A method and system for deriving fluid leak information are described. A plurality of sealing elements are introduced into the pipeline at a location upstream of the leak, and at least some of the sealing elements are drawn towards the leak to cause a sealing event which reduces fluid seepage. A pressure change caused by the sealing event is detected, and associated time data is used to derive information about the leak. In one embodiment, leak location information is derived. In an alternative embodiment, pressure transient information is used to derive leak size information.