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.

 

  1. Determine near which channel shot 163 was fired. Mark the shot location with a flag on the top margin of the plot (1 mark).
  2. Identify all linear events (2 mark)
    1. Refractions; note any changes in moveouts;
    2. Groundroll
  3. Locate the cross-over points if they exist (the distances where one refractions overtakes other refractions or direct wave). (1 mark)
  4. Pick the first breaks from the plots. Plot them on a T-X diagram (time vs. offset plot).  Use Matlab for plotting. (2 marks)
  5. Calculate the apparent velocity of each linear event, including the ground roll.  The apparent velocity equals 1/(inverse of the slope on the T-X diagram). (1 mark)
  6. Identify two reflection hyperbolas (1 mark)
  7. Pick the arrival times of these reflections and compute their stacking velocities using the X2 – T2 analysis (for explanations of the method, see below). (Use Matlab) (2 marks)
  8. Determine the velocity model. Comment on the agreement between refraction and reflection travel-time data (4 marks).
  9. What is the frequency of the noisy trace on channel 96?  What is this noise most likely caused by? (2 marks)
  10. Bonus 3 marks: Comment on the dispersive character of the ground roll. Dispersion describes the type of wave propagation in which the wavelet is continuously changing its shape. For a dispersive wave, its different frequency components propagate at different velocities. These velocities are called phase velocities. By contrast, group velocity is the velocity of propagation of the wave energy packet. For a dispersive wave, group and phase velocities should differ. Try and identify the group and phase velocities of the ground roll in the plots; are they different?

 

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.