## 2 Materials and Methods
### On-Field Implementation  
The gas measurement was performed in the boreholes dedicated to investigating soil-gas concentration in the unsaturated zone with two depths of 1 m and 6 m. Those boreholes were drilled in June 2019 using a geotechnical rig capable of performing rotary  
Figure 1: Infrared spectra of 3000 ppm of CO\({}_{2}\) (a) and 1800 ppm of CH\({}_{4}\) (b). For CO\({}_{2}\): frequency range for v3 area calculation (1). For CH\({}_{4}\): frequency range for v4 area calculation (2) and frequency range for v3 area calculation (3).  
drilling. A borehole consists of a perforated pipe and a permanent well that was fitted with drilling tubes. Figure 2 shows the subsoil measurement system composed of three main parts, including completion (A), gas circulation module (B), and calibrated FTIR Bruker Alpha (C), with an additional GC analysis for cross-validating purpose. This well is dedicated to recording the gas phase's geochemical properties in the subsoil, which operates continuously and simultaneously equipped with a logs data station. The configuration of the subsoil measurement system was subject to previous research [5,37].  
The soil gases monitoring equipment (A) consists of three significant subparts: chamber, packer, and circulation lines. The collection chamber is an essential component that allows continuous sampling of the gas in the soil using an electric pumping system and a manual sampling port. The length and the diameter of the collection chamber is 60 cm and 50 mm, respectively. The packer made of an annular inflatable device of 64 mm long prevents any interferences with the ambient atmospheric gases by fully isolating the collection chamber. The circulation module consists of two lines of stainless steel, which is 4 mm in diameter allowing the gas to be transferred to the sensor and reintroduced into the borehole. The other two lines are equipped with pressure and temperature sensor. The gas flow module permits the gas sample's transportation from the sensor's completion and controls all parameters associated with the fluid flow (pressure, temperature, and flow). The gas circulation module (Figure 2B) was installed equipped with (a) circulation pump with a flow rate of 20 mL/min allowing the extraction and reintroduction of the gas in the collection chamber; (b) a water trap ensuring the control of the potentially liquid water in  
Figure 2: Monitoring concept for simultaneous and continuous gas measurement, including schematic diagram (i) and realistic view (ii) of monitoring infrastructure. The schematic diagram describes in more detail the process flow, which includes completion/boreholes (**A**), the circulation module (**B**), and calibrated FTIR Bruker Alpha with an additional GC sensor (**C**) for cross-validating. The circulation module consists of (1) temperature sensor, (2) pressure sensor, (3) water trap, (4) fluxmeter, (5) packer pressure, (6) borehole temperature indicator, (7) atmospheric temperature sensor, (8) atmospheric pressure sensor, (9) CO\({}_{2}\) NDIR sensor, and (10) soil moisture sensor.  
the circuit by trapping the water and redirect its flow; (c) the flow meter and a pressure sensor measuring the flow rate and the pressure of the gas sample inside of the circulation module; (d) sample cells connected in parallel to the gas circuit allowing samples of gas for laboratory analysis without causing the ambient atmospheric gases to enter the system; (e) various types and sizes of valves; and (f) an inlet and an outlet to FTIR Bruker Alpha. Since the monitoring site had no access to grid power, three batteries with a total capacity of 300 Ah were used to supply the monitoring system's power.