A Remote Optical Active Gas Analyzer Combined with a Rangefinder

Abstract

The results of the research and development of an active remote methane gas analyzer combined with a rangefinder (to a reflector) are presented. The instrument, together with an aircraft, enables one to measure methane concentration with an accuracy of a few molecules of gas per million molecules of air.In view of the critical problem of the state of the atmosphere at the present time, the need arises for accurate monitoring of the atmosphere to prevent industrial emissions of different gases. This is particularly important in large cities close to factories, pipelines, and underground storehouses. An analysis of the gaseous composition of the atmosphere is also necessary when searching for gas and petroleum deposits in inaccessible regions. Laser active gas analyzers [1][2][3] are the most promising from the point of view of sensitivity, territorial coverage, and high selectivity. As a consequence of the availability of new miniature semiconductor lasers operating in the near infrared band, which have a narrow spectral line, it has become possible to construct highly accurate and miniature optical analyzers of impurity gases in the 1-20 μm spectral range. The absorption lines of methane, carbon dioxide, nitric oxide, ammonia, water vapor, etc., having an absorption line width of about 0.1 nm, lie in this band. The radiation line width of semiconductor lasers with distributed feedback (so-called DF-lasers) is 0.01 nm, which makes an accurate determination of the line shape of impurity gases possible.In active measurements, reflection occurs from natural objects -the ground, forests, and also from artificial media, structures, roads, etc. In a number of problems, the concentration distribution of impurity gases along the measurement path is estimated. In the case shown in Fig. 1, when measuring the specific concentration (the integral concentration divided by the distance) it can be seen that leakage of methane gas, for example, from a pipeline is to the left of the drop in height. Since methane is lighter than air, it does not fill the flat part on the right. If we were to measure the distance and, consequently, also the specific concentration, we would not know where the gas leakage has occurred, since the integral concentration of the gas in the region of the leakage both on the right and left of the drop is approximately the same.In the light of the above, we have proposed a gas analyzer in actual systems for the laser monitoring of gas leakages using the example of methane. The basis of the optimization is the fact that, first, in addition to the integral gas concentration we also measure the specific concentration N sp = N/L (where N is the integral concentration of the gas), using a gas analyzer, which also determines the distance L to the reflecting surface. This specific concentration enables one to judge whether there is an increased or reduced concentration of the gas being investigated. Second, to increase the measurement accuracy, synchronous detection is employed, and the amplitudes of the harmonics ...