 Home
 AZ Publications
 Geophysical Prospecting
 Previous Issues
 Volume 48, Issue 3, 2000
Geophysical Prospecting  Volume 48, Issue 3, 2000
Volume 48, Issue 3, 2000


A parallel laboratory for simulation and visualization of seismic wavefields
Authors F. Javier Sabadell, Francisco J. Serón and José BadalThe use of parallel computers makes simulation of elastic waves feasible throughout large structures by means of recent advances in domain decomposition methods. We introduce a competitive parallel algorithm for the propagation of elastic waves in complex heterogeneous media using finite‐element discretization. This parallel method, called the multiblock method, performs more efficiently than classical domain decomposition techniques based on substructuration, such as the Schur complement technique. It reduces considerably the amount of communication amongst processors because the interface problem between subdomains is solved by taking advantage of Huygens' principle for wave propagation. We provide some numerical examples and detailed studies on the efficiency and performance of the algorithm, proving that it is competitive and less costly, from the computational viewpoint, than algorithms based on the Schur technique.



Controlled sources for shear‐wave surveys in mines [Link]
Authors Gordon M. Holmes, Stuart Crampin and R. Paul YoungThe ability to analyse shear‐wave anisotropy in a mine environment is greatly aided by using multiple source orientations of a reproducible, impulsive shear‐wave source. The analysis of what is probably the first controlled source shear‐wave experiment in a mine environment demonstrates clearly that shear‐wave polarizations and time delays between split shear‐wave arrivals are reliably measured because of the use of multiple source orientations rather than a single shear‐wave source. Reliability is further aided by modelling the shear‐wave source radiation pattern, which allows for the unequivocal discrimination between seismic raypaths where shear‐wave splitting did and did not occur. The analysis also demonstrates the great importance of high reproducibility of the seismic source for the use of shear waves in time‐lapse surveys to monitor changes in a rockmass.



Seismic anisotropy in granite at the Underground Research Laboratory, Manitoba [Link]
Authors Gordon M. Holmes, Stuart Crampin and R. Paul YoungThe Shear‐Wave Experiment at Atomic Energy of Canada Limited's Underground Research Laboratory was probably the first controlled‐source shear‐wave survey in a mine environment. Taking place in conjunction with the excavation of the Mine‐by test tunnel at 420 m depth, the shear‐wave experiment was designed to measure the in situ anisotropy of the rockmass and to use shear waves to observe excavation effects using the greatest variety of raypath directions of any in situ shear‐wave survey to date. Inversion of the shear‐wave polarizations shows that the anisotropy of the in situ rockmass is consistent with hexagonal symmetry with an approximate fabric orientation of strike 023° and dip 35°. The in situ anisotropy is probably due to microcracks with orientations governed by the in situ stress field and to mineral alignment within the weak gneissic layering. However, there is no unique interpretation as to the cause of the in situ anisotropy as the fabric orientation agrees approximately with both the orientation expected from extensive‐dilatancy anisotropy and that of the gneissic layering. Eight raypaths with shear waves propagating wholly or almost wholly through granodiorite, rather than granite, do not show the expected shear‐wave splitting and indicate a lower in situ anisotropy, which may be due to the finer grain size and/or the absence of gneissic layering within the granodiorite. These results suggest that shear waves may be used to determine crack and mineral orientations and for remote monitoring of a rockmass. This has potential applications in mining and waste monitoring.



