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- Volume 58, Issue 2, 2010
Geophysical Prospecting - Volume 58, Issue 2, 2010
Volume 58, Issue 2, 2010
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Seismic data reconstruction using multidimensional prediction filters
Authors M. Naghizadeh and M.D. SacchiABSTRACTIn this paper we discuss a beyond‐alias multidimensional implementation of the multi‐step autoregressive reconstruction algorithm for data with missing spatial samples. The multi‐step autoregressive method is summarized as follows: vital low‐frequency information is first regularized adopting a Fourier based method (minimum weighted norm interpolation); the reconstructed data are then used to estimate prediction filters that are used to interpolate higher frequencies. This article discusses the implementation of the multi‐step autoregressive method to data with more than one spatial dimension. Synthetic and real data examples are used to examine the performance of the proposed method. Field data are used to illustrate the applicability of multidimensional multi‐step autoregressive operators for regularization of seismic data.
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Fast velocity analysis by wave path migration
Authors Yike Liu, Hongchuan Sun and Xu ChangABSTRACTIterative migration velocity analysis is computationally expensive, where most of the computation time is used for generating prestack depth images. By using a reduced form of Kirchhoff migration, denoted as wave path migration, we can significantly speed up the depth imaging process and reduce the entire velocity analysis expense accordingly. Our results with 2D synthetic and field data show that wave path migration velocity analysis can efficiently improve the velocity model and the wave path migration velocity analysis updated velocity correlates well with that from the Kirchhoff migration velocity analysis. The central processing unit comparison shows that, for a 2D synthetic and field data set, wave path migration velocity analysis is six times faster than Kirchhoff migration velocity analysis. This efficiency should be even greater for 3D data.
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Traveltime approximations for q‐P waves in vertical transversely isotropy media*
Authors Rafael Aleixo and Jörg SchleicherABSTRACTAs exploration targets have become deeper, cable lengths have increased accordingly, making the conventional two term hyperbolic traveltime approximation produce increasingly erroneous traveltimes. To overcome this problem, many traveltime formulas were proposed in the literature that provide approximations of different quality. In this paper, we concentrate on simple traveltime approximations that depend on a single anisotropy parameter. We give an overview of a collection of such traveltime approximations found in the literature and compare their quality. Moreover, we propose some new single‐parameter traveltime approximations based on the approximations found in the literature. The main advantage of our approximations is that some of them are rather simple analytic expressions that make them easy to use, while achieving the same quality as the better of the established formulas.
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Non‐uniqueness with refraction inversion – a syncline model study
More LessABSTRACTNon‐uniqueness occurs with the 1D parametrization of refraction traveltime graphs in the vertical dimension and with the 2D lateral resolution of individual layers in the horizontal dimension. The most common source of non‐uniqueness is the inversion algorithm used to generate the starting model. This study applies 1D, 1.5D and 2D inversion algorithms to traveltime data for a syncline (2D) model, in order to generate starting models for wave path eikonal traveltime tomography.
The 1D tau‐p algorithm produced a tomogram with an anticline rather than a syncline and an artefact with a high seismic velocity. The 2D generalized reciprocal method generated tomograms that accurately reproduced the syncline, together with narrow regions at the thalweg with seismic velocities that are less than and greater than the true seismic velocities as well as the true values.
It is concluded that 2D inversion algorithms, which explicitly identify forward and reverse traveltime data, are required to generate useful starting models in the near‐surface where irregular refractors are common. The most likely tomogram can be selected as either the simplest model or with a priori information, such as head wave amplitudes.
The determination of vertical velocity functions within individual layers is also subject to non‐uniqueness. Depths computed with vertical velocity gradients, which are the default with many tomography programs, are generally 50% greater than those computed with constant velocities for the same traveltime data. The average vertical velocity provides a more accurate measure of depth estimates, where it can be derived.
Non‐uniqueness is a fundamental reality with the inversion of all near‐surface seismic refraction data. Unless specific measures are taken to explicitly address non‐uniqueness, then the production of a single refraction tomogram, which fits the traveltime data to sufficient accuracy, does not necessarily demonstrate that the result is either ‘correct’ or the most probable.
