Introduction to Applied Geophysics
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Non-mathematical - but you will still need your calculators!!!
Basic Principles
Applications
Relevant Text
• Milsom: Field Geophysics, 1996. Open University Press.
• Kearey and Brooks: An Introduction to Geophysical Exploration., Blackwell
Science 1991(ISBN 0-632-02923-4).
• Telford, Geldhart, Sheriff & Keys: Applied Geophysics,1990. Cambridge
University Press
• Reynolds: An Introduction to Applied and Environmental Geophysics. Wiley
1997. (ISBN 0-471-95555-8)
Relevant Journals
• Geophysics
• Geophysical Prospecting
• Applied Geophysics
• Environmental and Engineering Geophysics
Other Relevant information Sources
• CSM see: http://magma.mines.edu/fs_home/tboyd/GP311/
• crb at St. Andrews Geoscinece web site
Applications
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Engineering
Environmental
Groundwater,
Mining
Geophysical Targets - Environmental
Problems for environmental engineers
•where did all those nasty contaminants go to?
•what will happen if there is a leak here?
•how can I design a contamination safety plan
Targets for environmental engineers
•Confining layers
•Barriers to water/contamination
•Fractured Bedrock
•Coarse channel fill
•Weathered bedrock
•Perched/permanent water tables
•High porosity/permeability confined
units
IN SUMMARY
•Rock Type
•Rock Fabric
•Geometry
•Fluid Content
Geophysical Targets - Mining
Problems mining engineers
•where is the primary resource
•How large is the primary resource
•How difficult is it to extract the primary resource
•How can the primary resource be extracted in an
environmentally sensitive manner
Targets for mining engineers
•Depth to target,size of target
•Physical nature of target
•Overlying material type and structure
•Perched/permanent water tables
IN SUMMARY
•Rock Type/mineral type
•Rock Fabric
•Geometry
•Fluid Content
Geophysical Targets - Engineering
Problems for Engineers
• How strong is the rock/soil
• How easily can it be
removed/dug into
Targets for engineers
•Depth to bedrock
•Fractured Bedrock
•Coarse channel fill
•Weathered bedrock
•Perched water tables
•High porosity/permeability confined
units
IN SUMMARY
•Rock Type
•Rock Fabric
•Geometry
•Fluid Content
Geophysical Targets - Groundwater
Ideal Well
• High Flow rate
• Good Quality
• Sustainable Yield
• Shallow (ish) Depth
Targets for hydrogeologists and
geologists
•Fractured Bedrock
•Coarse channel fill
•Weathered bedrock
•Perched water tables
•High porosity/permeability confined
units
Typical Well
• Variable Flow rate
• Variable Quality
• Seasonal (intermittent) Yield
• Medium to Deep
Targets parameters for geophysics
•Porosity - primary and secondary
•Density •Pore fluid - amount and type
IN SUMMARY
•Rock Type
•Rock Fabric
•Geometry
•Fluid Content
Typical Well Locations - Geophysical Targets
Common Well Conditions
1. Shallow perched aquifer in alluvium or weathered bedrock, discontinuous flow rate
2. Deep aquifer, seasonal recharge
3. Bedrock aquifer, sustainable yield, low flow rate
4. Bedrock aquifer, sustainable yield, high flow rate
Perched WT
3/4
1
2
3
1
4
WT
Fracture zone
Weathered
horizons
Clay
Alluvium
Sand and Gravel
Granite
Global
Scale
Regional
Local/ Field
Scale
Hand
specimen
Scale
Field Scale
Microscopic
clay rich
channeling
weathering
secondary
fracturing
sand rich
fracturing
Micro Scale
Factors influencing Porosity - fabric
Packing
100Vv
n=
Vt
Porosity = 47.65%
 ρb 
n = 1001 − 
 ρd 
Where
Vv - void volume
Vt - total volume
b - bulk density
d - particle density
Porosity = 25.95%
Density
(rock type)
is important
Factors influencing Porosity - fabric
Shape
mixed grain sizes
reduce porosity
Fabric
(rock type)
is important
Factors influencing Density
Mineral Type
Different minerals
have different
densities
Density of minerals
(rock type)
is important
Factors influencing Strength and Geophysical Signatures
In homogeneous, isotropic media the velocities of compression and
shear waves can be described in simple terms of elastic modulii and
density.
(4 µ + k)
3
Vp =
ρ
Bulk Modulus (k)- incompressibility of the medium
µ
Vs =
ρ
∆P
k=
∆v / V
Shear Modulus( µ ) - resistance to shearing; shear stress/shear strain. Note
that from the above equations, it is implied that fluids and gases do not
allow the propagation of S waves.
