1 refraction seismology 1-16

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1 refraction seismology 1-16

  1. 1. Introduction to Geophysics Short Course Assignments http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/main.html Definition Refraction Seismology - A program to map geologic structure by using head waves. Head waves involve energy that enters a high-velocity medium (refractor) near the critical angle and travels in the high-velocity medium nearly parallel to the refractor surface. Head wave arrivals are identified in terms of time after the shot and distance from the shot. The objective is to determine the arrival times of the head waves in order to map the depth to the refractors in which they traveled*. Useful References Burger, H. R., Exploration Geophysics of the Shallow Subsurface, Prentice Hall P T R, 1992. Robinson, E. S., and C. Coruh, Basic Exploration Geophysics, John Wiley, 1988. Telford, W. M., L. P. Geldart, and R. E. Sheriff, Applied Geophysics, 2nd ed., Cambridge University Press, 1990. Geosphere Seismic Page. This is a nice page that shows some examples of seismic reflection and refraction methods being used for near-surface applications. Return to the Introduction to Geophysical Exploration Homepage *Definition from the Encyclopedic Dictionary of Exploration Geophysics by R. E. Sheriff, published by the Society of Exploration Geophysics.1 of 1 7/19/99 11:40 AM
  2. 2. Seismic Methods: Refraction and Reflection http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/sintro.html Seismic Methods: Refraction and Reflection Like the DC resistivity method, seismic methods, as typically applied in exploration seismology, are considered active geophysical methods. In seismic surveying, ground movement caused by some source* is measured at a variety of distances from the source. The type of seismic experiment differs depending on what aspect of the recorded ground motion is used in the subsequent analysis. We do not mean to imply by this statement that any seismic experiment can be done from a given set of observations. On the contrary, the two types of experiments described below have very different acquisition requirements. These acquisition differences, however, arise from the need to record specific parts of the Earths ground motion over specific distances. One of the first active seismic experiments was conducted in 1845 by Robert Mallet, considered by many to be the father of instrumental seismology. Mallet measured the time of transmission of seismic waves, probably surface waves, generated by an explosion. To make this measurement, Mallet placed small containers of mercury at various distances from the source of the explosion and noted the time it took for the surface of the mercury to ripple after the explosion. In 1909, Andrija Mohorovicic used travel-times from earthquake sources to perform a seismic refraction experiment and discovered the existence of the crust-mantle boundary now called the Moho. The earliest uses of seismic observations for the exploration of oil and mineral resources date back to the 1920s. The seismic refraction technique, described briefly below, was used extensively in Iran to delineate structures that contained oil. The seismic reflection method, now the most commonly used seismic method in the oil industry, was first demonstrated in Oklahoma in 1921. A plaque commemorating this event was erected on the site by the Society of Exploration Geophysicists in 1971. Refraction Seismology -Refraction experiments are based on the times of arrival of the initial ground movement generated by a source recorded at a variety of distances. Later arriving complications in the recorded ground motion are discarded. Thus, the data set derived from refraction experiments consists of a series of times versus distances. These are then interpreted in terms of the depths to subsurface interfaces and the speeds at which motion travels through the subsurface within each layer. These speeds are controlled by a set of physical constants, called elastic p arameters that describe the material. Reflection Seismology - In reflection experiments, analysis is concentrated on energy arriving after the initial ground motion. Specifically, the analysis concentrates on ground movement that has been reflected off of subsurface interfaces. In this sense, reflection seismology is a very sophisticated version of the echo sounding used in submarines, ships, and radar systems. In addition to examining the times of arrival of these, reflection seismic processing extracts information about the subsurface from the amplitude and shape of the ground motion. Subsurface structures can be complex in shape but like the refraction methods, are interpreted in terms of boundaries separating material with differing elastic parameters. Each of these techniques has specific advantages and distadvantages when compared to each other and when compared to other geophysical techniques. For these reasons, different industries apply these techniques to differing degrees. For example, the oil and gas industries use the seismic reflection technique almost to the exclusion of other geophysical techniques. The environmental and engineering communities use seismic techniques less frequently than other geophysical techniques. When seismic methods are used in these communities, they tend to emphasize the refraction methods over the1 of 2 7/19/99 11:41 AM
