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
WhatsApp 9892124323 ✓Call Girls In Kalyan ( Mumbai ) secure service
Equipment tool box talks; commonly used (and useful) near surface geophysics techniques
1. Commonly used (and useful) geophysical techniques
Electro-magnetic (EM) ground conductivity
Tim Grossey
&
James Cotterill
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2. Types of EM instrument
Frequency domain
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Time domain
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3. Stacked 1D to 2D for interpretation
Localised zones of
particularly low resistivity
Laterally pervasive zone
of low resistivity
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Resisitivity (Ohm-m)
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4. Ga s main
Shallow ‘metal detector’ systems
Bur i
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5. EM response curves
Response magnitude
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Response curves are for
a particular coil type and
fixed frequency
(Geonics EM34 instrument)
12. 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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13. 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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14. 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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15. Commonly used (and useful) geophysical techniques
Magnetic surveys
Tom Chamberlain
&
Dan Drummond
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19. 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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20. 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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21. Magnetic data in Everton Park
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22. Magnetic data in Everton Park
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23. 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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24. 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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25. 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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26. Commonly used (and useful) geophysical techniques
Ground penetrating radar
Gerwyn Leigh
&
Paul Birtles
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28. 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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32. Accurately mapping your interpretation
Survey grid baseline
Direction of GPR survey lines
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33. Transfer into from each grid to CAD,
and connect the dots
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34. GPR in Everton Park
Strong reflector indicative of bedrock.
Data from topographical low where bedrock is
shallow.
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35. GPR in Everton Park
Two strong reflectors at different depths.
Indicative of buried foundations.
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36. GPR in Everton Park
Strong reflectors indicative of buried
foundations.
High amplitude hyperbolic reflection indicative of
a buried utility service.
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37. Common pitfalls
•Bad survey design
Wrong antenna(s)
Complicated grid layout
Insufficient coverage / density of coverage
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38. Common pitfalls
•Difficult ground conditions
Electrically conductive ground
Hetergeneous ground
Congested ground
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39. Common pitfalls
•Errors in interpretation
Not enough effort put in!
Lack of experience
Incorrect interpretation
Over or under interpretation
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40. GPR Deliverables
Factual
Reflections from sharp boundaries between materials with contrasting electrical
properties.
Good plan and depth location control
Interpretative
Map 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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41. Commonly used (and useful) geophysical techniques
Microgravity
Stephen Owen
&
Richard Hodgson
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44. Gravity data location
Jane Herdman Building
University of Liverpool
Data courtesy of Prof Peter Styles
Keele University
(formerly of Liverpool University)
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45. Williamsons Tunnels, Liverpool
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.
Data courtesy of Prof Peter Styles
Keele University
(formerly of Liverpool University)
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49. Common pitfalls
•Poor data quality
•Incomplete processing
•Over processing
•Topographic corrections
•Assumptions made in interpretation
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50. 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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51. 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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52. Gravity Deliverables
Factual
A map of the variation in the Earth’s gravitational field, corrected to remove
latitude, earth tide, height, and topographic effects
Interpretative
Variations in the density of the subsurface
Models of causative bodies, and estimates of geometry and volume
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53. Commonly used (and useful) geophysical techniques
Electrical resistivity
Matt Stringfellow
&
Liam Williams
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57. Electrical resistivity data examples
Stacked cross section and surface
electrical data define landfill extent,
depth and internal
structure
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61. Survey design
Choice of array type to suit target
Resolution / electrode spacing
Depth coverage
Lateral coverage
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62. 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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63. 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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64. 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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65. Resistivity Tomography Deliverables
Factual
Measurements of the potential differences measured at particular locations in
response to a current driven between each pair of electrodes
Interpretative
Tomographic 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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66. Commonly used (and useful) geophysical techniques
Seismic refraction
Joe Milner
&
Hannah Barker
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70. Seismic data from Everton Park
Shot record
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71. Seismic data from Everton Park
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72. Seismic data from Everton Park
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73. Seismic data from Everton Park
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74. Seismic data from Everton Park
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75. Seismic data from Everton Park
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76. Survey design
Geophone spacing and shot spacing - ray path density
Depth coverage required
Lateral coverage required
Shot energy source
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77. 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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79. Limitations
•Relies on robust inversion
•Resolution decreases with depth
•Poor lateral resolution
•Requires high energy sources to get depth penetration
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80. Seismic Refraction Deliverables
Factual
Lateral variations in the time taken for an elastic wave to travel from one point to
another point
Interpretative
Variation 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
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Notas del editor
Introduce yourselves – name, qualifications, time at RSK
Start with any H&S points for the location and the kit.
