Yanghua Wang

Inverse Q-filter for seismic resolution enhancement

A principal limitation on seismic resolution is the earth attenuation, or Q -effect, including the energy dissipation of high-frequency wave components and the velocity dispersion that distorts seismic wavelets. An inverse Q -filtering procedure attempts to remove the Q -effect to produce high-resolution seismic data, but some existing methods either reduce the S/N ratio, which limits spatial resolution, or generate an illusory high-resolution wavelet that contains no more subsurface information than the original low-resolution data. In this paper, seismic inverse Q -filtering is implemented in a stabilized manner to produce high-quality data in terms of resolution and S/N ratio. Stabilization is applied to only the amplitude compensation operator of a full inverse Q -filter because its phase operator is unconditionally stable, but the scheme neither amplifies nor suppresses high frequencies at late times where the data contain mostly ambient noise. The latter property makes the process invertible, differ...

Seismic time-frequency spectral decomposition by matching pursuit

A seismic trace may be decomposed into a series of wavelets that match their time-frequency signature by using a matching pursuit algorithm, an iterative procedure of wavelet selection among a large and redundant dictionary. For reflection seismic signals, the Morlet wavelet may be employed, because it can represent quantitatively the energy attenuation and velocity dispersion of acoustic waves propagating through porous media. The efficiency of an adaptive wavelet selection is improved by making first a preliminary estimate and then a localized refining search, whereas complex-trace attributes and derived analytical expressions are also used in various stages. For a constituent wavelet, the scale is an important adaptive parameter that controls the width of wavelet in time and the bandwidth of the frequency spectrum. After matching pursuit decomposition, deleting wavelets with either very small or very large scale values can suppress spikes and sinusoid functions effectively from the time-frequency spect...

Crustal structure across Longmenshan fault belt from passive source seismic profiling

[1] We analyse receiver functions from 29 broad-band seismographs along a 380-km profile across the Longmenshan (LMS) fault belt to determine crustal structure beneath the east Tibetan margin and Sichuan basin. The Moho deepens from about 50 km under Songpan-Ganzi in east Tibet to about 60 km beneath the LMS and then shallows to about 35 km under the western Sichuan basin. The average crustal Vp/Vs ratios vary in the range 1.75-1.88 under Songpan-Ganzi in east Tibet, 1.8-2.0 under the LMS, and decrease systematically across the NW part of the Sichuan basin to less than 1.70. A negative phase arrival above the Moho under Songpan-Ganzi and Sichuan basin is interpreted as a PS conversion from the top of a low-velocity layer in the lower crust. The very high crustal Vp/Vs ratio and negative polarity PS conversion at the top of lower crust in east Tibet are inferred to be seismic signatures of a low-viscosity channel in the eastern margin of the Tibetan plateau. The lateral variation of Moho topography, crustal Vp/Vs ratio and negative polarity PS conversion at the top of the lower crust along the profile seem consistent with a model of lower crust flow or tectonic escape.

Q analysis on reflection seismic data

[1] Q analysis refers to the procedure for estimating Q directly from a reflection seismic trace. Conventional Q analysis method compares two seismic wavelets selected from different depth (or time) levels, but picking clean wavelets without interferences from other wavelet and noise from a reflection seismic trace is really a problem. Therefore, instead of analysing individual wavelets, I perform Q analysis using the Gabor transform spectrum which reveals the frequency content changing with time in a seismic trace. I propose two Q analysis methods based on the attenuation function and compensation function, respectively, each of which may produce a series of average values of Q -1 (inverse Q), averaging between the recording surface (or the water bottom) and the subsurface time samples. But the latter is much more stable than the former one. I then calculate the interval or layered values of Q -1 by a constrained linear inversion, which produces a stable estimation of the interval-Q series.

Multichannel matching pursuit for seismic trace decomposition

The technique of matching pursuit can adaptively decompose a seismic trace into a series of wavelets. However, the solution is not unique and is also strongly affected by data noise. Multichannel matching pursuit (MCMP), exploiting lateral coherence as a constraint, might improve the uniqueness of the solution. It extracts a constituent wavelet that has an optimal correlation coefficient to neighboring traces, instead of to a single trace only. According to linearity theory, a wavelet shared by neighboring traces is the best match to the average of multiple traces, and therefore it might effectively suppress the data noise and stabilize the performance. It is found that the MCMP scheme greatly improves spatial continuity in decomposition and can generate a plausible time-frequency spectrum with high resolution for reservoir detection.

