- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 33, Issue 7, 1985
Geophysical Prospecting - Volume 33, Issue 7, 1985
Volume 33, Issue 7, 1985
-
-
FULL WAVE EQUATION DOWNWARD CONTINUATION OF SEISMIC REFLECTION DATA*
Authors D. J. FOSTER and Ph. M. CARRIONAbstractA new method for suppressing multiple reflections in seismograms is developed. It is based on a downward continuation procedure which uses the full acoustic wave equation (hyperbolic form) as a downward continuation operator. We demonstrate that the downward continuation of the recorded wave field maps a reflectivity function without multiply reflected events. The method is applied successfully to individual traces of plane‐wave decomposed (slant‐stacked) synthetic and field data.
-
-
-
SEISMIC SOURCES FOR SHALLOW REFLECTION SURVEYING*
Authors D. M. McCANN, E. M. ANDREW and C. McCANNAbstractA standard seismic reflection profile was shot along a disused railway track at Onley, near Rugby, U. K. Four different seismic sources including explosives, the propane/oxygen gas‐gun, the Bolt airgun, and the borehole sparker were used and compared with each other in terms of output energy, penetration and resolution. The results indicated that the resolution of the borehole airgun and the gas‐gun was slightly higher than that possible with gelignite. Both these sources had an output energy which was equivalent to 30 g of gelignite.
The borehole sparker was only useful for obtaining seismic information on the nearsurface weathered layer, since its output power (1 kJ) was very limited. However, McCann and McCann (1982) used a high‐power sparker source (14 kJ) on the nearby Grand Union Canal for a wide angle seismic reflection survey and achieved a maximum penetration of 250 m, which is comparable with the results obtained on land with the seismic sources mentioned above.
The seismic reflection profile, which was interpreted in the light of borehole information in the area and the results of McCann and McCann (1982), successfully identified the surface of the Palaeozoic rocks. The problem of detecting the presence of thin, high‐velocity layers in a seismic refraction survey without the availability of other information to calibrate the seismic section is also highlighted.
-
-
-
DETERMINISTIC ESTIMATION OF A WAVELET USING IMPEDANCE TYPE TECHNIQUE*
Authors D. LOEWENTHAL, S. S. LEE and G. H. F. GARDNERAbstractA new deterministic technique for estimating a wavelet suggested by Loewenthal and Jakubowicz requires measurement of both pressure and vertical particle velocity. Through construction of the impedance function a deterministic estimation of the wavelet and the reflectivity can be obtained.
This idea is tested for a one‐dimensional model. The test is carried out by forming a synthetic seismogram of both pressure and particle velocity and checking the formulas for obtaining the estimated wavelet under noisy conditions.
-
-
-
SEPARATION OF P‐ AND S‐WAVES*
More LessAbstractIn combined P‐ and SV‐observations with vertical and in‐line horizontal geophones, recognition and separation of the individual P‐ and S‐arrivals may form a considerable problem. Due to the geophone receiving characteristics, P‐ and S‐waves are observed on both horizontal and vertical geophones. A new method of separating P‐ and S‐waves is presented that inverts the geophone records for the receiving characteristics, thereby separating the two wavetypes.
The filter coefficients are determined from the near‐surface P‐ and S‐wave velocity and from the geometry of the geophone groups. The P/S‐separation filter was tested on synthetic data with good results. Even PS‐ and SP‐converted reflections, which are completely coinciding for horizontal reflectors, could be well separated. The filter appears to be stable and to have low sensitivity to noise in the data or to errors in the near‐surface P‐ and S‐wave velocity. The filter is superior to other separation methods, which were applied to the same dataset for comparison. This method can also be applied to VSP data.
-
-
-
ESTIMATION OF MARINE SOURCE SIGNATURES FROM DIRECT ARRIVALS TO HYDROPHONE GROUPS*
By S. P. KRAVISAbstractDeterministic deconvolution of seismic data recorded with a non‐minimum phase source, such as a sparker, requires an estimate of the source signature. Present methods of deterministic signature estimation and deconvolution require additional field equipment (near‐ or far‐field hydrophones), or else make a deterministic estimate of the effective source signature (the free‐field source signature convolved with source and receiver ghosts). By analyzing the direct arrival signal to normal hydrophone groups it is shown that extraction of the free‐field source signature from this signal is possible for a spherically symmetric source, and it is demonstrated that the use of this signature for a preliminary deterministic deconvolution gives better results on sparker data than minimum‐phase whitening deconvolution applied on its own. The applicability of the method to non‐spherically symmetric sources, such as arrays, is also discussed.
