Claudia Schütze

Fluid flow in the resurgent dome of Long Valley Caldera: implications from thermal data and deep electrical sounding

Abstract Temperatures of 100°C are measured at 3 km depth in a well located on the resurgent dome in the center of Long Valley Caldera, California, despite an assumed >800°C magma chamber at 6–8 km depth. Local downflow of cold meteoric water as a process for cooling the resurgent dome is ruled out by a Peclet-number analysis of temperature logs. These analyses reveal zones with fluid circulation at the upper and lower boundaries of the Bishop Tuff, and an upflow zone in the metasedimentary rocks. Vertical Darcy velocities range from 10 to 70 cm a −1 . A 21-km-long geoelectrical profile across the caldera provides resistivity values to the order of 10 0 to >10 3 Ωm down to a depth of 6 km, as well as variations of self-potential. Interpretation of the electrical data with respect to hydrothermal fluid movement confirms that there is no downflow beneath the resurgent dome. To explain the unexpectedly low temperatures in the resurgent dome, we challenge the common view that the caldera as a whole is a regime of high temperatures and the resurgent dome is a local cold anomaly. Instead, we suggest that the caldera was cooled to normal thermal conditions by vigorous hydrothermal activity in the past, and that a present-day hot water flow system is responsible for local hot anomalies, such as Hot Creek and the area of the Casa Diablo geothermal power plant. The source of hot water has been associated with recent shallow intrusions into the West Moat. The focus of planning for future power plants should be to locate this present-day flow system instead of relying on heat from the old magma chamber.

Comparative study of geophysical and soil–gas investigations at the Hartoušov (Czech Republic) natural CO2 degassing site

Abstract Our study at this natural analog site contributes to the evaluation of methods within a hierarchical monitoring concept suited for the control of CO2 degassing. It supports the development of an effective monitoring concept for geological CO2 storage sites—carbon capture and storage as one of the pillars of the European climate change efforts. This study presents results of comprehensive investigations along a 500-m long profile within the Hartoušov (Czech Republic) natural CO2 degassing site and gives structural information about the subsurface and interaction processes in relation to parameters measured. Measurements of CO2 concentrations and investigation of the subsurface using electrical resistivity tomography and self-potential methods provide information about subsurface properties. For their successful application it is necessary to take seasonal variations (e.g., soil moisture, temperature, meteorological conditions) into consideration due to their influence on these parameters. Locations of high CO2 concentration in shallow depths are related to positive self-potential anomalies, low soil moistures and high resistivity distributions, as well as high δ13C values and increased radon concentrations. CO2 ascends from deep geological sources via preferential pathways and accumulates in coarser sediments. Repetition of measurements (which includes the effects of seasonal variations) revealed similar trends and allows us to identify a clear, prominent zone of anomalous values. Coarser unconsolidated sedimentary layers are beneficial for the accumulation of CO2 gas. The distribution of such shallow geological structures needs to be considered as a significant environmental risk potential whenever sudden degassing of large gas volumes occurs.

Comparative study to evaluate three ground-based optical remote sensing techniques under field conditions by a gas tracer experiment

Today, ground-based optical remote sensing (ORS) has become an intensively used method for quantifying pollutant or greenhouse gas emissions from point or area sources, and for the validation of airborne or satellite remote sensing data. In this study, we present the results of a release experiment using acetylene (C2H2) as a tracer gas, where three ORS techniques were simultaneously tested for two main purposes: (1) the detection of emission sources and (2) the quantification of release rates. Therefore, passive and active open-path Fourier transform infrared spectroscopy (OP-FTIR) and open-path tunable diode laser absorption spectroscopy (TDLAS) were applied and evaluated. The concentration results of the active ORS methods are compared to those estimated by a Lagrangian stochastic atmospheric dispersion model. Our results reveal that passive OP-FTIR is a valuable tool for the rapid detection and imaging of emission sources and the spatial tracer gas distribution; while with active OP-FTIR and TDLAS, C2H2 concentrations in the sub-ppm range could be quantified that correlated well with the concentration data obtained by our modeling approach.

