Techniques for operating a tool include moving a tool in a magnetized tubular member that generates a stationary magnetic field. The tool includes a housing that defines an interior volume, a first magnetic field sensor positioned on or within the housing at a first location, and a second magnetic field sensor positioned on or within the housing at a second location apart from the first location by a separation distance. The techniques further include detecting a first magnetic field amplitude distribution sensed by the first magnetic field sensor started at a first time instant; detecting a second magnetic field amplitude distribution sensed by the second magnetic field sensor started at a second time instant; determining a time difference between the first and second time instants based on the identified magnetic field amplitudes; and determining a speed of the tool based on the separation distance and the time difference.