Geol 335.3
Lab
3 – Interpretation of Seismic Data
The purpose of this exercise is to
identify events and provide an interpretation of seismic shot records. You were given two plots of the same seismic
reflection record from one shot,: unscaled and with
exponential scaling applied to compensate the geometrical spreading. Also a plot of band-pass filtered records
with 200-ms Automatic Gain Control (AGC) is provided.
The data
set has symmetric split-spread geometry. The offset is in meters, and the trace
spacings are 20m.
Note that display is the key component of seismic processing – even with
a simple change in scaling, the records reveal different aspects of the data.
In the following, indicate all the events with colored pencils or
highlighters.
Theory:
For a
two-layer problem (a layer of thickness Z1 and velocity V1
overlying a medium of velocity V2):
Travel-time
equation of a Direct Wave is: .
The direct
wave thus allows estimation of V1.
First
Refraction (headwave) travel-time: .
Thus, in
the X-T method, from measuring the slope of the refraction in the (X,T),
your determine V2. Further, by extrapolating the refraction travel-time
line to zero offset, you will measure thee zero-offset intercept time, , and from it, determine Z1.
The X2-T2
method utilizes the
hyperbolic shape of a reflection in (X,T) plane in order to estimate
the optimum (“stacking”) velocity. Travel time of the reflection recorded at
offset x from the source is (from Pythagorean theorem):
.
This
equation describes a hyperbola t(x). If we consider t2
as a function of x2 instead, the relation becomes:
which is an
equation of a straight line in (x2,t2)
plane. From the slope of this line, you can determine V1, and
from its zero-offset intercept value, , determine Z1.
Hand in:
Annotated plots and write-up in a binder.