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9.5 Option - Geophysics: 3. Seismicity
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3. Seismic methods provide information about the
large scale structure of the Earth and the detailed
structure of its crust
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Students learn to:
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Students:
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Extract from Physics Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit 18 Aug 08]
Prior learning: Preliminary modules 8.2, 8.3, 8.4.
Background: Until the science of studying seismic waves the only clue to what was beneath the surface was the drill hole. The structure of the Earth was a complete mystery. Seismic waves study now provides clear images and clues as the structures within the crust and the structure of the Earth.
describe the properties of P waves and S waves
- P and S waves are both called body waves because they
have the capacity to travel through the body of the Earth.
- P waves are longitudinal compression waves similar to
sound and hence can travel through all states of matter. S
waves are shear waves and hence transverse. They cannot
pass through fluids that have no shear strength.
- The difference in P and S wave types means that the
shake direction of the P wave is in the direction of wave
propagation whereas an S wave is at 90° to the
direction of wave propagation. For more information and
diagrams
Earthquakes, The slinky and the rope
Scott Robinson, leader, The Hypertech online museum.
- P waves travel faster than S waves. Because of their lower speed, S waves are refracted slightly more at interface boundaries and density changes in materials within the Earth. This means S waves have slightly longer range than P waves for detection on the Earth's surface.
gather, process and present diagrammatic information to show the paths of P and S waves through the Earth
- Gather information from texts such as
Black, I.F. and Cook, B.J., (editors). Perspectives of
the Earth, Australian Academy of Science. Chapter 14 b
provides a good and easily read introduction to this topic.
Information can also be found at websites. U.S. Geological Survey, Earthquake Hazards Program could be a starting point.
Another web site is Seismic Waves and Earth's Interior
Department of Geosciences, Penn State University,
Pennsylvania, USA. Some of the information is beyond the level needed for this subject but much of it is relevent and explained well.
- Process the information you have
gathered by organising it into a series of diagrams that
show the path of P waves through the Earth as surmised from
the detection of the waves from different sites around the
world. You could then show the path of S waves and could
finish with a clearly labelled diagram showing the path of
P and S waves as has been worked out by the places that
each of the waves has been received.
- Present the information to the rest of the students on paper, using overhead transparencies or as a power point presentation.
outline how a seismic wave’s path is affected by the properties of the material it travels through
- Seismic waves are reflected and refracted at interfaces
within the materials in which they travel. Their velocity
of travel is affected by the properties of the material in
which they travel such as the density of the material, its
bulk modulus and shear modulus.
- In a situation such as the Earth where these properties change with depth within the Earth seismic waves tend to follow a curved path from their point of origin to detection.
explain how seismic waves are reflected and refracted at an interface
- Seismic waves follow the same laws of refraction and
reflection as any other wave at interfaces. That is the
angle of incidence will equal the angle of reflection.
Seismic waves hitting an interface that is relatively
transparent to them behave similarly to other wave types in
having some of their energy transmitted and some reflected.
Refraction follows Snell's law
. Seismic waves can also be critically
refracted at an interface.
outline the structure and function of geophones and seismometers
- Both geophones and seismometers work on the principle
of inertia of one component of the device measuring
movement of another component of the device that is
attached to the Earth.
- In the case of the geophone the device measures shakes
in the vertical plane causing the movement of a magnetic
field relative to a coil which generates an electric
current. The arrival time of the electric current so
generated to a computer provides a time difference between
the reception of the signal at the geophone and the time of
the original vibrations production (usually a small
explosion or thumping of the ground). Since geophones are
set out in lines the timing of the arrival of seismic
reflections from multiple geophones can be combined by the
computer to produce a picture of the subsurface.
Geophones (Micro Structures and
Sensors Lab, Stanford University, California, USA) are used
predominantly in exploration for petroleum and minerals.
- Seismometers detect seismic waves usually from
earthquakes though they can be used to detect explosions.
The size of the shake from an earthquake can be recorded on
a revolving drum. This information can be used to determine
the intensity of the earthquake.
- There are two types of seismometers. One measures the
earthquake intensity in the horizontal direction and the
other measures the earthquake intensity in the vertical
direction. A seismological observatory needs two
seismometers able to measure earthquake intensity in the
horizontal direction positioned at 90° to one another
and one able to measure earthquakes in the vertical
direction. The relative intensity of the earthquake
measurements on the two horizontal seismometers can be used
to determine the direction of the source of the earthquake
from the observatory.
