Passive Seismic Tomography: 3-D imaging in tectonically active regions
Archambeau, Charles B.
published: Jan 1, 2003
Open Access (paper can be downloaded for free)
Passive Seismic Tomography, which uses the numerous very small earthquakes that are common in tectonically active areas as energy sources, can produce rather detailed seismic velocity structure images throughout the crust and upper part of the mantle. The imaging method uses local and regional earthquake body wave signals to determine the intrinsic values of both the material compressional wave velocity (vp) and shear wave velocity (vs) within small spatial cells throughout a 3-D zone sampled by the signals. These cell velocities taken together, define variations of the seismic velocities in the material volume and so can provide a map, or image, of boundaries between differing material strata, as well as a means of inferring lithology from material velocity values. Because both compressional and shear velocities can be determined, it is possible to compute the intrinsic Poisson ratio of the material within cells and thereby be able to characterize fracture density and spatial variations of porosity within the composite medium. The value of the material Poisson ratio also indicates the presence of fluids and gas within fractures or pores; with higher than normal values indicating fluids filling fracture/pore voids while abnormally low values indicate gas filled voids.