- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 56, Issue 2, 2008
Geophysical Prospecting - Volume 56, Issue 2, 2008
Volume 56, Issue 2, 2008
-
-
On the classification of surface geoelectric arrays
Authors Sándor Szalai and László SzarkaABSTRACTWe collected approximately one hundred independent geoelectric arrays from published geophysical literature. We have presented them in a systematic way and with a unified notation. The classification of arrays is based on three divalent parameters: “superposition” of measurements, “focusing” of currents and “colinearity” of the array, creating 8 classes of geoelectric arrays.
Among the 102 independent arrays we found in the geophysical literature, we managed to classify 92 arrays in the aforementioned way. Ten further techniques fell beyond the proposed classification.
The classification we propose may open the way to new geoelectric arrays, hopefully providing improving responses to the infinite variety of field problems we may face. It may bring to daylight, exclusively in a logical way, currently unused arrays. In searching new geoelectric arrays this paper helps to avoid rediscovering the discovered. Although it might be thought that the modern multielectrode systems will supersede all former arrays, such systematization is not only for historical and tutorial interests: some of the “old” arrays can be perhaps built into new multielectode systems, further enhancing their effectivity in the future. Finally, this collection of arrays establishes the possibility of systematic intercomparisons of arrays on the basis of various theoretical or practical aspects.
-
-
-
Unification of single‐configuration seismic imaging processes
Authors H. Jaramillo and U. AlbertinABSTRACTIn this paper we derive an integral formula that encompasses all linear processes on seismic data. These include migration, demigration and residual migration, as well as data mapping procedures such as transformation to zero offset, inverse transformation to zero offset, residual transformation to zero offset and offset continuation. The derivation of the equation is different from all previous approaches to unification. Here we do not use a cascaded operation between two operators, but rather the superposition principle. In this regard, the derivation is not only more fundamental, but also simpler and more general. We study the kinematics and the dynamics of these processes and show that the signals can be reconstructed asymptotically either by finding the envelope of particular surfaces or by stacking energy along “adjoint” surfaces. For example, in the case of migration, the first set of surfaces are isochrons, while the “adjoint” surfaces are diffraction responses. In practice, the distinction between these two types of surfaces is equivalent to choosing the order of the computational loops with regard to the input and output seismic traces.
-
-
-
P‐ and S‐wave velocities of consolidated sediments from a seafloor seismic survey in the North Celtic Sea Basin, offshore Ireland
More LessABSTRACTA geophysical survey was conducted over a hydrocarbon prospect in the North Celtic Sea Basin using a small array of ocean‐bottom seismographs (OBSs). The purpose of this study was to determine the ratio of compressional (P)‐ to shear (S)‐wave velocity of consolidated sedimentary rocks in order to constrain possible subsurface variations in pore‐fluid content. The ratio of VP and VS is known to be particularly sensitive to lithology, porosity and pore‐fluid content, making it a useful parameter for evaluating hydrocarbon prospects. OBSs offer a relatively cheap and time‐effective means of acquiring multi‐component data compared with ocean‐bottom cables. In this contribution, we demonstrate the ability of an OBS survey comprising three pairs of two OBSs spaced at 1.6 km to recover lateral variations in the VP/VS ratio. A key requirement of this type of study is that S waves will be generated by mode conversions in the subsurface, since they cannot be generated in nor travel through fluids. In this survey, the contrast in physical properties of the hard seabed of the North Celtic Sea Basin provided a means of generating converted S waves. Two‐dimensional ray‐tracing and forward modelling was used to create both VP and VS models along a profile crossing the Blackrock prospect in the North Celtic Sea Basin. These models comprise four layers and extend to a maximum depth of 1.1 km. The observed northward decrease in the VP/VS ratio at depths of 500–1000 m below the seafloor in the study area is interpreted to represent lateral variation in the amount of gas present in the pore space of Upper Cretaceous chalks and shales overlying the prospective reservoir.
-
-
-
2D PP/PS‐stereotomography: Application to a real 2D‐OBC dataset
Authors M. Alerini, G. Lambaré, R. Baina, P. Podvin and S. Le BégatABSTRACTIt has been shown on an ‘ideal’ synthetic dataset that PP/PS‐stereotomography can estimate an accurate velocity model without any pairing of PP‐ and PS‐events. The P‐wave velocity model is first estimated using PP data and then, fixing this velocity field, the S‐wave velocity is estimated using the PS data. This method needed to be evaluated further and we present here the first application of PP/PS‐stereotomography to a real dataset: the 2D East‐West Mahogany OBC line (Gulf of Mexico). We are here confronted with data which do not fit our working assumptions: coherent noise (due to an approximate separation of PP‐ and PS‐events and some remaining multiples), probably some anisotropy and 3D effects. With a careful selection of the stereotomographic picks, which allows one to decrease the effect of the picked coherent noise by the automatic picker, our application can demonstrate the relevance of our approach in the upper part of the profile, where anisotropy and 3D effects might be low. We can thus estimate, without any pairing of PP‐ and PS‐events, a velocity field which provides not only flat common image gathers, but also PP‐ and PS‐depth migrated images located at the same positions. For the deeper part of the profile, a significant shift in depth appears. In addition to possible anisotropy, 3D effects and a more complex velocity field (‘salt body’), this is due to the quality of the PZ‐ and X‐components profiles: The PZ‐component profile where the PP‐stereotomographic picking is performed, is polluted by conflicting converted or multiple events and the X‐component profile, where the PS‐stereotomographic picking is performed, is highly noisy. This study emphasizes the need to develop accurate selection criteria for the stereotomographic picks.
