Margherita Maraschini

A new misfit function for multimodal inversion of surface waves

Higher-modecontributionisimportantinsurface-waveinversion because it allows more information to be exploited, increases investigation depth, and improves model resolution. A new misfit function for multimodal inversion of surface waves, based on the Haskell-Thomson matrix method, allows higher modes to be taken into account without the needtoassociateexperimentaldatapointstoaspecificmode, thus avoiding mode-misidentification errors in the retrieved velocity profiles. Computing cost is reduced by avoiding the need for calculating synthetic apparent or modal dispersion curves.Basedonseveralsyntheticandrealexampleswithinversion results from the classical and the proposed methods, wefind that correct velocity models can be retrieved through the multimodal inversion when higher modes are superimposedintheapparentdispersion-curveorwhenitisnottrivial todetermineaprioritowhichmodeeachdatapointoftheexperimental dispersion curve belongs. The main drawback of the method is related to the presence of several local minima inthemisfitfunction.Thisfeaturemakesthechoiceofaconsistentinitialmodelveryimportant.

Geotechnical Aspects of the L’Aquila Earthquake

On April 6, 2009 an earthquake (ML = 5.8 and MW = 6.3) stroke the city of L’Aquila with MCS Intensity I = IX and the surrounding villages with I as high as XI. The earthquake was generated by a normal fault with a maximum vertical dislocation of 25 cm and hypocentral depth of about 8.8 km. The deaths were about 300, the injured were about 1,500 and the damage was estimated as high as about 25 billion €. Both maximum horizontal and vertical components of the accelerations recorded in the epicentral area were close to 0.65 g. The paper summarises the activities in the field of earthquake geotechnical engineering aimed to the emergency and reconstruction issues. The ground motion recorded in the epicentral area is analysed; the geotechnical properties measured by in-situ and laboratory tests before and after the earthquake are summarised; site effects are preliminarily evaluated at accelerometric stations locations and damaged villages; the outstanding cases of ground failure are finally shown.

Surface wave analysis for S-wave static correction computation

Ground roll and mud roll in seismic reflection data can be analysed to infer a near surface S-wave velocity model from which converted-wave static correction can be computed. A workflow that exploits processing tools available in industrial processing codes has been implemented to extract dispersion curves and related uncertainties along a seismic line. The dispersion curves are then inverted with a laterally constrained inversion algorithm based on an initial model previously determined through a Monte Carlo inversion. The final S-wave velocity models are finally used for static computation. The application of the method to synthetic data supplied static correction values with error lower than 5%

A New Approach for Multimodal Inversion of Rayleigh and Scholte Waves

The knowledge of the near surface velocity can be of paramount importance for assessing near surface effects on exploration seismic data both on land and offshore surveying. Ground roll (Rayleigh waves) and mud roll (Scholte waves) present in seismic records can be processed and inverted to supply a shear velocity model of the first hundreds meters. In this paper an automatic multimodal inversion for Rayleigh and Scholte waves using a new approach is presented. The proposed algorithm minimizes the L1 norm of the modulus of the Haskell-Thomson matrix determinant of the synthetic profile evaluated on real data. This algorithm is very fast, and it allows considering simultaneously all modes. Tests on both synthetic and real data, relative to Rayleigh and Scholte waves, have shown the importance of accounting for higher modes and the capability of the proposed method to handle their inversion. Improvement on the reliability of the final model has been obtained with a fast and efficient technique.

An Interesting Property of the Seismic Wavefield – Scale Properties for Waveform Matching

In this paper we present the application of scale properties on waveform matching. Scale properties are properties of seismic waves which can be used to save computational time in full waveform modelling and inversion. The adimensionalization of motion equation allows the possible models to be divided in classes, such that the full waveform of one of the element in a class can be easily calculated from the full waveform of another element in the same class. These properties have two main applications: the first one concerns the full waveform modelling; when the full waveform is calculated from a given profile, it is straightforward to calculate the full waveform for any of the profiles in the same class. The second application concerns the full waveform matching: when a experimental seismogram is recorded, and a synthetic one is calculated, before comparing them, it can be useful to change the synthetic model to another in the same class which is closer to the experimental one.

Surface wave inversion in complex systems

Dispersion curves estimated from field data can be inverted to supply near surface velocity models. Since the problem is hill posed, strongly non linear and mix-determined, the inverse problem suffers from strong solution non uniqueness particularly for complex velocity models. Global search methods explore the solution space and supply a “picture” of the solution non uniqueness enabling a proper model parameterization to be used for linearised inversion. A possibility for reducing the broadness of the possible equivalent solutions is the introduction of constraints to the solution. Constraints are traditionally obtained from other geophysical tests or boreholes, but significant improvement in the inversion results can be obtained also introducing the higher modes of propagation or inverting simultaneously the dispersion curves relative to different locations in the case of complex 2D/3D systems.