  In this paper, we establish the theoretical framework for understanding the
sensing capabilities of optical fibers. We show the distinct advantage of
polarization over phase in detecting subhertz environmental processes.
Subsequently, we propose a scheme capable of extracting the spectrum of
perturbations affecting a specific section at any position along an optical
fiber by detecting the state of polarization of the backreflected light. We
discuss two examples of earthquake detection and the detection of sea swells
and ocean tides through the analysis of the state of polarization of an optical
signal reconstructed by the receiver of a transoceanic cable, obtained from an
online database.\cite{zhongwenzhan_2020} Finally, we provide the analytical
expression for the cross-correlation of the polarization perturbations of two
wavelength division multiplexed channels, and show that the analysis of the
polarization correlations between adjacent channels can provide valuable
insights into the localization of earthquakes.
