- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 57, Issue 6, 2009
Geophysical Prospecting - Volume 57, Issue 6, 2009
Volume 57, Issue 6, 2009
-
-
Lax‐Wendroff and Nyström methods for seismic modelling
By Jing‐Bo ChenABSTRACTLax‐Wendroff and Nyström methods are numerical algorithms of temporal approximations for solving differential equations. These methods provide efficient algorithms for high‐accuracy seismic modeling. In the context of spatial pseudospectral discretizations, I explore these two kinds of methods in a comparative way. Their stability and dispersion relation are discussed in detail. Comparison between the fourth‐order Lax‐Wendroff method and a fourth‐order Nyström method shows that the Nyström method has smaller stability limit but has a better dispersion relation, which is closer to the sixth‐order Lax‐Wendroff method. The structure‐preserving property of these methods is also revealed. The Lax‐Wendroff methods are a second‐order symplectic algorithm, which is independent of the order of the methods. This result is useful for understanding the error growth of Lax‐Wendroff methods. Numerical experiments based on the scalar wave equation are performed to test the presented schemes and demonstrate the advantages of the symplectic methods over the nonsymplectic ones.
-
-
-
Upward continuation with topographic datuming operator: the integrated wave equation datuming scheme revised
Authors Kai Yang, Yu‐Zhu Liu, Jian‐Hua Geng and Zai‐Tian MaABSTRACTThe integrated wave equation datuming scheme is an integrated datuming method to implement the wave equation velocity replacement with a one‐way extrapolator for overthrust data. The integrated wave equation datuming scheme is accomplished by downward continuing the data from the topography to a non‐planar base of the weathering layer, followed by an upward continuation from the base of the weathering layer to a final planar datum. Both the downward and the upward continuation are accomplished by a one‐way extrapolator by a classical ‘two‐step’ implementation. The topographic datuming operator is a distinctive technique by which the shots and receivers are downward continued simultaneously. Although its inadaptability to non‐planar datum and the strong lateral variation of the near‐surface structure constrains its application to the downward continuation of the overthrust data, we found that the ‘one‐step’ feature of the topographic datuming operator is extremely suitable for the upward continuation. In this paper, the topographic datuming operator is incorporated into the integrated wave equation datuming scheme to perform upward continuation instead of a one‐way extrapolator – thus a revised integrated wave equation datuming scheme is constructed. Compared with the previous scheme, the computatiol cost of the revised integrated wave equation datuming scheme is greatly reduced, thus the applicability of the integrated wave equation datuming scheme is further improved. The synthetic and real data examples demonstrate its effectiveness and efficiency.
-
-
-
Operator‐oriented CRS interpolation
Authors German Hoecht, Patrice Ricarte, Steffen Bergler and Evgeny LandaABSTRACTIn common‐reflection‐surface imaging the reflection arrival time field is parameterized by operators that are of higher dimension or order than in conventional methods. Using the common‐reflection‐surface approach locally in the unmigrated prestack data domain opens a potential for trace regularization and interpolation. In most data interpolation methods based on local coherency estimation, a single operator is designed for a target sample and the output amplitude is defined as a weighted average along the operator. This approach may fail in presence of interfering events or strong amplitude and phase variations. In this paper we introduce an alternative scheme in which there is no need for an operator to be defined at the target sample itself. Instead, the amplitude at a target sample is constructed from multiple operators estimated at different positions. In this case one operator may contribute to the construction of several target samples. Vice versa, a target sample might receive contributions from different operators. Operators are determined on a grid which can be sparser than the output grid. This allows to dramatically decrease the computational costs. In addition, the use of multiple operators for a single target sample stabilizes the interpolation results and implicitly allows several contributions in case of interfering events. Due to the considerable computational expense, common‐reflection‐surface interpolation is limited to work in subsets of the prestack data. We present the general workflow of a common‐reflection‐surface‐based regularization/interpolation for 3D data volumes. This workflow has been applied to an OBC common‐receiver volume and binned common‐offset subsets of a 3D marine data set. The impact of a common‐reflection‐surface regularization is demonstrated by means of a subsequent time migration. In comparison to the time migrations of the original and DMO‐interpolated data, the results show particular improvements in view of the continuity of reflections events. This gain is confirmed by an automatic picking of a horizon in the stacked time migrations.
-
-
-
Treatment of conflicting dips in the 3D common‐reflection‐surface stack
More LessABSTRACTThe 3D common‐reflection‐surface stack is a tool to simulate zero‐offset sections from the prestack data. In conventional implementations conflicting dip situations, i.e., the contribution of more than one stacking operator per output location, are not taken into account. This leads to stacked sections with fragmentary events and to incomplete sets of stacking parameters. Subsequent applications based on the results of the common‐reflection‐surface stack and its attributes will show deteriorated quality.
