Tarje Nissen-Meyer

Savani: A variable resolution whole‐mantle model of anisotropic shear velocity variations based on multiple data sets

We present a tomographic model of radially anisotropic shear velocity variations in the Earths mantle based on a new compilation of previously published data sets and a variable block parameterization, adapted to local raypath density. We employ ray-theoretical sensitivity functions to relate surface wave and body wave data with radially anisotropic velocity perturbations. Our database includes surface wave phase delays from fundamental modes up to the sixth overtone, measured at periods between 25 and 350 s, as well as cross-correlation traveltimes of major body wave phases. Before inversion, we apply crustal corrections using the crustal model CRUST2.0, and we account for azimuthal anisotropy in the upper mantle using ray-theoretical corrections based on a global model of azimuthal anisotropy. While being well correlated with earlier models at long spatial wavelength, our preferred solution, savani, additionally delineates a number of previously unidentified structures due to its improved resolution in areas of dense coverage. This is because the density of the inverse grid ranges between 1.25° in well-sampled and 5° in poorly sampled regions, allowing us to resolve regional structure better than it is typically the case in global S wave tomography. Our model highlights (i) a distinct ocean-continent anisotropic signature in the uppermost mantle, (ii) an oceanic peak in above average ξ<1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness, and (iii) a long-wavelength pattern of ξ<1 associated with the large low-shear velocity provinces in the lowermost mantle.

Elastic imaging and time-lapse migration based on adjoint methods

We have drawn connections between imaging in exploration seismology, adjoint methods, and emerging finite-frequency tomography. All of these techniques rely on spatial and temporal constructive interference between observed and simulated waveforms to map locations of structural anomalies. Modern numerical methods and computers have facilitated the accurate and efficient simulation of 3D acoustic, (an)elastic, and poroelastic wave propagation. Using a 2D cross section of the SEG/EAGE salt model, we have determined how such waveform simulations might be harnessed to improve onshore and offshore seismic imaging strategies and capabilities. We have found that the density sensitivity kernel in adjoint tomography is related closely to the imaging principle in exploration seismology, and that in elastic modeling the impedance kernel actually is a better diagnostic tool for reflector identification. The shear- and compressional-wave speed sensitivity kernels in adjoint tomography are related closely to finite-frequency banana-doughnut kernels, and these kernels are well suited for mapping larger-scale structure, i.e., for transmission tomography. These ideas have been substantiated by addressing problems in subsalt time-lapse migration.

Wave propagation in 3D spherical sections: effects of subduction zones

Abstract In order to understand details in the seismic wave field observed on regional and global scales on the Earth’s surface accurate modeling of 3D wave propagation is necessary. While numerical techniques are now routinely applied to local seismic wave propagation, only recently has the possibility of simulating wave propagation on larger scales in spherical geometry been investigated. We apply a high-order staggered-grid finite-difference scheme to the elastic wave equations in spherical coordinates [ϕ, θ, r]. Using regular grid spacing in a single domain the physical space is limited to spherical sections which do not include the axis θ=0. While the staggering of the space-dependent fields improves the overall accuracy of the scheme, some of the tensor elements have to be interpolated to the required grid locations. By comparing with quasi-analytical solutions for layered Earth models we demonstrate the accuracy of the algorithm. Finally, the technique is used to study the effects of a source located in a simplified slab structure. The 3D technique will allow us to study the wave field due to laterally heterogeneous structures, such as subduction zones, plumes or oceanic ridges.

Forward and adjoint simulations of seismic wave propagation on emerging large-scale GPU architectures

Computational seismology is an area of wide sociological and economic impact, ranging from earthquake risk assessment to subsurface imaging and oil and gas exploration. At the core of these simulations is the modeling of wave propagation in a complex medium. Here we report on the extension of the high-order finite-element seismic wave simulation package SPECFEM3D to support the largest scale hybrid and homogeneous supercomputers. Starting from an existing highly tuned MPI code, we migrated to a CUDA version. In order to be of immediate impact to the science mission of computational seismologists, we had to port the entire production package, rather than just individual kernels. One of the challenges in parallelizing finite element codes is the potential for race conditions during the assembly phase. We therefore investigated different methods such as mesh coloring or atomic updates on the GPU. In order to achieve strong scaling, we needed to ensure good overlap of data motion at all levels, including internode and host-accelerator transfers. Finally we carefully tuned the GPU implementation. The new MPI/CUDA solver exhibits excellent scalability and achieves speedup on a node-to-node basis over the carefully tuned equivalent multi-core MPI solver. To demonstrate the performance of both the forward and adjoint functionality, we present two case studies run on the Cray XE6 CPU and Cray XK6 GPU architectures up to 896 nodes: (1) focusing on most commonly used forward simulations, we simulate seismic wave propagation generated by earthquakes in Turkey, and (2) testing the most complex seismic inversion type of the package, we use ambient seismic noise to image 3-D crust and mantle structure beneath western Europe.

Seismic modeling and imaging based upon spectral-element and adjoint methods

The overarching goal of exploration geophysics is to map or “image” geological structures. Seismic migration, which attempts to map reflected and/or refracted signals to their actual spatial origin, plays a central role in imaging. Migration techniques are frequently based upon approximations to the seismic wave equation (e.g., ray-theory-based Kirchhoff migration or acoustic/first-arrival wavefield continuation methods).