Brief description of the operation of TDEM and FDEM.
Good, simple and quick first look at what lies beneath a site.
Coverage varies, but between 1 Ha and 6 Ha per day.
TDEM example: Cross section through the ground to identify the high conductivity (low resistivity) zone of ground water within a chalk section. Data need to be modelled to extract 1D profiles, and then stacked together to make a 2D section. Good control from known features heklpful, otherwise assumptions greatly affect the ground model produced.
TDEM example: buried infrastructure shallow variations and metal detection (used for UXO and buried services and obstructions mainly). Quick and high resolution data. Little depth control.
FDEM – depth of penetration depends of the frequency, and also on the coil separation. Averages large volumes of ground, and gives little or no depth control.
Data can be stripey. Needs to be georeferenced and flattened to remove artefacts, and to obtain a reliable picture of the variation of properties in the ground.
Describe example.
Interp is based on the recognition of patterns, so visualisation tools and the experience of the geophysicist is paramount.
Empty site post demolition – brief was to determine if any structures had been left in the ground. Surface was 2-4m of crushed concrete.
Lots still left in the ground.
Some foundations removed, but easternmost building footings appear to be entirely there.
Linear feature – metal pipe – severed at southern end and leaking into the ground.
Could be targeted with trial pit, sampling proved it to be mains water.
DEMO THE KIT HERE –DESCRIBE THE MAIN SITE LOGISTICS INVOLVED (ESP REGARDING HIGH NOISE AREAS, NEED FOR MAGNETIC FREE CLOTHING ETC.) GET SOMEONE TO DON THE KIT AND COLLECT SOME DATA – DESCRIBE THE PROCESS AS THE VOLUINTEER IS KITTED UP. Get them to describe what they see on the screen as they walk. Explain how that ends up in a map for interpretation.
EM data from this site
Describe main features of the data (point to flags on the ground – mobile map?)
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
DONT MISS THIS OUT IF YOU CAN HELP IT.
Talk through main points
Introduce yourselves – name, qualifications, time at RSK
Start with any H&S points for the location and the kit.
Describe the basic principles of the local variations in mag field caused by shallow changes in soil type, buried obstructions and ferrous objects.
Briefly describe the different types of magnetometer availabe.
Trade off of sensitivity and simplicity of data handling.
Describe origins of time varying fields, and explain how these are removed either by taking the gradient, or by using a fixed base station.
Describe basic processing steps
Describe basic processing steps
DEMO THE KIT HERE –DESCRIBE THE MAIN SITE LOGISTICS INVOLVED (ESP REGARDING HIGH NOISE AREAS, NEED FOR MAGNETIC FREE CLOTHING ETC.) GET SOMEONE TO DON THE KIT AND COLLECT SOME DATA – DESCRIBE THE PROCESS AS THE VOLUINTEER IS KITTED UP. Get them to describe what they see on the screen as they walk. Explain how that ends up in a map for interpretation.
Describe main features in data
Point out main features in the data
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
DONT MISS THIS OUT IF YOU CAN HELP IT.
Talk through main points
Introduce yourselves – name, qualifications, time at RSK
Start with any H&S points for the location and the kit.
Use this slide WITH THE KIT TO DEMO the basic operation
Can cover this quickly – there are a variety of antenna for each best suited for different applications – main message is on the next slide.
Briefly talk through lower frequency = greater depth penetration = lower resolution. Therefore, frequency must be chosen carefully, based on as much prior knowledge about the site and the targets as possible.
Data can contain a lot of detail – all of which should be examined by an experienced geophysicist to ‘pick’ reflection events from features of interest. There is no short cut for doing this properly.
All these features, locations and depths, need to be transposed onto a CAD drawing.
No detail should be lost, and ‘sanity checks’ for accuracy should be made.
Accuracy is important everywhere, but particularly in very congested areas. If care isnt taken at every stage of acquisition, processing and interpretation, then it is not possible for the final drawing to correctly detangle everything that was there.
THE ACCURACY OF THE TOPO IS VITAL HERE AS WELL – PAUL TO CHAT THROUGH THE LINK BETWEEN GOOD TOPO AND GOOD UTILITY DRAWINGS, AND REFER TO THE IMPORTANCE OF ACCURACTE LOCATION DATA FOR ALL ASPECTS OF GEOPHYSICS
Don’t dwell on these – but show them quickly – best to get the people pushing the kit about and seeing for themselves.
