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- Volume 56, Issue 4, 2008
Geophysical Prospecting - Volume 56, Issue 4, 2008
Volume 56, Issue 4, 2008
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Yardsticks for industrial tomography
By A. VesnaverABSTRACTSeismic tomography has been developed and applied for decades in seismological applications and for basic research purposes. During the last decade, large‐scale applications in the oil and gas industry became standard as tomostatics and velocity modelling for pre‐stack depth migration. In this paper, I take a snapshot of some current industrial applications, quantifying practical aspects by yardsticks such as data and model size and I try to draw a road map for the current decade.
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Iterative tomographic analysis based on automatic refined picking
Authors Claudio Satriano, Aldo Zollo and Charlotte RoweABSTRACTThe ever‐growing size of data sets for active and passive seismic imaging makes the availability of automatic procedures for rapid analysis more and more valuable. Such procedures are especially important for time‐critical applications like emergency decisions or re‐orienting of ongoing seismic surveys. In this paper a new, iterative scheme for 3D traveltime tomography is presented. The technique, based on a tool originally developed for earthquake data, uses cross‐correlation to examine waveform similarity and to adjust arrival times on seismic sections. A preliminary set of reference arrival times is first corrected by the cross‐correlation lag and then used to build an initial 3D tomographic velocity model through a standard inversion code; traveltimes calculated from this model are then taken as new reference arrivals and the process of pick adjustment is repeated. The result is a tomographic image, upgraded and refined at each iteration of the procedure. The test performed on the waveform data set recorded during the 2001 SERAPIS active seismic survey in the gulfs of Naples and Pozzuoli (Southern Italy) shows that the 3D iterative tomography scheme produces a velocity image of the structure of the Campi Flegrei caldera which is consistent with the results from previous studies, employing just a fraction of the time needed by a human analyst to identify first breaks. We believe that this technique can be effectively employed for rapid analysis of large data‐sets within time‐critical or time‐dependent tasks and for automatic 4D tomographic investigations.
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Potential and limits of double‐difference tomographic methods
Authors Jean‐Luc Got, Vadim Monteiller, Jean Virieux and Stéphane OpertoABSTRACTDouble‐difference tomographic methods use directly accurate time delays computed between similar signals. Such methods are designed to image very heterogeneous media, such as volcanoes or fault zones. In seismological applications, similar signals are recorded at a given station from earthquakes sharing similar and close‐by sources. In seismic exploration experiments, similar signals are often recorded at neighbouring receivers. After a brief presentation of the tomographic algorithm used, a seismological application is summarized. The potential and limits of double‐difference tomographic methods are explored using various numerical experiments. They show that two effects are competing in double‐difference tomography: (i) the degradation of the stability of the inversion due to the geometrical proximity of the rays used in the differentiation and (ii) the decrease in modelling error, which allows improving the stability of the inversion and using smaller quantities of a priori information when data are sufficiently accurate. The best resolution is obtained for an optimal value of the inter‐source or inter‐receiver distance. For optimal values of these distances and a priori information, tomography using traveltime differences provides significantly better resolved results than using traveltimes.
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Finite‐frequency tomography in a crustal environment: Application to the western part of the Gulf of Corinth
Authors S. Gautier, G. Nolet and J. VirieuxABSTRACTIn this paper we investigate finite‐frequency effects in crustal tomography. We developed an inversion procedure based on an exact numerical computation of the sensitivity kernels. In this approach we compute the 3D travel‐time sensitivity kernels by using (1) graph theory and an additional bending to estimate accurately both rays and travel‐times between source/receiver and diffraction points and (2) paraxial ray theory to estimate the amplitude along theses rays. We invert both the velocity and the hypocentre parameters, using these so‐called banana‐doughnut kernels and the LSQR iterative solver. We compare the ray‐theoretical and the finite‐frequency tomography to image the intermediate structures beneath the Gulf of Corinth (Greece), which has long been recognized as the most active continental rifting zone in the Mediterranean region. Our dataset consists of 451 local events with 9233 P‐ first‐arrival times recorded in the western part of the Gulf (Aigion area) in the framework of the 3F‐Corinth European project. Previous tomographic images showed a complex velocity crustal model and a low‐dip surface that may accommodate the deformation. Accurate velocity models will help to better constrain the rifting process, which is still a subject of debate. The main results of this study show that finite‐frequency tomography improves crustal tomographic images by providing better resolved images of the 3D complicated velocity structure. Because the kernels spread the information over a volume, finite‐frequency tomography results in a sharpening of layer boundaries as we observed for the shallower part of the crust (down to 5 km depth) beneath the Gulf of Corinth.
