- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 23, Issue 4, 1975
Geophysical Prospecting - Volume 23, Issue 4, 1975
Volume 23, Issue 4, 1975
-
-
DETECTION OF CAVES IN A KARST FORMATION BY MEANS OF ELECTROMAGNETIC WAVES*
More LessAbstractThe application of classical geophysical methods in locating karst caves did not yield always unambiguous results. It was our task to verify a new method that has not yet been tested. In investigating caves in a karst formation, electromagnetic waves in the band 2–10 MHz were applied.
The physical analysis of the problem is carried out, the measuring device used for the perradiation of the rocks between the boreholes is described, and the results of the measurements made in the karst area of East Slovakia (ČSSR) are shown. The verification from mines is included. The results mentioned are demonstrated in a space model. The work performed in the mines verified unambiguously the interpretation of the indications measured.
The results show the suitability of putting the electromagnetic method in question into the set of geophysical methods within the hydrogeological survey of karst areas.
-
-
-
TEST OF RECIPROCITY AND SUPERPOSITION IN TIME DOMAIN INDUCED POLARIZATION MEASUREMENTS*
Authors G. M. HABBERJAM and M. A. KHANAbstractThe principles of reciprocity and superposition, which hold in normal resistivity measurements, are sometimes considered to apply also to Induced Polarisation measurements.
In this paper, an account is given of experiments designed to test how far such a proposition is justified. The experiments comprise a limited field test and more extensive observations using a tank analogue.
Within acceptable limits, both tests showed that these basic theorems are applicable to I.P. potentials provided that these are measured at the same instant after switch off and that identical charging regimes are used.
The implications of these findings are discussed.
-
-
-
HIDDEN LAYER PROBLEM IN SEISMIC REFRACTION WORK*
Authors B. BANERJEE and S. K. GUPTAAbstractThe hidden layer problem in seismic refraction work has been studied for three velocity configurations – the intermediate layer having (a) lower, (b) intermediate and (c) higher velocity than the underlying and overlying beds. It has been shown that conventional methods fail to locate the presence of the intermediate layer for the cases (a) and (c) and lead to errors in calculating the depth to the bedrock. For the case (b), it is possible to interpret the first arrival travel time analytically as an alternative to Green's graphical approach. It has been suggested that the hidden layer may be detected in all the three cases if converted S waves are also recorded in the seismogram.
-
-
-
SIGN REVERSALS IN THE TRANSIENT METHOD OF ELECTRICAL PROSPECTING (ONE‐LOOP VERSION)*
By T. LEEAbstractThe transient electromagnetic fields observed by the MPPO‐1 equipment in practice over geological structures cannot always be adequately explained by Maxwell's equation when the imaginary part of the conductivity of the rocks is neglected.
If, for any reason, the conductivity of the rocks is a complex function of frequency such that the imaginary part is appreciable the model calculations must include this. This explains why some observed transient electromagnetic decay curves change sign with time.
-
-
-
INTERPRETATION OF MAGNETIC ANOMALIES USING SPECTRAL ESTIMATION TECHNIQUES*
Authors E. CASSANO and F. ROCCAAbstractThe depth of causative bodies may be derived from the power spectrum of their magnetic anomalies. After a short review of the theoretical basis of the method, several examples of its application to synthetic cases are shown. Disturbing effects due to improper choice of the sampling interval and to anomalies only partially contained in the examined segment of the profile are studied.
The spectral method is then applied to real cases; the tapering of the anomalies, the application of non‐linear filters and the effects of anomaly superpositions are investigated.
In conclusion, an appraisal of the method and of its possible practical impact has been given.
-
-
-
COMPATIBILITE ENTRE LA COMPRESSION DE L'INFORMATION SISMIQUE ET SON TRAITEMENT*
By P. BOISAbstractSome time ago, we described and implemented two methods of seismic data compression. In the first method a seismic trace is considered as being the convolution of a distribution made up of the trace peak values with a Gaussian pseudo‐pulse. The second method is performed through a truncation of the sequential (Walsh, Paley or Haar) spectrum of each trace.
