Detlef Lazik

Membrane Based Measurement Technology for in situ Monitoring of Gases in Soil

The representative measurement of gas concentration and fluxes in heterogeneous soils is one of the current challenges when analyzing the interactions of biogeochemical processes in soils and global change. Furthermore, recent research projects on CO2-sequestration have an urgent need of CO2-monitoring networks. Therefore, a measurement method based on selective permeation of gases through tubular membranes has been developed. Combining the specific permeation rates of gas components for a membrane and Daltons principle, the gas concentration (or partial pressure) can be determined by the measurement of physical quantities (pressure or volume) only. Due to the comparatively small permeation constants of membranes, the influence of the sensor on its surrounding area can be neglected. The design of the sensor membranes can be adapted to the spatial scale from the bench scale to the field scale. The sensitive area for the measurement can be optimized to obtain representative results. Furthermore, a continuous time-averaged measurement is possible where the time for averaging is simply controlled by the wall-thickness of the membrane used. The measuring method is demonstrated for continuous monitoring of O2 and CO2 inside of a sand filled Lysimeter. Using three sensor planes inside the sand pack, which were installed normal to the gas flow direction and a reference measurement system, we demonstrate the accuracy of the gas-detection for different flux-based boundary conditions.

Improved Membrane-Based Sensor Network for Reliable Gas Monitoring in the Subsurface

A conceptually improved sensor network to monitor the partial pressure of CO2 in different soil horizons was designed. Consisting of five membrane-based linear sensors (line-sensors) each with 10 m length, the set-up enables us to integrate over the locally fluctuating CO2 concentrations (typically lower 5%vol) up to the meter-scale gaining valuable concentration means with a repetition time of about 1 min. Preparatory tests in the laboratory resulted in a unexpected highly increased accuracy of better than 0.03%vol with respect to the previously published 0.08%vol. Thereby, the statistical uncertainties (standard deviations) of the line-sensors and the reference sensor (nondispersive infrared CO2-sensor) were close to each other. Whereas the uncertainty of the reference increases with the measurement value, the line-sensors show an inverse uncertainty trend resulting in a comparatively enhanced accuracy for concentrations >1%vol. Furthermore, a method for in situ maintenance was developed, enabling a proof of sensor quality and its effective calibration without demounting the line-sensors from the soil which would disturb the established structures and ongoing processes.

Near real-time reconstruction of 2D soil gas distribution from a regular network of linear gas sensors

A monitoring method is introduced that creates, in near real-time, two-dimensional (2D) maps of the soil gas distribution. The method combines linear gas sensing technology for in-situ monitoring of gases in soil with the mapping capabilities of Computed Tomography (CT) to reconstruct spatial and temporal resolved gas distribution maps. A weighted iterative algebraic reconstruction method based on Maximum Likelihood with Expectation Maximization (MLEM) in combination with a source-by-source reconstruction approach is introduced that works with a sparse setup of orthogonally-aligned linear gas sensors. The reconstruction method successfully reduces artifact production, especially when multiple gas sources are present, allowing the discrimination between true and non-existing socalled ghost source locations. A first experimental test indicates the high potential of the proposed method for, e. g., rapid gas leak localization.

Linear sensor for areal subsurface gas monitoring - Calibration routine and validation experiments

Membrane based linear gas sensors and fiber optical sensors feature similar geometries and complement each other in quantities to be measured. To the authors best knowledge, it is the first time that these sensors are combined to a multifunctional sensor for distributed measuring of gas concentrations, temperature, and strain. Objective is a comprehensive monitoring of underground gas storage areas. In the presented project a 400 m2 test site and a corresponding laboratory system were just built up to characterize, validate, and optimize the combined sensor. Application of the sensor lines in a grid structure should enable spatial resolution of the measurement data and early detection of relevant events, as gas leakage, temperature change, or mechanical impact. A Calibration routine was developed which can be applied subsequent to underground installation. First measurement results indicate the potential of the method, with regard to highly topical energy transport and storage issues.

Tomographic Reconstruction of Soil Gas Distribution From Multiple Gas Sources Based on Sparse Sampling

A monitoring method is introduced that creates 2-D maps of the soil gas distribution. The method combines linear gas sensing technology for in situ monitoring of gases in soil with the mapping capabilities of computed tomography to reconstruct spatial and temporal resolved gas distribution maps. A weighted iterative algebraic reconstruction method based on maximum likelihood with expectation maximization in combination with a source-by-source reconstruction approach is introduced that works with a sparse setup of orthogonally aligned linear gas sensors. The reconstruction method successfully reduces artifact production, especially when multiple gas sources are present, allowing the discrimination between true and non-existing the so-called ghost source locations. Experimental validation by controlled field experiments indicates the high potential of the proposed method for rapid gas leak localization and quantification with respect to pipeline or underground gas storage issues.

