Surface Wave Inversion Research Articles

Near-surface structure from ambient-noise tomography and horizontal-to-vertical spectral ratio beneath the Nankou-Sunhe fault*

Active faults pose a great threat to urban security. As the largest NW-trend active fault in Beijing area, the Nankou-Sunhe fault plays an important role in earthquake disaster and city construction. In this study, we collect continuous ambient noise data recorded by 43 temporary short-period seismograph between September 21th to October 12th 2019 to investigate the near-surface structure beneath the Nankou-Sunhe fault by using ambient noise tomography (ANT) and horizontal-to-vertical spectral ratio (HVSR) method. From ambient noise processing, fundamental-mode Rayleigh wave signals are clearly observed in the frequency band of 0.4–2.5 Hz. Then direct surface-wave inversion algorithm is applied to calculate the 3D shear-wave velocity model. Our results show that there is a sharp velocity contrast across the Nankou-Sunhe fault, with low velocities down to about 2 km on the hanging wall and high velocity on the footwall of the fault. According to the geological investigation, the low velocities are related to thicker sediments and Jurassic volcanic rock below which are the cap rock of the hydrothermal system. From the HVSR analysis, the HVSR curves of the sites near the fault shows double-peak, one less than 1 Hz and the other centered 7 Hz. After converting frequency to depth by the empirical equation, the results show that the thickness of sediments is thinned from southwest to northeast, which generally agrees well to field survey. Our results provide high-resolution near-surface structure for future study on disaster risk reduction and urban planning.

TRANSVERSE ISOTROPIC CRUST STRUCTURE BENEATH THE NORTHWEST AND CENTRAL NORTH ANATOLIA REVEALED BY SEISMIC SURFACE WAVES PROPAGATION

The Anatolian crust, which is abnormally hot, is widely deformed by subduction related volcanism. Suture zones, transform faults, thrusts and folds and metamorphic core complexes add to the geological complexity. Volcanic provinces such as Western, Central and Eastern Anatolia and Galatea are recognized as distinct features in the region. The middle-to-lower crust depths appear to be intruded by horizontal sills and the upper crust by vertical dykes. Both horizontal sills and vertical dykes leave anisotropic signs detected as Vertical Transverse Isotropy (VTI) that is explored by Love and Rayleigh surface wave inversions, i.e., Love-Rayleigh wave discrepancy which arises because the dykes and sills act differently against the Love and Rayleigh surface waves. The current study gives emphasis to the Northwest and Central North Anatolia utilizing both single-station and two-station tomography techniques to recover the two-dimensional group and phase speed charts from which one-dimensional dispersion inversions are implemented. The one-dimensional inversions are joined to construct the three-dimensional crust of the studied region. The shear-wave anisotropy is used to locate the anisotropy in the crust. The vertical dykes in the upper crust fit into negative VTI around -10% while the horizontal sills in the middle-to-lower crust yield positive VTI around 12%. The vertical magma flows within the vertical dykes and the horizontal magma flows within the horizontal sills contribute constructively to the anisotropy created by the special shape orientations of sills and dykes. The earthquakes hypocenter distribution and high and low speeds alongside the VTI provide significant clues to differentiate between diverse geological districts.

Revisiting Lunar Seismic Experiment Data Using the Multichannel Simulation with One Receiver (MSOR) Approach and Random Field Modeling

Major advancements in surface wave testing over the past 2 decades have led researchers to revisit and re-analyze archived seismic records, particularly those involving measurements on the Moon. The goal of such recent efforts with lunar seismic measurements has been to gain further insights into lunar geology. We examined the active seismic data from the Apollo 16 mission for their surface wave information using a multichannel approach. The inversion of Rayleigh surface waves provided a subsurface estimate for the uppermost 8 m of the lunar subsurface with the shear wave velocities varying from 40 to 50 m/s at the surface to velocities in the range of 95–145 m/s with an average of 120 m/s at a depth of about 7 m. Generally, the results from this inversion demonstrated good agreement with previous studies. Also, we carried out numerical modeling of wave propagation in a highly-heterogeneous domain to examine the effects of such anomalous features on the acquired seismograms. Results confirmed that a sharp-contrast bi-material domain can indeed produce significant coda wave as reflected on the lunar seismic traces.

