Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Extracting Physical Properties from the Shot Record and The Two Layer Refraction Problem

The critical distance and the crossover distance.

We now have several equations which contain quantities that we can measure directly from the shot record and use to determine layer thickness - h 1. h1h1 ?

Based on what you have learned so far what information could you extract from this record?

2. Suppose that a layer with a velocity of 1500 m/s and a thickness of 100 m lies above another layer with a velocity of 3000 m/s. Compute the expected crossover distance and intercept time for the critically refracted waves. Solve the Following Problem (see today’s handout)

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In the three layer problem the number of possible terms that could potentially be measured directly from the shot record includes - V 1, V 2 and V 3 two reflection time intercepts two refraction time intercepts one crossover distance, and two critical distances

Pitfalls - 1) We have assumed that our layers have successively higher and higher velocity. What happens if we have a velocity inversion - let’s say V 2 is less than V 1 and V 3 ? 2) Another assumption we have made here is that the refraction from the top of the third layer, for example, will actually show itself, and not get buried somewhere beneath the earlier refraction and reflections. This can happen if the 2 nd layer is too thin.

The presence of the velocity inversion delays the refraction from interface 2 and leads us to overestimate its depth. In addition, we have entirely missed the presence of the second layer. We overestimate thickness because we incorrectly assume that the refraction event traveled down to the refractor with a single velocity of 15000fps.

Dobrin and Savit, 1988 The next 4 slides contain some general information about the acoustic properties of various rock types.

Dobrin and Savit, 1988

Work the following problems and hand in next Wednesday 1. In your handout from the first day of class, you were given a shot record. Several events were highlighted on this display. We’ve derived the mathematical representation for refractions arriving from more than one constant thickness layer. At this point you should be able to take data off a shot record and determine the depths to the layers giving rise to the critical refractions. Using the shot record from the handout answer the following questions. a. How many layers are indicated by the shot record? b. What are their velocities? c. What are the time intercepts? c. Determine the thickness of each layer.

2. Suppose that a layer with a velocity of 1500 m/s and a thickness of 100 m lies above another layer with a velocity of 3000 m/s. Compute the expected crossover distance and intercept time for the critically refracted waves. (due next Wednesday)

3. Suppose that based on well information a structure consisting of four layers is recognized. The velocities and thicknesses are listed below: V 1 = 1500 m/s h 1 = 50 m V 2 = 2250 m/s h 2 = 100 m V 3 = 3500 m/s h 3 = 75 m V 4 = 4000 m/s a. What is the critical distance for the refracted wave from the interface between the layers defined by velocities of 3500 and 4000 m/s? b. If you change V 2 to 3000 m/s, what will be the new value for the critical distance? Robinson and Coruh (1988)

Lastly Next week hand in a one-or-two paragraph discussion of the topic for your term report. Include any references to the subject you may have collected at this point. For next time read over pages in Chapter 3 Bring any questions you might have about problems 1-3 to class next Monday.

In the time remaining we’ll discuss the problems 1-3 and any questions you might have concerning your term report.