Membrane-based characterization of a gas component--a transient sensor theory.

Based on a multi-gas solution-diffusion problem for a dense symmetrical membrane this paper presents a transient theory of a planar, membrane-based sensor cell for measuring gas from both initial conditions: dynamic and thermodynamic equilibrium. Using this theory, the ranges for which previously developed, simpler approaches are valid will be discussed; these approaches are of vital interest for membrane-based gas sensor applications. Finally, a new theoretical approach is introduced to identify varying gas components by arranging sensor cell pairs resulting in a concentration independent gas-specific critical time. Literature data for the N2, O2, Ar, CH4, CO2, H2 and C4H10 diffusion coefficients and solubilities for a polydimethylsiloxane membrane were used to simulate gas specific sensor responses. The results demonstrate the influence of (i) the operational mode; (ii) sensor geometry and (iii) gas matrices (air, Ar) on that critical time. Based on the developed theory the case-specific suitable membrane materials can be determined and both operation and design options for these sensors can be optimized for individual applications. The results of mixing experiments for different gases (O2, CO2) in a gas matrix of air confirmed the theoretical predictions.

Leak detection with linear soil gas sensors under field conditions — First experiences running a new measurement technique

A 400 m2 soil test field with gas injection system was built up, which enables an experimental validation of linear gas sensors for specific applications and gases in an application-relevant scale. Several injection and soil watering experiments with carbon dioxide (CO2) at different days with varying boundary conditions were performed indicating the potential of the method for, e. g., rapid leakage detection with respect to Carbon Capture and Storage (CCS) issues.

Setup of a large-scale test field for distributed soil gas sensors and testing of a monitoring method based on tomography

Abstract A 400 m2 soil test field with gas injection system was built up for the purpose of large-scale validation, optimization, and characterization of a novel comprehensive monitoring method for underground gas storage areas. The method combines gas sensing technology with linear form factor for in-situ monitoring of gases in soil with the mapping capabilities of Computed Tomography (CT) to reconstruct time-series of gas distribution maps based on samples of orthogonally-aligned linear gas sensors. Several injection experiments with carbon dioxide (CO2) at different days with varying boundary conditions indicates the potential of the method for, e.g., rapid leakage detection with respect to Carbon Capture and Storage (CCS) issues.

Approach for Self-Calibrating CO2 Measurements with Linear Membrane-Based Gas Sensors

Linear membrane-based gas sensors that can be advantageously applied for the measurement of a single gas component in large heterogeneous systems, e.g., for representative determination of CO2 in the subsurface, can be designed depending on the properties of the observation object. A resulting disadvantage is that the permeation-based sensor response depends on operating conditions, the individual site-adapted sensor geometry, the membrane material, and the target gas component. Therefore, calibration is needed, especially of the slope, which could change over several orders of magnitude. A calibration-free approach based on an internal gas standard is developed to overcome the multi-criterial slope dependency. This results in a normalization of sensor response and enables the sensor to assess the significance of measurement. The approach was proofed on the example of CO2 analysis in dry air with tubular PDMS membranes for various CO2 concentrations of an internal standard. Negligible temperature dependency was found within an 18 K range. The transformation behavior of the measurement signal and the influence of concentration variations of the internal standard on the measurement signal were shown. Offsets that were adjusted based on the stated theory for the given measurement conditions and material data from the literature were in agreement with the experimentally determined offsets. A measurement comparison with an NDIR reference sensor shows an unexpectedly low bias (<1%) of the non-calibrated sensor response, and comparable statistical uncertainty.

Visualization of surfactant solution transport in saturated soil: an experimental study to represent wastewater loss from sewers

Abstract Surfactants are the main active agents in detergents products. Our investigation dealt with the effects of surfactants as a wastewater constituent on the infiltration process of wastewater through saturated soil. In order to more closely observe the flow’s interaction, in a laboratory experiment, a 2D Plexiglas model was filled with fine-grained soil and saturated with degassed water. The particle-free artificial laboratory wastewater was created by adding a commercially available detergent to degassed tap water producing a surfactant concentration with the strength equivalent of up to about 15 times of its critical micelle concentration. The visualization of flow was improved by adding a brilliant blue dye tracer enhancing the color contrast. Photographs were taken from the 2D model using conventional imaging equipment and were processed by image analysis to distinguish the dynamic flow interface between dyed and non-dyed areas. Primarily, the images of vertical flows were analyzed after reducing the contrast range. Next, utilizing an image analysis method, 2D images were reconstructed into 3D visualization models. Three-dimensional and cross-sectional images of the fluid–fluid–soil boundary layer revealed a rapid solute transport prevailing at the dynamic interfaces. This was confirmed with image analysis showing geometric irregularities on the soil surface.