- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 30, Issue 5, 1982
Geophysical Prospecting - Volume 30, Issue 5, 1982
Volume 30, Issue 5, 1982
-
-
NON‐NORMAL INCIDENCE STATE‐SPACE MODEL AND LINE SOURCE REFLECTION SYNTHETIC SEISMOGRAM*
Authors F. AMINZADEH and J. M. MENDELAbstractThis paper is directed at modeling layered media. We extend the plane‐wave normal‐incidence state‐space model developed by Mendel, Nahi and Chan in 1979, to the non‐normal incidence case. To do this we introduce a shifting principle, a zero‐offset wavefront, and zero‐offset travel times for different layers. We also develop an algorithm for obtaining a synthetic line source reflection seismogram. In this algorithm non‐normal incidence plane‐wave seismograms are summed over a range of incident angles. The algorithm is based on a modified version of Sommerfield's (1896) theorem. Simulations of acoustic and elastic media are included which illustrate the applicability of our plane‐wave and line source seismograms for both elastic and acoustic cases.
-
-
-
A NEW DERIVATION OF THE WAVEFRONT CURVATURE TRANSFORMATION AT AN INTERFACE BETWEEN TWO INHOMOGENEOUS MEDIA*
By B. URSINAbstractThe principles for ray‐tracing and wavefront curvature calculations in a three‐dimensional medium are reviewed. A new derivation of the transformation of the wavefront curvature matrix at an interface between two inhomogeneous media is given. The derivation is based on a Taylor series expansion of the ray refraction equation at the interface between two inhomogeneous media, and only elementary geometric arguments are used. The wave‐front curvature transformation at the interface is obtained by neglecting all terms in the direction of the surface normal.
With proper definition of the variables, the derivation is also valid for a reflected wave‐front. A simplified transformation rule is derived for a reflected wave of the same type as the incident wave.
-
-
-
POROSITY PREDICTION FROM SEISMIC DATA*
Authors G. P. ANGELERI and R. CARPIAbstractThe inversion of seismic traces allows the estimation of reservoir porosity from an analysis of transit times derived from the pseudo‐velocity logs. A four‐step computational procedure is illustrated consisting of (i) inversion of seismic traces and calculation of interval velocities; (ii) accurate stratigraphic interpretation; (iii) determination of the petrophysical parameters for the porosity evaluation; (iv) analysis of the reliability of the results and final corrections.
Both the possibilities and the limits of the method are discussed. One of the causes of error is the fact that impedances—and not velocities—are readily obtainable from seismic data. Moreover, the porosity due to fracturation contributes only slightly to velocity, while it often contributes most importantly to the permeability.
Results are shown for two of the most significant reservoir types, i.e. carbonatic and clastic. Two cases belonging to the latter type will be examined. In the first case the primary porosity is dominant. The second case is very complex and both primary and secondary porosity are present.
-
-
-
PREDICTING ABNORMALLY PRESSURED SEDIMENTARY ROCKS*
Authors D. BILGERI and E. B. ADEMENOAbstractModern seismic processing techniques developed in recent years have provided the explorationist with more meaningful data than would have been predicted even by optimists. Correct migration of seismic data, relative amplitude preservation of reflections, and seismic trace inversion represent the necessary efforts to ensure that the best possible picture of in situ physical properties of the subsurface section is revealed. Moreover, compacted and over‐pressured zones can be predicted from surface data prior to drilling a well through them.
The basic tool for predicting overpressured zones from the surface is still the velocity analysis derived from good reflection data with few erratic multiples. The extraction of regional normal compaction trends from the seismic velocities allows one—where velocities deviate from the trend—to locate the top of overpressure. Moreover, the statistical behavior of the ratios of the sonic log vs pore pressure in existing boreholes enables one to convert the deviation from the trend of the seismic velocities into overpressure rates expected at the seismic reflection horizon.
This paper presents a field case study showing how the knowledge of well site lithology together with the more detailed information extracted from inverted seismic data enables the prediction to match well conditions with high reliability.
