Ohrnberger, Matthias

Ambient vibrations are nowadays considerably used worldwide for numerous types of engineering applications and scientific research. Geopsy and its companion tools are part of that landscape. Since the first release of the program package in 2005, as outcome of the European Union project Site Effects aSsessment from AMbient noisE, Geopsy has become a mature multiplatform open-source package (released under GNU Public License version 3) that has already been recognized as a reference tool for analyzing ambient vibration data in the context of site characterization studies. The community of users has grown from a core group of researchers up to thousands of seismologists and engineers on every career level and on all continents. The versatility of geopsy allows for the processing of all kinds of data needed in site characterization studies, that is, from single station single trace to three-component array recordings. In all of the aforementioned cases, the steps from field acquisition to the production of publication-ready figures are covered and supported by user-friendly graphical user interfaces or corresponding command-line tools for the automation of the complete processing chain. To avoid black-box usage, a number of lower-level tools guarantee maximum flexibility in accessing and controlling processing results at any stage of the analysis

Surface wave methods gained in the past decades a primary role in many seismic projects. Specifically, they are often used to retrieve a 1D shear wave velocity model or to estimate the VS,30 at a site. The complexity of the interpretation process and the variety of possible approaches to surface wave analysis make it very hard to set a fixed standard to assure quality and reliability of the results. The present guidelines provide practical information on the acquisition and analysis of surface wave data by giving some basic principles and specific suggestions related to the most common situations. They are primarily targeted to non-expert users approaching surface wave testing, but can be useful to specialists in the field as a general reference. The guidelines are based on the experience gained within the InterPACIFIC project and on the expertise of the participants in acquisition and analysis of surface wave data.

Mt. Amiata in Tuscany (Italy) is an extinct volcano whose last eruptive activity was dated about 200 ky ago. Being still characterized by a high geothermal gradient the area lends itself for geothermal exploitation. Beneath the Tuscan Geothermal Areas seismicity is exclusively observed in the upper crust and is confined in depth by the so called K-horizon (400°C isotherme). The structure above contains permeable layers of highly fractured, volcanic rocks saturated with hot water and steam. Geothermal exploitation from these layers started in the 1960’s. Shallow earthquakes have occurred close to the geothermal wells, and the question is raised whether these event are of natural origin or related to the exploitation of heat. To monitor the seismic activity inside the geothermal field, an 8 station seismic network and a 7 element small aperture seismic array were installed in 2015 in the vicinity of the geothermal power plants during a joint field experiment by the Istituto Nazionale di Geofisica e Vulcanologia, the University of Potsdam and the GFZ-German Research Center of Geoscience. Already during the first 24 hours of seismic recording the array and the neighboring network stations recorded a M0.5 seismic event in the vicinity of the geothermal field of Bagnore. Since then micro-earthquake activity was recorded regularly. One of the main challenges of the seismic array/network installation, deployed in direct proximity to the geothermal energy production, is to identify seismic events caused by human operations. As hypocenters are located close to the geothermal power plants, at a similar depth as the production level, it is very difficult - if not impossible - to discriminate between natural earthquakes and anthropogenic events. The main goal of the seismic array/network deployed in the framework of our project is to shed some additional light on this question. The monitoring capabilities of the recording system permit a lowering of the detection threshold for local seismic events, performing high-resolution hypocentral determination, especially in the vicinity of the industrial operations, and calculating focal mechanisms. Array techniques and relative location methods will be used for a precise hypocentral determination. Polarization and spectral analysis, will be applied to discriminate seismic recordings from Mt. Amiata that sometimes resemble rather volcano-seismic waveforms with long-period characteristics, than typical tectonic events.

