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 situation. 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 to the problem of the detection of meter size cavities situated at depths of 10 to 12 m. The success of the model mostly results from the specificity 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 seismic measurement and processing techniques for the detection of shallow diffracting objects. Results are first presented for a "Rubber-Plexiglass" structure which corresponds to rather good conditions of detection. It is shown, in this condition, that an appropriate choice of the geometry of the measurement system (Common-Offset with minimal offset) allows detection of the cavity without post-processing. A more complex structure situation is then considered by means of a "Plexiglass-Plexiglass" structure. In this second model, the tunnel reflection amplitude is always hidden by the coherent noise, whatever the geometry of the measurement system. It is shown, in the situation considered, that post-processing like bidimensional filtering can be applied after Common-Offset measurements (with minimal offset) with enough efficiency to enhance signal to noise ratio up to the detection of the diffracting object presence.