surely, refinement of this technology would enable visualisation and
analysis of planetary interiors by tweeking the input and output
frequencies.
------------------------
Image Reconstruction in Seismic and Medical Tomography
Ali Ismet Kanl
Istanbul University, Engineering Faculty, Department of Geophysical
Engineering 34320, Avclar, Istanbul-Turkey kanli@[EMAIL PROTECTED]
of Environmental & Engineering Geophysics; June 2008; v. 13;
issue. p. 85-97
Tomography has been used for more than 40 yr in medical science and
its success is well known. One of the most im****tant reasons for the
success of medical tomography compared to seismic tomography is the
data-acquisition geometry. A data-acquisition geometry similar to the
one used in medical tomography was designed and tested with synthetic
geological models. Geophysical-type data-acquisition geometries were
also applied to the geological models, and the resulting images were
compared to the medical images. The reconstructed image obtained in
the medical case resembled the synthetic geological model and had
higher resolution than the geophysical image. Secondary attention is
drawn to the determination of the differences between the low- and
high-velocity zones in the final image reconstruction of the
tomographic inversion process. Therefore, the step-like high- and low-
velocity embedded layer models were tested with different spreading
systems. The geological models were also tested with one-dimensional
(1-D) initial velocity models to understand the effect of the initial
velocity model on the final image reconstruction. In the high-velocity
layer case, the tomographic process is quite successful in image
reconstruction, but cannot achieve the same success in the low-
velocity layer case.
Because of its strong ability to display the spatial distribution of X-
ray attenuation through cross-sections of the human body, computerized
tomography has been widely used in medical X-ray imaging (Hounsfield,
1973; Dines and Lytle, 1979). Geophysical tomography (geotomography)
differs from medical tomography in both characteristics of scanning
geometry and physical scale. When comparing medical and geophysical
applications, the latter requires lower frequencies to obtain adequate
signal levels at long distances, with more sampling at larger physical
scales. In general, the dimensions of the images reconstructed from
geophysical signals vary from meters to kilometers, whereas medical
images deal in millimeters. Additionally, medical scanners are mostly
constructed with a fixed data-gathering geometry, whereas geophysical
applications require different scanning capabilities for each
application method (Dines and Lytle, 1979).
In its early days, seismic tomography techniques were a logical
extension of concepts used in medical tomography, which have the
potential to yield high-resolution structural information. However,
after 30 yr of testing (e.g., Dines and Lytle, 1979; Mason, 1981;
Peterson et al., 1985; Bregman et al., 1989; Pratt and Chapman, 1992;
Pratt et al., 1993; Rector et al., 1996; Washbourne et al., 1998;
Louis et al., 2005), it was realized that the applications of seismic
tomography are very different and more difficult than in medical
tomography because of insufficient ray coverage, multiple arrival
paths, and anisotropic and dispersive wave propagation (Louis et al.,
2005).
Because of unsatisfactory results and the difficulty in applying
tomographic methods to field data acquisition systems in the
geosciences, researchers prefer to solve these problems by working
with models and synthetic data sets, including simple constant
velocity layered models, gradient models, and models with local
heterogeneity. However, its ease of use and nearly 100% success in
medical applications make medical tomography an im****tant tool in
medical science.
A medical-type data acquisition geometry, inspired by Hardage (1992),
has been designed and tested using both the straight and curved ray
processes on two synthetic geological models to understand the data
acquisition geometry that is used so successfully in medical
tomography. An additional objective was to focus on the low-velocity
layer (LVL) case, which is a very common problem in seismic
investigations. Thus, the image reconstruction results of seismic
tomography on both the high- and low-velocity embedded step-like
geological models were investigated and tested with different data
acquisition geometries using curved ray modeling.
|
