Ambient noise tomography of Gran Canaria island (Canary Islands)

2021 ◽  
Author(s):  
Iván Cabrera Pérez ◽  
Jean Soubestre ◽  
Luca D'Auria ◽  
Germán Cervigón-Tomico ◽  
David Martínez van Dorth ◽  
...  
Keyword(s):  
Group Velocity ◽  
Canary Islands ◽  
Ambient Noise ◽  
Velocity Model ◽  
Dispersion Curves ◽  
Ambient Noise Tomography ◽  
Gran Canaria ◽  
Geothermal Reservoirs ◽  
Velocity Models ◽  
Non Linear

<p>The island of Gran Canaria is located in the Canarian Archipelago, with an area of 1560 km<sup>2 </sup>and a maximum altitude of 1956 m.a.s.l., being the third island of the archipelago in terms of extension and altitude. The island has two very well differentiated geological domains: the southwest domain or Paleo-Canarias, which is the geologically oldest part, and the northeast domain or Neo-Canarias, where are located the vents of the most recent Holocene eruptions. This volcanic island hosted Holocene eruptions. Therefore, apart from being affected by volcanic risk, it potentially hosts geothermal resources that could be exploited to increase the percentage of renewable energy in the Canary Islands.</p><p>The main objective of this work is to use Ambient Noise Tomography (ANT) for retrieving a high-resolution seismic velocity model of the first few kilometres of the crust, to improve local earthquake location and detect anomalies potentially related to active geothermal reservoirs. Currently, the 1-D velocity model of the island does not allow a correct determination of the hypocenters, being unable to take into account the substantial horizontal velocity contrasts correctly.</p><p>To realize the ANT, we deployed 28 temporary broadband seismic stations in two phases. Each campaign lasted at least one month. We also exploited data recorded by the permanent seismic network Red Sísmica Canaria (C7) operated by INVOLCAN. After applying standard data processing to retrieve Green’s functions from ambient noise cross-correlations, we retrieved the dispersion curves using the FTAN (Frequency Time ANalysis) technique. The inversion of dispersion curves to obtain group velocity maps was realized using a novel non-linear multiscale tomographic approach (MAnGOSTA, Multiscale Ambient NOiSe TomogrAphy). The forward modelling of surface waves traveltimes was implemented using a shortest-path algorithm that allows the topography to be taken into account. The MANgOSTA method consists of successive non-linear inversion steps on progressively finer grids. This technique allows retrieving 2-D group velocity models in the presence of substantial velocity contrasts with up to 100% of the relative variation. Then, we performed a depth inversion of the Rayleigh wave dispersion curves using a transdimensional Bayesian formulation. The final result is a 3-D model of P- and S-wave velocities of the island. The preliminary results show the presence of a low-velocity zone in the eastern part of the island that coincides spatially with anomalies observed in previous geophysical and geochemical studies and which could be related to actual or fossil geothermal reservoirs. Furthermore, the model shows the presence of high-velocity anomalies that are associated with the mafic core of the island.</p>

Velocity and attenuation models of Tenerife and La Palma (Canary Islands, Spain) through Ambient Noise Tomography.

2020 ◽  
Author(s):  
Iván Cabrera ◽  
Jean Soubestre ◽  
Luca D'Auria ◽  
Edoardo Del Pezzo ◽  
José Barrancos ◽  
...  
Keyword(s):  
Group Velocity ◽  
Canary Islands ◽  
Ambient Noise ◽  
Dispersion Curves ◽  
Ambient Noise Tomography ◽  
Geothermal Exploration ◽  
La Palma ◽  
Cross Correlations ◽  
Attenuation Models ◽  
Volcanic Islands

