Use of seismic methods for 2D soil dynamic characterization Bárbara Valente Peniche

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1 Use of seismic methods for 2D soil dynamic characterization Bárbara Valente Peniche July 215 Abstract: Key issue of this study is to apply different methodologies to the seismograms that result from MASW acquisitions, in order to get a better definition of lateral variation of the stratigraphy and a twodimensional profile of shear wave s velocity. This two-dimensional velocities profile was validated according to other methods performed in the same study areas. After obtaining and validating the twodimensional model, a numerical model was created using FLAC software (Itasca Consulting Group) and a MASW test was simulated. Dispersion curves obtained from the numerical models were after compared with the experimental ones. The described procedure was implemented in two different study areas, with different geotechnical characteristics: the experimental site at Faculdade de Engenharia da Universidade do Porto (FEUP) and a free field site in Forte da Casa, north Lisbon. The results from the two-dimensional numerical modelling present a good correlation with experimental results, where better results are shown in low frequencies range when comparing with the unidimensional models created. However, the acquisition methodology for two-dimensional characterization should be improved. Keywords: surface waves; MASW; two-dimensional; numerical modelling; FLAC 1. Introduction The geotechnical characterization is one of the most important aspects in engineering practice. In the last decades, the use of seismic methods is increasing for geotechnical characterization due to their fast execution, implementation simplicity and cost effectiveness. The surface wave method (MASW) is one of the more interesting seismic methods and it is used to get a unidimensional profile of shear wave s velocity. This parameter is the starting point for the determination of an important property of soil dynamic characterization, the initial shear modulus (G ). 2. Objectives The main goal of this study consists in applying different processing methodologies to the seismograms obtained from MASW acquisitions (Table 3.1), in order to get a more detailed definition of lateral variations of the stratigraphy and a two-dimensional profile of shear wave s velocity. The two-dimensional profile was validated and after, a numerical model was created and simulated. The comparison between the dispersion curves obtained from the twodimensional numerical modelling and the experimental tests was made in order to validate the model created. This procedure was implemented in two different areas: experimental site of Faculdade de Engenharia da Universidade do Porto (FEUP) and a free field in Forte da Casa (FC), north Lisbon. 3. Surface wave method Surface wave methods present some advantages when comparing with another seismic methods such as seismic refraction. They have high shallow resolution, are applicable to every stratification profile (normally or inversely dispersive) and are easy to perform, among others. These kind of methods explore the dispersive property of surface waves which have different velocities at different frequencies on heterogeneous media (Figure 3.1). Rayleigh waves are the most used because they are detected by any kind of geophones (vertical or horizontal), due to their elliptical retrograde 1

2 movement, and they carry the greater part of the energy. Table 3.1 Types of processing used for seismograms analysis Types of processing Acquisition line with 24 geophones 12 geophones Acquisition line with 6 geophones Complete seismogram Seismogram without noise Seismogram without P- waves Seismogram without noise and P-waves Description Complete seismogram analysis, including noise (coherent and incoherent and P-waves) removing noise (coherent and incoherent) removing P-waves removing noise (coherent and incoherent) and P- waves. considering only the 12 closest geophones from the source considering only the 12 farthest geophones from the source considering only the 6 closest geophones from the source considering only the second group of 6 geophones closest to the source considering only the third group of 6 geophones closest to the source considering only the farthest group of 6 The MASW Multichannel Acquisition of Surface Waves is a surface wave method and it is made out by 3 different steps: acquisition, processing and inversion (Figure 3.2). Figure 3.1 Rayleigh waves propagation on heterogeneous media (Lopes, I. et al., 28). Figure Scheme of the three steps of the method, with the corresponding result (Strobbia,23). Acquisition is the first step of the method, where data are obtained from the site. It is important to get quality data in order to get the best final results. There are some factors that could influence acquisition step such as signal/noise ratio, dispersion curve resolution and acquisition parameters (acquisition line length, geophones spacing and source distance). Processing data results in dispersion curve description. Dispersion curves lists Rayleigh waves velocity as a function of frequencies (or wavelength). There are some techniques to get dispersion curves as f-k transform, which is the one used on this study. Inversion phase s goal is to estimate a shear waves velocity profile. The inversion problem is nonlinear: the obtained profile of velocities 2

