Introduction to Applied Geophysics • 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 GeophysicsOther Relevant information Sources• CSM see: http://magma.mines.edu/fs_home/tboyd/GP311/• crb at St. Andrews Geoscinece web site

Applications

• Engineering• Environmental• Groundwater,• Mining

Geophysical Targets - Environmental

Targets for environmental engineers•Confining layers•Barriers to water/contamination•Fractured Bedrock•Coarse channel fill•Weathered bedrock•Perched/permanent water tables•High porosity/permeability confinedunits

IN SUMMARY•Rock Type•Rock Fabric•Geometry•Fluid Content

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

Geophysical Targets - Mining

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

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

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 confinedunits

IN SUMMARY•Rock Type•Rock Fabric•Geometry•Fluid Content

Geophysical Targets - Groundwater

Ideal Well• High Flow rate• Good Quality• Sustainable Yield• Shallow (ish) Depth

Typical Well• Variable Flow rate• Variable Quality• Seasonal (intermittent) Yield• Medium to Deep

Targets for hydrogeologists andgeologists

•Fractured Bedrock•Coarse channel fill•Weathered bedrock•Perched water tables•High porosity/permeability confinedunits

Targets parameters for geophysics•Porosity - primary and secondary•Density -•Pore fluid - amount and type

IN SUMMARY•Rock Type•Rock Fabric•Geometry•Fluid Content

Clay Sand and GravelAlluvium

Fracture zone

Perched WT

Granite

Typical Well Locations - Geophysical Targets

WT

Weathered horizons

Common Well Conditions1. Shallow perched aquifer in alluvium or weathered bedrock, discontinuous flow rate2. Deep aquifer, seasonal recharge3. Bedrock aquifer, sustainable yield, low flow rate4. Bedrock aquifer, sustainable yield, high flow rate

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ScaleGlobal

Regional

Local/ Field Scale

Hand specimen

MicroscopicScale

clay rich

sand rich

secondary fracturing

Micro Scale

Field Scale

weathering

fracturing

channeling

Factors influencing Porosity - fabric

Packing

Porosity = 47.65%

Porosity = 25.95%

−=

=

d

b

t

v

n

VVn

ρρ1100

100

Where Vv - void volumeVt - total volumeb - bulk density

d - particle density

Density (rock type)is important

Factors influencing Porosity - fabric

Shape

mixed grain sizesreduce porosity

Fabric(rock type)is important

Factors influencing Density

Mineral Type

Different mineralshave 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.

Bulk Modulus (k)- incompressibility of the medium

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.

Any changes in the shear or bulk modulii or the density will therefore cause a change in shear and compression velocity

ρµ )3

4( kVp

+= Vs =

µρ

VvPk/∆

∆=

ετµ =

Factors influencing Porosity - cements & fracturing

Secondary Porosity -NB these diagenetic changes also affect the material strength

FracturingCementatione.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

−==

d

b

t

v nV

Vnρρ1100,100

Hydraulic Conductivity and Specific YieldSpecific Yield in % (after Fetter)Material Maximum Minimum AverageClay 5 0 2Sandy Clay 12 3 7Silt 19 3 18Fine sand 28 10 21Medium sand 32 15 26Coarse sand 35 20 27Fine gravel 35 21 25Medium gravel 26 13 23Coarse gravel 26 12 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

Thus from 1) and 2)

G=6.67x10-11Nm2kg-2

F m g= 2

gGmr

= 12

Magnetic Fundamental Principles -Couloumb’s Equation

The expression for magnetic force experienced between two magneticmonpoles is given by

where µ is the magnetic permeability, p1and p2 are the strengths of two magnetic monopoles

Note similarity with Newton’s UniversalGravity Law

FGm m

rg = 1 22

Frm =

1 1 22µ

ρ ρ

Electrical Resistivity - ConductivityOhm’s LawEmpirical relationship between the current (I) flowing through a wire, of

resistance R and the voltage potential (V) required to propagate the current.

