the only safe way I know to get rid of gas odors is Ventilation .
If you know what type of odors, Fans & Ventilation.
but first you need to know if the Type of fan you use can set off a spark.
You can only smell leaking methane when commercial gas utility companies add a chemical smell to it
Blackdamp (---- ---) Has no odor so probably not that either. That just means there is No oxygen to Breathe.
Hydrogen sulfide maybe also called "Stink damp" has an odor of rotten eggs, and is heavier than air,
very poisonous, flammable and explosive. and maybe what it is, if it is natural to your area.
if not Hydrogen sulfide , more likely old Septic tank odor .
but would you want to find out after you blow up ? it wasn't old poop smell ?
then you would probably be mad at my answer. so be careful
.
According to our Homeland Terrorists
Facts about Cyanide | Homeland Security News
Cyanide gas sometimes is described as having a “bitter almond” smell, but it does not always give off an odor, and not everyone can detect this odor.
now on detectors , don't spend $6,000.00 on any detector !
if you do I'm not going to feel sorry for you after, I'm just going to Laugh
because I Warned you. I think that 50 meters deep translates to 180.4 feet deep.
If true, Not even Ground Penetrating Radar will help , because when you ask the question,
all you get is Double Speak
http://www.georentals.co.uk/gprsee.htm
add to that there are allot of scams out there.
If you have all that Money, Buy a Minelab GPX 5000 & settle for the
easy digging depth of a foot or 3 (depending on Size of Object) & have fun