Ambient Noise Seismic Interferometry 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.

Multichannel Analysis of Surface Waves based on Common Virtual Source Gathers of Seismic Ambient Noise Cross-Correlations: A Case Study at an Earth Dam in Brazil

The S-wave velocity (Vs) is a valuable parameter for assessing the mechanical properties of subsurface materials for geotechnical purposes. Seismic surface wave methods have become prominent for estimating near-surface Vs models. Researchers have proposed methods based on passive seismic signals as efficient alternatives to enhance depth of investigation, lateral resolution and reduce field effort. This study presents the Multichannel Analysis of Surface Waves (MASW) utilizing Common Virtual Source Gathers (CVSGs) derived from seismic ambient noise cross-correlations, based on Ambient Noise Seismic Interferometry concepts. The method is applied to passive data acquired with an array of receivers at the Paranoá earth dam in Brasília, Brazil, to construct a pseudo-2D Vs image of the massif for interpretation. Our findings showcase the adopted processing flow and combination of methods as an effective approach for near-surface Vs estimation, demonstrating its usability also for large earth dam embankments.

Utilization of ambient noise seismic interferometry to retrieve P‐wave reflections at Soma coal basin, Western Turkey

AbstractFollowing ambient noise seismic interferometry principles, we retrieved virtual zero‐offset vertical seismic profile shot gather using continuous seismic records obtained simultaneously during an active‐source data acquisition study from Soma coal basin, Western Turkey. We treated the data to include pure ambient noise by eliminating earthquakes, surface sweeps and quarry explosions due to mining activities. We have implemented a series of data processing steps, including spectral normalization and filtering to increase the signal‐to‐noise ratio. We produced a zero‐offset virtual shot gather by cross‐correlation and cross‐coherence methods in a coal basin for the first time and we observed the P‐wave first arrivals and reflections from the deepest coal layer (km2). Our results demonstrate that the ambient noise seismic interferometry method can serve as an alternative approach for seismic imaging and monitoring in areas where the use of active seismic sources is limited due to field conditions, provided that there are sufficient and appropriate noise sources at the wellhead.

In-situ monitoring of rock slope destabilization with ambient seismic noise interferometry in southwest China

The damage evolving on rock slopes, including unstable mass failure events, is expected to be associated with a rigidity loss of geomaterials. Therefore, velocity changes (dv/v) inferred from ambient noise interferometry allow remotely assessing the damage state of the slope. However, the time scale and amplitude of the dv/v evolution associated with the slope destabilizations are not well known, and their detection and characterization require both high precision and high temporal resolution in dv/v. We herein report an in-situ one-year monitoring of the dv/v on the slow-moving Pubugou rock slope with a high temporal resolution up to 20-min by using two seismic stations in the mountainous region of Sichuan, China, where the slope is affected by both earthquakes and rainfall. The results confirm the feasibility of (1) detecting a possible precursor dv/v drop (−0.5%) ∼40 min before a slope destabilization; (2) characterizing the dv/v caused by the superimposed effects of moderate earthquakes and rainfall; and (3) quantifying the daily dv/v fluctuations of the slope caused by the environmental thermal forcing through thermoelastic effects. In addition, the measurement error and the detectable depth resolution of the dv/v are also addressed in this study. Finally, we suggest the high-resolution technique as a complementary means to be used for operational rock slope damage surveillance in the future.

Correlations of Seismic Velocities and Elastic Moduli with Temperature in Superhot and Enhanced Geothermal Systems

This paper presents correlations derived by linear regression analysis of seismic velocities VP and VS and elastic moduli EP and ES with temperature in Los Humeros superhot (SHGS) and Acoculco enhanced (EGS) geothermal systems at depths down to 3 km below the surface and temperatures up to approximately 400 °C. In Los Humeros, the seismic velocity models were derived from the inversion of legacy active seismic survey data acquired in 1998, as well as from passive seismic monitoring and ambient seismic noise interferometry carried out during 2017–2019 by the GEMex consortium. In the Acoculco EGS, ambient seismic noise data were used. Steady-state formation temperatures were re-evaluated during and after the end of the GEMex project using measurements provided as a courtesy of the Federal Electricity Commission of Mexico (CFE). The density data needed for the calculation of elastic moduli were provided by the GEMex consortium, as derived from the inversion of regional and local gravity surveys. The analysis indicated that statistically significant correlations of seismic parameters to temperature exist in the vertical direction, namely exponential in Los Humeros superhot and logarithmic in Acoculco EGS, but no correlation was evident in the horizontal direction. This result suggests an indirect relationship among the considered variables due to interdependence on other parameters, such as pressure and vapor saturation. As the analysis was performed using only data obtained from sensing-at-surface methods, without direct geophysical calibration at depth, a distributed fiber-optic seismic and temperature sensing system at both surface and downhole is proposed for active-source and passive seismic monitoring, and seismic-while-drilling by the drill-bit source is considered for reverse vertical seismic profile (RVSP) recording whenever possible for future high-temperature geothermal applications.