<p>Tenerife and La Palma are active volcanic islands belonging to the Canarian archipelago. The island of La Palma is the most occidental and volcanically active island of the archipelago. The youngest volcanic rocks are located in the Cumbre Vieja volcanic complex, a fast-growing North-South ridge in the southern half part of the island. On the other hand, the central part of Tenerife island hosts the Teide composite volcano, the third tallest volcano on Earth measured from the ocean floor. The volcanic system of the island extends along three radial dorsals, where most of the historical eruptions occurred. Those two volcanic islands have potential geothermal resources that could be exploited to increase the percentage of renewable energy in the Canary Islands.</p><p> </p><p>The main objective of this work is the use of Ambient Noise Tomography (ANT) to determine high-resolution seismic velocity and attenuation models of the first few kilometres of the crust, in order to detect anomalies potentially related to active geothermal reservoirs. In the case of Tenerife, previous tomographic studies were performed on the island using active seismic data. They allowed to image the structure of the first 8 km depth. However, for the purpose of geothermal exploration, a higher spatial resolution is needed for the first few kilometres and the determination of the shear wave velocity has a particular importance when searching for fluid reservoirs. In the case of La Palma, no seismic tomography was performed yet.</p><p> </p><p>To realize the ANT, we deployed temporary broadband seismic networks in the two islands. In total, we deployed seismic stations on 41 measurements points in Tenerife and 23 points in La Palma. The campaigns lasted at least 1 month, using jointly the permanent seismic network Red Sísmica Canaria (C7) operated by INVOLCAN. After performing standard data processing to retrieve Green’s functions from cross-correlations of ambient noise, we retrieved the dispersion curves using the FTAN (Frequency Time ANalysis) technique. The inversion of dispersion curves to obtain group velocity maps was performed using a novel non-linear multiscale tomographic approach. The forward modelling of surface waves traveltimes was implemented using a shortest-path algorithm which takes the topography into account. The method consists of progressive non-linear inversion steps at increasing resolution. This technique allows retrieving 2D group velocity models in presence of strong velocity contrasts with up to 100% of relative variation.</p><p> </p><p>In parallel with velocity model, we retrieved maps of seismic attenuation (i.e. quality factor Q) retrieved from the coda envelope decay of noise cross-correlations (Q-coda). For each source-receiver pair, a Q-coda value was calculated, and mapped to the target area by using 2D empirical sensitivity kernels for diffusion (Del Pezzo and Ibañez, 2019). We compared 2D velocity and attenuation images at different dominant periods, evidencing structural features for Tenerife and La Palma islands which seem to be relevant for the purpose of geothermal exploration.</p>

Upper crustal structure at the KTB drilling site from ambient noise tomography

2020 ◽  
Author(s):  
Ehsan Qorbani ◽  
Irene Bianchi ◽  
Petr Kolínský ◽  
Dimitri Zigone ◽  
Götz Bokelmann
Keyword(s):  
Ambient Noise ◽  
Velocity Model ◽  
Ambient Noise Tomography ◽  
Data Set ◽  
Crustal Rocks ◽  
Velocity Models ◽  
Path Coverage ◽  
Passive Seismic ◽  
Cross Correlations ◽  
Drilling Site

<p>In this study, we show results from ambient noise tomography at the KTB drilling site, Germany. The Continental Deep Drilling Project, or ‘Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland’ (KTB) is at the northwestern edge of the Bohemian Massif and is located on the Variscan belt of Europe. During the KTB project crustal rocks have been drilled down to 9 km depth and several active seismic studies have been performed in the surrounding. The KTB area therefore presents an ideal test area for testing and verifying the potential resolution of passive seismic techniques. The aim of this study is to present a new shear-wave velocity model of the area while comparing the results to the previous velocity models and hints for anisotropy depicted by former passive and active seismological studies. We use a unique data set composed of two years of continuous data recorded at nine 3-component temporary stations installed from July 2012 to July 2014 located on top and vicinity of the drilling site. Moreover, we included a number of permanent stations in the region in order to improve the path coverage and density. Cross correlations of ambient noise are computed between the station pairs using all possible combination of three-component data. Dispersion curves of surface waves are extracted and are then inverted to obtain group velocity maps. We present here a new velocity model of the upper crust of the area, which shows velocity variations at short scales that correlate well with geology in the region.</p>

scholarly journals Velocity structure of the Whataroa Valley using Ambient Noise Tomography

2021 ◽  
Author(s):  
◽  
Andy McNab
Keyword(s):  
Rayleigh Wave ◽  
Group Velocity ◽  
Shear Wave Velocity ◽  
Correlation Functions ◽  
Ambient Noise ◽  
Cross Correlation ◽  
Velocity Dispersion ◽  
Group Velocity Dispersion ◽  
Velocity Model ◽  
Ambient Noise Tomography