3 results from the adjustment of a forward model synthetic curves to an experimental dispersion curve. All frequencies carry out information on the shallow layers while only the longer wavelengths have information on the deep ones.(figure 3.3) FEUP1_W_completo FEUP1_E_completo Figure Dispersion curves obtained for direct and inverse acquisitions using FEUP complete seismogram 6 4 Figure 3.3 Inversion step: overdetermination of the shallow layers comparing with the deep ones (Lopes, 25). 4. Data analysis 4.1 Data processing The unidimensional methodology consists in obtaining a shear wave velocity profile for each type of processing adopted. The dispersion curves are obtained in order to understand if there are significant variation on lateral stratigraphy. The dispersion curves were obtained using SWAN software (Geostudi Astier). FEUP For FEUP case study, sources from different side of the acquisition line were analysed, and a variation on lateral properties could be checked by comparing both sides dispersion curves. Figure 4.1 shows the comparison between west (direct) and east (inverse) acquisitions, considering the complete seismogram. Analysing the superposition of the dispersion curves, no high lateral variations are expected, especially for the shallow layers. Considering the acquisitions with 12 geophones, the dispersion curves show some differences, especially at the low frequencies range. Now, it is possible to conclude that probably some lateral variation at deep layers could occur (Figure 4.2) FEUP1_W 12 FEUP1_E_12_ FEUP1_W_12_ FEUP1_E 12 Figure Comparison of the dispersion curves (direct and inverse shot) considering groups of 6 geophones. When groups of 6 geophones are used, dispersion curves are not well defined, as the previous ones (Figure 4.3). East shot is noisier than the west one, as a consequence of an adjacent building basement. As the source distance increases, the signal to noise ratio is decreasing. However, it is possible to see that the dispersion curves present some differences on the low frequencies range too and there are some little differences on high frequencies range, but the global pace of the dispersion curves is similar. 3

4 Prof. [m] Prof. [m] with the one proposed by Lopes (25). It is possible to see in Figure 4.4 that velocity profiles show a good correlation with velocities range at shallow depth, however this correlation decreases with depth. The thickness of the layers is a more variable parameter V S [m/s] FEUP1_W_ FEUP1_E 6 FEUP1_W 6 FEUP1_E 6_ FEUP1_W_completo FEUP1_W_sem ruído FEUP1_W_sem ondas P 6 4 FEUP1_W 6_ FEUP1_E 6 FEUP1_W_sem ruído nem ondas P Modelo Lopes(25) V S [m/s] FEUP1_W 6 FEUP1_E_ FEUP1_E_completo FEUP1_E_sem ruído FEUP1_E_sem ondas P FEUP1_E_sem ruído nem ondas P Figure 4.3 Comparison of the dispersion curves (direct and inverse shot) considering groups of 6 geophones After the dispersion curves comparison, the shear waves velocity profiles were compared Modelo Lopes(25) Figure 4.4 Comparison between velocity profiles obtained for FEUP and the proposed by Lopes (25). 4

5 Forte da Casa In Forte da Casa case study the initially usedacquisitions had the source in the sameside of the acquisition line. These acquisitions differ only on the distance between the source and the first geophone. The distances were 2 and 2m. The complete seismogram from the two acquisitions show good correlation and it is possible to conclude that this site has a multimodal behaviour (Figure 4.5) Df=2m Df=2m Figure Dispersion curves obtained FC complete seismogram, considering different distances to the source FC_12_ Df=2m Df=2m FC 12 Df=2m Df=2m This type of behaviour is usual for sites where a layer with lower velocity appears beneath other with higher velocity. Considering the acquisitions with 12 geophones, shown in Figure 4.6, a similar pace can be seen. However the definition (in terms of characterizing points) is lower for source distance of 2m. For all the analysed data for Forte da Casa, it can be seen that there is a good definition of the dispersion curve until the twelfth geophone, for any source distance considered. With increasing distance to the source, although the definition is lower, there is more information on low frequencies range. Dispersion curves obtained for acquisition lines with groups of 6 geophones are shown in Figure 4.7. The velocity profiles obtained were compared to the one proposed by Lopes (25) (Figure 4.8). Comparing the results, it is possible to conclude that there is a good correlation between the obtained ones and the proposed by Lopes (25). However, on deeper layers, this correlation is smaller for both source distances. Figure Dispersion curves obtained for FC groups of 12 geophones, considering different distances to the source FC_6 Df=2m FC 6 Df=2m Df=2m Df=2m 5