  3. 3. Seismic Methods: Refraction and Reflection http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/sintro.html reflection methods. *Any of a variety of sources can be used. Typically these sources are manmade, thus satisfying our definition of an active geophysical survey. One could imagine using natural sources like earthquakes. Experiments that use natural sources to generate ground motion, however, are considered passive experiments.2 of 2 7/19/99 11:41 AM
  4. 4. Advantages and Disadvantages of Seismic Methods http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/sadv.html Advantages and Disadvantages of Seismic Methods When compared to the other geophysical methods weve described thus far, the seismic methods have several distinct advantages and several distinct disadvantages. Seismic Methods Advantage Disadvantage Can detect both lateral and depth variations in Amount of data collected in a survey can a physically relevant parameter: seismic rapidly become overwhelming. velocity. Data is expensive to acquire and the logistics of Can produce detailed images of structural data acquisition are more intense than other features present in the subsurface. geophysical methods. Data reduction and processing can be time Can be used to delineate stratigraphic and, in consuming, require sophisticated computer some instances, depositional features. hardware, and demand considerable expertise. Response to seismic wave propagation is dependent on rock density and a variety of Equipment for the acquisition of seismic physical (elastic) constants. Thus, any observations is, in general, more expensive than mechanism for changing these constants equipment required for the other geophysical (porosity changes, permeability changes, surveys considered in this set of notes. compaction, etc.) can, in principle, be delineated via the seismic methods. Direct detection of common contaminants Direct detection of hydrocarbons, in some present at levels commonly seen in hazardous instances, is possible. waste spills is not possible. If an investigator has deemed that the target of interest will produce a measurable seismic anomaly, you can see from the above list that the primary disadvantages to employing seismic methods over other methods are economically driven. The seismic methods are simply more expensive to undertake than other geophysical methods. Seismic can produce remarkable images of the subsurface, but this comes at a relatively high economic cost. Thus, when selecting the appropriate geophysical survey, one must determine whether the possibly increased resolution of the survey is justified in terms of the cost of conducting and interpreting observations from the survey.1 of 1 7/19/99 11:41 AM
  5. 5. Advantages and Disadvantages of the Refraction and Reflection Methods http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/sadv2.html Advantages and Disadvantages of the Refraction and Reflection Methods On the previous page, we attempted to describe some of the advantages and disadvantages of the seismic methods when compared to other geophysical methods. Like the electrical methods, the seismic method encompasses a broad range of activities, and generalizations such as those made on the previous page are dangerous. A better feel for the inherent strengths and weaknesses of the seismic approach can be obtained by comparing and contrasting the two predominant seismic methods, refraction and reflection, with each other. Refraction Methods Reflection Methods Advantage Disadvantage Advantage Disadvantage Because many source and receiver locations Refraction must be used to observations generally produce meaningful employ fewer source images of the Earths and receiver locations subsurface, reflection and are thus relatively seismic observations cheap to acquire. can be expensive to acquire. Reflection seismic Little processing is processing can be very done on refraction computer intensive, observations with the requiring sophisticated exception of trace computer hardware scaling or filtering to and a relatively help in the process of high-level of expertise. picking the arrival Thus, the processing of times of the initial reflection seismic ground motion. observations is relatively expensive. Because of the overwhelming amount of data collected, the possible complications imposed by the Because such a small propagation of ground portion of the motion through a recorded ground complex earth, and the motion is used, complications imposed developing models and by some of the interpretations is no necessary more difficult than simplifications our previous efforts required by the data with other geophysical processing schemes,1 of 3 7/19/99 11:41 AM
  6. 6. Advantages and Disadvantages of the Refraction and Reflection Methods http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/sadv2.html surveys. interpretations of the reflection seismic observations require more sophistication and knowledge of the process. Refraction seismic observations require relatively large Reflection seismic source-receiver offsets observations are (distances between the collected at small source and where the source-receiver offsets. ground motion is recorded, the receiver). Reflection seismic Refraction seismic only methods can work no works if the speed at matter how the speed which motions at which motions propagate through the propagate through the Earth increases with Earth varies with depth. depth. Refraction seismic Reflection seismic observations are observations can be generally interpreted in more readily terms of layers. These interpreted in terms of layers can have dip and complex geology. topography. Reflection seismic Refraction seismic observations use the observations only use entire reflected the arrival time of the wavefield (i.e., the initial ground motion time-history of ground at different distances motion at different from the source (i.e., distances between the offsets). source and the receiver). A model for the subsurface is The subsurface is constructed by directly imaged from attempting to the acquired reproduce the observed observations. arrival times. As you can see from the above list, the reflection technique has the potential for being more powerful in terms of its ability to generate interpretable observations over complex geologic structures. As stated before, however, this comes at a cost. This cost is primarily economic. Reflection surveys are more expensive to conduct than refraction surveys. As a consequence, environmental and engineering2 of 3 7/19/99 11:41 AM
