INTRODUCTION
CAT-SCAN IMAGING
The x-rays travel at more or less constant velocity so there is no refraction and the rays are straight. The amplitude, in contrast, drops as the ray encounters different tissues and bones. To make an image we divide the sample space into zillions of pixels or picture elements and assign an attenuation value to each. Then, based on the attenuation of each relevant pixel, we calculate the total attenuation along each ray. By solving a huge set of simultaneous equations, we adjust the attenuation in each pixel until the calculated attenuations matched the measured attenuations of all rays. Thus the final image is a map of x-ray attenuation in the whole sample space. To make three-dimensional views we move the patient a short distance normal to the rings of sources and receivers and repeat the experiment.
SEISMIC FAN SHOOTING
The seismographs were laid out in an arc with the shot at the center so the rays spread out in a fan-shaped pattern. In the shales and sandstones of the region the velocity generally increased downward and thus the ray paths were concave up. Travel-times at any given distance were similar over large areas.
In contrast, a wave that encountered a salt dome would speed up and arrive earlier than normal for a given distance. Thus they could conclude that a salt dome lay somewhere along a particular ray or set of rays. To remove the ambiguity a second fan of different orientation was used. The fan lay at the intersection of two or more "early" ray paths.
Seismos had great success for about a decade until they were driven from the scene by American companies using seismic reflection equipment with electronic amplifiers.
TOMOGRAPHY BETWEEN WELLS
Some uses include mapping fractures in bedrock, steam flooding in oil fields and in situ coal burning.
INTERIORS OF VOLCANOES
One technique uses large, temporary arrays of seismographs to record small local earthquakes resulting in part from magma movement. The earth beneath the volcano is divided into "voxels" (three dimensional equivalent of pixels). Each voxel is assigned an initial seismic velocity and each arrival an initial origin time and hypocenter. Using thousands of rays we can invert the whole data set for velocity, hypocenter location and origin time. The magma chamber should have low velocity (it’s hot), no S-waves (it’s liquid) and be surrounded by a swarm of little ‘quakes (magma motion cracking rock at the periphery of the chamber).
A second technique uses waves from far away. These teleseimic arrivals have almost horizontal, vertically ascending wavefronts under the volcanic region. Arrivals at stations above the magma chamber should be late (again high temperature reduces the wave speed) and there should be an S-wave shadow zone (liquid). The two methods can, of course, be combined.
TESTING GLOBAL WARMING
Preliminary study suggests that a source near Heard Island in the southern Indian Ocean would be heard (sorry about that!) throughout the world oceans. Additional source locations would help pin-point where warming has been occurring. At present the experiment is being delayed for fear that the sound source would damage marine mammals. (I just read in an old saga that in Viking times certain stretches of Icelandic beaches were prized for the large numbers of whales that would strand themselves there).
GLOBAL TOMOGRAPHY
Anderson and Dziewonski, 1984, Scientific American, 251, 60-68.
Bois, 1972, Geophysics, 37, 471-
Bregman, 1979, Geophysics, 54, 200-
Hawley et al., 1981, Journal of Geophysical Research, 86, 7073-
Tsokas et al., 1995, Geophysics, 60, 1735-1742.
Various, 1995, Geophysics, 60, (May-June)