No Slide Titlepages.geo.wvu.edu/~wilson/geol454/lect1/lec1.pdf · Applied Geophysics •Making and...

Tom Wilson, Department of Geology and Geography

Geol 454Environmental and Exploration Geophysics I

tom.h.wilson

[email protected]

Department of Geology and Geography

West Virginia University

Morgantown, WV

Introduction

Phone: 293-6431


• Syllabus (see handout)

• Class computer account

(next time)

• general information form

• writing section introduction

Hunting for Abandoned Wells

Hunting for caves

General information - handouts

http://www.amazon.com/Introduction-Applied-Geophysics-Exploring-Subsurface/dp/0393926370/ref=sr_1_1?ie=UTF8&s=books&qid=1251197643&sr=1-1


http://www.agiusa.com/brochure_schnabel_cave.shtml


Questions about the class


• Textbook – Applied Geophysics

• Class web page

• Grading 1) problem assignments (45% including

attendance), 2) computer labs (20%), 3) mid-term exam

(15%) and 4) final exam (20%).

• Grading breakdown for writing section - next slide

• Exams are problem solving. If you can do the

homework you will do good on the exams.

Hunting for Abandoned Wells

Hunting for caves





Writing section


Grading for those in the writing section differs as

follows:

• Exam grades in the writing section count 10%

(mid-term) and 15% (final).

• The remaining 10% of the grade for those in the

writing section is based on two 1600 word papers

(two chapter summaries) written during the semester.

Those in the writing section receive 5% of the total

semester grade for each of these two papers.

• Percentages for homework and labs are the same in

both sections.



Required text


Class web page at

http://pages.geo.wvu.edu/~wilson/geol454.htm


Note mid-term and final exam times


Click in this link

You can check your exam schedules on the finals link

or go directly to

http://registrar.wvu.edu/current_students/finals/


Note correction on last page of syllabus


GeophysicsObjectives

1. Obtain conceptual knowledge of the basic theory

of gravitational, magnetic, electric, and

electromagnetic fields.

2. Develop an understanding of how geophysical

observations can be integrated into subsurface

investigations.

3. Provide data analysis and modeling experience

necessary to oversee applications of these

methods in your specialty area.


GeophysicsDefinitions

Applied Geophysics

•Making and interpreting measurements of physical properties of the

earth to determine sub-surface conditions, usually with an economic

objective, e.g. discovery of fuel or mineral deposits.

Environmental Geophysics

•The applications of geophysical methods to the investigation of near-

surface physico-chemical phenomena which are likely to have

(significant) implications for the management of the local environment

Sheriff, R. E., Encyclopedic Dictionary

of Exploration Geophysics, SEG. &

Reynolds (1997)


Geophysical methods we will cover this semester

•Terrain Conductivity (chapter 8)

•Resistivity (chapter5)

•Gravity (chapter 6)

•Magnetics (chapter 7)

These methods represent only a few of the various

geophysical methods used to peer beneath the

surface in a non-invasive non-intrusive manner.

Geophysics also includes geophysical logging

from strat and sed, structure …?


A very direct and penetrative way to

gain information about the subsurface

The sensors

employed are similar

in may ways to those

used on the surface

but are designed for

borehole deployment

and operation.Tully Ls.

Lower Marcellus

Shale

Onondaga Ls and

Oriskany Ss.

Logging tools measure a variety of

geophysical parameters


Compressional

wave velocity

Shear wave

velocity

ray

emissionPoisson’s

ratio

What can we tell about

subsurface geology from

these surface measurements?

Terrain Conductivity

Inducing current flow – action at a distance


surface

What are variations in subsurface

conductivity associated with? How

extensive are they?

?


Information is in the induced EM field


surface

In the case of terrain conductivity

applications we are inducing current flow in

the subsurface, measuring and trying to

figure out what could be causing variations

in observed conductivity.


interpreting your findings depends on context


surface

Distribution of

contaminants?


Terrain Conductivity Instrumentation

Active Source

Two Commonly used Terrain Conductivity Meters -

You’ll hear a lot about these in the first 3-4 weeks of the class

EM31 EM34


Multifrequency terrain conductivity meter-

Same idea

Applications


Metal DetectionContaminant Plumes

Abandoned mine lands and mine spoil



An alkaline metal rich

hydroxide sludge


Terrain Conductivity over the Greer Site

The trench you

saw on the

previous slide

Other kinds of instrumentation

Gem2 – Aeroquest/GeoPhex


http://www.terraplus.ca/products/electromagnetics/gem2.htm

2nd method we will cover - Resistivity


Active source

Case Histories

TerraPlus

http://www.terraplus.ca/case-histories/index.html

http://www.terraplus.com/papers/resistivitycase.htm

Resistivity applications


Underground Storage Tanks

Leak detection (TerraPlus)

WESTEC Case Study

Map is contoured in units of 0.5 percent

change, which is a measure of the drop in

apparent resistivity from the baseline data set

to the data collected after 69 gallons of

solution was released. The largest observed

percent change in the data was 0.2%,

concentrated near the northeast margin of the

tank, and beneath the center of the tank.

http://www.terraplus.com/papers/westec.htm


Landfill Leak Detection

http://www.terraplus.com/papers/henderson.htm

A typical response to an introduced leak is shown on in the

figure. This calibration demonstrated that the sensitivity of

the particular system was approximately 20 gallons

in total introduced leachate, and the location ability of the

software was within 10 to 20 feet. This 20-gallon detection

level is equivalent to a 4-foot-square zone of saturated

soil, assuming a 1-foot thickness.

