- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 39, Issue 7, 1991
Geophysical Prospecting - Volume 39, Issue 7, 1991
Volume 39, Issue 7, 1991
-
-
AMPLITUDE, PHASE AND FREQUENCY VERSUS OFFSET APPLICATIONS1
By A. MAZZOTTIAbstractIn a layered earth the seismic reflection response for incidence at non‐normal angles is dependent upon the elastic constants and thicknesses of the layers.
The possibility is investigated of increasing the diagnostic value of seismic data by using the phase and the frequency v. offset information in addition to that from the amplitude v. offset.
The combined amplitude, phase and frequency versus offset (APF.VO) analysis is carried out through the computation of amplitude, phase and frequency indicators based on the analytical description of seismic traces.
Both synthetic and actual data are examined.
From the analyses of synthetic seismograms, it is shown that modifying the velocities and thicknesses of a given target layer, by introducing different pore fluids or lithological conditions, produces changes in APF.VO plots. In particular, the effects related to interference among reflections and to critical angle phenomena are clearly detected by both the phase and amplitude v. offset indicators in terms of phase shifts and amplitude variations. The frequency indicator is mainly controlled by the spectrum of the propagating wavelet.
Since the basic synthetic model is derived from an existing well, located close to a seismic line, some actual CDP gathers are analysed. Features related to interference and critical angle are again evident in the APF.VO plots of the actual data. The amplitude indicator appears to be reasonably stable while the phase shows a higher spatial variability and a stronger sensitivity to noise.
Differential interference with offset often occurs in actual layered structures and distorts seismic data significantly. Therefore AVO interpretation and AVO inversion procedures must also tackle this problem. Knowledge of phase and frequency variations v. offset may help classical AVO interpretation and yield further information for use in inversion techniques.
-
-
-
THE USE OF FORWARD‐ AND BACK‐SCATTERED P‐, S‐ AND CONVERTED WAVES IN CROSS‐BOREHOLE IMAGING1
Authors A. H. BALCH, H. CHANG, G. S. HOFLAND, K. A. RANZINGER and C. ERDEMIRAbstractThe principles of imaging, for example that of prestack migration, can be applied to cross‐borehole seismic geometry just as they can to surface seismic configurations. However, when using actual cross‐borehole data, a number of difficulties arise that are rarely or never encountered in imaging surface seismic data: discontinuities may reflect or diffract incident seismic waves in any direction. If a discontinuity lies between the lines of sources and receivers, forward‐scattered, or interwell, events may be recorded. If a discontinuity lies outside the interwell region, back‐scattered, or extra‐well, events may be recorded. Many angles of incidence are possible, and all possible reflected modes (P–P, P–S, S–P and S–S) are present, frequently in nearly equal proportions. The planes of the reflectors dip from 0 to ±90°.
In order to deal with these complexities we first separate propagation modes at the receiver borehole using both polarization and velocity. Next we compensate for phase distortion due to dispersion. Finally, and most importantly, we migrate or image the data in cross‐borehole common‐source gathers. To do this, a finite‐difference solution to the 2D scalar wave equation, using reverse time, for an arbitrary distribution of velocities, is used to project the separated, reflected‐diffracted wavefield back into the medium.
There are four reflection modes (P–P, P–S, S–P and S–S), so we can apply four different imaging conditions. The zones outside the boreholes as well as inside the boreholes can be imaged with these conditions.
These operations are repeated for each common‐source gather: each common‐source gather generates four partial images in each image space. This multiplicity of partial images can be stacked in various combinations to yield a final image of the subsurface.
Our experiments using solid (not fluid) physical models indicate that when these procedures are correctly applied, high quality cross‐borehole images can be obtained. These images appear with great clarity even though some of the weak diffractions causing diffraction images may be almost totally obscured by other high‐amplitude events on the raw data.
-
-
-
MAKING AVO SECTIONS MORE ROBUST1
By A. T. WALDENAbstractWhen large quantities of seismic data are involved it is impossible to examine all gathers by eye for AVO anomalies. The standard approach is to compute, for each amplitude profile (at a specific time) on each gather, the intercept and gradient of a straight‐line fit to seismic amplitudes. These intercepts and gradients are each plotted as a sort of seismic section ‐ an intercept section, and a gradient section.
Estimation of the intercept and gradient for a straight‐line fit to each amplitude profile proceeds traditionally via least‐squares. Two undesirable features can be hidden from the user by the fitting procedure, namely (i) the effect of outlying or uncharacteristic amplitudes on the intercept and gradient estimates, and (ii) complete breakdown of the straight‐line model for the amplitudes, thus rendering meaningless the intercept and gradient estimates. It should be remembered that least‐squares can always fit any sequence of numbers to any other sequence of numbers; checks are needed to show that the result is meaningful.
It is shown that statistically robust estimation methods can greatly limit the damage done by outlying amplitudes, and that a simple test on the model, the runs‐statistic, is capable of detecting breakdown of the straight‐line assumption. It is demonstrated using two seismic data sets that these two techniques, used in tandem, facilitate much better quality control of AVO intercept and gradient calculations.
-
-
-
NEAR‐SOURCE PULSE PROPAGATION: APPLICATION TO Q‐DETERMINATION1
By D. JONGMANSAbstractAmong the approaches generally used to measure attenuation from field data, the study of the first pulse broadening appears to be one of the more promising methods to estimate the quality factor Q for different geological formations including soils. Using a numerical scheme, we studied the evolution of the pulse shape in the neighbourhood of the source in order to establish the limits of the method. It was found that the pulse width variations depend strongly upon source depth. At short distances from the source, the pulse shape is controlled mainly by the near‐field terms and/or the onset of surface waves. The investigations proved that the pulse‐broadening method is reliable for distances greater than about 1.2 wavelengths. From numerical experiments, the maximum error in Q‐determination is found to be 10% in the half‐space case.
-
-
-
GEOELECTRICAL SURVEYS OF DIPPING STRUCTURES1
Authors M. BERNABINI and E. CARDARELLIAbstractAmong resistivity methods, models containing two dipping discontinuity surfaces with a conductive medium between them have been considered in this study. The theoretical apparent resistivity curves obtained for such models were calculated using Alfano's integral equation for various dip angles of planes at different array distances from the contacts. The results obtained showed that it is possible to achieve the dip values of the discontinuities under particular conditions, but ambiguities cannot be ruled out.
-
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)