Group Velocity Dispersion Measurements Research Articles

Ambient Seismic Noise Tomography Within the Floridan Aquifer System, Santa Fe River‐Sink Rise, Florida, US

AbstractThis study utilized ambient seismic noise tomography to investigate correlations of hydrogeological systems with heterogeneous porosity with the internal velocity structure of karst aquifers using the Floridan Aquifer System (FAS) as an example. We conducted a seismic signal analysis with 30 days of vertical component seismic data acquired within the Santa Fe River Sink‐Rise system (Sink‐Rise System), using power spectral density estimates and frequency‐wavenumber analysis to identify any noise source biases. Empirical Green's functions were derived through phase cross‐correlations and phase‐weighted stacking. The functions allowed extraction of group velocity dispersion measurements, which were inverted to generate 2D tomographic images at various periods. Tomographic images show multiple layers with varying group velocities that aligned with known hydrogeological features of the FAS, including its base, low‐porosity and low‐permeability dolostone semiconfining units characterized by elevated velocities, and zones of elevated porosity from karstification with reduced seismic velocities. Our results show how ambient seismic noise can be used to evaluate aquifer physical properties, identify unknown features, and identify the location and continuity of aquifer units.

Ultrafast pulse duration measurement method of near-infrared pulses for a broad range of wavelengths using two-photon absorption in a liquid and fluorescent dye solution.

A novel characterization method to measure the pulse duration of ultrafast near-IR pulses is introduced, which uses simple tabletop optics, is relatively inexpensive, and is expected to work in a broad wavelength range. Our diagnostic tool quantitatively characterizes the laser pulse duration of any near-IR wavelength assuming a Gaussian pulse shape with a linear chirp. We negatively prechirp near-IR pulses with a home-built broadband pulse compressor (BPC) and send this prechirped beam through a cell filled with a low-molar solution of a fluorescent dye in a liquid. After two-photon absorption, this dye fluoresces in the visible, and we record this visible signal as a function of the propagation distance in the liquid cell. We calibrate the group velocity dispersion (GVD) of our home-built BPC device against the known GVD of the compressor of our 800nm laser and confirm this value using geometric considerations. Now knowing the GVD of BPC and the recorded visible signal for various amounts of negative chirp, let us extract the smallest pulse duration of the near-IR pulse from this visible signal. As a useful corollary, our analysis also enables the direct measurement of the GVD for liquids and the indirect measurement of the absorption coefficient for liquids in the near-IR range, in contrast to indirect GVD measurements that rely on methods such as the double derivative of the refractive index.

One-dimensional regional shear velocity structure from joint inversion of fundamental mode group velocity dispersion measurements of Love and Rayleigh waves: application to the Uttarakhand Himalaya

One-dimensional regional shear velocity structure from joint inversion of fundamental mode group velocity dispersion measurements of Love and Rayleigh waves: application to the Uttarakhand Himalaya

Kerr nonlinearity and group velocity dispersion of InGaAs/InP and GaAsSb/InP waveguides in the mid-infrared

We report measurements of Kerr nonlinearity and group velocity dispersion in In0.53Ga0.47As/InP and GaAs0.51Sb0.49/InP ridge waveguides in the mid-infrared using four-wave mixing at λ ≈ 5 µm. Measured values of Kerr nonlinearity are significantly higher compared to those reported for any other materials systems suitable for building dielectric waveguides with low losses and low group velocity dispersion in the mid-infrared (λ ≈ 3–15 μm). Our measurements establish both In0.53Ga0.47As/InP and GaAs0.51Sb0.49/InP materials as promising platforms for the development of on-chip mid-infrared frequency comb generation and supercontinuum light sources.

Accurate modeling and measurement of pressure-induced group velocity dispersion variations in anti-resonant hollow-core fibers.

