@phdthesis{Lontsi2016, author = {Lontsi, Agostiny Marrios}, title = {1D shallow sedimentary subsurface imaging using ambient noise and active seismic data}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-103807}, school = {Universit{\"a}t Potsdam}, pages = {xix, 119}, year = {2016}, abstract = {The Earth's shallow subsurface with sedimentary cover acts as a waveguide to any incoming wavefield. Within the framework of my thesis, I focused on the characterization of this shallow subsurface within tens to few hundreds of meters of sediment cover. I imaged the seismic 1D shear wave velocity (and possibly the 1D compressional wave velocity). This information is not only required for any seismic risk assessment, geotechnical engineering or microzonation activities, but also for exploration and global seismology where site effects are often neglected in seismic waveform modeling. First, the conventional frequency-wavenumber (f - k) technique is used to derive the dispersion characteristic of the propagating surface waves recorded using distinct arrays of seismometers in 1D and 2D configurations. Further, the cross-correlation technique is applied to seismic array data to estimate the Green's function between receivers pairs combination assuming one is the source and the other the receiver. With the consideration of a 1D media, the estimated cross-correlation Green's functions are sorted with interstation distance in a virtual 1D active seismic experiment. The f - k technique is then used to estimate the dispersion curves. This integrated analysis is important for the interpretation of a large bandwidth of the phase velocity dispersion curves and therefore improving the resolution of the estimated 1D Vs profile. Second, the new theoretical approach based on the Diffuse Field Assumption (DFA) is used for the interpretation of the observed microtremors H/V spectral ratio. The theory is further extended in this research work to include not only the interpretation of the H/V measured at the surface, but also the H/V measured at depths and in marine environments. A modeling and inversion of synthetic H/V spectral ratio curves on simple predefined geological structures shows an almost perfect recovery of the model parameters (mainly Vs and to a lesser extent Vp). These results are obtained after information from a receiver at depth has been considered in the inversion. Finally, the Rayleigh wave phase velocity information, estimated from array data, and the H/V(z, f) spectral ratio, estimated from a single station data, are combined and inverted for the velocity profile information. Obtained results indicate an improved depth resolution in comparison to estimations using the phase velocity dispersion curves only. The overall estimated sediment thickness is comparable to estimations obtained by inverting the full micortremor H/V spectral ratio.}, language = {en} } @article{LontsiOhrnbergerKruegeretal.2016, author = {Lontsi, Agostiny Marrios and Ohrnberger, Matthias and Kr{\"u}ger, Frank and S{\´a}nchez-Sesma, Francisco Jos{\´e}}, title = {Combining surface-wave phase-velocity dispersion curves and full microtremor horizontal-to-vertical spectral ratio for subsurface sedimentary site characterization}, series = {Interpretation : a journal of subsurface characterization}, volume = {4}, journal = {Interpretation : a journal of subsurface characterization}, publisher = {Society of Exploration Geophysicists}, address = {Tulsa}, issn = {2324-8858}, doi = {10.1190/INT-2016-0021.1}, pages = {SQ41 -- SQ49}, year = {2016}, abstract = {We compute seismic velocity profiles by a combined inversion of surface-wave phase-velocity dispersion curves together with the full spectrum of the microtremor horizontal-to-vertical (H/V) spectral ratio at two sediment-covered sites in Germany. The sediment deposits are approximately 100 m thick at the first test site and approximately 400 m thick at the second test site. We have used an extended physical model based on the diffuse wavefield assumption for the interpretation of the observed microtremor H/V spectral ratio. The extension includes the interpretation of the microtremor H/V spectral ratio observed at depth (in boreholes). This full-wavefield approach accounts for the energy contribution from the body and surface waves, and thus it allows for inverting the properties of the shallow subsurface. We have obtained the multimode phase velocity dispersion curves from an independent study, and a description of the extracted branches and their interpretation was developed. The inversion results indicate that the combined approach using seismic ambient noise and actively generated surface-wave data will improve the accuracy of the reconstructed near-surface velocity model, a key step in microzonation, geotechnical engineering, seismic statics corrections, and reservoir imaging.}, language = {en} } @article{LontsiOhrnbergerKrueger2016, author = {Lontsi, Agostiny Marrios and Ohrnberger, Matthias and Kr{\"u}ger, Frank}, title = {Shear wave velocity profile estimation by integrated analysis of active and passive seismic data from small aperture arrays}, series = {Journal of applied geophysics}, volume = {130}, journal = {Journal of applied geophysics}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0926-9851}, doi = {10.1016/j.jappgeo.2016.03.034}, pages = {37 -- 52}, year = {2016}, abstract = {We present an integrated approach for deriving the 1D shear wave velocity (Vs) information at few tens to hundreds of meters down to the first strong impedance contrast in typical sedimentary environments. We use multiple small aperture seismic arrays in 1D and 2D configuration to record active and passive seismic surface wave data at two selected geotechnical sites in Germany (Horstwalde \& Lobnitz). Standard methods for data processing include the Multichannel Analysis of Surface Waves (MASW) method that exploits the high frequency content in the active data and the sliding window frequency-wavenumber (f-k) as well as the spatial autocorrelation (SPAC) methods that exploit the low frequency content in passive seismic data. Applied individually, each of the passive methods might be influenced by any source directivity in the noise wavefield. The advantages of active shot data (known source location) and passive microtremor (low frequency content) recording may be combined using a correlation based approach applied to the passive data in the so called Interferometric Multichannel Analysis of Surface Waves (IMASW). In this study, we apply those methods to jointly determine and interpret the dispersion characteristics of surface waves recorded at Horstwalde and Lobnitz. The reliability of the dispersion curves is controlled by applying strict limits on the interpretable range of wavelengths in the analysis and further avoiding potentially biased phase velocity estimates from the passive f-k method by comparing to those derived from the SPatial AutoCorrelation method (SPAC). From our investigation at these two sites, the joint analysis as proposed allows mode extraction in a wide frequency range (similar to 0.6-35 Hz at Horstwalde and similar to 1.5-25 Hz at Lobnitz) and consequently improves the Vs profile inversion. To obtain the shear wave velocity profiles, we make use of a global inversion approach based on the neighborhood algorithm to invert the interpreted branches of the dispersion curves. Within the uncertainty given by the apparent spread of forward models we find that besides a well defined sediment velocity range also a reasonable minimum estimate of bedrock depth and bedrock velocity can be achieved. The Vs estimate for the best model in Horstwalde ranges from similar to 190 m/s at the surface up to similar to 390 m/s in the bottom of the soft sediment column. The bedrock starts earliest around 200 m depth and bedrock velocities are higher than 1000 m/s. In Lobnitz, we observe slightly lower velocities for the sediments (similar to 165-375 m/s for the best model) and a minimum thickness of 75 m. (C) 2016 Elsevier B.V. All rights reserved.}, language = {en} }