A Hardware/Software System For Implementing Geophysical Diffraction Tomography

Abstract

Geophysical diffraction tomography is a technique for high resolution, quantitative imaging of subsurface cross-sections. The method is based upon an imaging process known as filtered backpropagation which is a generalization of the inverse straight ray tracing process referred to as backprojection. In backpropagation, an image of spatial variations in refractive index is formed by backpropagating data received along an array of detectors, by means of the reduced wave equation, from the array into the support volume of the host medium. Important steps in this imaging process include: (1) the synthesis of a coherent incident wave, (2) the determination of complex phase perturbations relative to a homogeneous background, and (3) the numerical application of a holographic lens. While there has been considerable development in the theory of backpropagation, less effort has been devoted to problems associated with its implementation. Problems of practical importance that must be considered include: (1) the development of a data acquisition system capable of resolving the necessary wave characteristics, (2) quantification of the incident field in a homogeneous host matrix, and (3) in-field, real-time signal processing. This paper describes a microprocessor-based data acquisition system specifically designed and fabricated for geophysical diffraction tomography, discusses signal processing algorithms which are implemented on the system and presents results of several field studies.