D. Köhn

Geophysical assessments of renewable gas energy compressed in geologic pore storage reservoirs

Renewable energy resources can indisputably minimize the threat of global warming and climate change. However, they are intermittent and need buffer storage to bridge the time-gap between production (off peak) and demand peaks. Based on geologic and geochemical reasons, the North German Basin has a very large capacity for compressed air/gas energy storage CAES in porous saltwater aquifers and salt cavities. Replacing pore reservoir brine with CAES causes changes in physical properties (elastic moduli, density and electrical properties) and justify applications of integrative geophysical methods for monitoring this energy storage. Here we apply techniques of the elastic full waveform inversion FWI, electric resistivity tomography ERT and gravity to map and quantify a gradually saturated gas plume injected in a thin deep saline aquifer within the North German Basin.For this subsurface model scenario we generated different synthetic data sets without and with adding random noise in order to robust the applied techniques for the real field applications. Datasets are inverted by posing different constraints on the initial model. Results reveal principally the capability of the applied integrative geophysical approach to resolve the CAES targets (plume, host reservoir, and cap rock). Constrained inversion models of elastic FWI and ERT are even able to recover well the gradual gas desaturation with depth. The spatial parameters accurately recovered from each technique are applied in the adequate petrophysical equations to yield precise quantifications of gas saturations. Resulting models of gas saturations independently determined from elastic FWI and ERT techniques are in accordance with each other and with the input (true) saturation model. Moreover, the gravity technique show high sensitivity to the mass deficit resulting from the gas storage and can resolve saturations and temporal saturation changes down to ±3% after reducing any shallow fluctuation such as that of groundwater table.

A combination of waveform inversion and reverse-time modelling for microseismic event characterization in complex salt structures

AbstractThe increased emission of greenhouse gases into the atmosphere, causing climate changes, leads to a strong requirement of renewable energy resources. However, they are intermittent and need buffer storage to bridge the time gap between production and public demands. The injection of gas (e.g. compressed air or hydrogen) in sealed underground structures like salt caverns is one approach to solve this problem. Possible risks related to cavern storage are gas leakages from the injection tube into the surrounding sediments, material failure in salt rock surrounding the cavern during irregular operation and in the most extreme case a partial collapse of the cavern. For the early detection of these problems, a geophysical monitoring strategy is required. The objective of this paper was to map possible leakage paths outside of the salt structures and local failures within the cavern walls by the localization of crack-induced microseismic events. Classical methods require arrival time picking and phase identification. An alternative approach is elastic reverse-time modelling (RTMOD), where the recorded microseismic events are numerically backpropagated from the receiver positions into the elastic underground model. The resulting seismic wavefield focuses at the location of the event, which can be subsequently imaged by estimating the maximum of the seismic energy at each underground point. However, the success of this approach highly depends on the used elastic background model. In case of complex salt bodies, the strong velocity contrast between the salt and the surrounding sediments is a major problem. Therefore, we propose a combined monitoring approach, consisting of a seismic full waveform inversion of active source reflection seismic data to accurately image the background velocity model and subsequent RTMOD for the microseismic event localization. Accuracy and sensitivity with respect to the acquisition geometry and random noise will be demonstrated using a complex benchmark model. Furthermore, the localization accuracy is discussed for three different scenarios covering the detection of a partial cavern collapse, a gas leakage and the occurrence of cracks within the cavern wall due to extreme loading conditions during irregular operation.

Monitoring gas leakages simulated in a near surface aquifer of the Ellerbek paleo-channel

