深部地层时深转换中的拟合式选择问题

对珠江口盆地白云凹陷多口探井的数据分别用多项式和幂函数进行了时深关系拟合, 并对结果进行了分析讨论.对于深度大于钻井控制范围的地层和地壳的时深转换, 常用的三次多项式时深关系式只有三次项系数为负和二次项系数为正时才可用, 但在深部必然产生速度倒转的问题, 因此在理论上就不适用于深部; 二次多项式时深关系式虽然不会导致速度倒转, 但其速度向下匀速增大, 也不符合速度向下减速增大的规律, 常常导致深部速度超高.乘幂在1和2之间的幂函数拟合式D=atb+c有可能同时近似浅部和深部的时深关系, 是相对最优的关系式, 但也不是对每口井都能拟合出可用于全区和全部深度的时深关系.分区和分深度进行幂函数拟合是最理想的办法.针对区域性的时深转换, 可以分区对多口井进行幂函数拟合, 从中择优选用. 

The Selection of Fitting Curve in Time-Depth Transformation of Deep-Seated Strata and Crust

Polynomial and power functions have been used to fit the time-depth relation based on data from wells in the Baiyun sag,Pearl River Mouth basin,northern South China Sea.For the time-depth relation of the strata deeper than the termination depth of the wells,the 3rd-order polynomial function may be applicable only if the coefficient of its 3rd-oder term is negative and the coefficient of its 2nd-order term is positive.This function,however,is not applicable to the depth because it inevitably leads to the velocity inversion at depth.The fitting of 2nd-order polynomial function does not cause the velocity inversion,but the velocity increases linearly towards the depth,violating the natural law that the velocity increment decreases downwards in the Earth's strata and crust.The power function D = at b c with 1