Hurstell Page 1 Feasibility Testing for Open-Path Cavity Ring-Down Spectroscopy Instrument Alexis Hurstell, Tanner Fretthold, Joel Schulz, Justin Weinmeister Colorado State University Dr. Azer Yalin Laurie McHale April 18 th , 2015 Abstract Over the past two decades the number of active well sites has doubled to meet natural gas demands [1]. Natural gas is a highly efficient form of energy comprised primarily of Methane, but as a Greenhouse gas, methane has 21 times the global warming potential of CO2 [2]. Previous studies investigating the process of obtaining natural gases have reported conflicting results on the amount of gas leaked into the atmosphere due to the process of drilling, transportation, and burning of this fuel. Traditional continuous wave cavity ring down spectroscopy (CRDS) instruments are capable of measuring gas species down to the parts per trillion level, and an available unit was used for this study. The aim of this study was to investigate the possibilities of mounting an open-path CRDS laser system on an unmanned aerial vehicle (UAV) to measure leaks of methane during extraction and transportation. Open path CRDS systems are underdeveloped and steps were taken to develop novel technologies to utilize a traditional CRDS instrument in an open-path configuration. A series of tasks were carried out to understand negative effects on the ring down signal once the cavity was removed from the instrument; mirror cleanliness in an exposed environment, particulate induced scattering, and the instrument’s ability to record data while moving were all key tasks required in characterizing the open path system. It was found that mirror cleanliness is not an issue for further development; however, particulate scattering and data recording during movement still pose development issues in dirty atmospheres and high speeds, respectively. Despite the observed influence of aerosols and pressure density gradients, our research demonstrated that open-path CRDS seems to be feasible for this use. 1 Introduction With expanded use of hydraulic fracturing techniques in the recovery of natural gas, the number of wells in the United States has grown rapidly. The Denver- Julesburg basin of Colorado is a local area that has seen this rapid growth. When comparing the energy production through burning fuels to the greenhouse gas emissions produced through combustion, natural gas is found to be a better alternative to other conventional energy sources like coal and oil. The primary component of natural gas is methane, which in its non-combusted form is a more potent greenhouse gas than carbon dioxide. However if the natural gas is combusted, the resulting net greenhouse gas emissions are comparatively less than the initial methane. Because of this, natural gas is a potent energy source which requires development of monitoring infrastructure to ensure that the methane leaks are contained and the greenhouse benefits of natural gas is preserved. Due to the risk of greenhouse gas pollution, the development of an effective way to detect and locate leaks is required. Conceptually it is possible to triangulate the leak using concentration measurements taken from a variety of locations on a mobile platform such as a UAV. Such a feat requires equipment sensitive enough to detect concentrations in the parts per billion scale, a fast data collection rate to gain sufficient resolution for a reasonable triangulation, and must maintain power and weight limits such that it can be mounted on a mobile platform. A measurement technique known as Cavity Ring-Down Spectroscopy (CRDS) is capable of making these measurements both with the required precision and resolution. Current CRDS sensors do not meet all requirements for implementation of a practical system though. In addition to a set of highly reflective mirrors (>99.99%) typical CRDS sensors use a partially evacuated sample cavity which requires both a vacuum pump and cavity structure which increase weight and energy requirements of the system, and particulate free gas samples, all things which lead to difficulty when converting the system to a mobile operation. Our sponsor gave our group the goal of working through some of the initial trial working sensor development, such as understanding the basics optics utilized in the CRDS sensors, and characterizing the nature of an open path device. All of which provided us grounds to assess the feasibility of such a sensor system. An explanation on the basic function of a CRDS sensor can be found in Appendix A.