Renewable energy resources are intermittent and need buffer storage to bridge the time-gap between production and demand peaks. The North German Basin has a very large capacity for compressed air/gas energy storage (CAES) in porous saltwater reservoirs and salt cavities. Even though these geological storage systems are constructed with high caution, accidental gas leakages occurred in the past. Stored gases migrated from deep reservoirs along permeable zones upwards into shallow potable aquifers. These CAES leakages cause changes in the electro-elastic properties, and density of the aquifers, and therefore justify investigations with the application of different geophysical techniques. A multiphase flow simulation has been performed to create a realistic virtual CAES leakage scenario into a shallow aquifer in Northern Germany. This scenario is used to demonstrate the detecting resolution capability of a combined geophysical monitoring approach, consisting of acoustic joint waveform inversion (FWI) of surface and borehole data, electrical resistivity tomography (ERT) and gravity. This combined approach of geophysical multi-techniques was able to successfully map the shape and determine the physical properties of the simulated gas phase body at a very early stage after leakage began. Techniques of FWI and ERT start to resolve CAES leakage anomalies only a few years and gravity even a few months after leakage began. Geophysical monitoring of vast areas may start by conducting time-effective aero-surveys (e.g. electromagnetic induction or gravity gradient methods) to isolate anomalous subareas of potential leakage risks. These subareas are then studied in detail using our combined high-resolution approach. In conclusion, our approach is sensitive to CAES leakages and can be used for monitoring.

The Role of Density in Acoustic Full Waveform Inversion of Marine Reflection Seismics

Full waveform inversion (FWI) is a data-fitting method that exploits the full information from the seismic data to provide high-resolution models of the subsurface. To reconstruct realistic models from field measurements the forward modeling should correctly account for wave propagation phenomena present in the recorded data. This mainly concerns the correct modeling of seismic amplitudes that are sensitive not only to the velocity variations, but also to the density, attenuation, seismic noise. The objective of this study is to investigate the role of density in the reconstruction of P-wave velocity models in the marine environment. We generated a realistic, synthetic data set with the conventional streamer geometry, and the frequency range from 3 to 20 Hz. We performed series of numerical experiments, testing various initial density models and different strategies for the density update. Our results suggest that it is important to include the realistic density information into the inversion scheme to bring improvement in the P-velocity reconstruction. Moreover, we investigated the potential benefits of multi-parameter inversion (P-velocity and density) of the noisy data, by considering random and spatially coherent noise. Density inversion has partly absorbed noise-related artefacts, which yielded better resolved P-wave models than the single-parameter inversion.

2D Elastic Full Waveform Tomography of Synthetic Marine Reflection Seismic Data

With the increasing performance of parallel supercomputers full waveform tomography (FWT) approaches can reduce the misfit between recorded and modelled data, to deduce a very detailed physical model of the underground. In recent years acoustic waveform tomography became a very popular tool to image the underground structures. However, acoustic waveform inversion has the disadvantage, that only P waves can be inverted. It can not invert for S-waves or surface waves. Here we will present the first inversion results of our elastic parallel time domain FWT code for two synthetic model examples and discuss problems which occurred during the code development like the choice of model parameters. Even though the problem is highly nonlinear and ill conditioned the elastic FWT is able to resolve very detailed images of all three elastic model parameters.

Comparison of acoustic full waveform tomography in the time - and frequency - domain

For better parameter estimation, both in active source and earthquakes seismology, we need to exploit the richness of full seismic waveforms. Full waveform tomography (FWT) is a powerful method to reach this goal. Although first implementations in the 1980s were conducted in the time-domain by Tarantola, the frequency-domain version of FWT developed in the 1990s by G. Pratt and coworkers has now emerged as an efficient imaging tool. The main advantage of the frequency-domain approach is the possibility of starting the inversion at low frequencies (large scale structures) and then moving to higher frequency compounds (smaller scale structures), thereby realizing a multiscale approach. The main advantage of the time-domain method is the efficient parallelization by domain decomposition leading to a significant speedup on parallel computers. In this study, we demonstrate the performance of our parallel acoustic time-domain code. We present the results for a very complex example - a random medium model. Last but not least, we compare our time-domain inversion results with the frequency-domain results calculated using the FULLWV code by G. Pratt et al.

