- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 17, Issue 3, 1969
Geophysical Prospecting - Volume 17, Issue 3, 1969
Volume 17, Issue 3, 1969
-
-
REMARKS ON THE SIGNAL TO NOISE RATIO IN THE VIBROSEIS SYSTEM*
By TH. KREYAbstractThe paper presents a discussion of some of the noise features of the VIBROSEIS SYSTEM*** and their bearing on the determination of the optimally weighted stack.
-
-
-
THE DELINEATION OF THE BEDROCK CONFIGURATION OF PART OF PORT JACKSON, NEW SOUTH WALES, WITH A BOOMER SYSTEM*
Authors D. W. EMERSON and C. V. G. PHIPPSAbstractA high resolution Boomer system was used to investigate the bedrock configuration of part of Port Jackson, New South Wales. The continuous reflection profiling technique was successful in delineating a channel incised in bedrock under a thickness of up to 60 m. of unconsolidated sediment covered by up to 20 m. of water.
-
-
-
IDENTIFICATION OF SEA‐FLOOR SEDIMENTS USING UNDERWAY ACOUSTICS*
Authors W. N. LI and D. TAYLOR SMITHAbstractLaboratory measurements, and a few in situ observations, show that saturated marine sediments have interdependent mechanical and acoustical properties, Acoustically, of particular importance are the acoustic impedance, velocity of sound and the sound attenuation coefficient of the sediment. The first two properties can be measured relatively easily from a surface ship; the measurement of attenuation however, is more problematical. It is suggested that this can be achieved by a quantitative treatment of the acoustic data collected during routine sub‐bottom profiling over a variable thickness of superficial sediments. In the assessment of four different sediment locations in the Irish Sea it was found that quantitative treatment of the acoustic signals yielded both a value of the attenuation coefficient as well as a measure of the frequency dependence of the attenuation. In addition a statistical analysis of the signal intensities seems to provide an indication of the relative roughness of the bottom and sub‐bottom interfaces. From the wide range of information provided the mechanical properties of the sea‐floor sediment may be estimated.
-
-
-
OPTIMUM DIGITAL FILTERS FOR SIGNAL TO NOISE RATIO ENHANCEMENT*
Authors SVEN TREITEL and ENDERS A. ROBINSONSummaryOne of the main objectives of seismic digital processing is the improvement of the signal‐to‐noise ratio in the recorded data. Wiener filters have been successfully applied in this capacity, but alternate filtering devices also merit our attention. Two such systems are the matched filter and the output energy filter. The former is better known to geophysicists as the crosscorrelation filter, and has seen widespread use for the processing of vibratory source data, while the latter is. much less familiar in seismic work.
The matched filter is designed such that ideally the presence of a given signal is indicated by a single large deflection in the output. The output energy filter ideally reveals the presence of such a signal by producing a longer burst of energy in the time interval where the signal occurs. The received seismic trace is assumed to be an additive mixture of signal and noise. The shape of the signal must be known in order to design the matched filter, but only the autocorrelation function of this signal need be known to obtain the output energy filter. The derivation of these filters differs according to whether the noise is white or colored. In the former case the noise autocorrelation function consists of only a single spike at lag zero, while in the latter the shape of this noise autocorrelation function is arbitrary.
We propose a novel version of the matched filter. Its memory function is given by the minimum‐delay wavelet whose autocorrelation function is computed from selected gates of an actual seismic trace. For this reason explicit knowledge of the signal shape is not required for its design; nevertheless, its performance level is not much below that achievable with ordinary matched filters. We call this new filter the “mini‐matched” filter.
With digital computation in mind, the design criteria are formulated and optimized with time as a discrete variable. We illustrate the techniques with simple numerical examples, and discuss many of the interesting properties that these filters exhibit.
-
-
-
REFLEXIONS SUR LES APPLICATIONS DE LA TRANSFORMEE DE FOURIER EN SISMIQUE ET EN GRAVIMETRIE*
Authors A. JACQMIN and L. PEKARAbstractNumerous studies have already been carried out on the Fourier Transform and its geophysical applications. The utilisation of computers has brought with it the digitalisation of the major method, reflection shooting, and the different handling techniques for numerical data have given birth to numerous papers on the subject. Gravimetric surveying has always been a numerical method, but it is evident that for it, too, new possibilities have been opened. Consequently, gravimetric experts are becoming increasingly interested in the theory and applications based on the study of frequencies spectra using the Fourier Transform. The researches of C. A. Schwartz, W. Sokoloff and W. C. Dean have been followed by other interesting ideas concerning the data processing of gravity measurements, such as: calculation of the first vertical derivative (V. Baranov), interpretation of the anomalies with the aid of their spectra (M. E. Odegard and J. Berg Jr.), isopach reduction in gravimetric surveying (J. L. Bible), etc.
