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地and震层析成像seismic tomography

2009-2-27 14:01| 发布者: geology| 查看: 6926| 评论: 0


老祖宗发明的候风地动仪。

现代地and震仪原理图

Introduction

Seismic tomography is an imaging technique that uses seismic waves generated by earthquakes and explosions to create computer-generated, three-dimensional images of Earth's interior. This is how seismologists infer the different layers in the Earth. How is this done? The time it takes for a seismic wave to arrive at a seismic station from an earthquake can be used to calculate the speed along the wave's ray path. By using first arrival times of P waves recorded by seismic stations all over the world, scientists are able to define slower or faster regions deep in the Earth. Those that come sooner travel faster. Those that come later are slowed down by something along the way. Human CAT scans are often used as an analogy.

以下是一段视频介绍:



不过由于地球的层圈结构不同于理想的等密度球体状态所以,实际情况还是与上面的视频略有不同的。


More

Various material properties control the speed and absorption of seismic waves. Careful study of the travel times and the amplitudes can be used to infer the existence of features within the planet.

Seismic waves travel at speeds of several kilometers per second in the Earth, with the speed of compressional waves (P waves), being about 1.7 times faster than that of shear waves (S wave). Seismic wave speeds are different in different kinds of rock., Speed increases with pressure (which is very nearly a function of depth alone) and decreases with an increase in temperature. What variables yield the best resolution to infer details at depth?

1) A high-density array of earthquake stations around the area being studied
2) Many earthquakes recorded by every station
3) Earthquake signals coming to stations from different parts of the world

Complications of This Technique

Seismic tomography is a rapidly evolving discipline. Observed anomalies within the Earth are open to discussion and controversy, making it a dynamic science. Because of this, creating an animation is a moving target. Our aim is merely to give a generalized picture of the underlying principles.

Two main complications of this techniquie are: 1) seismic waves don't move in a straight trajectory away from an earthquake, but refract, or bend in response to changes in density; and 2) seismic waves can bounce off of sharp boundaries such as the boundary between the core and mantle, and between the crust and the atmosphere. (Caveats.)

CAT Scans & Seismic Tomography

For lack of a perfect analogy, seismic tomography is often compared to a CAT scan (Computed Axial Tomography). The CAT scan uses computers to generate a three-dimensional image from a lot of flat X-ray pictures. The basic idea of a CAT scan is this: The X-ray beam is the energy source, which sends its signal (electromagnetic radiation) to the receiver (computer), which then captures and stores the data. Images are collected from hundreds of angles and the computer analyzes the information to produce a three-dimensional image. X-rays are absorbed unequally by different materials, and computer-aided tomography consists of studying the attenuation (reduction in intensity and amplitude) of X-rays that pass through the body.

The weakness in this analogy is that the observed quantity in a CAT scan is not a travel time, but rather the amount of X-ray absorption (attenuation). Seismic tomography uses the same principles, with the difference that the travel-times of the signals, rather than their attenuation, are observed. In terms of wave behavior, ultrasound imaging is more analogous to P-wave tomography, where compressive waves are reflected and refracted off materials of different composition and density. We use the CAT scan because it is commonly referred to without fully understanding how it works, plus it does provide an energy-receiver analogy.

Seismic tomography is much more difficult than X-ray tomography, because the ray paths are curved and initially unknown, and in some cases the locations of the sources are poorly known.

Compared to CAT scans, how does seismic tomography work?

Seismic tomography uses seismograms from thousands of local and global earthquakes to measure the speed of sound waves through the earth. With seismic tomography, the energy source is the earthquake. The earthquake sends its signal (seismic waves) to the receiver (seismograph), which records the data. Unlike an X-ray beam that shoots in one direction, the earthquake sends seismic waves in all directions, so instead of having to move the energy source around the Earth, scientists use multiple seismometers. One seismometer can only tell you that there has been an earthquake (as one X-ray can only tell you that you have bones). Many closely spaced seismometers yield three dimensional information about features below Earth's surface.

To determine how features seen at Earth's surface correlate with structural and compositional differences deep within the planet, seismologists need denser networks of seismic stations so that they are recording seismic waves that propagate through finer and finer slices of the earth beneath them. It's important to understand that tomographic techniques give a very coarse look at the subsurface.

Ongoing seismic-tomography experiments in the U.S. today

USArray, a component of EarthScope, is a fifteen-year program to place a dense network of permanent and portable seismographs across the continental United States. . The thousands of seismograph locations on public and private lands across the continental United States record local, regional, and distant (teleseismic) earthquakes. Hundreds of earthquakes occur throughout the world every day. By analyzing the seismograms of these earthquakes, scientists can learn about Earth structure and dynamics, and the physical processes controlling earthquakes and volcanoes. (LEARN MORE: USARRAY)


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