40_01 - 1.1 Exploding Reflectors T he basic equipment for...

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1.1 Exploding Reflectors The basic equipment for reflection seismic prospecting is a source for impulsive sound waves, a geophone (something like a microphone), and a mul- tichannel waveform display system. A survey line is defined along the earth's surface. It could be the path for a ship, in which case the receiver is called a hydrophone. About every 25 meters the source is activated, and the echoes are recorded nearby. The sound source and receiver have almost no direc- tional tuning capability because the frequencies that penetrate the earth have wavelengths longer than the ship. Consequently, echoes can arrive from several directions at the same time. It is the joint task of geophysicists and geologists to interpret the results. Geophysicists assume the quantitative, physical, and statistical tasks. Their main goals, and the goal to which this book is mainly directed, is to make good pictures of the earth's interior from the echoes. A Powerful Analogy Figure 1 shows two wave-propagation situations. The first is realistic field sounding. The second is a thought experiment in which the reflectors in the earth suddenly explode. Waves from the hypothetical explosion propagate up to the earth's surface where they are observed by a hypothetical string of geophones. Notice in the figure that the raypaths in the field-recording case seem to be the same as those in the exploding-reflector case. It is a great conceptual advantage to imagine that the two wavefields, the observed and the hypothet- ical, are indeed the same. If they are the same, then the many thousands of experiments that have really been done can be ignored, and attention can be focused on the one hypothetical experiment. One obvious difference between the two cases is that in the field geometry waves must first go down and then return upward along the same path, whereas in the hypothetical experiment they just go up. Travel time in field experiments could be divided by two. In practice, the data of the field experiments (two-way time) is analyzed
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1 .I Exploding Rejlectors Zero- oflset Section Exploding Reflectors FIG. 1.1-1. Echoes collected with a source-receiver pair moved to all points on the earth's surface (left) and the "exploding-reflectors" conceptual model (right). assuming the sound velocity to be half its true value. Huygens Secondary Point Source Waves on the ocean have wavelengths comparable to those of waves in seismic prospecting (15-500 meters), but ocean waves move slowly enough to be seen. Imagine a long harbor barrier parallel to the beach with a small entrance in the barrier for the passage of ships. This is shown in figure 2. A plane wave incident on the barrier from the open ocean will send a wave through the gap in the barrier. It is an observed fact that the wavefront in the harbor becomes a circle with the gap as its center. The difference between this beam of water waves and a light beam through a window is in the ratio of wavelength to hole size. Linearity is a property of all low-amplitude waves (not those foamy, breaking waves near the shore). This means that two gaps in the harbor bar-
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40_01 - 1.1 Exploding Reflectors T he basic equipment for...

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