A method for measuring strain along an optical fiber that includes first and second sets of gratings inscribed in a fiber core of the optical fiber, wherein the first set of gratings overlaps with the second set of gratings, and wherein the first set of gratings comprises one or more chirped gratings and the second set of gratings comprises one or more gratings, the one or more chirped gratings of the first set of gratings differing in a direction of chirp or a magnitude of a chirp rate from the one or more gratings of the second set of gratings, the method comprising: obtaining first time-delay-dependent reflection spectra resulting from combined reflections off the first and second sets of gratings when the optical fiber is in a reference state, wherein each time delay of the first time-delay-dependent reflection spectra corresponds to an associated position of a plurality of positions along the fiber; interrogating the optical fiber in a strained state of the fiber to obtain second time-delay-dependent reflection spectra resulting from combined reflections off the first and second sets of gratings; computing cross-correlations across frequency of the first time-delay-dependent reflection spectra with the second time-delay-dependent reflection spectra for multiple time-delay shifts between the first and second time-delay-dependent reflection spectra; determining correlation maxima across time delay and frequency in the computed cross-correlations, wherein each correlation maximum corresponds to a pair of a first spectrum and a second spectrum, and to a frequency shift between the first spectrum and second spectrum of the pair, wherein the first spectrum is of the first time-delay dependent reflection spectra and the second spectrum is of the second time-delay-dependent reflection spectra, and wherein the first spectrum and the second spectrum are associated with a same position along the fiber; and for each correlation maximum of the correlation maxima, computing a strain at the position along the fiber associated with the pair from the frequency shift associated with that correlation maximum. obtaining first time-delay-dependent reflection spectra resulting from combined reflections off the first and second sets of gratings when the optical fiber is in a reference state, wherein each time delay of the first time-delay-dependent reflection spectra corresponds to an associated position of a plurality of positions along the fiber; interrogating the optical fiber in a strained state of the fiber to obtain second time-delay-dependent reflection spectra resulting from combined reflections off the first and second sets of gratings; computing cross-correlations across frequency of the first time-delay-dependent reflection spectra with the second time-delay-dependent reflection spectra for multiple time-delay shifts between the first and second time-delay-dependent reflection spectra; determining correlation maxima across time delay and frequency in the computed cross-correlations, wherein each correlation maximum corresponds to a pair of a first spectrum and a second spectrum, and to a frequency shift between the first spectrum and second spectrum of the pair, wherein the first spectrum is of the first time-delay dependent reflection spectra and the second spectrum is of the second time-delay-dependent reflection spectra, and wherein the first spectrum and the second spectrum are associated with a same position along the fiber; and for each correlation maximum of the correlation maxima, computing a strain at the position along the fiber associated with the pair from the frequency shift associated with that correlation maximum. <EOS>