Seismic Tool Research Articles

Seismic signatures of stellar cores of solar‐like pulsators: Dependence on mass and age

AbstractUseful information from the inner layers of stellar pulsators may be derived from the study of their oscillations. In this paper we analyse three diagnostic tools suggested in the literature built from the oscillation frequencies computed for a set of main sequence models with masses between 1.0 M⊙ and 1.6 M⊙, to check what information they may hold about stellar cores. For the models with convective cores (M ≥ 1.2 M⊙) we find a relation between the frequency slopes of the diagnostic tools and the size of the jump in the sound speed at the edge of the core. We show that this relation is independent of the mass of the models. In practice, since the size of the jump in the sound speed is related to the age of the star, using these seismic tools we may, in principle, infer the star's evolutionary state. We also show that when combining two of the three diagnostic tools studied, we are able to distinguish models with convective cores from models without a convective core but with strong sound speed gradients in the inner layers (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Visibility analysis for target-oriented reverse time migration and optimizing acquisition parameters

To obtain the best possible image quality of complex geologic structures, prestack depth migration has evolved over the past several years from using one-way wave-equation migration (WEM) to using reverse time migration (RTM) in production seismic processing (Etgen et al., 2009). RTM extrapolates both source and receiver wavefields in time (Baysal et al., 1983; McMechan, 1983), rather than along a spatial (usually vertical) axis and is thus able to handle waves that propagate long distances horizontally, turning waves that propagate beyond 90°, and some special classes of multiple reflections, such as duplex waves and prism waves. The huge computational burden of RTM for high-frequency wavefields, especially in tilted anisotropic (TTI) media often prevents its use as an iterative seismic imaging tool. While certainly some imaging projects run multiple iterations of RTM, these projects often require long time frames and substantial expense, something justified only by the highest-profile projects. A wide class of seismic imaging problems being faced today are mostly “target-oriented” in their nature; they usually involve imaging particular details of salt features or other velocity complexities, or they involve creating the best possible image of the reservoir itself. In these cases, it is not necessary to migrate full-volume data to better reconstruct the image around the area of interest for each iteration because most of these data have no physical contributions to the target events and may even introduce noise and, thus, degrade the image quality of the target events. Rather, we should migrate those data having physical contributions to the target events. The visibility analysis we describe provides a solution to quantitatively select the data that need to be migrated for reconstructing the target events (Jin and Xu, 2009; Jin et al., 2010).

Sensitivity of the Calculated g-Mode Frequencies to Pulsation Codes and their Parameters

From the recent work of the Evolution and Seismic Tools Activity (ESTA: Monteiro et al., ESA, SP-1306, 363 – 371, 2006 and Lebreton et al., Astrophys. Space Sci. 316, 1 – 12, 2008), whose Task 2 is devoted to comparing pulsational frequencies computed using most of the pulsational codes available in the asteroseismic community, the dependence of the theoretical frequencies on non-physical choices is now quite well specified. To ensure that the accuracy of the computed frequencies is of the same order of magnitude or better than the observational errors, some requirements in the equilibrium models and the numerical resolutions of the pulsational equations must be followed. In particular, we have verified the numerical accuracy obtained with the Saclay seismic model, which is used to study the solar g-mode region (60 to 140 μHz). We have compared the results coming from the Aarhus adiabatic pulsation code (ADIPLS), with the frequencies computed with the Granada Code (GraCo) taking into account several possible non-physical choices and the effect of using two different outer mechanical boundary conditions. We have concluded that the present equilibrium models and the use of the Richardson extrapolation ensure an accuracy of the order of 0.01 μHz in the determination of the frequencies, which is quite enough for our purposes.

Comparisons between stellar models and reliability of the theoretical models

The high quality of the asteroseismic data provided by space missions such as CoRoT (Michel et al. in The CoRoT Mission, ESA Spec. Publ. vol. 1306, p. 39, 2006) or expected from new operating missions such as Kepler (Christensen-Dalsgaard et al. in Commun. Asteroseismol. 150:350, 2007) requires the capacity of stellar evolution codes to provide accurate models whose numerical precision is better than the expected observational errors (i.e. below 0.1 μHz on the frequencies in the case of CoRoT). We present a review of some thorough comparisons of stellar models produced by different evolution codes, involved in the CoRoT/ESTA activities (Monteiro in Evolution and Seismic Tools for Stellar Astrophysics, 2009). We examine the numerical aspects of the computations as well as the effects of different implementations of the same physics on the global quantities, physical structure and oscillations properties of the stellar models. We also discuss a few aspects of the input physics.

