refraction interpretation
TRANSCRIPT
Introduction to Geophysics
Ali [email protected].
saDepartment of Earth SciencesKFUPM
Refraction Interpretation
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Previous Lecture
1. Seismic Waves Through the Earth2. How do we know what’s inside the Earth?
• Direct observations: • Exposures on Surface• Drilling• Volcanic Material
• Indirect observations:• Gravity Field• Magnetic Field• Seismic Waves
LITHOSPHERErigid outer shell crust and upper mantle (~ 50 to 200 km thick)somewhat brittle, breakablecold (like butter out of fridge)
ASTHENOSPHEREwarmer, plastic layer under lithospheremantle from ~ 150 to 700 kmsquishy, plasticwarm (like softened butter)
LOWER MANTLEsolid~700 to 2900 km
OUTER COREliquid
INNER COREsolid
Interior of Earth by STRENGTH
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Park and Plates, 2005, Lillie
- Interpret depths to subsurface interfaces and the seismic wave velocities for each layer.
Refraction Seismology ObjectiveIn
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-Later arriving complications in the recorded ground motion are discarded.
Thus, the data set consists of a series of times of first arriving energy versus
distance.
Refraction SeismologyIn
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From Tom Boyd’s WWW Site - http://talus.mines.edu/fs_home/tboyd/GP311/introgp.shtmlIntr
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From Tom Boyd’s WWW Site - http://talus.mines.edu/fs_home/tboyd/GP311/introgp.shtml
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Crustal Thickness Depth to Bedrock
Fig 4.1 of Lillie
Then, receivers therefore must extend well beyond the Xcr for the deepest refractor of interest. A general rule,
Spread Length> 2 Xcr
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As a first arrival (see Xc), As a secondary arrival, the event may be masked by interference from direct, reflected, or other refracted events.
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Interpretation Models
Figure 4.3 of Lillie