Resistivity measurements made below the liner system

http://www.terraplus.com/papers/henderson.htm

The 3rd method looks at gravitational fields and

what their variation tells us about the subsurface


Passive source

LaCoste Romberg Gravimeter

Worden Gravimeter

We don’t stir things up or poke holes in

the ground


k

gmx s

x spring extension

ms spring mass

k Young’s modulus

g acceleration due to gravity

Colorado School of Mines web sites -

Mass and spring

Pendulum measurement

kxF From Hooke’s Law

sm

kxg we get

http://www.mines.edu/fs_home/tboyd/GP311/MODULES/GRAV/NOTES/spring.html

http://www.mines.edu/fs_home/tboyd/GP311/MODULES/GRAV/NOTES/pend.html

Newton’s Universal Law of Gravitation


Newton.org

212

2112

r

mmGF m1

m2

r12F12 Force of gravity

G Gravitational Constant

http://www.newton.org.uk/

We are interested in gobject: some subsurface feature like karst

systems, bedrock configuration and groundwater accumulations


212

E

E

sE

R

mG

m

Fg

gE represents the acceleration of gravity at

a particular point on the earth’s surface. The

variation of g across the earth’s surface

provides information about the distribution

of density contrasts in the subsurface since

m = V (density x volume).

ms spring mass

mE mass of the earth

RE radius of the earth


Comparison of bedrock depth map to the

residual gravity map

Form Stewart Residual gravity map

Lastly we will take a look at magnetic methods

and their applications to subsurface investigation


Magnetic Elements for your location

http://www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp

Today’s Space Weather

http://www.swpc.noaa.gov/

Again – we just measure what we find.

We measure the field we

find and try to figure out

what could be causing

the variations in

magnetic field we see.

http://www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp

http://www.swpc.noaa.gov/

Proton Precession Magnetometers


Tom Boyd’s Introduction to Geophysical Exploration Course

http://www.earthsci.unimelb.edu.au/ES304/index.html

http://www.earthsci.unimelb.edu.au/ES304/index.html

A variety of parameters influence what you

measure


Combined magnetic and electromagnetic

surveys


Coal mine refuse areaIt helps to constrain subsurface interpretations by

measuring more than one physical property.

Magnetic monopoles don’t exist. The basic

field is that of a dipole


212

2112

1

r

ppFm

p1

p2

r12Fm12 Magnetic Force

Magnetic Permeability

p1 and p2 pole strengthsCoulomb’s Law

http://talus.mines.edu/fs_home/tboyd/GP311/MODULES/MAG/NOTES/monopole.html

This gives rise to field line orientations and

magnitudes that vary spatially


The dipole field

+

-

Looking for abandoned wells


From Martinek

Critical to EOR, EGR, CO2 Sequestration, waste water disposal

A different perspective gives you a feel for the

relative sizes of the abandoned well anomaly


7000 gamma anomaly

No excavation planned since

the anomaly is located

adjacent to a water main

From Martinek

Start reading chapter 8


For general background on electromagnetic

methods read pages 499 through 514.

Most of our work will concentrate on concepts

associated with “non-contacting ground

conductivity measurements” pages 514 – 518.

Visit class page at

http://www.geo.wvu.edu/~wilson/geol454.htm



Additional reading material – see

EM Conductivity/Low induction number


Both readings are linked on lecture 1 – see class web page at

http://www.geo.wvu.edu/~wilson/geol454.htm

We will begin discussing basic terrain

conductivity theory on Thursday.

Begin reading through chapter 8 (499-

514) and skim through McNeil’s paper

Be prepared for an in-class discussion

of problem 8.4 next Tuesday


Calculate the apparent conductivity for a layer of

thickness 3m and conductivity15 mS/m (1) over a

lower layer of conductivity 100 mS/m (2).

Hint: 1 1 2 1(1 ( ) ( )a v vR z R z

Again, look over pages 514-518 and see if you

can figure it out.

Any general questions?


In the next lecture we will introduce

electromagnetic methods as discussed on

pages 499-518. A short section to read, so –

again - review before next time.

Writing section


If you are taking the writing section,

please remain for a few minutes so we

can have a brief discussion and answer

any basic questions you may have.

Grading for those in the writing section differs as follows: exam grades in the

writing section count 10% (mid-term) and 15% (final). The remaining 10% of

the grade for those in the writing section will be based on two 1600 word

papers (two chapter summaries) written during the semester. Those in the

writing section receive 5% of the total semester grade for each of these two

papers.

See you next Tuesday

No Slide Titlepages.geo.wvu.edu/~wilson/geol454/lect1/lec1.pdf · Applied Geophysics •Making and...

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