the importance of near surface geophysics in
TRANSCRIPT
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SITE
INVESTIGATIONS
Presented by
Dr Simon Hughes FGS
Operations Manager TerraDat (UK) Limited
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Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
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TALK CONTENTS
• What is geophysics?
• Benefits and advances in use
• Case study – mine shaft
• So you need a geophysical survey?
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
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Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 4
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
“…a suite of measurement techniques that can be used to
precisely measure physical “properties
of the subsurface and the contrast with the target”
chargeability thermal conductivity resistivity seismic
radioactivity velocity density magnetism
electrokinesis dielectricity acoustic mass
electromagnetic
What is geophysics?
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 5
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Benefits of using geophysics
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
1. Non-invasive
“Ability to measure subsurface
properties without need for
excavation or drilling”
• Ideal for urban / developed sites
• Can be used on SSSIs
• No reinstatement costs
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THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 7
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
2. Total site coverage
…………where would you drill?
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 8
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
3. Overcome errors in geological interpolation
…………where would you drill?0
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10
5
De
pth
(m
bgl)
50 100 150 200 250 metres
River
1750 m/s 1750 m/s
4600 m/s
4700 m/s
4750 m/s4800 m/s
1800 m/s
Original horizontal drilling path
BH 1BH 2
Geophysical data is continuous over a site or along a profile
Avoids misinterpretation of bedrock e.g. borehole down fracture or into boulder
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
4. Low risk to site operatives from
hazards
• Chemical contamination
• Unexploded ordnance
• Voids and caverns
• Pipelines or cables
• Soft ground
• Shafts or wells
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THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
ADVANCES
GPS positioning – greater speed &
anomaly positioning
In-field analysis of data
• Positioning of anomalies
• Data QA
• Optimal parameters
• Refining survey methodology on
the fly
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THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
RECENT ADVANCES
GIS and 3D integration of geophysics
with other sources of data e.g.
• Surveying
• Historical
• Geochemical
• Geological
• Aerial Photos
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THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 12
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Case study - mineshaft
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 13
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Case study - mineshaft
Aim: to locate
mineshaft under
playing field for
school extension
Techniques:
conductivity
magnetic
microgravity
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
EM Conductivity
Background:
Induced electromagnetic field gives measure
of subsurface electrical conductivity to depths
of 0.5-60 metres. Variation in conductivity due
to: clay minerals, metal, water saturation, void
space
Common Applications:
• Shallow geology
• Contamination plumes
• Foundations and services
• Buried pits or infilled quarries
Limitations
• Noise from services / buildings
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THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Case study - mineshaft
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
Magnetics
Background:
passive measurement of anomalies in the
Earth’s magnetic field cause by buried ferrous
objects
Common Applications:
• USTs and drums
• UXO
• Archaeology
Limitations
• Noise from services / buildings
• Non-ferrous metal targets
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THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Case study - mineshaft
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
Microgravity
Background:
Passive measurement of subtle variations in
Earth’s gravity due to subsurface density
changes caused by voids or different
geological compositions.
Common Applications:
• Void and fissure mapping
• Buried basements
• Abandoned mine workings
• Buried valleys
Limitations
• Acoustic noise & vibrations, wind,
earthquakes
• Speed of acquisition
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THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Case study - mineshaft
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 17
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Case study - mineshaft
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 18
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Case study - mineshaft
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 19
THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Case study - mineshaft
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
SO, YOU NEED A GEOPHYSICAL
SURVEY……
• Aims and why - depth, size and
nature of targets if known?
• Size of the site?
• Site cover (vegetated, building,
rebar…)?
• Geological setting?
• Geographical setting?
• Any existing desk study data?
• Access limitations?
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THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
WHEN TO USE GEOPHYSICS
• Desk study
• Site visit
• GEOPHYSICS
• Optimal design for borehole /
sampling
• Integration of data
• Final conclusions
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THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk
……WHAT YOU GET BACK……….
• Rapid / large site coverage
• Non-intrusive
• Low risk
• Low environmental impact
• Allows optimum use of intrusive
investigations
• Determination of sub-surface
properties
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THE IMPORTANCE OF NEAR SURFACE
GEOPHYSICS IN GEOTECHNICAL SI
But only if appropriate techniques are used and the survey designed to
match survey aims