Methods to eonstruet images of an objeet from "projeetions" of x-rays,
ultrasound or eleetromagnetie waves have found wide applieations in
eleetron mieroseopy, diagnostie medicine and radio astronomy.
Projeetions are measurable quantities that are a funetiona- usually
involving a line integral - of physieal properties of an objeet.
Convolutional methods, or iterative algorithms to solve large systems of
linear equations are used to reeonstruet the objeet. In prineiple, there
is no reasan why similar image reeonstruetions ean not be made with
seismie waves. In praetiee, seismic tomography meets with a number of
diffieulties, and it is not until the last deeade that imaging of
transmitted seismie waves has found applicatian in the Earth sciences.
The most important differenee between global seismie tomography and mare
eonventional applieations in the laboratory is the faet that the
seismologist is eonfronted with the lack of anything resembling a
well-eontrolled experimental set-up. Apart from a few nuelear tests, it
is not in our power to locate or time seismie events. Apart from a few
seabattom seismographs, our sensors are located on land -and even there
the availability of data depends on eultural and politieal factors. Even
in exploratian seismics, praetieal faetors such as the east of an
experiment put strong limitations on the eompleteness of the data set.