Seismic Refraction Method.docx

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Seismic Refraction Method

Applications:

depth of weathering zone (used

for statics correction to seismic

reflection data), i.e., depth to bedrock

depth of groundwater table

depth of basement

depth of Moho

depth of any faster unit



For our purposes, assume flat (not necessarily

horizontal), homogeneous layers. In order to get

a head wave, V2>V1!



The critical angle is the incident angle where the

head wave begins:

Snell's law for multiple parallel layers

Refracted angle into one layer becomes incident

angle into next layer:



Travel-time curve for single horizontal layer on a half-space:

so velocities gotten from reciprocal of

the slopes of the direct and refracted

segments, and depth gotten from

reflected time intercept (or cross-over



distance). However, often only first

arrivals are recorded:

Single horizontal layer on a half-space, V2>V1:

Alternatively, in terms of Ti2, the intercept time

from the second travel-time segment,



Two horizontal layers on a half-space, V3>V2>V1

where the depth to the lower interface is the sum

of z1 and z2, where z1 is computed by the single-layer formula above.

Single dipping layer on a half-space, V2>V1:

Example of ambiguity problem: Shooting up-dip

gives apparent velocity that is too fast; vice-

versa. VA, up-dip velocity, is too fast (shallow



slope), VB, down-dip velocity is too slow (steeper

dip). Note that, without reversing the profile,

could not distinguish from horizontal case. Notealso the total travel-time from end to end is same

in either direction: reciprocity theorem.



Ambiguity Problem! Low Velocity Layers

never get a refracted head-wave from a

slow layer underlying a fast layer

eventually get head-wave when faster

layer (V>V1) encountered: e.g., V2<V1<V3

The travel time curve will look like this (another

example of ambiguity):



interpreter assumes a layer-over--half-

space model (can't "see" layer 2)

t0 (delay in getting down to layer 3 and

back) is large because V2 is slow

causes layer to be interpreted thicker

than it really is

Typical Reversed Seismic Refraction Profile

Seismic Refraction Method.docx

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