In this paper it is shown that neither method has adverse effects on quality when traces with their information compressed undergo conventional data processing, such as stacking and deconvolution.
-
-
-
PSEUDO‐DIAGRAPHIES DE VITESSE EN OFFSHORE PROFOND*
By M. LAVERGNEAbstractPseudo‐velocity‐logs are tentative determinations of subsurface velocity variations with depth, using both information of seismic amplitude and reflection curvature.
A rigorous theoretical method would consist in
- a) deconvolving the seismic traces to remove the filtering effects of the ground and of the recording equipment
- b) demultiplying the deconvolved traces by a complete desynthesization with convergence criteria
- c) computing the velocities.
While this method works with synthetic examples, it is not generally applicable to field cases, one of the reasons being the poor reliability of desynthesization in the presence of noise.
The present method is a compromise between a rigorous and a practical process: the complete desynthesization is not performed; deconvolution and demultiplication are done by more classical techniques using real amplitudes; absolute velocities are determined to fit both the reflection coefficients and the rms velocities. It leads to pseudovelocity‐logs, accurate enough to show lithologic variations, smoothed enough to preserve the signal/noise ratio.
Examples are shown of Flexichoc profiles recorded in 2500 m (8000–9000 feet) deep areas of the Mediterranean Sea. Pseudo‐velocity‐logs show 1000 m (3000 feet) of a velocity‐increasing‐with‐depth Plio‐pleistocene marl formation, overlying Miocene evaporites. Intercalations of high and low‐velocity layers in the evaporites seem to indicate vertical facies variations.
The Pseudo‐velocity‐log, associated with other lithologic determination processes, should become a geological tool for deep offshore exploration.
-
-
-
HOMOMORPHIC FILTERING — THEORY AND PRACTICE*
By B. BUTTKUSAbstractThe application of homomorphic filtering in marine seismic reflection work is investigated with the aims to achieve the estimation of the basic wavelet, the wavelet deconvolution and the elimination of multiples. Each of these deconvolution problems can be subdivided into two parts: The first problem is the detection of those parts in the cepstrum which ought to be suppressed in processing. The second part includes the actual filtering process and the problem of minimizing the random noise which generally is enhanced during the homomorphic procedure.
The application of homomorphic filters to synthetic seismograms and air‐gun measurements shows the possibilities for the practical application of the method as well as the critical parameters which determine the quality of the results. These parameters are:
- a) the signal‐to‐noise ratio (SNR) of the input data
- b) the window width and the cepstrum components for the separation of the individual parts
- c) the time invariance of the signal in the trace.
In the presence of random noise the power cepstrum is most efficient for the detection of wavelet arrival times. For wavelet estimation, overlapping signals can be detected with the power cepstrum up to a SNR of three. In comparison with this, the detection of long period multiples is much more complicated. While the exact determination of the water reverberation arrival times can be realized with the power cepstrum up to a multiples‐to‐primaries ratio of three to five, the detection of the internal multiples is generally not possible, since for these multiples this threshold value of detectibility and arrival time determination is generally not realized.
For wavelet estimation, comb filtering of the complex cepstrum is most valuable. The wavelet estimation gives no problems up to a SNR of ten. Even in the presence of larger noise a reasonable estimation can be obtained up to a SNR of five by filtering the phase spectrum during the computation of the complex cepstrum. In contrast to this, the successful application of the method for the multiple reduction is confined to a SNR of ten, since the filtering of the phase spectrum for noise reduction cannot be applied. Even if the threshold results are empirical, they show the limits fór the successful application of the method.
-
-
-
ESSAIS D'AMELIORATION DU TRAITEMENT ET DE L'INTERPRETATION DES ENREGISTREMENTS MAGNETOTELLURIQUES*
Authors J. P. ROCROI and P. ANDRIEUXAbstractThe MT method has proved to be a useful tool for the resolution of some geophysical problems not well adapted to seismic prospecting. To meet the accuracy needed by such cases, the original field and processing techniques has to be somewhat improved.