Passive Seismic Structure Imaging of a Coal Mine by Ambient Noise Seismic Interferometry on a Dense Array

Passive Seismic Structure Imaging of a Coal Mine by Ambient Noise Seismic Interferometry on a Dense Array

2D body-wave seismic interferometry as a tool for reconnaissance studies and optimization of passive reflection seismic surveys in hardrock environments

Despite the unrivalled spatial resolution and depth penetration of active-source seismic methods used for mineral exploration in hardrock environment, economic and environmental restrictions (e.g., source permitting) may preclude its full-scale application. In such a case, 2D passive reflection seismics can be considered a cost-effective way to perform reconnaissance-type survey and provide body-wave structural imaging using ambient-noise seismic interferometry (ANSI). This is, however, conditional to the presence of noise sources in the subsurface, for example produced by underground mining activity. Here, we propose a 2D ANSI workflow as an intermediate step prior to a full-scale 3D ANSI survey and an affordable tool in brownfield exploration, e.g., when trying to update current geological models beyond the drilled area. We test the applicability of this approach by analysing selected receiver lines from a 3D passive dataset acquired over the Kylylahti mine in Finland. Our methodology aims at choosing the optimal processing strategy at possibly lowest acquisition (2D geometry) and computational (small amount of data) cost. We address the fundamental questions in ANSI, i.e., (i) how much AN should one record and (ii) which SI processing approach should one choose. Therefore, we test different processing steps necessary to produce virtual shot gathers (VSG): preprocessing, selection of the ambient-noise portion, and selection of the method for retrieving the impulse responses between the receivers (crosscorrelation - CC, crosscoherence - CCh, multidimensional deconvolution - MDD). We conclude that trace energy normalization and high-pass filtering are the preferred preprocessing steps, while the best imaging is obtained when VSGs are retrieved using MDD applied in the noise-volume approach or CC in the event-driven approach. An event-driven approach may significantly reduce the acquisition time: for the Kylylahti dataset, using 10 events with energetic body-wave arrivals, extracted from one hour of data, was enough to provide results comparable to the results from the noise-volume approach using the complete one hour of noise.

A linear algorithm for ambient seismic noise double beamforming without explicit crosscorrelations

Geoscientists and engineers are increasingly using denser arrays for continuous seismic monitoring, and they often turn to ambient seismic noise interferometry for low-cost near-surface imaging. Although ambient noise interferometry greatly reduces acquisition costs, the computational cost of pair-wise comparisons between all sensors can be prohibitively slow or expensive for applications in engineering and environmental geophysics. Double beamforming of noise correlation functions is a powerful technique to extract body waves from ambient noise, but it is typically performed via pair-wise comparisons between all sensors in two dense array patches (scaling as the product of the number of sensors in one patch with the number of sensors in the other patch). By rearranging the operations involved in the double beamforming transform, I have developed a new algorithm that scales as the sum of the number of sensors in two array patches. Compared to traditional double beamforming of noise correlation functions, the new method is more scalable, easily parallelized, and it does not require raw data to be exchanged between dense array patches.

Spatial and temporal influence of rainfall on crustal pore pressure based on seismic velocity monitoring

Crustal pore pressure, which controls the activities of earthquakes and volcanoes, varies in response to rainfall. The status of pore pressure can be inferred from observed changes in seismic velocity. In this study, we investigate the response of crustal pore pressure to rainfall in southwestern Japan based on time series of seismic velocity derived from ambient noise seismic interferometry. To consider the heterogeneity of the area, rainfall and seismic velocity obtained at each location were directly compared. We used a band-pass filter to distinguish the rainfall variability from sea level and atmospheric pressure, and then calculated the cross-correlation between rainfall and variations in S-wave velocity (Vs). A mostly negative correlation between rainfall and Vs changes indicates groundwater recharge by rainfall, which increases pore pressure. The correlations differ between locations, where most of the observation stations with clear negative cross-correlations were located in areas of granite. On the other hand, we could not observe clear correlations in steep mountain areas, possibly because water flows through river without percolation. This finding suggests that geographical features contribute to the imprint of rainfall on deep formation pore pressure. We further modelled pore pressure change due to rainfall based on diffusion mechanism. A strong negative correlation between pore pressure estimated from rainfall and Vs indicates that the Vs variations are triggered by pore pressure diffusion in the deep formation. Our modelling results show a spatial variation of diffusion parameter which controls the pore pressure in deep formation. By linking the variations in seismic velocity and crustal pore pressure spatially, this study shows that seismic monitoring may be useful in evaluating earthquake triggering processes or volcanic activity.

