A fiber optic array for the detection of sub-surface carbon dioxide at carbon sequestration sites

Abstract

A fiber sensor array for sub-surface CO 2 concentrations measurements was developed for monitoring geologic carbon sequestration sites. The fiber sensor array uses a temperature-tunable distributed feedback (DFB) laser outputting a nominal wavelength of 2.004 microns. Light from this DFB laser is directed to one of the four probes via an in-line 1x4 fiber optic switch. Each of the probes is placed underground and utilizes filters that allow only soil gas to enter the probe. Light from the DFB laser interacts with CO 2 within the probe before being directed back through the switch. The DFB laser is tuned across two CO 2 absorption features where a transmission measurement is made, allowing the CO 2 concentration to be retrieved. This process is repeated for each probe, allowing CO 2 concentration measurements to be made as a function of time for each probe. The fiber sensor array was deployed for fifty-eight days at the Zero Emission Research Technology (ZERT) field site and for a twenty-eight day period at the Kevin Dome geologic carbon sequestration site. Background measurements indicate the instrument can monitor background levels as low as 1,000 parts per million (ppm). During a thirty-four day sub-surface CO 2 release, elevated CO 2 concentrations were readily detected by each of the four probes with values ranging to over 60,000 ppm.