Experimental Preliminary Measurements of CO2 Flux for Exploring Hidden Geothermal Systems

Geothermal energy is a promising renewable energy source, and to enhance its use, identifying Hidden Geothermal Systems (HGS) without thermal manifestations on the surface is a challenging subject. Soil CO2 flux monitoring has become an effective method for detecting HGS, different from traditional methods that target thermal indicators. Expensive portable CO2 gas analyzers are commonly used for this purpose, but their high cost prevents wide applications. Thus, this study tries to design and test a cost-effective solution for measuring CO2 flux while keeping high accuracy and reliability of measured data. The method incorporates a self-made accumulation chamber connected to a relatively inexpensive CO2 portable meter, the GasLab Pro Carbon Dioxide Sampling Data Logger CM-1000. The device uses non-dispersive infrared (NDIR) to detect CO2 and is equipped with a data logger for continuous monitoring. The CO2 flux measurement is performed using the accumulation chamber method. The reliability of this tool for detecting CO2 flux is evaluated, and the experimental results are verified by comparing them with an intelligent gas flow meter, the Shimadzu Intelligent Flow Meter DFM-1000. The tool is tested in various conditions, with CO2 flux values ranging from 3.30 to 1013.02 g m-2 day-1, proving capable of measuring CO2 flux up to 1000 g m-2 day-1. Field tests were conducted at 60 sites to evaluate the tool’s performance. The results suggest that the lower measurement limit of the tool is approximately 0.1 g m-2 day-1. Overall, the cost-effective solution holds promise as a reliable tool for investigating HGS, with potential applications in other environments with similar or higher CO2 flux rates. In addition, conducting further comparison studies with a common sophisticated automatic flux tool such as LI-COR 850 can help improve the accuracy and reliability of the tool.


Introduction
Geothermal energy is a promising renewable energy source that has gained increasing attention worldwide.One of the challenges in developing geothermal energy is identifying Hidden Geothermal Systems (HGS) that lack thermal manifestations [1][2] [3].Traditional methods for identifying conventional geothermal systems rely on thermal indicators, such as hot springs or fumaroles.However, these indicators are not always present in HGS, making them challenging to locate.Recently, soil CO2 flux monitoring has emerged as a promising alternative method for identifying these HGS.This method is based on the principle that geothermal systems release more CO2 than surrounding areas, making it possible to locate them through CO2 flux measurements [4] [5] [6].Moreover, soil CO2 flux study has 2 been extended to not only for geothermal study, but also for volcanic study [7] or understanding the geological structure on the vicinity [8], and for agricultural purposes [9].
Portable CO2 gas analyzers, such as the LI-COR LI-8100 and EGM-5, are frequently utilized to measure CO2 flux in a variety of environments.However, the high cost of these instruments can be prohibitively expensive for many researchers and institutions (~30k USD for each instrument).To overcome this challenge, we developed a more affordable alternative for measuring CO2 flux.This study delves into the design and testing of our cost-effective solution (~2k USD), which aims to provide an affordable and reliable alternative to the more expensive portable CO2 gas analyzers typically used in the field, particularly in the identification of HGS.The method incorporates a self-made accumulation chamber (AC) connected to a relatively inexpensive CO2 portable meter, offering a dependable means of identifying HGS.The design process and subsequent field testing of this new tool have yielded promising results.In the following sections, we present our methodology in detail and share the outcomes of the first field applications, demonstrating the effectiveness of the alternative approach in accurately measuring CO2 flux and identifying HGS.

