According to the article linked by Hugh (great article btw):
"the average burial depth in the US is 1.16 meters (according to CAIC data). Below two meters, the chances of survival are extremely low: only 11 of the 126 people (9%) have survived burials deeper than 2 meters."
That being said, I have three probes of various length and weight and will select based on the terrain and conditions I'm traveling in.