Grading:
based on assignments, discussion, reports
from field work, and attendance. To pass the course you
must participate and satisfactorily complete all assignments.
This page (from 2000) will be modified as the semester progresses.
Scholarship
and summer field experience opportunities:
Introductory
material: course structure
The focus
of this course will be experimental design, data acquisition,
processing, interpretation and presentation aimed at investigating
physical properties of the shallow subsurface. The equipment
we will use includes:
| Trimble
GeoExplorer GPS receivers (sub-meter) |
Worden
gravimeter |
GEM
proton precession magnetometer/gradiometer |
|
|
|
Much of the equipment has its own processing software but
we will also use various software tools to facilitate interpretation.
A basic tool that will be necessary for many tasks is a common
spreadsheet such as Microsoft's Excel which is installed on
computers in our graphics lab (SC 305), computer teaching
room (SC 11), and my lab (SC 3). Simple notes and introductory
spreadsheet exercises
are available in my course notes for Computation
and Computers in Geology. If you need help, just ask because
you will need to be up to speed with a spreadsheet to accomplish
the assignments. A list of representative
textbooks, and a comment on PDF files, are at the bottom
of this page as are some links and references.
Week
by week events:
Week
one:
Review gravity and simple Bouguer anomalies, learn how to
operate the Worden gravimeter, and learn how to use GravCadW.
As a lab exercise, we will use the gravimeter to determine
the height of a table using 0.0877 scale divisions/mgal
for the gravimeter and 0.09406 mgals/ft for the gradient
of gravity with elevation.
Assignments:
Everybody
collects at least eight gravity measurements on the corner
of the concrete pier in my lab during the next week. Your
measurements should be at random times, they must be on
at least three different days and separated by at least
3 hours. We will use these data to evaluate instrument drift
and uncertainty. If you are curious, pack a bunch of readings
into 24 hours and see if you can see tides. Have your data
ready on 2/05/01 in an Excel spreadsheet using the following
format:
| Name |
Date |
Time |
Meter
Reading |
| Steve |
1/30/03 |
13:34 |
1632.7 |
If anything
is tricky about reading the gravimeter, it is the vernier
scale for the dial; here's an example of how
to read the Worden's scale.
GravCadW
is a 2D gravity modeling program;
it's mine so if you find bugs, let me know. One known
bug awakens if you try to print a model that has two vertices
with the same horizontal location, the print routine will
crash. Once you feel familiar with GravCadW, do
this problem:
Imagine
you are working in a groundwater system where the general
depth to bedrock (2800 kg/m3) is 500 meters, the average
density of the sediments in the basin is 2.2 g/cm3. A
divide in groundwater flow causes you to suspect a local
bedrock high in the basin. Your first guess is that the
bedrock high is 500 meters wide, very long, and sticks
up 200 meters into the sediments. What is the maximum
anomaly? How well do you need to know elevations (in +/-
meters) to contour this anomaly? How many gravity observations
should you collect to nicely define the anomaly?
Week
two:
First, a short lecture on error analysis, uncertainties,
propagation of uncertainties, precision
and accuracy. We'll analyze the measurements made over
the past week, further our discussion of uncertainties,
and show the need for drift curves. We will combine and
analyze the gravity data
(2000
data) you collected during last week. These observations
include daily tidal variations, monthly tidal variations,
operator error, and instrumental drift. To isolate instrument
drift and operator error from tidal variations use Excel
to fit a least-squares line to the data (meter readings
versus d'hour), remove the linear trend from the data (observation
- slope*d'hour), and calculate the mean (=Average() in Excel)
and standard deviation (=STDEV() in Excel) of the remaining
values.
Homework:
Reading assignment: Peter Dana's web page on geodetic
datums.
Problems:
-
Use
the gravimeter to determine the height of two different
tables, desks, or other surfaces in my lab. Make at least
three measurements of both the floor and table surface
for each surface - get the measurements as consistent
as you can. Use 0.0877 scale divisions/mgal for the gravimeter
and 0.09406 mgals/ft for the gradient of gravity with
elevation. Turn in a short, neat description of your experiment
including a table with your measurements, means, standard
deviations, and comparisons showing whether or not you
can tell the height of the two surfaces apart.
-
The
data in the linked spreadsheet
represent gravity meter readings collected on a circuit
using the lab as a base station. Calculate linear drift
corrections for each of the observations and the final
base reading. Turn in a neat explanation with a table
and graph of uncorrected and correct readings.