6 Prof. [m] Prof. [m] FC 6_ 6 V S [m/s] Df=2m Df=2m 15 FC Df=2m Df=2m FC_Df2_sismograma completo FC_Df2_sismograma sem ruído FC_Df2_sismograma sem ondas P FC_Df2_sismograma sem ruído nem ondas P Figure Dispersion curves obtained for FC groups of 6 geophones, considering different distances to the source. Modelo Lopes,25 Figure 4.8 Velocity profiles obtained for FC models with source distance of 2 and 2m V S [m/s] FC_Df2_sismograma completo FC_Df2_sismograma sem ruído FC_Df2_sismograma sem ondas P FC_Df2_sismograma sem ruído nem ondas P Modelo Lopes(25) 4.2 Two-dimensional Interpretation The applied methodology consists on the creation of a two-dimensional velocity profile, taking into account the different analysis done for the seismograms. The two-dimensional profile was compared with another tests performed on the same study areas. For FEUP, a tomography for S-waves, presented in Lopes (25), was the comparison test and acquisitions with groups of 6 geophones ere used to do the two-dimensional profile. Comparing these results, a good correlation can be assured, as Figure 4.9 shows. Figure 4.1 shows the two-dimensional model created for FEUP data. Figure 4.9 Comparison between S-wave tomography and the obtained results for two-dimensional model done for FEUP_6_6_6_6. 6

7 Figure 4.2 Two-di mensional model create d for FEUP Figure 4.1 Two-dimensional model created for FEUP The same methodology was applied to Forte da Casa site. The comparison profile was a geologic model created by Lopes (25). Forte da Casa model was created using acquisitions from two different sources, northeast and south-west, considering groups of 12 geophones. The data from south-west was treated after the sensitivity study. The comparison between northeast and southwest shots, Figure 4.11, shows that lateral variation of the stratigraphy was not expected. Figure 4.12 Two-dimensional model created for FC. 5. Numerical modelling The first step for the numerical modelling was the simulation of the unidimensional models proposed by Lopes (25) for each area of study. After that a two-dimensional models were created, based on SWAN 2D velocities profile. FLAC 7. was the software used to perform this simulations. An axisymmetric axis was defined and the horizontal displacement was locked at the origin of the model (x=). Quiet boundaries, in two directions, were applied on the inferior and left borders of the model (Figure 5.1). 6 4 FC_NE_Df2 FC_SW_Df Figure 4.11 Comparison between northeast and southwest acquisitions, considering complete seismograms. The two-dimensional model was created taking into account these two acquisitions and it is presented on Figure Figure 5.1 Boundary conditions applied on numerical models. FEUP The soil properties proposed by Lopes (25) for the unidimensional model are shown in Table 5.1 and the numerical model is presented in Figure 5.. Table 5.1 Soil properties aoolied on the one-dimensional model: FEUP_A Thickness [m] VS [m/s] ν ρ [kg/m 3 ] E [MPa]

8 Table Soil properties applied on the twodimensional model: FEUP_B. Figure 5.2 One-dimensional model created on FLAC for FEUP_A Average thickness [m] VS [m/s] ν ρ [kg/m 3 ] E [MPa] Figure 5.3 shows the comparison between experimental and numerical dispersion curves obtained for the numerical model and a good correlation between these results can be seen experimental_feup1_w experimental_feup1_e numerical_feup_a Figure 5.3 Comparison between experimental dispersion curves and the numerical one obtained for the unidimensional model. The soil properties for two-dimensional model are quite different from the proposed by Lopes (25). These velocities are adapted from the range proposed by SWAN 2D and adjusted to the experimental dispersion curves by gpdc application from Geopsy software. The soil properties proposed for the twodimensional model for FEUP are shown in Table 5.2 and the model is presented in Figure 5.4. Figure 5.4 Two-dimensional model created for FEUP using FLAC software The dispersion curve obtained for the twodimensional model presents a good correlation with the experimental ones. On the low frequencies range the dispersion curve obtained for the two-dimensional model presents better results comparing with the unidimensional one and the global pace is similar to experimental results (Figure 5.5) experimental_feup_w experimental_feup_e numerical_feup_a numerical_feupb Figure Comparison between experimental dispersion curves and the numerical ones. 8