Bayesian Rayleigh wave inversion with an unknown number of layers

Surface wave methods have received much attention due to their efficient, flexible and convenient characteristics. However, there are still critical issues regarding a key step in surface wave inversion. In most existing methods, the number of layers is assumed to be known prior to the process of inversion. However, improper assignment of this parameter leads to erroneous inversion results. A Bayesian nonparametric method for Rayleigh wave inversion is proposed herein to address this problem. In this method, each model class represents a particular number of layers with unknown S-wave velocity and thickness of each layer. As a result, determination of the number of layers is equivalent to selection of the most applicable model class. Regarding each model class, the optimization search of S-wave velocity and thickness of each layer is implemented by using a genetic algorithm. Then, each model class is assessed in view of its efficiency under the Bayesian framework and the most efficient class is selected. Simulated and actual examples verify that the proposed Bayesian nonparametric approach is reliable and efficient for Rayleigh wave inversion, especially for its capability to determine the number of layers.

SWinvert: a workflow for performing rigorous 1-D surface wave inversions

SUMMARYSWinvert is a workflow developed at The University of Texas at Austin for the inversion of surface wave dispersion data. SWinvert encourages analysts to investigate inversion uncertainty and non-uniqueness in shear wave velocity (Vs) by providing a systematic procedure and specific actionable recommendations for surface wave inversion. In particular, the workflow encourages the use of multiple layering parametrizations to address the inversion's non-uniqueness, multiple global searches for each parametrization to address the inverse problem's non-linearity and quantification of Vs uncertainty in the resulting profiles. While the workflow uses the Dinver module of the popular open-source Geopsy software as its inversion engine, the principles presented are of relevance to analysts using other inversion programs. To illustrate the effectiveness of the SWinvert workflow and to develop a set of benchmarks for use in future surface wave inversion studies, synthetic experimental dispersion data for 12 subsurface models of varying complexity are inverted. While the effects of inversion uncertainty and non-uniqueness are shown to be minimal for simple subsurface models characterized by broad-band dispersion data, these effects cannot be ignored in the Vs profiles derived for more complex models with band-limited dispersion data. To encourage adoption of the SWinvert workflow, an open-source Python package (SWprepost), for pre- and post-processing of surface wave inversion data, and an application on the DesignSafe-Cyberinfrastructure (SWbatch), for performing batch-style surface wave inversions with Dinver using high-performance computing, have been developed and released in conjunction with this work. The SWinvert workflow is shown to provide a methodical procedure and a powerful set of tools for performing rigorous surface wave inversions and quantifying the uncertainty in the resulting Vs profiles.

High‐Resolution 3‐D Shear Wave Velocity Model of the Tibetan Plateau: Implications for Crustal Deformation and Porphyry Cu Deposit Formation

AbstractThe high topography of the Tibetan Plateau was generated by the Cenozoic India‐Eurasia collision. A high‐resolution shear wave velocity model can provide improved understanding of the Tibetan structure and crustal deformation with complicated tectonic evolution. Based on continuous seismic observations at approximately 400 stations, we collected over 10,000 Rayleigh wave phase velocity dispersion curves extracted from ambient noise cross‐correlation functions. A direct surface wave inversion method was applied to obtain an S wave velocity model of the Tibetan crust. A heterogeneous structure including several prominent low‐velocity zones (LVZs) and relatively narrow low‐velocity bands connecting the LVZs is revealed. Our model shows significant crustal low‐velocity structures with lateral variations along the Himalayan front. In the eastern segment of the Lhasa terrane, the LVZ is spatially correlated with Miocene porphyry Cu deposits, which are probably related to strong tearing of the Indian lithosphere, while preexisting weak zones contribute to the LVZs in the western segment. Meanwhile, the LVZ along the Bangong‐Nujiang suture could be considered as a channel for eastward extrusion of ductile material, that is, crustal flow on geological timescales. This “flow” in the middle‐lower crust probably contributed to the formation of the V‐shaped conjugate strike‐slip fault system in central Tibet. The development of conjugate strike‐slip faults, however, ceased near 90°E, since the eastward “flow” was blocked by an intracrustal high‐velocity block in Amdo. This preexisting rigid zone (containing the Amdo microcontinent) hence influences the pattern of material transport inside the Tibetan crust and the deformation of the plateau.

