Optimized passive seismic interferometry for bedrock detection: A Singapore case study

Yunhuo Zhang1,2, Yunyue Elita Li1, Heng Zhang3 and Taeseo Ku1

1 National University of Singapore, 2 Land Transport Authority,

3Chinese Academy of Sciences, Institute of Tibetan Plateau Research16 Oct 2018

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Objective – bedrock detection

http://adeptconstruction.solutions/kvc-pile-geotechnical-parameters/

Bedrock detection is a common and criticalobjective in various Civil Engineeringdevelopments, despite onshore oroffshore.

Source: Straits Times2

Geology map of Singapore

Source: Defence Science and Technology Authority Singapore, (2009)

• Presence of igneous andsedimentary formations

• Top of bedrock variesfrom a few meters to50m below groundsurface.

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Current Challenges

https://www.kwangsing.com.sg/images/services/big/11438935175.jpg

Conventional site investigation --drilling boreholes

• Require an area allowable to drill. • Take times (2 weeks or longer).

Early developed geophysical survey

• Sometimes not accurate• Constrains due to active source• Large footprint and long

duration (passive) 4

Specific Aim:

1. Seek a passive alternative to detect the bedrock as accurate asdrilling a borehole.

2. Optimize acquisition to ease implementation (smaller footprint,shorter acquisition duration).

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Seismic Interferometry – turning a receiver to a virtual source

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Geophone A Geophone BImpulsive Source

Geophone A Geophone BImpulsive Source Virtual Source A’

Testing Site

Site Location

Array Configuration

Acquisition parameters:• 4.5 Hz geophone• 30m/40m array length• 30 mins duration• 3 L-shape arrays acquired at different time

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Raw signal

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Normalized the average power spectraAmbient noise recorded from Blue Array

Direction of arrival of ambient noise –Beam forming

Coherent semblance: • Plane wave direction (0 - 360)• Plane wave speed (100 – 1200 m/s)

Synthetic data

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#1#6

#11

C=500 m/s

Θ=0 ⁰

Coherent semblance

Synthetic data

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#1#6

#11

C=800 m/s

Θ=60 ⁰

Coherent semblance

Field data

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Blue Array

Red Array

Yellow Array

5 Hz 6 Hz 7 Hz 10 Hz

-1 0 1

Time lag (s)

0

2

4

6

Rec

eive

r num

ber

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Greens’ function

0 10 20 30

Time (s)

0

2

4

6

Rece

iver n

umbe

r

-1 0 1

Time lag (s)

0

2

4

6

Rec

eive

r num

ber

𝐶𝐶𝐶𝐶𝐴𝐴𝐴𝐴 = 𝑢𝑢 𝑟𝑟𝐴𝐴, 𝑠𝑠 𝑢𝑢′(𝑟𝑟𝐴𝐴,𝑠𝑠) 𝐶𝐶𝐶𝐶𝐴𝐴𝐴𝐴 = 𝑢𝑢 𝑟𝑟𝐴𝐴,𝑠𝑠 𝑢𝑢′(𝑟𝑟𝐵𝐵,𝑠𝑠)

|𝑢𝑢 𝑟𝑟𝐴𝐴,𝑠𝑠 |^2

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Phase velocity maps

5 10 15 20

Frequency (Hz)

200

400

600

800

1000

1200

Phas

e Ve

loci

ty(m

/s)

5 10 15 20

Frequency (Hz)

200

400

600

800

1000

1200

Phas

e Ve

loci

ty(m

/s)

by crosscorrelation by crosscoherence

1D shear wave velocity profile bedrock prediction

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Inferred Bedrock

Actual Bedrock

Backfill (Very soft to soft soil)Residual Soil (Firm soil)Completely weathered granite (Firm to stiff soil)Highly weathered granite (stiff soil)Moderately weathered granite (moderate strong rock)Moderately weathered granite (strong to very strong rock)

Specific Aim:

1. Seek a passive alternative to detect the bedrock as accurate asdrilling a borehole.

2. Optimize acquisition to ease implementation (smaller footprint,shorter acquisition duration).

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30 80 150 300 600 900 1200 1500

Duration (s)

2

4

6

8

10

12

14

MA

PE

(%)

Optimal acquisition duration

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Mean Absolute Percentage Error(MAPE)

𝐶𝐶0: reference dispersion curve𝐶𝐶𝑖𝑖: ith dispersion curve

Various length of data

Entire recorded data

𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 =100𝑛𝑛

�1

𝑛𝑛

|𝐶𝐶0 − 𝐶𝐶𝑖𝑖𝐶𝐶0

|

15minutes are optimal and sufficient to derive a accurate and stable dispersion curve.

Optimal acquisition array length

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0 5 10 15 20 25 30

Frequency (Hz)

0

0.5

1

1.5

Am

plitu

de (d

B)

Blue Array

Red Array

Yellow Array

Synthetic signal by wave equation:

Optimal acquisition array length

12m

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30m 60m

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30m Array

Optimal acquisition array length

30m long array is optimal and sufficient to derive a accurate and stable dispersion curve.

Specific Aim:

1. Seek a passive alternative to detect the bedrock as accurate asdrilling a borehole.

2. Optimize acquisition to ease implementation (smaller footprint,shorter acquisition duration).

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Thank you

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