Sensor transportation apparatus for a wireline logging tool string

A sensor transportation apparatus to convey an elongate sensor assembly through a wellbore, the sensor transportation apparatus comprising:

• at least one engagement structure to connect the sensor transportation apparatus to the sensor assembly, and
• a pair of wheels arranged to rotate about an axis of rotation substantially perpendicular to a longitudinal axis of the sensor assembly when the transportation apparatus is connected to the sensor assembly, and
• an orientation structure comprising at least one orientation projection, in a transverse outline of the sensor transportation apparatus a said orientation projection extending between the pair of wheels so that the shortest distance between a centre of mass of the elongate sensor assembly and the wellbore wall is when the pair of wheels are in contact with the wellbore wall and the sensor transportation apparatus has a single most stable orientation within the wellbore, in the single most stable orientation the sensor transportation apparatus oriented on the pair of wheels with the axis of rotation of the pair of wheels horizontal and with the elongate sensor assembly closest to the low side of the wellbore wall.