As per IS: 5249-19691 the natural frequency of soil-block system and hence the amplitude values of dynamic plot properties of soils can only be obtained if well defined peak in frequency is obtained by experimental data. But resonance peak may not be reached, in some cases such as in case of small block on stiff soils. In such cases reaults need be analysed by fitting frequency-amplitude plot by extrapolation and peak so obtained will be workable peak and not the true peak. A method is proposed here to fit the frequency- amplitude plot by extrapolation. An index to check as to which data would give a reliable peak has also been proposed developed. method. Field records from a few sites have been analysed to check the validity of the