Thin Layer AVO And Influence Of Frequency On Thin Layer Seismic Response

AVO technology is to predict and identify oil and gas reservoirs by using the relationship between the amplitude of reflected waves on prestack gathers and offset(or incident angle).However,in actual seismic data,the underground medium is generally thin layer or thin reservoir,which will affect the variation rule of AVO response.Therefore,this paper transits from the traditional single-interface AVO technology to thin-layer AVO response.Through forward modeling of thin-layer AVO and discussion of the influence of frequency on thin-layer,the limitation of AVO technology on single-interface is overcome and can be extended to thin-layer exploration.Therefore,the study of thin-layer AVO is of great significance for identifying thin reservoirs,thin layers and hidden reservoir detection.The main work includes the following points:Firstly,discuss and explain the basic concepts and principles of AVO,And the properties of petrophysical parameters in AVO analysis,Then,the reflection and transmission of incident waves on the single interface of homogeneous medium are analyzed,and Zoeppritz equation and its approximate formulas with different emphases are derived.Then,Ostrander gas-containing model and Goodway gas-containing model are used to discuss the response characteristics of reflection coefficients in these two cases.Considering the comprehensive factors,a high accuracy and concise AVO approximate formula-Shuey approximate formula is selected.This paves the way for the follow-up study of thin layer peak frequency,and then analyzes and obtains the reflection coefficient characteristics of four types of classical gas/water-bearing sandstone,namely AVO response.Secondly,when the incident wave propagates in a homogeneous layered medium,Brekhovski equation can be derived.After simplifying the equation,the reflection and transmission equations of incident waves in a three-layer medium can be obtained.The models of gas-bearing sandstone with different Poisson ratios are established,the influence of quality factor Q(formation absorption factor)on the reflection coefficient characteristics of thin layers is also discussed,and the reflection coefficient characteristics of thin layers under different incident wave frequencies and different intermediate layer thicknesses are obtained.It is found that the incident wave frequency,intermediate layer thickness and formation absorption factors will change the variation law of reflection coefficient characteristics of thin layers.Thirdly,the thin-layer AVO response is analyzed based on numerical simulation.Considering the influence characteristics of wavelet dominant frequency and thin layerthickness on AVO under the condition of formation absorption,the linear approximation analysis method is also used.The approximate value of AVO is obtained,and the conventional seismic analysis of the gas-bearing model is carried out by numerical simulation method.The reflection coefficients obtained by numerical simulation are compared with those obtained by AVO approximation method,which verifies the accuracy and credibility of AVO approximation method.Finally,according to the previous research foundation,the influence of thin layer thickness and wavelet incidence frequency on the peak frequency of thin layer is discussed.According to the reflection coefficient characteristics of the top-bottom interface of thin layer,thin layer is divided into four categories,and the relation expression between seismic peak frequency and thickness is derived,and the expression suitable for different types of thin layer is obtained by higher-order approximation.The characteristic curves of the peak frequencies of these four kinds of thin layers varying with the thickness are obtained,and the variation range of the relationship between the peak frequencies and the thickness,i.e.The gauge plate,is obtained by any combination according to the reflection coefficient characteristics of the top-bottom interface of the thin layers,which provides a new idea for us to predict the thickness of the thin layers.At the same time,the model is tested.The peak frequency characteristics of thin layers under AVO are also discussed.It is found that the thickness and wavelet incidence frequency have certain influence on the peak frequency of the thin layer.