Joint interpretation of geoelectrical and soil-gas measurements for monitoring CO2 releases at a natural analogue

The development and validation of hierarchic monitoring concepts is essential for detecting and assessing possible leakages from storage formations, especially for carbon capture and storage (CCS) applications. Joint interpretation of various techniques (such as carbon dioxide (CO2) concentration and flux measurements, self-potential (SP) and geoelectrical surveys) showed that the combination of geophysical methods with soil-gas analysis for mesoscale monitoring of the shallow subsurface above geologic CO2 storages can be a valuable tool for mapping and monitoring potential CO2 spread in the subsurface. Three measurement campaigns were undertaken – May 2011, July 2011 and April 2012 – at an analogue site in the Cheb Basin, Czech Republic, with the aim of studying CO2 leakages and their temporal and spatial behaviour. Results of geoelectrical investigations give an insight into the structural features of the subsurface. CO2 discharge into the atmosphere is mostly impeded by shallow, clay-rich, partly water-saturated zones, which can be seen in the electrical resistivity tomography (ERT) results. Several transport processes can be identified based on SP measurements. The SP results highlight the complex behaviour of temporal variations for the flow patterns. In particular, coupled migration of gas and water plays an important influencing role in this process. Site-specific, near surface geological features and meteorological conditions seem to exert great influence on the degassing pattern and measured CO2 values. Therefore, soil-gas measurements represent a snapshot which illustrates both a distinct typical pattern of the soil-gas distribution in the near subsurface and certain differences caused by soil and meteorological conditions. Observed CO2 soil-gas anomalies and modelled results suggest that the occurrence of gas discharge is much more localized around restricted areas, often controlled by local permeability contrasts. Hence, our results show that a proposed monitoring concept should integrate SP, time-lapse ERT, meteorological parameters and soil-gas measurements to provide a comprehensive insight into the subsurface structures and processes.

Challenges associated with the atmospheric monitoring of areal emission sources and the need for optical remote sensing techniques—an open-path Fourier transform infrared (OP-FTIR) spectroscopy experience report

In recent years, OP-FTIR spectrometry has been successfully used to monitor hazardous air pollutants, greenhouse gases and other emission products. This ground-based remote sensing method has proven itself to be a flexible long-path technique for the characterization of large atmospheric volumes, enabling simultaneous detection of various volatile atmospheric compounds relevant for environmental assessment via a single rapid measurement. Within our research, we applied both active and passive ground-based OP-FTIR spectroscopy as a screening tool for the detection of fugitive greenhouse gas emissions. The method was proven as offering a suitable ‘compliance toolbox’ for continuous monitoring of the complex systems at the ground surface–atmosphere boundary, allowing large-scale identification and quantification of atmospheric composition over large areas to be achieved, in terms of identifying zones of higher leakage vulnerability. In this paper, we compiled our research results and highlight the adaption options of the field technology, illustrated measuring options and case studies at urban, natural and industrial sites. Furthermore, this technology was validated regarding its applicability in environmental atmospheric monitoring. The results show that passive OP-FTIR measurements offer the chance to achieve robust and reliable surveys in various arbitrary measurement directions to gain a comprehensive overview. However, to improve quantitative analysis in the case of weak sources, the application of an active open-path spectrometer is recommended. It should be noted that our investigations demonstrate that site-specific parameters such as prevailing wind directions, meteorological conditions, topographic influences, infrastructure, other artificial emission sources, and biological background need to be measured both prior to and during atmospheric monitoring and should be taken into account for comprehensive interpretation.

MONACO—Monitoring Approach for Geological CO2 Storage Sites Using a Hierarchical Observation Concept

The reliable detection and assessment of potential CO2 leakages from storage formations require the application of assurance monitoring tools at different spatial scales. Such tools also play an important role in helping to establish a risk assessment strategy at carbon dioxide capture and storage (CCS) facilities. Within the framework of the MONACO project (“Monitoring approach for geological CO2 storage sites using a hierarchical observation concept”), an integrative hierarchical assurance monitoring concept was developed and validated with the aim of establishing a modular observation strategy including investigations in the shallow subsurface, at ground surface level, and in the atmosphere. Numerous methods and technologies from different disciplines (such as chemistry, hydrogeology, meteorology, and geophysics) were either combined or used complementarily to one another, with results subsequently being jointly interpreted. Patterns of atmospheric CO2 distributions in terms of leakage detection can be observed on large scales with the help of infrared spectroscopy or micrometeorological methods, which aim to identify zones with unexpected or anomalous atmospheric CO2 concentrations. On the meso-scale, exchange processes between ground surface level and subsurface structures need to be localized using geophysical methods and soil gas surveys. Subsequently, the resulting images and maps can be used for selecting profiles for detailed in situ soil gas and geophysical monitoring, which helps to constrain the extent of leakages and allows us to understand controlling features of the observable fluid flow patterns. The tools utilized were tested at several natural and industrial analogues with various CO2 sources. A comprehensive validation of the opportunities and limitations of all applied method combinations is given and it shows that large spatial areas need to be consistently covered in sufficient spatial and temporal resolutions.