Handheld seismometer Explorations in
earth science, Teaching and learning technologies, Purdue
University, Indiana, USA. This site uses a handheld
seismometer as a teaching resource as it is difficult to
see the parts of modern seismometers.
analyse information from a graph of travel time versus shot-to-geophone distance for a single layer
- When dong such an analysis it is important to look for the following things: The first arrival pulse time at each geophone compared to the shot time and the separation distances of the geophones. If these factors are constant then an analysis of the data should allow calculation of the depth of the underlying reflective layer.
summarise the evidence for a liquid outer core and a solid inner core of the Earth
- The first point to note is that the only evidence that
can conclusively indicate the presence of a liquid outer
and solid inner core is based on the study of seismic
refraction and reflection. The presence of the liquid outer
core is suggested by the inability of S-waves to travel
right through the interior of the Earth. The S waves are
thought to be unable to pass through the outer liquid core
and hence produce a shadow zone where no S waves are
detected. For S waves this zone exists from 103° in
all directions from the surface above the earthquake focus.
- P waves also form a shadow zone as a result of
refraction at the outer core mantle interface. Like S waves
this also produces a shadow zone but because the P waves
can actually pass into a liquid, the shadow zone is divided
into two intervals from 103° to 145° from the
earthquake focus.
More information can be found at this
website, Nick Strobel, Bakersfield College, Physical
Science Dept., Bakersfield, California, USA. (web site
current at 18 August 2008)
- The evidence for the solid inner core is based largely
on the detection of weak reflections thought to occur at
the outer core-inner core boundary that result in
reflection of some seismic wave energy into the shadow zone
for P waves and S waves as shown on the figure at the
website below(scroll down to figure 1-3). Some additional
evidence for a slid inner core also exists in the fact that
there are some seismic P waves that travel through the
Earth slower than expected if they were to remain as P
waves for their entire path. It is thought that these slow
P waves may be the result of some of the P wave energy
incident on the outer core - inner core interface being
converted to S wave energy that travels at a slower speed
through the inner core. Those S waves are then converted
back to P wave energy at the opposite inner core- outer
core interface.
Oceanography Department University of
Washington, Seattle, Washington, USA. (web site current at
18 August 2008)
perform an investigation to model the principles of the reflection and refraction of seismic methods
- One of the best ways to model the principles of the reflection and refraction of seismic waves is to use a ray of light from a ray box incident on a glass slab. Some of the light will be reflected from the surface of the slab while some will enter the slab. This is similar to the reflection of seismic energy at an interface between rock layers. Some of the seismic energy will pass into the deeper layer whereas some of the energy will be reflected back to the surface. If the angle of intersection of the ray with the slab is at the critical angle the ray will enter the slab but be unable to emerge from the layer and will travel along the surface just inside the slab. Any irregularities on the surface of the slab will result in some of the light energy escaping the slab in a similar way to the energy escaping from a rock layer interface.
outline the methods of seismic reflection and refraction
- In both seismic reflection and refraction methods a
source of seismic energy is artificially generated by an
explosion or ground-thumping device. That seismic energy is
refracted at or reflected by, rock layer interfaces and
detected by a string of geophone devices spaced at equal
intervals away from the source of the seismic energy. The
travel time of the refracted or reflected energy is used to
estimate the shape and depth of rock layers using complex
algorithms in computers.
- The reflection method relies on the law of reflection
to produce an image. The travel time allows computation of
depth to reflective layers.
- The refraction method relies on critical refraction of
seismic energy along an interface between a low velocity
and high velocity rock layer. Periodically some of that
trapped energy hitting an irregularity in the surface is
transmitted to the surface where it is detected by one of
the geophones. The travel time of this detected energy give
an indication of the amount of time the wave has spent in
the lower medium layer and hence the depth and continuity
of that layer. More information and a diagram can be found
at the Geo-Services
International (UK) website. Just click on
Seismic Refraction Profiling.
discuss the uses of seismic methods in the search for oil and gas
- Seismic methods are the tool of choice of oil and gas explorers seeking to identify structures within the crust of the Earth where oil or gas may be trapped. The reason for that is that this type of method is readily used in a variety of terrains including the marine environment. The method is relatively cheap and gives a relatively accurate representation of the arrangement of rock layers in the subsurface. The techniques used mainly involve the use of seismic reflection.
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