Tomography in various forms, including seismic,
radar,
and electrical resistivity, is starting to be used for
environmental studies. A new method of tomography based upon
making electrical resistivity measurements between boreholes has
been described by Daily and Owen [1991]. Because the
``ray paths'' followed by the electrical currents are highly dependent
upon the resistivity structure between the boreholes, the formation of
the tomograms for this type of data is highly nonlinear. The method
has been applied to hydrologic problems by Daily et
al. [1992] and Ramirez et al. [1993].
Advances in seismic tomography have centered on improvement of processing algorithms. When the data must be processed and interpreted in the field, a frequent requirement in environmental studies, the computer algorithm must be simple enough that it can be quickly executed by modern personal computers. For this case, investigators use either the algebraic reconstruction (ART) or simultaneous iterative reconstruction (SIRT) techniques [ Jessop et al., 1992; Elbring, 1993]. If enough time and computer resources are available, then the best results are obtained with matrix methods [ Block et al., 1994], because additional geological and geophysical information, such as velocity constraints, are easily incorporated into the processing.