Upper-Crustal Seismic Anisotropy in the Cantabrian Mountains (North Spain) from Shear-Wave Splitting and Ambient Noise Interferometry Analysis

AbstractThe upper-crustal anisotropy of the Cantabrian Mountains (North Spain) has been investigated using two independent but complementary methodologies: (a) shear-wave splitting and (b) ambient seismic noise interferometry. For this purpose, we have processed and compared seismic data from two networks with different scales and recording periods. The shear-wave splitting results show delay times between 0.06 and 0.23 s and spatially variable fast-polarization directions. We calculate that the anisotropic layer has a maximum effective thickness of around 7.5 km and an average anisotropy magnitude of between 4% and 8%. Consistently, our ambient noise observations point to an anisotropy magnitude between 4% and 9% in the first 10 km of the crust. Our results show a clear correlation between the fast directions from both methods and the orientations of the local faults, suggesting that the anisotropy is mainly controlled by the structures. Furthermore, in the west of the study area, fast-polarization directions tend to align parallel to the Variscan fabric in the crust, whereas to the east, in which the Alpine imprint is stronger, many fast directions are aligned parallel to east–west-oriented Alpine features.

Using Ambient Noise Seismic Interferometry and Local and Teleseismic Earthquakes to Determine Crustal Thickness and Moho Structure of the Northwestern Gulf of Mexico Margin

AbstractThe northwestern part of the Gulf of Mexico has undergone two episodes of continental rifting and collision and produced structural artifacts that are now buried under many kilometers of sediments, complicating investigations of the region. The deep sedimentary package precludes outcrops and points to a need for the application of seismic techniques, but low rates of seismicity in the region and sparse seismic monitoring limit the utility of traditional seismic methods. We therefore use diverse data to perform two‐dimensional seismic tomography across the dry land portion of the margin. Data are gleaned from teleseismic and regional earthquakes, postcritical SsPmp arrivals, and direct P wave energy identified with seismic interferometry that were recorded by a broadband, three‐component array and partially overlapping short‐period, vertical‐component array. The Pn and postcritical SsPmp phase help constrain the Moho discontinuity, which a previous receiver function study suggested was absent beneath the seaward portion of this transect. A high‐velocity body is observed in the crust at the same location as the Houston Magnetic Anomaly, possibly marking rocks from the Alleghenian continental assembly. The crust thins from NW to SE, indicating that extension occurred mostly to the south of the Ouachita orogeny. Our model indicates that the margin's sediment package reaches a maximum thickness of ~15 km at the coast and becomes unresolvably thin near the Llano Uplift.

Seismic velocity variations associated with the 2018 lower East Rift Zone eruption of K\u012blauea, Hawai\u02bbi

The 2018 lower East Rift Zone eruption of Kīlauea (Hawai‘i) marked a dramatic change in the volcano’s 35-year-long rift zone eruption. The collapse of the middle East Rift Zone vent Pu‘u ‘Ō‘ō was followed by one of the volcano’s most voluminous eruptions in 500 years. Over the course of this 3-month eruption, the draining of summit-stored magma led to near-daily collapses of a portion of the caldera and ultimately up to 500 m of summit subsidence. While deformation data indicated that the summit and middle East Rift Zone were inflating for the previous several years, why Pu‘u ‘Ō‘ō collapsed and what initiated down-rift dike propagation remains unclear. Using ambient noise seismic interferometry, we show that a Ml5.3 decollement earthquake beneath Kīlauea’s south flank in June 2017 induced a coseismic decrease of up to 0.30% in seismic velocity throughout the volcano. This velocity decrease may have been caused by dynamic stress–induced shallow crustal fracture, i.e., weakening to dilatant crack growth, and was greatest near Pu‘u ‘Ō‘ō. Additionally, we verify a pre-eruptive increase in seismic velocity, consistent with increasing pressurization in the volcano’s shallow summit magma reservoir. This velocity increase occurred coincident with the first in a series of lower-crustal earthquake swarms, 6 days before a 2-month period of rapid summit and middle East Rift Zone inflation. The increase in up-rift magma-static pressure, combined with the pre-existing weakness from the June 2017 earthquake, may have facilitated down-rift dike propagation and the devastating 2018 eruption.