Modified Kolsky model for seismic attenuation and dispersion

The decrease in the amplitude and resolution of seismic waves with depth, the so-called earth Q effects, can be conveniently defined in terms of frequency-dependent amplitude attenuation and velocity dispersion. In this paper, we modify Kolskys (1953 Stress Waves in Solids (Oxford: Clarendon)) attenuation–dispersion model so that it has an accurate representation of the velocity dispersion within the seismic band. Such a modification may lead to at least two advantages: (1) an accurate phase correction in inverse Q filtering that follows, and (2) a good match to other earth Q models. The latter suggests that, when applying them to design an inverse Q filter, filtered seismic sections should in principle be comparable with each other.

Inverse- Q filtered migration

An inverse- Q filtered migration algorithm performs seismic migration and inverse- Q filtering simultaneously, in which the latter compensates for the amplitudes and corrects the phase distortions resulting from the earth attenuation effect. However, the amplitudes of high-frequency components grow rapidly in the extrapolation procedure, so numerical instability is a concern when including the inverse- Q filter in the migration. The instability for each frequency component is independent of data and is affected only by migration models. The stabilization problem may be treated separately from the wavefield-extrapolation scheme. The proposed strategy is to construct supersedent of attenuation coefficients, based on given velocity and Q models, before performing wavefield extrapolation in the space-frequency domain. This stabilized algorithm for inverse- Q filtered migration is applicable to subsurface media with vertical and lateral variations in velocity and Q functions. It produces a seismic image with e...

Simultaneous inversion for model geometry and elastic parameters

Both traveltimes and amplitudes in reflection seismology are used jointly in an inversion to simultaneously invert for the interface geometry and the elastic parameters at the reflectors. The inverse problem has different physical dimensions in both data and model spaces. Practical approaches are proposed to tackle the dimensional difficulties. In using the joint inversion, which may properly take care of the structural effect, one potentially improves the estimates of the subsurface elastic parameters in the traditional analysis of amplitude variation with offset (AVO). Analysis of the elastic parameters estimated, using the ratio of S-wave to P-wave velocity contrasts and the deviation of this parameter from a normal background trend, promises to have application in AVO analysis. The inversion method is demonstrated by application to real data from the North Sea.

Multiple prediction through inversion: Theoretical advancements and real data application

Wave-equation-based multiple attenuation seismic methods may be divided into the two distinct phases of multiple modeling and multiple subtraction. These two are interrelated and must be optimized in order to produce an optimal final result. The multiple prediction through inversion (MPI) scheme updates the multiple model iteratively, as we usually do in a linearized inverse problem. The scheme models the multiple wavefield without an explicit knowledge of surface and subsurface structures or of the source signature; both are generally unknown in seismic surveys. However, compared to a conventional surface-related multiple attenuation method, the accuracy of the multiple model is improved both kinematically and dynamically. It is because the MPI scheme implicitly takes account of the spatial variation of the surface reflectivity, the source signature, the detector patterns and receiver ghosts, and other effects included in the so-called surface operator. When the MPI scheme is used in the first phase it also significantly reduces the nonlinearity of the problem in the second phase that involves attenuating multiples without removing or altering primaries. The effectiveness of the MPI scheme is demonstrated by examples involving real marine seismic data.

Crustal structure across the Dabie?Sulu orogenic belt revealed by seismic velocity profiles

The Tan-Lu fault (TLF) separates the Dabie and Sulu orogenic belts, well known for their ultra high pressure (UHP) metamorphic rocks in eastern China. We reinterpret one of the wide-angle seismic profiles traversing the TLF using traveltime tomography methods, and compare the results with the interpretation of three other seismic profiles across the TLF, to enable us to study the relationship of the five tectonic units comprising the North China plate (NCP), the Yangtze plate (YTZP), the TLF, the Dabie–Sulu orogenic belt (DSOB), and the ultra-high pressure metamorphic belt (UHPMB) that is exposed within the DSOB. The results demonstrate that there is strong lateral heterogeneity within the studied area. The TLFs penetrating depth deepens along a S–N direction. In the central section of the fault, the TLF can be traced to the middle crust but in the northern section it penetrates to the Moho. The average P-wave velocity in the UHPMB and DSOB is 0.1–0.4 km s−1 faster than that of the YTZP, NCP and TLF for upper crusts with depths 13 km. The bottom borders of the middle and lower crusts of the UHPMB and DSOB are apparently deeper than the other three tectonic units, and the Moho beneath UHPMB around Dabieshan may be deeper than 40 km. The general similarities of the crustal velocity structures between the Dabie and Sulu UHPMB may suggest a similar exhuming mechanism of UHP metamorphic rocks, before the large-scale TLF strike slip, driven by the subduction of the Yangtze block. The velocity gradient of the crust–mantle transition beneath the Sulu UHPMB implies the intrusion of basaltic melts from the upper mantle.