Research On The Method Of Normal Moveout Under Wide Angle Reflection

As the target of oil and gas exploration in China is gradually transferred to the deep layer and ocean,the shielded layers of high velocity among the underground medium in some area with complex geological structures makes the seismic wave hardly to travel down.Conventional seismic exploration technology can't get more deep reflection information.Wide angle reflection and refraction profiling(WARRP)which has large offset can solve the problem.In the processing of seismic data which is received by WARRP,the conventional Normal Moveout method will make the reflection events seriously lack of correction and waveform distortion for large offset distance of wide-angle reflection.The major reason is that the travel time accuracy can't meet the demand of the real travel time of seismic waves in the long offset.In this paper,firstly a study is about the characteristics of wide-angle reflected wave in energy,frequency,phase and occurrence rule,which is convenient for recognition and processing.Making a discussion on the advantages and disadvantages of ray-tracing method and finite difference forward modeling and choose ray-tracing method which focusing on the kinematical characteristics of seismic wave.This paper compares the advantages and disadvantages of six kinds of methods which are based on Taylor series and Chebyshev series truncation.Then,getting a new equation which avoids Taylor series' s truncation error by calculating all-order derivatives of travel time t on offset x around the shot-point through the parameter equation.Finally getting the formula of wide-angle reflection Normal Moveout correction under long-offset conditions by mathematical derivation.Under the method of separated-offset and according to different geological conditions,choosing different formulas in the area of short-offset and long-offset.So the time-distance relation expression with high precision is obtained.Forward modeling results and test correction results under multiple models both concluded that the time-distance relation expression under the method of separated-offset has high precision within the whole offset range.This method solve the problem of correction and stretching.