. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 74 KWINti AND NAFK [('HAP. r> in an area of thick sedimentary cover. (For a general description of time- distance graphs and of their relationship to various structures, the reader is referred to Chaj)ter 1.) This is ty|3ical of the seismic data from which sedi- mentary velocities and thicknesses are computed. The Ri, Rn and Rm curves are determined bv reflected waves from the first, second and fourth sub- surface interfaces respectively. The lines G3 and G4 are determined by refracted arrivals associated with the tops
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 74 KWINti AND NAFK [('HAP. r> in an area of thick sedimentary cover. (For a general description of time- distance graphs and of their relationship to various structures, the reader is referred to Chaj)ter 1.) This is ty|3ical of the seismic data from which sedi- mentary velocities and thicknesses are computed. The Ri, Rn and Rm curves are determined bv reflected waves from the first, second and fourth sub- surface interfaces respectively. The lines G3 and G4 are determined by refracted arrivals associated with the tops of Layers 4 and 5. Note that no Gi line (corres- ponding to refracted arrivals along the sea floor) is observed. This is tjrpical of practically all, if not all, deep-ocean areas. The significance of the absence of ^^ ^v\ PROFILE 98 Q z. - 8_ LlI to UJ -15 PROFILE 99 Rs ^ (ill 1 1 1 1 1 1 1 1 _l - UJ ^- \- . ^ G 1 1 1 - 10 "R5,..''' 1 1 1 1 1 1 1 64 1 1 , 1 , , 15 Velocity (km/sec) 3 4 ! ~i 1 r 10 5 0 5 10 DIRECT WATER WAVE TRAVEL TIME IN SECONDS 15. Loyer 5 Fig. 1. Tiine-distance graph, sLructui'e section and I'ay diagram for end-to-end profiles Sl-98 and 99. Velocity versus depth relationship shown at left of section. Raj' paths at left of receiving position are for waves refracted at interfaces. At the right are ray paths for reflected waves and waves refracted by the velocity gradient. such arrivals is that, in most places, the seismic wave velocity in the upper part of the deep-ocean sediments must be equal to or less than that in the water at the sea floor. The fact that reflected arrivals are received from this interface indicates that the acoustic impedance, pC (density x velocity), is different above and below the interface. The water and sediment mixture must be more. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemb
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