Passive Seismic Research Articles

Ambient noise surface-wave imaging in a hardrock environment: implications for mineral exploration

Summary The advancement of seismic methods is vital for mineral exploration in the ongoing energy transition. In this study, we investigate the application of ambient noise seismic interferometry and surface-wave analysis to characterize the subsurface in a mineral exploration context. We then confirm the results of the passive seismic investigation through an active source experiment. We collected ambient noise data using a 2D seismic line initially deployed for an active source reflection seismic study. By cross-correlating the signals, we retrieved the surface waves and constructed a 2D shear-wave velocity profile using conventional surface-wave analysis. We utilized the active source data to establish initial assumptions about the surveyed medium and then validated the passive seismic experiment. The passive seismic results are concordant with the active source results and allow for the interpretation of geological contacts and fault zones. Our work demonstrates the potential of passive seismic methods for investigating local tectonic settings and their role in hardrock mineral exploration.

Application Examples and Capabilities of Combining Passive Seismic Methods to Study Depth Structure of the Earth’s Crust

Application Examples and Capabilities of Combining Passive Seismic Methods to Study Depth Structure of the Earth’s Crust

GPUPRSI: GPU implementation of seismic interferometry for retrieving reflection responses from passive source seismic recordings

Passive seismic exploration is an environmentally friendly, economical, and highly accessible exploration method that is widely used in different-scale subsurface imaging. Retrieving reflections from ambient noise is a newly developed passive seismic method that can be used in many fields such as mineral exploration and near-surface imaging, but the interferometry calculation is time-consuming because it requires a longer period of data acquisition to improve the signal-to-noise ratio of the retrieved reflections. In this study, we introduced a graphical processing unit (GPU)- based implementation of seismic interferometry for retrieving reflection responses from passive source seismic recordings. Because all traces are involved in the computation process, but the size of passive source data often exceeds available memory, repeated disk reads lead to a decrease in computational efficiency. We design a strategy of grouping computations followed by stacking to minimize disk input and output, simultaneously keeping the memory requirements low. Passive source data is read and written only once, and there is no requirement for the memory size to be greater than the data size. Additionally, acceleration technologies such as asynchronous execution, asynchronous memory transfer, and GPU-accelerated libraries are used. We test the efficiency using short data where the number of sampling points per trace is on the order of 30,000, and long data, where the number of sampling points per trace is on the order of 30,000,000. For short data, the average speedup is 4 compared with a multi-core central processing unit (CPU); for long data, the speedup can reach 24. The GPU-based implementation of interferometry greatly reduces the calculation time for retrieving reflections from passive source seismic recordings, providing a solution to the problem of large calculation in three-dimensional (3D) passive source reflection exploration.

The Combination of Hvsr and Masw Methods with Landsat 8 Imagery to Assess the Changing Shoreline along the Coastal Area of Central Bengkulu

The Central Bengkulu Regency possesses a coastline measuring 21.8 km, situated in direct proximity to the Indian Ocean. The heightened wave and ocean current dynamics possess the capacity to induce detrimental effects in the form of coastal abrasion along the coastline. This study conducted an examination of the rate of shoreline changes resulting from coastal abrasion in Central Bengkulu Regency, Bengkulu, Indonesia. Horizontal-to-vertical spectral ratio (HVSR) analysis, multichannel analysis of surface waves (MASW), and Landsat-8 remote sensing were all used in the current investigation. The project gets started with conducting field investigations and measurements of geophysical techniques, including the collection of Landsat-8 satellite imagery spanning the period from 1990 to 2020. Additionally, an examination was conducted on the rate at which the shoreline undergoes changes in velocity. In order to determine the increase in change that occurs along coastlines over a period of several decades, a study was carried out during which Landsat-8 satellite images were analyzed. This study investigates the application of the HVSR technique for assessing the seismic vulnerability of the ground and, additionally, the MASW technique for measuring the shear wave velocity of the coastline’s soil. Both of these methodologies are compared and contrasted with one another in this research. The findings indicated that the mean pace of coastline change in Central Bengkulu Regency was 1.5 m/yr, with the maximum velocity recorded at 4.1 m/yr. This high velocity of shoreline change is correlated with the Vs30 value of MASW measurement and Kg of HVSR measurement, where the subsurface soil structure along the coast of Central Bengkulu Regency from Vs30 measurement shows that it is dominated by stiff soil structure that is susceptible to erosion. Outcomes from the study can inform decision-making processes about safeguards and preventative measures to prevent further coastal degradation. HIGHLIGHTS In this paper, we present a framework using application passive and active seismic method with Landsat 8 for knowing the changing of shoreline coastal area in Central Bengkulu Indonesia This study demonstrated that the high velocity of the shifting shoreline is correlated with Vs30 and Kg values The subsurface soil structure is predominantly composed of stiff soil, and the changing shoreline starts at 1.5 mm/yr, which makes the coast of Central Bengkulu vulnerable to erosion GRAPHICAL ABSTRACT

