2D Acoustic Full Waveform Inversion of the Asse II Mine Model

The Asse II is a salt mine in the district of Wolfenbüttel that lies within the Asse Hesseberg Hill. In the 1960s, the German federal government bought the mine and converted it into a nuclear waste storage facility. The salt chambers were filled with radioactive waste and intended to be used as a storage facility for millions of years. This idea of progressive waste disposal became a nightmare when ground water surrounding the geologic overburden of the mine began leaking into it. The widespread implications and potential damage of the water leaking into the chambers of the mine has caused the German government to devote significant resources into the mitigation of any potential escalation of the situation. The Bundesgesellschaft für Endlagerung (BGE), a German federal company responsible for research and extraction of the waste, in collaboration with the Karlsruhe Institute of Technology (KIT)'s Geophysics Institute, has preformed numerical modelling and geological research to gain insight on the geologic structure that makes up the Asse Heeseberg Hill.

Using data provided by BGE, a subsurface model referred to in this thesis as the Asse II Mine Model was developed by (\cite{Mejia}). ... mehrTo test the integrity and potential limits of inverse modelling with the Asse II Mine Model, 2D acoustic full waveform inversion (FWI) of the Asse II Mine Model was preformed in this thesis. FWI, as the name implies, is an inverse modelling technique that utilizes the comparison of complete seismic waveforms to recreate observed synthetic data or observed field data. All forward and inverse modelling was preformed by discretizing the p-wave velocity values on a 2D numerical grid. After it was clear that forward modelling could be accurately preformed on the model, attempts at inverse modelling began. All inverse modelling done in this thesis was done with FWI. Detailed inversion parameters tests were made in order to obtain the best possible inversion models. These parameters, among others, included gradient method, step length, LNORM, tapering, frequency filter, and abort criteria.

Once these parameters were set and able to get an accurate result other parameters such as starting model, shot frequency, acquisition geometry, and time windowing were tested. After combining these parameters and running numerous inversions, accurate inversion were obtained. Thus the question, "could acoustic 2D full waveform inversion could be used to successfully and accurately reconstruct the Asse II Mine Model?" was confirmed. The results of the inversions are indicative of the successful recreation of the observed synthetic model, and potential for further application of acoustic numerical modelling to 3D scenarios and even field data.