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. 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 Earth's 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 the
1 of 2 7/19/99 11:41 AM

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. 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 we've 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. 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 Earth's
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. 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 engineering
2 of 3 7/19/99 11:41 AM

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

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. 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 Earth's interior. In reflection and
refraction 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 propagate
away from the source in all directions. If the speed at which body waves propagate through the
Earth's 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. 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 Earth's 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. 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 wave
1 of 2 7/19/99 11:42 AM

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. 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.
Let's 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. 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 direct
2 of 3 7/19/99 11:42 AM

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
Snell's Law.
Finally, of fundamental importance to note is that if you were observing the ground's motion from any
point on the Earth's 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