The knowledge of the local soil structure is important for the assessment of seismic hazards. A widespread, but time-consuming technique to retrieve the parameters of the local underground is the drilling of boreholes. Another way to obtain the shear wave velocity profile at a given location is the inversion of surface wave dispersion curves. To ensure a good resolution for both superficial and deeper layers, the used dispersion curves need to cover a wide frequency range. This wide frequency range can be obtained using several arrays of seismic sensors or a single array comprising a large number of sensors. Consequently, these measurements are time-consuming. A simpler alternative is provided by the use of the ellipticity of Rayleigh waves. The frequency dependence of the ellipticity is tightly linked to the shear wave velocity profile. Furthermore, it can be measured using a single seismic sensor. As soil structures obtained by scaling of a given model exhibit the same ellipticity curve, any inversion of the ellipticity curve alone will be ambiguous. Therefore, additional measurements which fix the absolute value of the shear wave velocity profile at some points have to be included in the inversion process. Small-scale spatial autocorrelation measurements or MASW measurements can provide the needed data. Using a theoretical soil structure, we show which parts of the ellipticity curve have to be included in the inversion process to get a reliable result and which parts can be omitted. Furthermore, the use of autocorrelation or high-frequency dispersion curves will be highlighted. The resulting guidelines for inversions including ellipticity data are then applied to real data measurements collected at 14 different sites during the European NERIES project. It is found that the results are in good agreement with dispersion curve measurements. Furthermore, the method can help in identifying the mode of Rayleigh waves in dispersion curve measurements.

The inversion of surface-wave dispersion curve to derive shear-wave velocity profile is a very delicate process dealing with a non-unique problem, which is strongly dependent on the model space parameterization. When independent and reliable information are not available, the selection of most representative models within the ensemble produced by the inversion is often difficult. We present a strategy in the inversion of dispersion curves able to investigate the influence of the parameterization of the model space, and to select a ‘’best’’ class of models. We analyze surface-wave dispersion curves measured at 14 European strong-motion sites within the EC-project NERIES. We focus on the inversion task exploring the model space by means of four distinct parameterization classes composed of layers progressively added over a half-space. The classes differ in the definition of the shear-wave velocity profile; we consider models with uniform velocity as well as models with increasing velocity with depth. At each site and for each model parameterization, we perform an extensive surface-wave inversion (200100 models for 5 seeds) using the conditional neighbourhood algorithm. We address the model evaluation following the corrected Akaike’s Information Criterion (AICc) which combines the concept of misfit to the number of degrees of freedom (dof) of the system. The misfit is computed as least-squares estimation between theoretical and observed dispersion curve. The model complexity is accounted in a penalty term by AICc. By applying such inversion strategy on 14 strong-motion sites, we find that the best parameterization of the model space is mostly 3-4 layers over a half-space; where the shear-wave velocity of the uppermost layers can follow uniform or power-law dependence with depth. The shear-wave velocity profiles derived by inversion agree with shear-wave velocity profiles provided by borehole surveys at approximately 80% of the sites.

In the framework of the EU-NERIES project, 20 sites among all European strong motion sites in Italy, Greece, T urkey and France were selected to be representative of most common soil classes, and for which shear-wave velocities from borehole measurements (cross-hole and down-hole tests) are available. Passive (array noise) and active experiments have been carried out at these sites in order to evaluate the ability of surface waves technique to provide reliable estimates of shear-wave velocity profiles. In order to stay cheap and feasible, active seismic experiments involving 24 geophones and hammer source were carried out at all sites. Data were processed by using the MASW technique and Rayleigh and Love waves dispersion curves were retrieved from 5-10 Hz to 30-50 Hz. Passive array experiments were also performed by using 8 seismological stations linked with wireless connections and monitored with near real-time processing. Combining up to four different arrays with aperture ranging from 10 m and to 900 m, Rayleigh and Love waves dispersion curves were derived over a broad frequency range (from 0.5 Hz up to 45 Hz) by using the FK and MSPAC techniques. At about 75% sites, dispersion curves from ambient vibration and MASW are in good agreement over the overlapping frequency band. T he other 25% sites correspond to complex geometrical site structures. Whatever the site, passive experiments are shown to be very suitable to retrieve accurate estimates of phase velocities at high frequency (over 20-30 Hz). T his experiment also clearly outlined the limited penetration depth (comprised between 15 and 25 m) of the MASW technique. Inversion of dispersion curves to derive shear-wave profiles and EC8 site class (which is mainly based on Vs30) is a difficult and highly debated issue. Here we test an alternative to get average shear-wave profiles and especially Vs30 from the dispersion curves only. For these 20 sites, we show that site classes may be estimated directly from the dispersion curves. T heses results are confirmed by an extensive study involving about 800 velocity profiles from real sites.

