Lecture 14 Tomography

Body waves In the interior Of the Earth’s body P is direct P wave in mantle PcP is a reflection from the core Angle of incidence = Angle of reflection

P arrives at about 7.7 minutes after the origin of the earthquake PcP arrives at 9.7 minutes

P shadow and S shadow were the original evidence for a liquid outer core. P wave gets refracted downwards because of low P velocity in core. S has zero velocity in outer core.

Positron emission tomography (PET scan) MRI (magnetic resonance imaging) CAT Scan [computerized axial tomography (CAT) scan. A CT scanner directs a series of X-ray pulses through the body. ] CT was discovered independently by a British engineer named Sir Godfrey Hounsfield and Dr. Alan Cormack. It has become a mainstay for diagnosing medical diseases. For their work, Hounsfield and Cormack were jointly awarded the Nobel Prize in 1979. Positron emission tomography (PET) is a nuclear medicine medical imaging technique which produces a three-dimensional image or map of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radioisotope, which is introduced into the body on a metabolically active molecule.

MRI 20 Tesla (Earth’s field 50000 nT

Travel time= distanc/velocity

Theoretical Tomography The Radon transform is an integral transform whose inverse is used to reconstruct images from medical CT scans.

Seismic tomography uses earthquakes (or shots) to image lateral heterogeneity in the Earth’s interior.

Tomography beneath Yellowstone Caldera

Tomography shows shallow asthenosphere beneath Kenya dome on east African Rift

Tomography of the mantle and inner core. (from John Woodhouse) (from Adam Dziewonski) Tomography of the mantle and inner core. Note Africa and Pacific Super Plumes

Tomography

Major curculation in the mantle

UCSD Global Tomographic models

Slab graveyard 100-200 ma

Laboratory Experiments Lava Lamp Shows plumes rising

Two deep plumes Two shallow plumes Montelli et al., Science 2004

Deep plumes

Shallow plumes

Plumes from Montelli et al., 2004

http://www.mala.bc.ca/~earles/mantle-plume-depths-jan04.htm

From Albarede and VanderHilst

Tomography of Furnace Creek Fault zone Steps: 1. Pick 48x48 arrivals using RAS24.exe Save as ascii 2. Use linear velocity background model V=a+bz analytic solutions for T, X 3. Find average a b that fit data 4. Find nearest node points on 5 m grid for rays 5. Back project residuals along rays to form tomogram

Fit of background model

Rays One shot Analytic for V=a+bz=1200+45z m/s

Shot at 12 th geophone 55 m.

Rays from all 48 shots

Rays interpolated onto 48x48 5 meter grid

Hit count at each grid node

Tomogram

Tomogram after stacking travel times Note loader.m shows velocities

Axis (no vertical exaggeration) image view of tomogram