Precise control of group velocity dispersion (GVD) by pressure in a gas-filled hollow-core fiber (HCF) is of essential importance for many gas-based nonlinear optical applications. To accurately calculate the pressure-induced dispersion variations (∂β2/∂p) in anti-resonant types of HCF, an analytical model combining the contribution of the gas material, capillary waveguide, and cladding resonances is developed, with an insightful physical picture. Broadband (∼1000 nm) GVD measurements in a single-shot manner realize accuracy and precision as low as 0.1 ps2/km and 2 × 10-3 ps2/km, respectively, and validate our model. Consistent with our model, a pronounced negative ∂β2/∂p is observed experimentally for the first time, to our knowledge. Our model can also be extended to other HCFs with cladding resonances in predicting ∂β2/∂p, such as in photonic bandgap types of HCF.

3D-ambient noise surface wave tomography of Fogo volcano, Cape Verde

Fogo volcano belongs to the Cape Verde Archipelago, and it is one of the most active volcanoes in the Atlantic Ocean, which most recent eruption occurred from November 2014 to February 2015. We analyzed ambient seismic noise recordings of three different networks deployed in the island, totalizing 14 seismic stations, to derive a crustal 3D shear-wave crustal velocity model of the volcano. Through the phase cross-correlation technique followed by a time-domain phase weighted stack, we were able to measure Rayleigh wave group-velocity dispersion measurements in the period range from 1.0 to 10 s. These dispersion measurements were used to invert for 2D group velocity maps at selected periods, and then inverted to produce a 3D shear-wave velocity model of the island. The tomographic model shows three velocity domains. First, an asymmetric upper layer, above 5–6 km of depth, with lower velocities concentrated in the northeastern sector of the island and a clear higher-velocity horizontal body at 3–4 km of depth in the southwestern sector of the island; the spatial correlation between these two velocity zones and the Galinheiros normal fault suggests a genetic link between the high velocities and long-term surface deformation, which we related to sill intrusions between 3 and 4.5 km depth, beneath the southwestern sector of the island. Second, a marked higher-velocity horizontal layer in between 5 and 6 km and 8–9 km, interpreted as the seismic expression of pervasive sill and laccolith intrusions, now cooled, beneath the volcanic edifice and within the underlying oceanic crust. Third, a lower velocity layer below 8–9 km of depth, more pronounced beneath the northeastern sector, which could be explained by a hotter and possibly melt-rich zone beneath the volcano or a significantly altered/serpentinized crust. Finally, our study also confirms that Fogo lacks any sizable magma chambers (ancient or recent) within the volcanic edifice, in agreement with other geophysical and petrological studies. These observations demonstrate that 3D-ambient noise Rayleigh wave tomography is a powerful tool to image the crustal and upper mantle structure beneath volcanic islands, as shown here for Fogo volcano.

Lithospheric architecture below the Eastern Ghats Mobile Belt and adjoining Archean cratons: Imprints of India-Antarctica collision tectonics

The investigation of the rigid lithospheric mantle architecture floating over the weaker hotter asthenosphere is key to understanding the plate tectonic evolution of the Eastern Ghats Mobile Belt and the adjoining Archean cratons. We present shear-wave velocity (VS) structure for the crust and upper-mantle below the Eastern Ghats Mobile Belt (EGMB) and adjacent Archean cratons. The lithospheric structure is constrained through 4–150 s fundamental-mode Rayleigh and Love-wave group velocity dispersion measurements, using regional (2°–30°) earthquakes recorded at 27 seismic stations installed along two distinct profiles. Velocity models are improved by joint inversion of resultant dispersion curves with receiver functions computed from the teleseismic P-waves. Observed variations in crustal and lithospheric architecture across the domains of the Eastern Ghats Province are implication of the deformations due to rifting and collision of India and East Antarctica in the context of assembly and breakup of Rodinia and Gondwana supercontinents. The cratons and the mobile belt are characterised by 34–38 km, ∼45 km thick heterogeneous crust and 120–160 km, 90–120 km thick lithospheres, respectively. The abrupt changes in the crustal thickness owes its origin to the collisional thrusting of the Eastern Ghats Province against the Archean cratons. Significant variations in the nature of Moho are also noted. The flat sharp cratonic Moho is distinct from the gradational Moho below the Eastern Ghats Province, possibly a signature of magmatic underplating below the mobile belt. The nature of the lithosphere-asthenosphere boundary (LAB) interpreted from the gradient change in velocity appears to be poorly-resolved. Therefore, we utilise radial anisotropy determined from the discrepancy between VSH and VSV based on Rayleigh and Love-wave velocities respectively, as a proxy. Our findings confirm the LAB through positive radial anisotropy (VSH > VSV) prevalent in the asthenospheric-mantle that is likely explained by flow-induced shear. Overall, the lithospheric architecture below the study area has been shaped through several deformational episodes that distinguish the multiple litho-tectonic units within the mobile belt. The loss of cratonic lithospheric keel can be attributed to the mantle plume given the Gondwanan trajectory of the Indian subcontinent over Kerguelen and Crozet hotspots.

