EOSC 350 2010 Slide 1

Sept 14 : Goals Review quiz and team exercise

Case Histories Physical properties

EOSC 350 2010 Slide 2

CH1: Dublin Depth to Till

Setup: Find thickness of lodgement till. Hard, stiff, and low permeability layer (containing cobbles and boulders). Overlies limestone bedrock.

Properties: High stiffness related to elastic parameters. A seismic experiment would be appropriate. Till expected to have a variable velocity. Overlays a limestone basement with high velocity.

Bulk modulus Shear modulus P-wave velocity

EOSC 350 2010

2 body wave types – 2 surface waves

S-waves P-waves

Rayleigh waves Love waves

EOSC 350 2010

Physical Property: Seismic Velocity

Units: km/s (Compressional Wave)

EOSC 350 2010

CH1: Dublin Depth to Till Setup: Find thickness of till. Hard, stiff, and low permeability layer (containing

cobbles and boulders). Overlies limestone bedrock. Dublin is a noisy environment.

Properties: High stiffness related to elastic parameters. A seismic experiment would be appropriate. Till expected to have a variable velocity. Overlays a limestone basement with high velocity.

Survey: Seismic: Acoustic wave weight drop source. 24 receivers for each transmitter. “Shots” were at 6 meter intervals.

Data: Seismic signals at geophones. Surface waves:

EOSC 350 2010

CH1: Dublin Depth to Till Processing: Two steps: from the data acquire a phase velocity and

then invert to find a velocity depth profile. Composite the soundings into a 2D image.

Interpretation: Upper till region has 200

EOSC 350 2010

CH2: Sand and Gravel Quarries

Setup: Find sand and gravel quarries. Area has granitic mountains, rolling hills and lakes. Glacial deposits are responsible for potential sand and gravel resources. Some of the area is bog and agricultural land.

Properties: Bog material is wet and conductive. Gravel deposits are resistive (low conductivity). Gravels are unconsolidated and have a low seismic velocity.

EOSC 350 2010 Slide 8

Ohm’s law V = I X R ( R is resistance; units of Ohms) Resistance will

change if the measurement geometry or volume of material changes. It is NOT a physical property.

Volumetric version of R: Resistivity ρ (Ohm-m) ρ (Ohm-m) is the resistance per unit volume.

Or “Conductivity” =

Conductivity σ is in units of Seimens per meter or S/m

yresistivit1

EOSC 350 2010 Slide 9

Resistivity / Conductivity

8 orders of magnitude Matrix materials are mainly insulators Fluids and porosity are key (& minerals, sometimes)

EOSC 350 2010 Slide 10

Properties continued. Physical property Electrical resistivity ρ

Ohm-m

Electrical conductivity σ Siemens/meter

Affected by:

Metal content Porosity & fluid resistivity Archie’s law

Clay content

E= Jρ compare V=IR

EOSC 350 2010 EOSC 350 ‘06 Slide 11

Porosity and Archie’s Law

Porosity equals Volume pore /total volume

Archie’s Law

a, m both constants that depend on rock type.

F = “Formation factor”. a ~ 1 m ~ 1.0 in unconsolidated ~ 1.5 in sandstones ~ 2.0 in limestones

R for 3 rock types, log-lin

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7porosity

Ohm

-mFa mf

W == −φσσ

porosityf

W

=φ

σσ

EOSC 350 2010 EOSC 350 ‘06 Slide 12

Electrolytes: σw Temperature

Salinity

Month (Jan – Dec)

Con

duct

ivity

in

mS

/m

Salinity in grams per litre

Con

duct

ivity

in

mS

/m

EOSC 350 2010 EOSC 350 ‘06 Slide 13

Soils Saturated Unsaturated “clean” vs “clayey”

Quartz in sandstone: 0.1m2/gm

Illite: 100.0m2/gm

EOSC 350 2010

CH2: Sand and Gravel Quarries

Setup: Find sand and gravel quarries. Area has granitic mountains, rolling hills and lakes. Glacial deposits are responsible for potential sand and gravel resources. Some of the area is bog and agricultural land. (Picture)

Properties: Bog material is wet and conductive. Gravel deposits are resistive (low conductivity). Gravels are unconsolidated and have a low seismic velocity.

Survey: Preliminary EM survey (EM31) Logistically easy and gives an estimate of ground conductivity in the top few meters. Good reconnaissance tool. More detailed follow-up using DC resistivity to get 2D conductivity structure and seismic to find the base of the gravel.

