the first embodiment the clamp is attached to typically six thin rigid tether rods 4 i e three above and three below the other ends of the rods 4 connected to attachments 5 and 6 which are also affixed to the tubing 7 as in the first embodiment the method of installation is somewhat different to the first embodiment though the clamp shown in fig2 is contracted and expanded by physical manipulation of the members 13 at the ends of the c shape in practice a forked member not shown like in ep a 1370891 is used to hold the members 13 together to contract the clamp until it is inserted into the well the clamp may then either by positioned while the clamp is retracted and then expanded to hold it in position against the well casing or expanded near the top of the well and pushed down to the desired position against the well casing expansion of the clamp is effected by removing the forked member from engagement with members 13 so that the clamp moves into the expanded state sliding the expanded clamp down the casing does not damage either the clamp or the well casing as the clamp is typically made of a very hard material for example inconel u00ae and also due to the lubricating effect of fluid in the well it should be noted that both forms of clamp provide substantial force to press the sonde to the well casing to ensure good acoustic coupling experimental work with a prototype has demonstrated that the sliding friction force within the casing is sufficiently low for the.