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Kinematical characteristics of the factorized velocity model
More LessABSTRACTThe factorized velocity model that incorporates both vertical heterogeneity and constant anisotropy is one of the complicated analytical models used in seismic data processing and interpretation. In this paper, I derive the analytic equations for offset, traveltime and relative geometrical spreading for the quasi‐compressional (qP‐) waves that can be used for modelling and inversion of the traveltime parameters. I show that the presence of anelliptic anisotropy usually dominates over the vertical heterogeneity with respect to the non‐hyperbolicity of the factorized velocity model.
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Feasibility of joint 1D velocity model and event location inversion by the Neighbourhood algorithm
Authors Jaromir Jansky, Vladimir Plicka and Leo EisnerABSTRACTUsing a set of synthetic P‐ and S‐wave onsets, computed in a 1D medium model from sources that mimic a distribution of microseismic events induced by hydrofrac treatment to a monitoring geophone array(s), we test the possibility to invert back jointly the model and events location. We use the Neighbourhood algorithm for data inversion to account for non‐linear effects of velocity model and grid search for event location. The velocity model used is composed of homogeneous layers, derived from sonic logging. Results for the case of one and two monitoring wells are compared. These results show that the velocity model can be obtained in the case of two monitoring wells, if they have optimal relative position. The use of one monitoring well fails due to the trade‐off between the velocity model and event locations.
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Lateral velocity variation related correction in asymptotic true‐amplitude one‐way propagators
Authors Jun Cao and Ru‐Shan WuABSTRACTIt was mathematically proved that the asymptotic true‐amplitude one‐way wave equation could provide the same amplitude as the full‐wave equation in heterogeneous lossless media in the sense of high‐frequency asymptotics. Much work has been done on the vertical velocity variation related amplitude correction term but the lateral velocity variation related term has not received much attention, even being excluded in some asymptotic true‐amplitude one‐way propagator formulations. Here we analyse the effects of different amplitude correction terms in the asymptotic true‐amplitude one‐way propagator, especially the effect related to the lateral velocity variation, by comparing the wavefield amplitude from the one‐way propagator with that from full‐wave modelling. We derive a dual‐domain wide‐angle screen type asymptotic true‐amplitude one‐way propagator and evaluate two implementations of the amplitude correction. Numerical examples show that the lateral velocity variation related correction term can play a significant role in the asymptotic true‐amplitude one‐way propagator. Optimization of the expansion coefficients in the asymptotic true‐amplitude one‐way propagator can improve both the amplitude and phase accuracy for wide‐angle waves.
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On the propagation characteristics of tunnel surface‐waves for seismic prediction
Authors Stefan Jetschny, Thomas Bohlen and Denise De NilABSTRACTTo increase the safety and efficiency of tunnel constructions, online seismic exploration ahead of a tunnel has become a valuable tool. One recent successful forward looking approach is based on the excitation and registration of tunnel surface‐waves. For further development and for finding optimal acquisition geometries it is important to study the propagation characteristics of tunnel surface‐waves. 3D seismic finite difference modelling and analytic solutions of the wave equation in cylindrical coordinates reveal that at higher frequencies, i.e., if the tunnel‐diameter is significantly larger than the wavelength of surface‐waves, these surface‐waves can be regarded as Rayleigh‐waves confined to the tunnel wall and following helical paths along the tunnel axis. For lower frequencies, i.e., when the tunnel surface‐wavelength approaches the tunnel‐diameter, the propagation characteristics of these surface‐waves are similar to S‐waves. We define the surface‐wave wavelength‐to‐tunnel diameter ratio w to be a gauge for separating Rayleigh‐ from S‐wave excitation. For w > 1.2 tunnel surface‐waves behave like S‐waves, i.e. their velocity approaches the S‐wave velocity and the particle motion is linear and perpendicular to the ray direction. For w < 0.6 they behave like Rayleigh‐waves, i.e., their velocity approaches the Rayleigh‐wave velocity and they exhibit elliptical particle motion. For 0.6 < w < 1.2 a mixture of both types is observed. Field data from the Gotthard Base Tunnel (Switzerland) show both types of tunnel surface‐waves and S‐waves propagating along the tunnel.
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Drill‐bit seismic interferometry with and without pilot signals*
Authors Flavio Poletto, Piero Corubolo and Paolo ComelliABSTRACTWe use different interferometry approaches to process the seismic signals generated by a drill‐bit source in one well and recorded by seismic receivers located both in a second borehole and at the surface near the source well. We compare the standard interferometry results, obtained by using the raw drill‐bit data without a pilot signal, with the new interferometry results obtained by using the drill‐bit seismograms correlated with a reference pilot signal. The analysis of the stationary phase shows that the final results have different S/N levels and are affected by the coherent noise in the form of rig arrivals. The interferometry methods are compared by using different deconvolution approaches. The analysis shows that the results agree with the conventional drill‐bit seismograms and that using the reference pilot signal improves the quality of the drill‐bit wavefields redatumed by the interferometry method.
