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4th GeoPhysical Examples

# 4th GeoPhysical Examples - 8 Seismic Exploration...

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\ Seismic Exploration Fundamentals Fig. 1-8 Tilted surface 9 I I 8 equal angles to the perpendicular (Fig. 1-6). So the sound is reflected from a point on the rock layer directly beneath the midpoint between source and receiver. This is a basic principle of seismic exploration. A seismic shot yields data from midway between source and receiver. The rule just stated is true, though, only when both the ground and the reflecting layer are flat. They usually aren't. So what does happen? Suppose the reflecting horizon dips. Let's first consider the situation when the source and receiver are at about the same place, so the sound returns to its point of origin. The angle of incidence and the angle of reflection must be equal. The only way this can be true in this situation is for the sound to strike the layer perpendicularly. This agrees with things in ordinary living too. To throw a ball and have it return to your hand, you must throw it so it strikes something at a right angle. To have it bounce back from the floor, you throw it straight down (Fig. 1-7). But if you throw at a leaning sheet of plywood, then you have to throw outward so the ball hits the plywood perpendicularly (Fig. 1-8). The same common-sense reasoning applies when the source and receiver are some distance apart. If you want to throw a ball so another person can easily catch it after the first bounce, you throw it to hit the floor midway between you and the other person. And, although this is not as common an experience, if the floor between is tilted, you have

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4th GeoPhysical Examples - 8 Seismic Exploration...

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