Tracking Space and Time Changes of Physical Properties in Complex Geological Media

An important issue in seismology concerns the characterization of the propagation medium, aiming to analyze the behavior of rocks in relation to the generation of earthquakes (both natural and human-made). The basic idea is that seismic waves can be used to image the medium’s physical properties. In this context we placed our research project, concerning the reconstruction of the spatial and temporal changes of physical properties (velocity, attenuation, rock parameters) in complex geological media. In the first part of this thesis we present a detailed description of known and new methodologies useful to track the seismicity, the propagation medium’s features and their temporal variation. In particular, a new rock modelling approach is constructed, allowing the conversion of velocity and attenuation values in rock micro-parameters; and a new equalization procedure for the 4D tomography is developed, allowing at once to optimize the choice of time-windows in the case of massive data-sets and to completely handle seismic tomography issues. In the second part, we show the results obtained by applying this methodologies to three complex areas: the Irpinia fault zones, The Geysers geothermal area and the Solfatara volcano. The relevance of these three areas lies not only in their different physical nature, but also in their different dimension. The obtained results show how the described methodologies can be used in seismogenic and volcanic areas to improve the knowledge of the medium’s properties, in order to mitigate the risk associated to destructive events, and in geothermal areas, to monitor the induced seismicity through the tracking of the medium properties’ temporal variation. Therefore, this thesis represents a useful tool for the characterization of the propagation medium, by providing a compendium of different methodologies and by showing the results of their application to three complex areas characterized by different physical nature and dimensional scale

Reconstructing Hydrothermal Fluid Pathways and Storage at the Solfatara Crater (Campi Flegrei, Italy) Using Seismic Scattering and Absorption

Imaging of fluid pathways is crucial to characterize processes taking place in hydrothermal systems, a primary cause of volcanic unrest and associated hazards. The joint imaging of seismic absorption and scattering is an efficient instrument to map fluid flow at crustal scale, and specifically in volcanoes; however, this technique has so far been applied to image volcanoes and hydrothermal systems at the kilometre scale. Here, we use data from a meter-scale, active seismic survey inside the shallow structure of the Solfatara crater to obtain the first frequency-dependent near-surface scattering and absorption model of a hydrothermal system. The Solfatara crater is the place used to monitor historic unrest at Campi Flegrei caldera (Italy), a high-risk volcano under continuous surveillance due to its closeness to a densely populated area. Improving the imaging of the shallow part of this system is crucial to broaden the understanding of unrest processes that are progressively characterizing other portions of the eastern caldera. The scattering contrasts highlight the primary structural feature, a fault separating the hydrothermal plume from zones of CO2 saturation nearing fumaroles. While high-absorption anomalies mark zones of high soil temperatures and CO2 fluxes, low-absorption anomalies indicate zones of very shallow upflow and are caused by contrasts between liquid-rich and vapour-rich fluids coming from mud pools and fumaroles, respectively. All maps show a SW-NE trend in anomalies consistent with fluid-migration pathways towards the eastern fumaroles. The results provide structural constraints that clarify mechanisms of fluid migration inside the crater. The techniques offer complementary geophysical images to the interpretation of hydrothermal processes and prove that seismic attenuation measurements are suitable to map fluid pathways in heterogeneous media at a detailed scale

High Resolution Attenuation Images From Active Seismic Data: The Case Study of Solfatara Volcano (Southern Italy)

<p>The anelastic attenuation of rocks strongly depends on the contained fluid physical state and saturation. Furthermore, it is more sensitive than elastic parameters to changes in the physical state of materials. In a geologically complex  volcanic context, where fluids play a very important role, anelastic imaging of the subsoil is therefore a very powerful tool for a better understanding of its dynamics.</p><p>In this study we present a robust workflow aimed at retrieve accurate 1-D and 3-D anelastic models from the processing of active seismic data, in terms of lateral and depth variations of P-wave quality factors Q<sub>P</sub>. This methodology has been applied to data collected during a high resolution active seismic experiment in a very small-scale volcanic volume, the Solfatara crater, within Campi Flegri caldera, Southern Italy. The presented methodology is developed in three distinct steps: 1) the active seismic data have been properly processed and analyzed for measuring the t* attenuation parameter for all possible source-receivers couples. First, the source contribution has been removed by cross-correlating the recorded signal with the sweep function of the Vibroseis, which was the adopted active seismic source. Then, the spectral decay method has been applied in order to compute the t* values. 2) A reference 1-D attenuation model has been retrieved by means of a grid search procedure aiming at finding the 1-D Qp structure that minimizes the residual between the average observed t* and the theoretical t* distributions. The obtained starting reference model allowed to build a preliminary map of t* residuals through which the retrieved t* dataset has been validated. 3) The 15,296 t* measurements have been inverted by means of a linearized, perturbative approach, in a 160 x 160 x 45 m<sup>3 </sup>tomographic grid.</p><p>The retrieved 3-D attenuation model describes the first 30 m depths of Solfatara volcano as composed of very high attenuating materials, with Qp values ranging between 5 and 40. The very low Qp values, correlated with low Vp values retrieved by a previous tomographic work carried out in the area, indicate the low consolidation degree of very superficial volcanic materials of Solfatara volcano. Finally, in the NE part of the crater, lower attenuating bodies have been imaged: it is a further hint for characterizing this area of the volcano as the shallow release of the CO<sub>2 </sub>plume through the main fumaroles of the crater.</p&gt