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Identification of the shallow subsurface succession and investigation of the seawater invasion to the Quaternary aquifer at the northern part of El Qaa plain, Southern Sinai, Egypt by transient electromagnetic data
Authors Usama Massoud, Fernando Santos, Gad El Qady, Magdy Atya and Mamdouh SolimanABSTRACTThe phenomenon of seawater invasion is a very common and widespread environmental problem that is present in the majority of Egyptian coastal aquifers. Saltwater intrusion may occur due to human activities and by natural events such as climate change and sea level rise. El Qaa plain is a structural depression trending NNW‐SSE, parallel to the main rift system of the Gulf of Suez. The Quaternary aquifer in El Qaa plain is the main source for water supply in this region. So, this study has been conducted to shed more light on the description of the subsurface section and marking out the invasion of seawater to this aquifer at the northern part of El Qaa plain. In this concern, transient electromagnetic data were measured at 52 stations arranged along 2 parallel profiles, 10 km apart, trending NE‐SW to cross the major course of the plain and to be approximately perpendicular to the coast line of the Suez Gulf. In addition, information from a nearby borehole was used to confirm the results of transient electromagnetic data interpretation. Two apparent resistivity pseudosections and two geoelectrical cross‐sections were constructed along the selected profiles. Investigation of these sections revealed that four geoelectrical layers could be detected in the shallow subsurface medium at the studied area and the Quaternary aquifer has been affected by seawater invasion at the western and middle parts. Besides, three faults striking approximately NW‐SE parallel to the major track of the plain were inferred from this study. These faults may be controlling the groundwater accumulation and flow in this area.
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Fractured reservoirs with fracture corridors
Authors Jean‐Marie Questiaux, Gary D. Couples and Nicolas RubyABSTRACTOutcrop studies reveal a common occurrence of tabular zones of significantly‐increased fracture intensity affecting otherwise well‐lithified rocks. These zones, called fracture corridors, can have a profound effect on multi‐phase fluid flow in the subsurface. Using standard geo‐modelling tools, it is possible to generate 3D realizations of reservoirs that contain distributions of such fracture corridors that are consistent with observations, including the vertical frequency in pseudo‐wells inserted into the model at random locations. These models can generate the inputs to flow simulation. The approach adopted here is to run the flow simulations in a single‐porosity representation where the flow effects of fractures are upscaled into equivalent cell‐based properties, preserving a clear spatial relationship between the input geology and the resulting cellular model. The simulated reservoir performance outcomes are very similar to those seen in real oilfields: extreme variability between wells, early water breakthrough, disappointing recoveries and patchy saturation distributions. Thus, a model based on fracture corridors can provide an explanation for the observed flow performance of a suitable field. However, the use of seismics to identify fracture corridors is not an easy task. New work is needed to predict the seismic responses of fracture corridor systems to be able to judge whether it is likely that we can robustly detect and characterize these flow‐significant features adequately.
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Lithology and hydrocarbon mapping from multicomponent seismic data
Authors Hüseyin Özdemir, Kevin Flanagan and Emma TylerABSTRACTElastic rock properties can be estimated from prestack seismic data using amplitude variation with offset analysis. P‐wave, S‐wave and density ‘reflectivities’, or contrasts, can be inverted from angle‐band stacks. The ‘reflectivities’ are then inverted to absolute acoustic impedance, shear impedance and density. These rock properties can be used to map reservoir parameters through all stages of field development and production. When P‐wave contrast is small, or gas clouds obscure reservoir zones, multicomponent ocean‐bottom recording of converted‐waves (P to S or Ps) data provides reliable mapping of reservoir boundaries.
Angle‐band stacks of multicomponent P‐wave (Pz) and Ps data can also be inverted jointly. In this paper Aki‐Richards equations are used without simplifications to invert angle‐band stacks to ‘reflectivities’. This enables the use of reflection seismic data beyond 30° of incident angles compared to the conventional amplitude variation with offset analysis. It, in turn, provides better shear impedance and density estimates. An important input to amplitude variation with offset analysis is the Vs/Vp ratio. Conventional methods use a constant or a time‐varying Vs/Vp model. Here, a time‐ and space‐varying model is used during the computation of the ‘reflectivities’. The Vs/Vp model is generated using well log data and picked horizons. For multicomponent data applications, the latter model can also be generated from processing Vs/Vp models and available well data.