<p>This thesis applies ambient noise tomography to investigate the shallow structure of the Whataroa Valley. Ambient noise techniques are applied to continuous seismic recordings acquired on 158 geophones deployed during the Whataroa Active Source Seismic Experiment. Despite only having four days of data, a robust shear-wave velocity model is calculated using a phase-weighted stacking approach to improve the cross-correlation functions' signal-to-noise ratios, allowing for robust velocity measurements to be obtained between periods of 0.3 and 1.8\,s. This yields a database of 12,500 vertical component cross correlation functions and the corresponding Rayleigh wave phase and group velocity dispersion curves. Linearised straight-ray tomography is applied to phase and group velocity dispersion measurements at periods ranging from periods of 0.3 to 1.8\,s. The tomography reveals a velocity that decreases from the vicinity of the DFDP-2B borehole to the centre of the valley. This is interpreted to be the geologic basement deepening towards the centre of the valley. A Monte-Carlo inversion technique is used to jointly invert Rayleigh-wave phase and group velocity dispersion curves constructed from phase and group velocity tomography maps of successively higher periods. Linear interpolation of the resulting 1D shear-wave velocity profiles produces a pseudo-3D velocity model of the uppermost 1,000\,m of the Whataroa Valley. Using sharp increases in velocity to represent lithological change, we interpret two velocity contours at 1,150 and 1,250\,m/s as potential sediment-basement contacts. Depth isocontours of these velocities reveal that the basement deepens towards the centre of the valley, reaching a maximum depth of 400 or 600\,m for the 1,150 and 1,250\,m/s velocity contours respectively. These depths indicate strong glacial over-deepening and have implications for future drilling projects in the Whataroa Valley. A sharp velocity increase of 200\,m/s also occurs at 100\,m depth at the DFDP-2B borehole. We interpret this to be a change in sedimentary rock lithology from fluvial gravels to lacustrine silty sands, related to a change in sedimentary depositional environment.</p>

Upper crustal structure at the KTB drilling site from ambient noise tomography

2021 ◽  
Author(s):  
Ehsan Qorbani ◽  
Irene Bianchi ◽  
Petr Kolínský ◽  
Dimitri Zigone ◽  
Goetz Bokelmann
Keyword(s):  
Ambient Noise ◽  
Velocity Model ◽  
Ambient Noise Tomography ◽  
Data Set ◽  
Crustal Rocks ◽  
Velocity Models ◽  
Path Coverage ◽  
Passive Seismic ◽  
Drilling Site ◽  
Continental Deep Drilling

&lt;p&gt;In this study, we show results from ambient noise tomography at the KTB drilling site, Germany. The Continental Deep Drilling Project, or &amp;#8216;Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland&amp;#8217; (KTB) is at the northwestern edge of the Bohemian Massif and is located on the Variscan belt of Europe. During the KTB project crustal rocks have been drilled down to 9 km depth and several active seismic studies have been performed in the surrounding. The KTB area therefore presents an ideal test area for testing and verifying the potential resolution of passive seismic techniques. The aim of this study is to present a new shear-wave velocity model of the area while comparing the results to the previous velocity models and hints for anisotropy depicted by former passive and active seismological studies. We use a unique data set composed of two years of continuous data recorded at nine 3-component temporary stations installed from July 2012 to July 2014 located on top and vicinity of the drilling site. Moreover, we included a number of permanent stations in the region in order to improve the path coverage and density. We present here a new velocity model of the upper crust of the area, which shows velocity variations at short scales that correlate well with geology in the region.&lt;/p&gt;

scholarly journals Velocity structure of the Whataroa Valley using Ambient Noise Tomography

2021 ◽  
Author(s):  
◽  
Andy McNab
Keyword(s):  
Rayleigh Wave ◽  
Group Velocity ◽  
Shear Wave Velocity ◽  
Correlation Functions ◽  
Ambient Noise ◽  
Cross Correlation ◽  
Velocity Dispersion ◽  
Group Velocity Dispersion ◽  
Velocity Model ◽  
Ambient Noise Tomography