Constraining Europa's ice shell thickness with fundamental mode surface wave dispersion

Determining the thickness of Europa's outer ice shell is a key factor for understanding Europa's internal dynamics, evolution, and potential habitability. As such, one of the primary goals of any future lander mission to Europa would be to constrain the thickness of the ice shell and confirm the presence of a global subsurface ocean. Tidally induced ice fracturing events provide a natural source of seismic energy to illuminate the subsurface, thus a seismic instrument onboard a future lander mission could provide a promising means to probe ice shell thickness. A variety of seismic techniques could be used to constrain Europa's interior structure and dynamics, including body wave, surface wave, and normal mode seismology. Here, we use numerical simulations of seismic wave propagation on Europa in order to investigate the potential of using long period dispersion measurements of Rayleigh and flexural waves to constrain the ice shell thickness. Since the sensitivity kernels of group velocity dispersion measurements depend strongly on the structure of the ice shell, inverting for ice shell thickness is a non-linear problem.To address this, we use either a grid search or Markov chain Monte Carlo inversion approach, and test the method on a variety of plausible models of Europa's interior. Additionally, we demonstrate our approach in a “blind” inversion using the 1 week long synthetic catalogs of Europa's seismicity from Panning et al. (2018). We find that under most scenarios, group velocity dispersion measurements between periods of 25–250 s can constrain Europa's ice shell thickness to within several km uncertainty, although the method becomes increasingly inaccurate for thicker ice shells and at large epicentral distances. Our results, which suggest that surface waves from naturally occurring ice fracturing events on Europa can be used to help determine ice shell thickness, may help set instrument requirements for spaceflight capable seismometers aimed at exploring icy ocean worlds.

S‐Wave Velocity Structure of the Sichuan‐Yunnan Region, China: Implications for Extrusion of Tibet Plateau and Seismic Activities

AbstractThe Sichuan‐Yunnan region is located at the intersection between the South China Block, the Indian plate, and the Tibet Plateau and is crisscrossed with deep and large faults and is characterized by strong seismic activities and the extrusion of the Tibetan Plateau. Here we employ 1‐year continuous waveforms of the vertical component of 89 broadband seismic stations using noise tomography in this region to evaluate the velocity structure and its implications. Through single station data preprocessing, cross‐correlation calculation, stacking, group velocity dispersion measurement, and quality evaluation, the group velocity dispersion curves of Rayleigh waves for the different periods were extracted in this area. We then use the surface wave tomography method to obtain the Rayleigh wave group velocity distribution of 9–40s. Finally, the S‐wave velocity structure in the depth range of 0–60 km is calculated by pure path dispersion inversion. The results show that a large‐scale low‐velocity anomaly in the lower crust runs through the Sichuan‐Yunnan rhombus block (SYRB) to Yangtze Block from west to east, which might correspond to the extrusion of the Tibetan Plateau or channel flow. On the other hand, the seismic activities are mainly concentrated at the western part of the region, with the earthquakes distributed at the boundary between the low‐ and high‐velocity structures, as well as the adjacent region, which we correlate with the extrusion of the Tibet Plateau.

Milliradian precision ultrafast pulse control for spectral phase metrology

A pulse-shaper-based method for spectral phase measurement and compression with milliradian precision is proposed and tested experimentally. Measurements of chirp and third-order dispersion are performed and compared to theoretical predictions. The single-digit milliradian accuracy is benchmarked by a group velocity dispersion measurement of fused silica.

