3D PHYSICAL MODELING FOR HIGH-RESOLUTION SEISMIC PROSPECTION OF DIFFRACTING TARGETS

A 3D physical model was built to study the possibilities of detection of shallow underground diffracting objects by means of high resolution seismic methods. The model structure is inspired from a real field si tuation. It is composed of two superposed layers, several centimeters thick, the lower one presenting a cylindrical hole of 3 cm diameter to simulate a diffracting tunnel. This situation typically corresponds t o the problem of the detection of meter size cavities situated at dept hs of 10 to 12 m. The success of the model mostly results from the spe cificity of the source developed (Mini-sparker). This source generates a very short, omnidirectional and high power acoustic pulse, which is difficult to obtain with conventional piezoelectric sources, but must be as close as possible to real shallow seismic sources. The model is used in this paper to discuss the efficiency of typical large depth s eismic measurement and processing techniques for the detection of shal low diffracting objects. Results are first presented for a ''Rubber-Pl exiglass'' structure which corresponds to rather good conditions of de tection. It is shown, in this condition, that an appropriate choice of the geometry of the measurement system (Common-Offset with minimal of fset) allows detection of the cavity without post-processing. A more c omplex structure situation is then considered by means of a ''Plexigla ss-Plexiglass'' structure. In this second model, the tunnel reflection amplitude is always hidden by the coherent noise, whatever the geomet ry of the measurement system. It is shown, in the situation considered , that post-processing like bidimensional filtering can be applied aft er Common-Offset measurements (with minimal offset) with enough effici ency to enhance signal to noise ratio up to the detection of the diffr acting object presence.