In this paper, I present a modified workflow for the handling of the conflicting dip problem in context of the 3D common‐reflection‐surface stack. The strategy utilizes the path‐summation technique to obtain an improved input for the conflicting dip search in the zero‐offset domain. The actual detection is done by means of an adapted peak detection algorithm. For each detected event consistent sets of attributes are obtained by a newly introduced search step.
Two 3D real data applications show the applicability of the proposed technique. The strategy proves to resolve most of the conflicting dip situations even for poor signal‐to‐noise ratio. With only moderate additional computational cost the presented method provides superior results compared to the conventional 3D common‐reflection‐surface stack.
-
-
-
Seismic refraction traveltime inversion for static corrections in a glaciated shield rock environment: a case study
More LessABSTRACTA traveltime inversion technique is applied to model the upper ∼40 m of the subsurface of a glaciated shield rock area in order to calculate static corrections for a multi‐azimuth multi‐depth walk‐away vertical seismic profile and a surface seismic reflection profile. First break information from a seismic refraction survey is used in conjunction with a ray‐tracing program and an iterative damped least‐squares inversion algorithm to create a two‐dimensional model of the subsurface. The layout of the seismic survey required crooked seismic lines and substantial gaps in the source and receiver coverage to be accounted for. Additionally, there is substantial topographical variation and a complex geology consisting of glaciofluvial sediment and glacial till overlying a crystalline bedrock. The resolution and reliability of the models is measured through a parameter perturbation technique, normalized χ2 values, root means square traveltime residuals and comparison to known geology.
-
-
-
Simultaneous core sample measurements of elastic properties and resistivity at reservoir conditions employing a modified triaxial cell – a feasibility study
Authors Zhong Wang, Leiv‐J. Gelius and Fan‐Nian KongABSTRACTAdaptations of existing triaxial cells for ultrasonic P‐ and S‐wave measurements are well documented. This paper proposes further modification of such a cell so that also resistivity measurements can be carried out simultaneously at reservoir conditions. By employing the top cap and the pedestal of the cell as electrodes, axial resistivity measurements are now feasible. In order to minimize the polarization effect of this two‐electrode arrangement, careful analyses have been carried out to optimize the choice of electrode coating and measurement frequency band. Radial resistivity measurements are also included in the system by introducing a strap‐electrode system.
In a reservoir under production changes in both saturations, temperature (if steam injection) and stresses can take place. Therefore the modified triaxial system should be able to measure the integrated effects on the acoustic parameters and electric responses caused by variations in each of these parameters. The feasibility of the system to obtain such reliable information is demonstrated, employing a small selection of core samples. In the future such combined measurements on reservoir core samples can be used to link both seismic and electromagnetic observations to the actual earth model and constrain both modelling and inversion.
-
-
-
Experimental research on the rock electrical parameter dispersion properties in the displacement process
Authors Zhanshan Xiao, Shengwen Zhang, Tiexuan Zhang, Shihe Zhu and Jing MaABSTRACTBased on experiments on rock electrical parameter dispersion properties in both water displacing oil and oil displacing water processes, it is found that the frequency dispersion properties of electrical parameters in water‐bearing rocks are quite different from those in oil‐bearing rocks. Oil saturation can severely impact the rock electrical parameter dispersion properties and as the oil saturation increases, the properties are more distinguishing. The variation of formation water salinity has a significant effect on the rock resistivity, hence the modulus of the rock complex resistivity varies with water saturation and a U‐shaped curve occurs over a single frequency, just like the normal resistivity. However, the effect of formation water salinity on the rock frequency dispersion is less than that of oil saturation. As a result, the rock electrical parameter dispersion properties will have a significant potential of application in the evaluation of watered‐out zones as well as identification of oil‐bearing zones and water‐bearing zones.
-
-
-
The use of reference models from a priori data to guide 2D inversion of electrical resistivity tomography data
Authors Lucy M. L. Catt, L. Jared West and Roger A. ClarkABSTRACTClay‐rich till plains cover much of the UK. Such sites are attractive locations for landfills, since clay aquitards lower the risk of landfill leachate entering groundwater. However, such tills often contain sand and gravel bodies that can act as leachate flow routes. Such bodies may not be detected by conventional site investigation techniques such as drilling boreholes and trial pitting. A method of guided inversion, where a priori data are used to construct structural reference models for use in inverting electrical resistivity tomography data, was proposed as a tool to improve the detection of sand and gravel bodies within clay‐rich till sequences.
Following a successful 2D guided inversion synthetic modelling study, a field study was undertaken. Wenner 2D electrical resistivity tomography lines, resistivity cone penetrometry bores and electromagnetic induction ground resistivity data were collected over a site on the East Yorkshire coast, England, where sand and gravel lenses were known to exist from cliff exposures. A number of equally valid geoelectrical models were constructed using the electromagnetic and resistivity cone data. These were used as structural reference models in the inversion of the resistivity tomography data. Blind inversion using an homogenous reference model was also carried out for comparison.