Data: EM31data. Also DC and seismic are acquired along selected line profiles.

EOSC 350 2010

CH2: Sand and Gravel Quarries

Processing: EM31 data is converted to ground conductivity. (Picture). DC resistivity data is inverted to get a 2D cross section. Seismic data are inverted to provide location of refracting interfaces.

Interpretation: Areas of low conductivity are identified from the EM survey. The inversion of DC and seismic data outline a gravel lens along one of the transects. Gravel lens is 5-8 meters in thickness and 40-50 meters in length.

Synthesis: Seems successful. Have found gravel lenses and results have helped assess the potential tonnage across the site.

EOSC 350 2010

CH3: Mapping Peat Thickness Setup: Bog material in raised bogs is used for energy production. Need to

map out thickness of the bog over 35,000 Ha. Properties: Peat is a porous carbon material with large water content (they

need to dry it before using). Region below is listed as lake deposits. Possibly a difference is water content and texture and this may provide a difference in dielectric permittivity.

.

EOSC 350 2010 EOSC 350 ‘06 Slide 17

Dielectric permittivity, ε

See GPG section 3.g.

This physical property quantifies how easily material becomes polarized in the presence of an electric field.

Qualitative diagram of permittivity vs frequency

Log frequency

EOSC 350 2010

Relative permittivity

Value of electrical permittivity (ε) in freespace (ε0) is 8.844E-12 Farads/meter

Relative permittivity εr = ε/ε0 Where ε is the electrical permittivity of the

geologic material

εr sometimes called dielectric constant)

EOSC 350 2010 EOSC 350 ‘06 Slide 19

Electrical conductivity Water again has a strong effect – but depends on TYPE

of water Attenuation of radar signals is most affected by σ.

Define attenuation?? Signal energy loss per meter of travel.

EOSC 350 2010

EOSC 350 2010

Electrical Permittivity Properties

GPR experiment and data

EOSC 350 2010

CH3: Mapping Peat Thickness Setup: Bog material in raised bogs is used for energy production. Need to

map out thickness of the bog over 35,000 Ha. Properties: Peat is a porous carbon material with large water content (they

need to dry it before using). Region below is listed as lake deposits. Possibly a difference is water content and texture and this may provide a difference in dielectric permittivity.

Survey: GPR (Ground Penetrating Radar) Towed 100MHz antenna, with RTK GPS for positional accuracy. (20mm)

Data: Profiles collected every 60 m and plotted as distance-time sections.

EOSC 350 2010

CH3: Mapping Peat Thickness Data: Profiles collected every 60 m and plotted as distance-time sections. Processing: Processed to remove topography effects and identify

correlated reflection events. Interpretation: Peat augur (borehole device) was used to calibrate the

data. The base of the peat was identified at various checkpoints and then the associated reflector interpolated throughout the section. The thickness of the peat is provided in ms.

EOSC 350 2010

CH3: Mapping Peat Thickness Interpretation: Peat augur (borehole device) was used to calibrate the

data. The base of the peat was identified at various checkpoints and then the associated reflector interpolated throughout the section. The thickness of the peat is provided in ms. The 2D sections are interpolated and presented as a 3D image. (Picture)

Synthesis: Survey results are listed as being invaluable in the future planning of the remaining peat resources.

EOSC 350 2010

CH4: Karst Investigations

Setup: Karstic limestones create numerous problems for engineers. A proposed road could be compromised by cavities. (Figure)

Properties: Voids have low mass and would respond to a gravity survey. Limestone is a poor conductor. Karstified rock or overburden will have a high conductivity compared to competent limestone.

EOSC 350 2010 Slide 26

Physical Property: Density

Density – total mass/ total volume

Units: grams/cc (gm/cc) Kilograms/cubic meter (kg/m3) Symbol: or d

ρ

EOSC 350 2010

Density of Rocks and Minerals

EOSC 350 2010

CH4: Karst Investigations

Setup: Karstic limestones create numerous problems for engineers. A proposed road could be compromised by cavities. (Figure)

Properties: Voids have low mass and would respond to a gravity survey. Limestone is a poor conductor. Karstified rock or overburden will have a high conductivity compared to competent limestone.

Survey: Gravity survey over an area encompassing the proposed road. DC resistivity survey along the center of the road.

Data: Micro gravity data were acquired on a 10m X 10m grid. 2D DC resistivity were acquired along profiles in the center of the proposed road.

