GNGTS 2014 SESSIONE 3.1 3 L-SHAPED ARRAY REFRACTIONS MICROTREMORS (LeMi) J. Boaga 1 , C. Strobbia 2 , G. Cassiani 1 1 Dipartimento di Geoscienze, Università degli Studi di Padova, Italy 2 Total S.A., Pau, France Introduction. Surface wave methods are nowadays the main testing tools for the site characterization concerning engineering applications. Their use has been growing in past years especially due to the increasing interest in shear wave velocity measurement, which is essential in geotechnical earthquake engineering applications (Beavers, 2002; Chaillat et al. 2009). One of the reasons is the progression of several internationals a-seismic building codes that propose simplified seismic scenario based on Vs classification (e.g. Vs30 parameter as in Moss, 2008; EC8, 2003). Surface wave methods based on dispersion properties studies are nowadays widely adopted in local subsoil Vs characterization: different frequencies involve different soil thicknesses, and consequently travel at different velocities. Dispersion properties of surface waves are then used to define vertically heterogeneous media (Thomson, 1950; Tokimatsu, 1995; Foti, 2003, 2011; Socco and Strobbia, 2004; Strobbia and Cassiani, 2011), and they represent by now the most diffused techniques for Vs modeling. Surface wave methods are free from many practical and theoretical limitations of the classical body-wave analyses and they are free from the logistical effort of drilling (Boaga et al., 2010, 2011; Vignoli et al. 2010, 2012; Foti et al., 2011). The technique requires an accurately recorded Rayleigh/Love wavefields to be analyzed for its dispersive properties, and the consequent inversion of the dispersion curve (e.g. phase velocity versus frequency). Surface wave methods can be divided in active methods, relying on the use of controlled sources, and passive methods, basing on the analysis of ambient noise, or microtremors. The active (controlled source) surface wave methods retrieve dispersion properties using several procedures: linear array methods as MASW (Park et al., 1999), coupled receivers methods as SASW (Nazarian et al., 1983) or single receiver methods as FTAN (Levshin et al., 1972; Nunziata et al., 1999; Boaga et al., 2010). Active methods basing on the use of controlled sources are accurate, but have limited exploration depth linked the ability of generating low frequencies with adequate sources. Passive methods have no control of the sources but, with the same array geometry, can reach deeper investigation depth thanks to the low frequency content of seismic noise and microtremors. The standard approaches for passive surface wave methods in shallow engineering applications derive for the seismological array processing. In seismological applications microtremors techniques use mainly 2D arrays of low frequency receivers in various and irregular geometries (Aki, 1957; Frosh and Green 1966; Tokimatsu et al., 1992; Okada, 2003). 2D arrays deployment identifies in fact the source direction and it is required in order to identify the velocity and the direction of a wavefield which is not controlled and comes from unknown directions (Bonnefoy-Claudet et al., 2006). In exploration seismology common passive arrays techniques are: i) beamforming methods as F-K, ii) spatial autocorrelation SPAC (Aki, 1957), iii) extended Spatial autocorrelation ESAC (Ohori et al., 2002), iv) high resolution arrays like MUSIC (Schmidt, 1981) and v) cross-correlation methods (Sabra et al., 2005; Shapiro et al., 2005). The logistic constraints of typical engineering applications often do not allow deploying large 2D arrays. Moreover, the required processing techniques, despite their relative simplicity, are often not available to the ‘practitioners’ community. This is the main reason for the success of the simple linear array passive method called ReMi (Refraction Microtremors: Louie, 2001). A linear array in a diffused wavefield has a response which is not function of its direction, but only of its size (length and receiver spacing). An averaged kinematics spectrum, such as the spectral density in the frequency wavenumber domain f-k, can be used to estimate the local propagation properties. The most important limitation of the ReMi approach is related to the basic assumption that the recorded data consist of a uniform wavefield. When this is not the case, the ReMi spectrum depends on the unknown source distribution, and its interpretation or
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GNGTS 2014 sessione 3.1
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L-SHAPED ARRAY REFRACTIONS MICROTREMORS (LeMi)J. Boaga1, C. Strobbia2, G. Cassiani1
