Vibroacoustics

Academics: Jen Muggleton, Emiliano RustighiResearchers: Michal Kalkowski, Michele Iodice

Aim

To develop novel vibro-acoustic methodologies to assess the condition of the buried utility service, geotechnical and transport infrastructures

Objectives

• To use a pipe vibration method to assess the condition of buried pipework

• To investigate a variety of ground excitation methods to interrogate both the ground and the buried infrastructure

• To explore a tree excitation method to determine the location of tree roots in order to identify areas of pipe network at risk of damage

• To develop vibro-acoustic methods to measure relevant wavespeeds (including variation with depth) in situ

Pipe Excitation Method: Background

In MTU, pipe excitation technique developedWhen pipe is excited, wave propagation in the pipe mirrored at the ground surface and the run of the pipe can be determined from ground vibration contours

In addition, bends and holes in the pipe wall may be detected as well as changes in soil condition

35Hz

-6 -4 -2 0 2 4 6-6

-4

-2

0

2

4

6

8

-10

-5

0

5

10

Example Pipe Discontinuities

Lateral distance from pipe, y(m)

Axi

al d

ista

nce

alon

g pi

pe, x

(m)

-2 0 2

2

4

6

8

10

12

14

16

18

20

-60

-55

-50

-45

-40

-35

-30

-25

-20

pipe end

32mm hole

Axi

al d

ista

nce

alon

g pi

pe, x

(m)

Lateral distance from pipe, y (m)-2 0 2

0

1

2

3

4

5

6

7

8

9

10

11

-30

-25

-20

-15

-10

-5

bend in pipe

Example Soil Discontinuities19Hz

Lateral distance from excitation location (m)

Axi

al ra

nge

(m)

-2 0 20

2

4

6

8

10

12

-30

-25

-20

-15

-10

-5

0

19Hz

Lateral distance from excitation location (m)

Axi

al ra

nge

(m)

-2 0 20

2

4

6

8

10

12

-20

-15

-10

-5

0

5

10

15

20

changes in soil type

Development of theoretical models

Prediction of pipe wavenumbers (wavespeed and attenuation) for different soil coupling conditions

Development of theoretical models

Prediction of ground surface response arising from wave motion in pipe

Torsional Motion

Torsional motion may be linked to certain types of pipe failure, in particular spiral fracture of cast iron pipes

Ultrasonic inspection techniques frequently exploit torsional waves but little is known about their behaviour at audio frequencies

Modelling work has been undertaken to predict

• dispersion characteristics (wavespeed & attenuation) for buried cast iron/plastic pipes

• ground surface response as a result of torsional wave motion in pipe

1

HH/1

01

0102

220 ak

akakhakk r

r

rrr

rp

mT

The individual terms contributing to the wavenumber expression can be readily identified as:• the in-vacuo torsional wavenumber, kT;• a pipe wall mass component, ω2ρph;• a soil shear stiffness component, μm/a;• and a shear wave radiation component associated with the Hankel function ratio,

akakak r

r

rrr

r01

010 HH

Next Steps

• To investigate effects of pipe discontinuities on wave propagation – In pipe– At ground surface

• To undertake experiments on test sites to– Locate pipe defects– Detect areas of flooded ground

Ground excitation methods: BackgroundPoint Vibration Technique

Applicable when no direct access to the pipe is available At low frequencies, ground exhibits classic mass-spring behaviour with a well-defined resonanceChanges in resonance frequency can be used to detect the presence of a buried object close to the surfaceVertical excitation is applied at the ground surface at several points along a line and accelerance (acceleration/force) is measured at each pointPotentially extremely quick to implementHas been used successfully to detect a number of shallow-buried services

101 102 10310-2

10-1

100

101

102

Frequency (Hz)|A

ccel

eran

ce| (

m/s

2 /N)

measurement not over pipemeasurement directly over pipe

Ground excitation methods: BackgroundShear wave Technique

Applicable when no direct access to the pipe is availableDirectional shear waves generated at ground surface using exciter attached to rake (excitation direction is perpendicular to measurement line)