Material And Methods
This study used the GasLab Pro Carbon Dioxide Sampling Data Logger CM-1000 (Figure 1a) as the primary tool for measuring CO 2 concentrations.The sensing method uses non-dispersive infrared (NDIR) to detect CO2.The device is equipped with a data logger, which allows for continuous monitoring of CO2 concentration at regular intervals, providing a comprehensive understanding of the fluctuations in CO2 levels over time.In this research, the CO 2 flux is measured by the accumulation chamber method, which is already established and vastly used [3][4].The construction of the AC was simply built and cost-effective (Figure 1b).The CO2 flux measurement is performed by connecting an AC to a portable CO2 meter, as depicted in the schematic diagram in Figure 2. The CO2 flux is calculated, proportional to this observed rate of change which is derived from the ideal gas law in the closed system [9].The equation to calculate the CO2 flux is given in Equation 1.To validate the data, we compare the pure CO2 results obtained using our device with an intelligent gas flow meter.The Shimadzu Intelligent Flow Meter DFM-1000 was used in this comparison, and it provides gas flow in the unit of ml min -1 .Following the validation of the results, measurements of CO2 flux have been performed in several places, including open areas, locations near the geothermal manifestations, and the vicinity of hidden geothermal zones.The purpose of this field test was to determine whether the tool works effectively in different environments.No specific geological settings and features were chosen for this purpose.

Field test
Sixty measurements from several areas, such as open-space at Katsura Campus-Kyoto Japan, active geothermal area at Kagoshima prefecture Japan, and hidden geothermal at Mt. Endut, Banten, Indonesia, were conducted to evaluate the tool's performance in the field (Figure 3).The field tests yielded CO2 flux values ranging from 0.03 to 3.98 g m -2 day -1 , as depicted in Figure 4.The results reveal a relationship between the R 2 (coefficient of determination) values and CO 2 flux.For CO 2 flux values below 0.1, the R 2 values are generally below 0.8, suggesting a relatively poor correlation.In contrast, CO2 flux values above 0.1 exhibit R 2 values above 0.8, indicating a good correlation.Furthermore, flux values above 4 1.00 g m -2 day -1 correspond to R 2 values near 1.This suggests that the tool's lower measurement limit is approximately 0.1 g m -2 day -1 .Figure 5 shows the comparison graph between low, mid, and good R2 values from several measurements.The most noticeable characteristic is the fluctuations or "wiggle" in the low R2 measurement.This phenomenon occurs due to the resolution of the portable CO2 meter, which is ten ppm.At low CO2 content, the sensor struggles to measure the gas, resulting in observed fluctuations.As the flux value increases, the wiggle diminishes, indicating a better correlation between measurements.At R2 values above 0.8, a trendline can be observed forming, suggesting a more reliable relationship between the measurements.Despite the slight visibility of the wiggle, this improvement in correlation demonstrates the tool's capability to measure higher CO2 flux values with better accuracy.Further advancements in the tool's resolution could help mitigate these fluctuations and enhance its performance at lower CO2 flux values.

