Multifrequency microwave diffraction tomography: Application to ground imagery by transmission

Among the important number of geophysical methods dedicated to ground analysis, those based on the propagation of electromagnetic waves at radar frequencies have gained a lot of interest these last ten years. Their non-destructive capabilities in addition to the technological advances in antenna design and signal processing played an important role in their success, especially for the Ground Penetrating Radar, which is commonly used from Field Geophysics field to Civil Engineering, and also for tomographic applications [1]. The multifrequency microwave diffraction tomography, presented in this paper, is based on the measurement of the scattered electric field for several frequencies after it has diffracted in the ground. There are several applications for this technique, such as human body imaging [2]. Applied to ground analysis, its goal is to provide an image of the ground that is relevant of its electromagnetic properties. The technique is applicable, in theory, in reflection from the surface as well as in transmission around walls, or from boreholes [3]. In the latter case, we would obtain an image of the vertical section of ground between the two boreholes, enabling to reduce the number and therefore the cost of the investigations. This technique has already been studied trough several PhD theses in the LPC network (Laboratoires des Ponts et Chaussées) [4,5]. In the present work, the interest will be mainly focused on the transmission mode from boreholes, with an approach of numerical modeling using laboratory experimental results. The goal is to develop a cross-hole microwave imagery technique that could be applied to the localization of objects like cavities or utilities.