Integrating near-surface geophysical methods and remote sensing techniques for reconstructing fault-bounded valleys (Mellieha valley, Malta)

The island of Malta (central Mediterranean) is dissected by several WSW-trending fault-line valleys related to Miocene-Pliocene extensional tectonics. Some valleys host remnants of alluvial deposits that could provide information on possible Quaternary faulting, but the thickness of these deposits and their subsurface extent is poorly constrained. Our study aimed to investigate the structural configuration of the southern sector of the Mellieha valley, which is located in the north-west part of the island and is limited by the ENE-WSW general trending Mellieha and Ghadira faults, and their relation with a thin layer of infill sediments. We employed different near-surface geophysical techniques (electrical resistivity tomography, active and passive seismic methods, ground-penetrating radar), as well as remote sensing techniques (unmanned aerial vehicle digital photogrammetry and ground-based Light Detection and Ranging) to complement classic structural surveys. The valley structure has an asymmetric graben configuration, being bounded by a normal fault to the southeast (Mellieha Fault) and other to the north-west (Ghadira Fault). Our study provides insights of possible Quaternary fault displacements and revealed the presence of an additional fault splay, the previously unmapped NNW-dipping Mellieha Fault 2 at the south-eastern edge of the valley. Additionally, by integrating results from geophysical surveys, we estimated the thickness of the valley's recent deposits, being thicker towards the Mellieha Bay sector, with maximum values of 8–10 m. Our study aligns with the general model of horst and graben structuring with associated regional tilting since the late Miocene, and supports the hypothesis that some segments of NW-trending normal faults within the North Malta Graben exhibit Quaternary activity, although with minimal throw rates (<< 0.1 mm/yr).

Sediment thickness across Australia from passive seismic methods

SUMMARY Around the world the Earth’s crust is blanketed to various extents by sediment. For continental regions, knowledge of the distribution and thickness of sediments is crucial for a wide range of applications including seismic hazard, resource potential and our ability to constrain the deeper crustal geology. Excellent constraints on the sediment thickness can be obtained from borehole drilling or active seismic surveys. However, these approaches are expensive and impractical in remote continental interiors such as central Australia. Recently, a method for estimating the sediment thickness using passive seismic data, the collection of which is relatively simple and low cost, was developed and applied to seismic stations in South Australia. This method uses receiver functions, specifically the time delay of the P-to-S converted phase generated at the sediment-basement interface, relative to the direct-P arrival, to generate a first-order estimate of the thickness of sediments. In this work, we expand the analysis to the vast array of over 1500 seismic stations across Australia, covering an entire continent and numerous sedimentary basins that span the entire range from Precambrian to present day. We compare with an established yet separate method to estimate the sediment thickness, which utilizes the autocorrelation of the radial receiver functions to ascertain the two-way traveltime of shear waves reverberating in a sedimentary layer. Across the Australian continent the new results match the broad pattern of expected sedimentary features based on the various geological provinces. We are able to delineate the boundaries of many sedimentary basins, such as the Eucla and Murray Basins, which are Cenozoic, and the boundary between the Karumba Basin and the mineral rich Mount Isa Province. Contrasts in seismic delay time across these boundaries are upwards of 0.4 $\, \mathrm{s}$. The delay signal is found to diminish to &amp;lt;0.1 $\, \mathrm{s}$ for older Proterozoic basins, likely due to compaction and metamorphism of the sediments over time. As an application of the method, a comparison with measurements of sediment thickness from local boreholes allows for a straightforward predictive relationship between the delay time and the cover thickness to be defined. This offers future widespread potential, providing a simple and cheap way to characterize the sediment thickness in underexplored areas from passive seismic data.

