Abstract: A single-shot or multi-shot measurement probe incorporates a magnetic field sensor for sensing the earth's magnetic field, a processing unit for determining from the output of the sensor that the probe has moved from within a section of a drill pipe made predominantly of magnetizable material to within a collar made of substantially non-magnetizable material, and a switching unit for initiating the taking of one or more survey measurements by the probe when an output is received from the processing unit indicating that the probe has entered the collar. The probe further incorporates a gravity sensor for sensing the earth's gravitational field and a further processing unit for determining from the output of the gravity sensor that movement of the probe has stopped and for controlling the switching unit such that the taking of one or more survey measurements by the probe is not initiated until after receipt of an output from the further processing unit indicating that movement of the probe has stopped.