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- Volume 30, Issue 6, 1982
Geophysical Prospecting - Volume 30, Issue 6, 1982
Volume 30, Issue 6, 1982
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SOME ASPECTS OF HANDLING VELOCITY INVERSION AND HIDDEN LAYER PROBLEMS IN SEISMIC REFRACTION WORK*
By R. SCHMÖLLERAbstractThere are two types of masked layers in seismic refraction work: the velocity reversal (low‐velocity layer) and the hidden layer (insufficient velocity contrast or layer thickness). On the basis of an analytical formulation of the general case of a masked layer under an overburden of plane and parallel multiple refractors the two limiting cases are discussed: the solution resulting from an uncritical interpretation of the measured time‐distance curve and the blind zone solution. Between these two limiting cases there is a variety of possible masked layer solutions. These no‐blind zone solutions—as well as the blind zone solution itself—are formulated separately for the velocity inversion and the hidden layer case.
For the evaluation of some no‐blind zone solution a diagram is presented which can be used for any case of multiple refractors in the overburden of the masked layer. However, it is only for the three‐ and the four‐layer case that a blind zone interpretation by use of diagrams is advisable. Such diagrams are presented together with the basic sets of formulae which contain as parameters only ratios of velocities and layer thicknesses. As the velocity of the masked layer is usually unknown the diagrams are principally constructed to show the dependence on the masked layer velocity. This is useful for estimation of the largest possible error.
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DEVELOPMENT OF MORE EFFICIENT AIRGUN ARRAYS: THEORY AND EXPERIMENT*
More LessAbstractSource strength of an airgun array may be increased by:
- — utilizing higher pressure,
- — increasing total array volume,
- — employing more guns,
- — improving gun efficiency.
One measure of gun efficiency is “specific source strength”, Pa*, defined as source strength per unit quantity of air used. Typical units are MPa m/l. Most developments are directed toward increasing gun pressure and/or gun volume to increase source strength of the array. These efforts require that more air compressors be installed onboard the ship. Consequently, a larger ship may be needed for the additional compressors, guns, and auxiliary equipment.
A development program was initiated in 1976 to increase source strength of the array without using a larger ship. New guns were designed and built—one for 41.4 MPa and 7.37 liter (6000 p.s.i./450 in3) operation and another with 13.8 MPa and 4.92 liter (2000 p.s.i./300 in3) capability. Experiments were conducted with these new guns (and existing guns) over a range of pressures from 13.8 to 41.4 MPa (2000 to 6000 p.s.i.).
Design of the new guns was aided by a mathematical model. The model relates physical dimensions of the airgun to acoustic pressure in the water. It consists of four nonlinear differential equations relating
- — shuttle motion,
- — bubble pressure,
- — chamber pressure,
- — bubble radius.
The last equation is the “free‐bubble‐oscillation equation” and represents the ideal case of a pressurized bubble released instantaneously in water. The three other equations modify this ideal case; the four equations together model an airgun of the type manufactured by Bolt Associates, Inc.
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A CONTRIBUTION TO THE INVESTIGATION OF AMPLITUDE CHARACTERISTICS OF VIBRATOR SIGNALS*
More LessAbstractThe characteristic and information content of a seismic signal depends considerably on the bandwidth of the signal. The Vibroseis system gives the possibility of selecting a certain frequency range. This selection is mostly done in the field by visual comparison, mainly taking into account the characteristic of the signal and the signal/noise ratio. A signal with a good characteristic has, in some cases, an arbitrary amplitude and phase characteristic which is not by itself optimum for the transmission of the desired stratigraphic information. The different factors which influence the amplitude characteristic of a Vibroseis signal are discussed. The present situation is illustrated by some results of measurements of vibrators in different areas. An outlook of how the characteristic of vibrators may be improved is given.
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COMBINED SWEEP SIGNALS FOR CORRELATION NOISE SUPPRESSION*
Authors H. A. K. EDELMANN and H. WERNERAbstractComparison of both synthetic and field data shows that considerable suppression of correlation noise can be achieved with the Combisweep technique and with the Encoded Sweep Technique. In the first technique, the spectrum is shaped by superposition of linear sweeps with different frequency range; in the second technique, short sweeps of different polarity are combined to form the “alphabet” of a code.
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OFFSET CONTINUATION OF SEISMIC SECTIONS*
Authors G. BOLONDI, E. LOINGER and F. ROCCAAbstractGeometrical acoustic and wave theory lead to a second‐order partial differential equation that links seismic sections with different offsets. In this equation a time‐shift term appears that corresponds to normal moveout; a second term, dependent on offset and time only, corrects the moveout of dipping events.