All these ideas and techniques have the same purpose: to make the interpretation easier. But, although they have the same aim and tackle the same type of difficulty, it would seem that the gravimetric and the seismic experts have developed their own tools independently of each other, with no consideration for the fact that both geophysical methods, in particular the methods of treatment of raw measurements, are, if not identical, at least very close to one another and that, consequently, any improvement in one method may be useful to the other.
The purpose of the present paper is to reconsider the philosophy of the seismic and gravimetric methods, starting with data recording, then dealing with the most important data processing systems and finally ending with the interpretation. The paper bases its approach on two points of view which are in fact complementary:
– Although digital data processing is almost always effected in the functional sphere by convolutions, it is much easier to understand and to conceive these systems if one reasons alternately in the functional and frequency spheres; this is possible by using the Fourier Transform.
– By considering the problem in frequencies, there is no fundamental difference between the seismic and gravimetric methods. A curve plotted in gravity units, as a function of the distance, and a seismic trace which represents the variations of the output of a galvanometer, as a function of the time, are identical from the point of view of the Fourier Transform.
With these ideas in mind, the following problems are dealt with:
– Seismic and gravity signals.
– The sampling problem in gravimetry (data sampling rate non‐constant).
– Presentation and discussion of spectra of some synthetic and practical examples:
•Wave number filtering.
•Frequencies filtering.
•The problem of the frequency o (horizontal and vertical derivatives).
•Continuation = deconvolution.
– Other applications of the Fourier Transform in gravimetry.
-
-
-
AN ANALYSIS OF EQUIVALENCE IN RESISTIVITY SOUNDING*
By O. KOEFOEDAbstractA mathematical analysis is given of the phenomenon of equivalence in resistivity sounding, which is based upon the properties of the raised kernel function. Analysis of this function instead of the apparent resistivity function is justified because, as has been shown in a previous publication, variations in the apparent resistivity function lead to variations in the raised kernel function with relative values of the same order of magnitude The expression for the raised kernel function is expanded into a Mac Laurin series. Equivalence can occur only if the second order term of this series is negligible. The coefficient of the first order term depends on the resistivity and the thickness of the layer under consideration. There is an infinite set of combinations of values for these two quantities, for which the coefficient of the first order term has the same value. All these combinations represent equivalent layer distributions.
-
-
-
AERIAL RADIOMETRY OF EL ALAMEIN OILFIELD, EGYPT, U.A.R.*
Authors E. M. EL SHAZLY, W. M. MESHREF, K. M. FOUAD, A. A. AMMAR and M. L. MELEIKAbstractTotal gamma airborne radiometric survey of El Alamein area in the Western Desert has been carried out by the Atomic Energy Establishment. The flight lines are directed north‐south with 500 m spacing, while the ground clearance is about 50 m. Superimposition of the radioactivity profiles over El Alamein Oilfield shows a general decrease of gamma radiation in the central part of the structure framed by zones of relatively higher values. The aerial radioactivity of El Alamein Oilfield is described and it is correlated with some reported radiometric characteristics of other oil bearing structures. The distribution of aeroradiometric measurements over El Alamein Oilfield and its environment has been found to be lognormal.
-
-
-
ESTIMATION OF SPECTRUM AND CROSS‐SPECTRUM OF AEROMAGNETIC FIELD USING FAST DIGITAL FOURIER TRANSFORM (FDFT) TECHNIQUES*
More LessAbstractAn efficient method of computing spectrum and cross‐spectrum of large scale aero‐magnetic field (or of any other two‐dimensional field) has been developed and programmed for a digital computer. The method uses fast Fourier transform techniques. Briefly, the method is as follows: a digitized aeromagnetic map is divided into a number of rectangular blocks. Fourier transforms of these blocks are computed using a two‐dimensional fast Fourier transform method. Finally, the amplitude of the Fourier transforms is averaged to give the desired spectrum. Computation of cross‐spectrum follows the same lines. In fact, the same programme may be used to a compute the spectrum as well as cross‐spectrum. The method has a number of computational advantages, in particular it reduces greatly computational time and storage requirements. The programme has been tested on synthetic data as well as on real aeromagnetic data. It took less than 30 seconds on an IBM 360/50 computer to compute the spectrum of an aeromagnetic map covering an area of approximately 4500 square miles.
-
-
-
BOOK REVIEW
Book reviewed in this article:
Courant‐Hilbert, Methoden der mathematischen Physik I, Heidelberger Taschen‐bucher Vol. 30
-
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)