A sensor transportation apparatus to convey an elongate sensor assembly through a wellbore, the sensor transportation apparatus comprising: a main body comprising at least one engagement structure adapted to connect the sensor transportation apparatus to an elongate sensor assembly, and two wheels each arranged to rotate about an axis of rotation substantially perpendicular to a longitudinal axis of the elongate sensor assembly when the transportation apparatus is connected to the elongate sensor assembly, and an orientation structure comprising the wheels and at least one laterally extending orientation projection , to define a form having a transverse outline which has a rotational centre , wherein lateral extremities of the orientation structure substantially lie on a substantially circular curve centred on the rotational centre , wherein, in use, relative rotation between the sensor transportation apparatus and the elongate sensor assembly is prevented and the rotational centre is offset from a centre of mass of the elongate sensor assembly, characterised in that : the orientation projection is configured so that in use the elongate sensor assembly has an unstable position when the orientation projection is in contact with the low side of the wellbore and the elongate sensor assembly is configured to naturally rotate from this unstable position so that the sensor transportation apparatus is oriented in a most stable position with the elongate sensor assembly centre of mass below the rotational centre of the sensor transportation apparatus and with the elongate sensor assembly carried on the wheels of the transportation apparatus in contact with the low side of the well bore. a main body comprising at least one engagement structure adapted to connect the sensor transportation apparatus to an elongate sensor assembly, and two wheels each arranged to rotate about an axis of rotation substantially perpendicular to a longitudinal axis of the elongate sensor assembly when the transportation apparatus is connected to the elongate sensor assembly, and an orientation structure comprising the wheels and at least one laterally extending orientation projection , to define a form having a transverse outline which has a rotational centre , wherein lateral extremities of the orientation structure substantially lie on a substantially circular curve centred on the rotational centre , wherein, in use, relative rotation between the sensor transportation apparatus and the elongate sensor assembly is prevented and the rotational centre is offset from a centre of mass of the elongate sensor assembly, characterised in that : the orientation projection is configured so that in use the elongate sensor assembly has an unstable position when the orientation projection is in contact with the low side of the wellbore and the elongate sensor assembly is configured to naturally rotate from this unstable position so that the sensor transportation apparatus is oriented in a most stable position with the elongate sensor assembly centre of mass below the rotational centre of the sensor transportation apparatus and with the elongate sensor assembly carried on the wheels of the transportation apparatus in contact with the low side of the well bore. <EOS>