3D S-wave velocity modelling with surface waves in oil seismic prospecting

It is a convenient and effective way to infer near-surface S-wave velocity () structures by using seismic surface waves. In spite of many successful applications on the geotechnical or engineering scale, surface-wave analysis and inversion methods are still not widely used in oil seismic exploration. Particularly, there are few researches reported on the three-dimensional (3D) structure modelling with the surface wave methods on this exploration scale. In this paper, we proposed a seismic surface wave data processing and inversion scheme for 3D near-surface modelling, and applied it to a field seismic data acquired for oil prospecting in Eastern China. Firstly, we analysed and adjusted the acquisition geometry to suit surface-wave analysis. Next, the interpolation and stacking processing was applied to the seismic data to eliminate spatial aliasing and improve the quality of dispersion images. In term of phase velocity dispersion imaging method, we adopted the cross-correlation and phase-shifting (CCPS) method to acquire accurate dispersion images. Simultaneous linearisation inversion of and layer thickness was used to inverse the surface wave dispersion curves. This inversion method reduces dependence of the initial models and has ability to detect the top interface of high-velocity layer. At last, the 3D near-surface structure was constructed by interpolating the all of 360 1D structures. We contrasted the surface wave inversion result with the first-arrival tomography inversion result, and the geological stratification results of both were coincident. Within the near-surface range, the surface wave inversion result has a higher resolution. This confirms that 3D modelling with surface waves in oil seismic prospecting is effective and practical.

Random objective waveform inversion of surface waves

Full-waveform inversion (FWI) of surface waves is becoming increasingly popular among shallow-seismic methods. Due to a huge amount of data and the high nonlinearity of the objective function, FWI usually requires heavy computational costs and may converge toward a local minimum. To mitigate these problems, we have reformulated FWI under a multiobjective framework and adopted a random objective waveform inversion (ROWI) method for surface-wave characterization. Three different measure functions were used, whereas the combination of one measure function with one shot independently provided one of the [Formula: see text] objective functions ([Formula: see text] is the total number of shots). We have randomly chose and optimized one objective function at each iteration. We performed a synthetic test to compare the performance of the ROWI and conventional FWI approaches, which showed that the convergence of ROWI is faster and more robust compared with conventional FWI approaches. We also applied ROWI to a field data set acquired in Rheinstetten, Germany. ROWI successfully reconstructed the main geologic feature, a refilled trench, in the final result. The comparison between the ROWI result and a migrated ground-penetrating radar profile further proved the effectiveness of ROWI in reconstructing the near-surface S-wave velocity model. We also ran the same field example by using a poor initial model. In this case, conventional FWI failed whereas ROWI still reconstructed the subsurface model to a fairly good level, which highlighted the relatively low dependency of ROWI on the initial model.

Structure of the Ecuadorian forearc from the joint inversion of receiver functions and ambient noise surface waves

SUMMARYThe Ecuadorian forearc is a complex region of accreted terranes with a history of large megathrust earthquakes. Most recently, a Mw 7.8 megathrust earthquake ruptured the plate boundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an international collaboration arranged by the Instituto Geofisico at the Escuela Politécnica Nacional mobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. We combine this new seismic data set with 14 permanent stations from the Ecuadorian national network to better understand how variations in crustal structure relate to regional seismic hazards along the margin. Here, we present receiver function adaptive common conversion point stacks and a shear velocity model derived from the joint inversion of receiver functions and surface wave dispersion data obtained through ambient noise cross-correlations for the upper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slow velocity anomaly we interpret as subducting oceanic crust, which shallows near the projected centre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversion in the Ecuadorian forearc near the Borbon Basin indicating a major discontinuity at a depth of ∼7 km. This conversion is not ubiquitous and may be the top of the accreted terranes. We also observe significant north–south changes in shear wave velocity. The velocity changes indicate variations in the accreted terranes and may indicate an increased amount of hydration beneath the Manabí Basin. This change in structure also correlates geographically with the southern rupture limit of multiple high magnitude megathrust earthquakes. The earthquake record along the Ecuadorian trench shows that no event with a Mw >7.4 has ruptured south of ∼0.5°S in southern Ecuador or northern Peru. Our observations, along with previous studies, suggest that variations in the forearc crustal structure and subducting oceanic crust may influance the occurrence and spatial distribution of high magnitude seismicity in the region.