Week
three:
Basics
of the GPS system. These figures (2D
and 3D) compare
real-time differentially corrected GPS results from the
PRO-XRS system with non-corrected results. The standard
deviations fro real-time corrected and uncorrected data
are about the same. These two figures show GPS measurements
before and
after differential
corrections on the code phase before selective availability
(SA) was removed from the GPS system. The USFS provides
a useful GPS
link; Peter Dana at the University of Colorado has an
excellent GPS
page, as well as pages for map
projections, geodetic
datums, and coordinate
systems.
Homework:
Reading assignment: The Trimble book - Differential
GPS Explained.
Problems:
-
This
linked spreadsheet has 699 points collected over two
hours with a Pathfinder PRO XRS system. The data have
no real-time corrections and are not post-processed. Thus
they are a measure of what the XRS system will do on a
stand alone basis. Calculate the mean and standard deviations
of the X, Y, and Z components and then find the square
root of the sum of the squares of those standard deviations
(SQRT(sX^2 + sY^2 + sZ^2)); this provides a reasonable
expectation for the error from one of the PRO XRS systems.
-
Make
graphs and or plots that do a nice job of depicting the
location results from the GPS
observations above. You might first transform the
mean to the origin (Xi-meanX, Yi-meanY, Zi-meanZ) and
plot X vs. Y and Z vs. SQRT(X^2 + Y^2) - think about your
results, what do they mean?, why do they look like that?
Next, use SURFER
to make a contour or image map using X and Y as the map
coordinates and Z as elevation. I want you to find a way
to make a good visualization of the variance in the GPS
data - we'll compare and contrast your ideas in class.
-
Make
a graph of PDOP vs. time. Next modify your map from SURFER
to overlay contours of elevation on top of an image map
of PDOP vs. X & Y; use blue for low values and red
for high PDOP. Or, find a better or different way to show
X, Y, Z and their correlation with PDOP on the same image;
click the thumbnail for one example. What does this tell
you?
- figure
Week
four: (after President's day holiday). We
have two different GPS systems, both of which allow sub-meter
accuracy. The Trimble
PRO XRS systems (Bobolink and Avocet) are newer, bulkier
and more accurate (~ 2-5 meter precision) in real-time with
good conditions and conservative parameter
settings. Setup to acquire carrier-phase data for gravity
stations without having to wait for optimal conditions (>=5
visible satellites) the PRO-XRS system has a standard deviation
of 3-5 meters and differentially corrects to submeter (0.1
- 0.3 meters) accuracy. Comparison
of two trials, each over a couple of hours, shows the
improvement from differentially correcting carrier-phase
data (figure
1). The Geoexplorers
(Amelia and Sir John) are handheld units capable of 30 centimeter
precision when used in the carrier-phase mode. For both,
you need to occupy your station for at least 10 minutes.
Homework:
Reading assignment: Application of
the gravity method to the investigation of a landfill
in glaciated mid-continent, USA, a case history.
R.L. Roberts, W.J. Hinze, and D.I. Leap, in Geotechnical
and Environmental Geophysics, SEG,
S.H. Ward, editor, 1990.
Assignment:
Everybody
will be responsible for ten combined GPS/gravity determinations
on a line west
of Mount Sentinel; it will be easiest to work in groups
of two. Before class on 3/5/99 you need to drift
correct your data. Remember that due to tidal changes
and instrumental drift you need to return to the base station
within three hours. Your field notes must include: date,
time, meter reading, GPS file name, and a short description
of each site (so you or I could return to it).
We
will combine all of your data to make a map of the complete
Bouguer anomaly for the area. Thus it is imperative that
all of you go through the same steps, use the same GPS
parameters, Export
parameters, and accurately drift
correct your data.
Week
five: The gravity
flow chart outlines the whole procedure for
a gravity survey and shows the general steps for this exercise.
Assignment:
Prepare
a graph of your simple Bouguer anomaly by 3/12/01. Next
week we'll go over the remaining parts of the gravity
flow chart.
The
next step is gravity terrain corrections using 30-meter
digital elevation data (DEMs)
and HAMXYZ2 from Gradient Geophysics. In SC 11, we will
calculate terrain
corrections for a point at the base of Mount Sentinel
and for one about a kilometer west. The question to consider
is "how far away from the stations to we have to
evaluate terrain contributions?" My Pentium 266
took about 2.5 hours to calculate 3,172 terrain
corrections (out to 22 km).
Week
Six: Continuation of the gravity project
- Corpscon,
etc.
Assignment:
during
the week following spring break turn
in a report including: field notes, description of your
data acquisition and processing, a 2D
graph of your raw, drift corrected, and terrain
corrected data, a map of the complete Bouguer anomaly
for the class data from Surfer, and a simple interpretation
including some modeling and a figure using GravCadW. For
your 2D gravity model use a density contrast of -0.85 g/cm3.