EOSC 350 2010

CH4: Karst Investigations Data: Micro gravity data were acquired on a 10m X 10m grid. 2D DC resistivity were

acquired along profiles in the center of the proposed road. Processing: Gravity data were processed using a base station as reference. DC

data were inverted to produce a 2D cross-section. (Picture) Interpretation: Pronounced gravity low in a region that had no surface expression of

karstification. This coincided with a low resistivity zone. (Picture) Synthesis: Gravity data indicate the location of the anomaly as it trends at an angle

across the proposed road. Resistivity shows the horizontal and vertical extend of the anomalous rock.

EOSC 350 2010

CH5: Mineral Exploration Setup: Pallas Green area is geologically favorable to host Pb-Zn

mineralization. Delineate drill targets. Pattern drilling on a 1km x 1km grid indicated mineralization and a step out hole intersected 3m massive sulfides at 150 m depth. This was the “discovery hole” and the goal was to explore this region in more detail.

Properties:Mineralization is normally found at the base of a highly resistive sequence of mudbank limestones. These overlay a less resistive argillaceous limestone sequence. Mineralization is associated with alteration units containing sulfides that are chargeable.

EOSC 350 2010

Physical Property: Chargeability Chargeable materials act like capacitors when an electric field is applied. Units of chargeability

• dimensionless (0

EOSC 350 2010

Chargeability: Rocks

Material type Chargeability (ms.)

20% sulphides 2000 - 3000

8-20% sulphides 1000 - 2000

2-8% sulphides 500 - 1000

volcanic tuffs 300 - 800

sandstone, siltstone 100 - 500

dense volcanic rocks 100 - 500

shale 50 - 100

granite, granodiorite 10 - 50

limestone, dolomite 10 - 20

Material type Chargeability (ms.)

ground water 0

alluvium 1 - 4

gravels 3 - 9

precambrian volcanics 8 - 20

precambrian gneisses 6 - 30

schists 5 - 20

sandstones 3 - 12

argilites 3 - 10

quartzites 5 - 12

EOSC 350 2010

Chargeability: minerals

Material type Chargeability (ms.)

pyrite 13.4

chalcocite 13.2

copper 12.3

graphite 11.2

chalcopyrite 9.4

bornite 6.3

galena 3.7

magnetite 2.2

malachite 0.2

hematite 0.0

EOSC 350 2010

CH5: Mineral Exploration Setup: Pallas Green area is geologically favorable to host Pb-Zn mineralization.

Delineate drill targets. Pattern drilling on a 1km x 1km grid indicated mineralization of and a step out hole intersected 3m massive sulfides at 150 m depth. This was the “discovery hole” and the goal was to explore this region in more detail.

Properties:Mineralization is normally found at the base of a highy resistive sequence of mudbank limestones. These overlay a less resistive argillaceous limestone sequence. Mineralization is associated with alteration units containing sulfides that are chargeable.

Survey: Electrical conductivity separating the units might be delineated with DC resistivity. IP (Induced Polarization) survey can find chargeable zones.

Data: A gradient array DC resistivity and IP survey were carried out. Survey area was centered on the discovery hole (500m X 500m area). A mise-a-la-masse (MALM) experiment was conducted using the discovery hole.

Processing: DC resistivity data are converted to apparent resistivity and plotted as a map image. The apparent chargeability is also plotted in this format.

Apparent resistivity Apparent chargeability

EOSC 350 2010

CH5: Mineral Exploration Processing: DC resistivity data are converted to apparent resistivity and plotted as a

map image. The apparent chargeability is also plotted in this format. (Picture) Interpretation: NW-SE grain in apparent resistivity image is consistent with thinning

of the mudbank sequence. Chargeability has a number of distinct anomalies. Drill hole B is caused by the massive sulfides and is 12m thick. (8 m of 3.9% Zn) Chargeabilities from the MALM survey were proximal to drill hole B.

Synthesis: The IP has been successful for locating a drill hole but not all IP anomalies are associated with sulfides.

Apparent resistivity Apparent chargeability Apparent chargeability From MLAM

EOSC 350 2010

CH6: Mineral Exploration Setup: Raglan area has known mineralization. Where are the deposits and what is

the ore deposit model. Mineralization is thought to have a cylindrical geometry and deposits are not connected. There is a need to test that hypothesis.

Properties: Surface rocks are volcanics (basalts) in contact with sedimentary rocks. Ultramafics, which contain the mineralization. The ultramafics have a high susceptibility, volcanics are weakly susceptible, and sedimentary rocks have low susceptibilty.