Modelagem numérica do efeito sismoelétrico em meios 2D

Este trabalho utiliza as equações dos fenômenos poroelástico-eletromagnéticos acoplados para simular pelo método das diferenças finitas no domínio do tempo as respostas sismoelétricas de modelos porosos saturados bidimensionais. As fontes sísmicas são do tipo explosivo com freqüência de corte 100 Hz.Os receptores foram localizados na direção horizontal e na vertical. Foram simulados cinco modelos porosos heterogêneos cujas respostas serviram para definir os tipos de interfaces que geram sinais sismoelétricos, as dimensões mínimas da heterogeneidade para detectabilidade sismoelétrica e o raio de investigação máxima. Observou-se resposta sismoelétrica irradiada em interfaces com contraste de porosidade, salinidade, litologia e fluido óleo-água. Os resultados obtidos sugerem a possibilidade de se planejar uma aquisição sismoelétrica conjuntamente com uma aquisição sísmica de superfície para objetivos rasos ou com uma aquisição sísmica em poço e interpoços para objetivos dentro do raio de investigação máxima.

Concluding remarks and perspectives on the evolution and seismic tools activity (ESTA) of the CoRoT community

A first part gives a summary of the main conclusions stemmed from the different comparisons reported in this book. One main conclusion is: with some care in the computations, one is able to obtain frequency differences between different computations which are smaller than required by Corot challenges. This is true for high frequency modes. Small frequency differences are much less easily obtained for low frequencies around the fundamental radial mode frequency. Care here means: having the same physics, same constants of physics, same input stellar parameters, computing models and oscillations with enough self numerical consistent accuracy. The ESTA group has built reference grids of models and associated oscillation frequencies and made them available to the community. In a second part of the present paper, a study case is considered in order to show the need and the use of such reference grids. Finally some perspectives concerning the remaining tasks are suggested.

The CoRoT evolution and seismic tools activity

The forthcoming data expected from space missions such as CoRoT require the capacity of the available tools to provide accurate models whose numerical precision is well above the expected observational errors. In order to secure that these tools meet the specifications, a team has been established to test and, when necessary, to improve the codes available in the community. The CoRoT Evolution and Seismic Tool Activity (ESTA) has been set up with this mission. Several groups have been involved. The present paper describes the motivation and the organisation of this activity, providing the context and the basis for the presentation of the results that have been achieved so far. This is not a finished task as future even better data will continue to demand more precise and complete tools for asteroseismology.

Reference grids of stellar models and oscillation frequencies

We present grids of stellar models and their associated oscillation frequencies that have been used by the CoRoT Seismology Working Group during the scientific preparation of the CoRoT mission. The stellar models have been calculated with the CESAM stellar internal structure and evolution code while the oscillation frequencies have been obtained from the CESAM models by means of the ADIPLS adiabatic oscillation programme. The grids cover a range of masses, chemical compositions and evolutionary stages corresponding to those of the CoRoT primary targets. The stellar models and oscillation frequencies are available on line through the Evolution and Seismic Tools Activity (ESTA) web site.

Grids of stellar evolution models for asteroseismology (cesam + posc)

In this paper we present a grid of stellar evolution models, computed with an up-to-date physical description of the internal structure, using the Code d’Evolution Stellaire Adaptatif et Modulaire (cesam). The evolutionary sequences span from the pre-main sequence to the beginning of the Red Giant Branch and cover an interval of mass typical for low and intermediate mass stars. The chemical composition (both helium and metal abundance) is the solar one. The frequencies of oscillation, computed for specific stellar models of the grid using the Porto Oscillations Code (posc), are also provided. This work was accomplished in order to support the preparation of the CoRoT mission within the Evolution and Seismic Tools Activity (CoRoT/ESTA). On the other hand, the grid can also be used, more generally, to interpret the observational properties of either individual stars or stellar populations. The grids (data and documentation) can be found at http://www.astro.up.pt/corot/models/cesam .

Winner: Geophysics Solving the Oil Equation

In this paper, the Kaleidoscope code is developed and used as a seismic imaging tool for oil exploration.

Tuning seismic resolution by heterodyning

This paper presents a new seismic data analysis and interpretation tool—the heterodyning technique of radio engineering. When applied to seismic data, it enables us to tune up seismic apparent resolution by shifting the dominant frequency of the signal with a given frequency bandwidth. Heterodyning provides a trade-off between detection precision and the signal-to-noise structure of the seismic data field. This is realized by tuning the amount of sidelobe energy and the phase characteristics of the seismic signals. Moderate phase rolls introduced by heterodyning do not affect the interpretation process because the subtlety in the behaviour of individual peaks and troughs is preserved and the eye tolerates gradual changes in phase characteristics. For steeply dipping beds, horizon flattening can be used to minimize the phase roll. In this paper, we introduce the principles of heterodyning and illustrate the process using both synthetic and real data.