Fiber Seismic Network on the Moon

Abstract Internal structures of the Moon are key to understanding the origin and evolution of the Earth–Moon system and other planets. The Apollo Passive Seismic Experiment detected thousands of lunar seismic events and vastly improved our understanding of the Moon’s interior. However, some critical questions like the state and composition of the core remain unsolved largely due to the sparsity of the Apollo seismic stations and the strong scattering of seismic waves in the top layer of the Moon. In this study, we propose the concept of a fiber seismic network on the Moon and discuss its potential in overcoming the challenges in imaging deep Moon structures. As an emerging technique, distributed acoustic sensing (DAS) can provide a cost-efficient solution for large-aperture and dense seismic network deployment in harsh environments. We compute lunar synthetic seismograms and evaluate the performance of DAS arrays of different configurations in retrieving the hidden core reflected seismic phase ScS from the strong scattered waves. We find that, compared to a sparse conventional seismic network, a fiber seismic network using tens of kilometers of cable can dramatically increase the chance of observing clear ScS by array stacking. Our results indicate that DAS could provide new opportunities for the future lunar seismic surveys, but more efforts and further evaluations are required to develop a space-proof DAS.

Characterization of anisotropy in basin-scale subsurface using teleseismic receiver function analysis

Teleseismic receiver function (RF) analysis offers a passive-source analogue to detect impedance boundaries using the converted body waves generated by earthquakes. While the technique traditionally has targeted deep earth structures such as the Moho and transition zones, there is growing interest in assessing its applicability in basin-scale seismic characterization, ultimately aimed for onshore commercial integration as a cost-effective complement to existing active-source seismic surveys. Specifically, the conventional broadband seismometers used in global observational seismic experiments are not only logistically simple in terms of data acquisition, but they also record ground motions in three mutually orthogonal time series, enabling effective detection of shear waves and directional variations of observed signals. Here, we perform teleseismic RF analysis to detect shear-wave anisotropy and related symmetry axes orientations in a basin setting, using open-source seismic data recorded at 55 closely spaced seismic stations in the LaBarge Passive Seismic Experiment deployed in Wyoming between November 2008 and June 2009. We find that the strengths and geometry of the observed anisotropy are variable along the array. Significantly, not only can anisotropy effectively delineate subsurface fault interfaces, it can also substantiate and reveal additional interpretable signals that are otherwise disregarded. The estimated fast axes orientations compare favorably with the complex fracture systems documented in the region. Finally, we show that P-to-S amplitude variations with P incidence are systematic and modelable using existing computational tools, offering an opportunity to develop an analysis technique similar to amplitude variation with offset with the products of RF analysis.

Tectonics and Seismicity of the Lunar South Polar Region

The lunar south pole regions are subjected to global stresses that result in contractional deformation and associated seismicity. This deformation is mainly expressed by lobate thrust fault scarps; examples are globally distributed, including polar regions. One small cluster of lobate scarps falls within the de Gerlache Rim 2 Artemis III candidate landing region. The formation of the largest de Gerlache scarp, less than 60 km from the pole, may have been the source of one of the strongest shallow moonquakes recorded by the Apollo Passive Seismic Network. The scarp is within a probabilistic space of relocated epicenters for this event determined in a previous study. Modeling suggests that a shallow moonquake with an M w of ∼5.3 may have formed the lobate thrust fault scarp. We modeled the peak ground acceleration generated by such an event and found that strong to moderate ground shaking is predicted at a distance from the source of at least ∼40 km, while moderate to light shaking may extend beyond ∼50 km. Models of the slope stability in the south polar region predict that most of the steep slopes in Shackleton crater are susceptible to regolith landslides. Light seismic shaking may be all that is necessary to trigger regolith landslides, particularly if the regolith has low cohesion (on the order of ∼0.1 kPa). The potential of strong seismic events from active thrust faults should be considered when preparing and locating permanent outposts and pose a possible hazard to future robotic and human exploration of the south polar region.