3D wave-equation dispersion inversion of surface waves recorded on irregular topography

Irregular topography can cause strong scattering and defocusing of propagating surface waves, so it is important to account for such effects when inverting surface waves for shallow S-wave velocity structures. We have developed a 3D surface-wave dispersion inversion method that takes into account the topographic effects modeled by a 3D spectral element solver. The objective function is the frequency summation of the squared wavenumber differences [Formula: see text] along each azimuthal angle of the fundamental mode or higher-order modes of Rayleigh waves in each shot gather. The wavenumbers [Formula: see text] associated with the dispersion curves are calculated using the data recorded along the irregular free surface. Numerical tests on synthetic and field data demonstrate that 3D topographic wave equation dispersion inversion (TWD) can accurately invert for the S-wave velocity model from surface-wave data recorded on irregular topography. Field data tests for data recorded across an Arizona fault demonstrate that, for this example, the 2D TWD model can be as accurate as the 3D tomographic model. This suggests that in some cases, the 2D TWD inversion is preferred over 3D TWD because of its significant reduction in computational costs. Compared to the 3D P-wave velocity tomogram, the 3D S-wave tomogram agrees much more closely with the geologic model taken from the trench log. The agreement with the trench log is even better when the [Formula: see text] tomogram is computed, which reveals a sharp change in velocity across the fault. The localized velocity anomaly in the [Formula: see text] tomogram is in very good agreement with the well log. Our results suggest that integrating the [Formula: see text] and [Formula: see text] tomograms can sometimes give the most accurate estimates of the subsurface geology across normal faults.

Geophysical approach to study the potential ocean wave-induced liquefaction: an example at Magoodhoo Island (Faafu Atoll, Maldives, Indian Ocean)

This paper presents an application of geophysical techniques to determine the liquefaction potential of the surface sand layer under wave loading in an atoll island, Magoodhoo Island (Faafu Atoll). Measurements of background noise (H/V technique), seismic refraction profiles (tomography), surface wave inversion and electrical resistivity tomography were applied to study the properties of the substratum. Geotechnical analysis provided the relative soil density (Dr = 55%) and the grading curves; the comparison of these parameters with geophysical data, allowed to identify indicative features related to ground liquefaction phenomena. Microtremor measurements calculated a fundamental site resonance in the frequency range of 1.42–23.19 Hz. Velocity profiles showed that bedrock interface depth varies from approximately z = 35 m north-east of the island, to z = 25 m south-west, while in the most north-eastern external area, it was not detected. These results suggested a significant lateral variability of the site seismic response throughout the study area. Liquefaction triggering with wave heights varying between 2.0 and 3.0 m and 70–300 cycles was calculated. Also, considering an average wave period of 6 s, liquefaction was more likely after 7–30 min. The soil vulnerability index Kg was also used to evaluate the soil liquefaction hazard suggesting that the north-eastern island sector is more susceptible to liquefaction, confirmed also by the building damage status as compared to other areas of the island. The results obtained in the survey highlight that in an atoll island, where potential disasters can be caused by several natural causes, saturated loose sands lose their strength and stiffness if subjected to wave loading.

