I would hope so, but that might not be easy. The pipelines were pressurised to over 100 bar when the explosions occurred, so it's quite possible that an external explosion (say detcord or similar wrapped around the pipe) might look like an internal one, as the pipe bursts. 1500psi is a fair old pressure, and still a differential pressure of close to 1400psi at the depth of the pipe (it's no deeper than 100m along most of its length). My guess is that this would be enough to splay the ends of the fractured pipe outwards and blow debris some distance, even from an external blast.
I had been wondering if these blasts were trials of a technique, given that there seem to have been four separate blast locations, two on each pipeline. Might sound a bit wacky, but neither of these pipelines was likely to be used for some time, if ever. As such, they could be viewed as candidates to test ways to blow up pipelines without the perpetrator being proven.
The method in this madness is that this then turns pipelines into covert weapon delivery systems. An "accidental" pipeline explosion within Ukraine, or even within Slovakia, Hungary or Poland, that couldn't be proved to have been caused by a particular state, could be another useful weapon to have.