Introduction to Geophysics

Ali Onceloncel@kfupm.edu.

saDepartment of Earth SciencesKFUPM

Refraction Seismology: Field Work

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Previous Lecture

Earthquake Waves: As Probes of Earth's Interior Travel Time Models of Earthquakes for flat earth Travel Time Models of Earthquakes for Curved earthField Work: Refraction

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Types of Refraction

Seismometer

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Geometrics StrataView OYO Geospace DAS 1

Refraction Survey In

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Geophone spread for a refraction survey with shot locations indicated

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What does gravity surveying or “gravity”

measure?

Variation in gravitational acceleration

? km ? km

Shot #1 Shot #2

Bedrock = V2 = xxxx ft/sec

Alluvium = V1 = xxxx ft/sec

x km

Corrected Dip =?o

Ground slope = xo

Develop a Geophysical Model

?

We made a single shot and ignored the reversed one? What is the significant of reverse shot through the developing geophysical model?

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Determine slopes and intercept values

http:/ / www.mines.edu/ f s_home/ tboyd/ GP311/ MODULES/ SEI S/ NOTES/ deriv1.html

Observational DataDistance Direct Refracted Reflected

Arrival Arrivalft ft/sec

0 0.00 0.17 0.20500 0.10 0.22 0.22

1000 0.20 0.27 0.281500 0.30 0.32 0.362000 0.40 0.37 0.452500 0.50 0.42 0.543000 0.60 0.47 0.633500 0.70 0.52 0.734000 0.80 0.57 0.824500 0.90 0.62 0.925000 1.00 0.67 1.02

Single-Layer Model

XV

tdirect

1

1

21

21

21

2 4

V

h

V

Xtreflection

21

22

21

1

2

21VV

VV

hX

Vtrefraction

Equations

You should have already had experience in estimating required parameters which are needed for geophysical model.

What about your homework? Deadline is already finished!

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Controlled Seismic Sources

Impact Sources Vibrating Explosive Sources

Seismic waves are generated using a number of differentsource types:• weight-drop sources (e.g. hammer strike on steel plate)• accelerated weight drop• vibrating sources (‘vibroseis’)• explosives (dynamite, downhole shotgun)

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2m

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Refraction Profile

Generating Seismic SourceIn

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Shot 1

Shot 1

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Pick First Arrivals

Example: Santa Teresa Hills

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Example: Santa Teresa HillsSanta Teresa Hills Forward Profile

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Distance (feet)

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You can plot the measured data under EXCELL and show the data in this way.

Forward ProfileGeophone Distance (ft) Time (msec)

0 0 0.01 30 14.22 60 17.83 90 24.64 120 27.45 150 28.66 180 32.27 210 33.08 240 34.29 270 36.610 300 39.811 330 41.012 360 45.0

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Example: Santa Teresa Hills

What is the geophone distance in this example?30 mWhat was the geophone distance considered in our work?3 m

What makes differences between geophones?0.0

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Reverse Profile

Forward Profile

Reverse ProfileGeophone Distance (ft) Time (msec)

0 0 0.01 30 11.42 60 18.23 90 20.24 120 21.45 150 24.66 180 29.07 210 31.48 240 34.29 270 37.010 300 38.211 330 43.412 360 45

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Example: Forward and ReversedSanta Teresa Hills Forward Profile

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Santa Teresa Hills Reverse Profile

0.05.0

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Distance (feet)

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We have just made Forward, so you can ignore the Reversed

Example: Santa Teresa Hills

What is the geophone distance in this example?30 mWhat was the geophone distance considered in our work?3 m

What makes differences between geophones?0.0

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Reverse Profile

Forward Profile

Reverse ProfileGeophone Distance (ft) Time (msec)

0 0 0.01 30 11.42 60 18.23 90 20.24 120 21.45 150 24.66 180 29.07 210 31.48 240 34.29 270 37.010 300 38.211 330 43.412 360 45

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Both on Same Plot

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Reverse Profile

Forward Profile

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Example: Santa Teresa Hills

What is the geophone distance in this example?30 mWhat was the geophone distance considered in our work?3 m

What makes differences between geophones?0.0

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Reverse Profile

Forward Profile

Reverse ProfileGeophone Distance (ft) Time (msec)

0 0 0.01 30 11.42 60 18.23 90 20.24 120 21.45 150 24.66 180 29.07 210 31.48 240 34.29 270 37.010 300 38.211 330 43.412 360 45

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Generate Data TableIn

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Santa Teresa Hills Experiment

T = 0.332xR2 = 0.8731

T = 0.0705x + 18.367R2 = 0.9849

T = 0.0954x + 10.985R2 = 0.9913

T = 0.3033x R2 = 0.9792

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Direct Wave Forward

Head Wave Forward

Direct Wave Reverse

Head Wave Reverse

Linear (Direct Wave Forward)

Linear (Head Wave Forward)

Linear (Head Wave Reverse)

Linear (Direct Wave Reverse)

Example: Interpreted Plot

Include T intercepts and crossover distances

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Determine slopes and intercept values

y = 0.0954x + 10.985

R2 = 0.9913

y = 0.3187x

R2 = 0.975

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Data TableParameterV1

V2

Ti

Xcross

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? km

Bedrock = V2 = xxxx ft/sec

Alluvium = V1 = xxxx ft/sec ? km

Bedrock = V2 = xxxx ft/sec

Alluvium = V1 = xxxx ft/sec

Your Model for Flat Layer

Your model is very simple since we forward shot and assumed the

surfaced layer which is flat.

Thus, you are asked to estimate velocities and thickness.

Use the data you measured in the to calculate unknown variables.

Field Study Report Format, Due March 18

Statement of Geologic Problem (1/2 page)Theoretical Background (1-2 pages with 2-3 diagrams)Data Acquisition (1-2 pages, 1-2 diagrams)Data Processing (1-2 pages, 1 diagram - processing flow)Interpretation (1 page, 1 diagram - model)Conclusions (1/2 page)References (3 or more citations)

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