This paper examines the meaning of principal methods of system identif
ication (SI). System identification is seen as a unique procedure to e
stimate in situ soil properties, especially soil subject to large stra
in. When applicable, parametric modeling of system processes is found
to be superior to traditional Fourier methods. Several different metho
ds have been presented to directly assess nonstationary data. These me
thods cover a wide range, from transforming the data into a stationary
signal to full-fledged nonlinear, nonstationary analysis. The method
used will depend on the nature of the data available and the nature of
the requisite information. Segmentation of the data into stationary p
ieces gives sound results and is widely used. However, if the process
of interest has time-varying parameters, a recursive technique should
be used.