Fig. 7 Comparison of the exact solution and the approximate solution of travel time in a sandy soil. 
sk = 0.4, rk = 0.05, 
k = 12 m-1, Ksk = 2.8 x 10-5 m s-1, and q = 1 x 10-6 m2 s-1. Subscript k stands for the true value of the corresponding parameter