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