High Resolution Mantle Tomography using Advanced Modeling Methods

Project description

Motivation (background):

The structure and dynamics of the Earth’s mantle are fundamental for understanding structure and dynamics of the Earth’s surface. This includes fundamental, and to a large extent unanswered questions, concerning energy, resources, the environment and natural disasters. The large scale structure of the mantle, especially subduction zones and mantle plumes, has been imaged successfully using seismic tomography. However, the resolution of these images is still quite small (tens of kilometers in most parts of the mantle). One of the main fundamental research areas in geoscience is to improve this resolution. There are various ways in which this can done. In this description the focus is on improving the methodology in two fundamental ways. Each of these constitutes a master thesis.

Hypothesis (scientific problem):
The two main methods to determine the structure of the mantle are surface wave tomography and travel time
tomography. The projects described here are part of a larger effort where these two methods are combined. However, here the focus is on travel time tomography. Usually, travel time tomography is done using millions of manual picks of mantle phases (such as P, S, PP, SS, PcP etc.). These picks contain a number of uncertainties and errors and are only done for larger earthquakes. Picking the travel times using automated methods results in better and more picks and hence better images. This has been the topic of a couple of previous master theses. The next steps to improve the tomographic results consist of improving the methodology. There are two different and fundamental ways in which this can be done, each of which would be the topic of research for one master thesis: 1. by inverting for anisotropic, rather than isotropic, mantle structure and 2. by using finite, rather than infinite (ray), sensitivity kernels.

Test (work):
These methdological projects have a focus on theory and computer programming. Starting point for the inclusion of anisotropy in the tomography is an anisotropic ray tracing code in Cartesian codes. This code needs to be modified so that it can be used in mantle tomography (which is done in spherical coordinates) and includes ray tracing with a number of interfaces (such as the Moho, 410 km and 660 km discontinuities). Once the code it is written it can be used to test anisotropic tomography. The second project, which involves the computation of finite frequency sensitivity kernels, uses techniques from seismic exploration, in particular the ray-Born approximation, to compute the kernels. In
this case the focus is on making the computation of the kernels as efficient as possible. Once the sensitivity kernels can be computed, tests will be devised to compare the traditional and the finite frequency tomography. If time allows both methods will be applied to real data.

Proposed course plan during the master's degree (60 ECTS)

Mat212 (10sp) or Geov274 (10 sp)
Geov276 (10sp)
Geov300 (5sp)
Geov355 (10sp)
Spesial pensum (5sp)
AG335 (10sp)
Geov375 (10sp)

Prerequisites

Bachelor in geophysics

Field-, lab- and analysis work

NA