Laser Sensing for Monitoring Headspace Methane

The migration of methane gas in groundwater is a concern for many domestic well users, especially in areas of natural gas development.  Commercial gas sensors for monitoring methane in groundwater wells typically utilize electrochemical devices.  Laser sensing has proven to be a more reliable and accurate technology for detecting methane and other gases in air.  A tuned laser produces light at wavelengths equal to the spectral absorption bands of the gas in question.  The presence of the gas in the path of the laser beam attenuates the light as a function of concentration, which is then measured with a photocell.  Laser gas detectors are solid state devices that only require the introduction of the gas sample into the laser beam path.  Laboratory tests with a commercial unit had an accurate detection range from 10 ppm methane to the lower explosive limit of 5%.

Although these sensors perform well in a laboratory environment, deploying them in the field for environmental monitoring is challenging.  Along with power supply and data storage issues, the major problem is moisture condensation in the sensor, which can create data inconsistencies.  Field deployment of the laser sensor for continuous monitoring of methane in the headspace of a groundwater well required the use of a commercial gas drying method to remove humidity.  Low power requirements were critical.  As configured, the field-deployable device collects an air sample from the well headspace, pumps it through a drying system, and then into the laser sensor to measure methane concentration.  Ambient temperature and pressure are also measured before discharging the gas.  The entire unit is contained within a medium-size utility box, and field sampling requires only the insertion of a 5 mm (1/4 inch) collection tube into the monitoring well headspace.  Field tests are ongoing in support of a patent application.