Abstract: Apparatus is disclosed for securing a sonde to the inner wall of a well casing (3). The sonde (2) comprises a clamp (2) expandable from a contracted position to an expanded position in which the clamp can engage the inner wall. The securing apparatus comprises a member (5,6) for releasably engaging with the clamp (2) to hold the clamp in the contracted position; a spring (7) connected to the member such that when the spring is in a compressed state, the member is constrained into engagement with the clamp to hold the clamp in the contracted position; and a mechanism (8,9,10) for holding the spring (7) in its compressed state during a 10 first mode of operation and, in a second mode of operation, for releasing the spring (7) from its compressed state thus disengaging the member (5,6) from the clamp to permit the clamp to expand into its expanded position, the mechanism including a material (11) which in the first mode of operation, is in a solid state and, in the second mode of operation, is in a liquid state.