Page 107 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

89 for 4.14b) the of from downdip

Models solving by (Fig. graph slope the (V,) layer shooting : downdip

Interpretation determined be (V,/Vou) (Vi/ Vo.) travel-time of inverse the refracting the (X), distance shooting updip: shooting

easy = see V,,, 2a «) — sin (8, downdip shooting updip shooting layer overlying can critical angle the sin“ — sin™'(V,/V2q) 2 sin”! + (V,/Voa) 2 the from determined as (V,) layer overlying of velocity the for 1 arrival of direct slope Vi sin®, horizontal at receiver het Dy + | when interface to Viti 2 hi, 4" 20s @, when interface to distance te Vi =

1

&

=
apparent velocity velocity apparent of the velocity angle critical interface. the of dip and interface of the simultaneously: a= sin”! i _ be can velocities the of velocity solving then and Y= Va a to times (T,), are: ee 0. L 6. 2h, ae l perpendicular distance perpendicular

Va Vou V, = 6, = = a dip The V,, true The the reading arrival, Law: travel The updip and — em = hy = h,

where: and V,, by direct Snell’s (Ty) where:

the
B
reversed refraction the combination of surveys downdip and updip directions the source at A, from traverses B, at the shot A. The are in (b) the same. Apparent in (b) are different, rays emerge at different for the downdip (Fig. 4.13a) and is deeper beneath intercept time (t,,) for updip when shooting as uti- refrac- times

A ray receiver at as a ray from receiver at times T,, and Ty, angles. b) Superposition of travel-time updip (Fig. 4.13b) surveys. Note that, interface is greater than same the is raypath if downdip
4.14 is (Fig. 4.13). a) A to a therefore exactly velocities V,, and V,, the the the (t,y). be verify reciprocal the in
FIGURE profile shooting in shooting same course shooting to a reciprocal because curves because B, Shot shooting downdip to has the exact can one T,,. If shooting

however, B A, 4.14b), = when

s oS B, at at A, because at at (Fig. if T,, arrival a)
Interpretation city Velo receiver a receiver equal: be Tea = Tas receiver to at A receiver to at B plot refraction determining will erroneous. be refracted Vv, + in (0, =

Refraction ——> App arent igh Hi shot the to A at shot to a B at must from time shot from time shot reversed by interface below analysis velocities the for are: Vea

Seismic Velocity 4 from time the from time reciprocal times the = travel = travel events a on same the from the same, apparent directions

Chapter4 Low Apparent travel The travel the Thus lized. where: Tg Tpa picking tions are the not The and updip

88 es In are