Method for wave equation velocity replacement of the low-velocity-layer in seismic data processing

Abstract

A method for correcting seismic data for the effects of the low-velocity-layer. According to the method, estimates of the seismic velocity in the low-velocity-layer and the depth of the low-velocity-layer are used to extrapolate the seismic data to simulated source and receiver positions located at the base of the low-velocity-layer. The data are then extrapolated to an arbitrary reference datum using a selected replacement velocity. The principles of wave equation datuming are used for these extrapolations.

What is claimed is: 
     
       1. A method for correcting seismic data for the effects of the low-velocity-layer, comprising the steps of: a) obtaining estimates of the seismic velocity in the low-velocity-layer and the depth of the low-velocity-layer;   b) using said estimates and the principles of wave equation datuming to extrapolate said seismic data to simulated source and receiver positions located at the base of the low-velocity-layer;   c) selecting a replacement seismic velocity for said low-velocity-layer; and   d) using said replacement seismic velocity and the principles of wave equation datuming to extrapolate said seismic data from said simulated source and receiver positions to a selected reference datum.   
     
     
       2. The method of claim 1, wherein said estimates of the seismic velocity in the low-velocity-layer and the depth of the low-velocity-layer are obtained by a first-arrival refraction analysis of said seismic data. 
     
     
       3. The method of claim 1, wherein said replacement velocity is the seismic velocity in the subsurface formation immediately below the low-velocity-layer. 
     
     
       4. The method of claim 1, wherein said selected reference datum is a horizontal datum plane. 
     
     
       5. The method of claim 4, wherein said horizontal datum plane is located above the highest surface location represented in said seismic data. 
     
     
       6. The method of claim 1, wherein said estimates of the seismic velocity in the low-velocity-layer and the depth of the low-velocity-layer are obtained by a) selecting at least three gathers of seismic traces from said seismic data, the traces within each of said gathers having a selected offset;   b) determining for each trace within said seismic trace gathers the first break traveltime, said first break traveltime representing the transit time from source to receiver along the refraction interface at the bottom of the low-velocity-layer;   c) developing for each trace gather an equation relating the known transit time to the sum of (i) the source-receiver distance divided by the seismic velocity at the refraction interface,   (ii) a source contribution, and   (iii) a receiver contribution; and     d) solving said equations using least squares simultaneous solution methods.   
     
     
       7. A method for correcting seismic data for the effects of the low-velocity-layer, comprising: a) performing a first-arrival refraction analysis of said seismic data to obtain estimates of the seismic velocity in the low-velocity-layer and the depth of the low-velocity-layer;   b) using said estimates and the principles of wave equation datuming to extrapolate said seismic data to simulated source and receiver positions located at the base of the low-velocity-layer;   c) selecting a replacement seismic velocity for said low-velocity-layer; and   d) using said replacement seismic velocity and the principles of wave equation datuming to extrapolate said seismic data from said simulated source and receiver positions to a selected reference datum.   
     
     
       8. The method of claim 7, wherein said replacement velocity is the seismic velocity in the subsurface formation immediately below the low-velocity-layer. 
     
     
       9. The method of claim 7, wherein said selected reference datum is a horizontal datum plane. 
     
     
       10. The method of claim 9, wherein said horizontal datum plane is located above the highest surface location represented in said seismic data. 
     
     
       11. The method of claim 7, wherein said first-arrival refraction analysis further comprises the steps of: a) selecting at least three gathers of seismic traces from said seismic data, the traces within each of said gathers having a selected offset;   b) determining for each trace within said seismic trace gathers the first break traveltime, said first break traveltime representing the transit time from source to receiver along the refraction interface at the bottom of the low-velocity-layer;   c) developing for each trace gather an equation relating the known transit time to the sum of (i) the source-receiver distance divided by the seismic velocity at the refraction interface,   ( ii ) a source contribution, and   (iii) a receiver contribution; and     d) solving said equations using least squares simultaneous solution methods.