SDI-12 looks to be a high level interface running on a 1200 baud, bidirectional serial link, (Automotive K-line / Key Word Protocol, sounds almost like an exact match)
For decoding this before i have just used a 8 bit logic analyser, with some resistors to define the correct high / low thresholds (K-line logic 0 is 25-35% of VCC)
You would then decode it as Async Serial, and all the data would be there, You would then just have to review which side is requesting to nut out which side is talking at a given time.