Further

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.

V IR=

ρ = RAL IL

VA=ρ

ALR ∝

or

Summary of Geophysical Target Properties

• 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 hydrogeologicconditions

• 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 aneconomic 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

Oil and Gas• Structural Highs• Reservoir Seals

• High porosity-permeability formations

• Station spacing >25m

• Resolution 5-15m• Seismic Reflection

dominant• Targets 1-6km

Groundwater• Structural Lows• Reservoir Seals &

leaks• High porosity-

permeability formations

• Station spacing 1-25m

• Resolution 0.5-10m• Multi-technique

• Targets 10m-1.5km

Environmental• Structural Lows• Reservoir Leaks

• Low porosity-permeability formations

• Station spacing <3m

• Resolution 0.5-2m• Multi-technique

• Targets 1m-500m

Adapted from Steeples

Exploration Characterization

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 signale.g.

– Seismic reflection and refraction surveying

– Direct current electric methods– controlled source

electromagnetics

Passive• Detect variations in natural

fields associated with Earthe.g.

– Gravity surveying– Magnetic surveying

Geophysical Methods and Physical Properties

Method Property Major Influence Typical Ranges

Electrical &Electromagnetic

ElectricalConductivity(resistivity)

Lithology (claycontent)Moisture (dissolvedsolids)

104 (sea water) to 10-4 (dry sand)millimohs/m

Gravity Density Lithology (magneticmineral)

0 (air filled void) to 1 (sediments) to 3(massive rocks) gm/km

Magnetic MagneticSusceptability

Lithology (mineral,porosity)

10-6 (sediments) to 102 (iron alloys)

Seismic Seismicvelocity/attentuation

Lithology (porosity,saturation, pressure)

102 (soil) to 104 (massive rocks) m/sec

GroundPenetratingRadar

Dielectric constant Lithology,watercontent,density

10 (ice) to 102 (water)

Note: Geophysics measures properties that are not uniqueto 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• Insurance - liability• Overhead - administrative, consumables• Development - skills, software• Contingencies - something unplanned for will always happen!

Planning a SurveyDefine Objectives

Desk Top SurveyCost evaluation

Recommend No Geophysics•Resolution•Cultural factors•Cost•QA/QC•Safety•Data reduction

Forward Model Site Check

Select Geophysics Methodology

GOOD

BAD

Field Operationsplan•Line/Station/GridProcessing/interp•Integration

Survey Design

Data Collection, Processing, Interpretation

Data Integration, Presentation and Recommendation

Recommend No Geophysics

Data Reduction - Data Processing - Data PresentationHow 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 acquisitionHow 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

Dynamic Static

Cultural (manmade)• Electrical power• Radio transmitters• Vehicle

• Buried pipes• Drains• Foundations

Natural

• Rain• Wind• Wave• Electrical Storms• Magnetic Storms

• Any geologic-hydrogeologic noisenot related to target

Noise Sources

The Geophysical Survey - Typical Survey TypesSounding - 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

initiation4D - 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

Rule of ThumbGeophysical signature (anomaly) typically at least twice actual size of feature.

too small: spatially undersampledtoo small: waste time and moneytoo large: miss target completely

• Station spacing/station interval lead to spatial aliasing

• line interval lead to issues of spatial aliasing

Spatial Aliasing

• (spatial) loss of high frequency information

2D - True Profile Data

3D example - “Bulls Eye” Effect

Aliased (undersampled)

Optimally sampled

Oversampled

Data Interpretation and PresentationQualitative• 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

Linear position

0

10

20

30

0

10

20

30

Fracture Zone

EM 34 Horizontal Coils, 20m spacing

EM 34 Vertical Coils, 20m spacing

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Data Presentation - 3D data

Simple Contour Map

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Colour Contour Map

Shaded Relief Map

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3D Relief