In this work an attempt is made to compare the spectrum of ac conductivity time series, on various frequencies, of low porosity geomaterial samples that were contaminated by hydrocarbon compounds and others that were not influenced by contaminators. The used geomaterials were Mt. Penteli marbles. The power spectral density was calculated for both, contaminated and not, samples with the aid of wavelet transform in order to achieve time-frequency localization. The shape of the log-log plot of the power spectral density show clear differences between the contaminated samples and those that had not been affected by the hydrocarbon contamination compounds.