Equipment tool box talks; commonly used (and useful) near surface geophysics techniques
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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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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 Copyright of RSK 1
- 2. Types of EM instrument Frequency domain Copyright of RSK Time domain 18 November 2013 2
- 3. Stacked 1D to 2D for interpretation Localised zones of particularly low resistivity Laterally pervasive zone of low resistivity 0 20 40 60 80 100 20 0 -20 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 Resisitivity (Ohm-m) Copyright of RSK 120
- 4. Ga s main Shallow ‘metal detector’ systems Bur i ed o bst ructio ns Pi le s Fe nc e Copyright of RSK
- 5. EM response curves Response magnitude 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 -5 -10 n= ati o par e il s Co 10m -15 Depth (m ) -20 -25 ti ra pa e il s Co on = m 20 l oi C n io at ar ep s = m 40 -30 -35 -40 -45 Copyright of RSK -50 Response curves are for a particular coil type and fixed frequency (Geonics EM34 instrument)
- 6. EM main processing steps Copyright of RSK
- 7. EM interpretation – pattern recognition Landfill Lateral boundaries and internal variations Copyright of RSK
- 8. EM data examples Copyright of RSK
- 9. EM data examples Copyright of RSK
- 10. EM data in Everton Park Copyright of RSK 18 November 2013 10
- 11. Survey planning Coil separation Coil orientation Line spacing Coverage Limitations of access and environmental noise Copyright of RSK
- 12. FDEM benefits A quick and low cost Can deliver a great deal of useful information Relatively simple operation Relatively simple data processing Copyright of RSK
- 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. Copyright of RSK
- 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. Copyright of RSK
- 15. Commonly used (and useful) geophysical techniques Magnetic surveys Tom Chamberlain & Dan Drummond Copyright of RSK 15
- 16. Magnetic mapping Copyright of RSK
- 17. Magnetic instrument types Fluxgate magnetometer Copyright of RSK Alkali vapour magnetometer
- 18. Magnetic temporal variations Copyright of RSK
- 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 Copyright of RSK
- 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 Copyright of RSK
- 21. Magnetic data in Everton Park Copyright of RSK November 18, 2013 21
- 22. Magnetic data in Everton Park Copyright of RSK November 18, 2013 22
- 23. Survey planning Instrument type Gradient or total field Configuration Line spacing and coverage Access limitations Sources of noise (near surface metal / EM noise) Copyright of RSK
- 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. Copyright of RSK
- 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 Copyright of RSK
- 26. Commonly used (and useful) geophysical techniques Ground penetrating radar Gerwyn Leigh & Paul Birtles Copyright of RSK 26
- 27. Ground penetrating radar (GPR) E-plane Copyright of RSK
- 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 Copyright of RSK
- 29. Ground penetrating radar (GPR) Resolution & Depth Penetration - Higher Freqency GPR 10 100MHz 9 8 Depth Penetration (m) 7 6 5 200MHz 4 3 450MHz 2 900 MHz 1 1.2 GHz 0 0 0.1 0.2 0.3 0.4 0.5 0.6 Half Wavelength Resolution (m) Copyright of RSK 0.7 0.8 0.9 1
- 30. What to pick, and what to do next… Copyright of RSK
- 31. Accurately mapping your interpretation Copyright of RSK
- 32. Accurately mapping your interpretation Survey grid baseline Direction of GPR survey lines Copyright of RSK
- 33. Transfer into from each grid to CAD, and connect the dots Copyright of RSK
- 34. GPR in Everton Park Strong reflector indicative of bedrock. Data from topographical low where bedrock is shallow. Copyright of RSK November 18, 2013 34
- 35. GPR in Everton Park Two strong reflectors at different depths. Indicative of buried foundations. Copyright of RSK November 18, 2013 35
- 36. GPR in Everton Park Strong reflectors indicative of buried foundations. High amplitude hyperbolic reflection indicative of a buried utility service. Copyright of RSK November 18, 2013 36
- 37. Common pitfalls •Bad survey design Wrong antenna(s) Complicated grid layout Insufficient coverage / density of coverage Copyright of RSK
- 38. Common pitfalls •Difficult ground conditions Electrically conductive ground Hetergeneous ground Congested ground Copyright of RSK
