S. A. al Hagrey

Resistivity and percolation study of preferential flow in vadose zone at Bokhorst, Germany

Traditional nondestructive resistivity techniques have been applied in combination with tracer displacement and conventional soil moisture recording methods [i.e., buried tensiometer and time domain reflectometry (TDR)] for studying flow processes at an arable site in Bokhorst, Germany. Three water infiltration experiments were carried out using tap water spiked with a nonreactive tracer at different concentrations. The study aimed at exploring the capabilities of these combined techniques for tracing the preferential movement of water in the uppermost 1.5 m of the highly heterogeneous vadose zone. The results illustrate that the applied tensiometer and TDR techniques can detect the relatively fast flow process at their position points and thus only tentatively trace the preferential flow. The additional application of the resistivity method can trace the preferential flow paths continuously along the plane of measurements. The results of the individual applied methods complement and confirm each other.

Ground-penetrating radar tomography for soil-moisture heterogeneity

Many ground-penetrating radar (GPR) studies incorporate tomographic methods that use straight raypaths for direct model reconstruction, which is unrealistic for media with gradually changing petrophysics. Ray-bending algorithms can sometimes lead to unreliable resolution, especially at interfaces of abrupt dielectric changes. We present an improved GPR tomography technique based on a combination of seismic tomographic methods and a finite-difference solution of the eikonal equation. Our inversion algorithm uses velocity gradient zones and bending rays that represent realistic geology in the subsurface. We tested the technique on theoretical and experimental models with anomalous bodies of varying saturations and velocity and applied it to data from a GPR field experiment that analyzed the root zones of trees. Synthetic results showed that the resolution of our technique is better than that of published methods, especially for local anomalies with sharp velocity contacts. Our laboratory experiments consisted of four objects buried in sand with various water saturations. The GPR tomogram could map the objects and determine their degree of saturation. The velocities are compatible with those of the complex refraction index method; their relationship to the water content fits a previously published empirical equation. Our original field experiment around a poplar tree could map the heterogeneous subsurface and distinguish a central low velocity beneath the tree from the peripheral negative anomaly of a refill. This zone reflects the whole root zone and is caused by its bulk water content of both the organic root network and its surrounding soils.

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.

Combined Waveform Inversion and Reverse Time Modeling for Microseismic Event Characterization in Complex Salt Structures

Increased emission of greenhouse gases into the atmosphere lead 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 storage of compressed gas energy in sealed underground structures like salt caverns is one approach to bridge this time gap. The early detection of possible gas leakage paths in the surrounding of caverns can be mapped by the localization of crack-induced microseismic events. One approach is based on elastic reverse-time modeling, where the recorded seismograms of a microseismic event are numerically backpropagated and the seismic wavefield focuses at the location of the event. The success of this approach depends on the used elastic background model. In case of complex salt bodies, the strong velocity contrast between salt and 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 a subsequent reverse time modeling for microseismic event localization. The accuracy and sensitivity with respect to random noise is demonstrated using the complex SEG/EAGE BP 2004 benchmark model.

A Combined Elastic Waveform and Gravity Inversion for Improved Density Model Resolution Applied to the Marmousi-II Model

In recent years the elastic full waveform inversion (FWI) was successfully applied to synthetic and field data to compute high resolution velocity models. While seismic velocities are derived from recorded phase information, density models can be estimated from the amplitudes. However, due to the complexity of the inverse problem a long wavelength initial model is required for a good reconstruction of the density. The inclusion of gravity data into the FWI concept can solve this problem. In this study a two-step hierarchic joint inversion of seismic waveforms and gravity data is tested using the Marmousi-II model. In step 1 FWI is performed for all elastic parameters. Gaussian filtered velocity models of the true model and a constant halfspace density model (CDH) are used as initial models. While the velocities can be reconstructed well, the density shows large deviations from the true model. In step 2 joint inversion is applied to optimize only the density model, while the velocity inversion results of the first step and the CDH are used as initial models. The results of this combined approach show a significant improvement of the density model compared to the results of a pure FWI.

Combined Seismic Waveform Inversion and Geoelectrical Monitoring for CO2 Storage Using a Synthetic Field Site

We present the application and verification of a combined geophysical approach for monitoring and quantifying the storage of CO2 in deep saline formations using numerical simulations. Supercritical CO2 is injected into a deep thin saline aquifer below a synthetic site of the North German Basin. The displacement of formation brine by CO2 yields changes in bulk density, elastic moduli and electric resistivity. This justifies the application of the seismic full waveform inversion (FWI) and electric resistivity tomography (ERT) to monitor and quantify the thin, deep gas plume. These goals are real challenges for the applied geophysical monitoring techniques. Densities and saturations are obtained from a numerical simulation of the injection process and are introduced into geophysical forward models to simulate the geophysical data acquisition. These synthetic geophysical datasets are then inverted and evaluated with respect to changes in CO2 saturation and are compared to the fully known CO2 saturation of the numerical process model. Inversion results show that both seismic FWI as well as ERT techniques are capable to detect and map the thin CO2 phase body within the target storage formation (~2.2 km depth) from the beginning of the injection process, if accurate baseline models are available.

Comparative Study of CO2 Mass Quantification Using Process Modelling and Geophysical Techniques

This paper presents the application and verification of a combined seismic and geoelectrical monitoring approach for CO2 storage using a virtual test side in the North German Basin. It is found, that the sensitivity of both methods to CO2 phase saturation is complementary, with the seismic reflection coefficient being most sensitive at low CO2 saturations and the resistivity being most sensitive at high saturations promising a comprehensive monitoring of the sequestration process. An integrated workflow is developed for the method assessment. Thereby, results of a numerical flow simulation (phase saturation, density, pore pressure, porosity and permeability) are used to simulate synthetic seismic and resistivity data for a realistic geophysical survey. From the synthetic data, changes of P- and S-wave velocities are calculated using 2D elastic time-lapse full waveform inversion and resistivity changes are calculated using borehole resistivity tomography. With that, the original CO2 phase distribution can be determined. The resulting CO2 mass estimation is then compared to the results of the CO2 mass distribution of the numerical flow simulation.