1 refraction seismology 1-16

$: 1 refraction seismology 1-16$
Definition

Refraction Seismology - A program to map geologicstructure by using head waves. Head waves involve energythat enters a high-velocity medium (refractor) near thecritical angle and travels in the high-velocity mediumnearly parallel to the refractor surface. Head wave arrivalsare identified in terms of time after the shot and distancefrom the shot. The objective is to determine the arrivaltimes of the head waves in order to map the depth to therefractors in which they traveled*.

Useful References

Burger, H. R., Exploration Geophysics of theShallow Subsurface, Prentice Hall P T R, 1992. Robinson, E. S., and C. Coruh, Basic ExplorationGeophysics, John Wiley, 1988. Telford, W. M., L. P. Geldart, and R. E. Sheriff,Applied Geophysics, 2nd ed., Cambridge UniversityPress, 1990. Geosphere Seismic Page. This is a nice page thatshows some examples of seismic reflection andrefraction methods being used for near-surfaceapplications.

Return to the Introduction to Geophysical Exploration Homepage

*Definition from the Encyclopedic Dictionary of Exploration Geophysics by R. E. Sheriff, published bythe Society of Exploration Geophysics.

1 of 1 7/19/99 11:40 AM

Introduction to Geophysics Short Course Assignments http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/main.html

Seismic Methods: Refraction and ReflectionLike the DC resistivity method, seismic methods, as typically applied in exploration seismology, areconsidered 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 dependingon what aspect of the recorded ground motion is used in the subsequent analysis. We do not mean toimply by this statement that any seismic experiment can be done from a given set of observations. On thecontrary, 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 Earth's groundmotion over specific distances.

One of the first active seismic experiments was conducted in 1845 by Robert Mallet, considered bymany to be the father of instrumental seismology. Mallet measured the time of transmission of seismicwaves, probably surface waves, generated by an explosion. To make this measurement, Mallet placedsmall containers of mercury at various distances from the source of the explosion and noted the time ittook for the surface of the mercury to ripple after the explosion. In 1909, Andrija Mohorovicic usedtravel-times from earthquake sources to perform a seismic refraction experiment and discovered theexistence 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 the1920s. The seismic refraction technique, described briefly below, was used extensively in Iran todelineate structures that contained oil. The seismic reflection method, now the most commonly usedseismic method in the oil industry, was first demonstrated in Oklahoma in 1921. A plaquecommemorating 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 initialground movement generated by a source recorded at a variety of distances. Later arrivingcomplications in the recorded ground motion are discarded. Thus, the data set derived fromrefraction experiments consists of a series of times versus distances. These are then interpreted interms of the depths to subsurface interfaces and the speeds at which motion travels through thesubsurface within each layer. These speeds are controlled by a set of physical constants, calledelastic p arameters that describe the material.

Reflection Seismology - In reflection experiments, analysis is concentrated on energy arriving afterthe initial ground motion. Specifically, the analysis concentrates on ground movement that hasbeen reflected off of subsurface interfaces. In this sense, reflection seismology is a verysophisticated version of the echo sounding used in submarines, ships, and radar systems. Inaddition to examining the times of arrival of these, reflection seismic processing extractsinformation about the subsurface from the amplitude and shape of the ground motion. Subsurfacestructures can be complex in shape but like the refraction methods, are interpreted in terms ofboundaries separating material with differing elastic parameters.

Each of these techniques has specific advantages and distadvantages when compared to each other andwhen compared to other geophysical techniques. For these reasons, different industries apply thesetechniques to differing degrees. For example, the oil and gas industries use the seismic reflectiontechnique almost to the exclusion of other geophysical techniques. The environmental and engineeringcommunities use seismic techniques less frequently than other geophysical techniques. When seismicmethods are used in these communities, they tend to emphasize the refraction methods over the

1 of 2 7/19/99 11:41 AM

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 ourdefinition 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 passiveexperiments.

2 of 2 7/19/99 11:41 AM

Seismic Methods: Refraction and Reflection http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/sintro.html

Advantages and Disadvantages of SeismicMethodsWhen compared to the other geophysical methods we've described thus far, the seismic methods haveseveral distinct advantages and several distinct disadvantages.

Seismic Methods

Advantage Disadvantage

Can detect both lateral and depth variations ina physically relevant parameter: seismic

velocity.

Amount of data collected in a survey canrapidly become overwhelming.

Can produce detailed images of structuralfeatures present in the subsurface.

