GB1317765A - Interval velocity determination - Google Patents

Abstract

1317765 Processing seismic data MOBIL OIL CORP 4 Aug 1970 37594/70 Heading H4D A method of computing the internal velocities of reflecting zones from common depth point seismic data comprises using the time dip of the Nth reflection to specify the ray path in the zones thereabove, determining the ray path in in the Nth zone to determine the dip of the Nth reflecting zone, computing arrival times of reflected energy at spaced receivers for a plurality of assumed values of interval velocity, comprising with the actual arrival time from said seismic data, choosing the value of interval velocity giving the least error and iterating the process for successive zones. The average velocity of seismic waves travelling paths to a number of reflecting interfaces and the dip of the interfaces is determined by assuming values of velocity and dip and plotting the zero-lag crosscorrelation function for every trace (Fig. 7, not shown). Peaks in these plots indicate the average velocity Va and dip #. By plotting the maxima in the curves the zero offset time T 0 of each interface may also be found. Using this information a computer is used to generate a layered model of the subsurface. Using the average velocity in the first layer and the dip of the first interface and assuming a plurality of valves of the interval velocity in the second layer the corresponding depth and dip of the second interface can be calculated using Snell's Law or the Eikonal equations. This enables a series of curves to be computed representing arrival times at points along the line of investigation which is compared to the field data. The curve giving minimum error indicates the average velocity in the second layer. The error is checked for magnitude and if above a predefined threshold the layer is ignored. If the error criterion is satisfied and the velocity is above a predefined minimum the values of V N and # are stored and used to calculate the values for the next layer.