The spatial resolution of a fiber optical Distributed Acoustic Sensing (DAS) assembly is enhanced by:arranging an optical DAS fiber (1) with a series of contiguous channels (C1-C14) that are sensitive to vibration in a U-shaped loop (U1) such that substantially parallel fiber sections (IA, IB) comprise pairs of channels (C1&C14, C2&C13,C3&C12 . . . etc) that are arranged at least partially side by side;transmitting a series of light pulses (5A, 5B) through the optical fiber (1) and receiving back reflections (6A, 6B) of said light pulses (5A, 5B) by a light transmission and receiving assembly (7); andprocessing the received back reflections (6A, 6B) such that back reflections stemming from at least one pair of channels (C1, C14; C2,C13) that are arranged at least partially side by side are correlated to each other.