Integrating earthquake-based passive seismic methods in mineral exploration: Case study from the Gerolekas bauxite mining area, Greece

As the global need for aluminum constantly rises, bauxite is considered to be a critical mineral, and the mining industry is in search of new and effective exploration solutions. In this context, we design and implement a purely earthquake-based passive seismic survey at the Gerolekas bauxite mining site in Greece. It is a very difficult exploration setting, characterized by rough topography, limited accessibility, and a very complex geotectonic regime. We gather a passive seismic data set consisting of four months of continuous recordings (May to August 2018) from 129 stand-alone 3C seismological stations. We then analyze this data set and extract 848 microearthquakes that will serve as sources for the application of local earthquake tomography (LET) and transient-source seismic interferometry (TSI) by autocorrelation. We apply LET to estimate the 3D P- and S-wave velocity models of the subsurface below the study area and TSI by autocorrelation to retrieve the zero-offset virtual reflection responses below each of the recording stations. The velocity models provide a relatively coarse image of a previously completely unexplored part of the mining concession, whereas the higher-resolution virtual reflection imaging illuminates in detail the different interfaces. We also reprocess three lines of legacy active seismic data that were shot in 2003, using the LET P-wave velocity model for depth migration, and confirm the improvement of seismic imaging. Finally, we evaluate the obtained results using well data and jointly interpret them, extracting useful information on the expected target depths and indicating that earthquake-based passive seismic techniques can be an innovative and environmentally friendly option for mineral exploration.

Soil characterization of Babol City using single-station ambient seismic noise method

The interrelation between the local site conditions and the severity of destruction during earthquakes has increased the necessity for more investigations on soil characterization in recent years. In residential areas, where active seismic methods are problematic for defining soil properties, passive seismic methods have attracted considerable attention due to their cost-effective and non-invasive features. Therefore, we have utilized 35 single-station ambient seismic noise recordings in the seismically vulnerable city of Babol, Iran, to evaluate the site response (including azimuthal variation and resonance frequency), estimate the seismic vulnerability index (Kg), and classify the soil based on the Vs30 of the region. To address the aim, we extract Rayleigh wave ellipticities from ambient seismic noise using their polarization features and employ them to define the azimuthal variations, resonance frequencies, and Kg values of the region. In each station, the ellipticity curve is inverted with the support of supplementary geotechnical information to obtain a shear velocity profile. The inversion results have good consistency with borehole and downhole test information.The study identifies a low resonance frequency of 0.9 Hz with a NE directivity for the majority of the city and a Vs30 range of 180 m/s to 350 m/s. Moreover, the seismic bedrock of Babol City is represented by a 3D Vs model at a depth of 60 m. Our results reveal that the city is vulnerable to seismic hazards due to the thick and low-velocity deposits responsible for high amplification affecting the study region, which combines with a high seismic vulnerability index, particularly in the southern part. This work has applications for future structural design, urban development, and seismic hazard reduction.

Noise and site response characterization in Makassar Strait and Celebes Sea based on Ocean Bottom Seismometer

To overcome the limited information in the northern waters of Sulawesi Island, ocean bottom seismometers (OBS) were installed in the northern part of the Makassar Strait and the Sulawesi Sea. OBS data were analysed using passive seismic methods to determine the characteristics of noise and site response in the Makassar Strait and the Celebes Sea. Probabilistic Power Spectral Density (PPSD) from OBS station showed microtremor in the period 0.06 – 0.7 s, secondary microseismic peak in the period 0.9 – 2 s, secondary microseismic peak in the period 4 – 8 s, and hum in the period more than 10 s. The amplitude of the noise power is in the range of New Low Noise Model (NLNM) and New High Noise Model (NHNM) for a period <10 s and slightly above NHNM for a period >10 s for OBS IGGCAS F-type. For OBS IGGCAS G-type, the noise power amplitude is in the period < 0.7 s in the NLNM and NHNM ranges. However, there is a tendency for the noise power amplitude to be flat with an amplitude above NHNM for a period > 0.7 s. The noise power’s amplitude depends on the station’s location, climate, instruments, daily variations, and seasonal variations. Horizontal-to-Vertical Spectral Ratio (HVSR) in the North Makassar Strait shows an H/V curve with an unclear peak in the middle of the strait and multiple peaks in the eastern part of the strait with a frequency range of 1.35 – 4.7 Hz. Between the Makassar Strait and the Celebes Sea, the H/V curve shows a flat peak and multiple peaks with a frequency range of 1.10 – 4.03 Hz. Meanwhile, in the Celebes Sea, the H/V curve shows an unclear peak (unclear peak) and several peaks (multiple peaks) with a frequency range of 0.65 – 6.25 Hz.

