Abstract
An all-passive, multipoint, and multiparameter optical monitoring system was developed and deployed in an industrial environment for the simultaneous measurement of methane concentration and other physical parameters. Methane is detected via rapid wavelength modulation spectroscopy (WMS) at 1648.2 nm and 4 MHz frequency. An attenuation invariant quantity defined by the peaks at 0, 4, and 8 MHz of the fast Fourier transform (FFT) of temporal signal is employed, characterized, and validated. Other parameters can concomitantly be measured by fiber Bragg grating (FBG) sensors operating in the 1520-1590 nm range. In the deployed system, the tested parameter was the temperature, which is an important quantity for gas monitoring. The system features a modular architecture that enables scalability up to 16 384 sensing points with an estimated less than 20-min acquisition cycle. In its current deployment, it monitors methane and temperature at eight locations using a single optical network. The system is intended to be used onshore and offshore platforms where the usual monitoring protocol consists of manual measurements usually performed three to four times a year and involves personal displacement and risky situations. Field tests at an onshore natural gas treatment unit (NGTU) demonstrated reliable performance and effective event detection, including undocumented nocturnal emissions, maneuvers at main shut-off valve, and partial plant shutdowns and restarts.

Abstract
We present a novel architecture for large-scale optical methane (CH4) sensing by integrating an Optical Time-Domain Reflectometer (OTDR) as a low-cost, high-speed, and high-sensitivity detector. By employing an optical converter to modulate a dedicated laser source with the OTDR's pulse, we achieve high spatial resolution and scalability. Experimental results demonstrate the interrogation of up to 64 channels per block, with a projected capacity of 200 sensors over 5 km using 200 ns pulses. A cost-benefit analysis confirms that this modular approach significantly reduces expenses compared to traditional MEMS-only switching solutions or pulse modulated alternatives by 12.5 times, while achieving a useful minimum Limit of detection of 0.53%.

Abstract
Methane (CH4) monitoring is vital for climate goals and industrial safety, yet high-pressure and fluctuating temperature environments often compromise sensor accuracy. This paper presents a numerical investigation into a 3D-MA-WMS technique, which expands Multiple Amplitude Wavelength Modulation Spectroscopy by integrating two-line thermometry. Using the HITRAN database, 27,000 absorption spectra were simulated to create a 3D parameter space of P,C, and T. Two retrieval methods-a geometric crossing algorithm and a Artifical Neural Network (ANN)-were evaluated. Results: The ANN outperformed the geometric approach, achieving average errors of 0.15% for concentration, 4.5 mbar for pressure, and 0.78 K for temperature. Significance: These results demonstrate the theoretical feasibility of a self-correcting, autonomous optical sensor capable of decoupling P, C, and T without auxiliary electronic sensors.