Abstract: A detector that can detect a gas which has leaked, particularly from buried and or exposed pipelines. The detector can be placed on an aircraft and flown at heights, e.g. up to about 500 feet, or other heights, and at relatively high speeds along the length of the pipeline. A tunable light source is programmed to switch between a first frequency, which is known to be absorbed by the gas in question, and a second frequency, which is known not to be as readily absorbed by the gas in question. A laser rangefinder is also provided to measure the distance between the pipeline and the detector. A computer is preferably provided to interpret readings from the sensors, based on distances measured by the laser rangefinder. A GPS receiver is also preferably provided.