ALBERT

All Library Books, journals and Electronic Records Telegrafenberg

feed icon rss

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: A sensitivity study of elastic parameters in amplitude-variation-with-slowness (AVS) for small- and large-offset seismic data is presented. In order to handle the non-linearity associated with waveform or amplitude beyond the critical slowness, an inversion algorithm based on Bayes' theory is used. A genetic algorithm was used to obtain the a posteriori probability density (PPD) function. The sensitivity analysis is performed on synthetic data containing P-wave as well as converted S-wave reflections. Four different two-layer models, which represent the typical range of AVS responses associated with the gas-sands normally encountered in exploration, were used to examine how well the elastic parameters can be inverted for different parametrizations by comparing the PPD functions. The sensitivity study results suggest that including wide-angle data in the inversion can greatly enhance the quality of inversion. The converted S-wave reflections can provide valuable extra information that can be used to extract elastic parameters. The results with noisy data demonstrate that the contrast of density and three velocity ratios can be estimated robustly with wide-angle reflection data.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 2
    Electronic Resource
    Electronic Resource
    PO Box 1354, 9600 Garsington Road , Oxford OX4 2XG , UK . : Blackwell Science Ltd
    Geophysical prospecting 53 (2005), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Seismic waveforms contain valuable information about the media, but the waveform inversion is a non-linear problem. We present a waveform inversion method that combines a local optimization method with a genetic algorithm to determine the anisotropic parameters of a horizontally stratified medium. Synthetic seismograms for a horizontally stratified anisotropic medium are calculated using the reflectivity technique. In the initial stage of the inversion, the global space-sampling properties of the genetic algorithm are used to direct the search to the region close to the global solution. This solution is then further improved using a conjugate-gradient method. The numerical experiments performed with noisy synthetic data show that our hybrid optimization method satisfactorily reconstructs the anisotropic parameters at a reasonable computing cost while the range of slowness is adequate. We found that (i) for small-angle data neither single- nor multiple-component data are sufficient to determine the anisotropic parameters uniquely; (ii) for medium-angle data the multiple-component data are sufficient to determine the anisotropic parameters exactly whereas the single-component data are not sufficient; and (iii) for wide-angle data, either single- or multiple-component data are sufficient to determine the anisotropic parameters accurately.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Geophysical prospecting 51 (2003), S. 0 
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: Wide-angle multicomponent ocean-bottom cable (OBC) data should further enhance sub-basalt imaging by using both compressional and converted shear wavefields. The first step in analysing multicomponent OBC data is to decompose the recorded wavefields into pure P- and pure S-wavefields, and extract the upgoing P- and S-waves. This paper presents a new scheme to separate P- and S-wavefields from wide-angle multicomponent OBC data in the τ–p domain. By considering plane-wave components with a known horizontal slowness, the P- and S-wavefields are separated into the directions of observed P- and S-wave oscillations using the horizontal and vertical components of the data. The upgoing P- and S-waves are then extracted from the separated P- and S-wavefields. The parameters used in the separation are the seismic wave velocities and the density at the receiver location, which can be estimated from the first reflection phase observed on the horizontal and vertical components. Numerical tests on synthetic data for a plane-layered model show good performance and demonstrate the accuracy of the scheme. Separation of wavefields from a basalt model is performed using synthetic wide-angle multicomponent OBC data. The results show that both near-offset and wide-angle reflections and conversions from within and below basalt layers are enhanced and clearly identified on the separated wavefields.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 4
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers. The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast-spreading ...
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 5
    facet.materialart.
    Unknown
    In:  [Poster] In: AGU Fall Meeting 2018, 10.-14.12.2018, Washington, D.C., USA .
    Publication Date: 2018-12-11
    Description: The global Mid-Ocean Ridge system is a 65000 km long continuous mountain range and is a central part of the plate tectonic theory. New oceanic lithosphere is continuously being generated at these centers of magma upwelling and then propagated away from the ridge axis by the forces of ridge push and slab pull. Though these processes of crustal generation and propagation are well studied at spreading ridges, much less is known about how crustal accretion changed over time and how the crust and lithosphere evolve with age. Also, the Lithosphere-Asthenosphere Boundary (LAB), which occurs around Moho near the ridge axis, deepens with age and is poorly defined in terms of depth and thickness. To quantify the variation of velocity structure in the crust and lithosphere and to image the LAB from 0 to 50 Ma using wide-angle reflections, LITHOS experiment was conducted in late 2017 near the equatorial Mid-Atlantic Ridge on R/V Maria S. Merian. A total of 71 Ocean Bottom Seismometers (OBS) and hydrophones were deployed to acquire refraction and wide angle reflection data with an average spacing of 15 km along 1100 km long seismic transect coincidental with a deep seismic reflection profile acquired in 2015 using a 12 km long streamer. The OBS profile runs across the Mid-Atlantic Ridge covering 75 km on the South American plate and the rest of 1025 km lies on the African plate over 0 to approximately 50 Ma old lithosphere. In this study we analyze 600 km long profile in the western part of the seismic transect crossing the ridge-axis with age variation from 0 to 25 Ma. The OBS data was band pass filtered between 3 and 30 Hz and predictive deconvolution was applied. Pn arrivals were observed till offsets of up to 150 km on some OBS stations away from ridge axis. We present the velocity structure of the crust and upper mantle down to a depth of 10 km from seafloor by using first arrival travel time tomography. We also observe secondary arrivals on most of the OBS stations starting from offsets around 35 - 40 km and ongoing analysis will determine the nature of such events. Detailed analysis of velocity structure and heat flow data will shed light on the effect of cooling of lithosphere with age on the crustal and mantle velocities and presence of hydrothermal circulation near the ridge axis.
