Page 81 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

63 between are waves criti- the as t, at the than critical the direct the how- receivers, layer the critically critically the

Techniques Selected raypaths and for the direct (dashed) (solid) arrivals. crossover = relationships refracted source, known the T-intercept less at arrive of function after arrives distant through is and direct distance is: distance

Seismic 2.27 graph critically refracted distance; X,, ~ show critically the to X, from waves a is present, at wave path travel the (X,,) crossover crossover

Source FIGURE travel-time and critical X, = distance. with: time 3.27 Fig. that Note from extending refracted distance (h): where direct refracted distance the + 2 ys 2h \ V,-YV;
Controlled 3.26b) (Fig, T-intercept slope Js = V, in curves arrivals. distance certain segment critically critical the that layer upper 2htan@, = X, wave, the before the of crossover time; beyond direct wave. The = X,,
line straight a is equation = t, travel-time and raypaths refracted critically and a beyond and at only line dotted (X,). The no because fictitious, shows 3.26 Fig. distance. of the thickness the and refracted the source the appears wave refracted more proportionally the At (V2). velocity same the at arrive waves the before

above The and e direct the observed diStance cal therefore js critical (6,) angle Near wave. The because ever, higher with refracted refracted

for
velocity the segments are:

the
by 1n 8,
divided times h/ cos Vv; +2

travelled travel receiver. # 2htan®, -Mane)
distance the (h) h/cos®, Vi 2hran8, ————_—_$_———— h/cos®@, Vv, to source is: receiver 2htan®, Vy X

the of thickness — — X = R= from to - T; Ty X= , , (1 constant) a
is segment layer i distance source + T; h/cos®, Vv, 2h/cos®, 2h T, = _ ? is (t,

Seismic Waves for an horizontal time = Law: aes
each upper from = T.. sin®, cos 8, = hcos8 i

time segment. For = travel 1an@, Snell's t,=
3 The X total that: that: Let:
Chapter that where: The But: so By so

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