Evidence of reactivation of a hydrothermal system from seismic anisotropy changes

Seismic velocity measurements have revealed that the Tohoku-Oki earthquake affected velocity structures of volcanic zones far from the epicenter. Using a seismological method based on ambient seismic noise interferometry, we monitored the anisotropy in the Mount Fuji area during the year 2011, in which the Tohoku-Oki earthquake occurred (Mw = 9.0). Here we show that even at 400 km from the epicenter, temporal variations of seismic anisotropy were observed. These variations can be explained by changes in the alignment of cracks or fluid inclusions beneath the volcanic area due to stress perturbations and the propagation of a hydrothermal fluid surge beneath the Hakone hydrothermal volcanic area. Our results demonstrate how a better understanding of the origin of anisotropy and its temporal changes beneath volcanoes and in the crust can provide insight into active processes, and can be used as part of a suite of volcanic monitoring and forecasting tools.

3D S-wave velocity imaging of Reykjanes Peninsula high-enthalpy geothermal fields with ambient-noise tomography

Tomographic imaging based on ambient seismic noise measurements has shown to be a powerful tool, especially in areas like Iceland, where the microseism illumination is excellent. In this paper, we produce a 3D S-wave tomographic image over the western Reykjanes Peninsula high-enthalpy geothermal fields and evaluate the reliability of the tomographic results for different resolutions through simulated and real data. We use 30 broadband stations operating for approximately one-and-a-half year and apply ambient noise seismic interferometry for each station-pair. This results in empirical Green's functions in which especially the ballistic surface waves (BSW) are well resolved. The retrieved BSW exhibit a high signal-to-noise ratio between 0.1 and 0.5 Hz, and the beamforming analysis indicates an apparent surface-wave velocity of 3 km/s over a broad azimuthal range. For the tomographic inversion, we invert the estimated phase velocities between all station pairs to frequency-dependent phase velocity maps in four different resolutions (1, 2, 3, and 4 km) using a Tikhonov regularisation. With the estimated regularisation parameter per frequency per resolution, we invert simulated data for checkerboard sensitivity tests per frequency for different combinations of velocity anomaly sizes and resolutions.Finally, after the inversion to depth, we detect S-wave velocity anomalies with variations between −15% and 15% with reference to an estimated average velocity using 1 km and 3 km of lateral resolutions and 1 km of vertical resolution. This study shows the potential of ambient noise tomography as complementary seismological tool for reservoir characterization.

Decrease in Seismic Velocity Observed Prior to the 2018 Eruption of Kīlauea Volcano With Ambient Seismic Noise Interferometry

AbstractThe 2018 Kīlauea eruption was a complex event that included deformation and eruption at the summit and along the East Rift Zone. We use ambient seismic noise interferometry to measure time‐lapse changes in seismic velocity of the volcanic edifice prior to the lower East Rift Zone eruption. Our results show that seismic velocities increase in relation to gradual inflation of the edifice between 1 March and 20 April. In the 10 days prior to the 3rd of May eruption onset, a rapid seismic velocity decrease occurs even though the summit is still inflating. We confirm that intereruptive velocity change is correlated with surface deformation, while the velocity decrease prior to eruption is likely due to accumulating damage induced by the pressure exerted by the magma reservoir on the edifice. The accumulating damage and subsequent decrease in bulk edifice strength may have facilitated increased transport of magma from the summit reservoir to the Middle East Rift Zone.

Imaging Torfajökull's Magmatic Plumbing System With Seismic Interferometry and Phase Velocity Surface Wave Tomography

AbstractTorfajökull volcano, Iceland, has not erupted since 1477. However, intense geothermal activity, deformation, and seismicity suggest a long‐lasting magmatic system. In this paper, we use ambient noise tomography to image the magmatic system beneath Torfajökull volcano. One hundred days of ambient noise data from 23 broadband seismometers show the consistent presence of double‐frequency microseism noise with significant power between ∼0.1 and 0.5 Hz. Beamforming results indicate microseism noise with persistent higher energy propagating from west and SE directions and apparent velocities below 3 km/s. We use ambient noise seismic interferometry to retrieve Rayleigh waves, and we introduce a method to estimate the reliability of the retrieved surface waves. We find stable estimation of surface wave phase velocities between 0.16 and 0.38 Hz. Azimuthal velocity variations show a trend of higher velocities in the NE/SW direction, the strike of the rift zone intersecting Torfajökull, and orientation of erupted lavas on a NE‐SW fissure swarm. Tomographic results indicate low‐velocity anomalies beneath the volcano caldera (between −5% and −10%) and even lower velocity variations in the southeast and southwest study area (below −10%), outside the volcano caldera. Low anomalies may indicate the existence of hot material, more prominent outside the caldera outskirts. High‐velocity variations (between 5% and 10%) outline the volcano caldera between 4‐ and 5‐km depth and more pronounced velocities (between 10% and 15%) up to 5‐km depth in the north of the volcano caldera. We interpret the former as possible caldera collapse structure and the latest as solidified intrusive magma from the old preferred magma paths.

