Transcript Geophysics - West Virginia University

Geol 454
Environmental and Exploration Geophysics I
Introduction
tom.h.wilson
[email protected]
Phone: 293-6431
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography
General information - handouts
• Syllabus (see handout)
Hunting for Abandoned Wells
• Example paper summary
Hunting for caves
Tom Wilson, Department of Geology and Geography
General information
• Syllabus (see handout)
Hunting for Abandoned Wells
• Textbook (current status ?)
• Class web page
• Grading (32% (pbs),28% (labs), 10%
(paper summaries), 10% (mid term),
20% (final))
• Papers and paper summaries (see
handout and link)
Tom Wilson, Department of Geology and Geography
Hunting for caves
Textbook ?
Tom Wilson, Department of Geology and Geography
Geophysics
Objectives
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 your
discipline or study.
3. Provide analytical experience necessary to
oversee applications of these methods in your
specialty area.
Tom Wilson, Department of Geology and Geography
Geophysics
Definitions
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 nearsurface 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
•Terrain Conductivity
•Resistivity
•Gravity
•Magnetics
These methods represent only a few of the
various geophysical methods used to peer
beneath the surface in a
manner.
Tom Wilson, Department of Geology and Geography
Terrain Conductivity
Inducing current flow – action at a distance
Tom Wilson, Department of Geology and Geography
Terrain Conductivity Instrumentation
Active Source
EM31
EM34
Two Commonly used Terrain Conductivity Meters You’ll hear a lot about these in the first 3-4 weeks of the class
Tom Wilson, Department of Geology and Geography
Multifrequency terrain conductivity meterSame idea
Tom Wilson, Department of Geology and Geography
Applications
Contaminant Plumes
Tom Wilson, Department of Geology and Geography
Metal Detection
Abandoned mine lands and mine spoil
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Terrain Conductivity over the Greer Site
Tom Wilson, Department of Geology and Geography
Other kinds of instrumentation
Gem2 – Aeroquest/GeoPhex
http://www.terraplus.ca/products/electromagnetics/gem2.htm
Tom Wilson, Department of Geology and Geography
Resistivity
Active source
TerraPlus
Case Histories http://www.terraplus.ca/case-histories/index.html
Tom Wilson, Department of Geology and Geography
Resistivity applications
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
Underground Storage Tanks
Leak detection (TerraPlus)
WESTEC Case Study
Tom Wilson, Department of Geology and Geography
Resistivity measurements made below the liner system
Landfill Leak Detection
http://www.terraplus.com/papers/henderson.htm
Tom Wilson, Department of Geology and Geography
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.
Gravitational fields
Passive source
LaCoste Romberg Gravimeter
Worden Gravimeter
Tom Wilson, Department of Geology and Geography
From Hooke’s Law
F  kx
we get
kx
g 
ms
ms g
x
k
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
Tom Wilson, Department of Geology and Geography
Newton’s Universal Law of Gravitation
F12  G
m1m2
m1
2
r12
F12 Force of gravity
G Gravitational Constant
r12
m2
Newton.org
Tom Wilson, Department of Geology and Geography
We are interested in gobject: some subsurface feature like karst
systems, bedrock configuration and groundwater accumulations
F12
mE
gE 
G
ms
RE2
ms spring mass
mE mass of the earth
RE radius of the earth
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).
Tom Wilson, Department of Geology and Geography
Comparison of bedrock depth map to the
residual gravity map
Form Stewart
Tom Wilson, Department of Geology and Geography
Residual gravity map
Magnetic Methods
Passive source
Magnetic Elements for your location
http://www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp
Today’s Space Weather
http://www.swpc.noaa.gov/
Tom Wilson, Department of Geology and Geography
Proton Precession Magnetometers
Tom Boyd’s Introduction to Geophysical Exploration Course
http://www.earthsci.unimelb.edu.au/ES304/index.html
Tom Wilson, Department of Geology and Geography
See
http://www.earthsci.unimelb.edu.au/ES304/MODULES/MAG/DESIGN/MagForwardJava.html
Tom Wilson, Department of Geology and Geography
Field Operation
Coal mine refuse area
Tom Wilson, Department of Geology and Geography
Magnetic monopoles
1 p1 p2
Fm12 
2
 r12
p1
r12
Fm12 Magnetic Force
 Magnetic Permeability
p1 and p2 pole strengths
Coulomb’s Law
p2
Tom Wilson, Department of Geology and Geography
Magnetic Monopoles do not exist. The fundamental
magnetic element is a dipole or combination of one
positive and one negative magnetic monopole.
Tom Wilson, Department of Geology and Geography
Looking for abandoned wells
Critical to EOR, EGR, CO2 Sequestration, fracking and frack water storage and disposal
From Martinek
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
Tom Wilson, Department of Geology and Geography
Start reading chapter 8
Visit class page at
http://www.geo.wvu.edu/~wilson/geol454.htm
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.
Tom Wilson, Department of Geology and Geography
Also look over McNeill’s technical note on 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
Tom Wilson, Department of Geology and Geography
Any questions?
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.
Tom Wilson, Department of Geology and Geography
Writing Section – please remain after class
Read through the handout with brief
question/answer session.
Writers should start working on their
first paper this week.
Tom Wilson, Department of Geology and Geography