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First‐arrival traveltime tomography using second generation wavelets
Authors Matthieu Delost, Jean Virieux and Stéphane OpertoABSTRACTWavelet decomposition of the slowness model allows a multiscale description of the seismic first‐arrival time tomography. We propose the introduction of the so‐called second generation wavelets that could be used for any mesh structure and do not require a number of samples, such as the power of two in each direction for fast wavelet transform. A linearized procedure for inverting delayed travel‐times considering either slowness coefficients or wavelet coefficients has been set up with an efficient ray tracing at each iteration of the inversion procedure. Wavelet decomposition over constant patches (Haar wavelet) or over linear patches (Battle‐Lemarie wavelet) of coefficients at different scales are inverted as unknowns of the tomographic linearized system. Reconstruction of these coefficients depends dynamically on the local resolution when considering dense ray coverage. On simple synthetic examples, it has been found necessary to perform a local resolution analysis for specifying wavelet coefficients to be inverted. This resolution analysis could be performed for an initial smooth reconstructed medium and by designing a bit mask operator it allows fine scales to be inverted in specific areas of the model where the resolution is high while not being inverted in other areas where the resolution is poor: the wavelet decomposition will ease the multiscale reconstruction. A few synthetic examples, such as crosshole tomography or surface‐surface tomography illustrate the multiscale feature of wavelet tomography. The second generation wavelet approach seems to be a flexible and rather promising tool for controlling the resolution variation of seismic first‐arrival tomography.
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Depth and morphology of reflectors from the non‐linear inversion of arrival‐time and waveform semblance data. Part I: method and applications to synthetic data
Authors Maurizio Vassallo and Aldo ZolloABSTRACTWe propose a two‐dimensional, non‐linear method for the inversion of reflected/converted traveltimes and waveform semblance designed to obtain the location and morphology of seismic reflectors in a lateral heterogeneous medium and in any source‐to‐receiver acquisition lay‐out. This method uses a scheme of non‐linear optimization for the determination of the interface parameters where the calculation of the traveltimes is carried out using a finite‐difference solver of the Eikonal equation, assuming an a priori known background velocity model. For the search for the optimal interface model, we used a multiscale approach and the genetic algorithm global optimization technique. During the initial stages of inversion, we used the arrival times of the reflection phase to retrieve the interface model that is defined by a small number of parameters. In the successive steps, the inversion is based on the optimization of the semblance value determined along the calculated traveltime curves. Errors in the final model parameters and the criteria for the choice of the best‐fit model are also estimated from the shape of the semblance function in the model parameter space. The method is tested and validated on a synthetic dataset that simulates the acquisition of reflection data in a complex volcanic structure. This study shows that the proposed inversion approach is a valid tool for geophysical investigations in complex geological environments, in order to obtain the morphology and positions of embedded discontinuities.
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Depth and morphology of reflectors from the non‐linear inversion of arrival times and waveform semblance data. Part II: modelling and interpretation of real data acquired in Southern Apennines, Italy
Authors M. Corciulo, A. Zollo, M. Vassallo, P. Dell'Aversana and S. MorandiABSTRACTIn order to retrieve a 2D background velocity model and to retrieve the geometry and depth of shallow crustal reflectors in the Southern Apennines thrust belt a separate inversion of first arrival traveltimes and reflected waveforms was performed. Data were collected during an active seismic experiment in 1999 by Enterprise Oil Italiana and Eni‐Agip using a global offset acquisition geometry. A total of 284 on‐land shots were recorded by 201 receivers deployed on an 18 km line oriented SW–NE in the Val D'Agri region (Southern Apennines, Italy).