Using multicomponent ambient seismic noise cross-correlations to identify higher mode Rayleigh waves and improve dispersion measurements

SUMMARYAmbient seismic noise cross-correlation with three-component sensors yields a nine-component empirical Green's tensor, in which four components of the radial–vertical plane contain Rayleigh waves. We exploit the retrograde elliptical nature of particle motion of the fundamental mode Rayleigh wave to correct the phase of the four radial–vertical components and stack them to obtain an average fundamental mode Rayleigh-wave time-series. This technique can suppress incoherent noise and wave packets that do not follow the targeted elliptical particle motion. The same technique can be used to isolate the first higher mode Rayleigh wave that follows prograde elliptical particle motion. We first demonstrate the effectiveness of the method on synthetic waveforms and then apply it on noise cross-correlations computed in Central California. Using this method, we isolate 1st higher mode Rayleigh waves on noise cross-correlations in the Great Valley, California, which provides new phase velocity constraints for estimating velocity structure in the sedimentary basin. We also obtain improved estimates of fundamental mode Rayleigh-wave dispersion for surface-wave tomography. The waveforms stacked assuming retrograde particle motion return at least ∼20 per cent more group velocity dispersion measurements satisfying a minimum signal-to-noise ratio (SNR) criterion than the individual components for periods ∼4–18 s. For equivalent group velocity measurements, SNR for the stacked estimate of the fundamental mode Rayleigh wave is on average 40 per cent greater than that measured on the individual components at periods less than 10 s. The technique also provides an easy way to detect large errors in sensor orientation.

Ambient noise Love wave tomography of China

We first report on the Love wave tomography of China based on ambient noise cross-correlations. We used 3 years of continuous waveform data recorded by 206 broadband seismic stations on the Chinese Mainland and 36 neighboring global stations and obtained Love wave empirical Green’s functions from cross-correlations of the horizontal components. The Love wave group velocity dispersion measurements were used to construct dispersion maps of 8- to 40-s periods, which were then inverted to obtain a three-dimensional horizontally polarized S-wave (SH) velocity structure. The resolution was approximately 4° × 4° and 8° × 8° for eastern and western China, respectively, and extended to a depth of approximately 50 km. The SH model was generally consistent with a previously published vertically polarized S-wave (SV) model and showed large-scale features that were consistent with geological units, such as the major basins and changes in the crustal thickness across the north-south gravity lineament. The SH and SV models also showed substantial differences, which were used to examine the subsurface radial anisotropy. We define the radial anisotropy parameter as \begin{document}$\psi = 2\left( {{V_{\rm SH}} - {V_{\rm SV}}} \right)/\left( {{V_{\rm SH}} + {V_{\rm SV}}} \right)$\end{document} . At a shallow depth, we observed significant radial anisotropy under major basins, which may be related to thick sedimentary layers. At the mid to lower crust, most of the Chinese continent showed strong positive radial anisotropy (SH > SV). Central and southern Tibet showed strong positive anisotropy, whereas the radial anisotropy was relatively weak at the northern and eastern margins, which suggests a change in deformation style from the plateau interior to its margins. The North China craton showed prominent positive radial anisotropy, which may be related to decratonization and strong extension since the Mesozoic Era. Love waves are less well retrieved than Rayleigh waves from ambient noise cross-correlations. Increasing the duration of the cross-correlation data beyond 4 to 8 years may not aid in retrieving Love waves of longer periods, for which improved methods need to be explored.

Pulse delay and group velocity dispersion measurement in V-type electromagnetically induced transparency of hot atom

Pulse delay with the group velocity dispersion (GVD) characteristics was studied in the V-type electromagnetically induced transparency in the hyperfine levels of atoms with a closed system configuration. The phase coherency between the pump and the probe laser beams was maintained. We studied the pulse delay and the GVD characteristics with the variation of the pump Rabi frequency taking optical density (OD) as a parameter. We observed a maximum of 268 ns pulse delay for 21.24 MHz pump Rabi frequency at OD 5.04 of the Rb vapor cell. For a better understanding of the experimental results, we have derived an analytical solution for the delay characteristics considering the thermal averaging. The analytical solution was derived for a three level V-type system. The theoretical plots of the delay and the GVD show the same characteristics as we observed in the experiment. This analytical approach can be further generalized for the higher level schemes to calculate different quantities such as susceptibility, group velocity delay or GVD characteristics.