Microscopic Diffusion in Stellar Evolution Codes: First Comparisons Results of ESTA-Task 3

We present recent work undertaken by the Evolution and Seismic Tools Activity (ESTA) team of the CoRoT Seismology Working Group. The new ESTA-Task 3 aims at testing, comparing and optimis- ing stellar evolution codes which include microscopic diffusion of the chemical elements resulting from pressure, temperature and concentra- tion gradients. The results already obtained are globally satisfactory, but some differences between the different numerical tools appear that require further investigations.

Modelling of the fast rotatingδScuti star Altair

We present an asteroseismic study of the fast rotating star HD 187642 (Altair), recently discovered to be a 0 Scuti pulsator. We have computed models taking into account rotation for increasing rotational velocities. We investigate the relation between the fundamental radial mode and the first overtone in the framework of Petersen diagrams. The effects of rotation on such diagrams, which become important at rotational velocities above 150 km s -1 , as well as the domain of validity of our seismic tools are discussed. We also investigate the radial and non-radial modes in order to constrain models fitting the five most dominant observed oscillation modes.

JaTS: a fully portable seismic tomography software based on Fresnel wavepaths and a probabilistic reconstruction approach

JaTS, a Java 2D seismic tomography software, is presented. It implements original algorithms achieving optimal accuracy with reasonable computing costs. A second-order Fast Marching Method (FMM) is used for solving the eikonal equation, therefore enabling a fast and robust computation of seismic traveltimes between sources and receivers. The wavepaths are materialized by Fresnel volumes rather than by conventional rays. This approach accounts for complex velocity models and has the advantage of considering the effects of the wave frequency in the velocity model resolution. The model is computed by a Simultaneous Iterative Reconstruction Technique (SIRT) which has been reformulated to integrate Fresnel wavepaths by using a probabilistic approach. In addition, various utilities are implemented, such as a tapering filter, used to decrease artifact effects occurring in the vicinity of the sources and receivers. The software also offers the possibility of reconstructing the velocity field on a grid larger than the one used for the wave propagation computation. This contributes to stabilize the estimated values. All of the seismic processing tools have been integrated with a user-friendly graphical interface. JaTS represents a tightly integrated tool suite that supports the entire process of importing the SG2 field records, first-break picking, forward modeling and velocity-field computing across multiple platforms.

3D Seismic Technology: Are We Realising Its Full Potential?

Abstract Three-dimensional (3D) seismic data have had a substantial impact on the successful exploration and production of hydrocarbons. Although most commonly acquired by the oil and gas exploration industry, these data are starting to be used as a research tool in other Earth sciences disciplines. However despite some innovative new directions of academic investigation, most of the examples of how 3D seismic data have increased our understanding of the structure and stratigraphy of sedimentary basins come from the industry that acquired these data. The 3D seismic tool is also making significant inroads into other areas of Earth sciences, such as igneous and structural geology. However, there are pitfalls that parallel these advances: geoscientists need to be multidisciplined and true integrators, and at the same time have an ever-increasing knowledge of geophysical acquisition and processing. Notably the utility of the 3D seismic tool seems to have been overlooked by most of the academic community, and we would submit that academia has yet to take full advantage of this technology as a research tool. We propose that the remaining scientific potential far exceeds the advances made thus far and major opportunities, as well as challenges, lie ahead.

Preventing a "noisequake" [substrate noise analysis

Substrate noise analysis can identify potential problems in mixed-signal and RF designs. In this paper substrate noise analysis was described and the methodology used in Cadence's SeismIC tool, a 3D solver, was discussed in detail. Several results were presented to demonstrate the accuracy of substrate extraction. Good correlation was shown between SeismIC and the 2D solver, Medici, for two structures in a TSMC 0.18 μm process. The overall average error in magnitude of the impedance for the two structures was 1.4 dB. Excellent correlation was demonstrated between SeismIC and measured impedances. The average magnitude of error between simulations and measurements was 6.4%. Another example of comparison of SeismIC with silicon was shown for a mixer fabricated in 0.6 μm technology. Measured gain showed excellent correlation to that simulated using a circuit simulator, with a substrate annotated netlist computed using SeismIC extraction. The SeismIC simulation flow was demonstrated for an Ethernet transceiver chip containing one million devices. This example shows the utility of using substrate analysis in the debug phase of a design, the value of identifying the worst noise contributors for a given design, and using analysis in the design phase to optimize the noise immunity of a design.