  7. 7. Advantages and Disadvantages of the Refraction and Reflection Methods http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/sadv2.html concerns generally opt for performing refraction surveys when possible. On the other hand, the petroleum industry uses reflection seismic techniques almost to the exclusion of other geophysical methods. In this set of notes, we will only consider seismic refraction methods. Those interested in reflection surveying are directed to any number of introductory geophysical texts, some of which are listed on the module homepage.3 of 3 7/19/99 11:41 AM
  8. 8. Elastic Waves http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/waves.html Elastic Waves When the is Earth rapidly displaced or distorted at some point, the energy imparted into the Earth by the source of the distortion can be transmitted in the form of elastic waves. A wave is a disturbance that propagates through, or on the surface of, a medium. Elastic waves satisfy this condition and also propagate through the medium without causing permanent deformation of any point in the medium. Elastic waves are fairly common. For example, sound propagates through the air as elastic waves and water waves propagate across the surface of a pond as elastic waves. In fact, water waves on the surface of a pond offer a convenient analogy for waves propagating through the earth. When a pebble is thrown into a pond, the disturbance caused by the pebble propagates radially outward in all directions. As the ripples move away from their source, notice that there are two distinct ways of looking at the waves as they travel. These two distinct viewpoints are called frames of reference. We can view the waves propagating across the surface of the pond from above the pond. At any time, the waves form a circular ring around the source with some radius that is governed by the speed at which the wave propagates through the water and the time elapsed since the wave originated at the source. In this viewpoint, we fix time and we view the wavefield at any location across the entire surface. We can view these same waves as they propagate through some fixed location on the surface of the pond. That is, imagine that instead of observing the waves from above the pond, we are in a small boat on the surface of the pond, and we record how the boat moves up and down with respect to time as the wave propagates past the boat. In this viewpoint, we fix our spatial location and view the wavefield at this location at all times. These two viewpoints give us two fundamentally different pictures of the exact same wave. Assume that our ripple propagating outward from the source can be approximated by a sine wave. From the first perspective, we can examine the wave at any location on the surface of the pond at some fixed time. That wave would then be described as shown in the figure below. In this reference frame, the wave is defined by two parameters: amplitude and wavelength. Amplitude is1 of 2 7/19/99 11:41 AM
  9. 9. Elastic Waves http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/waves.html the peak to trough height of the wave divided by two. Wavelength is the distance over which the wave goes through one complete cycle (e.g., from one peak to the next, or from one trough to the next). From our second perspective, we can examine the wave at a fixed location on the surface of the pond as it propagates past us. That is, as time varies. That wave would be described as shown below. In this frame of reference the wave is described by an amplitude and a period. The amplitude described in this frame is identical to the amplitude described previously. Period is the time over which the wave is observed to complete a single cycle. Another commonly used description related to period is the frequency. Frequency is nothing more than the reciprocal of the period. If the period is measured in seconds (s), frequency has the units of Hertz (Hz), 1/s. As you might expect, period and wavelength are related. They are related by the speed at which the wave propagates across the surface of the pond, c, where c equals the wavelength divided by the period of the wave.2 of 2 7/19/99 11:41 AM
