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Towards an understanding of Earth's structure - The development of adaptive optics approaches for Seismology

Dr Sebastian Rost (SEE), Dr Neil Selby (AWE Blacknest), Dr Andy Nowacki (SEE)

Project partner(s): AWE Blacknest (CASE)

Contact email: s.rost@leeds.ac.uk

The Earth is heterogeneous on many scale-lengths - Pick up a crustal rock and you see the heterogeneity on a grain scale, look at a geological map to see the heterogeneity of Earth’s crust and look at the differences between oceans and continents to see evidence for the heterogeneity on a global scale. This heterogeneity we see at the surface of the Earth can also be found in the interior of our planet and is the expression of the dynamics and evolution of our planet. Imaging this expected heterogeneity in the Earth’s interior is one of the grand challenges of Seismology.

Over the last few decades our understanding of the large-scale structure of the Earth’s interior, as imaged e.g. with seismic tomography, has started to converge and our understanding of the dynamics and origin of many features in the models has improved. On the other hand, the small-scale structure (i.e. smaller than a few hundred kilometres) of Earth’s interior remains controversial and ill-resolved. This is partly due to the limitations of seismology to resolve small-scale structure due to the seismic wavelength. To understand the structure, evolution and dynamics of our planet we require knowledge and understanding of structures across the full spectrum of heterogeneities found in the Earth. This project aims to improve our understanding of the fine-scale structure of the Earth and will contribute to a better understanding of Earth’s dynamics.

Earthquake seismology is one of the main remote sensing tools for the interior structure of the Earth. Using information extracted from the seismic wavefield we are able to understand both the source mechanisms and the structure along the propagation path. E.g. the large-scale structure along the path can be resolved using seismic tomography and smaller scale structure and information on the source mechanism can be resolved using waveform information. This project will focus on extracting information on the Earth’s fine scale structure from the seismic wavefield by using scattered seismic energy. The scattered wavefield contains structural information on the km scale and allows insight into many processes at Earth’s surface and in its interior (Revenaugh, 1999). Elastic waves scatter at heterogeneities with scale lengths on the same order as the seismic wavefield and many sections of recorded seismograms are dominated by scattered energy (Rost et al., 2015; Shearer, 2007; Shearer and Earle, 2008).  

 

Figure 1: (top) Typical scale lengths of Earth heterogeneity and seismic probe to image the heterogeneity. The largest scales are resolvable using traveltime tomography with shorter scales being imaged using waveform inversions. The shortest resolvable scales manifest in the scattered seismic wavefield and will be used here. After Rost et al., (2015). (Bottom) Geodynamic simulation of Earth structure showing the different scale structures (e.g. LLSVPs, subduction, plumes and crustal remnants) imaged using these probes. After (Van Keken et al., 2002)

The strongest seismic heterogeneities can typically be found in the Earth’s lithosphere (Korn, 1988, 1990). Since most seismic stations are located at the Earth’s surface (Figure 2), these heterogeneities restrict our ability to image and understand Earth structure and source mechanisms further along the seismic raypath. This project aims to develop a methodology similar to adaptive optics to remove the effects of the shallow heterogeneities on the seismic waveform to improve imaging of path structure and source mechanism.

Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions from atmospheric conditions and is commonly used with astronomical telescopes. For seismology we will characterize the structure beneath a station using a large database of seismic recordings and will use wavefront simulation techniques to predict the influence of the structure on the waveform. The project will then develop methodology to remove this influence from the incoming wave field, which will allow us to image structure along the path and the source mechanism better. Similar approaches have recently been adopted in hydrocarbon reservoir imaging (Etgen et al., 2014).

Figure 2: Schematics of the effect of scattering media of different thicknesses, scattering attenuation (Qs), and anelastic attenuation (Qa) on a wavefront in terms of time (shape of wavefront) and amplitude (shading of wavefront) for scenarios with (a) shallow scattering only and (b) shallow and deep scattering. In this case, the effect of the known shallow structure on the wavefront (determined using shallowly penetrating waves) can be taken into account to determine the effect of the deep structure along (image courtesy of D. Frost (UC Berkeley)).

Objectives

The project can be divided in three parts that will roughly map onto the three years of the candidature.

Part 1 will characterize the small-scale heterogeneities beneath the primary arrays of the Comprehensive Test Ban Treaty (CTBT) Organization (https://www.ctbto.org) to monitor underground nuclear explosions. These arrays are used to monitor underground nuclear explosions. AWE Blacknest, the CASE partner, is the UK National Data Centre for the CTBT, and one of its roles is to develop enhanced methods for the monitoring of the Treaty. The information on small-scale structure beneath the arrays will be important for CTBTO scientists for better monitoring of explosions globally. The CTBTO arrays are ideal for method development since they cover a wide range of tectonic regions, are typically installed in seismically quiet regions, have large data catalogues and allow 3D analysis of the incident seismic wavefield (Dainty and Toksoz, 1990).

The second part of the project will develop a methodology to remove the near-receiver structure from the seismic wavefield – the adaptive optics process. This will include synthetic modelling of the seismic wavefield in 3D media and theoretical development of the seismic adaptive optics.