The zero‐offset stacked section can thus be obtained by continuing the section with maximum offset towards zero, and stacking along the way the other common‐offset sections.
Without the correction for dip moveout, the spatial resolution of the section is noticeably impaired, thus limiting the advantages that could be obtained with expensive migration procedures. Trade‐offs exist between multiplicity of coverage, spatial resolution, and signal‐to‐noise; in some cases the spatial resolution on the surface can be doubled and the aliasing noise averaged out.
Velocity analyses carried out on data continued to zero offset show a better resolution and improved discrimination against multiples. For instance, sea‐floor multiples always appear at water velocity, so that their removal is simplified.
This offset continuation can be carried out either in the time‐space domain or in the time‐wave number domain. The methods are applied both to synthetic and real data.
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OFFSET CONTINUATION FOR SEISMIC STACKING*
Authors L. SALVADOR and S. SAVELLIAbstractWave equation migration techniques have shown the limits of traditional stacking methods with data from tectonically complicated areas. An improved stack can be obtained utilizing the dip‐moveout correction technique based on offset continuation. The properties and the limits of the algorithms used are summarized briefly.
Several synthetic and real data examples are shown and compared with the results obtained using conventional processing in order to show the focusing effects and the strong improvement in signal‐to‐noise ratios, both at the stacked and migrated section level. The possibility of exploiting this technique to transform multiple coverage into increased spatial resolution is illustrated with examples.
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USE OF THE SEISMIC DYNAMIC DECONVOLUTION ALGORITHM IN DIRECT RESISTIVITY INTERPRETATION*
By E. SZARANIECAbstractSeismic dynamic deconvolution is the mathematical basis on which a degree of unification in different prospecting methods is possible, relative to the parameter identification in horizontally stratified media. There is a basic structure which has some immediate applications to the inversion of resistivity data and possibly to other problems. For resistivity soundings there exists a key equation which is parallel to the energy conservation law in the theory of synthetic seismograms.
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QUADRATIC CORRECTIONS FOR THE Z‐EFFECT APPLIED TO THE INVERSION OF BROADBAND GAMMA RAY LOGS FROM URANIFEROUS ZONES*
By R. H. DUFFINAbstractNonlinear correction parameters have been determined to correct broadband gamma ray logs for the effects of photoelectric absorption in zones of high‐grade uranium mineralization. These corrections have been determined empirically by logging artificial test pits. The corrections are a function of grade of mineralization, hole casing thickness, hole medium, and probe energy thresholds. Comparisons between core assays from a high‐grade mineralized zone and corrected log inversions recorded with both low‐threshold and high‐threshold filtered probes are made. The best correlations were obtained between assays and inversion solutions for the low‐threshold probe. This reflects the superior counting statistics of this type of probe. Filtered probes reduce z‐effect errors but do not eliminate them, and have poorer counting statistics than unfiltered probes.
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MAGNETOTELLURIC RESPONSE ON VERTICALLY INHOMOGENEOUS EARTH HAVING CONDUCTIVITY VARYING LINEARLY WITH DEPTH*
By D. KAOAbstractMagnetotelluric response is studied for an inhomogeneous medium having conductivity varying linearly with depth as σ(z) =σ1+αz. For a medium having conductivity increasing linearly with depth, the phase of the impedance approaches 60° at long periods and the apparent resistivity becomes log (ρa) = 2 log (1.36/α1/3) — 1/3 log (T'). The asymptote of log (ρa, T'→∞) when plotted against log (T') has a constant gradient —1/3 and has an intercept on the log (T') axis, which equals 6 log (1.36/α1/3). When a homogeneous layer with a moderate thickness overlies an inhomogeneous half‐space, this layer does not affect the asymptote, but it affects the cut‐off period and pushes this toward the long period direction. For a medium having conductivity decreasing linearly with depth, the impedance is equivalent to that of a Cagniard two‐layer model; the intercept period related to the thickness is T'0=σ1(h2/2)2. Homogeneous multilayer approximations to an inhomogeneous layer are also investigated, and it is shown that the fit to the model variation depends on the number of layers and the layer parameters chosen.
