Abstract: A time of arrival of a pressure wave front travelling through fluid in a pipeline due to an event which causes fluctuation of pressure in such fluid is determined. The characteristic related to the pressure of the fluid is measured at a given position along the pipeline. A rate of change of the fluid pressure is determined for each measured pressure value by calculating a straight line derived from a plurality of such pressure values. The slope of the current straight line is compared to a threshold derived from a plurality of slopes obtained for previous pressure values, and a signal is outputted when the slope exceeds the threshold. Such signal is indicative of the time of arrival of the pressure wave front. The time of arrival is utilized to obtain the magnitude of the event, the location on the pipeline at which the event occurred, and its time of occurrence.