Tom Wilson, Department of Geology and Geography
Geol 454Environmental and Exploration Geophysics I
tom.h.wilson
Department of Geology and Geography
West Virginia University
Morgantown, WV
Introduction
Phone: 293-6431
Tom Wilson, Department of Geology and Geography
• Syllabus (see handout)
• Class computer account
(next time)
• general information form
• writing section introduction
Hunting for Abandoned Wells
Hunting for caves
General information - handouts
Questions about the class
Tom Wilson, Department of Geology and Geography
• 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
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
Class web page at
http://pages.geo.wvu.edu/~wilson/geol454.htm
Tom Wilson, Department of Geology and Geography
Note mid-term and final exam times
Tom Wilson, Department of Geology and Geography
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/
Tom Wilson, Department of Geology and Geography
Note correction on last page of syllabus
Tom Wilson, Department of Geology and Geography
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.
Tom Wilson, Department of Geology and Geography
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)
Tom Wilson, Department of Geology and Geography
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 …?
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
surface
What are variations in subsurface
conductivity associated with? How
extensive are they?
?
Terrain Conductivity
Information is in the induced EM field
Tom Wilson, Department of Geology and Geography
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.
Terrain Conductivity
interpreting your findings depends on context
Tom Wilson, Department of Geology and Geography
surface
Distribution of
contaminants?
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
Multifrequency terrain conductivity meter-
Same idea
Applications
Tom Wilson, Department of Geology and Geography
Metal DetectionContaminant Plumes
Abandoned mine lands and mine spoil
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
An alkaline metal rich
hydroxide sludge
Tom Wilson, Department of Geology and Geography
Terrain Conductivity over the Greer Site
The trench you
saw on the
previous slide
Other kinds of instrumentation
Gem2 – Aeroquest/GeoPhex
Tom Wilson, Department of Geology and Geography
http://www.terraplus.ca/products/electromagnetics/gem2.htm
2nd method we will cover - Resistivity
Tom Wilson, Department of Geology and Geography
Active source
Case Histories
TerraPlus
http://www.terraplus.ca/case-histories/index.html
Resistivity applications
Tom Wilson, Department of Geology and Geography
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.
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
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
The 3rd method looks at gravitational fields and
what their variation tells us about the subsurface
Tom Wilson, Department of Geology and Geography
Passive source
LaCoste Romberg Gravimeter
Worden Gravimeter
We don’t stir things up or poke holes in
the ground
Tom Wilson, Department of Geology and Geography
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
Newton’s Universal Law of Gravitation
Tom Wilson, Department of Geology and Geography
Newton.org
212
2112
r
mmGF m1
m2
r12F12 Force of gravity
G Gravitational Constant
We are interested in gobject: some subsurface feature like karst
systems, bedrock configuration and groundwater accumulations
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
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.
Proton Precession Magnetometers
Tom Wilson, Department of Geology and Geography
Tom Boyd’s Introduction to Geophysical Exploration Course
http://www.earthsci.unimelb.edu.au/ES304/index.html
A variety of parameters influence what you
measure
Tom Wilson, Department of Geology and Geography
Combined magnetic and electromagnetic
surveys
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
212
2112
1
r
ppFm
p1
p2
r12Fm12 Magnetic Force
Magnetic Permeability
p1 and p2 pole strengthsCoulomb’s Law
This gives rise to field line orientations and
magnitudes that vary spatially
Tom Wilson, Department of Geology and Geography
The dipole field
+
-
Looking for abandoned wells
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
7000 gamma anomaly
No excavation planned since
the anomaly is located
adjacent to a water main
From Martinek
Start reading chapter 8
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
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?
Tom Wilson, Department of Geology and Geography
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
Tom Wilson, Department of Geology and Geography
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