-
-
-
THE INFLUENCE OF PERMEABILITY ON COMPRESSIONAL WAVE VELOCITY IN MARINE SEDIMENTS*
Authors F. HAMDI and D. TAYLOR SMITHAbstractTo investigate the effect of permeability on the propagation of seismo‐acoustic waves through marine sediments, a theoretical model based on Biot's equations is established which relates the compressional wave velocity measured at a fixed frequency to computed velocities at zero and infinite frequencies in terms of sediment porosity and permeability. The model is examined experimentally in a standard soil mechanics consolidation test (itself dependent, among other things, on sediment porosity and permeability) which has been modified to include measurements of compressional wave velocity at 1 MHz and shear‐wave velocity at 5 kHz. This test allows the elastic modulus of the sediment frame to be assessed under different load conditions simultaneous with the velocity determinations.
From a number of tests on different samples, five samples are chosen to typify the range of sediment sizes. The results show that the difference between the measured velocity at 1 MHz and the model‐derived velocity at zero frequency increases with increasing particle size (from clays to fine sand), with decreasing porosity, and with increasing permeability. For sediments coarser than fine sand the simple model breaks down, possibly because of the dominance of scattering/diffraction effects at the high frequency of the experiment. Within this limitation the model seems satisfactory to offer a capability of predicting the permeability of a sea floor sediment to an order of magnitude by the in situ measurement of seismic velocities over a wide range of frequencies; the prediction process requires a good in situ determination of sediment porosity such as that offered by electrical formation factor measurements.
-
-
-
SINGLE‐TRACE PROCESSING USING ITERATIVE CDP‐STACKING*
By O. E. NAESSAbstractThe use of conventional CDP‐stacking in the processing of reflection data imposes restrictions on the horizontal and vertical resolution. Ideally, the final seismic section should consist only of short offset or, in practice, near‐trace primary energy. Through the use of the iterative stacking algorithm, the signal‐to‐noise ratio on a single trace in the CDP‐gather may be improved to an extent comparable to what occurs on a conventional stacked trace. By using this approach and treating the near‐trace section after iterative stacking as the final section, the seismic resolution can be improved.
-
-
-
RECIPROCAL AVERAGING TECHNIQUES IN THE GEOELECTRICAL BOUNDARY ELEMENT APPROACH*
By M. OKABEAbstractA Fredholm integral equation of the first kind with respect to the surface charge density is presented via a weighted residual formulation in the standard isotropic problem. The surface charge densities are numerically obtained by solving the well‐known Fredholm integral equation of the second kind, where the new equation can be regarded as a constraint. The accuracy of boundary element solutions is examined in connection with the violation of such a constraint and the “modified reciprocal averaging techniques” are proposed.
-
-
-
THE INFLUENCE OF SHALE EFFECTS UPON THE ELECTRICAL RESISTIVITY OF RESERVOIR ROCKS*
More LessAbstractThe roots of the so‐called shaly‐sand problem in hydrocarbon evaluation lie in the effect of relatively fine‐grained minerals upon measured electrical parameters of granular reservoirs. This influence manifests itself as an excess conductivity, over and above that due to the purely geometric effects of electrolyte distribution within the pore space. For formations with low shaliness, this excess conductivity is usually insignificant in typical oilfield situations.
The influence of shaliness upon observed values of formation resistivity has been appraised by collating core‐sample data from four different reservoirs. It has been demonstrated that during the course of electrical measurement under conditions of full electrolyte saturation, any given lithology can exhibit both negligible and highly significant shale effects depending upon the resistivity of the interstitial aqueous electrolyte. The effects of shaliness are also governed by the degree of water saturation.
Because the manifestation of shaliness in electrical data is not a function of lithology alone, recourse is made to a more realistic concept of shale effects whereby a formation, or section of a formation, is classified as “effectively clean” or “effectively shaly” according to whether it obeys or defies, respectively, the fundamental empirical laws of Archie (1942). In particular, since an intrinsic formation factor can be obtained directly in fully‐saturated effectively clean reservoirs, whereas only an apparent quantity may be recorded directly in fully‐saturated effectively shaly reservoirs, the ratio of apparent to intrinsic formation factor serves as a useful conceptual indicator of shale effects, attaining the limiting value of unity only under effectively clean conditions.