The final part will test the feasibility to apply these methods to installation of seismometer e.g. as part of USArray (http://www.earthscope.org) or using dense deployments in Europe. Using stations deployed over a wide range of tectonic terranes will not only allow characterization of the lithosphere in these regions but will also allow tests how widely the adaptive optics approach is applicable to other seismic imaging techniques.

The work will be presented at international conferences such as the annual meeting of the American Geophysical Union (San Francisco/New Orleans), the European Geosciences Union (Vienna) or the Science and Technology Conference Series of the CTBTO (Vienna). Extended visits to CTBTO (Vienna) or national institutions for explosion monitoring (e.g. NORSAR in Norway or the US Geological Survey) are also possible.

Training

This project is suited for students with a background in geophysics, natural sciences, geosciences, physics or mathematics with a strong interest in seismic data analysis, Earth structure and dynamics. The successful candidate will develop a deep understanding of seismic wave propagation through the analysis of seismic data and synthetic modelling of seismic wave propagation in heterogeneous media. The project will entail several placements at AWE Blacknest allowing interaction with experts in source localisation and characterization. The skills obtained in this project are transferrable to a variety of fields including the hydrocarbon industry and global earthquake seismology. All three objectives will lead to high quality publications and the methods developed in this project aim to be applied within the CTBTO.  

Potential for high impact outcome

The project will represent a significant contribution to our understanding of the effects of Earth structure on the seismic wavefield. The adaptive optics approach will allow us to account for several imaging errors and will sharpen our images of Earth structure elsewhere along the seismic raypath. The work will be easily divisible into peer-reviewed publications that will form consecutive chapters of a PhD thesis as described under Objectives.

Research Environment

The applicant will be located in the Institute of Geophysics and Tectonics (http://www.see.leeds.ac.uk/research/igt/) at the contact point between the Deep Earth Research Group (http://www.see.leeds.ac.uk/research/igt/deep-earth-research/) and the research group for Geodynamics & Tectonics (http://www.see.leeds.ac.uk/research/igt/geodynamics-and-tectonics-group/).

The Deep Earth Research Group is one of the world’s largest groups of scientists studying the structure and dynamics of Earth’s core and mantle. Research topics include the dynamics and structure of the Earth’s magnetic field and convection in the outer core, material properties under high pressure and temperature and Global Seismology. The Group collaborates closely with the Department of Applied Mathematics in Leeds and Deep Earth research groups worldwide.

The Geodynamics & Tectonics group aims to understand the processes that form and change the surface of the Earth through remote sensing, seismology, structural geology and geodynamics.

The Institute of Geophysics and Tectonics comprises several research group including Geodynamics & Tectonics, Volcanology, High Temperature Geochemistry as well as Deep Earth Research. Several of the research groups use seismology as a research tool and the institute offers a diverse and supportive research environment.

CASE Partner

This project is a potential CASE project in collaboration with AWE Blacknest and one of the supervisors will be located there.  Successful applicants will spend time at AWE Blacknest during several placements. Blacknest’s roles include seismic monitoring of adherence to the Comprehensive Test Ban Treaty (CTBT) designed to prevent nuclear explosions, such as the recent nuclear tests in North Korea. AWE Blacknest will provide additional funding for research and travel expenses and will provide access to the data of the CTBTO.

References and further reading

Dainty, A., Toksoz, M., 1990. Array analysis of seismic scattering. Bull. Seismol. Soc. Am. 80, 2242–2260.

Etgen, J.T., Ahmed, I., Zhou, M., 2014. Seismic Adaptive Optics. Presented at the 2014 SEG Annual Meeting, Society of Exploration Geophysicists.

Korn, M., 1990. A modified energy flux model for lithospheric scattering of teleseismic body waves. Geophys. J. Int. 102, 165–175. doi:10.1111/j.1365-246X.1990.tb00538.x

Korn, M., 1988. P-Wave Coda Analysis of Short-Period Array Data and the Scattering and Absorptive Properties of the Lithosphere. Geophys. J.-Oxf. 93, 437–449. doi:10.1111/j.1365-246X.1988.tb03871.x

Revenaugh, J., 1999. Geologic applications of seismic scattering. Annu. Rev. Earth Planet. Sci. 27, 55–73.

Rost, S., Earle, P.S., Shearer, P.M., Frost, D.A., Selby, N.D., 2015. Seismic Detections of small-scale heterogeneities in the deep Earth, in: The Earth’s Heterogeneous Mantle, Springer Geophysics. Springer, pp. 367–390.

Shearer, P.M., 2007. Deep Earth Structure - Seismic Scattering in the Deep Earth, in: Treatise on Geophysics. Elsevier, Amsterdam, pp. 695–729.

Shearer, P.M., Earle, P.S., 2008. Chapter 6 Observing and Modeling Elastic Scattering in the Deep Earth, in: Earth Heterogeneity and Scattering Effects on Seismic Waves. Elsevier, pp. 167–193.

Van Keken, P.E., Hauri, E.H., Ballentine, C.J., 2002. Mantle Mixing: The Generation, Preservation, and Destruction of Chemical Heterogeneity. Annu. Rev. Earth Planet. Sci. 30, 493–525. doi:10.1146/annurev.earth.30.091201.141236

Related undergraduate subjects:

  • Earth science
  • Geophysics
  • Geoscience
  • Natural sciences
  • Physics