CMG Research: Adjoint, Migration, and Velocity Analysis Methods for Imaging the Upper Mantle: Theory and Application
Regional and global seismic travel time tomography studies show that the Earth's upper mantle has a high degree of lateral heterogeneity, with fluctuations in seismic velocities up to ±6% in Vp and ±8–9% in Vs. These anomalies are often found in bodies whose size is at the edge of resolution using ray-theoretical methods. Recent work to incorporate aspects of wave character (i.e., fat rays or banana-donuts) in the tomographic inversions has illuminated smaller scale, previously hard to image anomalies, such as those from low velocity zones in the deeper parts of the mantle. The larger scale fluctuations in the mantle are indicative of both chemical variations and physical state differences (e.g. melt phases) as well as temperature fluctuations. Although seismic velocity is often taken as a proxy for temperature, many observations cannot be explained as temperature variations only. This is consistent with our understanding of the composition of the upper mantle from geochemistry, geodynamics, seismology and geologic field observations: 1) Beneath cratonic cores of continents the lithospheric mantle forms a buoyant iron depleted chemical and thermal boundary layer that has largely resisted convective return to the deeper mantle for 2.5 Ga, whereas the oceanic lithosphere is a thermal boundary layer whose residence time at the Earth's surface is ~200 Ma. Girdling the continents are orogenic belts whose upper mantle properties show the largest variability, and are at present the least well understood. 2) Several models of upper mantle structure posit that young oceanic lithosphere (< 20 Ma) will remain in the upper mantle as relatively small scale heterogeneities due to its low strength during subduction, promoting fragmentation, and thermally induced positive buoyancy. 3) High frequency (f > 1Hz) seismic scattering in the continental upper mantle is relatively strong, and also suggests as high degree of heterogeneity. 4) Field observations of ophiolites show multi-scale (in fact self-affine) separation between units of upper mantle lherzolite/harzburgite and dunites or garnet pyroxenites over a broad range of scales. The relative coarseness of tomography images has led to a situation in which vastly different models of mantle hetereogeneity and both large and small scale mantle convection structures are equally well supported.
Intellectual Merit: We can gain greater understanding of Earth geodynamics by increasing the resolution of seismic images of the upper mantle (to ~1000 km depth) by one to two orders of magnitude. Development of dense seismic arrays in the U.S., Asia, Europe, and elsewhere are providing data amenable to more rigorous inversion than ray-based travel-time tomography. We are proposing development of elastic/anelastic wave equation adjoint methods for waveform inversion of teleseismic data using P to S and S to P scattered waves that can increase seismic resolution by more than an order of magnitude.
Broader Impacts: Levander heads a group of Earthscope researchers who will hold 3 workshops on direct imaging for the seismological community from 2006–2008, in part to train graduate students in the theory and use of direct imaging methods. Levander will also lecture on direct imaging methods at the multi-disciplinary 2006 CIDER [Cooperative Institute for Deep Earth Research] workshop with graduate student and post-doctoral education being a primary goal. We expect the development of waveform tomography generally, and of extended variants incorporating velocity analysis in particular, to significantly affect the development of industrial seismic technology over the coming decade. Graduate student and post-doc training in this field for dissemination of the imaging systems is of particular importance to both academia and industry.