Identification of a nascent tectonic boundary in the San-in area, southwest Japan, using a 3D S-wave velocity structure obtained by ambient noise surface wave tomography

We derived a three-dimensional S-wave velocity model for the San-in area of southwest Japan to examine heterogeneous structures such as tectonic faults. Many earthquakes occur in this area, but much of the activity has been relatively recent, so the fault distribution has yet to be fully clarified. Here, we used continuous ambient noise data from a dense seismic network, deployed from November 2009 to extract Rayleigh and Love wave dispersion data between station pairs, and then applied a direct surface wave inversion to the phase velocities of each station pair to determine a three-dimensional S-wave velocity model. In the resulting model, faults and a previously unrecognized tectonic boundary appeared as low-velocity anomalies or velocity boundaries, and the velocity anomalies were also associated with many past earthquake hypocenters. These results contribute to our understanding of heterogeneous structures caused by recent tectonic motion and of possible future tectonic activity, such as intraplate earthquakes. Surface wave tomography using ambient noise recorded in dense seismic networks could also be applied in other parts of the world to reveal new heterogeneous geological structures (i.e., unrevealed tectonic faults) and could contribute to disaster mitigation.

Nevşehir Castle Region in Turkey Interpreted by the Use of Seismic Surface Wave and Electrical Resistance Measurements Together

Abstract The underground city beneath the Nevşehir Castle located in the middle of Cappadocia region in Turkey with approximately cone shape is investigated by jointly utilizing the modern geophysical techniques of seismic surface waves and electrical resistivity. The systematic void structure under the Nevşehir Castle of Cappadocia, which is known to have widespread underground cities, is studied by the use of 33 separate two-dimensional profiles ~4-km long where electrical resistivities and seismic surface waves are concurrently measured. Seismic surface wave measurements are inverted to establish the shear-wave velocity distribution while resistivity measurements are inverted to resolve the resistivity distribution. Several high-resistivity anomalies with a depth range 8-20 m point to a systematic void structure beneath the Nevşehir Castle. We were able to effectively isolate the void structure from the embedding structure since the currently employed resistivity instrument has provided us high resolution quality measurements. Associated with the high resistivity anomalies there exist low-velocity depth zones acquired from the surface wave inversions also pointing to a systematic void structure where three-dimensional visualization techniques are used to show the extension of the void structure under the studied area.

Seismic Characterization of the Nevada National Security Site Using Joint Body Wave, Surface Wave, and Gravity Inversion

ABSTRACTAs a part of the series of Source Physics Experiments (SPE) conducted on the Nevada National Security Site in southern Nevada, we have developed a local-to-regional scale seismic velocity model of the site and surrounding area. Accurate earth models are critical for modeling sources like the SPE to investigate the role of earth structure on the propagation and scattering of seismic waves. We combine seismic body waves, surface waves, and gravity data in a joint inversion procedure to solve for the optimal 3D seismic compressional and shear-wave velocity structures and earthquake locations subject to model smoothness constraints. Earthquakes, which are relocated as part of the inversion, provide P- and S-body-wave absolute and differential travel times. Active source experiments in the region augment this dataset with P-body-wave absolute times and surface-wave dispersion data. Dense ground-based gravity observations and surface-wave dispersion derived from ambient noise in the region fill in many areas where body-wave data are sparse. In general, the top 1–2 km of the surface is relatively poorly sampled by the body waves alone. However, the addition of gravity and surface waves to the body-wave dataset greatly enhances structural resolvability in the near surface. We discuss the methodology we developed for simultaneous inversion of these disparate data types and briefly describe results of the inversion in the context of previous work in the region.

3D wave-equation dispersion inversion of Rayleigh waves

The 2D wave-equation dispersion (WD) inversion method is extended to 3D wave-equation dispersion inversion of surface waves for the shear-velocity distribution. The objective function of 3D WD is the frequency summation of the squared wavenumber [Formula: see text] differences along each azimuth angle of the fundamental or higher modes of Rayleigh waves in each shot gather. The S-wave velocity model is updated by the weighted zero-lag crosscorrelation between the weighted source-side wavefield and the back-projected receiver-side wavefield for each azimuth angle. A multiscale 3D WD strategy is provided, which starts from the pseudo-1D S-velocity model, which is then used to get the 2D WD tomogram, which in turn is used as the starting model for 3D WD. The synthetic and field data examples demonstrate that 3D WD can accurately reconstruct the 3D S-wave velocity model of a laterally heterogeneous medium and has much less of a tendency to getting stuck in a local minimum compared with full-waveform inversion.