Application of 2D (visco)-elastic Rayleigh Waveform Inversion to Ultrasonic Data from the Porta Nigra in Trier (Germany)

Beside geophysical applications from the near-surface to global scale, seismic full waveform inversion (FWI) can be applied to ultrasonic data on the centimeter and decimeter scale for non-destructive testing (NDT) of pavements, facades, plaster, sculptures and load-bearing structures like pillars. Classical NDT approaches are based on the inversion of body-wave travel-times to deduce P-wave velocity models. In contrast, surface waves (Rayleigh or Love waves) are well suited to quantify superficial alterations of material properties, e.g. due to weathering. In this paper we demonstrate the potential of 2D Rayleigh waveform inversion on the ultrasonic scale using a very low coverage acquisition geometry consisting of 1 shot and a few dozen receiver positions. For a 2D elastic FWI with a passive visco-elastic modelling approach the resolution is illustrated using a ultrasonic field data example from the weathered facade of the Porta Nigra, a large Roman city gate from the 2nd century AD, in Trier (Germany).

Feasibility Study of Applying Geoelectric and Gravity Techniques for CAES Reservoir Monitoring

Mitigations of anthropogenic GHG demand developments of renewable energy resources. These energy resources are intermittent and need buffer storage to bridge the time-gap between production and demand peaks. North German Basin has a very large capacity for CAES in porous saltwater aquifers and salt cavities (natural and artificial). Replacement of brine by compressed gas in saline formations cause strong changes in electrical resistivity and density, and therefore justify the application of geoelectrics, electromagnetics and gravity etc. In this study we study the applicability of these geophysical techniques in mapping CAES reservoirs (pore and cavern) in the underground of NW Germany. Our constrained techniques of electric resistivity tomography in boreholes are able to highly resolve the unusual problem of super resistive air caverns within the extreme resistive salt rocks. For gravity techniques, we could determine the lowest detectable mass deficit and offset of double caverns (2-fold their depth) in the underground resulting from pore and salt cavern CAES at some study sites.

2D Full Waveform Inversion Applied to a Strongly-Dispersive Love Wave Field Dataset

The full waveform inversion (FWI) of strongly dispersive Love wave data is a challenging task. Amplitude, phase and dispersion information not only depends on the density and shear modulus distribution in the subsurface, but also significantly on intrinsic damping. This is especially a problem in near surface data applications with complex underground structures and low Qs values. Therefore, the FWI of a dispersive Love wavefield demands an accurate initial visco-elastic model and careful data pre-processing. Another key ingredient of a successful time-domain FWI is the sequential inversion of frequency filtered data in order to mitigate the non-linearity of the inverse problem. Common FWI strategies are based solely on either low- or bandpass filtered data. In this study we introduce a workflow consisting of a combined low- and bandpass filter strategy to achieve an appropriate data fit of the low-frequency Love wave and high-frequency refracted SH-wavefield. The applicability of this FWI strategy and the importance of a visco-elastic medium description is demonstrated for SH field data from the transect over a medieval 2D canal structure in southern Germany. The resolved canal shape and small scale structures in the inversion results are verified by an archaeological excavation.

Investigating Surficial Alterations of Natural Stone by Ultrasonic Surface Measurements

The surface of buildings and monuments is essential for their visual appearance and their protection. Usually the surface is carefully designed and strongly related to the purpose of the object. Moreover, it is carrying information on the artistic and technical skills of the builders. It is however also prone to weathering and deterioration. A prerequisite for a careful restoration of the surface of historic buildings and monuments is a thorough analysis of surficial alterations. Quantitative measurements support visual inspections, but nondestructive characterization of material properties in the uppermost centimeters of an object represents a challenge. Here, the potential of ultrasonic surface measurements for the quantification of surficial alterations of natural stone is demonstrated by forward modeling, laboratory tests, and case studies. The fundamental Rayleigh mode is shown to dominate the waveform in the case of surface measurements. It is sensitive to changes in the shear wave velocity with depth and in anelastic damping of shear wave propagation. We report on ultrasonic surface measurements at facades made of marble (Neptungrotte, Park Sanssouci, Potsdam), sandstone (Porta Nigra, Trier), and tuff (Campidoglio, Rome). Average Rayleigh wave velocities are roughly proportional to measured P-wave velocities and are related to the overall state of the object in the uppermost centimeters. Rayleigh wave group velocities indicate changes in the material properties with depth, and waveform inversions allow for estimating depth profiles of velocity and damping of shear waves. The strong variability of weathering of natural stone is illustrated.