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GEOELECTRIC INVESTIGATIONS OF A FAULT SYSTEM IN QUATERNARY DEPOSITS*
More LessAbstractGeoelectric soundings were used to investigate the younger fault activity (Middle and Early Pleistocene) of the southwestern fault margin area of the Central Graben (and extension of the Rhine Graben) in the southern Netherlands and northern Belgium. Some effects of this fault activity can still be observed in the present geomorphology and hydrography. Investigations have been concentrated on the uppermost 20–30 m, consisting mainly of Middle and Early Pleistocene fluviatile deposits. Marshy clays and loams are most conductive. They alternate with large sand beds and gravelly gully infillings, characterized by high specific resistivities. Problems in the interpretation of the geoelectric data, caused by the variability of the deposits, are partially solved by in‐situ resistivity measurements (“mini‐electric” measurements). In spite of the lithologic inhomogeneity, a few marker horizons allow the geologic structure to be determined. Some new tectonic boundaries have been traced. The movements along the faults during the Quaternary are very small (less than 10 m). Therefore, the network of soundings has to be very dense and a very intensive analysis of the soundings is necessary. The results also have a hydrogeologic significance. This project illustrates the possibility of locating small tectonic structures in relatively inhomogeneous deposits by a detailed and carefully designed geoelectric survey.
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A NUMERICAL METHOD TO COMPUTE THE RESISTIVITY TRANSFORM FROM WENNER SOUNDING DATA*
Authors RAKESH KUMAR and M. V. RAMANAIAH CHOWDARYAbstractA numerical technique to compute the resistivity transform directly from the observed Wenner sounding data has been developed. In principle, the procedure is based on a decomposition method and consists of two steps: the first step determines a function that approximates the apparent resistivity data and the second step transforms this function into the corresponding kernel by an analytical operation. The proposed method is tested on some theoretical master curves. A high degree of precision is achieved with very little computer time. The applicability is shown on two field examples.
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APPLICATION OF BOREHOLE GEOPHYSICS AT AN EXPERIMENTAL WASTE STORAGE SITE*
Authors P. H. NELSON, K. A. MAGNUSSON and R. RACHIELEAbstractA suite of electrical, radiation, and mechanical borehole probes were run in a 76‐mm‐diameter borehole drilled to a slant depth of 380 m in leptite and granite. The hole is located in Precambrian bedrock in central Sweden where a site is dedicated to in‐situ experiments pertaining to the disposal of radioactive wastes. The challenge to borehole logging methods for such site investigations is to resolve geological features and fluid flow parameters in geological sites which are initially chosen for their homogeneity, low porosity, and minimal fracturing. The Stripa borehole is characterized by high electrical resistivity values in the 20–100 kΩm range, by acoustic velocities around 5800 m s‐1 (which is close to laboratory values on intact specimens), and by total porosity of around one volume percent. In this context, probe resolution was adequate to produce interpretable information on almost all of the logs.
Two principal rock types were encountered in the hole: granite, of quartz monzonitic composition, and leptite. The granite and leptite intercepts are subdivided into units characterized by mafic mineral content, sulfide mineral content, and electrical and radiation properties. Iron‐rich zones in the leptite are highly anomalous on the gamma‐gamma and neutron logs; thin mafic zones in the granite can also be distinguished. Occurrences of a few percent pyrite are detected by the electrical, gamma‐gamma, and neutron logs. Although overall porosity is quite low throughout the hole, analysis of the resistivity and neutron logs indicates the porosity increases by a few volume percent at fracture zones. The differential resistance and caliper probes detect borehole diameter roughness of less than 1 mm, helping to confirm acoustic waveform anomalies which are indicative of fracture zones. Compres‐sional wave transit time and shear‐wave interference patterns usually occur coincident with open fractures observed in core, the correlation being especially good at major fracture zones.
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Volume 72 (2023 - 2024)
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Volume 46 (1998)
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Volume 45 (1997)
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Volume 44 (1996)
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Volume 42 (1994)
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Volume 41 (1993)
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Volume 40 (1992)
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Volume 39 (1991)
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Volume 38 (1990)
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Volume 37 (1989)
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Volume 36 (1988)
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Volume 34 (1986)
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Volume 33 (1985)
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Volume 32 (1984)
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Volume 31 (1983)
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Volume 30 (1982)
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Volume 29 (1981)
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Volume 28 (1980)
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Volume 27 (1979)
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Volume 26 (1978)
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Volume 25 (1977)
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Volume 24 (1976)
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Volume 23 (1975)
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Volume 22 (1974)
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Volume 21 (1973)
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Volume 20 (1972)
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Volume 19 (1971)
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Volume 17 (1969)
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Volume 16 (1968)
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Volume 15 (1967)
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Volume 14 (1966)
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Volume 13 (1965)
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Volume 12 (1964)
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Volume 11 (1963)
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Volume 10 (1962)
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Volume 9 (1961)
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Volume 8 (1960)
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Volume 7 (1959)
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Volume 6 (1958)
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Volume 5 (1957)
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Volume 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)