Improving modelling and inversion in refraction seismics with a first‐order Eikonal solver
Authors Isabelle Lecomte, Håvar Gjøystdal, Anders Dahle and Ole Christian PedersenA first‐order Eikonal solver is applied to modelling and inversion in refraction seismics. The method calculates the traveltime of the fastest wave at any point of a regular grid, including head waves as used in refraction. The efficiency, robustness and flexibility of the method give a very powerful modelling tool to find both traveltimes and raypaths. Comparisons with finite‐difference data show the validity of the results. Any arbitrarily complex model can be studied, including the exact topography of the surface, thus avoiding static corrections. Later arrivals are also obtained by applying high‐slowness masks over the high‐velocity zones. Such an efficient modelling tool may be used interactively to invert for the model, but a better method is to apply the refractor‐imaging principle of Hagedoorn to obtain the refractors from the picked traveltime curves. The application of this principle has already been tried successfully by previous authors, but they used a less well‐adapted Eikonal solver. Some of their traveltimes were not correct in the presence of strong velocity variations, and the refractor‐imaging principle was restricted to receiver lines along a plane surface. With the first‐order Eikonal solver chosen, any topography of the receiving surface can be considered and there is no restriction on the velocity contrast. Based on synthetic examples, the Hagedoorn principle appears to be robust even in the case of first arrivals associated with waves diving under the refractor. The velocities below the refractor can also be easily estimated, parallel to the imaging process. In this way, the model can be built up successively layer by layer, the refractor‐imaging and velocity‐mapping processes being performed for each identified refractor at a time. The inverted model could then be used in tomographic inversions because the calculated traveltimes are very close to the observed traveltimes and the raypaths are available.



Electrical resistivity tomography to investigate geological structures of the earth's upper crust [Link]
More LessIt is important to have detailed knowledge of the electrical properties of the earth's crust in order to recognize geological structures and to understand tectonic processes. In the area surrounding the German Continental Deep Drilling Project (KTB), we have used DC dipole–dipole soundings to investigate the electrical conductivity distribution down to a depth of several kilometres. We have adapted the electrical resistivity tomography (ERT) technique, a well‐established near‐surface method, to large‐scale experiments. Independent transmitting and receiving units were used to realize the concept of simultaneous multichannel registration of the scalar electrical potential at 44 dipoles. The measured data yielded apparent resistivities which were inverted to a 2D resistivity model ranging from the surface down to a depth of 4 km. Two highly conductive structures with steep inclination were detected. They are expected to be major fault zones embedded in a metamorphic body. The rather low resistivity (ρ < 10 Ωm) can be explained by the existence of graphitic minerals and/or electrolytic fluids.



Constant normal‐moveout (CNMO) correction: a technique and test results [Link]
Authors Andrew Shatilo and Fred AminzadehWe introduce a processing technique which minimizes the ‘stretching effects’ of conventional NMO correction. Unlike conventional NMO, the technique implies constant normal moveout (CNMO) for a finite time interval of a seismic trace. The benefits of the proposed method include preservation of higher frequencies and reduction of spectral distortions at far offsets. The need for severe muting after the correction is reduced, allowing longer spreads for stack, velocity and AVO analysis. The proposed technique has been tested on model and real data. The method may improve the resolution of CMP stack and AVO attribute analysis. The only assumptions for this stretch‐free NMO correction are (i) all time samples of a digital reflected wavelet at a particular offset have the same normal moveout, and (ii) reflection records have an interference nature.



A first attempt at monitoring underground gas storage by means of time‐lapse multichannel transient electromagnetics [Link]
More LessTwo successive transient electromagnetic surveys were carried out over an underground gas storage site in France. The idea was to monitor changes in the gas bubble from the differences in the data. If successful, the new methodology could help to reduce the number of monitoring wells and finally reduce costs. Preliminary 3D modelling indicated that the resistivity changes caused by movements of the gas/water contact should be detectable in the electric field transients provided that the signal‐to‐noise ratio is at least 100:1. The surveys were performed with the TEAMEX multichannel acquisition system, adapted from a seismics system. The highly redundant data were analysed by calculating the relative differences in the electric field transients. The differences were common‐midpoint‐sorted and spatially stacked. Another approach was the calculation of electric field time derivatives in a log–log domain, to eliminate static shift effects which are present in the data. Even though the data quality is excellent from a classical point of view, neither of the two approaches reveals changes in the data which might be caused by changes in the gas reservoir. In future applications to monitoring, transmitters and receivers should be installed permanently, and the transmitter input waveform should be monitored continuously, to avoid some of the problems encountered here. Moreover, the signal‐to‐noise ratio will have to be further increased by at least one order of magnitude.



