Fast Imaging of Thin, Curve-Like Electromagnetic Inhomogeneities without a Priori Information

It is well-known that subspace migration is a stable and effective non-iterative imaging technique in inverse scattering problem. However, for a proper application, a priori information of the shape of target must be estimated. Without this consideration, one cannot retrieve good results via subspace migration. In this paper, we identify the mathematical structure of single- and multi-frequency subspace migration without any a priori of unknown targets and explore its certain properties. This is based on the fact that elements of so-called multi-static response (MSR) matrix can be represented as an asymptotic expansion formula. Furthermore, based on the examined structure, we improve subspace migration and consider the multi-frequency subspace migration. Various results of numerical simulation with noisy data support our investigation.

The Heat Conductivity of Rubber at Low Temperatures

The experimental evidence points to the fact that a transition of some kind takes place at about 210° K, which, however, in consequence of supercooling, may take effect at a considerably lower temperature. As is mentioned in the Introduction, there is other evidence of such a transition. The thin curve in Figure 2, which is taken from the paper of Bekkedahl and Matheson, gives the specific heat of amorphous rubber. A sudden rise of this quantity occurs just below 200° K, and we feel sure that this near coincidence is more than accidental. The temperature lag between the two may be accounted for by the fact that Bekkedahl and Matheson worked with pure rubber hydrocarbon, whereas the material used for our investigation was vulcanized rubber. Bekkedahl's experiments on the temperature-volume relationship of various kinds of rubber showed that this transition is shifted towards higher temperatures when the rubber has been vulcanized.