Study On RVSP Seismic Imaging For Coalbed Methane Exploration

To improve the capability of coalbed methane abundance prediction by boreholeseismic exploration, enhance the level of coalbed methane exploration anddevelopment technology, complement China’s energy structure and reduce theincidence of coal mine gas outburst accident quickly, this dissertation, based on thetraditional VSP seismic data processing and imaging methods, systemically studiedCBM RVSP seismic data imaging method. Currently, the main problems of thetraditional borehole seismic processing and imaging includes:①RVSP seismic datahas serious impact due to tube wave and multiple wave;②Traditional RVSP velocityanalysis methods cannot build accurate velocity model of the stratums under thedeepest shot position in well;③Existing RVSP seismic data imaging methods arealmost isolated with velocity analysis, so image quality is not as good as the desiredeffect should be.To address the issues described above, this dissertation carried out in-depthseveral study points as following:①RVSP reflected wave time-distance curveequations and reflection point calculation formulas in the homogeneous medium,horizontally layered medium and continuous change medium have been derived indetail.②According to the typical CBM reservoir geology characteristics, seismicwave propagation under observation system of RVSP has numerically simulated,besides the wave field characteristics of RVSP record and the source and nature ofnoise also have been analyzed to study how to separate the up reflected wave andsuppress noise effectively.③Based on two-way acoustic wave equation RVSPpre-stack time migration, as well as analysis of migration noise sources andcharacteristics, boundary condition of RTM has been studied to suppress themigration noise and improve the imaging quality by solving the issue of large storagespace requirement of RTM.④New velocity analysis methods have been studied toobtain velocity information of the stratums under the maximum excitation depth andthe characteristics and scopes of RVSP-CDP transform stack imaging, based oncommon reflection point gathers and common shot gathers speed spectral analysismethod.⑤A completely new imaging method has been studied for RVSP data, basedon the RVSP-CDP gathers extraction, which could apply surface seismic dataprocessing ideas, such as velocity spectral analysis, NMO correction, residual staticsand stack migration.Based on the above research contents, this dissertation has made some important achievements as follows:①Hybrid boundary condition for RVSP pre-stack timereverse migration is proposed, which combines the adaptive stochastic nonlinearboundary and PML absorbing boundary conditions.②Coherent velocity spectrumanalysis methods, based on common reflection point gathers and common shotgathers, as well as surface seismic and RVSP time-distance curve equations, areproposed.③RVSP perfectly equivalent surface seismic data processing and imagingmethod is proposed, based RVSP-CDP gathers extraction.Finally, using the above research results, a set of experimental CBM RVSP data,acquired in Wanbaoshan regions of Qinshui Basin, was analyzed and processednarrowly. By the analysis of the characteristics of wave field and noise of the originaldata, reflected waves are separated effectively, and receiver static correction iscompleted by applying tomographic static correction technique, lastly RVSP-CDPgathers extraction, velocity analysis, residual static correction, NMO correction andstack imaging process are realized, this imaging result is the best profile comparedwith the imaging result from surface seismic data on the same survey line and theresult achieved from the same RVSP data but by a simple processing method.Through the above research achievements, a set of CBM RVSP seismic dataprocessing and imaging technology processes is eventually formed preliminarily. Insummary, these achievements of this study has practical significance in furtherenhancing the level of CBM abundance prediction by borehole seismic exploration.