Research On Methods Of Velocity Modeling For Deep Seawater

The P-wave velocity of seawater is not only a function of time,but also a function of space.More and more studies from the field of marine seismic exploration have shown that improper seawater velocity values will lead to deviations in the imaging results of stratigraphic structures,which will become more obvious at larger water depths.The seafloor reflection,as the reflection that passes through the water body and has the strongest energy in seismogram,is of great significance for the study of the spatial distribution of seawater velocity.In order to build a correct seawater velocity model to improve the accuracy of migration imaging,the seafloor reflection traveltime is used as the constraint information,and the research results in hydroacoustics are reference to express the seawater velocity as a Munk-type sound speed profile or a simplified sound speed profile.Different realization methods of seawater velocity modeling are introduced:(1)Based on Fast Marching Mehtod(FMM)and the multi-stage traveltime algorithm,the method of inverting the Munk-type sound speed profile using Simulated Annealing algorithm is given;(2)The simultaneous inversion method of seafloor depth and seawater velocity using Newton method is given after the traveltime and traveltime derivative formulas under two-layer simplified sound speed profile are deduced,also we proposed a rayparameter extraction method for static correction data;(3)For 3D velocity modeling of seawater,firstly,the 3D double-layer velocity model is established,and the raypath and traveltime formula under the double-layer model are derived;Secnodly,the semi-analytical seafloor reflection traveltime calculation method is propsed for the specific source and receiving point positions;Finally,a fast3 D velocity modeling method based on Simulated Annealing algorithm and the semi-analytical method is given.In addition,the paper quantitatively calculates the plane wave reflection signal of water layer based on the reflection coefficient of continuous medium,analyzes and compares the difference in reflection characteristics between the Munk-type sound speed profile and the simplified sound speed profile,and explains why the velocity modeling of seawater uses seafloor reflections instead of water reflections.The Kirchhoff depth migration of the horizontal seafloor model and the undulating seafloor model verifies the effectiveness of different velocity modeling methods of deep seawater.Comparing the Munk-type velocity modeling method with the simplified one,it is found that the latter not only has higher calculation efficiency,but also has a calculation accuracy equivalent to the former.Besides,the simplified velocity modeling method can simultaneously invert the seawater velocity and seafloor depth,and shows a good appliacation effect for actual data.