detection of underground voids in the vicinity of … · •recent development of the ohmmapper and...

DETECTION OF UNDERGROUND

VOIDS IN THE VICINITY OF PIPELINES

Prof Chan Lung-sang

Department of Earth Sciences

The University of Hong Kong

Engineering Geophysical Methods

Mechanical - seismic methods, surface wave dispersion, downhole seismic, acoustic televiewer, microgravity, gamma density and neutron porosity, hydro-fracturing, ultrasonic systems, impact echo

Electrical - resistivity, self potential, induced polarization, electrical cylinder

Electromagnetic - ground penetrating radar, crosshole radar, locators, electromagnetic, conductivity meter, permittivity

Magnetic – magnetometer, locator

Radiometric – natural gamma spectroscopy

Thermal – thermography, infrared thermography

Void detection using

geophysical methods (HK

experience)

• Microgravity: specifically for subsurface

caverns

• GEO Geophysical Site Characterisation

Study Phases 1 & 2 (1995-97): GPR,

seismic, EM and Resistivity imaging

• Site trial experiment in Tai Po 1995: GPR

• Various attempts; DSD 2013 trials in

Kowloon Bay and Sha Tau Kok Road

Factors to be considered in selection of

methods:

1. Ability to identify contrasting physical properties

between the void and surrounding ground;

2. Non-invasive;

3. Ability to produce a continuous section;

4. Fast and inexpensive;

5. The method should give some indications on the depth,

size and extent of the voids;

6. No significant disruption to traffic;

7. Not affected by electromagnetic interferences and radio

noises in city.

Methods GEO

(1997)

Reynolds

(1997) Technos (2004)

DSD (2009)

WSD (2010)

High-speed adaptability

Seismic refraction C B B Yes, land streamer

Seismic reflection, P-wave

C B B B Yes, land streamer

Seismic reflection, S-wave

C Yes, land streamer

Surface wave spectral analysis

B A C A Yes, land streamer

Surface wave reflectometry

B Maybe

Microgravity B A B No

Frequency domain EM B B C B Yes

Time domain EM C A C C Yes

Ground penetrating radar

A A A B A Yes, air horne

Electrical resistivity A A B C A Yes, OhmMapper

Self potential B B No

Magnetic methods B C Yes

Infrared thermography C C C B No

Gamma density C C No

Sonar method B Yes

Effectiveness Ratings of Various Surface Geophysical Techniques in Void Detection

Electrical and electromagnetic

methods applications: soil resistivity:

subsurface detection of changes in

conductivity

• Ground profile

• Voids and moisture zone

• Water table

• Clay layers

• Contaminated ground

• Metallic objects ELECTRICAL

SURVEY

Drainage Service Department, HKSAR, 2013

Field survey to detect existence of

voids and utilities –

electromagnetic and electrical

resistivity measurements

Consultant: Black & Veatch Water

Geophysics Contractors: EGS (Asia) Limited, HKU

Credit: Northwest Mining Association and Klein

Frequency-domain EM method:

Depth and size of the conductor affect the

amplitude of the secondary field.

Conductivity of the conductor affects the

ratio of in-phase to out-of-phase

amplitudes (AR/AI), a good conductor

having a higher ratio

Electrical imaging method:

A set amount of current in injected into the

ground through a pair of current electrodes

Potential differences between a pair of

potential electrodes are measured with a

resistivity meter

Electrodes are moved to achieve various

depth of penetration

Resistivity section

1st Site Trial at Kowloon Bay

EM results from

Area A, Kowloon

Bay Site

General findings:

High negative in-

phase variations and

polarity reversal

resistivity values

Trend axis aligned

with buried pipes

Resistivity Section, Line B

Resistivity Section, Line A

General findings:

No particular

resistivity anomalies

attributable to buried

pipes and voids

Resistivity results

from Kowloon Bay

Site

HKU Resistivity survey in

Area B

HKU Resistivity survey in

Area B, Kowloon Bay Site

General findings:

Prominent negative

resistivity zone likely

associated with

buried pipes

Method works better

when survey line is

perpendicular to

buried utilities

Resistivity results

from Area B,

Kowloon Bay Site

(HKU)

2nd Site Trial at Sha Tau Kok Road

EM results from

Sha Tau Kok

Road Site

EM results from

Sha Tau Kok

Road Site

General findings:

High negative in-

phase variations and

polarity reversal

resistivity values

aligned with survey

line

Probably due to

buried pipes and

cables

General findings:

Multiple resistivity

anomalies identified

No particular

evidence they were

voids-induced

Resistivity results

from Sha Tau Kok

Road Site

Summary of Findings

The geophysical trials were successful but did not detect convincingly

underground voids in the vicinity of underground pipes due to site

constraints;

The geophysical methods may plausibly produce useful results only at

very favourable site conditions;

The methods should be regarded as indicative measures and not for

definitive determination.

Chen and Scullion, Geophysical

Testing Journal, 2010

GSSI air horne system

GEOVision system

Alternative geophysical methods

Void detection using

Land Streamer (Overseas Cases)

Van der Veen et al., Geophysics, 2001

Ohmmapper Surveys

Prof YH Wang, Univ of Science &

technology, HK

Way Forward

• Recent development of the Ohmmapper and Land Streamer,

respectively for electrical resistivity and seismic reflection survey,

may enable field surveys to be conducted in a mobile and fast

manner. Their efficacy for detecting underground voids in Hong Kong

is uncertain.

• A rational way to proceed with the survey is to design a control, trial

site for testing the feasibility of the methods.

Thank You!

chanls@hku.hk Department of Earth Sciences,

University of Hong Kong