Validation of data accuracy
To validate the experimental results, a total of 14 measurements were carried out by comparing the results of the AC and the intelligent gas flow meter.The choice of using pure CO 2 as the test gas was deliberate, as it minimized the potential for erroneous readings that could arise from the gas flow meter detecting other gases, such as methane (CH4) or nitrogen (N2).The measurement procedure was executed sequentially, following a three-step process.First, the gas flow was measured using the intelligent gas flow meter to establish the initial flow rate.Next, the measurement was shifted to the AC setup, where the flux value was recorded.Finally, the gas flow measurement was conducted once again using the intelligent gas flow meter to obtain the final flow rate.The comprehensive results of these measurements are presented in Table 1, which provides a detailed comparison between the AC and intelligent gas flow meter readings.This comparative analysis aims to establish the accuracy and reliability of the AC method while also highlighting the performance of the intelligent gas flow meter in measuring the gas flow rates under test conditions.The measurement process employs a regulator to manage the gas flow.The gas flow is set up within a range of 1 to 340 ml min-1, and when compared to the AC method, it is equivalent to 19.5 to 4940 ppm s -1 (see Table 1).However, during the measurement using the gas flow meter, maintaining a consistent flow between the initial and final results proved challenging, leading to a lower flow rate in the final measurement.It is suspected that the cause of this issue lies within the gas regulator.To achieve better accuracy, a more precise gas regulator and controller should be utilized.To address this drawback, we will compare the initial, final, and average results of the measurements.It is important to note that three measurements in the initial flow of Pure 1, Pure 5, and CO2 4 could not be determined due to technical problems encountered during the process.The calculated flux values obtained during validation ranged from 3.30 to 1013.02 g m -2 day -1 .These results demonstrate that the measuring tools employed in the study are capable of measuring CO2 flux up to 1000 g m -2 day -1 .This finding is significant, as it indicates that the tools are well-suited for assessing CO2 flux values in geothermal areas, where typical flux rates generally range from 50 to 500 g m -2 day -1 [3][4].The tools' performance in this validation study suggests that they may also apply to other environments with similar or higher CO2 flux rates, broadening the scope of potential research applications and contributing to a better understanding of CO2 emissions in various geological settings.
Figure 6 illustrates the correlation between the results obtained from the AC measurements and the intelligent gas flow meter using pure CO2.The high R2 value, close to 1, demonstrates a strong correlation between the two methods, indicating that the validation results are reliable.When analyzing the low flow range (Pure 1 to 10 with flows between 0-40 ml min -1 ), the slopes of the initial, final, and average data differ slightly.However, the slopes of the initial, final, and average measurements become more consistent when incorporating higher flows from CO2 1, CO2 2, CO2 4, and CO2 5 (flows up to 340 ml min -1 ).Although the values have minor differences, they are relatively small and can be reasonably expected.

Conclusion
This research aimed to develop an affordable cost-effective device and method of portable CO2 gas analyzer for detecting hidden geothermal systems (HGS).The device was composed of a self-made accumulation chamber (AC) connected to a relatively inexpensive CO2 portable meter.The findings demonstrated that the self-made AC and the system performed well in measuring CO2 flux, with a strong correlation between our method and an intelligent gas flow meter.Furthermore, the tool's performance in the validation study suggested applicability to other environmental issues with similar or higher CO2 flux rates, contributing to a better understanding of CO2 emissions in various geological settings.The experimental results demonstrated the possible measurement range from approximately 0.1 g m - 2 day -1 to ~1000 g m -2 day -1 .This tool offers a more accessible option for researchers and institutions with limited budgets, thereby promoting the exploration and identification of HGSs.Future improvements to the tool's design, such as increased resolution, could enhance its performance and broaden its utility in identifying HGS and other research applications.In addition, conducting further comparison studies with a common sophisticated automatic flux tool such as LI-COR 850 can help improve the accuracy and reliability of the tool, contributing to the advancement of geothermal energy research and development.
Further recommendations for field measurement, as demonstrated by [7][8], systematical grid sampling is highly recommended for field measurements in geothermal studies where feasible, as it ensures systematic coverage and detailed data.However, in scenarios where grid sampling is impractical due to terrain or other constraints, random sampling serves as a viable alternative.This approach, while less structured, can still provide a representative overview of the geothermal characteristics of the area.The choice between these methods should be tailored to the study's goals, the nature of the geothermal system, and the practicalities of the field environment, ensuring an effective and insightful exploration of hidden geothermal systems.

Figure 1 .
Figure 1.Photos of (a) CO2 handheld meter, (b) Self-made AC, and (c) CO2 meter connected with accumulation chamber to measure CO2 flux.

Figure 2 .
Figure 2. Schematic diagram of an AC measurement system of soil CO2 flux.

Figure 4 .
Figure 4. Relationship between field test data of CO2 flux at 60 locations and their R 2 values.The square is from the open-space, the triangle is from the geothermal area, and the circle is from hidden geothermal area.Black color means the measurement was conducted on a dug hole and red was not dig.

Figure 5 .
Figure 5. Examples of measured data with low, medium, and high R 2 .

Figure 6 .
Figure 6.Correlations between the AC measurement and gas flow meter using the data of Pure 1 -10 (top) and all the data inTable 1 (bottom).