Below critical angle detection of buried objects

This paper deals with the detection of buried objects using a parametric sonar working at grazing angles below the critical angle beneath which, according to the Snell theory, no penetration into the sediment is possible. Two experiments were performed in October 1995 and March 95 in France at the GESMA test site in order to assess the feasibility of detecting buried objects in sub-critical angle conditions. The TOPAS sonar from Simrad was used. Proud (unburied) objects were also considered in order to compare the results between the high frequency and the low frequency. For the proud objects it is shown that, for this type of bottom type, the peak signal to bottom reverberation ratio (PSSR) is 9 dB higher for the low frequency than for the high frequency. For the buried objects their detection is shown to be possible even if the grazing angle (16 degrees) is lower than the critical angle (28 degrees). Comparing the theoretical target strength (TS) to the measured TS only a few dB loss is observed, corresponding to both the absorption and the attenuation into the sediment, which is very promising. Almost no detection was observed with the high frequency. While the sonar was moving on the rail it was possible to perform synthetic aperture sonar processing with the parametric array resulting in significant improvements. In particular, all the objects were precisely located within the minefield and gains of 10 dB and 8 dB were obtained for the proud objects and the buried objects respectively.