Waves in strata |

From the assumption that experimental data
can be fit to hyperbolas
(each with a different velocity and each with a different apex )
let us next see how
we can fit an earth model of layers,
each with a constant velocity.
Consider the horizontal reflector
overlain by a stratified **interval velocity**
shown in Figure 3.10.

stratrms
Raypath diagram for normal moveout in a stratified earth.
Figure 10. | |
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The separation between the source and geophone, also called the offset, is and the total travel time is . Travel times are not be precisely hyperbolic, but it is common practice to find the best fitting hyperbolas, thus finding the function .

An example of using equation (3.24) to stretch into is shown in Figure 3.11. (The programs that find the required and do the stretching are coming up in chapter .)

nmogath
If you are lucky and get a good velocity,
when you do NMO, everything turns out flat.
Shown with and without mute.
Figure 11. |
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Equation (3.21) shows that
is
the ``root-mean-square'' or
``RMS'' velocity defined by
an average of over the layers.
Expressing it for a small number of layers we get

The two-way vertical travel time in the th layer is related to the thickness and the velocity by

Next we examine an important practical calculation,
getting interval velocities from measured RMS velocities:
Define
in layer ,
the interval velocity
and the two-way vertical travel time .
Define the RMS velocity
of a reflection
from the bottom of the -th layer
to be .
Equation (3.25) tells us that for
reflections from the bottom of the first, second, and third layers we have

Normally it is easy to measure the times of the three hyperbola tops, , and . Using methods in chapter we can measure the RMS velocities and . With these we can solve for the interval velocity in the third layer. Rearrange (3.30) and (3.29) to get

and subtract getting the squared interval velocity

For any real earth model we would not like an imaginary velocity which is what could happen if the squared velocity in (3.33) happened to be negative. You see that this means that the RMS velocity we estimate for the third layer cannot be too much smaller than the one we estimate for the second layer.

Waves in strata |

2009-03-16