Method of high rate fibre optical distributed acoustic sensing, the method comprising: injecting a first coherent light pattern into an optical fibre; injecting a second coherent light pattern into the optical fibre while first Rayleigh backscatter light of the first light pattern is propagating in the optical fibre, wherein the first coherent light pattern and the second coherent light pattern have a light pattern power above a nonlinear effect related power limit; detecting the first Rayleigh backscatter light; and detecting second Rayleigh backscatter light of the second light pattern, wherein the first light pattern and the second light pattern is composed of light having same wavelength or wavelength range, wherein the first Rayleigh backscatter light has substantially the same wavelength as first light of which the first coherent light pattern is composed, wherein the second Rayleigh backscatter light has substantially the same wavelength as second light of which the second coherent light pattern is com posed, wherein the light pattern power is adjusted to enable acoustic sensing over a useful range of the fibre length (ur, 49) equal to or shorter than an effective length (el) and shorter than a physical fibre length (pl), wherein the effective fibre length (el) is given by a distance from a light entry position into the fibre to a position in the optical fibre at which the light power/intensity is attenuated to be lower than a threshold, the threshold being 1/10 or less of the injected light power, the method further comprising: analysing the detected light, to determine, over the useful range (ur, 49) of optical fibre, at least one of: strain, stress, acoustic disturbance, wherein backscatter light from positions beyond the useful range marginally disturb the measurement results relating to optical disturbance within the useful range of the fibre. injecting a first coherent light pattern into an optical fibre; injecting a second coherent light pattern into the optical fibre while first Rayleigh backscatter light of the first light pattern is propagating in the optical fibre, wherein the first coherent light pattern and the second coherent light pattern have a light pattern power above a nonlinear effect related power limit; detecting the first Rayleigh backscatter light; and detecting second Rayleigh backscatter light of the second light pattern, wherein the first light pattern and the second light pattern is composed of light having same wavelength or wavelength range, wherein the first Rayleigh backscatter light has substantially the same wavelength as first light of which the first coherent light pattern is composed, wherein the second Rayleigh backscatter light has substantially the same wavelength as second light of which the second coherent light pattern is com posed, wherein the light pattern power is adjusted to enable acoustic sensing over a useful range of the fibre length (ur, 49) equal to or shorter than an effective length (el) and shorter than a physical fibre length (pl), wherein the effective fibre length (el) is given by a distance from a light entry position into the fibre to a position in the optical fibre at which the light power/intensity is attenuated to be lower than a threshold, the threshold being 1/10 or less of the injected light power, strain, stress, acoustic disturbance, wherein backscatter light from positions beyond the useful range marginally disturb the measurement results relating to optical disturbance within the useful range of the fibre.