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These toolbox talks will cover the basic physical principles and the application of each near surface geophysical technique to common site investigations. For more information contact George Tuckwell, gtuckwell @ rsk.co.uk
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Commonly used (and useful) geophysical techniques
Electro-magnetic (EM) ground conductivity
Tim Grossey&James Cotterill
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Types of EM instrument
11 April 2023 2
Frequency domain Time domain
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Stacked 1D to 2D for interpretation
Resisitivity (Ohm-m)
0 20 40 60 80 100 120
-20
0
20
20 30 40 50 60 70 80 90 100110120130140150160170180190
Localised zones ofparticularly low resistivity
Laterally pervasive zoneof low resistivity
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Shallow ‘metal detector’ systems
Fence
Piles
Ga
s m
ain
Buried obstructions
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EM response curves
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Response magnitude
Dep
th(m
)
Coil separation = 10m
Coil se
paratio
n = 20m
Coil se
para
tion =
40m
Response curves are for a particular coil type and fixed frequency(Geonics EM34 instrument)
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EM main processing steps
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EM interpretation – pattern recognition
Landfill
Lateral boundaries and internal variations
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EM data examples
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EM data examples
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EM data in Everton Park
11 April 2023 10
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Survey planning
Coil separation
Coil orientation
Line spacing
Coverage
Limitations of access and environmental noise
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FDEM benefits
A quick and low cost
Can deliver a great deal of useful information
Relatively simple operation
Relatively simple data processing
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FDEM limitations
As sensitive to above ground features as to below ground features
Limited or no depth control
Averages the electrical properties of the ground
Interpretation relies heavily on the experience of the geophysicist, and the availability of contextual information.
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EM deliverables
Factual
Map of the lateral variations in the bulk electrical properties for the volume of ground sampled by the instrument.
Indication of the presence of very high conductivity (metallic) features.
Interpretative
Interpretation based on ‘pattern recognition’, using the relative values and geometry of the variations recorded. Relies heavily on the context, and on additional information to be confident of attributing specific interpretations.
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Commonly used (and useful) geophysical techniques
Magnetic surveys
Tom Chamberlain&Dan Drummond
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Magnetic mapping
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Magnetic instrument types
Fluxgate magnetometer Alkali vapour magnetometer
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Magnetic temporal variations
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Data processing – filtering and flattening
The signal of interest is often the smallest amplitude signal in the data
•Heading stripes
•Temporal variations
•Geology
•Cultural noise
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Data processing - filtering and flattening
The signal of interest is often the smallest amplitude signal in the data
•Heading stripes
•Temporal variations
•Geology
•Cultural noise
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Magnetic data in Everton Park
April 11, 2023 21
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Magnetic data in Everton Park
April 11, 2023 22
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Survey planning
Instrument type
Gradient or total field
Configuration
Line spacing and coverage
Access limitations
Sources of noise (near surface metal / EM noise)
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Magnetic survey limitations
As sensitive to above ground features as to below ground features
Only indicative depth control
Interpretation relies heavily on the experience of the geophysicist, and the availability of contextual information.
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Deliverables
Factual
Map of the local variation of the Earth’s magnetic field.
Interpretative
Origin and nature of features determined from interpretation of the pattern and geometry of the feature, the strength of the magnetic signal, and the context.
Numerical inversion can deliver some additional constraint on the causative bodies for specific magnetic anomalies
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Commonly used (and useful) geophysical techniques
Ground penetrating radar
Gerwyn Leigh&Paul Birtles
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E-plane
Ground penetrating radar (GPR)
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Ground Penetrating Radar (GPR) equipment
There are a number of manufacturers, each provide a number of equipment configurations
Each configuration has its advantages and disadvantages
Data location can be by odometer/distance measurement or by GPS
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Resolution & Depth Penetration - Higher Freqency GPR
0
1
2
3
4
5
6
7
8
9
10
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Half Wavelength Resolution (m)
Dep
th P
en
etr
ati
on
(m
)
100MHz
200MHz
450MHz
900MHz
1.2GHz
Ground penetrating radar (GPR)
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What to pick, and what to do next…
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Accurately mapping your interpretation
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Survey grid baseline
Dire
ction
of G
PR
surve
y line
s
Accurately mapping your interpretation
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Transfer into from each grid to CAD, and connect the dots
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GPR in Everton Park
April 11, 2023 34
Strong reflector indicative of bedrock.
Data from topographical low where bedrock is shallow.
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GPR in Everton Park
April 11, 2023 35
Two strong reflectors at different depths.Indicative of buried foundations.
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GPR in Everton Park
April 11, 2023 36
Strong reflectors indicative of buried foundations.High amplitude hyperbolic reflection indicative of a buried utility service.
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Common pitfalls
•Bad survey design Wrong antenna(s)
Complicated grid layout
Insufficient coverage / density of coverage
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Common pitfalls
•Difficult ground conditions Electrically conductive ground
Hetergeneous ground
Congested ground
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Common pitfalls
•Errors in interpretationNot enough effort put in!
Lack of experience Incorrect interpretation
Over or under interpretation
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GPR Deliverables
FactualReflections from sharp boundaries between materials with contrasting electrical properties.
Good plan and depth location control
InterpretativeMap view and depth view information on the presence of buried features
Good control on geometry, sufficient in most cases to give confident interpretations
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Commonly used (and useful) geophysical techniques
Microgravity
Stephen Owen&Richard Hodgson
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Microgravity
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Gravity data corrections
instrument reading +
drift correction +
Free Air anomaly +
Bouguer anomaly =
Simple Bouguer Anomaly
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Gravity data location
April 11, 2023 44
Data courtesy of Prof Peter StylesKeele University(formerly of Liverpool University)
Jane Herdman BuildingUniversity of Liverpool
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Williamsons Tunnels, Liverpool
April 11, 2023 45
Data courtesy of Prof Peter StylesKeele University(formerly of Liverpool University)
In addition to the standard corrections, this data sets needed to have the effects of the local buildings, and the railway tunnel removed before the gravitational effects of the Williamson Tunnels were revealed.
