D. De Nil

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.

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.

Full-waveform inversion of SH- and Love-wave data in near-surface prospecting

We develop a two-dimensional full waveform inversion approach for the simultaneous determination of S-wave velocity and density models from SH - and Love-wave data.We illustrate the advantages of the SH/Love full waveform inversion with a simple synthetic example and demonstrate the method’s applicability to a near-surface dataset, recorded in the village Cˇ achtice in Northwestern Slovakia. Goal of the survey was to map remains of historical building foundations in a highly heterogeneous subsurface. The seismic survey comprises two parallel SH-profiles with maximum offsets of 24 m and covers a frequency range from 5 Hz to 80 Hz with high signal-to-noise ratio well suited for full waveform inversion. Using the Wiechert–Herglotz method, we determined a one-dimensional gradient velocity model as a starting model for full waveform inversion. The two-dimensional waveform inversion approach uses the global correlation norm as objective function in combination with a sequential inversion of low-pass filtered field data. This mitigates the non-linearity of the multiparameter inverse problem. Test computations show that the influence of visco-elastic effects on the waveform inversion result is rather small. Further tests using a monoparameter shear modulus inversion reveal that the inversion of the density model has no significant impact on the final data fit. The final full waveform inversion S-wave velocity and density models show a prominent low-velocity weathering layer. Below this layer, the subsurface is highly heterogeneous.Minimum anomaly sizes correspond to approximately half of the dominant Love-wavelength. The results demonstrate the ability of two-dimensional SH waveform inversion to image shallow small-scale soil structure. However, they do not show any evidence of foundation walls.

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.

Seismic Prediction Ahead of Tunnel Construction Using Rayleigh-waves

To increase safety and efficiency of tunnel constructions, online seismic exploration ahead of a tunnel can become a valuable tool. We developed a new forward looking seismic imaging technique e.g. to determine weak and water bearing zones ahead of the constructions. Our approach is based on the excitation and registration of tunnel surface-waves. These waves are excited at the tunnel face behind the cutter head of a tunnel boring machine and travel into drilling direction. Arriving at the front face they generate body-waves propagating further ahead. Reflected S-waves are back-converted into tunnel surface-waves.