. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. EWINO [chap. 1 explosions is capable of penetrating to dc])ths of thousands of meters, and the seisinic-reriection techni(iiie using explosions has been one of the major tools of sub-surface exploration in the past few decades. The propagation of seismic energy can be considered adequately for most purposes to behave according to ray theory. The shot point is the origin of ray paths which extend in all directions, and each ray can be considered to be a bundle of energy travelling that particular ])ath. Only the rays that l
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. EWINO [chap. 1 explosions is capable of penetrating to dc])ths of thousands of meters, and the seisinic-reriection techni(iiie using explosions has been one of the major tools of sub-surface exploration in the past few decades. The propagation of seismic energy can be considered adequately for most purposes to behave according to ray theory. The shot point is the origin of ray paths which extend in all directions, and each ray can be considered to be a bundle of energy travelling that particular ])ath. Only the rays that leave the origin and eventually arrive at a detector are of interest. In a multi-layered medium, there are several possible paths, most of them involving reflection or refraction at the interfaces between the layers. The simplest case to consider is that involving reflections from horizontal plane interfaces. The lower jjart of Fig. 1 shows the ray paths of the direct. Fig. 1. Ray diagram and time-distance graph foi- reflections in a tliree-layer model, Ci<C2<C'3. O = detector X = shot Oi = Of waves and of the first-order reflected waves from two shots to a detector. The shots and detector are in Layer 1, which has a seismic velocity Ci and thickness ^1. Underneath are two layers with velocities C2 and C3. The upper part of the figure shows the travel time vs. distance graph for the arrivals. The direct waves determine the straight line, D. The reflected waves determine the hyper- boUc curves, Ri and Rn.^ The inverse slope of D, ADfAT, gives the seismic velocity in Layer 1. The Ri curve is asymptotic to D. Its intercept on the ordinate, 2Ai/C*i, gives the travel time for a reflected wave from the top of 1 In this and succeeding discussions, Rj, Rh, Rm- . .designate reflected waves from the 1st, 2nd, 3rd. .interfaces below the surface. Arabic subscripts R2, Ra- â .designate second, third. . order reflections from the sea Please note that these images are extracted
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