SEIS++

Predominant industrial practice has evolved from general-purpose class libraries to domain-specific frameworks" and design patterns. Both of them are a means to achieve large-scale reuse by capturing successful software development strategies within a particular context. Design patterns focus on reuse of recurring architectural design themes and mainly consist of predefined design structures that can be used as building blocks to compose the architecture of software system. Together the patterns in a specific domain form a pattern language, which can be used to approach a certain class of problems in the application domain. In this paper, we propose a pattern language SEIS++, a set of design patterns, for seismic tool construction and integration in oil and gas exploration domain. The language uses Tools and Materials as the new design conception to guide domain-specific application development, and to enhance software architecture reusability.

Perspectives on the Field of Physics of Earthquakes

This article reports on the state of physics governing the behavior of earthquakes and faults, based on discussions held during a workshop of the Southern California Earthquake Center (SCEC) suggested by Tom Henyey and convened by Yehuda Ben-Zion and Charles Sammis at Snowbird, Utah, June 21-23, 1998. The objective of the workshop was to assess the current state of understanding of earthquake processes including event nucleation, propagation and arrest of ruptures, spatiotemporal seismicity patterns, interactions between faults, and evolution of fault systems. A better understanding of the earthquake process should enable scientists to develop seismic hazard assessment tools based upon improved estimates of the locations and sizes of future earthquakes and the time-dependent probabilities of their occurrence. It will allow incorporation of realistic simulations of dynamic rupture and wave propagation into hazard models so that time histories of strong ground-shaking from scenario earthquakes needed in performance-based seismic design of structures...

Constraints on the shallow seismic structure at Ocean Drilling Program Site 1107, Ninetyeast Ridge, from implosive bottom sources and airgun shots

On Ninetyeast Ridge the seismological community drilled Ocean Drilling Program (ODP) Site 1107, coincident with ODP Site 757, for the future host of a long‐term seismological observatory in the Central Indian Ocean. To study the shallow seismic structure at the drill site, we developed a unusual on‐bottom seismic tool. Five implosive sources were placed along a 2‐km line. Four ocean bottom sensors recorded seismic waves over epicentral distances of 400 m to 2200 m, revealing seismic velocities of the immediately subjacent sediments. Velocities within this portion of crust are generally not detectable by traditional marine seismic techniques. Airgun shots, however, can be used to determine velocities of basement rocks. One‐dimensional travel‐time inversion of both bottom sources and water path corrected surface shots yields a velocity structure which is in excellent agreement with ODP drilling results. Three lithological units are imaged. A 150 m thick sediment section with velocities of 1600–1720 m/s. Underneath, a layer of 230 m thickness with velocities ranging from 2080 to 2250 m/s, corresponding to volcanic ashes and tuffs. The volcanic basement occured at 380 m below seafloor. Velocities rapidly increase from 3500 m/s to values of more than 4500 m/s within a few hundred metres.

Discriminating Between Onlap and Lithologic Interfingering in Seismic Models of Outcrops

Seismic models of outcrops have shown that near the limits of resolution the seismic reflection tool records rapid facies changes as false lap-outs. We examined one such outcrop, the Triassic platform flank of Picco di Vallandro (Durrenstein) in northern Italy, to develop criteria for recognizing false lap-out patterns. Using measured sections, photo mosaics of the continuous outcrop, and petrophysical measurements on samples, two contrasting impedance models were constructed. One model, supported by field observation, shows interfingering between basin marls and slope carbonates. The second model shows onlap of basin marls against slope carbonates, but with similar overall geometries to the first model. Vertical incidence seismic models were produced at frequencies between 50 and 150 Hz and with displays of seismic amplitude, as well as two seismic attributes-instantaneous phase and reflection strength. At 50 Hz, the two lithologic models are seismically indistinguishable, both showing onlap of basin units onto the slope. At 90 Hz, displays of instantaneous phase begin to reveal the stratigraphic interfingering, whereas seismic amplitude displays require frequencies of 150 Hz to do so. Reflection strength accurately portrays the distribution of the two major facies in the entire range of examined frequencies, but does not show sufficient detail to reveal interfingering vs. onlap. The same trends were visible with finite difference modeling of the full waveform. Here, the differences between interfingering and onlap become visible at 150 Hz in the phase model, where seismic amplitude displays at this frequency are still ambiguous. The phase model, however, fails to differentiate between slope dip and the basin sediment wedge, whereas this wedge is clearly mapped by the reflection strength. A combination of instantaneous phase and reflection strength provides a decisive advantage over conventional amplitude displays in recognizing rapid facies changes in seismic reflection data.