Can't see it. Can't touch it. How can we know what's going on inside? EAPS geoscientists have the tools and tenacity to tease apart just how the interior of the Earth works.
Seismic imaging—the technique of using sound waves to look inside the earth—has applications in the search for and responsible extraction of natural resources like oil and gas, in improving scientists' understanding of geologic hazards, such as enabling them to predict the ground motion in an earthquake, and in providing the data leading to a deeper understanding of the fundamental processes at work in the heart of our planet.
EAPS boasts world-class expertise in many areas of seismology. Most practitioners are affiliated with the Earth Resources Laboratory (ERL), MIT’s primary home for research and education focused on sub-surface energy resources. Geophysical imaging is only one among a variety of methodologies (also including rock physics and chemistry, multiphase flow, geomechanics, microseismics, and remote sensing) ERL’s faculty, research staff, and students use to obtain a holistic understanding of sub-surface reservoirs—their structure, the geological materials of which they are made, the fluids that flow through them, and changes that occur in response to production.
For example, Cecil and Ida Green Professor of Earth Sciences and Director of the ERL Brad Hager uses seismic data in conjunction with other geophysical measurements like GPS to understand surface deformation, earthquakes, and dynamical processes in the Earth’s interior. Assistant Professor of Geophysics Alison Malcolm focuses on imaging complex structures such as the fracture systems produced in hydraulic fracturing to help understand the ﬂuids that may be contained within. Senior Research Scientist Mike Fehler (ERL's Deputy Director) develops and tests novel methods for seismic imaging that can be applied to a number of problems in reservoir characterization including identification of changes in reservoirs, identification of fractures, and the relationship of induced seismicity and reservoir structure. Associate Director and Chair of EAPS Program in Geophysics Dale Morgan's research interests, which run the gamut from geoelectromagnetics to rock physics, applied seismology to environmental and engineering geophysics, also often rely heavily on seismic imaging. Schlumberger Professor of Geosciences and Head of EAPS Rob van der Hilst, besides an interest in subsurface reservoir science, images the Earth's deep interior to understand more about the thermal and chemical state of our planet and its links to surface processes. Finally, Assistant Professor Germán Prieto (more below and later in this issue) seeks an improved understanding of the diversity of earthquakes and the associated ground motions expected on the surface.
Here we focus on the work of three particular EAPS geophysicists developing and applying the tools and techniques of seismic imaging to turn the Earth: Inside-Out.
Using Math to See
Advised by Assistant Professor Alison Malcolm, graduate student Lucas "Bram" Willemsen (also ERL Honors Founder, this issue) is exploring new techniques for estimating elements in the approximate Hessian, a matrix operator fundamental in numerical techniques used to construct velocity models of the subsurface from seismic measurements. The more accurate a velocity model is, the more focused the seismic images will be.
Exactly calculating and storing all the terms in this matrix is often too computationally demanding. Willemsen is seeking to develop a more tractable reduced form, extracting the essence of the full expression while minimizing the computational overhead.
Willemsen benefits from ERL's interdisciplinary focus, often turning to mathematicians affiliated with the lab, such as Laurent Demanet, Assistant Professor of Mathematics.
Listening to See: Using Noise to Understand Earthquake Ground Motions
Along with destruction, earthquakes provide scientists with important information about the structure of the Earth's crust and upper mantle.
EAPS new assistant professor of geophysics Germán Prieto grew up in the seismic hot zone of Colombia, an experience which he freely admits developed in him an early interest in seismology.
Among Prieto's interests are ambient seismic fields. Once regarded as nothing more than noise, these signals have recently been shown to provide important information about Earth's structure. Surface wave tomography, body wave tomography both for crustal and deep interfaces, crustal anisotropy (directional dependence), attenuation tomography, and basin amplification have all been studied using these signals.
By looking at the spatially coherent signals between seismic stations, under certain conditions, researchers are able to extract an impulse response record similar to the Green's function of the medium; as if one station behaved like a source and the others were recording the response of the Earth's crust and upper mantle. In current work, Prieto is exploring what additional information can be gleaned from the amplitudes of these empirical Green's functions, both in time and frequency domains, to see even more clearly structural variations within the ground. You can learn more about Prieto's work in Three New Faculty elsewhere in this issue.
Doubly Deep: Deepening our Understanding of the Deep Earth
There remain deep unanswered questions about the large scale processes that occur inside our planet, even fundamental processes that are important to the evolution of life are not necessarily well understood.
Postdoctoral Associate Elizabeth Day, working with Rob van der Hilst and graduate student Chunquan Yu, uses seismic imaging to seek a deeper understanding of mantle dynamics and Earth's evolution over geologic time.
Focusing specifically on the seismic structure of the mantle beneath Hawaii, Day, Yu and van der Hilst are trying to understand its source within the deep Earth. What is the path of the hot plume that causes Hawaii through the mantle? What is the composition of the mantle in this region? Answers to these and questions like them will hopefully lead to a better overall understanding of how the mantle convects.
Since without mantle convection it is unlikely that there would be plate tectonics, and without plate tectonics it is unlikely that life as we know it would have developed on Earth, this work could have profound implications for our deeper understanding of the early Earth and its subsequent evolution, as well as our understanding for other rocky planets. You can follow Day on Twitter at https://twitter.com/lizzieday.
Is your name on the Geophysics Program Can?
Graduate student Alan Richardson proudly points to his entry on the EAPS Geophysics Can
For close to thirty years, EAPS geophysics graduate students, on passing their General Exams, have traditionally added their names to a ceremonial soup can. Diligently passed down from generation to generation, and currently on a shelf in an undisclosed student office in the ERL: Is your name among this auspicious roll call of former EAPSters? You can check out all the faces of the can here. Where are you now? Get in touch to let us know where your post-EAPS life has taken you (eapsnewsletter [at] mit [dot] edu).
All images credit: Helen Hill
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