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- Volume 46, Issue 2, 1998
Geophysical Prospecting - Volume 46, Issue 2, 1998
Volume 46, Issue 2, 1998
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Very high resolution 3D marine seismic data processing for geotechnical applications
More LessThe processing of a small‐scale, very high resolution (VHR) shallow marine 3D data volume is described. The data were acquired over a small clay diapir, on the river Schelde, in 1990. Using an array of 12 dual‐channel microstreamers towed from a catamaran, a network of 1 m × 1 m bins could be produced over an area of 50 m × 180 m (< 100 m penetration). Positioning was performed with an auto‐tracking laser ranging system, assuring an absolute accuracy of a few decimetres.
Preliminary processing steps included tidal correction and multiple removal. An important step concerned the application of 3D prestack Kirchhoff depth migration. Indeed this processing allows easy handling of the exact positions of both source and receivers as the latter were not set out on a conventional regular grid due to navigation difficulties. Because of the restricted data volume and the more‐or‐less stratified medium, a 1D velocity model could be used. This allowed a considerable simplification of the migration algorithm, based on summation. Traveltimes were calculated only once, using a 2D time grid with 0.1 m intervals.
This migration method proved very efficient, greatly improving the seismic image, and involved only limited CPU time on a small computer (Sparc 10 workstation). It clearly demonstrates that advanced seismic processing can form a valuable and economically feasible tool for VHR shallow subsurface 3D seismics, as long as the velocity field is not too complex. This method should therefore no longer be restricted to large computers and hydrocarbon exploration, but should also become a routine for VHR 3D shallow seismic work.
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Surface and underground measurements of geomagnetic variations in the micropulsations band
Geomagnetic fluctuations in the frequency band 2–70 mHz recorded simultaneously at a depth of about 1200 m in the underground Gran Sasso Laboratory (central Italy) and on the earth's surface (approximately along the Laboratory vertical) are compared using multivariate spectral analysis. Experimental problems and analytical techniques adopted for the signal processing are discussed. In particular, a modification of the standard least‐squares method for estimating multiple‐input transfer functions is proposed. The results show apparent different skin effects for each magnetic field component and a significant coherence between the underground vertical signal and the horizontal signals, suggesting the presence of lateral inhomogeneities in the underground conductivity structure. The results are also consistent with an average resistivity of the intervening medium of the order of 10–20 Ωm and with the presence of a more conductive layer at greater depth.
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iSPITM— the improved source parameter imaging method
Authors Richard S. Smith, Jeffrey B. Thurston, Ting‐Fan Dai and Ian N. MacLeodInterpretation of an anomalous magnetic response involves determining the parameters that characterize the source of the anomaly. The depth to the top of the structure is a parameter that is commonly sought, and the Source Parameter ImagingTM (SPITM) method is one way of determining this depth estimate. One advantage of the SPI method is that the depths can be displayed on an image. Typically there can be one image for an assumed contact (fault) model and another image for an assumed dipping thin sheet (dike) model. The depth estimate obtained will depend on the model assumed. An improvement to the source parameter imaging method extends the method to horizontal cylinders and at the same time allows the most appropriate model to be determined automatically. This model can be displayed on an image and the correct depth estimate for each anomaly can also be determined. The depth estimates can therefore be summarized on one map independent of an assumed model. The images generated from synthetic and field data show that the improved SPI method makes the task of interpreting magnetic data significantly easier.
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A modelling study of open‐hole single‐well seismic imaging
More LessSingle‐well, or uni‐well, imaging uses an acoustic source and an array of receivers located in the same borehole to image local geological structure. Due to the intrinsic attenuation of the formation it is likely that a source emitting frequencies in the typical cross‐well range would be necessary to illuminate structure at distances above 100 m from the borehole. At these frequencies a significant proportion of the source energy is converted into tube‐waves which are, for the purpose of these surveys, noise. This paper reports the results of a modelling study designed to assess the feasibility of using existing cross‐well hardware, i.e. a piezo‐electric source and hydrophone array, modified to run in a single borehole, to perform single‐well surveys. In particular we study the case of an open borehole in a gas‐filled, low‐permeability sandstone reservoir. Our results suggest that the amplitudes of the tube‐wave reverberations generated by calliper variations are such that reflections of interest arriving in the time window after the first tube‐wave arrival will not be visible. However, reflections may be visible in the time window preceding the first tube‐wave arrival provided tube waves from previous shots are not still present and the long source–receiver offsets required to make observations in this window can be incorporated into the tool design.
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Microlevelling procedures applied to regional aeromagnetic data: an example from the Transantarctic Mountains (Antarctica)
Authors Fausto Ferraccioli, Marco Gambetta and Emanuele BozzoThe extensive application of digital enhancement and filtering as a powerful tool for aeromagnetic interpretation, not only of high resolution but also of regional data, requires an improved levelling. Two microlevelling techniques were thus compared in order to find an effective but relatively simple procedure to remove, or at least to reduce, residual magnetic errors remaining after standard levelling processes. This study was carried out on regional aeromagnetic data recently acquired at high magnetic latitudes along the Transantarctic Mountains in Antarctica, where it is particularly critical to remove time‐dependent magnetic variations. Two‐dimensional FFT filters applied to the gridded data, namely the Butterworth and a directional cosine filter, proved to be more effective than previously proposed one‐dimensional space‐domain filters in the reduction of the ‘residual corrugation’ not removed by statistical levelling. Tectonic interpretation of trends detected in the total field magnetic anomaly map and in the 3D analytic signal improved after application of frequency‐domain microlevelling. However, we also show that when interpreting microlevelled data, two factors must be considered: (i) the possible presence of real geological trends aligned along the flight lines; (ii) modifications in the results yielded by depth estimates of magnetic sources due to the FFT filters applied during the microlevelling procedure. Such changes were seen both in the well‐established 2D FFT method, based on the slope of the energy spectrum, and in the more recent 3D Euler deconvolution technique. Overall our results indicate that microlevelling could profitably be applied to older gridded aeromagnetic data sets in Antarctica, thus improving the accuracy and geological significance of future regional magnetic compilations, as already seen in other continents.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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Volume 63 (2015)
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Volume 62 (2014)
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Volume 61 (2013)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 58 (2010)
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Volume 57 (2009)
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Volume 56 (2008)
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Volume 55 (2007)
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Volume 53 (2005)
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Volume 52 (2004)
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Volume 51 (2003)
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Volume 50 (2002)
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Volume 49 (2001)
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Volume 48 (2000)
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Volume 47 (1999)
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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 43 (1995)
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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 35 (1987)
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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 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 18 (1970)
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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)