Detecting gas upwelling hazards in coastal areas through integration of active and passive electrical and seismic methods (Fiumicino, Central Italy)

The accurate location of gas upwelling flows is still an open problem for non-invasive imaging techniques in populated areas. Gas blowouts of deep origin may represent a serious threat to human health in urban areas and should be correctly imaged with high-resolution for assessing the related hazards. In this work, we propose an integration of active (electrical resistivity tomography and high-resolution sub-bottom profiling complemented with the multibeam bathymetry) and passive (self-potential and ambient noise recordings) geophysical methods to image gas upwelling flows in the coastal area of Fiumicino (Central Italy), where the gas presence is well-documented by previous works. We demonstrate that merging seismic sub-bottom profiling and electrical resistivity tomography has enormous diagnostic potential for gas detection, since they combine the high resolution needed to correctly image the subsurface and the interfaces between different media with the high diagnostic capability of electrical methods to detect anomalies associated with the gas emissions. Passive seismic methods complement the analysis enabling an estimation of the shear-wave velocity through array measurements. Finally, the reconstruction of the natural electrical sources, inferred from the inversion of self-potential data, confirms the location of the near-surface gas upwelling flows assessed through the resistivity model. This work demonstrates that the integration of high-resolution active and passive seismic and electrical methods can be an effective choice for the accurate location of risk-prone areas by imaging the near surface gas pathways where borehole drilling is strongly limited if not forbidden.

Dictionary Learning for Single-Channel Passive Seismic Denoising

Abstract Passive seismic denoising is mostly performed using a simple band-pass filter, which can be problematic when signal and noise share the same frequency band. More advanced passive seismic denoising methods take advantage of fixed-basis transforms, for example, the wavelet, to remove noise. Here, we present an open-source package for data-driven denoising based on adaptively learning sparse transform. Contrary to the fixed-basis transforms, the proposed method belongs to the adaptive-basis transforms. We learn the 1D features embedded in the passive seismic data from all the available waveform data sets without requiring spatial coherency in a data-driven way. Thus, the new method is flexible to apply in any passive seismic monitoring project because of its data-driven and single-channel nature when implemented. Considering the computationally expensive K-singular-value-decomposition (KSVD) in the traditional dictionary learning framework, we suggest applying a fast SVD-free dictionary learning method that can be readily applicable to process massive seismic data during passive seismic monitoring. The proposed method is applied to two synthetic data examples and three real passive seismic data sets to demonstrate its effectiveness in improving the signal-to-noise ratio, and its potential in applications like arrival picking. The open-source reproducible package can be found in the Data and Resources section.

Pre-commercial baseline passive seismic monitoring around CarbonNet's Pelican Site in the offshore Gippsland Basin, Victoria—The first five years

We undertook the design, development, and operation of a two-scale high-performance seismic network in SE Victoria to establish a baseline for natural seismicity around CarbonNet's Pelican Site. This is particularly important for providing assurance of the safety of the CO2 storage space and discriminating between natural and induced seismicity. The new high-precision earthquake catalogue developed from this work demonstrates a significant improvement in monitoring passive seismicity; a 77% average increase in annual event detections relative to pre-2017, magnitude of completeness lowered from 1.2 to 0.7, a minimum detection magnitude of -0.5, and location errors of ≤ ∼1 km within the network. Using the new earthquake catalogue, we also discovered the hitherto unknown deepest seismogenic zone in Australia in the offshore Gippsland Basin, with event depths exceeding 25 km. The spatial distribution of earthquakes indicates no seismic activity within a ∼10 km radius around the Pelican Site, providing confidence in its selection for potential CCS development. Together with the spatial distribution of earthquakes, newly estimated and more robust earthquake magnitude-frequency parameters enable the discrimination of natural versus induced seismicity with greater confidence. Continuous monitoring is recommended to overcome the limitations of the short monitoring time window.