Data is expensive to acquire and the logistics ofdata acquisition are more intense than other

geophysical methods.

Can be used to delineate stratigraphic and, insome instances, depositional features.

Data reduction and processing can be timeconsuming, require sophisticated computer

hardware, and demand considerable expertise.

Response to seismic wave propagation isdependent on rock density and a variety of

physical (elastic) constants. Thus, anymechanism for changing these constants(porosity changes, permeability changes,

compaction, etc.) can, in principle, be delineatedvia the seismic methods.

Equipment for the acquisition of seismicobservations is, in general, more expensive thanequipment required for the other geophysical

surveys considered in this set of notes.

Direct detection of hydrocarbons, in someinstances, is possible.

Direct detection of common contaminantspresent at levels commonly seen in hazardous

waste spills is not possible.

If an investigator has deemed that the target of interest will produce a measurable seismic anomaly, youcan see from the above list that the primary disadvantages to employing seismic methods over othermethods are economically driven. The seismic methods are simply more expensive to undertake thanother geophysical methods. Seismic can produce remarkable images of the subsurface, but this comes ata relatively high economic cost. Thus, when selecting the appropriate geophysical survey, one mustdetermine whether the possibly increased resolution of the survey is justified in terms of the cost ofconducting and interpreting observations from the survey.

1 of 1 7/19/99 11:41 AM

Advantages and Disadvantages of Seismic Methods http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/sadv.html

Advantages and Disadvantages of the Refractionand Reflection MethodsOn the previous page, we attempted to describe some of the advantages and disadvantages of the seismicmethods when compared to other geophysical methods. Like the electrical methods, the seismic methodencompasses a broad range of activities, and generalizations such as those made on the previous page aredangerous. A better feel for the inherent strengths and weaknesses of the seismic approach can beobtained by comparing and contrasting the two predominant seismic methods, refraction and reflection,with each other.

Refraction Methods Reflection Methods

Advantage Disadvantage Advantage Disadvantage

Refractionobservations generallyemploy fewer source

and receiver locationsand are thus relatively

cheap to acquire.

Because many sourceand receiver locations

must be used toproduce meaningfulimages of the Earth'ssubsurface, reflectionseismic observationscan be expensive to

acquire.

Little processing isdone on refraction

observations with theexception of trace

scaling or filtering tohelp in the process of

picking the arrivaltimes of the initialground motion.

Reflection seismicprocessing can be very

computer intensive,requiring sophisticated

computer hardwareand a relatively

high-level of expertise.Thus, the processing of

reflection seismicobservations is

relatively expensive.

Because such a smallportion of the

recorded groundmotion is used,

developing models andinterpretations is nomore difficult thanour previous efforts

with other geophysical

Because of theoverwhelming amountof data collected, the

possible complicationsimposed by the

propagation of groundmotion through a

complex earth, and thecomplications imposed

by some of thenecessary

simplificationsrequired by the dataprocessing schemes,

1 of 3 7/19/99 11:41 AM

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 thereflection seismic

observations requiremore sophistication

and knowledge of theprocess.

Refraction seismicobservations require

relatively largesource-receiver offsets(distances between thesource and where the

ground motion isrecorded, the receiver).

Reflection seismicobservations arecollected at small

source-receiver offsets.

Refraction seismic onlyworks if the speed at

which motionspropagate through theEarth increases with

depth.

Reflection seismicmethods can work nomatter how the speed

at which motionspropagate through the

Earth varies withdepth.

Refraction seismicobservations are

generally interpreted interms of layers. These

layers can have dip andtopography.

Reflection seismicobservations can be

more readilyinterpreted in terms of

complex geology.

Refraction seismicobservations only usethe arrival time of theinitial ground motionat different distancesfrom the source (i.e.,

offsets).

Reflection seismicobservations use the

entire reflectedwavefield (i.e., the

time-history of groundmotion at different

distances between thesource and the

receiver).

A model for thesubsurface is

constructed byattempting to

reproduce the observedarrival times.

The subsurface isdirectly imaged from

the acquiredobservations.

As you can see from the above list, the reflection technique has the potential for being more powerful interms of its ability to generate interpretable observations over complex geologic structures. As statedbefore, however, this comes at a cost. This cost is primarily economic. Reflection surveys are moreexpensive to conduct than refraction surveys. As a consequence, environmental and engineering

2 of 3 7/19/99 11:41 AM


concerns generally opt for performing refraction surveys when possible. On the other hand, thepetroleum industry uses reflection seismic techniques almost to the exclusion of other geophysicalmethods.

