Seismic geotomography is one of the computer tomographic techniques, which enable to visualize the inner structure and/or the distribution of physical properties of an objective body (rock mass) by means of the travelling manners of elastic wave. The authors have been developing the seismic attenuation tomography which can show more details of the distribution of discontinuities in the rock mass than the conventional seismic tomography. Several fundamental models are illustrated in terms of the numerical simulation. In the latter half of this paper, the application of seismic geotomography is described and the validity of seismic geotomography is discussed. Judging from the reality, the application of attenuation geotomography provides more useful information about fracture distribution.
Geotomography is a kind of remote sensing technique which enables us to visualize the inner structure and/or the distribution of the physical properties of an objective body (rock and/or soil) in a certain cross section in terms of the transmission of elastic and electromagnetic waves, electric currents, and water pressure pulses to be generated around the objective space or area. The tomographies for obtaining the two-dimensional distribution of propagation velocity and attenuation of elastic waves (called ‘seismic velocity’ and ‘seismic attenuation’ tomographies, respectively) and electrical resistivity are well applied in the field of rock engineering (Ishii, 1990; Sassa, 1990; Tanimoto and Shibata, 1992). The geotomographic technique has been employed in various fields to improve its ability to map sections of the earth's interior. Interpretations of underground structures are subject to being made with a very limited amount of known factors. Thus, rock engineers should create their own methods, from data acquisition to the final interpretation, depending on their objectives. From an engineer's point of view, seismic velocity geotomography and seismic attenuation geotomography have been developed to pursue further details on the respective joints than the available seismic geotomography which gives no consideration to an accurate joint distribution, As for seismic velocity geotomography, the capability of analysis, especially with respect to such boundary conditions as tile source and receiver points, relationship between raypaths, and inclination of the lower velocity zone, will be discussed. Then, several fundamental models are illustrated in terms of the numerical simulation. Judging from the reality obtained through in-situ observation, the application of attenuation geotomography provides more reliable information on fracturing than that of velocity tomography for the case described in this paper.
When a seismic survey is carried out in the field, traveltime between two points (the shot and the receiver points) is gained. Based on many sets of traveltimes along the various raypaths which densely propagate through an objective medium (rock mass), the seismic velocity geotomographic technique (SVT) shows the distribution of velocities in profile. In SVT, the concept of slowness is conveniently employed instead of velocity. The sources are set up into one borehole with locations of source generation, receivers are set up in alignment into another borehole.