1. A method for measuring the flow velocity of a fluid in an elongated body, the fluid traversing the elongated body substantially along the longest axis of the elongated body, the fluid bearing a vortical disturbance at a velocity correlated to the flow velocity, the vortical disturbance causing fluctuations in the pressure of the fluid near to the disturbance, the method comprising the steps of: a) providing an array of at least two sensors disposed at predetermined locations along the elongated body, each sensor for sampling the pressure of the fluid at the position of the sensor at a predetermined sampling rate; b) accumulating the sampled data from each sensor at each of a number of instants of time spanning a predetermined sampling duration; and c) constructing from the accumulated sampled data at least a portion of a k-&ohgr; plot, indicative of a dispersion relation for the propagation of acoustic pressures emanating from the vortical disturbances, the constructing of the k-&ohgr; plot based on correlating the sampled data provided by at least one of the sensors with the sampled data acquired by at least one other of the sensors, and by autocorrelating the sampled data acquired by each sensor.