In this set of notes, we will only consider seismic refraction methods. Those interested in reflectionsurveying are directed to any number of introductory geophysical texts, some of which are listed on themodule homepage.

3 of 3 7/19/99 11:41 AM


Elastic WavesWhen the is Earth rapidly displaced or distorted at some point, the energy imparted into the Earth by thesource of the distortion can be transmitted in the form of elastic waves. A wave is a disturbance thatpropagates through, or on the surface of, a medium. Elastic waves satisfy this condition and alsopropagate 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 andwater 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 throughthe earth. When a pebble is thrown into a pond, the disturbance caused by the pebble propagates radiallyoutward in all directions. As the ripples move away from their source, notice that there are two distinctways 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 anytime, the waves form a circular ring around the source with some radius that is governed by thespeed at which the wave propagates through the water and the time elapsed since the waveoriginated at the source. In this viewpoint, we fix time and we view the wavefield at any locationacross the entire surface.

We can view these same waves as they propagate through some fixed location on the surface ofthe pond. That is, imagine that instead of observing the waves from above the pond, we are in asmall boat on the surface of the pond, and we record how the boat moves up and down withrespect to time as the wave propagates past the boat. In this viewpoint, we fix our spatial locationand view the wavefield at this location at all times.

These two viewpoints give us two fundamentally different pictures of the exact same wave. Assume thatour 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 somefixed 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 is

1 of 2 7/19/99 11:41 AM

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 wavegoes 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 asit 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 describedin this frame is identical to the amplitude described previously. Period is the time over which the wave isobserved to complete a single cycle. Another commonly used description related to period is thefrequency. Frequency is nothing more than the reciprocal of the period. If the period is measured inseconds (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 wavepropagates across the surface of the pond, c, where c equals the wavelength divided by the period of thewave.

2 of 2 7/19/99 11:41 AM

Elastic Waves http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/waves.html

Seismic WavesWaves that propagate through the earth as elastic waves are referred to as seismic waves. There are twobroad categories of seismic waves: body waves and surface waves.

Body waves - These are elastic waves that propagate through the Earth's interior. In reflection andrefraction prospecting, body waves are the source of information used to image the Earth's interior.Like the ripples on the surface of the pond example described previously, body waves propagateaway from the source in all directions. If the speed at which body waves propagate through theEarth's interior is constant, then at any time, these waves form a sphere around the source whoseradius is dependent on the time elapsed since the source generated the waves. Shown below is across section through the earth with body waves radiated from a source (red circle) shown atseveral different times. In the figure below, ms stands for milli-seconds. One milli-second equalsone 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 zeroamplitude, red is a large positive amplitude, and purple is a large negative amplitude. Notice thatthis plot is explicitly constructed in a reference frame that fixes time, thus allowing us to examinethe spatial variations of the seismic wave. At any given time, notice that the wave is circular withits center located at the source. This circle is, of course, nothing more than a two-dimensionalsection 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 themedium faster than the other wave types. In P waves, particles consistituting the medium aredisplaced 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 themedium. P waves are analogous to sound waves propagating through the air.

1 of 3 7/19/99 11:41 AM

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 throughthe medium slower than P waves. In S waves, particles consistituting the medium aredispaced in a direction that is perpendicular to the direction that the wave is propagating. Inthis example, as the wave propagates radially, the medium is being deformed alongspherical surfaces.

Most exploration seismic surveys use P waves as their primary source of information. The figureshown above could, however, represent either P or S waves depending on the speed chosen togenerate the plot.

Surface Waves - Surface waves are waves that propagate along the Earth's surface. Theiramplitude at the surface of the Earth can be very large, but this amplitude decays exponentiallywith depth. Surface waves propagate at speeds that are slower than S waves, are less efficientlygenerated by buried sources, and have amplitudes that decay with distance from the source moreslowly than is observed for body waves. Shown below is a cross section through a simplified Earthmodel (the speed of wave propagation is assumed to be constant everywhere) showing howsurface waves would appear at various times in this medium.

Like body waves, there are two classes of surface waves, Love and Rayleigh waves, that aredistinquished by the type of particle motion they impose on the medium. For our purposes, it is notnecessary 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 ofsurface waves generated from shallowly buried sources are often observed to be larger than theamplitudes 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 firstwave. Surface waves are never the first arrival. In all of the remaining discussion about seismicwaves, we will consider only body waves.