1D complete calculation for electrostatic soundings interpretation
Authors Alain Tabbagh and Cédric PanissodSoundings achieved with the electrostatic method cannot be interpreted correctly with 1D electrical programs if the extension of the array or the conductivity of the ground are too large. A complete calculation taking into account the induction effect due to the frequency must be performed. This paper presents the solutions for overcoming the difficulties encountered in this calculation: the iterative processes which make it possible to determine the kernel functions of the Hankel transforms and the analytic integration of the terms of the electric field. Application to practical cases first illustrates the distortion of the electrostatic curves by reference to DC sounding curves. The limitation in depth of investigation is then emphasized: in practice, the investigation is limited by the skin depth corresponding to the frequency used. The examples of the soundings obtained in the city of Alexandria (Egypt) demonstrate the importance of using the complete calculation for very conductive grounds.



Non‐equilibrium compaction and abnormal pore‐fluid pressures: effects on rock properties^{1}
Authors José M. Carcione and Anthony F. GangiKnowledge of pore pressure using seismic data will help in planning the drilling process to control potentially dangerous abnormal pressures. Various physical processes cause anomalous pressures on an underground fluid. Non‐equilibrium compaction is a significant process of overpressure generation. This occurs when the sedimentation rate is so rapid that the pore fluids do not have a chance to ‘escape’ from the pore space.
The model assumes a closed system and that the pore space is filled with water and hydrocarbon in a liquid state. Balancing mass and volume fractions yields the fluid pressure versus time of deposition and depth of burial. Thermal effects are taken into account. The pore pressure, together with the confining pressure, determines the effective pressure which, in turn, determines the bulk moduli of the rock matrix.
We assume a sandstone saturated with hydrocarbons and water, for which calibration of the model with experimental data is possible. The seismic velocities and attenuation factors are computed by using Biot’s theory of dynamic poroelasticity and the generalized linear solid. The example shows that the formation can be overpressured or underpressured depending on the properties of the saturating fluid. Wave velocities and quality factors decrease with decreasing differential pressure. The effect is important below approximately 20 MPa. The model is in good agreement with experimental data for Berea sandstone and provides a tool for predicting pore pressure from seismic attributes.



A generalized Biot–Gassmann model for the acoustic properties of shaley sandstones^{1}
Authors José M. Carcione, Boris Gurevich and Fabio CavalliniWe obtain the wave velocities of clay‐bearing sandstones as a function of clay content, porosity and frequency. Unlike previous theories, based simply on slowness and/or moduli averaging or two‐phase models, we use a Biot‐type three‐phase theory that considers the existence of two solids (sand grains and clay particles) and a fluid. The theory, which is consistent with the critical porosity concept, uses three free parameters that determine the dependence of the dry‐rock moduli of the sand and clay matrices as a function of porosity and clay content.
Testing of the model with laboratory data shows good agreement between predictions and measurements. In addition to a rock physics model that can be useful for petrophysical interpretation of wave velocities obtained from well logs and surface seismic data, the model provides the differential equation for computing synthetic seismograms in inhomogeneous media, from the seismic to the ultrasonic frequency bands.



A new concept in Euler deconvolution of isolated gravity anomalies [Link]
More LessEuler's homogeneity equation has been used to develop a new technique to interpret the gravity anomalies over some simple geometrical sources, namely a finite horizontal line/vertical line, a finite vertical ribbon, a semicircular dome/basin and an isosceles triangle approximating an anticline/syncline. A linear over‐determined system of equations has been solved to compute the depth, the horizontal location and the structural index, all treated as free parameters. The concept of a variable structural index provides better depth estimates and helps to identify the source geometry. Nomograms have been prepared to compute an additional model parameter, namely the horizontal/vertical extent of a line, the vertical extent of a ribbon and the radius of a dome/basin. The efficacy of the proposed method has been evaluated using two real field examples.



