  The phase lag between an applied forcing and a response to that forcing is a
fundamen tal parameter in geophysical signal processing. For solid deforming
materials, the phase lag between an oscillatory applied stress and the
resulting strain response encapsulates information about the dynamical behavior
of materials and attenuation. The phase lag is not directly measured and must
be extracted through multiple steps by carefully comparing two time-series
signals. The extracted value of the phase lag is highly sensitive to the
analysis method, and often there are no comparable values to increase
confidence in the calculated results. In this study, we propose a method for
extracting the phase lag between two signals when either one or both include an
underlying nonlinear trend, which is very common when measuring attenuation in
creeping materials. We demonstrate the robustness of the method by analyzing
artificial signals with known phases and quantifying their absolute and
relative errors. We apply the method to two experimental datasets and compare
our results with those of previous studies