1 Dipartimento di Geoscienze, Università degli Studi di Padova, Italy2 Total S.A., Pau, France
Introduction. Surface wave methods are nowadays the main testing tools for the site characterization concerning engineering applications. Their use has been growing in past years especially due to the increasing interest in shear wave velocity measurement, which is essential in geotechnical earthquake engineering applications (Beavers, 2002; Chaillat et al. 2009). One of the reasons is the progression of several internationals a-seismic building codes that propose simplified seismic scenario based on Vs classification (e.g. Vs30 parameter as in Moss, 2008; EC8, 2003). Surface wave methods based on dispersion properties studies are nowadays widely adopted in local subsoil Vs characterization: different frequencies involve different soil thicknesses, and consequently travel at different velocities. Dispersion properties of surface waves are then used to define vertically heterogeneous media (Thomson, 1950; Tokimatsu, 1995; Foti, 2003, 2011; Socco and Strobbia, 2004; Strobbia and Cassiani, 2011), and they represent by now the most diffused techniques for Vs modeling. Surface wave methods are free from many practical and theoretical limitations of the classical body-wave analyses and they are free from the logistical effort of drilling (Boaga et al., 2010, 2011; Vignoli et al. 2010, 2012; Foti et al., 2011). The technique requires an accurately recorded Rayleigh/Love wavefields to be analyzed for its dispersive properties, and the consequent inversion of the dispersion curve (e.g. phase velocity versus frequency). Surface wave methods can be divided in active methods, relying on the use of controlled sources, and passive methods, basing on the analysis of ambient noise, or microtremors. The active (controlled source) surface wave methods retrieve dispersion properties using several procedures: linear array methods as MASW (Park et al., 1999), coupled receivers methods as SASW (Nazarian et al., 1983) or single receiver methods as FTAN (Levshin et al., 1972; Nunziata et al., 1999; Boaga et al., 2010). Active methods basing on the use of controlled sources are accurate, but have limited exploration depth linked the ability of generating low frequencies with adequate sources. Passive methods have no control of the sources but, with the same array geometry, can reach deeper investigation depth thanks to the low frequency content of seismic noise and microtremors. The standard approaches for passive surface wave methods in shallow engineering applications derive for the seismological array processing. In seismological applications microtremors techniques use mainly 2D arrays of low frequency receivers in various and irregular geometries (Aki, 1957; Frosh and Green 1966; Tokimatsu et al., 1992; Okada, 2003). 2D arrays deployment identifies in fact the source direction and it is required in order to identify the velocity and the direction of a wavefield which is not controlled and comes from unknown directions (Bonnefoy-Claudet et al., 2006). In exploration seismology common passive arrays techniques are: i) beamforming methods as F-K, ii) spatial autocorrelation SPAC (Aki, 1957), iii) extended Spatial autocorrelation ESAC (Ohori et al., 2002), iv) high resolution arrays like MUSIC (Schmidt, 1981) and v) cross-correlation methods (Sabra et al., 2005; Shapiro et al., 2005). The logistic constraints of typical engineering applications often do not allow deploying large 2D arrays. Moreover, the required processing techniques, despite their relative simplicity, are often not available to the ‘practitioners’ community. This is the main reason for the success of the simple linear array passive method called ReMi (Refraction Microtremors: Louie, 2001). A linear array in a diffused wavefield has a response which is not function of its direction, but only of its size (length and receiver spacing). An averaged kinematics spectrum, such as the spectral density in the frequency wavenumber domain f-k, can be used to estimate the local propagation properties. The most important limitation of the ReMi approach is related to the basic assumption that the recorded data consist of a uniform wavefield. When this is not the case, the ReMi spectrum depends on the unknown source distribution, and its interpretation or
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inversion is not possible. This induces to an overestimation in retrieving shear wave velocities, that can be dangerous for seismic hazard evaluation. The proposed data processing method, based on refraction microtremors L-shaped arrays (LeMi), can easily and appropriately solve the problem.
