geo 468k geophys for geosc majors lab 2 seismogram interpretation

GEO 468K GEOPHYS FOR GEOSC MAJORS

Lab 2

Seismogram Interpretation


• 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. Thus the data set consists of a series of times versus distances.

• Reflection Seismology – In reflection experiments, analysis is concentrated on energy arriving after the initial ground motion. The analysis focuses on the waves reflected from the subsurface layers.


Elastic waves

• Amplitude is the peak to trough height divided by two.

• Wavelength is the distance over which the wave goes through one complete cycle.

• Period is the time over which the wave is observed to complete a single cycle.


Snell’s Law

• Snell’s Law governs the path by which a wave

would take the least amount of time to propagate

between two fixed points.

• In this case, the velocity of the overlying layer is

less than that of the underlying layer.


Seismic Velocity

• Seismic velocity is a material property (like density).

• There are two kinds of waves – Body and Surface waves.

• There are two kinds of body wave velocity – P and S wave velocities.

• P waves always travel faster than S waves.

• Seismic velocities depend on quantities like chemical composition, pressure, temperature, etc.

Faster Velocities

• Lower temperatures

• Higher pressures

• Solid phases

Slower Velocities

• Higher temperatures

• Lower pressures

• Liquid phases


Seismic Velocity and Travel Times

• A common method to determine the structure of the earth’s interior is to analyze the variations in the travel times of seismic waves:– Earthquake occurs and generates seismic waves

– Earthquake is identified and located

– Travel times of seismic waves are compared to a reference model

– Anomalous travel times are converted to heterogeneities inside the earth (inverse modeling)

• Travel time of a seismic wave is the time taken to travel from the hypocenter of the earthquake to the seismometer

• Seismologists use the travel time curve to identify seismic phases (P, S, etc.) by determining when they will arrive on a seismogram given how far away the earthquake epicenter is from the station. It can also be used in reverse.


Seismic Velocity-Depth Model for Whole Earth

• Pressure and temperature increase

as we go deeper into the earth

• These have opposite effects on

seismic velocity

• Seismic velocity tends to increase

with depth (increasing pressure)

• Exceptions include regions of

partial melt (LVZ-asthenosphere),

and total melt (the outer core)

•What is the relationship of seismic

velocities with density?


Possible Ray Paths for Seismic Waves Penetrating the Earth

• In the mantle and inner core, the

velocities increase with depth, so

the ray bend away from the normal

• At the mantle-outer core (fluid) boundary the decrease in velocity causes those rays refracted into the core to bend towards the normal

• What is mode conversion?


Seismogram Example

Pick two seismic phases and measure the time interval

P wave S wave


Jeffreys-Bullen Travel Time Curve for Earthquake Focus at the Surface

Epicentral distance (1 degree = 111 km) is the angle, subtended by the earthquake epicenter and seismometer, at the center of the earth (Bullen and Bolt, 1985)

Slide right or left on the graph below until that amount of time (vertical axis) fits on the curves representing the two seismic phases identified on the seismogram


Surface Waves

• Surface waves propagate along the earth’s surface.

• Surface waves are larger in amplitude and longer in duration than body waves.

• Surface waves propagate at a speed lower than body waves and are recorded after the P and S waves.

• There are two types of surface waves: Rayleigh and Love waves.

• Rayleigh waves are denoted by LR or R, and Love waves are denoted by LQ or Q (L for long; R for Rayleigh; Q for Querwellen, German, ‘transverse waves’).

• Surface wave amplitudes decays exponentially with depth.


Surface Wave Dispersion

• Surface waves are dispersive, which means that their velocities depend on frequency.

• Longer wavelength (longer-period, lower-frequency) surface waves contain more information about the deep velocity structure.

• Shorter wavelength (shorter-period, higher-frequency) surface waves provide information about the shallow structure.

• The first surface wave energy to arrive at any seismometer is of those frequencies that have the greatest velocities.

• The other frequencies will arrive later according to their frequencies.

• So, seismometers at increasingly greater distances from an earthquake record surface waves that are increasingly spread out.


Seismogram Example


Group Velocity of Surface Waves

• The velocity with which a surface wave energy associated with a particular frequency travels is called the group velocity (Delta/Time).

• The group velocity of surface waves is constant for a given frequency.

• Generally, Love wave group velocities are greater than Rayleigh wave group velocities, which means that on seismograms Love waves usually arrive before Rayleigh waves.

• A plot of velocity against period is called the dispersion curve.

• Dispersion curves contain much information about the velocity structure of the crust and upper mantle.

• A linearized inversion technique, is often used to obtain a velocity-depth structure appropriate for particular dispersion curves.


The subsurface is directly imaged from the acquired observations

A model for the subsurface is constructed by attempting to reproduce the observed arrival times.

Reflection seismic observations use the entire reflected wavefield (i.e., the time-history of ground motion at different distances between the source and the receiver).

Refraction seismic observations only use the arrival time of the initial ground motion at different distances from the source (i.e., offsets).

Reflection seismic observations can be more readily interpreted in terms of complex geology.

Refraction seismic observations are generally interpreted in terms of layers. These layers can have dip and topography.

Reflection seismic methods can work no matter how the speed at which motions propagate through the Earth varies with depth.

Refraction seismic only works if the speed at which motions propagate through the Earth increases with depth.

Reflection seismic observations are collected at small source-receiver offsets.

Refraction seismic observations require relatively large source-receiver offsets (distances between the source and where the ground motion is recorded, the receiver).

Because of the overwhelming amount of data collected, the possible complications imposed by the propagation of ground motion through a complex earth, and the complications imposed by some of the necessary simplifications required by the data processing schemes, interpretations of the reflection seismic observations require more sophistication and knowledge of the process.

Because such a small portion of the recorded ground motion is used, developing models and interpretations is no more difficult than our previous efforts with other geophysical surveys.

Reflection seismic processing can be very computer intensive, requiring sophisticated computer hardware and a relatively high-level of expertise. Thus, the processing of reflection seismic observations is relatively expensive.

Little processing is done on refraction observations with the exception of trace scaling or filtering to help in the process of picking the arrival times of the initial ground motion.

Because many source and receiver locations must be used to produce meaningful images of the Earth's subsurface, reflection seismic observations can be expensive to acquire.

Refraction observations generally employ fewer source and receiver locations and are thus relatively cheap to acquire.

Disadvantage Advantage Disadvantage Advantage

Reflection Methods Refraction Methods

Comparison of Refraction and Reflection Methods


Earthquakes and Aftershocks

• An earthquake occurs as the result of a slow build up of strain (deformation) in rock, usually caused by the relative motion of adjacent plates.

• When a fault or volume of rock can no longer resist movement, the stored strain energy is released, causing an earthquake.

• A strong earthquake is generally followed by a sequence of aftershocks, which can continue for months.

• The aftershocks occur during a period of readjustment of the earth following the main shock, in which small localized strains on the fault are released.

• Deep focus earthquake usually do not have aftershocks.

geo 468k geophys for geosc majors lab 2 seismogram interpretation

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