    Type: Conference or Workshop Item , NonPeerReviewed
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 6
    facet.materialart.
    Unknown
    In:  [Poster] In: AGU Fall Meeting 2018, 10.-14.12.2018, Washington, D.C., USA .
    Publication Date: 2018-12-11
    Description: The lithosphere-asthenosphere boundary (LAB) is the most prevalent plate boundary on earth, but its definition and nature remains a matter of debate. Using wide-angle ocean bottom seismic (OBS) data, here we present the very first reflection image of the LAB over young oceanic lithosphere. The OBS data were acquired in 1996 to image the crustal structure, but advanced processing of these data show the presence of two wide-angle reflections originating from within the mantle. We first perform forward modeling to exclude the possibility of multiples and show these events are real. Based on tomographic velocity model, we perform migration and find that these reflections are continuous over 25-30 km distance and originate at 14 km and 19 km below the sea surface for ~2.6 Ma Oceanic Lithosphere. These seismic reflections lie several kilometers below the Moho, and the seismic forward modeling requires these reflections to originate from velocity discontinuities, which could relate to the presence of melts in the mantle that create the low velocity anomalies, therefore we suggest that they may define the LAB. These observations provide constrain of the depth, distribution and physical properties of the LAB transition zone, combined with the recent study of Mehouachi and Singh (2018), suggest that the LAB could consists of melt rich layers.
    Type: Conference or Workshop Item , NonPeerReviewed
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 7
    Publication Date: 2018-12-20
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: text
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 8
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We present velocity constraints for the upper-mantle transition zones beneath Central Siberia based on observations of the 1982 RIFT Deep Seismic Sounding (DSS) profile. The data consist of seismic recordings of a nuclear explosion in north-western Siberia along a 2600 km long seismic profile extending from the Yamal Peninsula to Lake Baikal. We invert seismic data from the mantle transition zones using a non-linear inversion scheme using a genetic algorithm for optimization and the WKBJ method to compute the synthetic seismograms. A statistical error analysis using a graph-binning technique was performed to provide uncertainty values in the velocity models.Our best model for the upper-mantle velocity discontinuity near 410 km depth has a two-stage velocity-gradient structure, with velocities increasing from 8.70–9.25 km s−1 over a depth range of 400–415 km, a gradient of 0.0433 s−1, and from 9.25–9.60 km s−1 over a depth range of 415–435 km, a gradient of 0.0175 s−1. This derived model is consistent with other seismological observations and mineral-physics models. The model for the velocity discontinuity near 660 km depth is simple, sharp and includes velocities increasing from 10.15 km s−1 at 655 km depth to 10.70 km s−1 at 660 km depth, a gradient of 0.055 s−1.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 9
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The pre-critical wide-angle seismic reflection data collected during the BABEL survey using multireceiver spreads contain high-amplitude P-wave and mode-converted S-wave reflected arrivals. The two phases can be partially separated and their signal-to-noise ratio emphasized by exploiting the multifold nature and the increased sensitivity of velocity analysis afforded by these data to produce zero-offset-corrected stacked sections. Using these sections, a method has been developed to derive estimates of Poisson's ratio by optimizing the cross-correlation of P-wave and S-wave arrivals, and hence to constrain petrological models of the crust sampled by these data. Although the apparently high degree of temporal and spatial resolution of reflected arrivals that are visible within the stacked sections must be reduced to achieve a satisfactory degree of correlation robustness, the results shed some light on crustal petrology, and concur with existing estimates of Poisson's ratio for the Baltic shield.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: WKBJ theory has been widely used to solve wave equations. When the medium is 1-D, it is possible to obtain an approximate analytical expression for the Green's function of an elastic wave equation in the plane-wave domain using this theory. As the expression is analytical, it is a very efficient way to compute synthetic seismograms. If the medium is also smoothly varying in the lateral directions, the WKBJ theory can be extended to compute the Green's function for 3-D. This can be achieved by dividing the model parameters into two parts: vertically varying and a small lateral deviation from the vertical. In this paper, we derive an approximate analytical expression for the Green's function for such 3-D media. The solution is valid for pre-critical waves and can be used to compute the scattered wavefields in 3-D. Synthetic seismograms have been computed for direct waves in 2-D media and are found to be in agreement with the ray theory results.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...