Ambient noise Love wave tomography at a gold mine tailings storage facility

Tailings storage facilities (TSFs) are some of the most challenging structures to operate in the mining industry. Some of these structures are susceptible to liquefaction and piping failure, and need to be monitored carefully. In this study, ambient seismic noise interferometry is applied to image the internal structure of a tailings storage facility (TSF) that showed signs of increased seepage. Twenty geophones were deployed along a roughly 100 m section of the TSF and recorded continuous seismic data. The ambient noise was used to create Love wave dispersion curves between sensor pairs, which were in turn inverted to estimate the shear wave velocity of the dam wall as a function of depth. The velocity profile indicated the phreatic surface roughly 10 m below the surface, with regions near the centre of the array showing the phreatic surface as close as 3 m below the surface. These areas are spatially well correlated with the area where increased seepage was identified and the results were comparable with cone penetration tests that were performed in the area. The study showed that the analysis of ambient seismic noise can be a cost-effective, fast and non-invasive method to image the internal structure of TSFs.

Studying CO2 storage with ambient-noise seismic interferometry: A combined numerical feasibility study and field-data example for Ketzin, Germany

Seismic interferometry applied to ambient-noise measurements allows the retrieval of the seismic response between pairs of receivers. We studied ambient-noise seismic interferometry (ANSI) to retrieve time-lapse reflection responses from a reservoir during [Formula: see text] geologic sequestration, using the case of the experimental site of Ketzin, Germany. We applied ANSI to numerically modeled data to retrieve base and repeat reflection responses characterizing the impedances occurring at the reservoir both with and without the injection of [Formula: see text]. The modeled data represented global transmission responses from band-limited noise sources randomly triggered in space and time. We found that strong constraints on the spatial distribution of the passive sources were not required to retrieve the time-lapse signal as long as sufficient source-location repeatability was observed between the base and the repeat passive survey. To illustrate the potential of the technique, ANSI was applied to three days of passive field data recorded in 2012 at Ketzin. Comparison with the modeled results illustrated the potential to retrieve key reflection events using ANSI on field data from Ketzin. This study supports the idea that the geologic setting and characteristics of ambient noise at Ketzin may be opportune to monitor [Formula: see text] sequestration.

Four Years of Experience with a Permanent Seismic Monitoring Array at the Ketzin CO2 Storage Pilot Site

CO2 was injected into a saline aquifer near the town of Ketzin in Germany from July 2008 to August 2013. To monitor CO2- migration close to the injection well, TNO installed a fixed 2D seismic array of 120 meters length in 2009, with 3- component (3- C) geophones at the surface, 4-component receivers at 50 meters depth and a central vertical array of 4-component receivers. The test-bed is used both for the recording of high-quality active time-lapse seismic data as well as for continuous passive seismic data recording. Here we focus on the latest results obtained from active source experiments, micro-seismic data analysis and ambient noise seismic interferometry (ANSI) of passive seismic data. Data from two experiments with a source driven by linear motors (permanent) and an impact hammer source (semipermanent) were evaluated in order to assess the repeatability of the two source types.The repeatability of the first source was relatively high compared to the second source. After the stop of injection in August 2013, active seismic shots were acquired repeatedly on a daily basis. These data did not show significant impedance changes at reservoir level. Microseismic event analysis of passive seismic data detected a few local events originating from reservoir depth. In order to quantify these events the seismic array was successfully calibrated by using independent earthquake recording. This yielded estimates of the local magnitude for local events within the range [-2.5,0.5]. Recent work on ANSI focused on the application of this method on both synthetic and field data. The resulting reflection response shows a strong resemblance with outcomes from active shot reflection profiles, and the key reflectors at reservoir level have comparable characteristics

Ambient seismic noise interferometry in Hawai'i reveals long-range observability of volcanic tremor

Ambient seismic noise interferometry in Hawai'i reveals long-range observability of volcanic tremor