This paper is published with the permission of the Director, Bureau of Mineral Resources, Geology and Geophysics, Canberra, Australia.
-
-
-
AN EXTENSION OF THE BORN INVERSION METHOD TO A DEPTH DEPENDENT REFERENCE PROFILE*
Authors N. BLEISTEIN and S. H. GRAYAbstractAn extension of the multidimensional Born inversion technique for acoustic waves is described. In earlier work, a perturbation in reference sound velocity was determined by assuming that the reference velocity was constant. In this extension, we allow the reference velocity to be a function of the depth variable z. The output of this method is a high‐frequency bandlimited reflectivity function of the subsurface. The reflectivity function is an array of bandlimited singular functions scaled by the normal reflection strength. Each singular function is a Dirac delta function of a scalar argument which measures distance normal to a reflecting interface. Thus, the reflectivity function is an indicator map of subsurface reflectors equivalent to the map produced by migration. In addition to the assumption of small perturbation, the method requires that the reflection data reside in the high frequency regime in a well‐defined sense.
The method is based on the derivation of an integral equation for the perturbation in sound velocity from a known reference velocity. When the reference velocity is constant, the integral equation admits an analytic solution as a multifold integral of the reflection data. Further high frequency asymptotic analysis simplifies this integral considerably and leads to an extremely efficient numerical algorithm for computing the reflectivity function. The development of a computer code to implement this constant‐reference‐velocity solution is published elsewhere.
For a reference velocity c(z) we can no longer invert the integral equation exactly. However, we can write down an asymptotic high‐frequency approximation for the kernal of the integral equation and an asymptotic solution for the perturbation. The computer implementation of this result is designed along the same lines as the code for constant background velocity. In tests the total processing time for this algorithm with depth‐dependent background velocity is usually considerably less than that required by a standard Kirchhoff migration algorithm. The method is implemented as a migration technique and compared with alternative migration algorithms on the flanks of the salt dome.
-
-
-
DATA QUALITY ASSESSMENT OF TIME‐DOMAIN IP DECAY CURVES*
By J. P. BUSBYAbstractData quality assessments have been made on time‐domain induced polarization decay curves taken over sulphide ore bodies of Cyprus. The data were taken with the dipole‐dipole, pole‐dipole and symmetrical quadripole arrays. The results indicate that at larger array separations decay curves taken with the dipole‐dipole and pole‐dipole arrays are often seriously affected by noise, but that curves taken with the symmetrical quadripole array are not so affected. Optimum pseudo‐sections designed to minimize the noise show that the noise results in spurious highs and lows.
This work was carried out as part of a NERC research studentship at the University of Leicester.
-
-
-
THE COMPLEX RESISTIVITY SPECTRA OF MODELS CONSISTING OF TWO POLARIZABLE MEDIA OF DIFFERENT INTRINSIC PROPERTIES*
More LessAbstractTheoretical model study shows that when an earth model is composed of two (Cole‐Cole) polarizable media, its normalized complex resistivity spectrum is approximately a multiplicative combination of the contributions of the two media. This also applies on inversion, but the two dispersions thus obtained are apparent rather than intrinsic dispersions.
In models consisting of two media, either a multiplicative or an additive combination of Cole‐Cole functions fits the complex apparent resistivity spectrum. On inversion each combination gives similar parameters except for the apparent chargeability of the component with the shorter time constant. However, this can be compensated simply. In this sense the two representations are almost equivalent.
We show that the apparent spectrum due to a finite polarizable body is actually a true Cole‐Cole dispersion, as is usually assumed in practice.
The behavior of a complex apparent resistivity spectrum and its corresponding apparent Cole‐Cole dispersion parameters is influenced by the variation of the dilution factor with frequency. Hence when estimating intrinsic parameters from the nomogram, based on constant (frequency‐independent) dilution factors, particular care is required to correctly relate apparent parameters to intrinsic parameters.
-
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)