On-demand custom broadband synthetic seismograms

ABSTRACT We present a new webservice, Syngine, running at the Incorporated Research Institutions for Seismology Data Management Center (IRIS‐DMC), that offers on‐demand and custom‐tailored seismograms served over HTTP. The free service produces full seismic waveforms, including effects like attenuation and anisotropy, that are calculated in commonly used spherically symmetric Earth models (preliminary reference Earth model [PREM], ak135‐f, IASP91). Users can freely adjust sources and receivers, retrieve seismograms from finite sources, convolve with arbitrary source time functions, and download Green’s functions suitable for moment tensor inversions. Syngine extracts and processes seismograms in as fast as fractions of a second, making it suitable for applications demanding short iteration times and a large number of waveforms. For the first time, researchers without large computational resources or specialized knowledge can easily access high‐quality, custom, broadband seismograms. In this article, we present the rational and basic principles of our method, including its limitations. Additionally, we demonstrate the features of Syngine and the included Earth models, showcase several applications, and discuss future possibilities.

Seismic Wave Propagation in Icy Ocean Worlds

Seismology was developed on Earth and shaped our model of the Earths interior over the 20th century. With the exception of the Philae lander, all in situ extraterrestrial seismological effort to date was limited to other terrestrial planets. All have in common a rigid crust above a solid mantle. The coming years may see the installation of seismometers on Europa, Titan and Enceladus, so it is necessary to adapt seismological concepts to the setting of worlds with global oceans covered in ice. Here we use waveform analyses to identify and classify wave types, developing a lexicon for icy ocean world seismology intended to be useful to both seismologists and planetary scientists. We use results from spectral-element simulations of broadband seismic wavefields to adapt seismological concepts to icy ocean worlds. We present a concise naming scheme for seismic waves and an overview of the features of the seismic wavefield on Europa, Titan, Ganymede and Enceladus. In close connection with geophysical interior models, we analyze simulated seismic measurements of Europa and Titan that might be used to constrain geochemical parameters governing the habitability of a sub-ice ocean.

A Lattice Boltzmann Method for Elastic Wave Propagation in a Poisson Solid

The lattice Boltzmann (LB) method is a numerical method that has its origins in discrete mechanics. The method is based on propagating discrete density distributions across a fixed lattice and implementing conservation laws between the density distributions at lattice intersections. The method has been successfully applied to a wide variety of problems in fluid dynamics but has yet to be applied to elastic wave propagation. In this article we outline a new 2D and 3D LB solution to the elastic wave equation in a Poisson solid using a regular lattice, in 2D a square geometry and in 3D a cubic geometry. We outline the theory behind the method and derive the elastic wave equation from a Chapman–Enskog expansion about the Knudsen number. The scheme is shown to give rise to the elastic wave equation with a fixed Poisson ratio of 0.25 with a Knudsen number truncation error of order two. We have performed a von Neumann plane‐wave analysis and found that the numerical dispersion is comparable to other discrete methods for modelling wave propagation. We have compared the numerical method to two problems, a 3D infinite homogeneous medium and a 2D heterogeneous block model. In both cases, we found the solutions agreed, thus showing that the LB method can be used to model elastic wave propagation. The scheme offers the potential to model the interaction of several continuum equations within one solver as the continuum equation is solely dependent on the equilibrium distribution.

Classifying elephant behaviour through seismic vibrations

Seismic waves - vibrations within and along the Earths surface - are ubiquitous sources of information. During propagation, physical factors can obscure information transfer via vibrations and influence propagation range [1]. Here, we explore how terrain type and background seismic noise influence the propagation of seismic vibrations generated by African elephants. In Kenya, we recorded the ground-based vibrations of different wild elephant behaviours, such as locomotion and infrasonic vocalisations [2], as well as natural and anthropogenic seismic noise. We employed techniques from seismology to transform the geophone recordings into source functions - the time-varying seismic signature generated at the source. We used computer modelling to constrain the propagation ranges of elephant seismic vibrations for different terrains and noise levels. Behaviours that generate a high force on a sandy terrain with low noise propagate the furthest, over the kilometre scale. Our modelling also predicts that specific elephant behaviours can be distinguished and monitored over a range of propagation distances and noise levels. We conclude that seismic cues have considerable potential for both behavioural classification and remote monitoring of wildlife. In particular, classifying the seismic signatures of specific behaviours of large mammals remotely in real time, such as elephant running, could inform on poaching threats.

3-D Seismic Wave Propagation on a Global and Regional Scale: Earthquakes, Fault Zones, Volcanoes

For computational seismology the present years are extremely exciting. The reason is, that with the current supercomputer technology, the frequency band in which seismic waves are observed following regional or global earthquakes, can be simulated numerically for realistic 3D earth models for the first time. Depending on the spatial scales under consideration (whole planet, a sedimentary basin at risk from local earthquakes, a volcano with high risk for future eruptions) this will lead to considerable improvement (1) in the understanding of the structural properties (e.g. the Earth’s mantle, the inside of a sedimentary basin or a volcano) and (2) in forecasting strong ground motion for realistic earthquake scenarios. The latter point may have considerable long-term societal benefits, as the short-term prediction of large earthquakes seems out of sight. During the first phase of this project some of the highest-resolution simulations ever done were carried out with important implications for future directions in computational seismology. The most important scientific results can be summarized as: (1) 3D Simulations of several earthquakes in the Cologne Basin in Germany demonstrate that the main characteristics of ground motion (e.g. peak motion amplitude, shaking duration) are successfully predicted through numerical simulations; (2) The low seismic velocities inside active faults (e.g. San Andreas Fault, California) may act as an amplifier for ground motion. This has implications for buildings in the vicinity of faults; (3) Large scale simulations of strong earthquakes in subduction zones show that the local 3D structure at depth strongly influences the waves propagating to the surface. Ignoring this will lead to severe misinterpretations. (4) Including topography to understand wave propagation inside volcanoes is crucial. Our simulations demonstrate the scattering effects due to topography. If we want to understand the state of a volcanic system prior to eruptions from seismic waves these effects have to be taken into account.