9 Forte da Casa The same procedure was applied to Forte da Casa area of study. The soil properties for the unidimensional model proposed by Lopes (25) is show in Table 5.3 and the numerical model is shown in Figure 5.6. The dispersion curve obtained for this model shows the typical multimodal behaviour for Forte da Casa site (Figure 5.8). Table Soil properties applied on the unidimensional model: FC_A Thickness[m] V S [m/s] ν ρ [kg/m 3 ] E [MPa] As it occurs for FEUP data, Forte da Casa s velocities for two-dimensional model are quite different from the unidimensional velocities as well as the average thickness. The soil properties for two-dimensional model of Forte da Casa are shown in Table 5.4 and the numerical model is shown in Figure 5.7. Table 5.4 Soil properties applied on the two-dimensional model: FCB. Average thickness [m] V S [m/s] ν ρ [kg/m 3 ] E [MPa] Figure 5.7 Two-dimensional model created on FLAC for FC. Figure 5.6 One-dimensional model created on FLAC for FC_A experimental_fc_ne_complete experimental_fc_sw_complete numerical_fca Figure 5.8 Comparison between experimental dispersion curves and the numerical one obtained for the unidimensional model The dispersion curve obtained for the twodimensional model (Figure 5.9) does not show the multimodal behaviour expected. This is now the most important aspect to be discussed. However, it can be seen that velocities only fit in the low frequencies range. Analysing the unidimensional model proposed by Lopes, the inversion layer (with lower velocity than the adjacent) has higher thickness then another model was created. This new model has the same soil properties, however the inversion layer has higher thickness and equal to the proposed for the unidimensional model. After this simulation, the multimodal behaviour can be seen approximately on the same frequency (Figure 5.1) like the experimental results. However, the velocities are underestimated. 9

10 6 4 2 experimental_fc_ne_complete experimental_fc_sw_complete numerical_fcb Figure Comparison between experimental dispersion curves and the numerical one obtained for the unidimensional model experimental_fc_ne_df2_complete expeirmental_fc_sw_df2_complete numerical_fcb_model Figure 5.1 Comparison between experimental dispersion curves and the numerical one obtained for the twodimensional model with higher inversion layer thickness. 6. Conclusions and recommendations The application of two-dimensional profiles from MASW is nowadays still an unusual procedure for soil dynamic characterization. This study tries to present some advantages on using this type of methodology: Different processing types are usual to evaluate the real lateral variation of the properties; Numerical modelling present good results for one-dimensional and twodimensional models; The inversion layer thickness was an important aspect on higher modes occurrence in Forte da Casa case study. However, this methods can be improved and some recommendations are done to develop their performance: Propose a two-dimensional acquisition methodology; Complement the geotechnical characterization using different types of seismic methods; Numerical modelling of different source positions, in order to better evaluate the numerical results; Test the presence of groundwater level and its influence on the numerical results; Validation of this methodology for different sites with different geotechnical conditions. Bibliography Itasca consulting group, Inc. 211, Fast Lagrangian Analysis of Continua, Version 7.. Minneapolis, ICG. Lopes, I. (25). Carcaterização geotécnica de solos no domínio das pequenas deformações. Aplicação do método das ondas superficiais. Tese de Doutoramento. Lisboa: Faculdade de Ciências - Departamento de Geologia: Universidade de Lisboa. Lopes, I., Santos, J.A., and Almeida, I.M.d. 28, O método das ondas superficiais: aquisição, processamento e inversão, in Revista Geotecnia. p Strobbia, C. (23). Surface wave methods: acquisition, processing and inversion. Dottorato di Ricerca in Ingegneria Geotecnica. Torino: Politecnico di Torino. Wathelet, M. (n.d.). Geospy Project. Retrieved from 1