Abstract

The Montserrat Massif (Catalonia, NE of Spain) is a natural and cultural heritage where rockfall risk arises. Therefore, a risk mitigation plan is underway, including rockfall monitoring at different scales using different techniques. Particularly, the rock block A3-6, menacing the rack railway leading to the monastery, has been monitored since 2010 by extensometers that show the cyclic movement due to the annual thermal cycle, which is mainly recoverable, but small residual plastic derivations have been detected in a varying amount, along years. At the end of 2021, stabilization work has been carried out. The block has been monitored with passive seismic techniques before, during, and after these works in order to detect signs or evidence of evolving stability similar to previous experiences in the Alps. Two main results have been found. On the one hand, a slight difference in the recorded ambient noise between the potentially unstable block and the rear massif was detected, for both the H/V spectral ratio and the polar spectrogram. A characteristic resonance frequency of the block is observed around 20 Hz in the direction of the toppling instability mechanism. On the other hand, during the drilling works of the anchor bolts, it was possible to clearly detect when the drilling hammer crossed the rear joint of the block. This allows confirming the assumed geometry of the block and the required anchor length, as well as a qualitative assessment of the persistence of the joint and its mechanical contact.

Abstract

In recent decades, microtremor and surface wave explorations have been widely applied for geotechnical investigations to estimate the S-wave velocity profile. The estimated S-wave velocity profile provides essential information for site characterization. Since we need to solve an inverse problem with the observed phase velocity of the surface waves to estimate the S-wave velocity profile, acquiring high-quality data is the most critical part. To ensure the data quality and the corresponding results, ISO 24057:2022, array measurement of microtremors to estimate shear wave velocity profile, was developed in 2022. The document specifies appropriate equipment, procedures, data analysis, and reporting for the array measurement of microtremors as a passive geotechnical survey. Since we cannot control the frequency contents in ambient noises, the frequency range with the high power may be limited in the microtremor explorations. In this situation, surface wave exploration can supplement this shortcoming. An example of combining the two results shows that it gives us a dispersion curve in a wide frequency range to estimate the S-wave velocity profile from shallow to deep subsurface structure. Developing a new ISO standard for surface wave explorations by following the microtremor exploration enhances the quality of the estimated S-wave velocity profile. Accordingly, we expect non-destructive and cost-effective investigations to be widely accepted worldwide.

Abstract

The horizontal to vertical spectral ratio (HVSR) passive seismic method consists in processing measurements of ambient noise performed in three perpendicular directions. The objective is to define the primary resonant frequency of the site and estimate its amplification characteristics, which can be deduced from visible peaks or troughs in the HVSR curves. The shape of HVSR curves depend on the interaction of waves with the interface between sediments and the formation bedrock and, hence, can be related to the bedrock depth through derived relationships. This work presents 4 study cases in the Paraguayan oriental region where HVSR of microtremors were obtained. Furthermore, the obtained HVSR curves are inverted to get approximate shear wave velocity profiles under the assumption of sub-vertically propagating P and S body waves. HVSR curves were calculated through Geopsy (Wathelet et al. 2020) and the inversion process was made through OpenHVSR (Bignardi et al. 2016), both open-source tools for ambient vibration processing. In general, the method proves to give sufficient accurate estimates of sediment thickness and stratigraphy in typical Paraguayan formations of the oriental region and presents the advantage of being an economical and fast survey option, especially for early stages in a geotechnical campaign

Abstract

Recent examples of tailings dam failure remind us how the monitoring of such sites is important. One major factor affecting the stability of tailings storage facilities (TSFs) is the presence and movement of water within the facility. Dam failures are often caused by seepage or the presence of weak layers within the tailings dam or the foundation soil. Thus, monitoring for the presence or movement of water within TSFs is becoming increasingly important, and solutions for permanent, non-intrusive and cost-effective monitoring of dams are a major challenge. One such solution is represented by geophysical imaging techniques such as ambient seismic noise interferometry. Through a research project, a nodal network system of accelerometer sensors was installed in a dense and random array on a TSF embankment wall at an Australian mine. Simultaneously, a fiber optic cable was deployed in a trench at the top of the TSF. Ambient seismic noise was recorded over a few weeks using the nodal network and with the fiber optic cable over several months using Distributed Acoustic Sensing (DAS) technology. Following data acquisition, benchmarking passive seismic imaging from the accelerometer network and the DAS system was conducted to evaluate the DAS capabilities for long-term TSF monitoring.

Abstract

Geophysical methods based on surface waves have become very popular in recent decades due to their versatility and reduced cost of execution compared to other invasive techniques. Due to space constraints in urban environments, measurements with linear arrays using active and passive techniques are usually combined to reach the exploration depth required by seismic site classification regulations. Although several good practice guidelines have been developed for performing this type of geophysical explorations, one of the remaining major challenges is to relate the results of these explorations to uncertainty metrics. In the case of active tests, there are effects associated with the filtering method used to eliminate the near and far-field effects, as well as other difficulties related to higher Rayleigh-modes or heterogeneities of the site. In this paper, we study the effect of body waves unavoidably induced in active tests on the proper determination of the dispersion curve of a site. For this purpose, active tests are carried out using triaxial geophones to extract from the records the motion effectively corresponding to Rayleigh waves by means of the Normalized Inner Product (NIP) technique. The results show that the effects of body waves are negligible for frequencies above about 8 Hz, but below this value, in the transition zone with ESPAC passive tests, the differences can be more significant. These results are used to introduce uncertainty indicators in this type of explorations