Inferring Crustal Temperatures Beneath Italy From Joint Inversion of Receiver Functions and Surface Waves

AbstractTemperature distribution at depth is of key importance for characterizing the crust, defining its mechanical behavior and deformation. Temperature can be retrieved by heat flow measurements in boreholes that are sparse, shallow, and have limited reliability, especially in active and recently active areas. Laboratory data and thermodynamic modeling demonstrate that temperature exerts a strong control on the seismic properties of rocks, supporting the hypothesis that seismic data can be used to constrain the crustal thermal structure. We use Rayleigh wave dispersion curves and receiver functions, jointly inverted with a transdimensional Monte Carlo Markov Chain algorithm, to retrieve the VS and VP/VS within the crust in the Italian peninsula. The high values (>1.9) of VP/VS suggest the presence of filled‐fluid cracks in the middle and lower crust. Intracrustal discontinuities associated with large values of VP/VS are interpreted as the α−β quartz transition and used to estimate geothermal gradients. These are in agreement with the temperatures inferred from shear wave velocities and exhibit a behavior consistent with the known tectonic and geodynamic setting of the Italian peninsula. We argue that such methods, based on seismological observables, provide a viable alternative to heat flow measurements for inferring crustal thermal structure.

Ambient noise multimode Rayleigh and Love wave tomography to determine the shear velocity structure above the Groningen gas field

SUMMARYThe Groningen gas field is one of the largest gas fields in Europe. The continuous gas extraction led to an induced seismic activity in the area. In order to monitor the seismic activity and study the gas field many permanent and temporary seismic arrays were deployed. In particular, the extraction of the shear wave velocity model is crucial in seismic hazard assessment. Local S-wave velocity-depth profiles allow us the estimation of a potential amplification due to soft sediments.Ambient seismic noise tomography is an interesting alternative to traditional methods that were used in modelling the S-wave velocity. The ambient noise field consists mostly of surface waves, which are sensitive to the Swave and if inverted, they reveal the corresponding S-wave structures.In this study, we present results of a depth inversion of surface waves obtained from the cross-correlation of 1 month of ambient noise data from four flexible networks located in the Groningen area. Each block consisted of about 400 3-C stations. We compute group velocity maps of Rayleigh and Love waves using a straight-ray surface wave tomography. We also extract clear higher modes of Love and Rayleigh waves.The S-wave velocity model is obtained with a joint inversion of Love and Rayleigh waves using the Neighbourhood Algorithm. In order to improve the depth inversion, we use the mean phase velocity curves and the higher modes of Rayleigh and Love waves. Moreover, we use the depth of the base of the North Sea formation as a hard constraint. This information provides an additional constraint for depth inversion, which reduces the S-wave velocity uncertainties.The final S-wave velocity models reflect the geological structures up to 1 km depth and in perspective can be used in seismic risk modelling.

Site characterization using passive seismic techniques: A case of Suez city, Egypt