Refractions Microtremors with L-shaped arrays. The processing of passive data on two dimensional arrays is well known and widely discussed in the literature (Okada, 2003). Some of the processing techniques can be considered extensions of the transform-based methods typically used in active multichannel surface wave testing. As conventional MASW data are transformed from T-X to F-K (or F-P, F-V), with two dimensional arrays the data are transformed from T-X-Y into the F-Kx-Ky domain. The transforms can be FKK transforms or other 2D beamformers. In the following we show the results obtained with different 2D arrays in processing non-uniform wavefield for engineering practice both for synthetic and real cases. The objective of this work is not the evaluation of optimal arrays for passive surface wave testing, but simply suggest 2D array procedure consisting of two linear-arrays evenly spaced receivers. In our proposal the array geometry consists of two straight branches, to simplify the deployment of receivers and cables and the surveying: the use of evenly spaced arrays is chosen to simplify the processing, and to allow the use of the spectral analysis methods typically available in commercial software. An L-shaped array has the advantage of allowing the acquisition of active multichannel data with a limited extra effort. A far-field linear plane wave propagating across a dual-linear array is detected with two different apparent wavenumbers by the two branches: each identifies the apparent velocity and apparent wavenumber in its direction assuming that a single plane wave is recorded simultaneously. If the two directions are orthogonal, and they correspond to a local reference systems x-y, the true wavenumber can be determined simply as
Ktrue = sgrt (kx2 – ky2) (1)In the case of passive measurements, this allows to overcome the trouble issue of oriented
noise source, not easy detectable on site. If a source direction is strongly dominant on the recorded data, then the two averaged measurements represent the same direction and can be combined as in Eq. (1) to retrieve the true wavenumber Ktrue. From true wavenumber we can define the surface wave phase velocity versus frequency dispersion property and then, after the inversion process, the shear wave profile. In the synthetic case of Fig. 1a it is visible how a predominant unknowns orientated source make impossible the common ReMi linear array detection of the true wavenumber (and then of the true seismic velocity). In this peculiar case, considering the y branch linear orientation could bring to an over-estimation of the seismic velocity of 10-20% because the array is almost perpendicular to the main noise source (Fig. 1b). Since orientation noise source is unknown, one operator can potentially consider to use only a linear array as the x branch, leading to a huge misleading over-estimation of the velocity (Fig. 1b) that can have serious consequence in seismic hazard studies.
The use of an L-shaped array can on the contrary retrieve the true wavenumber (and then the true velocity) starting from any orientation of the perpendicular branches (Fig. 1b). The combination of the results of two linear arrays can overcome the limitations of ReMi in cases of strongly directional sources. If the sources are stationary, the acquisition of the two arrays could even be done separately, one after the other, processing the arrays separately for the best k resolution. Since common seismograph channels numbers adopted in engineering practice is increasing, it is however recommended to acquire the arrays simultaneously. This ensures the absence of variation of the dominant sources, and allows, if needed, a proper 2D processing as beamformer analysis (Capon, 1969). We do not propose complex theoretical approach but provide a simple modification of the ReMi procedure that can be successfully implemented by practitioners in the field and still be rigorous enough to provide useful estimates of true shear wave velocity.
Real data L-shaped array test. The test site was located in Badia Polesine (Ro), N-E Italy, in the southern part of the ‘Po’ river plain. The site was selected for its geological conditions,
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which can be well assimilated to 1D profile, being characterized by deep spatial homogeneous alluvial deposits (Fontana et al., 2008). It is expected that in such environment passive linear arrays retrieve same results for any orientation the operator can choose, being on not laterally variable environment. The acquisition has been performed using an L-shaped array: a linear array of 24 receivers (4.5 Hz natural frequency) along the E-W (x) direction and a perpendicular linear array of 24 receivers (4.5 Hz natural frequency) along the N-S (y) direction. The receiver spacing was 4m for both branches, sampling rate was 2 ms. Controlled source records were acquired besides the passive data, to test the validity of LeMi approach, using a seismic gun as source with record lengths of 2 s. For the passive analysis record lengths was 30 s for each noise window. One of the main advantage of the proposed L-shaped array stays in fact in the possibility to locate a shot point at the vertex of the L-shaped array, in order to simultaneously provide two active tests along the 2 directions with common multichannel analysis of surface wave. Active data results to be used as benchmark are plotted in Fig. 2.
