Seismic cavity detection: comparison between field data and synthetic data

Seismie reflection technique, based on the common midpoint method, commonly gives good results to image long smooth subsurface features. Irregular geological features, and particularly objects with small size compared to the main wavelength and/or the geophone spread, are not or badly imaged on the zero offset section. Nevertheless, both theoretical and field experiments show that, for high resolution seismics, such small objects locally affect the wavefronts. Our goal is to analyze these wavefront-perturbations and to extract informations conceming the disturbing objects. Metric size cavities located at lower depth have been chosen as example of small objects, allowing us to do field acquisitions on a weIl known target. This study is based on two approaches: one consists of modeling (ray tracing and wave equation modeling) in order to determine the possible phases, delay time and/or amplitude anomaly for different situations. The other one is based on the processing of real data acquired on several situations (man-made cavity, karstic cave). We present here our first observations obtained on both real and synthetic data. The first real dataset have been acquired above a weIl known man-made cavity (Belbèze caves, Ariège) digged for limestone exploitation purpose. Good results have also been obtained on a karstic area (Moulis, Ariège) where an unknown cave has been detected by integrated geophysical surveys, including high resolution seismics.