Tomography is the study of the internal properties of a body by observing the behavior of rays passing through the body. Seismic tomography uses mathematical modeling of P and S wave travel times to map velocity perturbations in the interior of the Earth. The primary energy source used in global seismic tomography is seismic waves generated by earthquakes which pass through the Earth in all directions, and are recorded on seismograms around the world. Inversion of arrival time data is used to determine the speed of the waves at any given point in the Earth. Using seismic tomography to interpret the internal structure of the Earth is similar in technique to a CAT-scan. Computer assisted tomography (CAT) uses X-rays transmitted through the body in many different directions. A mathematical method is then applied to explain the loss in intensity of the X-rays due to the varying absorptive properties of different parts of the body. The comparison between CAT-scans and seismic tomography differs because X-rays travel in straight paths, whereas the ray paths of sound waves bend with changes in the velocity structure of the medium.
Seismic tomography has several applications in exploration and global geophysics. Crosshole transmission tomography is used to image subsurface features between boreholes with greater accuracy than conventional surface reflection methods. Seismic tomography can also be used to characterize fractured bedrock, map groundwater reservoirs, and locate ore bodies. Global seismic tomography is used to interpret the presence of ancient subducted slabs, locate the source of hotspots, and model convection patterns in the mantle.
Global seismic tomography is limited by the irregularity in time and space of the source, and by the incomplete coverage of recording stations. The primary source is earthquakes, which are impossible to predict and only occur at certain locations around the world. In addition, the global coverage of recording stations is limited due to economic and political reasons. Because of these limitations, seismologists must work with data that contains crucial gaps. Experimental data can not accurately replicate conditions deep in the Earth's interior, therefore making comparisons with real world data difficult. Another limitation in imaging deep structures is attenuation and absorption of energy due to the long distances waves travel through the Earth, which reduces the resolution which can be attained. Due to the problem of attenuation, the minimum sizes of features in the mantle which can be resolved are blocks 100-200 km on each side.
Mapping of velocity perturbations in the Earth's mantle results in an indication of mantle temperature variation. Waves tend to travel faster in colder regions than in hotter regions. This is due to density contrasts related to temperature. Colder materials are more dense than hotter materials, allowing them to transmit waves at a higher speed. Figures 1 & 2 show maps of the mantle generated from tomographic data. These map are color coded with red correlating to slower velocities and blue indicating faster velocities. Looking at these maps, one can discern a strong correlation between tectonic features and the velocity of waves. Areas of younger, hotter material, such as actively spreading ridges, correspond to red (slow) areas on the velocity map. Areas of old, colder material, such as the interiors of continents, correspond to blue (fast) areas on the velocity map. See Figures 1 & 2.
Velocity anisotropy means that waves propagate at different rates in different directions. This property can be used to infer mantle convection patterns. The primary component of the mantle is believed to be olivine. The anisotropic structure of olivine crystals allows waves to propagate faster along the long (a) axis of the crystal than along the short axes (perpendicular to a). Flow in the mantle due to convection is believed to align the olivine crystals with their long axis oriented in the direction of the flow. Therefore, mantle convection patterns can be mapped using velocity perturbations, because the waves will travel faster along paths where crystal lattices have been aligned due to flowing of mantle material. Verically and horizontally polarized shear-wave velocities determined by tomography indicate that vertical flow is dominant under ridges and subduction zones, and horizontal flow dominates under cratonic areas.
III. Application of seismic tomography for solving geologic questions:
As seen in figures 1 & 2, seismic tomography can be used to delineate plate boundaries. Tomographic velocity maps of the mantle show areas of high and low velocity, and additionally with temperature variation and anisotropy, these velocity variations have been used to infer the depth of mountain roots and ancient subducted slabs. Three dimensional models have been generated to show the velocity structure of the mantle underneath the continents. Near the center of the continents, and under mountain ranges, velocity contours can be seen to curve. This seems to support the theory of isostasy, or compensation of a thick unit of continental crust by a large mass of buoyant material beneath. Tomography has also been used to infer the location of ancient subducted slabs. See figure 3. Figure 4 shows P and S wave models which illustrate global high and low velocity areas. High velocity areas, where ancient subduction zones are believed to have existed, have been interpreted to represent remnants of old slabs of dense subducted oceanic crust. See figure 4. Another application of seismic tomography to plate tectonic theory lies in the imaging of low velocity zones below proposed hot spots, or areas of constant volcanic activity. Hot spots are believed to originate deep in the mantle. Images generated from tomographic studies have shown low velocity zones extending deep into the mantle under the Hawaiian and other hot spots. This could indicate higher temperatures in these areas and possibly melt generation at depth. This can also help to disprove some proposed hot spots which do not correlate to low velocity zones.
Seismic tomography has potential for many aspects of exploration geophysics. These include shallow, high resolution techniques used in environmental and economic exploration, such as crosshole transmission tomography used to image the flanks of salt domes, and better evaluate ore bodies between boreholes. The applications used to image the deep structure of the Earth are limited in their usefulness, and can only be realistically improved by increasing the density of seismogram coverage. The only other option to improve the available data is to detonate large explosions, such as nuclear bombs, at locations where gaps in the data exist.