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Gravity example
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0m 50m
school building
reis
du
al B
oug
ue
r a
no
ma
ly (
mG
al)
1
2 3
4
5
6
Gravity example
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0m 50m
school building
-0.13
-0.12-0.11
-0.1
-0.09-0.08
-0.07
-0.06-0.05
-0.04-0.03
-0.02-0.01
00.01
0.02
0.030.04
0.05
0.060.07
0.08
0.09
reis
du
al B
oug
ue
r a
no
ma
ly (
mG
al)
DP6
DP3
1
2
3
4
5
6
7
8
9
5
de
pth
(m
)
DP4
1
2
3
4
5
6
7
8
9
5
dep
th (
m)
DP12
1
2
3
4
5
6
7
8
9
5
dep
th (
m)
N100
DP2
1
2
3
4
5
6
7
8
9
5 10
de
pth
(m)
N100
DP1
1
2
3
4
5
6
7
8
9
5 10
de
pth
(m)
N100
DP7
1
2
3
4
5
6
7
8
9
dep
th (
m)
N100
5 10 15 20
DP8
1
2
3
4
5
6
5 10 15 20 25
de
pth
(m
)
N100
DP55 10 15 20 25
1
2
3
4
5
6
7
8
9
dep
th (
m)
N100 DP9
1
2
3
4
5
6
7
8
5 10 15 20 25
dep
th (
m)
N100 DP10
1
2
3
4
5
6
7
8
9
5 10 15
dep
th (
m)
N100
DP11
1
2
3
4
5
6
7
8
9
5 10 15
de
pth
(m
)
N100
1
2 3
4
5
6
existingdoline
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Common pitfalls
•Poor data quality
•Incomplete processing
•Over processing
•Topographic corrections
•Assumptions made in interpretation
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Benefits
•The only technique that measures what a void is – absence of mass
•Can look deep (it’s a passive technique)
•All surface (and above surface) features can be removed from the data
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Limitations
•Complex subsurface gives complex data
•Relatively slow to acquire data, so perceived as more expensive
•Resolution decreases with the depth of the feature
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Gravity Deliverables
FactualA map of the variation in the Earth’s gravitational field, corrected to remove latitude, earth tide, height, and topographic effects
InterpretativeVariations in the density of the subsurface
Models of causative bodies, and estimates of geometry and volume
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Commonly used (and useful) geophysical techniques
Electrical resistivity
Matt Stringfellow&Liam Williams
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Electrical resistivity
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Electrical resistivity
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Electrode array types
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Stacked cross section and surface electrical data define landfill extent, depth and internalstructure
Electrical resistivity data examples
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Electrical resistivity data examples
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Resistivity data from Everton Park
April 11, 2023 59
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Resistivity data from Everton Park
April 11, 2023 60
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Survey design
Choice of array type to suit target
Resolution / electrode spacing
Depth coverage
Lateral coverage
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Common pitfalls
•Noisy data from external field and signals
•Heterogeneous or high resistivity ground
•Undersampling
•Data QC and repeats
•Over-trusting the inversion process
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Benefits
•Relatively quick and easy and reliable
•Good lateral and vertical resolution
•Detects variations in solid soils and geology, and groundwater / pore fluids
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Limitations
•Relies on robust inversion, which can be quirky in some circumstances
•Resolution decreases with depth
•Requires long spread lengths to get depth penetration
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Resistivity Tomography Deliverables
FactualMeasurements of the potential differences measured at particular locations in response to a current driven between each pair of electrodes
InterpretativeTomographic inversion of the observed data to produce a ground model of the distribution of electrical properties in the subsurface
An interpretation of geological and ground water variations can be made from the tomographic inversion. These can be based on assumptions, or on existing information available for the site.
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Commonly used (and useful) geophysical techniques
Seismic refraction
Joe Milner&Hannah Barker
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Seismic investigations
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PRESSURE WAVE
SHEAR WAVE
SURFACE WAVE
Seismic waves
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Seismic waves
Wave front
Refracted wave ‘ray path’
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Seismic data from Everton Park
April 11, 2023 70
Shot record
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Seismic data from Everton Park
April 11, 2023 71
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Seismic data from Everton Park
April 11, 2023 72
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Seismic data from Everton Park
April 11, 2023 73
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Seismic data from Everton Park
April 11, 2023 74
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Seismic data from Everton Park
April 11, 2023 75
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Survey design
Geophone spacing and shot spacing - ray path density
Depth coverage required
Lateral coverage required
Shot energy source
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Common pitfalls
•Noisy data from external sources (often drilling or plant!)
•Assumes a layered subsurface
•Undersampling, too few raypaths
•Data QC and stacking
•Over-trusting the inversion process
•Using a spurious or unjustified layered model
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Benefits
•Relatively quick and easy
•Reliable and proven for depth to bedrock / rippability
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Limitations
•Relies on robust inversion
•Resolution decreases with depth
•Poor lateral resolution
•Requires high energy sources to get depth penetration
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Seismic Refraction Deliverables
FactualLateral variations in the time taken for an elastic wave to travel from one point to another point
InterpretativeVariation of seismic velocity laterally and with depth, based on the inversion of travel times along modelled raypaths.
Ground model based on layer intervals with constant internal velocities
Location and magnitude of remaining uncertainties