The processing of the natural EM field (with two electrical and three magnetic components recorded) is made in the frequency domain, and the usual MT parameters are calculated. However, the dispersion of the results is given a great importance and the final plots (apparent resistivity vs. frequency, azimuths of the main directions vs. frequency, etc.) illustrate the distribution of the calculated values.
Average curves can then be selected and studied by means of the known master curves. However, if the earth can be assumed to consist of parallel layers, a computer program derives from the apparent resistivity diagram an automatic solution consisting of a great number of layers with the same ratio of thickness to square root of resistivity. This comprehensive solution is then simplified through an auxiliary diagram called the MT Dar Zarrouk curve and a set of geologically consistent solutions can be found.
-
-
-
CONTRIBUTION A L'ETUDE DE L'EQUIVALENCE EN PROSPECTION ELECTRIQUE (COURANT CONTINU ET MAGNETOTELLURIQUE)*
By J. P. ROCROIAbstractIn all electrical methods (EM, MT, DC), a theoretical one‐to‐one relation links the apparent resistivity diagram at a station with the local distribution of the resistivities in a tabular earth. Unfortunately, the actual field diagrams derive from a few measurements of limited accuracy and the one‐to‐one relation vanishes. Calculating the distribution of the earth resistivities from the field diagrams is thus an undetermined problem, which experience has shown to be partly controlled by approximate laws of “equivalence”. The use of these rough rules can produce only a very restricted number of the resistivity distributions that correspond to calculated diagrams which do not differ from a given field curve by more than a tolerable discrepancy. On the other hand, the full set of such “equivalent” solutions is given automatically by a computer program. Tests made on field examples prove that a few geological data generally reduce the wide range of undetermination.
-
-
-
DYNAMIC PREDICTIVE DECONVOLUTION*
More LessAbstractDynamic predictive deconvolution makes use of an entire seismic trace including all primary and multiple reflections to yield an approximation to the subsurface structure. We consider plane‐wave motion at normal incidence in an horizontally layered system sandwiched between the air and the basement rock. Energy degradation effects are neglected so that the layered system represents a lossless system in which energy is lost only by net transmission downward into the basement or net reflection upward into the air; there is no internal loss of energy by absorption within the layers. The layered system is frequency selective in that the energy from a surface input is divided between that energy which is accepted over time by net transmission downward into the basement and the remaining energy that is rejected over time by net reflection upward into the air. Thus the energy from a downgoing unit spike at the surface as input is divided between the wave transmitted by the layered system into the basement and the wave reflected by the layered system into the air. This reflected wave is the observed seismic trace resulting from the unit spike input. From surface measurements we can compute both the input energy spectrum, which by assumption is unity, and the reflection energy spectrum, which is the energy spectrum of the trace. But, by the conservation of energy, the input energy spectrum is equal to the sum of the reflection energy spectrum and the transmission energy spectrum. Thus we can compute the transmission energy spectrum as the difference of the input energy spectrum and the reflection energy spectrum. Furthermore, we know that the layered system acts as a pure feedback system in producing the transmitted wave, from which it follows that the transmitted wave is minimum‐delay. Hence from the computed energy spectrum of the transmitted wave we can compute the prediction‐error operator that contracts the transmitted wave to a spike. We also know that the layered system acts as a system with both a feedback component and a feed‐forward component in producing the reflected wave, that is, the observed seismic trace. Moreover, this feedback component is identical to the pure feedback system that produces the transmitted wave. Thus, we can deconvolve the observed seismic trace by the prediction‐error operator computed above; the result of the deconvolution is the wave‐form due to the feedforward component alone. Now the feedforward component represents the wanted dynamic structure of the layered system whereas the feedback component represents the unwanted reverberatory effects of the layered system. Because this deconvolution process yields the wanted dynamic structure and destroys the unwanted reverberatory effects, we call the process dynamic predictive deconvolution. The resulting feedforward waveform in itself represents an approximation to the subsurface structure; a further decomposition yields the reflection coefficients of the interfaces separating the layers. In this work we do not make the assumption as is commonly done that the surface as a perfect reflector; that is, we do not assume that the surface reflection coefficient has magnitude unity.
-
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)