<p>This thesis applies ambient noise tomography to investigate the shallow structure of the Whataroa Valley. Ambient noise techniques are applied to continuous seismic recordings acquired on 158 geophones deployed during the Whataroa Active Source Seismic Experiment. Despite only having four days of data, a robust shear-wave velocity model is calculated using a phase-weighted stacking approach to improve the cross-correlation functions' signal-to-noise ratios, allowing for robust velocity measurements to be obtained between periods of 0.3 and 1.8\,s. This yields a database of 12,500 vertical component cross correlation functions and the corresponding Rayleigh wave phase and group velocity dispersion curves. Linearised straight-ray tomography is applied to phase and group velocity dispersion measurements at periods ranging from periods of 0.3 to 1.8\,s. The tomography reveals a velocity that decreases from the vicinity of the DFDP-2B borehole to the centre of the valley. This is interpreted to be the geologic basement deepening towards the centre of the valley. A Monte-Carlo inversion technique is used to jointly invert Rayleigh-wave phase and group velocity dispersion curves constructed from phase and group velocity tomography maps of successively higher periods. Linear interpolation of the resulting 1D shear-wave velocity profiles produces a pseudo-3D velocity model of the uppermost 1,000\,m of the Whataroa Valley. Using sharp increases in velocity to represent lithological change, we interpret two velocity contours at 1,150 and 1,250\,m/s as potential sediment-basement contacts. Depth isocontours of these velocities reveal that the basement deepens towards the centre of the valley, reaching a maximum depth of 400 or 600\,m for the 1,150 and 1,250\,m/s velocity contours respectively. These depths indicate strong glacial over-deepening and have implications for future drilling projects in the Whataroa Valley. A sharp velocity increase of 200\,m/s also occurs at 100\,m depth at the DFDP-2B borehole. We interpret this to be a change in sedimentary rock lithology from fluvial gravels to lacustrine silty sands, related to a change in sedimentary depositional environment.</p>

scholarly journals Ambient-noise tomography of the Greater Geneva Basin in a geothermal exploration context

2019 ◽  
Vol 220 (1) ◽  
pp. 370-383 ◽  
Author(s):  
Thomas Planès ◽  
Anne Obermann ◽  
Verónica Antunes ◽  
Matteo Lupi
Keyword(s):  
Group Velocity ◽  
Ambient Noise ◽  
Large Scale ◽  
Broad Band ◽  
Vertical Component ◽  
Dispersion Curves ◽  
Geophysical Data ◽  
Data Sets ◽  
Ambient Noise Tomography ◽  
Geothermal Exploration

SUMMARYThe Greater Geneva Basin is one of the key targets for geothermal exploration in Switzerland. Until recently, information about the subsurface structure of this region was mostly composed of well-logs, seismic reflection lines, and gravity measurements. As part of the current effort to further reduce subsurface uncertainty, and to test passive seismic methods for exploration purposes, we performed an ambient-noise tomography of the Greater Geneva Basin. We used ∼1.5 yr of continuous data collected on a temporary seismic network composed of 28 broad-band stations deployed within and around the basin. From the vertical component of the continuous noise recordings, we computed cross-correlation functions and retrieved Rayleigh-wave group-velocity dispersion curves. We then inverted the dispersion curves to obtain 2-D group-velocity maps and proceeded to a subsequent inversion step to retrieve a large-scale 3-D shear-wave velocity model of the basin. We discuss the retrieved features of the basin in the light of local geology, previously acquired geophysical data sets, and ongoing geothermal exploration. The Greater Geneva Basin is an ideal natural laboratory to test innovative geothermal exploration methods because of the substantial geophysical data sets available for comparison. While we point out the limits of ambient-noise exploration with sparse networks and current methodology, we also discuss possible ways to develop ambient-noise tomography as an affordable and efficient subsurface exploration method.

Extracting Long-Period Surface Waves and Free Oscillations Using Ambient Noise Recorded by Global Distributed Superconducting Gravimeters

2020 ◽  
Vol 91 (4) ◽  
pp. 2234-2246
Author(s):  
Hang Li ◽  
Jianqiao Xu ◽  
Xiaodong Chen ◽  
Heping Sun ◽  
Miaomiao Zhang ◽  
...  
Keyword(s):  
Surface Waves ◽  
Group Velocity ◽  
Ambient Noise ◽  
Velocity Dispersion ◽  
Group Velocity Dispersion ◽  
Dispersion Curves ◽  
Free Oscillations ◽  
Long Period ◽  
Superconducting Gravimeters ◽  
Noise Data

Abstract Inversion of internal structure of the Earth using surface waves and free oscillations is a hot topic in seismological research nowadays. With the ambient noise data on seismically quiet days sourced from the gravity tidal observations of seven global distributed superconducting gravimeters (SGs) and the seismic observations for validation from three collocated STS-1 seismometers, long-period surface waves and background free oscillations are successfully extracted by the phase autocorrelation (PAC) method, respectively. Group-velocity dispersion curves at the frequency band of 2–7.5 mHz are extracted and compared with the theoretical values calculated with the preliminary reference Earth model. The comparison shows that the best observed values differ about ±2% from the corresponding theoretical results, and the extracted group velocities of the best SG are consistent with the result of the collocated STS-1 seismometer. The results indicate that reliable group-velocity dispersion curves can be measured with the ambient noise data from SGs. Furthermore, the fundamental frequency spherical free oscillations of 2–7 mHz are also clearly extracted using the same ambient noise data. The results in this study show that the SG, besides the seismometer, is proved to be another kind of instrument that can be used to observe long-period surface waves and free oscillations on seismically quiet days with a high degree of precision using the PAC method. It is worth mentioning that the PAC method is first and successfully introduced to analyze SG observations in our study.

