Generally, the traditional spatial autocorrelation (SPAC) method utilizes the fundamental-mode surface-wave dispersion curve for inversion. Ignoring the higher-mode surface-wave dispersion curves will affect the accuracy of the SPAC method in detecting the shear-wave (S-wave) velocity structures, especially for the inversion of deeper structures. To fully and accurately utilize the multi-mode nature of surface waves, we investigate a direct inversion method of subsurface S-wave velocity structures from the spatial autocorrelation coefficients of multi-mode surface waves extracted from the seismic ambient noise. In this new method, the SPAC coefficient is directly inverted to obtain the S-wave velocity. The key step is to obtain the theoretical SPAC coefficients of a layered model by taking into account both the energy ratio of each mode of surface waves in the studied frequency band and the influence of the finite station pairs, and by weighting the Bessel functions corresponding to the phase velocities of each mode with the energy ratio as the weighting factor. The direct inversion method for the SPAC coefficient significantly improves the efficiency of data processing by omitting the extraction of the dispersion curves. Meanwhile, joint inversion of the fundamental and higher-mode SPAC coefficients of surface waves ensures the reliability of inversion results, increases the exploration depth, strengthens the stability of inversion, and significantly reduces the inversion misfit.
Manganese nodule coverage is estimated based on multi-beam and deep-towed video nodule survey profiles of about 1,700 km in the China Pioneer Area of Eastern Pacific. Two statistical equations for estimating nodule coverage are derived separately from the multi-beam normal incidence amplitude data and angular amplitude data based on theoretical analysis of influence factors on multi-beam amplitude data. Predictions generated by the normal incidence amplitude model fall within 5% of real nodule coverage, and theoretically calculated angular amplitude data fits well with the original multi-beam amplitudes at incident angles larger than 20° according to nodule coverage estimated from the deep-towed video data.
The full waveform inversion (FWI) method utilizes the kinematic and dynamic information of prestack seismic data to rebuild underground velocity structure.It has the potential of revealing detailed structure and lithology characteristics under complex geological background.FWI can be carried out in the either frequency or time domain. Frequency-domain FWI has the advantages of high computation efficiency and data selection flexibility over its time-domain counterpart.Recent advances in frequency-domain FWI including waveform modeling methods,frequency selection strategies,object function configuration styles,source wavelet processing methods,and gradient precondition methods make it attractive for realistic problems.The multiple local minimal nature of the object function js one of the major obstacles to good reflection waveform inversion results.The frequency-domain damped wavefield inversion method fills the gap between the Laplace-domain and frequency-domain FWI methods.The combination of the three methods can mitigate the strong nonlinearity of the waveform inversion problem.
Summary Seismic illumination plays an important role in subsurface imaging. A better image can be expected either through optimizing acquisition geometry or introducing more advanced seismic migration and/or tomographic inversion methods involving illumination compensation. Vertical cable survey is a good supplement/replacement for traditional marine seismic survey for its flexibility and data quality. Conventional vertical cable data processing requires separation of primaries and multiples before migration. We proposed to use multi-scale full waveform inversion (FWI) to improve illumination coverage of vertical cable survey. A deep water velocity model is built to test the capability of multi-scale FWI in detecting low velocity anomalies below seabed. Synthetic results show that multi-scale FWI is an effective model building tool in deep-water exploration. Geometry optimization through target oriented illumination analysis and multi-scale FWI may help to mitigate the risks of vertical cable survey. The combination of multi-scale FWI, low-frequency data and multi-vertical-cable acquisition system may provide both high resolution and high fidelity subsurface models.
Unconventional reservoirs, such as tight gas sand, coal‐bed methane and shale gas reservoirs will play an important role in stable domestic nature gas production in China during the next decade. The complex geological conditions and drilling risks are the major difficulties to the successful development of these resources. Integration of multi‐discipline analysis including geophysics, rock physics and geology will help to reduce the costs. We present a multistage, target oriented prestack full waveform inversion procedure to estimate tight gas sand in Xujiahe Formation in the northeast Sichuan basin, taking account of prior information from well log data and some basic geological knowledge. Spatial distributions of tight gas sand estimated from the final inverted elastic models are consistent with current well interpretation and gas production test results. The study results indicate that the multi‐stage prestack full waveform inversion strategy is practical for tight gas sand detection in this region.
Elastic properties of subsurface reservoirs often change during fracking, fluid ejection and production. Through repeated monitoring of seismic response of the same geological target at different stages, future hydrocarbon production plan may be optimized to reduce risks. Full waveform inversion is an effective method to extract high resolution, time-lapse elastic parameters using both seismic amplitude and phase information. However, the complexity of wave propagation phenomenon in real world and illness of the inversion problem make it difficult to get reliable and geological plausible model. We propose a target oriented constrained double difference waveform inversion method to enhance image quality of marine streamer and OBC data for deep buried target. A complex deep-water model is built to demonstrate the illumination properties for time-lapse monitoring of deep buried target. A spatial window filter in model space is used to apply the implicit constrains during each inversion iteration. Synthetic time-lapse waveform inversion tests showed that the new inversion strategy is more efficient for recovery of velocity change of deep-water reservoir.
Summary Unconventional resources play an important role for stable domestic energy supply in China. Recent years has seen great effects to reduce the high risks and high costs in seismic exploration and exploitation of the unconventional targets. The demands for high resolution, geological plausible reservoir models push the industry to work from post-stack domain to pre-stack domain, from a single parameter model, such as macro velocity or compressional wave impedance, to more complicated elastic parameters sensitive to lithology and stratum change. We present a target oriented, integrated multi-parameter seismic full waveform inversion (FWI) and interpretation workflow for unconventional targets. Real data tests showed Rock physics analysis, including well logging data analysis and virtual rock physics, may provide proper information to build multi-parameter waveform inversion constrains and for calibration of lithology and fluid for unconventional.
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