Site effect assessment is an important step in seismic risk mitigation. There is therefore a drastic need for co proxies to site effects estimates. In that context, a new promising approach was proposed, using the time-avera over the top z meters with z varying form 5, 10, 20 and 30 meters (Vsz) and the fundamental resonance frequen two-parameters characterization of a site. Then to assess site effect, a Site Amplification Prediction Equat completely defined by these two parameters was build-up based on Japanese data from the KiK-Net network. Thu to be validated using other dataset. For that aim the EUROSEIS-test data is a suitable one. The EUROSE sedimentary basin in northern-Greece that has been thoroughly investigated through grants from the European C mainly to study site effects. Fourteen accelerometric stations have been installed since 1995, that to date recorded 100 events. After a review of the main available information over the EUROSEIS-test, we end up with a poor Vs for some of the accelerometric stations. Thus eight accelerometric stations were selected for noise array me surveys to provide more details information about Vsz and f0 parameters. The noise array technique has been pro decades ago but its development is still in progress, particularly regarding the inversion step. Different approaches el al. 2009, Renalier et al. 2009) were tested in this study to provide Vsz. These two inversion strategies provide Vsz for z equals to 10, 20 and 30 meters. With the resulting Vsz and f0 from noise analysis, a validation-test of th SAPE was realized. The results of such a comparison are encouraging and indicate as well limitations of the SAP It is a promising tool for engineering and seismic risk management.

In the present study, we investigated the dispersion characteristics of medium-to-long period Rayleigh waves (2 s < T < 20 s) using both singlestation techniques (multiple-filter analysis, and phase-match filter) and multichannel techniques (horizontal slowness [p] and angular frequency [~] stack, and cross-correlation) to determine the velocity structure for the Mt. Etna volcano. We applied these techniques to a dataset of teleseisms, as regional and local earthquakes recorded by two broad-band seismic arrays installed at Mt. Etna in 2002 and 2005, during two seismic surveys organized by the Istituto Nazionale di Geofisica e Vulcanologia (INGV), sezione di Napoli. The dispersion curves obtained showed phase velocities ranging from 1.5 km/s to 4.0 km/s in the frequency band 0.05 Hz to 0.45 Hz. We inverted the average phase velocity dispersion curves using a non-linear approach, to obtain a set of shear-wave velocity models with maximum resolution depths of 25 km to 30 km. Moreover, the presence of lateral velocity contrasts was checked by dividing the whole array into seven triangular sub-arrays and inverting the dispersion curves relative to each triangle.

Within the scope of the EC-projects NERIES and ITSAK-GR we have applied a procedure able to combine a multi-model space parameterization and an information theoretic approach in analysis of dispersion curve inversion. In detail we considered the dispersion curve assessed at 14 strong motion European sites. At each site we investigated the model space through four different parameterization groups within the wavelength range estimated by actual dispersion curves. In order to explore the influence of model space we increased progressively the number of layers for each parameterization. We therefore addressed the model evaluation among a set of competing models obtained by inversion following the corrected Akaike’s Information Criterion(AICc). By using such information-theoretic approach, we found an acceptable agreement between the inverted shear-velocity profiles of the best models and the available borehole results.

For an optimal analysis of the H/V curve, it appears necessary to check the instrument signal to noise ratio in the studied frequency band, to ensure that the signal from the ground noise is well above the internal noise. We assess the reliability and accuracy of various digitizers, sensors and/or digitizer-sensor couples. Although this study is of general interest for any kind of seismological study, we emphasize the influence of equipment on H/V analysis results. To display the impact of the instrumental part on the H/V behavior, some series of tests have been carried out following a step-by-step procedure: first, the digitizers have been tested in the lab (sensitivity, internal noise...), then the three components sensors, still in the lab, and finally the usual user digitizers-sensors couple in lab and outdoors. In general, the digitizer characteristics, verified during this test, correspond well to the manufacturer specifications, however, depending on the digitizer, the quality of the digitized waveform can be very good to very poor, with variation from a channel to another channel (gain, time difference etc.). It appears very clearly that digitizers need a warming up time before the recording to avoid problems in the low-frequency range. Regarding the sensors, we recommend strongly to avoid the use of “classical” accelerometers (i.e., usual force balance technology). The majority of tested seismometers (broadband and short period, even 4.5 Hz) can be used without problems from 0.4 to 25 Hz. In all cases, the instrumentation should be checked first to verify that it works well for the defined study aim, but also to define its limit of use (frequency, sensitivity...).