Time-domain gradient-type methods for inversion of the subsurface

We discuss the inverse problem of reconstructing a scattering object in a two-media background. The measurement equipment is located in the first medium while the scattering object is located in the second medium. We present two time-domain inversion methods based on the time-domain volume integral equation. The methods give promising results, even in a situation of very limited illumination and observation angles. An example shows a situation in which the Born-type method is much faster than the contrast source inversion (CSI) method. In another situation it was found that the CSI method produced better results. Which method to choose is a question for further research.