VOIDS IN THE VICINITY OF PIPELINES Prof Chan Lung-sang Department - - PowerPoint PPT Presentation

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