It was shown for the first time that the best solution model produced by 2D inversion of one data set with a range of structural reference models could be determined by using the l2 model misfit between the solution models and associated reference models (reference misfit) as a proxy for the l2 misfit between the solution models and the synthetic model or ‘best‐guess’ geoelectrical model (true misfit). The 2D methodology developed here is applicable in clay‐rich till plains containing sand and gravel bodies throughout the UK.
-
-
-
Artificial neural networks applied to mineral potential mapping for copper‐gold mineralizations in the Carajás Mineral Province, Brazil
Authors Emilson Pereira Leite and Carlos Roberto De Souza FilhoABSTRACTMotivated by recent successful results of artificial neural network modelling in a variety of problems related to the geosciences, we have applied a radial basis functional link net to a regional‐scale mapping of the potential for Cu‐Au mineralizations in the Carajás Mineral Province, northern Brazil. To derive the input feature vectors, we have used geological and both radiometric and magnetic geophysical data. A k‐fold cross‐validation method was employed in order to tune the parameters of the network and to select the best radial basis functional link net model amongst several others. Subsets of the available data set were used for training and validation and the estimated overall accuracy of the selected model is 91.7%. The plotting of a cumulative area versus favourability curve allowed us to define favourability zones of occurrences of Cu‐Au mineralizations and to assess the efficiency and the predictive power of the model. A binary map showing high and low favourability sectors was produced for the study area as an end product that can be used to guide and support more detailed exploration efforts. Our results show that 4.18% of the study area has an extremely high potential to contain Cu‐Au mineralizations, especially those of iron‐oxide Cu‐Au type, which are related to volcanic rocks and hydrothermal alteration.
-
-
-
Terracing potential field data
Authors G.R.J. Cooper and D.R. CowanABSTRACTTerracing is an operator that is applied to potential field data to produce regions of constant field amplitude that are separated by sharp boundaries, as an aid to interpretation. When applied to map data, the boundaries are defined by the zero contour of the 2D Laplacian derivative operator. An improved method is described here that defines the boundaries by the zero contour of the profile curvature. This approach gives superior results because the 2D Laplacian operator is composed only of derivatives in the EW and NS directions, while the profile curvature uses the curvature in the ‘uphill’ direction at each point, whatever that direction may be. The method is demonstrated on gravity data from South Africa. Source code in Matlab format is available from the authors on request.
-
-
-
Estimating the near surface amplification factor to minimize earthquake damage: a case study at West Wadi Hagoul area, Egypt
More LessABSTRACTThe damage caused by earthquake occurrences in different localities all over the world necessitates the evaluation of the underground and/or sub‐soil structure for strategic projects. The case study is the proposed area for a new factory construction that lies 3 km west of Suez Cement factory, in the area between Kottamiya and Ain Sokhna. The shallow seismic refraction survey was carried out through 10 major lines (grid lines), comprising 30 seismic profiles. The microtremor measurements were done at 25 locations at the grid intersection points. The obtained compressional (P) and horizontal shear (SH) wave velocities, with the horizontal to vertical (H/V) spectral ratios, are used to deduce the subsurface structures, to evaluate the elastic and dynamic properties and the transfer function at both the surface and bedrock. The results illustrate that there is a good correlation between the fundamental frequencies deduced through the theoretical transfer function and the H/V spectral ratio. The maximum value of ground motion amplification factor of the study area is 3.3 at a corresponding frequency of 7.8 Hz.
-
-
-
Saturation effect on electrical properties of hematitic sandstone in the audio frequency range using non‐polarizing electrodes
More LessABSTRACTThis paper is devoted to study the effect of saturation, with distilled water, on AC electrical conductivity and dielectric constant of a fully and partially saturated hematitic sandstone sample (Aswan area, Egypt). The saturation of the sample was changed from full saturation to partial saturation by air drying. Complex resistivity measurements at room temperature (∼16° C) were performed in the frequency range from 10 Hz to 100 kHz. We used non‐polarizing Cu/CuSO4 gel electrodes. Experimental electrical spectra indicate, generally, that the electrical conductivity and dielectric constant vary strongly with water saturation and frequency. The low‐frequency electrical conductivity and dielectric constant are supposed to be mainly controlled by surface conduction and polarization of the electrical double layer. Power law behaviours with frequency were noticed. The change in electrical conductivity and dielectric constant with increasing water content is fast at low saturations and slow at high saturations. The behaviour of the electrical conductivity and dielectric constant, with increasing water content, was argued to be the orientational polarization of bound water for very low saturations, displacement of the excess surface charges for relatively low saturations and free exchange of excess ions in double layer with the bulk electrolyte and generation of transient diffusional potentials, which lag behind the applied field for high saturations in addition to membrane polarization on clay and at inter‐grain and grain surface water throats having selective charge transport properties. Also, from the data a semi‐percolation behaviour was found that has a peak of dielectric constant at a certain concentration and an abrupt change in conductivity at another saturation.
-
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)