Imaging subsurface objects by seismic P-wave tomography: numerical and experimental validations

We tackle the problem of characterizing the subsurface, more specifically detecting shallow buried objects, using seismic techniques. This problem is commonly encountered in civil engineering when cavities or pipes have to be identified from the surface in urban areas. Our strategy consists of processing not only first arrivals, but also later ones, and using them both in tomography and migration processes, sequentially. These two steps, which form the basis of seismic imaging, can be carried out separately provided that the incident and diffracted wavefields are separated in the data space. Tomography is implemented here as an iterative technique for reconstructing the background velocity field from the first-arrival traveltimes. The later signals are then migrated by a Kirchhoff method implemented in the space domain. To study the reliability of this methodology, it is first applied to synthetic cases in the acoustic and elastic approximation. Both the background velocity field and the local impedance contrasts are reconstructed as defined in the predicted model. An experimental case, specifically designed for the purpose, is then considered in order to test the algorithms under real conditions. The resulting image coincides well with the predicted model when only P-waves are generated. In the elastic mode, surface waves make P-wave extraction difficult, so that the reconstruction remains incomplete. This is confirmed by the real data example. Finally, we demonstrate the appropriateness of the proposed method under such circumstances, provided that suitable preprocessing of data is carried out, in particular, the removal of surface waves. tackled. The former is dedicated to estimating large wavelength velocity contrasts, here called the background velocity. By extending the normal moveout (NMO) analyses, the tomography concept has the advantage of being applicable to laterally heterogeneous velocity fields. Moreover, seismic antenna geometry can account for complex configurations making the data independently indexed to a source–receiver pair, in contrast to classical seismic profiling where data refer to CDP gathers. Experimental seismology provides good examples of the possibilities, as shown by Nolet (1987). The migration process is often considered for the step following tomography because it relies upon the background velocity field to re-allocate the seismic events to their real positions. Numerous techniques can be used, depending on the complexity of the sounded medium, the kind of data considered, i.e. prestack or post-stack, and the available a priori information on the velocity field. For complex media, the prestack depth migration is generally used for ensuring the focusing of reflectors (Robein 1999). In particular, reverse-time migration has been successfully tested for detecting cavities from radar-wave measurements (Hui Zhou and Sato 2004). New methods have been developed more recently, either for reducing the computation times (Ecoubtet et al. 2002) or improving the inverse problem conditioning. For example, the INTRODUCTION Among the many difficulties encountered in the environmental and civil-engineering fields, characterization of the subsurface from a structural and a mechanical point of view constitutes the major challenge. Seismic sounding is an appropriate geophysical method to tackle this problem because of its ability to produce high-resolution images of the first 100 m of the subsurface. Numerous examples illustrate the diverse applications where seismic methods are effective, whether for detecting cavities (Grandjean et al. 2002), imaging active faults (Pratt et al. 1998) or delineating aquifer structure (Liberty 1998; Grandjean 2006). Such seismic images are generally difficult to obtain, especially when the subsurface is characterized by strong velocity variations with heterogeneities similar to the seismic signal wavelength. Nevertheless, we show that such media can be studied by this means, provided that both the first and the later arrivals are considered. Our work is particularly focused on applications related to civil engineering, such as the detection of shallow buried objects, like cavities or pipes. As Mora (1989) has already described, the complex problem of tomography and migration in subsurface imaging needs to be * [email protected]