558 

 Also, 



and the time gradient is 



SEISMIC METHODS 



^'-k^^O' 



dx 



Vi 



/^ - m 



[Chap. 9 

 (9-716) 

 (9-71c) 



_ _ 100 

 Ratio of Depth to Spread 



Fig. 9-82. Time gradient of reflection from horizontal bed. 



Another correction method is based on an expansion of the second 

 expression in eq. (9-72) into a binomial series. Consideration of only the 

 first two terms gives 



^■'('"21?)' 



SO that, with U as vertical time, 



d = ^Vi(^„ - At), 

 where A^ = x^/2y\L 



" = -2 