Reconstruction of Fault Architecture in the Natural Thermal Spring Area of Daruvar Hydrothermal System Using Surface Geophysical Investigations (Croatia)

The sustainable utilization of geothermal energy mostly depends on the characteristics of the geothermal resource from which it is extracted. Among others, detailed geological modeling is a key factor for estimating the potential of a geothermal resource. This research focuses on the modeling and reconstruction of the geological setting of the Daruvar thermal spring area using geophysical techniques. An integrated geophysical approach based on electrical resistivity tomography (ERT) and both active and passive seismic (MASW and HVSR) methods was used. Based on ERT results and the stratigraphic logs of the wells in Daruvar, three resistivity layers/geological units were identified. The deepest layer with resistivity &lt; 150 Ωm is the Triassic carbonate that constitutes the thermal aquifer. Sharp lateral variations in the resistivity distributions within the bedrock were interpreted as fault damage zones saturated with thermal waters. Integrating the results of the seismic methods, the thickness of the first seismic layer that corresponds to the Quaternary cover was estimated from 5 to 20 m. Here, results of the geophysical investigations were combined into a 3D geological model highlighting the occurrence of subvertical N-S and E-W trending faults in the Daruvar spring area. The N-S-trending fault was interpreted as a fault plane parallel to the regionally mapped Daruvar fault. This fault juxtaposes the Triassic carbonate complex of the thermal aquifer with a Neogene sedimentary sequence of significantly lower permeability. Neogene–Quaternary tectonic activity further increased the fracturing and the permeability field in the Daruvar spring area, as proven by the smaller scale E-W faults and the well logs. This fracture network permits a quick upwelling of thermal fluids resulting in thermal springs with temperatures up to 50 °C. This work proves that the construction of a detailed geological model is crucial for assessing the reservoir and fault geometries in thermal systems hosted in fractured carbonate rocks.

Preliminary Results on Low Frequency Passive Seismic (LFPS) Study in Lake Toba

Sumatra Island is an island located in the subduction zone of the Indo-Australian plate towards Eurasia. The geological structure on the island of Sumatra is the Semangko Fault. There is active volcanic activity, such as the Bukit Barisan Mountains, which makes Sumatra Island vulnerable to volcanic eruptions. The study was conducted in Lake Toba, North Sumatra, using earthquake data from the IRIS station. The purpose of this study is to investigate the amplitude in determining the presence of fluid reservoirs (magma) and to analyze the volcanics of the Lake Toba region using the Low Frequency Passive Seismic (LFPS) method by utilizing attributes such as maximum amplitude Vertical-to-Horizontal Spectral Ratio (VHSR), maximum amplitude of Power Spectral Density (PSD) Z, and the maximum amplitude spectrum of the Z component, which are used as indicators to connect the subsurface fluid reservoir by looking at the spectrum curve. Previous research has linked the presence of a reservoir to the peak amplitude spectrum in the vertical component in the frequency range 1-6 Hz. Based on the research results from the analysis of all attribute parameters obtained, the maximum anomaly of the maximum VHSR amplitude spectrum, the maximum PSD Z amplitude, and the maximum Z component amplitude spectrum indicate that the location with the highest subsurface fluid reservoir potential is in the Northwest-Southeast around Lake Toba and Samosir island. These results support previous research. Volcanic activity in Lake Toba is relatively lower because the amplitude spectrum values of all attribute parameters are not too high.