  10. 10. Seismic Waves http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/swaves.html Seismic Waves Waves that propagate through the earth as elastic waves are referred to as seismic waves. There are two broad categories of seismic waves: body waves and surface waves. Body waves - These are elastic waves that propagate through the Earths interior. In reflection and refraction prospecting, body waves are the source of information used to image the Earths interior. Like the ripples on the surface of the pond example described previously, body waves propagate away from the source in all directions. If the speed at which body waves propagate through the Earths interior is constant, then at any time, these waves form a sphere around the source whose radius is dependent on the time elapsed since the source generated the waves. Shown below is a cross section through the earth with body waves radiated from a source (red circle) shown at several different times. In the figure below, ms stands for milli-seconds. One milli-second equals one one-thousandth of a second (i.e., there are one thousand milli-seconds in a second). Click Here for Movie Version (127Kb) The color being plotted is proportional to the amplitude of the body wave. Light blue-green is zero amplitude, red is a large positive amplitude, and purple is a large negative amplitude. Notice that this plot is explicitly constructed in a reference frame that fixes time, thus allowing us to examine the spatial variations of the seismic wave. At any given time, notice that the wave is circular with its center located at the source. This circle is, of course, nothing more than a two-dimensional section of the spherical shape the wave has in three-dimensions. Seismic body waves can be further subdivided into two classes of waves: P waves and S waves. P Waves - P waves are also called primary waves, because they propagate through the medium faster than the other wave types. In P waves, particles consistituting the medium are displaced in the same direction that the wave propagates, in this case, the radial direction. Thus, material is being extended and compressed as P waves propagate through the medium. P waves are analogous to sound waves propagating through the air.1 of 3 7/19/99 11:41 AM
  11. 11. Seismic Waves http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/swaves.html S Waves - S waves are sometimes called secondary waves, because they propagate through the medium slower than P waves. In S waves, particles consistituting the medium are dispaced in a direction that is perpendicular to the direction that the wave is propagating. In this example, as the wave propagates radially, the medium is being deformed along spherical surfaces. Most exploration seismic surveys use P waves as their primary source of information. The figure shown above could, however, represent either P or S waves depending on the speed chosen to generate the plot. Surface Waves - Surface waves are waves that propagate along the Earths surface. Their amplitude at the surface of the Earth can be very large, but this amplitude decays exponentially with depth. Surface waves propagate at speeds that are slower than S waves, are less efficiently generated by buried sources, and have amplitudes that decay with distance from the source more slowly than is observed for body waves. Shown below is a cross section through a simplified Earth model (the speed of wave propagation is assumed to be constant everywhere) showing how surface waves would appear at various times in this medium. Like body waves, there are two classes of surface waves, Love and Rayleigh waves, that are distinquished by the type of particle motion they impose on the medium. For our purposes, it is not necessary to detail these differences. Suffice it to say that for virtually all exploration surveys, surface waves are a form of noise that we attempt to suppress. For reflection surveys in particular, suppression of surface wave energy becomes particularly important, because the amplitudes of surface waves generated from shallowly buried sources are often observed to be larger than the amplitudes of the body waves you are attempting to record and interpret. For refraction surveys, surface waves are less of a problem because we are only interested in the time of arrival of the first wave. Surface waves are never the first arrival. In all of the remaining discussion about seismic waves, we will consider only body waves.2 of 3 7/19/99 11:41 AM
  12. 12. Wavefronts and Raypaths http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/raypath.html Wavefronts and Raypaths In the previous geophysical methods explored, in particular magnetics and resistivity, we often employed two different descriptions of the physical phenomena being observed. For example, when discussing magnetism we looked at both the strength of the magnetic field and the direction of the magnetic field. When discussing resistivity, we discussed both the electrical potential and current flow. Similarly, there are two equally useful descriptions of seismic waves: wavefronts and raypaths. The relationship between these two descriptions is shown below. Raypaths - Raypaths are nothing more than lines that show the direction that the seismic wave is propagating. For any given wave, there are an infinite set of raypaths that could be used. In the example shown above, for instance, a valid raypath could be any radial line drawn from the source. We have shown only a few of the possible raypaths. Wavefront - Wavefronts connect positions of the seismic wave that are doing the same thing at the same time. In the example shown above, the wavefronts are spherical in shape. One such wavefront would be the sphere drawn through the middle of the dark blue area. This surface would connect all portions of the wave that have the largest possible negative amplitude at some particular time. In principle and in practice, raypaths are equivalent to the directions of current flow, and wavefronts are equivalent to the equipotential lines described in the resistivity section. They are also equivalent to field direction and strength in magnetism. Notice that in this example, wavefronts are perpendicular to raypaths. This is in general always true. So, given either a set of wavefronts or a set of raypaths, we can construct the other. This was also true for current flow and equipotential surfaces in resistivity and for field strength and field direction in magnetism. Through much of the development to follow, we will use a raypath description of seismic wave1 of 2 7/19/99 11:42 AM