The two‐step procedure allows for the retrieval of a reliable velocity model by using a non‐linear tomographic inversion and reflected waveform semblance data inversion. The tomographic model shows that the P wave velocity field varies vertically from approximately 3 km/s to 6 km/s within 4 km from the Earth's surface. Moreover, at a distance of approximately 11 km along the profile, there is an abrupt increase in the velocity field. In this zone indeed, an ascent from 2 km depth to 0 km above sea level of the 5.2 km/s iso‐velocity contour can be noted. The retrieved velocity can be associated with Plio‐Pleistocene clastic deposits outcropping in the basin zone and with Mesozoic limestone deposits. The inversion of waveform semblance data shows that a P‐to‐P reflector is retrieved at a depth of approximately 2 km. This interface is deeper in the north‐eastern part of the profile, where it reaches 3 km depth and can be associated with a limestone horizon.
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Merging active and passive data sets in traveltime tomography: The case study of Campi Flegrei caldera (Southern Italy)
Authors Jean Battaglia, Aldo Zollo, Jean Virieux and Dario Dello IaconoABSTRACTWe propose a strategy for merging both active and passive data sets in linearized tomographic inversion. We illustrate this in the reconstruction of 3D images of a complex volcanic structure, the Campi Flegrei caldera, located in the vicinity of the city of Naples, southern Italy. The caldera is occasionally the site of significant unrests characterized by large ground uplifts and seismicity. The P and S velocity models of the caldera structure are obtained by a tomographic inversion based on travel times recorded during two distinct experiments. The first data set is composed of 606 earthquakes recorded in 1984 and the second set is composed of recordings for 1528 shots produced during the SERAPIS experiment in 2001. The tomographic inversion is performed using an improved method based on an accurate finite‐difference traveltime computation and a simultaneous inversion of both velocity models and earthquake locations. In order to determine the adequate inversion parameters and relative data weighting factors, we perform massive synthetic simulations allowing one to merge the two types of data optimally. The proper merging provides high resolution velocity models, which allow one to reliably retrieve velocity anomalies over a large part of the tomography area. The obtained images confirm the presence of a high P velocity ring in the southern part of the bay of Pozzuoli and extends its trace inland as compared to previous results. This annular anomaly represents the buried trace of the rim of the Campi Flegrei caldera. Its shape at 1.5 km depth is in good agreement with the location of hydrothermalized lava inferred by gravimetric data modelling. The Vp/Vs model confirms the presence of two characteristic features. At about 1 km depth a very high Vp/Vs anomaly is observed below the town of Pozzuoli and is interpreted as due to the presence of rocks that contain fluids in the liquid phase. A low Vp/Vs body extending at about 3–4 km depth below a large part of the caldera is interpreted as the top of formations that are enriched in gas under supercritical conditions.
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Crustal‐scale prestack depth imaging of the 1994 and 1999 LARSE surveys
Authors Michael Thornton and Hua‐wei ZhouABSTRACTWhile seismic imaging for crustal and mantle structures has traditionally relied on surface wave and refraction data, the use of reflection data for crustal‐scale targets has been largely limited to the common midpoint (CMP) stack techniques. The rapid increase in the number of seismograph array deployments in recent years in crustal and mantle seismology has reached a level such that a re‐examination of the imaging techniques is becoming necessary. In this paper we show the advantage of prestack depth imaging for crustal reflection studies, based on data from two reflection surveys of the Los Angeles Regional Seismic Experiment (LARSE) to map faults and crustal‐scale structures. Our analysis indicates that the quality of the previous images of these surveys is limited by the CMP stack technique. For comparison, we present here depth images of the same LARSE data using wave equation prestack depth imaging and a tomographic velocity model based on first arrivals of the LARSE surveys and local earthquakes. Our new images are considerably improved over previous images in terms of resolution and reflector continuity. The new images show reflectors throughout the crust and suggest truncations in the Moho associated with the San Andreas Fault. A series of bright reflector segments, which are associated with the San Gabriel and San Andreas faults have been identified and might represent reflections from the fault zones. Our results suggest that the presence of high noise level, strong lateral velocity heterogeneity and wide angle geometry argue for, rather than against, the use of prestack depth imaging over the simple CMP stack techniques. As demonstrated in this study, it is now viable to conduct prestack depth imaging of crustal reflection data using a velocity model based on earthquake first arrivals thanks to the dense acquisition deployment.