Reservoir rock properties such as λρ, μρ, Poisson's ratio and bulk modulus can be computed from acoustic impedance, shear impedance and density for pore fill and lithology identification. λ and μ are the Lamé constants and ρ is density. These estimations can also be used for a more efficient log property mapping. Vp/Vs ratio or Poisson's ratio, λρ and weighted stacks, such as the one computed from λρ and λ/μ, are good gas/oil and oil/water contact indicators, i.e., pore fill indicators, while μρ mainly indicates lithology. μρ is also affected by pressure changes.
Results from a multicomponent data set are used to illustrate mapping of gas, oil and water saturation and lithology in a Tertiary sand/shale setting. Whilst initial log crossplot analysis suggested that pore fill discrimination may be possible, the inversion was not successful in revealing fluid effects. However, rock properties computed from acoustic impedance, shear impedance and density estimates provided good lithology indicators; pore fill identification was less successful. Neural network analysis using computed rock properties provided good indication of sand/shale distribution away from the existing wells and complemented the results depicted from individual rock property inversions.
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Discovery of the causeway and the mortuary temple of the Pyramid of Amenemhat II using near‐surface magnetic investigation, Dahshour, Giza, Egypt
Authors T. Abdallatif, A.E. El Emam, M. Suh, I.A. El Hemaly, H.H. Ghazala, E.H. Ibrahim, H.H. Odah and H.A. DeebesABSTRACTThe main outbuildings of the Amenemhat II pyramid complex in Dahsour were yet to be discovered due to a very long subjection of the area to the military authorities and also the demolition of the pyramid itself. We describe the discovery of some of these outbuildings using near‐surface magnetic investigations. A gradiometer survey was conducted in the area east of the pyramid to measure the vertical magnetic gradient with a high resolution instrument at 0.5 m sampling interval. The data showed some undesirable field effects such as grid discontinuities, grid slope, traverse stripe effects, spikes and high frequencies originating from recent ferrous contamination. These undesirable effects were addressed to produce an enhanced display. We have successfully detected four main structures in the area east of the pyramid; the causeway that connected the mortuary temple with the valley temple during the Middle Kingdom of the 12th Dynasty, the mortuary temple and its associated rooms, ruins of an ancient working area and an Egyptian‐style tomb structure called a Mastaba. An improved recognition for these structures was accomplished by using the analytic signal and Euler deconvolution techniques. Excavation of a small part within the study area has proven the reliability of magnetic discoveries and the shallowness and composition of the detected features.
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Magnetic basement: gravity‐guided magnetic source depth analysis and interpretation
Authors Serguei A. Goussev and John W. PeirceABSTRACTWe present a new integrated approach to the interpretation of magnetic basement that is based on recognition of characteristic patterns in distributions and alignments of magnetic source depth solutions above and below the surface of magnetic basement. This approach integrates a quantitative analysis of depth solutions, obtained by 2D Werner deconvolution of the magnetic data, with a qualitative evaluation of the Bouguer gravity anomalies. The crystalline/metamorphic basement and sedimentary cover have different origins, tectonic histories, lithologies and magnetic properties. These differences result in different geometries of magnetic sources associated with faults, fracture zones, igneous intrusions, erosional truncations, subcrop edges and other structural discontinuities.
Properly tuned, 2D Werner deconvolution is able to resolve the intra‐sedimentary and intra‐basement magnetic source geometries into distinctly different distributions and alignments of calculated depth solutions. An empirical set of criteria, basement indicators, was developed for identification and correlation of the basement surface. The ambiguity of basement correlation with limited or non‐existent well control, which is common for onshore frontier and offshore explorations, can be reduced by incorporating the Bouguer gravity data into the process of correlation.
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Reply to comment on ‘Low‐frequency microtremor anomalies at an oil and gas field in Voitsdorf, Austria’ by Marc‐André Lambert, Stefan M. Schmalholz, Erik H. Saenger and Brian Steiner, Geophysical Prospecting 57, 393–411
Authors Marc‐André Lambert, Stefan M. Schmalholz, Erik H. Saenger and Brian Steiner
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