Hello from the California desert!
Check with your natural gas supplier supplier to find out what is acceptable NOW, since materials, standards and codes change. They are usually helpful and free since they want to sell gas.
High desert or low desert? There may be different soil conditions/problems for buried lines and this may be a local situation.
If you are 18" below grade (which is smart) and will probably do more work in the area, do it right the first time, so you can forget about it and go about living in the desert - it is great at twilight and at night in the undeveloped areas.