Inversion of normal moveout for monoclinic media^{1}
Authors Vladimir Grechka, Pedro Contreras and Ilya TsvankinMultiple vertical fracture sets, possibly combined with horizontal fine layering, produce an equivalent medium of monoclinic symmetry with a horizontal symmetry plane. Although monoclinic models may be rather common for fractured formations, they have hardly been used in seismic methods of fracture detection due to the large number of independent elements in the stiffness tensor. Here, we show that multicomponent wide‐azimuth reflection data (combined with known vertical velocity or reflector depth) or multi‐azimuth walkaway VSP surveys provide enough information to invert for all but one anisotropic parameters of monoclinic media.
In order to facilitate the inversion procedure, we introduce a Thomsen‐style parametrization for monoclinic media that includes the vertical velocities of the P‐wave and one of the split S‐waves and a set of dimensionless anisotropic coefficients. Our notation, defined for the coordinate frame associated with the polarization directions of the vertically propagating shear waves, captures the combinations of the stiffnesses responsible for the normal‐moveout (NMO) ellipses of all three pure modes. The first group of the anisotropic parameters contains seven coefficients (ε^{(1,2)}, δ^{(1,2,3)} and γ^{(1,2)}) analogous to those defined by Tsvankin for the higher‐symmetry orthorhombic model. The parameters ε^{(1,2)}, δ^{(1,2)} and γ^{(1,2)} are primarily responsible for the pure‐mode NMO velocities along the coordinate axes x1 and x2 (i.e. in the shear‐wave polarization directions). The remaining coefficient δ^{(3)} is not constrained by conventional‐spread reflection traveltimes in a horizontal monoclinic layer. The second parameter group consists of the newly introduced coefficients ζ^{(1,2,3)} which control the rotation of the P‐, S1‐ and S2‐wave NMO ellipses with respect to the horizontal coordinate axes. Misalignment of the P‐wave NMO ellipse and shear‐wave polarization directions was recently observed on field data by Pérez et al.
Our parameter‐estimation algorithm, based on NMO equations valid for any strength of the anisotropy, is designed to obtain anisotropic parameters of monoclinic media by inverting the vertical velocities and NMO ellipses of the P‐, S1‐ and S2‐waves. A Dix‐type representation of the NMO velocity of mode‐converted waves makes it possible to replace the pure shear modes in reflection surveys with the PS1‐ and PS2‐waves. Numerical tests show that our method yields stable estimates of all relevant parameters for both a single layer and a horizontally stratified monoclinic medium.



Computing three‐dimensional gravitational fields with equivalent sources [Link]
More LessExisting techniques for computing the gravitational field due to a homogeneous polyhedron all transform the required volume integral, expressing the field due to a volume distribution of mass, into a surface integral, expressing the potential due to a surface mass distribution over the boundary of the source body. An alternative representation is also possible and results in a surface integral expressing the potential due to a variable‐strength double layer located on the polyhedral source boundary. Manipulation of this integral ultimately allows the gravitational field component in an arbitrary direction to be expressed as a weighted sum of the potentials due to two basic source distributions. These are a uniform‐strength double layer located on all faces and a uniform‐strength line source located along all edges. The derivatives of the gravitational field components can also be expressed in a similar form as can the magnetic field components due to a homogeneous magnetic polyhedron. It follows that the present approach can be used to generate a universal program capable of modelling all the commonly used potential field responses due to 3D bodies of arbitrary shape.



Removal of DC power‐line magnetic‐field effects from airborne total magnetic‐field measurements [Link]
Authors Mehran Gharibi and Laust B. PedersenPower lines carrying DC current can strongly affect total magnetic‐field measurements. A simple algorithm using Biot–Savart's law was made to remove magnetic‐field components due to a DC power line from airborne total magnetic‐field measurements in the Gävle area, Sweden. The power‐line location was estimated from observed data and then split into short line segments. The magnetic‐field components due to each segment were calculated and summed together to give the total magnetic effect due to the power line at each observation point. The corrected total magnetic field was calculated by subtracting the power‐line magnetic‐field vector, projected on to the direction of the main field, from the measured total field. The results show a successful removal of the power‐line magnetic effect from the total magnetic‐field measurements. However, an error in the estimation of the power‐line location can result in a magnetic‐field residual after correction. A non‐linear median filtering was used to remove this residual when needed.

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)