Page 105 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

87 apparent arrival are shoots updip pro- ver- also updip
X surface velocity extend- seismic and (see
Models <= receivers in a downdip apparent velocity (V.,) lower a steeper angle; an velocity slope. the source true the shooting refraction receivers The below and downdip

interpretation Updip Apparent Velocity - an 7 higher The negative of its at observed the When than lower from results reversed a to A at 4.14a). Fig. A; same horizontal the presented the in B) to (from A A). to B

Shooting High “ the source, rays emerge at basement. because velocities layer. is velocity velocity recording shotpoint a to B with equations shooting 5 downdip (from updip

b) emerge at a shallow angle, resulting in updip from from crystalline out stands apparent refracting the apparent true the by resolved from (as (from direction superimposed the to 1976, p. 281-284). times intercept by # ty shooting shooting

\ Seismic survey of a dipping refractor. a) Selected raypaths shooting toward receivers are be Moho) the 4.13), (Fig. of velocity the receivers, than higher be can direction one other are 4.14b) according al., et the interface, when intercept when intercept

Downdip Low Apparent Velocity \ \ refraction the (V,).b) When than V, results. might refraction from probably Interface interface true the the toward velocity a interface dipping in shot is the in then (Fig. records analyzed then 80-85; Telford dipping equal: not T-axis T-axis

\ km/s; Dipping dipping to equal 4.13a); 4.13b). The profile B), travel-time scales, 1992, p. a For are = tg = t,,
Shooting ==> X 4.13 direction. Raypaths for the critical velocity the true higher (V.,) km/s 6 (8 Single a For not downdip (Fig. (Fig. file. A to ing tical Burger, directions where:
_

a) FIGURE than velocity

Positions of direct and arrivals on seismic Same profile as (a), reduced travel time of apparent velocity graph. the 6 km/s have positive slopes 6 km/s > the (toward in plotted time travel an to relative have positive slopes; have km/s refrac- first The strata).

a) profile. b) a are horizontal on the right); velocities negative direction is 4.12a actual 6 > V,, and sedimentary
4.12 critically refracted but plotted with 6 km/s. Arrivals with < a Fig. events with arrival
FIGURE refraction of 6 km/s Velocities (toward in slope left). in of plotting identify km/s 6 events direct

graph Instead «. =e km/s to easier < V,,, with 3) and the that indicating

Interpretation X => time. travel-time the km/s. of 6 time plotted, where: is Teg = T - 6 it makes events 1) horizontal; show thus slopes (perhaps km/s 6

Refraction Source From travel for information travel reduced (T,,4) time display km/s: of 6 (V,,) are km/s 6 positive than of less

Seismic Distance reduced = same a with travel reduced travel-time velocity = V,, with velocities

4 Teg The 4.12b reduced events negative slopes. The have
Chapter where: Fig. (T), the The apparent 2) tion

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