Page 85 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

67 Selected raypaths and reflected waves for a interface separating a higher a lower velocity the source from and X,, the crossover distance arrives before the direct sum- are arrivals refracting interface
Seismic Techniques 3.31 travel-time graph of direct, critically refracted, and horizontal velocity (V,) layer from (V,) surficial layer. X, is the critical distance (closest distance where the critically refracted wave is observed) (beyond that distance the critically refracted wave ; three the receiver from below refracting interface.

Source FIGURE wave). for times to source layer. up and t

Controlled travel x Vi; wave from of upper to, along, of layer

b< —= > compute direct of distance down time seismic yelocity intercept: Ee 8, V1

—— to used time travel horizontal seismic velocity travel = = T-axis = 4

x x or Equations = T, = X = V, Refracted Wave: T, V, t,

3.31). below.
Cc Direct Wave: :
x ; ; marized 7 Critically
(Fig.

.

.

:

and other arrivals. the for 3.30). (Fig. time (to) therefore is the (T,); layer critical the At ee time; re ection. the . horizontal, a For and reflected graph travel-time

reflected the source. b) The critically refracted graph arrivals large, the T-intercept distances the upper distance. a a of yperbola . refracted, a on

travel-time the refracted very Th , large <at through Critical b) — have the to Equations and Curves, critically plotted

Refracted graphs illustrating relationships between arrival curves are asymptotic at large distance from (X,)- the critical distance at 3.29¢), with: (Fig. intercept time. T-axis = how illustrate critically and direct becomes X As x he+ = == v, T= \ 2 0 Vy recorded reflections wave direct the for wave. direct the to refracted ee tangent

Critica lly and travel-time direct arrival of the hyperbola a of ty circumstances the to relates the source. that: insignificant, So for (T,) curve as same the asymptotic iS and ae a racted re the for 3.20), (Fig. geometric simple

Seismic Waves Reflected Selected raypaths and reflected that to equation the special Two wave distance from travel-time approximately wave reflected a be line straight Summary model two-layer show waves

Chapter 3 3.30 and is tangent curve is This reflected Long a) becomes The the

FIGURE a) The direct arrival

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