τ
µ=
Any changes in the shear or bulk modulii or the density will therefore
cause a change in shear and compression velocity
ε
Factors influencing Porosity - cements & fracturing
Secondary Porosity NB these diagenetic changes also affect the material strength
Fracturing
Cementation
e.g. calcite, dolomite, silica
Diagenesis
(rock type)
is important
Hydrogeological factors of geophysical interest
Specific yield - ratio of the volume of water that drains from a
saturated rock owing to attraction of gravity, to the total
rock volume (Sy)
Specific retention - ration of water retention to total rock
volume (Sr)
specific retention
specific yield
Porosity,
n = Sy + Sr, also remember
 ρb 
100Vv
n=
, n = 100 1 − 
Vt
 ρd 
Hydraulic Conductivity and Specific Yield
Specific Yield in % (after Fetter)
Material
Clay
Sandy Clay
Silt
Fine sand
Medium sand
Coarse sand
Fine gravel
Medium gravel
Coarse gravel
Maximum
5
12
19
28
32
35
35
26
26
Minimum
0
3
3
10
15
20
21
13
12
Average
2
7
18
21
26
27
25
23
22
Other Geophysical Properties
• Thermal conductivity
• Radioactivity
Newton’s Second Law of Gravitation (motion)
However, when measuring the Earth’s gravity we measure the
acceleration (g) resulting from the gravitational attraction.
Newton’s Second Law
Force is proportional to acceleration
F = m2 g
Thus from 1) and 2)
Gm1
g= 2
r
G=6.67x10-11Nm2kg-2
Magnetic Fundamental Principles Couloumb’s Equation
The expression for magnetic force experienced between two magnetic
monpoles is given by
1 ρ1 ρ2
Fm =
2
µ r
where µ is the magnetic permeability, p1and p2 are the strengths of two
magnetic monopoles
Note similarity with Newton’s Universal
Gravity Law
Gm1m2
Fg =
2
r
Electrical Resistivity - Conductivity
Ohm’s Law
Empirical relationship between the current (I) flowing through a wire, of
resistance R and the voltage potential (V) required to propagate the
current.
V = IR
Further
L
R ∝
A
where L is the length and A the cross sectional area of wire.
However, as we are not concerned with wires in the Earth, and electrical
current is not constrained, the resistivity, ρ of a material is a more
useful concept where.
ρ = RA
L
or
ρ = VA
IL
Summary of Geophysical Target Properties
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Density
Magnetic Susceptibility
Velocity (p and s wave)
Attenuation
Resistivity
Relative Dielectric Constant
• Rock Type
• Pore (fluid) Content
• Geometry
Geophysics
The Study of the Earth Using Quantitative Physical Methods
Remote Insight into the Earth
Objectives of Geophysical Investigation
• Remotely map changes in subsurface geologic and hydrogeologic
conditions
• Optimise locations for drilling wells
• Recognize and map economic resources
• Extend “Ground Truth” knowledge from boreholes into formations
Geophysical applications
• Whole Earth Geophysics - Classical Geophysics
• Exploration Geophysics - measure specific physical properties of the
earth to determine subsurface conditions and typically locate an
economic resource (typically oil, gas and minerals but also includes
water)
• Characterization Geophysics - remotely map changes in subsurface
geologic, engineering and hydrogeologic conditions (map distribution
and properties of aquifers and aquicludes)
Exploration, Groundwater and Environmental Geophysics
Exploration
Oil and Gas
• Structural Highs
• Reservoir Seals
• High porositypermeability
formations
• Station spacing
>25m
• Resolution 5-15m
• Seismic Reflection
dominant
• Targets 1-6km
Adapted from Steeples
Characterization
Groundwater
• Structural Lows
• Reservoir Seals &
leaks
• High porositypermeability
formations
• Station spacing 125m
• Resolution 0.5-10m
• Multi-technique
Environmental
• Structural Lows
• Reservoir Leaks
• Targets 10m-1.5km
• Targets 1m-500m
• Low porositypermeability
formations
• Station spacing <3m
• Resolution 0.5-2m
• Multi-technique
Geophysics doesn’t/didn’t Work!!
The geophysical methods are/were not used in an appropriate
manner/setting
Key points
• Geophysics is just another tool to help solve geologic/hydrogeologic
problems
• Geophysics measures physical parameters that must be interpreted in
terms that the end user will understand
• There is rarely a unique geophysical solution
• To ensure success, every geophysical survey must be conducted within
an appropriate geologic framework
Geophysical Methods
Active
• Artificially generate a signal
• Transmit this through the Earth
and record changes to signal
e.g.
– Seismic reflection and
refraction surveying
– Direct current electric methods
– controlled source
electromagnetics
Passive
• Detect variations in natural
fields associated with Earth
e.g.