In the context of electrical measurement the terms “shaliness” and “shale effects” are evidently not synonymous and it is the latter which should be considered when selecting equations for the computation of water saturation. The implications for well‐log analysis follow through formulated guidelines that describe the relative levels of shale effects in different zones of lithologically uniform reservoirs.
-
-
-
COMPARISON OF FIVE LEAST‐SQUARES INVERSION TECHNIQUES IN RESISTIVITY SOUNDING*
Authors G. M. HOVERSTEN, A. DEY and H. F. MORRISONAbstractA brief history of the development of the inverse problem in resistivity sounding is presented with the development of the equations governing the least‐squares inverse. Five algorithms for finding the minimum of the least‐square problem are described and their speed of convergence is compared on data from two planar earth models. Of the five algorithms studied, the ridge‐regression algorithm required the fewest numbers of forward problem evaluations to reach a desired minimum.
Solution space statistics, including (1) parameter‐standard errors, (2) parameter correlation coefficients, (3) model parameter eigenvectors, and (4) data eigenvectors are discussed. The type of weighting applied to the data affects these statistical parameters. Weighting the data by taking log10 of the observed and calculated values is comparable to weighting by the inverse of a constant data error. The most reliable parameter standard errors are obtained by weighting by the inverse of observed data errors. All other solution statistics, such as dataparameter eigenvector pairs, have more physical significance when inverse data error weighting is used.
-
Volumes & issues
-
Volume 72 (2023 - 2024)
-
Volume 71 (2022 - 2023)
-
Volume 70 (2021 - 2022)
-
Volume 69 (2021)
-
Volume 68 (2020)
-
Volume 67 (2019)
-
Volume 66 (2018)
-
Volume 65 (2017)
-
Volume 64 (2015 - 2016)
-
Volume 63 (2015)
-
Volume 62 (2014)
-
Volume 61 (2013)
-
Volume 60 (2012)
-
Volume 59 (2011)
-
Volume 58 (2010)
-
Volume 57 (2009)
-
Volume 56 (2008)
-
Volume 55 (2007)
-
Volume 54 (2006)
-
Volume 53 (2005)
-
Volume 52 (2004)
-
Volume 51 (2003)
-
Volume 50 (2002)
-
Volume 49 (2001)
-
Volume 48 (2000)
-
Volume 47 (1999)
-
Volume 46 (1998)
-
Volume 45 (1997)
-
Volume 44 (1996)
-
Volume 43 (1995)
-
Volume 42 (1994)
-
Volume 41 (1993)
-
Volume 40 (1992)
-
Volume 39 (1991)
-
Volume 38 (1990)
-
Volume 37 (1989)
-
Volume 36 (1988)
-
Volume 35 (1987)
-
Volume 34 (1986)
-
Volume 33 (1985)
-
Volume 32 (1984)
-
Volume 31 (1983)
-
Volume 30 (1982)
-
Volume 29 (1981)
-
Volume 28 (1980)
-
Volume 27 (1979)
-
Volume 26 (1978)
-
Volume 25 (1977)
-
Volume 24 (1976)
-
Volume 23 (1975)
-
Volume 22 (1974)
-
Volume 21 (1973)
-
Volume 20 (1972)
-
Volume 19 (1971)
-
Volume 18 (1970)
-
Volume 17 (1969)
-
Volume 16 (1968)
-
Volume 15 (1967)
-
Volume 14 (1966)
-
Volume 13 (1965)
-
Volume 12 (1964)
-
Volume 11 (1963)
-
Volume 10 (1962)
-
Volume 9 (1961)
-
Volume 8 (1960)
-
Volume 7 (1959)
-
Volume 6 (1958)
-
Volume 5 (1957)
-
Volume 4 (1956)
-
Volume 3 (1955)
-
Volume 2 (1954)
-
Volume 1 (1953)