scholarly journals Structure of the crust and uppermost mantle beneath the Sicily channel from ambient noise and earthquake tomography

2020 ◽  
Vol 63 (Vol 63 (2020)) ◽  
Author(s):  
Radia Kherchouche ◽  
Merzouk Ouyed ◽  
Abdelkrim Aoudia ◽  
Billel Mellouk ◽  
Ahmed Saadi
Keyword(s):  
High Velocity ◽  
Ambient Noise ◽  
Velocity Structure ◽  
Dispersion Curves ◽  
Mount Etna ◽  
Uppermost Mantle ◽  
Velocity Models ◽  
Phase Velocity Dispersion ◽  
Sicily Channel ◽  
Tyrrhenian Basin

•  In this work, we study the crust and the uppermost mantle structure beneath the Sicily Channel, by applying the ambient noise and earthquake tomography method. After computing cross-correlation of the continuous ambient noise signals and processing the earthquake data, we extracted 104 group velocity and 68 phase velocity dispersion curves corresponding to the fundamental mode of the Rayleigh waves. We computed the average velocity of those dispersion curves to obtain tomographic maps at periods ranging from 5 s to 40 s for the group velocities and from 10 s to 70 for the phase velocities. We inverted group and phase speeds to get the shear-wave velocity structure from the surface down to 100 km depth with a lateral resolution of about 200 km. The resulted velocity models reveal a thin crust with thickness value of 15 km beneath the southern part of the Tyrrhenian basin and a thickness value of 20 km beneath Mount Etna. The obtained thickness values are well correlated with the reported extension of the Tyrrhenian lithosphere due to the past earthquake tomography subduction and rollback of the Ionian slab beneath the Calabrian Arc. The crustal thickness increases and reaches values between 28 and 30 km beneath the Tunisian coasts and Sicily Channel. The S-wave models reveal also the presence of high velocity body beneath the island of Sicily. This finding can be interpreted as the presence of the Ionian slab subducting beneath the Calabrian Arc. Another high velocity body is observed beneath the southern part of the Tyrrhenian basin, it might be interpreted as the presence of fragments of the African continental lithosphere beneath the  Tyrrhenian basin.

First step towards an integrated geophysical-geological model of the W-Alps: A new Vs model from transdimensional ambient-noise tomography

2021 ◽  
Author(s):  
Ahmed Nouibat ◽  
Laurent Stehly ◽  
Anne Paul ◽  
Romain Brossier ◽  
Thomas Bodin ◽  
...  
Keyword(s):  
Group Velocity ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Ambient Noise ◽  
Receiver Functions ◽  
Time Inversion ◽  
Geological Model ◽  
Ambient Noise Tomography ◽  
Low Velocity

