Systematic errors in Impact-Echo thickness estimation due to near field effects

Near field effects are found to create systematic errors, within the range of about 5-15%, in thickness estimations from Impact-Echo (IE) testing. This paper studies near field effects from a point source in a combined Multichannel Analysis of Surface Waves (MASW) and Impact-Echo analysis. The near field creates deviations in the measured velocity of the P-wave and the Rayleigh-wave, which lead to an underestimated thickness. This systematic error is identified in both a numerical and a real field case. The results are also compared with the conventional Impact-Echo method.