A thrust recovery valve has two flaps, with each flap controlled independently via its own rotary actuator. Such a borehole image may be formed, for example, by processing logging sensor data with a predetermined sensor response function to acquire probabilistically distributed sensor data. A stationary excitation core spaced apart from the uncluttered rotor by an axial air gap and a radial air gap substantially encloses the stationary excitation core. The trigger includes an upper trigger piece and a lower trigger piece that are connected to each other.