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Three‐dimensional kinematic depth migration of converted waves: application to the 2002 Molise aftershock sequence (southern Italy)
ABSTRACTMigration techniques, currently used in seismic exploration, are still scarcely applied in earthquake seismology due to the poor source knowledge and sparse, irregular acquisition geometries. At the crustal scale, classical seismological studies often perform inversions based on the arrival time of primary phases (P‐ and S‐waves) but seldom exploit other information included in seismic records. Here we show how migration techniques can be adapted to earthquake seismology for converted wave analysis. As an example, we used data recorded by a dense local seismic network during the 2002 Molise aftershock sequence. In October and November 2002, two moderate magnitude earthquakes struck the Molise region (southern Italy), followed by an aftershock sequence lasting for about one month. Local earthquake tomography has provided earthquake hypocenter locations and three‐dimensional models of P and S velocity fields. Strong secondary signals have been detected between first‐arrivals of P‐ and S‐waves and identified as SP transmitted waves. In order to analyse these waves, we apply a prestack depth migration scheme based on the Kirchhoff summation technique. Since source parameters are unknown, seismograms are equalized and only kinematic aspects of the migration process are considered. Converted wave traveltimes are calculated in the three‐dimensional (3D) tomographic models using a finite‐difference eikonal solver and back ray tracing. In the migrated images, the area of dominant energy conversion corresponds to a strong seismic horizon that we interpreted as the top of the Apulia Carbonate Platform and whose geometry and position at depth is consistent with current structural models from existing commercial seismic profiles, gravimetric and well data.
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Using an airgun array in a land reservoir as the seismic source for seismotectonic studies in northern China: experiments and preliminary results
Authors Yong Chen, Lanbo Liu, Hongkui Ge, Baojun Liu and Xuelin QiuABSTRACTThis paper reports the field setup and preliminary results of experiments utilizing an airgun array in a reservoir in north China for a seismotectonic study. Commonly used in offshore petroleum resource exploration, the airgun source was found to be more useful than a traditional explosive source for large‐scale and long offset land seismic surveys. The airgun array, formed by four 1,500 in3 airguns (a total of 6,000 in3 in volume) was placed at a depth of 6–9 m into the reservoir to generate the pressure impulse. No direct evidence was found that the airgun source adversely affected the fish in the reservoir. The peak ground acceleration recorded on the top of the reservoir dam 100 m away was 17.8 gal in the horizontal direction; this is much less than the designed earthquake‐resistance threshold of 125 gal for this dam. The energy for one shot of this airgun array is about 6.68 MJ, equivalent to firing a 1.7 kg explosive. The seismic waves generated by the airgun source were recorded by receivers of the regional seismic networks and a temporary wide‐angle reflection and refraction profile formed by 100 short‐period seismometers with the maximum source‐receiver offset of 206 km. The seismic wave signature at these long‐offset stations is equivalent to that generated by a traditional blast source in a borehole with a 1,000–2,000 kg explosive. Preliminary results showed clear seismic phases from refractions from the multi‐layer crustal structures in the north China region. Forward modelling using numerical simulation confirms that the seismic arrivals are indeed from lower crustal interfaces. The airgun source is efficient, economical, environmentally friendly and suitable for being used in urbanized areas. It has many advantages over an explosive source for seismotectonic studies such as the high repeatability that is supreme for stacking to improve signal qualities. The disadvantage is that the source is limited to existing lakes or reservoirs, which may restrict experimental geometry.
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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 64 (2015 - 2016)
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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 54 (2006)
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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 29 (1981)
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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 16 (1968)
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Volume 15 (1967)
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Volume 12 (1964)
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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)