Don’t dwell on these – but show them quickly – best to get the people pushing the kit about and seeing for themselves.
Don’t dwell on these – but show them quickly – best to get the people pushing the kit about and seeing for themselves.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
DONT MISS THIS OUT IF YOU CAN HELP IT.
Talk through main points
Introduce yourselves – name, qualifications, time at RSK
Start with any H&S points for the location and the kit.
Bref intro to gravity method – INTRODUCE THE INSTRUMENT
Explain what is measured, and what must be corrected for to obtain the information wanted.
RICHARD TO EMPHASISE THE NEED FOR GOOD POSITIONAL DATA, AND HEIGHTS IN PARTICULAR – WHAT KIT DO WE NEED TO GET THE ACCURACIES NEEDED.
Explain we don’t have gravity data from inside the park, but we do have some excellent local data – it is also in the mobile map app so they can wander over that way later if they would like.
Describe the main features of the data – explain that the effects of the buildings and the mainline rail tunnel have been removed, and what is left is variations in density of the subsurface – which is dominated by the Williamson Tunnels.
Chat through this example if time – hole appeared in primary school playground. Clay over chalk.
Gravity survey located local lows indicating low density or voided ground.
Targeted dynamic probe investigation proved the voids and soft ground.
BEFORE THIS SLIDE – DEMO THE KIT. GET A VOLUNTEER TO LEVEL THE INSTRUMENT AT A LOCATION AT THE BOTTOM OF THE SLOPE, MAKE A NOTE OF THE READING.
GET SOMEONE ELSE TO LEVEL THE KIT AT THE TOP OF THE SLOPE – NOTE THAT THE READINGS ARE DIFFERENT, AND REITERATE ALL THE FACTORS THAT CONTRIBUTE TO THAT.
EXPLAIN THE BASIC FIELD PROCEDURE IN TERMS OF REPEAT READINGS, ETC.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
DONT MISS THIS OUT IF YOU CAN HELP IT.
Talk through main points
Introduce yourselves – name, qualifications, time at RSK
Start with any H&S points for the location and the kit.
SHOW THE KIT – EXPLAIN MAIN FEATURES.
Explain the sequence of active electrodes to build up a picture of the ground.
Don’t spend ages on this –
Different electrode configurations give different trade-offs between depth penetration, horizontal and vertical resolutions.
TALK ABOUT THE KIT NOW, AND EXPLAIN THA BASIC FIELD PROCEDURES, LIMITATIONS AND LOGISTICS. SHOW THE KIT CYCLING THROUGH THE SEQUENCE, AND EXPLAIN WHAT CAN BE SEEN ON THE SCREEN.
Briefly describe
Data over a landfill site – shows waste extents, depth, zoning, and location of leachate.
Briefly describe –
clay over chalk.
Wind farm development.
Confirming depth to rock head, and looking for variations in the top of the chalk, and evidence for natural voiding within the chalk.
Talk through the ‘raw’ data plots as a way of briefly describing the processing steps required to get a ‘final’ res section for interpretation.
Explain the features in the data in terms of the features that they can see on the ground.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
DONT MISS THIS OUT IF YOU CAN HELP IT.
Talk through main points
Introduce yourselves – name, qualifications, time at RSK
Start with any H&S points for the location and the kit.
SHOW THE KIT AND TALK THROUGH THE MAIN COMPONENTS, SURVEY LAYOUT, LOGISTICAL CONSTRAINTS, SOURCE TYPES, AND HOW THE SURVEY LAYOUT AND SOURCE ARE TAILORED TO A PARTICULAR APPLICATION AND TARGET TYPE.
Briefly describe the three wave types – explain that all can be used to extract information, today we will concentrate on the first arrival of the pressure wave.
Explain the basic principal of refraction.
GET SOMEONE OT HAVE A GO WITH THE HAMMER, AND LET EVERYONE IN THE GROUP SEE THE SHOT RECORTD ON THE SCREEN
Use this slide to talk through what is in the shot record. Show the p-wave first arrivals, and explain how these are picked. Also show the s wave and surface wave arrivals.
Explain what this is. (briefly!)
Explain the tomography inversion.
Explain how analysis of the tomography inversion, and measurements taken directly from the first arrival time graphs can give layers of equal velocity.
A look at the ray paths indicates the depths to which you have obtained reliable data, and how well constrained the results are by multiple shot records (multiple ray paths).
Explain the data from Everton park in terms of what they can see on the ground.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
Talk through main points – can miss this, or make it very brief, if short of time.
DONT MISS THIS OUT IF YOU CAN HELP IT.
Talk through main points