Application of open-path Fourier transform infrared spectroscopy for atmospheric monitoring of a CO 2 back-production experiment at the Ketzin pilot site (Germany)

During a controlled “back-production experiment” in October 2014 at the Ketzin pilot site, formerly injected CO2 was retrieved from the storage formation and directly released to the atmosphere via a vent-off stack. Open-path Fourier transform infrared (OP FTIR) spectrometers, on-site meteorological parameter acquisition systems, and distributed CO2 point sensors monitored gas dispersion processes in the near-surface part of the atmospheric boundary layer. The test site provides a complex and challenging mosaic-like surface setting for atmospheric monitoring which can also be found at other storage sites. The main aims of the atmospheric monitoring of this experiment were (1) to quantify temporal and spatial variations in atmospheric CO2 concentrations around the emitting vent-off stack and (2) to test if and how atmospheric monitoring can cope with typical environmental and operational challenges. A low environmental risk was encountered during the whole CO2 back-production experiment. The study confirms that turbulent wind conditions favor atmospheric mixing processes and are responsible for rapid dilution of the released CO2 leading to decreased detectability at all sensors. In contrast, calm and extremely stable wind conditions (especially occurring during the night) caused an accumulation of gases in the near-ground atmospheric layer with the highest amplitudes in measured gas concentration. As an important benefit of OP FTIR spectroscopic measurements and their ability to detect multiple gas species simultaneously, emission sources could be identified to a much higher certainty. Moreover, even simulation models using simplified assumptions help to find suitable monitoring network designs and support data analysis for certain wind conditions in such a complex environment.

Line-averaging measurement methods to estimate the gap in the CO 2 balance closure – possibilities, challenges and uncertainties

An imbalance of surface energy fluxes using the eddy covariance (EC) method is observed in global measurement networks although all necessary corrections and conversions are applied to the raw data. Mainly during nighttime, advection can occur, resulting in a closing gap that consequently should also affect the CO2 balances. There is the crucial need for representative concentration and wind data to measure advective fluxes. Ground-based remote sensing techniques are an ideal tool as they provide the spatially representative CO2 concentration together with wind components within the same voxel structure. For this purpose, the presented SQuAd (Spatially resolved Quantification of the Advection influence on the balance closure of greenhouse gases) approach applies an integrated method combination of acoustic and optical remote sensing. The innovative combination of acoustic travel-time tomography (A-TOM) and open-path Fourier-transform infrared spectroscopy (OPFTIR) will enable an upscaling and enhancement of EC measurements. OP-FTIR instrumentation offers the significant advantage of real-time simultaneous measurements of lineaveraged concentrations for CO2 and other greenhouse gases (GHGs). A-TOM is a scalable method to remotely resolve 3-D wind and temperature fields. The paper will give an overview about the proposed SQuAd approach and first results of experimental tests at the FLUXNET site Grillenburg in Germany. Preliminary results of the comprehensive experiments reveal a mean nighttime horizontal advection of CO2 of about 10 μmol m−2 s−1 estimated by the spatially integrating and representative SQuAd method. Additionally, uncertainties in determining CO2 concentrations using passive OP-FTIR and wind speed applying A-TOM are systematically quantified. The maximum uncertainty for CO2 concentration was estimated due to environmental parameters, instrumental characteristics, and retrieval procedure with a total amount of approximately 30 % for a single measurement. Instantaneous wind components can be derived with a maximum uncertainty of 0.3 m s−1 depending on sampling, signal analysis, and environmental influences on sound propagation. Averaging over a period of 30 min, the standard error of the mean values can be decreased by a factor of at least 0.5 for OPFTIR and 0.1 for A-TOM depending on the required spatial resolution. The presented validation of the joint application of the two independent, nonintrusive methods is in the focus of attention concerning their ability to quantify advective fluxes.