Research On High Resolution Velocity Analysis Methods Of Converted Wave

With the development of lithological exploration,the P-wave and S-wave joint exploration attracts more and more people's attention.Compared to the sole P-wave exploration,the P-SV converted wave exploration is easy and feasible and has the low cost. Converted wave data can provide more extensive information on underground rock,these information play an important role at lithology inversion,fractured parameters extraction and gas detection.Now P-SV converted wave exploration has become the main means of S-wave exploration,particularly along with the multi-component submarine cable technology's use,the processing of converted wave is gradually developed to the conventional production and is becoming a new hot spot in Geophysical field.In seismic exploration,high resolution seismic data processing has been widely used,along with the high request of exploration precision.But in the processing of high resolution seismic data,high resolution stack velocity analysis is one of the important links.In converted wave data processing, velocity parameter is most important and most difficult to grasp.Therefore, how to seek the exact and reasonable velocity is the key point in the entire converted wave processing.In particular,after picking up the high-precision, how to pick up the exact S-wave velocity parameter is becoming a major challenge in converted wave data processing. Because of the ray asymmetry of the downgoing and upgoing waves, the velocity analysis based on the CMP gather cannot be adapted to that of the converted wave(CCP). Main difficulties are from special characteristics of the CCP bin and CCP time-distance relation. For one thing,even if we assume that the reflection layer is homogeneous and horizontal,the position of the common converting reflection point will be changed with the offset,the depth of reflection layer and ratio of the S to P wave velocity. For another, the time-distance relation to make out velocity analysis,we cannot get the correct value of S-wave velocity ,especially with the shallow-middle events or large offsets. On the other hand ,if we analyze the velocities using all traces traditionally ,the resolution of velocity spectra will decrease greatly when one wishes to distinguish between neighboring primary events from reflectors with conflicting dip,or to identify primaries in the presence of multiples.This paper,based on the foundation of predecessor's work,through to the above question's analysis,research and summary,has obtained certain innovations.In the conventional velocity analysis algorithm foundation, this paper introduces two velocity examination factor and has realize two high resolution converted wave velocity analysis methods:the convariance measure and ECM(Enhanced Complex Matched filter).Simultaneously, it aims at the conversion point formula and chooses the best conversion point exact solution formula.,which is applied to CCP gather separation based on the time window .This process is going along with the two high resolution velocity analysis method in order to achieve the converted wave velocity analysis arithmetic.The results on theoretical models and actual material show that the two velocity analysis methods proposed in this paper obtain a higher resolution in time and velocity. Coherency measures for velocity estimation are currently widely in use,the majority of coherency measures are based on beam forming or crosscorrelations between traces along hyperbolic trajectories in CDP gathers.These method don't account for the noise associated with nearby and interfering events, residual statics,event moveouts and other nonrandom noise that commonly occurs.Resolution between events closely spaced in time and velocity is precluded because interfering events stack constructively at a wide range of times and velocities.These effects result in the broad maxima and poor velocity resolution characteristic of stack-based measures.The complex matched filter,as a velocity analysis method in complex field,is based on semblance coefficient rule and need to obtain the data by Hilbert transform.This method goes forward a step than the tranditonal method,but hasn't made the substantive change.Enhanced Complex Matched filter analysis is an improvement over Complex Matched filter analysis.It could be obtained by removeing contribution from interfering events and noise for each time corrected window.The SVD decomposition is proposed for the coherent weighted stack of NMO corrected data for velocity analysis.This method improve the resolution in time and velocity,but ECM method needs wavelet information.The convariance measure uses the eigenvalues of the covariance matrix to estimate signal-to-noise ratio and construct energy function.When velocity searches are performed about a single event,the energy within each window at incorrect velocities is spread among all the eigenvalues.When the correct velocity is reached all the energy of the signal is expressed in the major eigenvalue of R,resulting in a sharp decrease in the magnitude of the minor eigenvalues.This results in the confined maxima and enhanced resolution provided by the convariance measure.This measure avoids the fault of the traditional velocity analysis method,due to the method's sensitivity and resolution.It can obtain better resolution for interfer events due to the sensitivity of signal correlation between windows.The improvement offered by the convariance method over standard crosscorrelation-based techniques are indicative of the importance of noise estimation problem.This paper aims the converted-wave seismic data processing by applying the technique more efficiently based on the achievements gotten already and the technique exists.By comparison with P-wave,the partial key points, the conversion point,CCP binning and converted-wave velocity analysis,are discussed emphatically which are different from those of the P-wave seismic data processing.Based on the previous work,by analyzing,researching and concluding the problem above,this paper has finished the following researches:First,by the consult and collection of data,this paper analyzes and researches converted-wave data processing and the related content:(a) the transmission character of converted wave;(b)the affecting factors and the formula of CCP location;(c) converted wave's travel-time calculating formula;(d)the travel time curve relation of converted wave;(e)velocity analysis methods of converted wave.Second,in isotropic medium,we set up several underground media models and simulate the P-wave seismic records,which use the semblance, the convariance,the complex matched filter and ECM method to make velocity analysis.At last this paper compares the result of several velocity spectrum curves.Third,comparing several formula's accuracy and effect, we test the effect in the theoretical model in order to exacting CCP gathers.In the matlab platforms, we carry out the CCP gather selection.Fourth, we set up several underground media models and simulate the converted-wave seismic records.After exacting CCP gathers,this paper make the converted wave velocity.Fifth, research on the implementation algorithms of velocity analysis method, concluding the semblance, the convariance and ECM.Sixth,this paper summarizes the above work, and three methods above were checked and compared in the theoretical data.we make feasibility test of this two high resolution velocity analysis and summarize the advantage and applicable cases of several velocity analysis methods.