  13. 13. Wavefronts and Raypaths http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/raypath.html propagation. This description will allow for a much easier computation of the propagation times of specific seismic phases, because we will be able to explicitly construct the path along which the seismic wave has travelled before being recorded by our receiver. As we will see next, although the raypaths for the waves shown above are very simple, as we begin to construct models of the Earth that contain speed variations, these raypaths will become more complex.2 of 2 7/19/99 11:42 AM
  14. 14. Wave Interaction with Boundaries http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/bound.html Wave Interaction with Boundaries Thus far we have considered body wave propagation through media that has a constant speed of seismic wave propagation. What happens if the media consists of layers, each with a different speed of seismic wave propagation? Consider the simple model shown below. Although more complex than the homogeneous models considered previously, this model is still very simple, consisting of a single layer over a halfspace. In this particular example, the speed* at which seismic waves propagate in the layer is faster than the speed at which they propagate in the halfspace. Lets now watch the seismic waves propagate through this medium and see how they interact with the boundary at 150 meters. Shown below are three snapshots of the seismic wave at times of 25, 50, and 75 ms**.1 of 3 7/19/99 11:42 AM
  15. 15. Wave Interaction with Boundaries http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/bound.html Click Here for Movie Version (129Kb) From 0 to 50 ms, the wave propagates solely within the upper layer. Thus, our pictures of the wavefield look identical to those generated previously. After 50 ms, the wave begins to interact with the boundary at 150 meters depth. Part of the wave has penetrated the boundary. The portion of the wavefield that has penetrated the boundary is referred to as the refracted wave***. Also notice that part of the wave has bounced off, or reflected off, of the boundary. This part of the wavefield is referred to as the reflected wave***. This is the portion of the wavefield that is used in reflection surveying. Finally, part of the wavefield has not interacted with the boundary at all. This part of the wavefield is called the direct wave. There are several interesting features to note about the refracted arrival. First, notice that the wavefront defining the refracted arrival is still circular, but its radius is no longer centered on the source. Geophysicists would describe this as a change in the curvature of the wavefront. Second, notice that the apparent wavelength of the refracted arrival is much shorter than the direct2 of 3 7/19/99 11:42 AM
  16. 16. Wave Interaction with Boundaries http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/bound.html arrival. Both of these phenomena are related to the presence of the discontinuity. Remember that the period of a wave is related to its wavelength through the speed at which the wave propagates through the medium. The wavelength is equal to the speed times the period. Thus, if the period of the wave remains constant and the speed of the medium decreases, the wavelength of the wave must also decrease. The change in curvature of the wavefront as the wave passes through the interface implies that the raypaths describing the direction of propagation of the wave change direction through the boundary. This change in direction of the raypath as it crosses a boundary is described by a well-known law known as Snells Law. Finally, of fundamental importance to note is that if you were observing the grounds motion from any point on the Earths surface, you would observe two distinct waves. Initially, you would observe an arrival that is large in amplitude and that is the direct wave. Then, some time later, you would observe a smaller amplitude reflected wave. The time difference between your observation of these two arrivals is dependent on your distance from the source, the speed of wave propagation in the layer, and the depth to the boundary. Thus, by observing this time difference we may be able to learn something about the subsurface structure. *Unless otherwise indicated, we will now assume that we are looking at P wave propagation through the Earth. Thus, the speeds indicated are appropriate for P waves. **ms stands for milliseconds. One millisecond is one one-thousandth of a second. ***We have simplified the situation a bit here. In general, when a P wave interacts with a boundary, it generates not only a reflected and a refracted P wave, but it can also generate a reflected and a refracted S wave. Conversely, S waves that interact with boundaries can generate reflected and refracted P waves. These conversions of P waves to S waves and S waves to P waves are called mode conversions. We will assume that no mode conversions occur. For refraction surveys, this is not a seriously flawed assumption, because again, we are considering only the time of arrival of the initial wave. P to S wave mode conversions will never be the first arrival. For reflection surveys, unless we were interested in recording S wave arrivals or mode conversions, we design our survey and choose the recording equipment to minimize their effects.3 of 3 7/19/99 11:42 AM

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