Group refractive index and group velocity dispersion measurement by complex master slave interferometry.

This paper demonstrates that the complex master slave interferometry (CMSI) method used in spectral domain interferometry (SDI) can efficiently be used for accurate refractive index and group velocity dispersion measurements of optically transparent samples. For the first time, we demonstrate the relevance of the phase information delivered by CMSI for dispersion evaluations with no need to linearize data. The technique proposed here has been used to accurately measure the group refractive index and the group velocity dispersion of a strong dispersive sample (SF6 glass), and a weak dispersive one (distilled water). The robustness of the technique is demonstrated through the manipulation of several sets of experimental data.

Dispersion measurement of ultra-high numerical aperture fibers covering thulium, holmium, and erbium emission wavelengths

We present broadband group velocity dispersion (GVD) measurements of commercially available ultra-high numerical aperture fibers (UHNA1, UHNA3, UHNA4, UHNA7, and PM2000D from Coherent-Nufern). Although these fibers are attractive for dispersion management in ultra-fast fiber laser systems in the 2 μm wavelength region, experimental dispersion data in the literature is scarce and inconsistent. Here we demonstrate the measurements using the spectral interferometry technique covering the typically used erbium, thulium, and holmium emission bands. The results are characterized in terms of the standard-deviation uncertainty and compared with previous literature reports. Fitting parameters are provided for each fiber allowing for the straightforward replication of the measured dispersion profiles. This work is intended to facilitate the design of ultra-fast fiber laser sources and the investigations of nonlinear optical phenomena.

Dispersion-managed Ho-doped fiber laser mode-locked with a graphene saturable absorber.

In this Letter, we demonstrate an all-fiber holmium-doped laser operating in the stretched-pulse regime. As a result of dispersion management, the laser is capable of generating 190fs pulses with a bandwidth of 53.6nm. The pulses centered at 2060nm reach 2.55nJ of energy. Mode-locking is achieved with a multilayer graphene saturable absorber (SA). The Letter also presents the measurement of group velocity dispersion of active (Nufern SM-HDF-10/130), passive (SMF28), and dispersion-compensating (Nufern UHNA4) fibers in a 1.8-2.1μm range. To the best of our knowledge, this is the first report on an all-fiber, stretched-pulse laser operating beyond 2μm with nanomaterial-based SA.

A new algorithm for three‐dimensional joint inversion of body wave and surface wave data and its application to the Southern California plate boundary region

AbstractWe introduce a new algorithm for joint inversion of body wave and surface wave data to get better 3‐D P wave (Vp) and S wave (Vs) velocity models by taking advantage of the complementary strengths of each data set. Our joint inversion algorithm uses a one‐step inversion of surface wave traveltime measurements at different periods for 3‐D Vs and Vp models without constructing the intermediate phase or group velocity maps. This allows a more straightforward modeling of surface wave traveltime data with the body wave arrival times. We take into consideration the sensitivity of surface wave data with respect to Vp in addition to its large sensitivity to Vs, which means both models are constrained by two different data types. The method is applied to determine 3‐D crustal Vp and Vs models using body wave and Rayleigh wave data in the Southern California plate boundary region, which has previously been studied with both double‐difference tomography method using body wave arrival times and ambient noise tomography method with Rayleigh and Love wave group velocity dispersion measurements. Our approach creates self‐consistent and unique models with no prominent gaps, with Rayleigh wave data resolving shallow and large‐scale features and body wave data constraining relatively deeper structures where their ray coverage is good. The velocity model from the joint inversion is consistent with local geological structures and produces better fits to observed seismic waveforms than the current Southern California Earthquake Center (SCEC) model.