Survey: Magnetic survey. Acquire data along multiple lines to cover the area of interest.

Data: Ground magnetic data are acquired and plotted as a 2D map.

EOSC 350 2010 Slide 37

Physical Property: Magnetic Susceptibility

Magnetic susceptibility:

Ability for a rock to become “magnetized” when an external magnetic field is applied.

Units: dimensionless

EOSC 350 2010 EOSC 350 ‘06 Slide 38

Relating property values to geology

EOSC 350 2010 EOSC 350 ‘06 Slide 39

Magnetic susceptibility of rocks

Why such broad ranges of this property? Magnetism is a complicated phenomenon

Atomic, molecular, crystalline and grain-size scales. Interaction of fields. Remanence (permanent magnetism).

A few minerals dominate, but magnetite is by far the most important for most rocks.

(Diagram is in magnetics notes. It is for illustration only.)

TiO2

FeO Fe2O3 Fe3O4

magnetite

FeTiO3

EOSC 350 2010

CH6: Mineral Exploration Setup: Raglan area has known mineralization. Where are the deposits and what is

the ore deposit model. Mineralization is thought to have a cylindrical geometry and deposits are not connected. There is a need to test that hypothesis.

Properties: Surface rocks are volcanics (basalts) in contact with sedimentary rocks. Ultramafics, which contain the mineralization. The ultramafics have a high susceptibility, volcanics are weakly susceptible, and sedimentary rocks have low susceptibilty.

Survey: Magnetic survey. Acquire data along multiple lines to cover the area of interest.

Data: Ground magnetic data are acquired and plotted as a 2D map.

EOSC 350 2010

CH6: Mineral Exploration Data: Airborne and ground magnetic data are acquired over a large area. Airborne

data are collected along lines. Processing: Data are processed to remove time variations. Also regional field is

removed. Decimated data are inverted to generate a 3D distribution of magnetic susceptibility.

Interpretation: 3D inversion shows magnetic material at surface with co-locate with mineralized outcrops. These units join at depth.

Synthesis: Drilling and direct sampling was used to test the interpretation. The drill hole encountered 150 meters of ultramafic rock and 12 meters of mineralization. This revised the ore deposit model and greatly increased the estimate of mineral reserve on the property.

EOSC 350 2010

What have we learned from the Case Histories

Earth materials have a range of physical properties.

Application of geophysics is carried out in a 7 Step process. Physical property of the target must be different from host material

Knowledge of a single physical property does not uniquely identify a material. Interpretation was aided by using multiple surveys.

Examples: Gravel quaries: conductivity, elastic parameters Karst investigations: density, conductivity Mining: conductivity, chargeability

EOSC 350 2010

Summary of Readings so far

Foundations: GPG 2.b Summary 7 Steps GPG 2.c Seeing Underground GPG 2.d Physical Properties GPG 2.e Plotting Case History: Geophysical Journey in Ireland

EOSC 350 2010

Summary for Introductory Section

What has been learned?

EOSC 350 2010

|Next Readings: Magnetics

Geophysical Surveys GPG 3.d.0 Susceptibility GPG 3.d.1 Measurements

Sept 14 : GoalsCH1: Dublin Depth to Till2 body wave types – 2 surface waves Physical Property: Seismic VelocityCH1: Dublin Depth to TillCH1: Dublin Depth to TillCH2: Sand and Gravel QuarriesOhm’s lawResistivity / ConductivityProperties continued.Porosity and Archie’s LawElectrolytes: σwSoils CH2: Sand and Gravel QuarriesCH2: Sand and Gravel QuarriesCH3: Mapping Peat ThicknessDielectric permittivity, εRelative permittivityElectrical conductivitySlide Number 20Electrical Permittivity PropertiesCH3: Mapping Peat ThicknessCH3: Mapping Peat ThicknessCH3: Mapping Peat ThicknessCH4: Karst InvestigationsPhysical Property: DensityDensity of Rocks and� MineralsCH4: Karst InvestigationsCH4: Karst InvestigationsCH5: Mineral ExplorationPhysical Property: ChargeabilityChargeability: RocksChargeability: mineralsCH5: Mineral ExplorationCH5: Mineral ExplorationCH6: Mineral ExplorationPhysical Property: Magnetic SusceptibilityRelating property values to geology Magnetic susceptibility of rocksCH6: Mineral ExplorationCH6: Mineral ExplorationWhat have we learned from the Case HistoriesSummary of Readings so farSummary for Introductory Section |Next Readings: Magnetics