California Code, Labor Code - LAB § 7966

In any tunnel classified as gassy by the division, there shall be tests for gas or vapors taken prior to each shift and at least hourly during actual operation.  If a mechanical excavator is used, gas tests shall be made prior to removal of muck or material and before any cutting or drilling in tunnels or underground mines where explosives are used.  A log shall be maintained for inspection by the division showing results of each test.  Whenever a tunnel excavation or underground mine operation approaches a geologic formation in which there is a likelihood of encountering gas or water, a probe hole at least 20 feet ahead of the tunnel face or area where material is being mined shall be maintained.