Seismic structure beneath the Gulf of California: a contribution from group velocity measurements

Rayleigh wave group velocity dispersion measurements from local and regional earthquakes are used to interpret the lithospheric structure in the Gulf of California region. We compute group velocity maps for Rayleigh waves from 10 to 150 s using earthquakes recorded by broad-band stations of the Network of Autonomously Recording Seismographs in Baja California and Mexico mainland, UNM in Mexico, BOR, DPP and GOR in southern California and TUC in Arizona. The study area is gridded in 120 longitude cells by 180 latitude cells, with an equal spacing of 10 × 10 km. Assuming that each gridpoint is laterally homogeneous, for each period the tomographic maps are inverted to produce a 3-D lithospheric shear wave velocity model for the region. Near the Gulf of California rift axis, we found three prominent low shear wave velocity regions, which are associated with mantle upwelling near the Cerro Prieto volcanic field, the Ballenas Transform Fault and the East Pacific Rise. Upwelling of the mantle at lithospheric and asthenospheric depths characterizes most of the Gulf. This more detailed finding is new when compared to previous surface wave studies in the region. A low-velocity zone in northcentral Baja at ∼28oN which extends east–south–eastwards is interpreted as an asthenospheric window. In addition, we also identify a well-defined high-velocity zone in the upper mantle beneath central-western Baja California, which correlates with the previously interpreted location of the stalled Guadalupe and Magdalena microplates. We interpret locations of the fossil slab and slab window in light of the distribution of unique post-subduction volcanic rocks in the Gulf of California and Baja California. We also observe a high-velocity anomaly at 50-km depth extending down to ∼130 km near the southwestern Baja coastline and beneath Baja, which may represent another remnant of the Farallon slab.

Verifying single-station seismic approaches using Earth-based data: Preparation for data return from the InSight mission to Mars

The planned InSight mission will deliver a single seismic station containing 3-component broadband and short-period sensors to the surface of Mars in 2016. While much of the progress in understanding the Earth and Moon’s interior has relied on the use of seismic networks for accurate location of sources, single station approaches can be applied to data returned from Mars in order to locate events and determine interior structure. In preparation for the data return from InSight, we use a terrestrial dataset recorded at the Global Seismic Network station BFO, located at the Black Forest Observatory in Germany, to verify an approach for event location and structure determination based on recordings of multiple orbit surface waves, which will be more favorable to record on Mars than Earth due to smaller planetary radius and potentially lower background noise. With this approach applied to events near the threshold of observability on Earth, we are able to determine epicentral distance within approximately 1° (corresponding to ∼60km on Mars), and origin time within ∼30s. With back azimuth determined from Rayleigh wave polarization, absolute locations are determined generally within an aperture of 10°, allowing for localization within large tectonic regions on Mars. With these locations, we are able to recover Earth mantle structure within ±5% (the InSight mission requirements for martian mantle structure) using 1D travel time inversions of P and S travel times for datasets of only 7 events. The location algorithm also allows for the measurement of great-circle averaged group velocity dispersion, which we measure between 40 and 200s to scale the expected reliable frequency range of the InSight data from Earth to Mars data. Using the terrestrial data, we are able to resolve structure down to ∼200km, but synthetic tests demonstrate we should be able to resolve martian structure to ∼400km with the same frequency content given the smaller planetary size.

Measurement of group velocity dispersion in a solid-core photonic crystal fiber filled with a nematic liquid crystal.

Liquid crystal-filled photonic crystal fibers (PCFs) are promising candidates for electrically tunable integrated photonic devices. In this Letter, we present group velocity measurements on such fibers. A large mode area PCF, LMA8, was infiltrated with the liquid crystal mixture, E7. The measurements were performed with an interferometric setup. The fiber exhibits several spectral transmission windows in the visible wavelength regime that originate from the bandgap guiding mechanism. The dispersion of these windows is very unusual compared to typical fibers. Our measurements show that it can change from -2500 ps km(-1) nm(-1) to +2500 ps km(-1) nm(-1) within a spectral range of only 15 nm. This leads to multiple zero dispersion wavelengths in the visible wavelength range.