surface crack and stress anomalies. Consequently, it has been suggested that shear‐waves should be recorded subsurface in vertical seismic profiles (VSPs), in order to extract accurate information about the in situ crack and stress geometry contained in shear‐wave splitting. This paper compares the information in synthetic shear‐waves in reflection gathers and VSPs, in order to assess the relative merits of the two techniques for investigating shear‐wave splitting. Synthetic seismograms demonstrate that in the presence of even very simple surface layers, shear‐waves recorded in reflection surveys at the surface have polarizations which may not indicate crack and stress geometry at depth. In contrast, shear‐waves recorded in VSPs are relatively unaffected by surface layers and near‐surface stress and crack anomalies, and the behaviour of shear‐wave splitting is dominated by the structure of the rock mass in the vicinity of subsurface geophones. Matrix rotations of multicomponent‐multisource shear‐wave reflection data to extract the information contained in the split shear‐waves, are found to be directly meaningful only in situations where crack orientations do not change with depth.
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COMPARISON OF SIGNAL PROCESSING TECHNIQUES FOR ESTIMATING THE EFFECTS OF ANISOTROPY1
Authors COLIN MACBETH and STUART CRAMPINAbstractThree‐component recordings of shear‐waves in exploration surveys provide an opportunity to measure crustal anisotropy, which may be important in estimating the geometrical and physical parameters of reservoir rocks. VSPs are particularly important for this purpose as they are less subject to the complex interactions of the shear wavefield with the free surface. The first stage in characterizing the subsurface anisotropy requires that the distinctive phenomenon of shear‐wave splitting must be examined for every arrival at each geo‐phone. This effect may be defined by two parameters: the polarization of the leading shear‐wave and the time‐delay between corresponding split shear‐waves. A variety of techniques have been designed to estimate these parameters of shear‐wave splitting. Here, we classify the published techniques into four main categories and review their properties. Representative procedures from each group are applied to a common synthetic data set contaminated with signal‐generated noise. The results allow some general statements to be made about the utility of these methods for processing shear‐waves in VSP data.
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CONVERSION POINTS AND TRAVELTIMES OF CONVERTED WAVES IN PARALLEL DIPPING LAYERS1
Authors GISA TESSMER and ALFRED BEHLEAbstractFor converted waves, stacking as well as AVO analysis requires a true common reflection point gather which, in this case, is also a common conversion point (CCP) gather. The coordinates of the conversion points for PS or SP waves, in a single homogeneous layer can be calculated exactly as a function of the offset, the reflector depth and the ratio vp/vs. An approximation of the conversion point on a dipping interface as well as for a stack of parallel dipping layers is given. Numerical tests show that the approximation can be used for offsets smaller than the depth of the reflector under consideration.
The traveltime of converted waves in horizontal layers can be expanded into a power series. For small offsets a two‐term truncation of the series yields a good approximation. This approximation can also be used in the case of dipping reflectors if a correction is applied to the traveltimes. This correction can be calculated from the approximated conversion point coordinates.
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AUTOMATIC INTERPRETATION OF GRAVITY GRADIOMETRIC DATA IN TWO DIMENSIONS: VERTICAL GRADIENT1
Authors E. E. KLINGELÉ, I. MARSON and H.‐G. KAHLEAbstractThe magnetic and gravity field produced by a given homogeneous source are related through Poisson's equation. Starting from this consideration, it is shown that some 2D interpretation tools, widely applied in the analysis of aeromagnetic data, can also be used for the interpretation of gravity gradiometric data (vertical gradient). This paper deals specifically with the Werner deconvolution, analytic signal and Euler's equation methods. After a short outline of the mathematical development, synthesized examples have been used to discuss the efficiency and limits of these interpretation methods. These tools could be applied directly to airborne gravity gradiometric data as well as ground gravity surveys after transformation of the Bouguer anomalies into vertical gradient anomalies. An example is given of the application of the Werner deconvolution and Euler's equation methods to a microgravity survey.
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GRAVITY GRADIENT TENSORS DUE TO A POLYHEDRON WITH POLYGONAL FACETS1
More LessAbstractUsing the conjugate complex variables formulation, closed‐form formulae for the gravity gradient tensors of the gravitational potential due to a homogeneous polyhedral body composed of polygonal facets are derived. The treatise considers the cases of the observation point being inside the polyhedron, on the surface of a facet, or outside the polyhedron.
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