Tectonic Interpretation of Seismic Refraction Profiles 2

Introduction to Geophysics

Ali Onceloncel@kfupm.edu.

saDepartment of Earth SciencesKFUPM

Tectonic Interpretation of Seismic Refraction Tectonic Interpretation of Seismic Refraction ProfilesProfiles

Previous Lecture

•Hidden Layers •Problem Situations • Low Velocity layer (V2<V1<V3) •Thin layer (V3>V2>V1 but h2 very small) •Velocity Inversion-Hidden Layers

•Dipping Layer Refractions•Single Dipping Refractor Surface

•Quiz

ΔX=10 m

Can you pick out the direct arrival?

• A= Direct Arrival, tA1=0.003 sec, tA2 …………

• Can you pick out the refraction arrivals? How much critical refraction is there? 2 Critical Refractions: B and C

• tB1=0.01msec , tc1=0.016 sec• How would you determine the refraction velocity?• V2= 45m/0.007 sec =6750m/sec• V3= 148m/0.001 =14800 m/secHow would you determine the air wave or direct arrival

velocity?• Vdirect= 33m/0.01 sec =3300 m/sec• Vair= 33m/0.034 sec =963 m/sec

45 m0.01

A= Direct Wave

B= First Ref raction

C= Second Ref raction

Problem 3.2 in pp. 127 Berger's book.

Figure is a field seismograph from a region with horizontal interfaces. Interpret the data.

First geophone is at 3 meters from the source, the remainder are spaced at 10m intervals.

•What can you see?

•How many layers indicated by the shot record?

•What are their velocities?

•What are the time intercepts?

•Determine the thickness of each layer?

Homework due to April 15

P.C: 1 milisecond is 10^-3 seconds

First geophone is at 3 meters from the source, the remainder are spaced at 10m intervals.

What can you see?

•How many layers indicated by the shot record?

•What are their velocities?

•What are the time intercepts?

•Determine the thickness of each layer?

Problem 3.3 in pp. 127 Berger's book.

Using the information in Figure, determine as much as possible about the subsurface geology at the site where the information was recorded.

P.C: 1 milisecond is 10^-3 seconds

Plate BoundariesIn

Raised Relief Map of North America and Adjacent Oceans

Due to shallow

hot mantle

Due to cold lithospheric

Map of P-wave Velocity (km/s)

Active Continental

RiftsAdvanced Rifting

(Lower Elevations)

Early Rifting (Higher

Elevations)

National Park

Map of P-wave Velocity (km/s)

Crustal thinning due to Continental rifting

Remember that Red Sea is a good example for Continental Rifting.

Map of Crustal Thickness (km)

Crustal Thinning at Continental Rifts

As the lithosphere rips apart, the underlying asthenosphere becomes shallow while the crust still fairly thick. The buoyancy of the shallow asthenosphere and thick crust results in high overall elevation of the Basin.

Grand Teton National Park, WyomingGreat Basin National Park, Nevada

As the continental crust thins, it is less buoyant. Elevations are lower through the region.

Sagauro National Park, ArizonaDeath Valley National Park, California

Salton SeaGulf of California

Continental Rifting eventually open an Ocean Basin

Red Sea is a good example for Continental Rifting

(see pp. 50, Lillie 2005).

Parks and Plates

East Pacific Rise

Mid-Atlantic Ridge

A Continental Rift can be Viewed as the On-land Continuation of a Mid-Ocean Ridge

Parks and Plates

Basin and Range Province

East Pacific Rise

Mid-Atlantic Ridge

Gulf of California

A Continental Rift can be Viewed as the On-land Continuation of a Mid-Ocean Ridge

Parks and Plates

Active Continental

Death Valley

Salton Sea

National Park

Moho DEPTH relate to tectonic history?

Fig.4.17 of Lillie

Fig 4.18 of Lillie

Formation of Oceanic Crust

DIVERGENT PLATE BOUNDARYDEVELOPMENT

Ocean/Continent Subduction ZoneIn

Fig 4.19 of Lillie

CONVERGENT PLATE BOUNDARY

Subduction Zone

AccretionaryWedge Volcanic

CONVERGENT PLATE BOUNDARY

Continental Collision

Problems

4-3, 4-5, 4-6, 4-7

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