Fig. 3 summarizes the advantage of application of the passive LeMi approach in the Badia test site. As it could be seen from data, despite the strictly geological 1D conditions confirmed by several boreholes, the Kx and Ky wavenumbers referred to the two branches of the L shaped passive array present different values. This implies the site was afflicted by
Fig. 1 – a) Synthetic example of ReMi spectra on two orthogonal arrays with a concentration of source in a sector. Left: plan view of the simulated geometry, with sources in red, receivers in blue. Centre: ReMi f-k spectrum on the y (N-S) array branch. Right: ReMi f-k spectrum of the x (E-W) array branch. b) Picking of the wavenumber for the case of Figure1a and phase velocity/frequency transform. Kx (black) refers to the x direction and Ky (red) refers to the y direction. In blue the combination to estimate the true wavenumber from LeMi procedure. Note as in this case the blind orientation of a linear array can lead to great overestimation of the phase velocity while the L-shaped array allows to retrieve the true phase velocity.
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an unknown orientated noise source (in this case toward y direction), which does not admit common linear array ReMi treatment. The combination of the results of two L-shaped linear arrays can overcome the limitations of ReMi in cases of these strongly directional sources. The use of LeMi L-shaped array approach allows to detect the orientation of the predominant source and correctly estimate Ktrue and the related phase velocities (Fig. 3). In this test site L-shaped array results proved to be in agreement with the controlled source test results of
Fig. 2 – Controlled source f-k spectrum for the L shaped array, (branch x and y) and dispersion curve obtained averaging the results of the two branches, for the real dataset collected in Badia Polesine (Ro).
Fig. 3 – F-k maxima picking for the N-S (y) branch (blue); E-W (x) branch (red) and combination of the two apparent wavenumbers (EW and NS) to estimate the Ktrue values. In right panel are visible the relative dispersion curves in the Phase velocity/ frequency.
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Fig. 2, in the common range of frequency, retrieving the K true and overcoming the difficult orientated noise source condition. The use of two linear arrays allows the identification of the true propagation direction, and the correction for the error that a single array would produce. For the Badia Polesine case these passive results were supported by the controlled source test and by the geotechnical local informations (personal communication of Dr. F. Marinoni). For the presented real case we tested with success also the response of the L-shaped array with use of classical beamforming techniques thus identifying true velocity and direction of events, independently on the source distribution been more or less spatially polarized.
Conclusion. ReMi is based on the hypothesis of a uniform wavefield, and only in this case the results can be considered representative of the local propagation properties. The properties of the spectrum should be analyzed and tested to verify the validity of the basic hypothesis site by site. To overcome the limitations, an alternative and quick acquisition solution consist in L–shaped arrays practice. The proposed LeMi combination of two linear evenly spaced arrays presents the advantages of practical fieldwork, allowing the acquisition of simple multichannel data on the same spread. The use of LeMi approach allows: i) the detection of predominant orientated noise source; ii) the identification of the true wavenumber, overcoming the error that a single array would produce. It must be underlined that combining 2 arrays simultaneously practically limits the number of available receivers and consequently the f-K resolution. Narrow directional source admits theoretically LeMi treatment even if record of the 2 L-shaped branches are not simultaneously but the issue can become critical with a more complex source distribution (different than uniform or narrow directional noise sources). If LeMi data are based on the simultaneous analysis of the data of the two arrays, the proposed acquisition scheme allows in any case more advanced 2D treatment approaches as beamformer. These should be adopted in the cases where there are evidences of more variable source distribution.
In conclusion the passive LeMi methods, characterized by easy deployment and treatment, can considerable improve the correct estimation of seismic velocity for site characterization fronting narrow oriented and variable source orientation. This avoids the overestimation of the subsoil velocities, typical of linear passive array as ReMi, in the current case of oriented noise source. This can play a relevant role in all the seismic hazard characterization studies, since the overestimation of parameter as Vs can lead to totally misleading seismic response estimation.
Acknowledgments. Authors thank the geologist F. Marinoni for the Badia Polesine site geological and geotechnical information. Jacopo Boaga work is supported by the European FP-7 project CLIMB:’ Climate Induced Changes on the Hydrology of Mediterranean Basins’.
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