Hình thành nước ngầm do nước trên bề mặt ngấm xuống, do không thể ngấm qua tầng đá mẹ nên trên nó nước sẽ tập trung trên bề mặt, tùy từng kiến tạo địa chất mà nó hình thành nên các hình dạng khác nhau. | 40 Wolfgang Rabbel Fig. . Ray geometry to be considered to derive the Plus-minus method of Hagedoorn 1959 . The meaning of the plus-time t x can be understood by regarding 1 the refracted rays connecting the geophone at x with shot points A and B and 2 the positions of the refracted wave fronts when they reach the same point P beneath the geophone. P is the foot point of the interface normal pointing towards geophone position x. The refracted wave fronts emerging from shot points A and B arrive at P at the same travel time as they arrive at points QA and QB on the up-going parts of the respective refracted rays. Since in addition P-x-QA and P-x-QB form congruent rectangular triangles the following travel time equations apply to respective segments of the refracted rays T2 x xA t2 x QA t2 QA xa T2 x XB T2 x Qb T2 Qb xb T2 xa xb T2 P xa T2 P xb T2 QA xa T2 Qb xb t2 x QA t2 x Qb . Therefore t x can be identified as t x 2-T2 x QA 2-T2 x Qb This deviation will cause some inaccuracy in cases were the refracting interface is not locally plane. To image the interface segment vertically below x the slopes of the refraction time derivatives would have to be computed at points x x z-tan iC ỗ for shot points A and B respectively. Alternatively the ray diagram Fig. shows that the same range of interface points will be covered by both shot observations if right and left hand segments of the respective travel time curves are considered namely - - - T2 x xA for x x x s and T2 x xB for x-s x x where s 2 r cos iC 2 J cos iC Ổ The velocity v2 x of the bottom layer can be computed from averaging the slopes of these curve segments. 2 Seismic methods 41 Fig. . Horizontal shift of segments of reversed refraction travel time curves belonging to the same of interface segment cf. Eqs. and Consistency criteria of seismic refraction measurements The above consideration of the two-layer case shows that there are a number of criteria of field layout and travel
