Yinhe Luo

Advantages of Using Multichannel Analysis of Love Waves (MALW) to Estimate Near-Surface Shear-Wave Velocity

As theory dictates, for a series of horizontal layers, a pure, plane, horizontally polarized shear (SH) wave refracts and reflects only SH waves and does not undergo wave-type conversion as do incident P or Sv waves. This is one reason the shallow SH-wave refraction method is popular. SH-wave refraction method usually works well defining near-surface shear-wave velocities. Only first arrival information is used in the SH-wave refraction method. Most SH-wave data contain a strong component of Love-wave energy. Love waves are surface waves that are formed from the constructive interference of multiple reflections of SH waves in the shallow subsurface. Unlike Rayleigh waves, the dispersive nature of Love waves is independent of P-wave velocity. Love-wave phase velocities of a layered earth model are a function of frequency and three groups of earth properties: SH-wave velocity, density, and thickness of layers. In theory, a fewer parameters make the inversion of Love waves more stable and reduce the degree of nonuniqueness. Approximating SH-wave velocity using Love-wave inversion for near-surface applications may become more appealing than Rayleigh-wave inversion because it possesses the following three advantages. (1) Numerical modeling results suggest the independence of P-wave velocity makes Love-wave dispersion curves simpler than Rayleigh waves. A complication of “Mode kissing” is an undesired and frequently occurring phenomenon in Rayleigh-wave analysis that causes mode misidentification. This phenomenon is less common in dispersion images of Love-wave energy. (2) Real-world examples demonstrated that dispersion images of Love-wave energy have a higher signal-to-noise ratio and more focus than those generated from Rayleigh waves. This advantage is related to the long geophone spreads commonly used for SH-wave refraction surveys, images of Love-wave energy from longer offsets are much cleaner and sharper than for closer offsets, which makes picking phase velocities of Love waves easier and more accurate. (3) Real-world examples demonstrated that inversion of Love-wave dispersion curves is less dependent on initial models and more stable than Rayleigh waves. This is due to Love-wave’s independence of P-wave velocity, which results in fewer unknowns in the MALW method compared to inversion methods of Rayleigh waves. This characteristic not only makes Love-wave dispersion curves simpler but also reduces the degree of nonuniqueness leading to more stable inversion of Love-wave dispersion curves.

Inversion stability analysis of multimode Rayleigh-wave dispersion curves using low-velocity-layer models

Surface wave inversion is increasingly applied to estimate near-surface shear (S)-wave velocities for geological structures. Our study addresses the sensitivity and stability of the Rayleigh-wave multimode dispersion-curve inversion for earth models containing a low-velocity layer. Due to the dependence of Rayleigh-waves on the S-wave velocity over a range of depth, the inverted Vs generally have uncertainties that vary with depth and structure. An evolutionary algorithm was used to provide a population of final models from inversion of multimode Rayleigh-wave dispersion curves. We analysed the uncertainties of inversion results for three irregular Vs structures, i.e., three models with a low S-wave velocity layer. The results show that 1) because the low-velocity layer traps the energy of Rayleigh-waves and makes the wave travel within it, Rayleigh-wave phase velocities are insensitive to variations in layers beneath a low-velocity layer. This characteristic can influence the inversion stability for these layers’ parameters. 2) The high degrees of uncertainties of inverted Vs for these layers still remain although the higher mode Rayleigh-wave data are included in the inversion procedure. It can be concluded that Vs, estimates for layers beneath a low-velocity layer are with a low degree of confidence and need to be treated with extra caution.

Magnetic study of the UHP eclogites from the Chinese Continental Scientific Drilling (CCSD) Project

Received 7 July 2008; revised 4 December 2008; accepted 2 January 2009; published 24 February 2009. [1] To establish the relationships between rock magnetism of eclogites and the corresponding retrograded metamorphic processes, this study integrated both rock magnetism and metamorphic petrology of 171 eclogite samples with different degree of retrograded metamorphism at the depth range of 100–2050 m of the main hole of the Chinese Continental Scientific Drilling (CCSD) Project, located in the Sulu ultrahigh pressure (UHP) metamorphic belt, eastern China. Results show that the average density is a suitable indicator for the degree of the retrograded metamorphism. Opaque minerals in variable eclogites are mainly magnetite, ilmenite, pyrite, and hematite. With increasing the metamorphic retrogression, the concentration of opaque minerals generally increases; however, magnetic properties of eclogites first increase from well-preserved eclogite to partially retrogressed eclogites because of the neoformation of coarse grained magnetite particles, and then decrease for the completely retrogressed eclogites because of the consumption of the magnetite particles and the formation of weakly magnetic minerals (e.g., hematite). These processes can be summarized as garnet + omphacite + rutile + SiO2 +H 2O ! amphibole + plagioclase + magnetite + ilmenite ! amphibole + epidote + hematite + ilmenite. The retrograded metamorphic eclogites with elevated magnetic properties take up 25% of all rocks and thus are one of major magnetic rocks in the CCSD main hole located in the Sulu UHP area. Therefore the significant volumes of retrograded eclogites may account for the magnetic anomalies flanking the northeastern part of the Sulu UHP metamorphic belt.

Rayleigh-wave dispersive energy imaging and mode separating by high-resolution linear Radon transform

In recent years, multichannel analysis of surface waves (MASW) has been increasingly used for obtaining vertical shear-wave velocity profiles within near-surface materials. MASW uses a multichannel recording approach to capture the time-variant, full-seismic wavefield where dispersive surface waves can be used to estimate near-surface S-wave velocity. The technique consists of (1) acquisition of broadband, high-frequency ground roll using a multichannel recording system; (2) efficient and accurate algorithms that allow the extraction and analysis of 1D Rayleigh-wave dispersion curves; (3) stable and efficient inversion algorithms for estimating S-wave velocity profiles; and (4) construction of the 2D S-wave velocity field map.

Mantle Transition Zone Structure Beneath Southeastern China and its Implications for Stagnant Slab and Water Transportation in the Mantle

We determined depth variation of the 410- and 660-km discontinuities beneath southeastern China by common-converted-point stacking of

Body waves revealed by spatial stacking on long-term cross-correlation of ambient noise

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Heat shielding effects in the Earth’s crust

P-wave receiver functions of 121 permanent Chinese seismic stations. We then combined the results with seismic velocity variation to estimate temperature and water content variations in the mantle transition zone of the region. Previous tomographic studies have shown a stagnant slab in the mantle transition zone in eastern Asia that is connected to subduction of the western Pacific. Temperature variations obtained clearly outline the shape of the stagnant slab, with its western edge at 113.5