&lt;p&gt;&lt;span&gt;We have successfully derived a new &lt;/span&gt;&lt;span&gt;3-D&lt;/span&gt;&lt;span&gt; high resolution shear wave velocity model of the crust and uppermost mantle of a large part of W-Europe from transdimensional&lt;/span&gt;&lt;span&gt;&lt;strong&gt; &lt;/strong&gt;&lt;/span&gt;&lt;span&gt;ambient-noise tomography. This model is intended to contribute to the development of the first &lt;/span&gt;&lt;span&gt;3-D&lt;/span&gt;&lt;span&gt; crustal-scale integrated geophysical-geological model of the W-Alps to deepen understanding of orogenesis and its relationship to mantle dynamics. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;We used an exceptional dataset of 4 years of vertical-component, daily seismic noise records (2015 - 2019) of more than 950 permanent broadband seismic stations located in and around the Greater Alpine region, complemented by 490 temporary stations from the AlpArray sea-land seismic network and 110 stations from Cifalps dense deployments.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;We firstly performed a &lt;/span&gt;&lt;span&gt;2-D&lt;/span&gt;&lt;span&gt; data-driven transdimensional travel time inversion for group velocity maps from 4 to 150 s (Bodin &amp; Sambridge, 2009). The data noise level was treated as a parameter of the inversion problem, and determined within a Hierarchical Bayes method. We used Fast Marching Eikonal solver (Rawlinson &amp; Sambridge, 2005) jointly with the reversible jump algorithm to update raypath geometry during inversion. In the inversion of group velocity maps for shear-wave velocity, we set up a new formulation of the&lt;/span&gt;&lt;span&gt; approach proposed by Lu et al (2018) by including group velocity uncertainties. Posterior probability distributions on &lt;/span&gt;&lt;span&gt;Vs&lt;/span&gt;&lt;span&gt; and interfaces were estimated by exploring a set of 130 millions synthetic &lt;/span&gt;&lt;span&gt;4-&lt;/span&gt;&lt;span&gt;layer &lt;/span&gt;&lt;span&gt;1-D Vs&lt;/span&gt;&lt;span&gt; models that allow for &lt;/span&gt;&lt;span&gt;low-velocity zones&lt;/span&gt;&lt;span&gt;&lt;em&gt;.&lt;/em&gt;&lt;/span&gt;&lt;span&gt; The obtained probabilistic model was refined using a linearized inversion&lt;/span&gt;&lt;span&gt;&lt;em&gt;. &lt;/em&gt;&lt;/span&gt;&lt;span&gt;For the ocean-bottom seismometers of the Ligurian-Provencal basin, we applied a specific processing to clean daily noise signals from instrumental and oceanic noises (Crawford &lt;/span&gt;&lt;span&gt;&amp;&lt;/span&gt;&lt;span&gt; Webb, 2000) and adapted the inversion for Vs to include the water column.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;Our Vs model evidences strong variations of the crustal structure along strike, particulary in the subduction complex. The European crust includes lower crustal low-velocity zones and a Moho jump of ~8-12 km beneath the W-boundary of the external crystalline massifs. We observe a deep LVZ&lt;em&gt; &lt;/em&gt;structure (50 - 80 km) in the prolongation&lt;em&gt; &lt;/em&gt;of the European continental subduction beneath the Ivrea body. The striking fit between the receiver functions ccp migrated section across the Cifalps profile and this new Vs model validate its reliability.&lt;/p&gt;

Two robust imaging methodologies for challenging environments: Wave-equation dispersion inversion of surface waves and guided waves and supervirtual interferometry + tomography for far-offset refractions

2018 ◽  
Vol 6 (4) ◽  
pp. SM27-SM37 ◽  
Author(s):  
Jing Li ◽  
Kai Lu ◽  
Sherif Hanafy ◽  
Gerard Schuster
Keyword(s):  
Wave Equation ◽  
Velocity Distribution ◽  
Surface Waves ◽  
Guided Waves ◽  
Velocity Model ◽  
Dispersion Curves ◽  
P Wave ◽  
Head Wave ◽  
Near Surface ◽  
Velocity Models

Two robust imaging technologies are reviewed that provide subsurface geologic information in challenging environments. The first one is wave-equation dispersion (WD) inversion of surface waves and guided waves (GW) for the shear-velocity (S-wave) and compressional-velocity (P-wave) models, respectively. The other method is traveltime inversion for the velocity model, in which supervirtual refraction interferometry (SVI) is used to enhance the signal-to-noise ratio of far-offset refractions. We have determined the benefits and liabilities of both methods with synthetic seismograms and field data. The benefits of WD are that (1) there is no layered-medium assumption, as there is in conventional inversion of dispersion curves. This means that 2D or 3D velocity models can be accurately estimated from data recorded by seismic surveys over rugged topography, and (2) WD mostly avoids getting stuck in local minima. The liability is that WD for surface waves is almost as expensive as full-waveform inversion (FWI) and, for Rayleigh waves, only recovers the S-velocity distribution to a depth no deeper than approximately 1/2 to 1/3 wavelength of the lowest-frequency surface wave. The limitation for GW is that, for now, it can estimate the P-velocity model by inverting the dispersion curves from GW propagating in near-surface low-velocity zones. Also, WD often requires user intervention to pick reliable dispersion curves. For SVI, the offset of usable refractions can be more than doubled, so that traveltime tomography can be used to estimate a much deeper model of the P-velocity distribution. This can provide a more effective starting velocity model for FWI. The liability is that SVI assumes head-wave first arrivals, not those from strong diving waves.

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