Line of geophones used to measure ground surface vibrationGeneralized cross correlation functions used to extract time delay informationTime domain stacking technique employed to generate cross-sectional images of the groundMethod has been successful at detecting both plastic and metal water pipes and empty metal pipes

shaker with rake attachment

geophones

approximate run of pipe

80m/s

Distance along surface (m)

Dept

h (m

)

0 1 2 3 4 5 6

0

0.5

1

1.5

2

2.5

Current Work

Signal processing enhancements– Various signal processing

enhancements have been investigated including• data apodization• data enveloping

– The robustness of the algorithms to errors in wavespeed estimation has also been considered

Use of surface waves to detect road surface cracks

– Using wave decomposition– Exploiting MASW/MISW

differences

0 50 100 1500

0.2

0.4

0.6

0.8

1

1.2

1.4

frequency [Hz]

refle

ctio

n co

effic

ient

ratio direct/reflected wave amplitude node 160

with boundary (with reflections)without boundary (without reflections)

Next Steps

• Measurements on test site(s)• Comparison of point vibration technique with falling weight

deflectometer• Experimental measurements on road surface delamination

with point vibration technique

Tree excitation: Background

Tree roots well known to be disruptive to underground pipe and cable networksDamage can occur due to a number of different mechanisms, e.g.

• via direct penetration of the pipework, resulting in leakage

• through alterations in the ground water content locally

• by means of a gradual displacing of the pipework from its original location, resulting in pipe fracture

Detecting extent of root development of individual trees from ground surface could identify areas of infrastructure at risk

Field measurements

Measurements on real trees demonstrate that energy can effectively be transmitted from the trunk into the root system and thence the ground

25Hz

-4 -2 0 2 4-4

-2

0

2

4

626Hz

-4 -2 0 2 4-4

-2

0

2

4

627Hz

-4 -2 0 2 4-4

-2

0

2

4

628Hz

-4 -2 0 2 4-4

-2

0

2

4

629Hz

-4 -2 0 2 4-4

-2

0

2

4

6

30Hz

-4 -2 0 2 4-4

-2

0

2

4

631Hz

-4 -2 0 2 4-4

-2

0

2

4

632Hz

-4 -2 0 2 4-4

-2

0

2

4

633Hz

-4 -2 0 2 4-4

-2

0

2

4

634Hz

-4 -2 0 2 4-4

-2

0

2

4

6

35Hz

-4 -2 0 2 4-4

-2

0

2

4

636Hz

-4 -2 0 2 4-4

-2

0

2

4

637Hz

-4 -2 0 2 4-4

-2

0

2

4

638Hz

-4 -2 0 2 4-4

-2

0

2

4

639Hz

-4 -2 0 2 4-4

-2

0

2

4

6

Modelling & laboratory measurements

Root radius variation along the length is often close to exponentialWave propagation in exponentially tapered rods studied, to understand the phenomena expected in real tree rootsA purpose built root model was used in lab experiments to estimate the wavenumbers for longitudinal and flexural waves from equidistant FRF measurements

Next steps

The rod will now be buried in a sandbox allowing for an investigation of the soil effect on wave propagation

Interrogation of the soil

• Comparison of soil excitation methods to excite different wavetypes• Combining vertical & horizontal ground vibration responses to

extract the Rayleigh surface wave information• Comparison of geotechnical/geophysical properties derived using

seismic and electromagnetic methods• Use of inversion methods to extract near-surface wavespeed

information in both– homogenous soil– a layered soil

Summary

• Significant progress made in all four areas and all show considerable promise– Pipe vibration– Ground vibration– Tree excitation– Soil interrogation

• Next steps– Continue developing our fundamental understanding via• Analytical & numerical modelling• Laboratory & field experiments

– Testing on real sites

And finally .….

Thankyou &

Merry Christm

as !