-
-
-
A frequency criterion for optimal node selection in smoothing with cubic splines
Authors Jörg Schleicher and Ricardo BilotiABSTRACTWhen smoothing a function with high‐frequency noise by means of optimal cubic splines, it is often not clear how to choose the number of nodes. The more nodes are used, the closer the smoothed function will follow the noisy one. In this work, we show that more nodes mean a better approximation of Fourier coefficients for higher frequencies. Thus, the number of nodes can be determined by specifying a frequency up to which all Fourier coefficients must be preserved and increasing the number of nodes until this criterion is met. A comparison of the corresponding smoothing results with those obtained by filtering using moving average and moving median filters of corresponding length and a low pass with corresponding high‐cut frequency shows that optimal cubic splines yield better results as they preserve not only the desired low‐frequency band but also important high‐frequency characteristics.
-
-
-
Laboratory estimates of normal and shear fracture compliance
Authors R. Lubbe, J. Sothcott, M.H. Worthington and C. McCannABSTRACTLaboratory estimates of the normal (Bn) and shear (Bt) compliance of artificial fractures in samples of Jurassic and Carboniferous limestone under wet and dry conditions are presented. The experiments were performed over a range of confining pressures (from 5 MPa up to 60 MPa), at ultrasonic frequencies in a Triaxial Hoek cell, using the pulse‐echo reflection technique. The results of this study confirm that the Bn/Bt ratio of a fracture is dependent on the fluid fill. A value of Bn/ Bt of less than 0.05 was obtained for our wet (honey saturated) sample which is consistent with the prediction that this ratio should be close to zero for fluid saturated fractures. Values of Bn/Bt for the dry sample are significantly higher and increase with confining pressure from 0.2 to 0.5. It is suggested that a Bn/Bt ratio of 0.5 is probably a more representative value to use in modelling studies of gas filled fractures than the common assumption that Bn ≈ Bt.
-
-
-
An algorithm to calculate the gravity anomaly of sedimentary basins with exponential density‐depth relationships
Authors Alex Chappell and Nick KusznirABSTRACTWe derive wavenumber domain expressions to calculate the gravity anomaly of a body with irregular bounding surfaces and an exponential density‐depth relationship. We apply the method to sedimentary basins, which commonly have this type of geometry and density distribution. The mathematical formulation also allows the exponential density‐depth relationship to be measured from an arbitrary irregular surface rather than the top surface. Using this arrangement, the gravity anomaly of exhumed sedimentary basins can be predicted if the amount of eroded section can be estimated. The corresponding inverse algorithms are also derived. Examples of the use of the forward algorithms, from the Galicia Interior Basin and the Central Irish Sea Basin, are used to illustrate these methods.
-
-
-
Inversion of magnetotelluric data in the presence of strong bathymetry/topography
Authors Shenghui Li, John R. Booker and Claudia ApreaABSTRACTWe describe an algorithm for inverting magnetotelluric (MT) data in the presence of strong bathymetry or topography. Instead of correcting distortions due to bathymetry or topography we incorporate them directly into the inversion. To achieve a high accuracy in computing MT responses we use finite‐difference approximations that permit sloping discrete boundaries inside model elements. The same approach is applicable to any seafloor electromagnetic calculation and can also be used to incorporate steep topography on land. We test our approach on various topographic features and compare our results to that of a finite element approach. Finally, we present inversion examples that illustrate the effectiveness of our inversion algorithm in recovering true subsurface structures in the presence of strong bathymetry and topography.
-
-
-
Redox potential distribution inferred from self‐potential measurements associated with the corrosion of a burden metallic body
Authors J. Castermant, C.A. Mendonça, A. Revil, F. Trolard, G. Bourrié and N. LindeABSTRACTNegative self‐potential anomalies can be generated at the ground surface by ore bodies and ground water contaminated with organic compounds. These anomalies are connected to the distribution of the redox potential of the ground water. To study the relationship between redox and self‐potential anomalies, a controlled sandbox experiment was performed. We used a metallic iron bar inserted in the left‐hand side of a thin Plexiglas sandbox filled with a calibrated sand infiltrated by an electrolyte. The self‐potential signals were measured at the surface of the tank (at different time lapses) using a pair of non‐polarizing electrodes. The self‐potential, the redox potential, and the pH were also measured inside the tank on a regular grid at the end of the experiment. The self‐potential distribution sampled after six weeks presents a strong negative anomaly in the vicinity of the top part of the iron bar with a peak amplitude of −82 mV. The resulting distributions of the pH, redox, and self‐potentials were interpreted in terms of a geobattery model combined with a description of the electrochemical mechanisms and reactions occurring at the surface of the iron bar. The corrosion of iron yields the formation of a resistive crust of fougerite at the surface of the bar. The corrosion modifies both the pH and the redox potential in the vicinity of the iron bar. The distribution of the self‐potential is solved with Poisson's equation with a source term given by the divergence of a source current density at the surface of the bar. In turn, this current density is related to the distribution of the redox potential and electrical resistivity in the vicinity of the iron bar. A least‐squares inversion method of the self‐potential data, using a 2D finite difference simulation of the forward problem, was developed to retrieve the distribution of the redox potential.
-
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)