Characteristic evaluation of passive seismic ellipticity method in urban shield tunnel

In recent years, engineering examples have shown that traditional surface-wave detection methods are easily limited in areas with small space and heavy traffic. The horizontal-to-vertical spectral ratio survey method has become increasingly prevalent in site measurements and is favored by geophysicists for its ability to detect near-surface velocity structures with greater sensitivity than phase velocities. Based on the phase difference method and polarization filtering, we proposed a passive seismic ellipticity method. The objective of this method is to extract the Rayleigh waves from ambient noise recordings and the analysis of ellipticity dispersion characteristic. This method was applied to an actual case study. Specifically, the velocity dispersion profile of the site is obtained using the active-source surface-wave method. Then, we compared the results obtained by the passive seismic ellipticity method with those obtained by the active-source surface-wave method and the field data, to evaluate the accuracy of results. The results demonstrate that the proposed method can effectively detect the location of subsurface shield tunnels and has the characteristics of simple data processing steps, less interference, and high stability of results. This demonstrates the necessity of using the phase difference method to select segments dominated by Rayleigh waves in ambient noise recordings and applying polarization filtering before calculating ellipticity. The paper provides an actual case study for the application and promotion of the passive seismic ellipticity method and has significant practical value and reference significance.

Understanding slow-moving landslide triggering processes using low-cost passive seismic and inclinometer monitoring

Landslides are a major natural hazard, threatening communities and infrastructures worldwide. Mitigation of these hazards relies on understanding their causes and triggering processes, which critically depend on subsurface characteristics and their variations over time. In this study, we present a novel approach combining passive seismic and low-cost inclinometer monitoring methods to improve the understanding of landslide activation mechanisms and their controls. We evaluate the efficiency of this approach on a shallow, slow-moving landslide directly endangering a road bridge, a bridge that is part of an important emergency response route. Results show the value of combining the two approaches for observing and monitoring landslide hazards. Passive seismic monitoring captures the variation in soil properties (rigidity and density) over time by sensing the variations of the seismic wave velocity (dV/V and its associated correlation coefficient). At the same time, novel low-cost inclinometers are monitoring subsurface deformation (from millimetric to pluricentimetric scale) and temperature. Seismic precursors detected at the bottom sensor a few hours prior to the reactivation are followed by the reactivation of the landslide toe, releasing stresses in the top part that lead to the reactivation of the whole landslide. This reactivation occurs during an episode of heavy rainfall following a 7-month drought. Meanwhile, temperature monitoring enables us to track water infiltration and to highlight its role in the landslide mechanisms. Overall, the combination of the two monitoring methods shows promise for quantifying the sliding mechanisms of landslide reactivations and for designing landslide early warning systems.

Real-time tunneling risk forecasting using vibrations from the working TBM

Tunnel Boring Machines (TBMs) are effective tools for excavating tunnels through various soil and rock strata, especially in complex urban areas. During excavating, hazards overlooked during site investigation, such as abrupt geological changes, buried obstacles like boulders and building foundations, fault zones, and water-bearing regions, significantly increase the risks of cutter damage and tunneling accidents. Geophysical methods have been proposed during tunnel excavation to predict these potential hazards ahead of the cutter face. With the existence of a metallic TBM and subsequently installed concrete tunnel lining, the performance of electromagnetic geophysical methods such as ground-penetrating radar and DC resistivity tomography are greatly hampered, despite their success in the traditional open tunnels. In this study, we present a passive seismic method to forecast the tunneling risks ahead of the TBM cutterhead. This method uses the working TBM as the source and predicts the velocity/density changes in front of the TBM by delineating the reflected seismic energy from the noisy recordings. The key to this technology is seismic interferometry, which transforms the chaotic ambient noise into interpretable seismic profiles. We utilize beamforming and semblance analysis to extract the traveltime information from the passive records. After velocity inversion and calibration, we map the arrival-time of a reflection event to a seismic impedance interface ahead of the cutterface. To test the proposed method, we demonstrate the process on a simulated passive seismic record after laying out its theoretical foundations. We present a field application of our method where geological interfaces are predicted in real time as the excavation progresses. The field data results match well with the muck density and TBM parameters measured after excavation and hence demonstrate the applicability of our method in complex urban environments for tunneling risk reduction.