– Gravity surveying
– Magnetic surveying
Geophysical Methods and Physical Properties
Method
Property
Electrical
Electrical &
Electromagnetic Conductivity
(resistivity)
Major Influence
Typical Ranges
Lithology (clay
content)
Moisture (dissolved
solids)
Lithology (magnetic
mineral)
104 (sea water) to 10-4 (dry sand)
millimohs/m
Lithology (mineral,
porosity)
10-6 (sediments) to 102 (iron alloys)
Gravity
Density
Magnetic
Magnetic
Susceptability
Seismic
Seismic
Lithology (porosity,
velocity/attentuation saturation, pressure)
102 (soil) to 104 (massive rocks) m/sec
Ground
Penetrating
Radar
Dielectric constant
10 (ice) to 102 (water)
Lithology,
watercontent,
density
0 (air filled void) to 1 (sediments) to 3
(massive rocks) gm/km
Note: Geophysics measures properties that are not unique
to a particular soil or rock type!
The Geophysical Survey - Budget
• Staffing
• Operating Costs
– general logistics - non-specific equipment, transportation, access,
damages, politics, social constraints,
– geophysical equipment - cost of rental, depreciation
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Insurance - liability
Overhead - administrative, consumables
Development - skills, software
Contingencies - something unplanned for will always happen!
Planning a Survey
Define Objectives
•Resolution
•Cultural factors
•Cost
•QA/QC
•Safety
•Data reduction
Forward Model
Field Operations
plan
•Line/Station/Grid
Processing/interp
•Integration
Desk Top Survey
Cost evaluation
Recommend No Geophysics
Select Geophysics Methodology
Site Check
BAD
Recommend No Geophysics
GOOD
Survey Design
Data Collection, Processing, Interpretation
Data Integration, Presentation and Recommendation
Data Reduction - Data Processing - Data Presentation
How is data to be reduced?
• Computer aided?
• Hand analysis and drafting?
How is data to be processed?
• Computer aided?
• Don’t collect more data than you can process - this is a great
temptation with digital acquisition
How is data to be interpreted?
• If computer aided interpretations used are the results
geologically/hydrogeologically realistic?
• Contouring is a particular problem with some sparse data sets
Final data presentation?
• How will the information finally be presented? Can the data be
converted into a useable form for presentation to the client?
Noise
•Coherent - systematic noise that can be filtered e.g. power line
•Incoherent - random noise that can be stacked e.g. wind
Noise Sources
Cultural (manmade)
Natural
Dynamic
• Electrical power
• Radio transmitters
• Vehicle
Static
• Buried pipes
• Drains
• Foundations
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• Any geologichydrogeologic noise
not related to target
Rain
Wind
Wave
Electrical Storms
Magnetic Storms
Noise Sources
The Geophysical Survey - Typical Survey Types
Sounding - 1D
• measure variation in properties (usually with depth) at one physical
location on surface, e.g. electrical sounding giving “borehole like”
result
Profiling - 2D
• measure variation in properties along the surface of a 2D cross section
• must consider line orientation (usually perpendicular to anticipated
major anomaly or strike of target)
Mapping - 2 ½D
• usually involves extrapolating between a number of parallel profiles
• join all points of equal value with isolines (equivalent to contours on a
map)
Mapping - 3D
• grid of survey points simultaneously recording (live) for every source
initiation
4D - 3D
• 3D data acquired using time lapse
Resolution
Critical to all types of survey is the issue of required survey resolution. This is a
function of sampling and can be either a time criteria or a distance criteria
• Station spacing/station interval
lead to spatial aliasing
• line interval lead to issues of
spatial aliasing
too small: spatially undersampled
too small: waste time and money
too large: miss target completely
Rule of Thumb
Geophysical signature (anomaly) typically at least twice actual size of feature.
Spatial Aliasing
Aliased (undersampled)
• (spatial) loss of high frequency
information
2D - True Profile Data
Optimally sampled
Oversampled
3D example - “Bulls Eye” Effect
Data Interpretation and Presentation
Qualitative
• Pattern recognition
– Can be applied to any data (property) set
– Correlate a certain geologic (hydrogeologic) condition with a geophysical
character or range or values.
– Change in values is usually the important criteria
– Target will not be identified if the variations in properties of the
background material are similar in contrast and scale to those associated
with the target.
Quantitative
• inversion
• numerical modeling
• neural networks
Line Profiling - 2D data
30
20
10
0
30
Fracture Zone
20
10
EM 34 Horizontal Coils, 20m spacing
EM 34 Vertical Coils, 20m spacing
0
Linear position
Data Presentation - 3D data
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3D Relief