- 39. Common pitfalls •Errors in interpretation Not enough effort put in! Lack of experience Incorrect interpretation Over or under interpretation Copyright of RSK
- 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 Copyright of RSK
- 41. Commonly used (and useful) geophysical techniques Microgravity Stephen Owen & Richard Hodgson Copyright of RSK 41
- 43. Gravity data corrections instrument drift Free Air Bouguer Simple Bouguer + + + = reading correction anomaly anomaly Anomaly Copyright of RSK
- 44. Gravity data location Jane Herdman Building University of Liverpool Data courtesy of Prof Peter Styles Keele University (formerly of Liverpool University) Copyright of RSK November 18, 2013 44
- 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) Copyright of RSK November 18, 2013 45
- 46. Gravity example Copyright of RSK
- 47. Gravity example 2 1 3 4 5 reisdual Bouguer anomaly (mGal) school building 6 0m Copyright of RSK 50m
- 48. DP4 1 2 2 3 3 1 2 4 depth (m) depth (m) 4 5 5 4N 8 100 9 DP7 N1 00 5 10 15 DP9 5 N10 0 10 15 20 25 DP10 1 1 2 3 N1 00 10 2 3 5 1 2 3 4 4 4 4 5 5 6 6 6 7 7 7 8 8 8 8 9 9 5 5 9 DP8 2 20 7 9 9 25 6 8 8 20 3 7 7 15 2 6 7 6 10 1 5 6 N100 3 5 N1 00 10 15 20 25 1 2 4 1 3 depth (m) dept h (m ) 3 5 dept h (m ) 1 DP5 5 dept h (m ) 5 dept h (m ) DP3 N1 00 5 10 depth (m) DP2 4 5 6 5 DP6 school building 0.09 5 1 0.04 0.03 -0.01 -0.02 -0.08 -0.09 8 -0.1 9 Copyright of RSK 6 7 5 N1 00 10 15 DP12 1 2 3 5 1 2 4 5 -0.13 DP11 3 -0.03 -0.04 -0.05 -0.06 -0.07 -0.1 1 -0.12 10 2 0.02 0.01 0 N10 0 3 4 5 depth (m) DP1 dept h (m) 0.08 0.07 0.06 0.05 dept h (m ) reisdual Bouguer anomaly ( mGal) existing doline 4 6 5 6 6 7 7 8 8 9 9 0m 50m 15
- 49. Common pitfalls •Poor data quality •Incomplete processing •Over processing •Topographic corrections •Assumptions made in interpretation Copyright of RSK
- 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 Copyright of RSK
- 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 Copyright of RSK
- 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 Copyright of RSK
- 53. Commonly used (and useful) geophysical techniques Electrical resistivity Matt Stringfellow & Liam Williams Copyright of RSK 53
- 56. Electrode array types Copyright of RSK
- 57. Electrical resistivity data examples Stacked cross section and surface electrical data define landfill extent, depth and internal structure Copyright of RSK
- 58. Electrical resistivity data examples Copyright of RSK
- 59. Resistivity data from Everton Park Copyright of RSK November 18, 2013 59
- 60. Resistivity data from Everton Park Copyright of RSK November 18, 2013 60
- 61. Survey design Choice of array type to suit target Resolution / electrode spacing Depth coverage Lateral coverage Copyright of RSK
- 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 Copyright of RSK
- 63. Benefits •Relatively quick and easy and reliable •Good lateral and vertical resolution •Detects variations in solid soils and geology, and groundwater / pore fluids Copyright of RSK
- 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 Copyright of RSK
- 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. Copyright of RSK
- 66. Commonly used (and useful) geophysical techniques Seismic refraction Joe Milner & Hannah Barker Copyright of RSK 66
- 68. Seismic waves SURFACE WAVE SHEAR WAVE PRESSURE WAVE Copyright of RSK
- 69. Seismic waves Wave front Refracted wave ‘ray path’ Copyright of RSK
- 70. Seismic data from Everton Park Shot record Copyright of RSK November 18, 2013 70
- 71. Seismic data from Everton Park Copyright of RSK November 18, 2013 71
- 72. Seismic data from Everton Park Copyright of RSK November 18, 2013 72
- 73. Seismic data from Everton Park Copyright of RSK November 18, 2013 73
- 74. Seismic data from Everton Park Copyright of RSK November 18, 2013 74
- 75. Seismic data from Everton Park Copyright of RSK November 18, 2013 75
- 76. Survey design Geophone spacing and shot spacing - ray path density Depth coverage required Lateral coverage required Shot energy source Copyright of RSK
- 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 Copyright of RSK
- 78. Benefits •Relatively quick and easy •Reliable and proven for depth to bedrock / rippability Copyright of RSK
- 79. Limitations •Relies on robust inversion •Resolution decreases with depth •Poor lateral resolution •Requires high energy sources to get depth penetration Copyright of RSK
- 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 Copyright of RSK
Notes de l'éditeur
- 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