This article introduces a site investigation study using quick and low-cost passive seismic techniques at Suez city which located in the northeastern part of Egypt. The Spatial Auto Correlation (SPAC) method is applied, as well as the newly developed Centerless Circular Array (CCA) method for estimating the shear wave velocity (Vs) structures of the subsurface layers using microtremor array data. The Horizontal/Vertical Spectral Ratio (HVSR) approach is employed in determination of predominant frequency (f0) of a site and its equivalent peak amplitude of ground motion (A0) using a single station microtremor measurements.In this study, microtremor array measurements have been carried out at five sites distributed in Suez city. In order to cover a wide frequency band, different array sizes are carried out at each observation site. Measurements are made using high performance accelerometers with a sampling rate 100 sample per second. In both SPAC and CCA methods, only the vertical component of microtremor data is considered in the analysis. The analysis includes two main stages, i.e., calculation of surface wave dispersion curves and inversion process to infer the Vs structure of the ground. The obtained dispersion curves cover frequency band (2.0–21.0 Hz) and show a good fitting between the observed and the calculated ones at all measured sites, proving the reliability of the inversion process performed in this work. In HVSR analysis, the E-W, N-S, and vertical components of microtremors are considered, and the ratio between the horizontal and the vertical components spectra is calculated, then the spectral ratio curves are produced. The predominant frequency (f0) and the peak amplitude (A0) of HVSR curve are reported for each site of observation. The inferred Vs profiles are used for calculating the average velocity value of shear wave up to 30 m depth (AVS30) which in turn employed in site classification at the investigated area.The results demonstrate that the parameter AVS30 varies between 248 m/s and 310 m/s in Suez city. The created site classification incorporates only one class (Class D), appearing no significant variation in the characteristics of the superficial and shallow soils in Suez city. The site predominant frequency (f0) ranges between 0.35 Hz and 5.0 Hz with relevant amplification (A0) between 1.6 and 4.2.These outputs represent a crucial site response model for future seismic hazard assessment and risk decrease studies in Suez area which is currently witnesses large investments and rapid land use planning.

Physical Properties and Structure of the Near Subsurface in the Săcel Area, Romania, from Shallow Seismic Measurements

Results are presented for shallow seismic reflection measurements performed southwest of Săcel village in Romania for the purpose of obtaining information about the geological structure in the near subsurface. The P-wave and S-wave velocity distributions were also obtained below the soil surface. The measurements were performed along a nearly linear profile on the top of an elongated hill. Most of the shot gathers were characterized by a good signal-to-noise ratio. A depth-converted migrated section was obtained after the processing of shot gathers, on which an image of sedimentary deposits with various thicknesses, separated by shallow faults until a depth of about 80 m, were observed. The P-wave and S-wave velocity-depth models for two segments were of considerable interest for a geotechnical study proposed for the construction of a windmill park. The two- and three-layered P-wave velocity-depth models were comparable until depths of about 10 m after first-arrival traveltime inversions. The lateral variations in the subsurface geological structure and lithology reflected the variations in the P-wave velocity values from both models. The S-wave velocity-depth models for comparable depth intervals were similar to those from the P-wave velocity-depth models. Reliable S-wave velocity distributions were obtained after inversion of fundamental-mode and higher-mode surface waves.

High-Resolution Characterization of Near-Surface Structures by Surface-Wave Inversions: From Dispersion Curve to Full Waveform

Surface waves are widely used in near-surface geophysics and provide a noninvasive way to determine near-surface structures. By extracting and inverting dispersion curves to obtain local 1D S-wave velocity profiles, multichannel analysis of surface waves (MASW) has been proven as an efficient way to analyze shallow-seismic surface waves. By directly inverting the observed waveforms, full-waveform inversion (FWI) provides another feasible way to use surface waves in reconstructing near-surface structures. This paper provides a state of the art review of MASW and shallow-seismic FWI and a comparison of both methods. A two-parameter numerical test is performed to analyze the nonlinearity of MASW and FWI, including the classical, the multiscale, the envelope-based, and the amplitude-spectrum-based FWI approaches. A checkerboard model is used to compare the resolution of MASW and FWI. These numerical examples show that classical FWI has the highest nonlinearity and resolution among these methods, while MASW has the lowest nonlinearity and resolution. The modified FWI approaches have an intermediate nonlinearity and resolution between classical FWI and MASW. These features suggest that a sequential application of MASW and FWI could provide an efficient hierarchical way to delineate near-surface structures. We apply the sequential-inversion strategy to two field data sets acquired in Olathe, Kansas, USA, and Rheinstetten, Germany, respectively. We build a 1D initial model by using MASW and then apply the multiscale FWI to the data. High-resolution 2D S-wave velocity images are obtained in both cases, whose reliabilities are proven by borehole data and a GPR profile, respectively. It demonstrates the effectiveness of combining MASW and FWI for high-resolution imaging of near-surface structures.