2 of 3 7/19/99 11:41 AM

Seismic Waves http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/swaves.html

Wavefronts and RaypathsIn the previous geophysical methods explored, in particular magnetics and resistivity, we often employedtwo different descriptions of the physical phenomena being observed. For example, when discussingmagnetism 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. Therelationship between these two descriptions is shown below.

Raypaths - Raypaths are nothing more than lines that show the direction that the seismic wave ispropagating. For any given wave, there are an infinite set of raypaths that could be used. In theexample shown above, for instance, a valid raypath could be any radial line drawn from thesource. 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 thesame time. In the example shown above, the wavefronts are spherical in shape. One suchwavefront would be the sphere drawn through the middle of the dark blue area. This surface wouldconnect all portions of the wave that have the largest possible negative amplitude at someparticular time.

In principle and in practice, raypaths are equivalent to the directions of current flow, and wavefronts areequivalent to the equipotential lines described in the resistivity section. They are also equivalent to fielddirection 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 forcurrent flow and equipotential surfaces in resistivity and for field strength and field direction inmagnetism.

Through much of the development to follow, we will use a raypath description of seismic wave

1 of 2 7/19/99 11:42 AM

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 ofspecific seismic phases, because we will be able to explicitly construct the path along which the seismicwave has travelled before being recorded by our receiver. As we will see next, although the raypaths forthe waves shown above are very simple, as we begin to construct models of the Earth that contain speedvariations, these raypaths will become more complex.

2 of 2 7/19/99 11:42 AM

Wavefronts and Raypaths http://www.mines.edu/fs_home/tboyd/GP311/MODULES/SEIS/NOTES/raypath.html

Wave Interaction with BoundariesThus far we have considered body wave propagation through media that has a constant speed of seismicwave propagation. What happens if the media consists of layers, each with a different speed of seismicwave propagation?

Consider the simple model shown below.

Although more complex than the homogeneous models considered previously, this model is still verysimple, consisting of a single layer over a halfspace. In this particular example, the speed* at whichseismic waves propagate in the layer is faster than the speed at which they propagate in the halfspace.Let's now watch the seismic waves propagate through this medium and see how they interact with theboundary at 150 meters. Shown below are three snapshots of the seismic wave at times of 25, 50, and 75ms**.

1 of 3 7/19/99 11:42 AM

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 wavefieldlook identical to those generated previously. After 50 ms, the wave begins to interact with the boundaryat 150 meters depth. Part of the wave has penetrated the boundary. The portion of the wavefield that haspenetrated the boundary is referred to as the refracted wave***. Also notice that part of the wave hasbounced off, or reflected off, of the boundary. This part of the wavefield is referred to as the reflectedwave***. This is the portion of the wavefield that is used in reflection surveying. Finally, part of thewavefield 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 nolonger centered on the source. Geophysicists would describe this as a change in the curvature ofthe wavefront.

Second, notice that the apparent wavelength of the refracted arrival is much shorter than the direct

2 of 3 7/19/99 11:42 AM


arrival.

Both of these phenomena are related to the presence of the discontinuity. Remember that the period of awave 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 constantand 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 theraypaths describing the direction of propagation of the wave change direction through the boundary. Thischange in direction of the raypath as it crosses a boundary is described by a well-known law known asSnell's Law.

Finally, of fundamental importance to note is that if you were observing the ground's motion from anypoint on the Earth's surface, you would observe two distinct waves. Initially, you would observe anarrival that is large in amplitude and that is the direct wave. Then, some time later, you would observe asmaller amplitude reflected wave. The time difference between your observation of these two arrivals isdependent on your distance from the source, the speed of wave propagation in the layer, and the depth tothe boundary. Thus, by observing this time difference we may be able to learn something about thesubsurface structure.

*Unless otherwise indicated, we will now assume that we are looking at P wave propagation through theEarth. 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, itgenerates not only a reflected and a refracted P wave, but it can also generate a reflected and a refractedS 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 willassume that no mode conversions occur. For refraction surveys, this is not a seriously flawedassumption, because again, we are considering only the time of arrival of the initial wave. P to S wavemode conversions will never be the first arrival. For reflection surveys, unless we were interested inrecording S wave arrivals or mode conversions, we design our survey and choose the recordingequipment to minimize their effects.

3 of 3 7/19/99 11:42 AM


1 refraction seismology 1-16

Technology