Monitoring the Microseismicity through a Dense Seismic Array and a Similarity Search Detection Technique: Application to the Seismic Monitoring of Collalto Gas-Storage, North Italy

Seismic monitoring in areas where induced earthquakes could occur is a challenging topic for seismologists due to the generally very low signal to noise ratio. Therefore, the seismological community is devoting several efforts to the development of high-quality networks around the areas where fluid injection and storage and geothermal activities take place, also following the national induced seismicity monitoring guidelines. The use of advanced data mining strategies, such as template matching filters, auto-similarity search, and deep-learning approaches, has recently further fostered such monitoring, enhancing the seismic catalogs and lowering the magnitude of completeness of these areas. In this framework, we carried out an experiment where a small-aperture seismic array was installed within the dense seismic network used for monitoring the gas reservoir of Collalto, in North Italy. The continuous velocimetric data, acquired for 25 days, were analysed through the application of the optimized auto-similarity search technique FAST. The array was conceived as a cost-effective network, aimed at integrating, right above the gas storage site, the permanent high-resolution Collalto Seismic Network. The analysis allowed to detect micro-events down to magnitude Ml = −0.4 within a distance of ~15 km from the array. Our results confirmed that the system based on the array installation and the FAST data analysis might contribute to lowering the magnitude of completeness around the site of about 0.7 units

Insight on the 2014 {MJMA} 6.7, northern Nagano earthquake sequence evolution in space and time through high resolved earthquake locations

The estimation of spatial and temporal changes in the host medium physical properties is a quest to improve risk evaluation and hazard forecasting application. The space-time evolution of the seismicity gives information about stress variations, fluid content, and pore-pressure changes inside the medium. Thus, the accuracy of arrival-time measurements is crucial for travel-time-based seismological applications, such as earthquake location and travel(delay)-time tomography, especially when double-difference times are used. Standard monitoring networks and tools implements single-station, STA/LTA-based, automatic event detection/location procedures, which may produce inconsistent arrival-times of the same phase among stations. To overcome this problem, refined cross-correlation (CC) techniques for time picking have been recently developed. Their basic approach is to use CC to refine picks of event pairs with high waveform similarity. Similar events are grouped in families, considering the max CC values, the inter-distance and/or the focal mechanism similarity. Two drawbacks of this common approach are (1) the impact of noise from individual receiver levels on the quality of reference trace (RT) and (2) the inability to adjust the systematic shift of automatic picks. Here we propose a new, fully automatic approach to refine the phase time picks. The CC is used to identify family members with a hierarchical clustering procedure. In each family, after the trace alignment, we build the RT by stacking the events weighted by the signal-to-noise ratio and the polarity. We applied this technique to a catalog of 3574 events of the 2014 MJMA 6.7 sequence occurred at the Northern Nagano prefecture. The results indicate that we can improve the precision of phase picks of similar events and to adjust the systematic shift introduced by the automatic picker with mean differences between refined and automatic picks up to 0,5-1 s. The high consistency of the phase picks allows to increase the accuracy of absolute location by reducing the mean location error from 0,6 km to 0,1 km and the root-mean-square from 0,15 to 0,075. Consequently, we observe an alignment of the seismicity respect to the main fault plane with an 30°-45° east-dipping angle for the shallow part while the deeper part dips at 50°-65°. Then, the double difference location provides highly resolved hypocenter locations and medium parameters by considering events of the same family as events pair. This improvement allows to use fast-tracking methods, as the Vp/Vs in time and the Coda-Wave Interferometry, to get information about the velocity variations before and during the sequence. By using those methods, we expect to get accurate information of the physical properties evolution and especially about the role of fluid in the triggering of the sequence