This isn't due on the Monday after spring break but I want
it during that week; it is due on or before 3/29/03.
When you model the Sentinel fault, consider how you would
project its dip without the aid of your gravity data.
3/14/2003
- Get your 2D stuff all ready and written up. I'm still
trying to get all the class data and it looks like there
is a problem with some of your elevations. Make sure you:
-
corrected
your elevations for the height of the antennae off the
ground
-
added
1.16 meters to your NAD83/GEOID96 values (as exported
from Pathfinder Office)
-
used
the elevations as above to calculate free air corrections
and simple Bouguer corrections.
-
used
the NGVD29 vertical datum (Corpscon) for the elevations
of your stations when you calculated your terrane corrections
Here
is the "final" spreadsheet
from your (2001) surveys. Take a look at it (using SURFER)
and we will chat about it some more on April 9th. I think
there is still a problem with some of the elevations in
the northern part.
Here
are some results from the 1999 class as a guide. See me
with questions regarding Corpscon, Hammerxyz, Gravcad,
or how to make maps like the ones
linked to below.
1.
Maps of elevation,
simple Bouguer anomaly, terrain corrections, and complete
Bouguer anomaly. The 1999
data seem pretty good - look at the elevation contours
and see how nicely the terrain correction smoothes out
the anomaly which is now coherent from south to north.
I want you to make your own terrain corrections but
you can look at the maps to see what the results should
look like. You can use the longest, central profile
for your modeling as that will make it more interesting.
2.
The 1999 data file
with latitude, longitude and HAG transformed to a coordinate
system suitable for terrain corrections (State Plane
coordinates and NAVD29 elevations). These data went
through one more corrective iteration since class and
now look pretty good.
3.
I projected all of your data onto a central line roughly
perpendicular to the west face of Mount Sentinel. I
then used Grapher
to calculate a least-squares best-fit 5th degree polynomial
through those data to give you a sense of what a mean
profile though all the data would look like. Here are
the smoothed results;
use them for your modeling if you want to. Note that
in my figure
and the smoothed results that the variance of observations
from the line is not all uncertainty; much of the apparent
noise is a result of projecting the data some distance
onto the line. The
map of the complete Bouguer anomaly is smooth and
coherent..
I
will miss nest week due to a trip to Washington D.C.;
read
for the following week: L. Barrows and J. E. Rocchio,
Magnetic Surveying for Buried Metallic Objects,
Ground Water Monitoring Review, Summer 1990, p. 204-211.
Week
Seven (4/9/03): Review of your gravity reports
and learn how to operate the GEM proton precession magnetometer.
The quick
sheet on operating the magnetometer presents the basics
of operation. The complete
document provides more detail. After collecting data you'll
have to download
those data from the magnetometer to the lab computer.
Field
Assignment:
Take
the magnetometer to an open area (no cultural artifacts
like buildings, power lines, etc.) and determine the magnetic
anomaly around a car or a small pile of mountain bikes.
This is another exercise that will be best completed in
small groups. You need to determine the background magnetic
field and the appropriate sample spacing to best characterize
the magnetic anomaly from your source. That is, you want
to collect just enough data to adequately and accurately
characterize the anomaly. Turn
in a report by Monday, 4/21/2003 including a description
of your source, sample spacing, data acquisition and processing,
maps of the total field anomaly and field-gradient anomalies
using Surfer,
and some thoughts on magnetic survey design in areas with
cultural artifacts.
Week
Eight (4/16/03): Short discussion/demo on
edge effects in modeling gravity data - i.e. further discussion
of your reports. I will discuss two case studies of magnetic
exploration:
-
A
near-surface minerals exploration project in Alaska
-
An
environmental project in the Philippines
and
run through a demonstration of MagCad (DOS-based 2D magnetic
modeling program). We will start to solve problems with your
magnetometer surveys and data.
Week
Nine(4/23/03): Discuss magnetic reports.
Introduction to electromagnetic exploration and the EM31
ground conductivity meter from Geonics.
Read:
C.M.
Schlinger, Magnetometer and Gradiometer Surveys for
Detection of Underground Storage Tanks, Bull Assoc.
Engineering Geologists, 1990, v. 27, n1, p. 37.
Field
Assignment:
Take
the EM31
and do one of the following: 1) duplicate the magnetometer
exercise, 2) design your own experiment to decide if the
inphase or quadrature signal is best for detecting metal,
3) make maps of the area north of the Science Complex, or
4) any reasonable experiment you think will be interesting.
Turn
in a report on your experiment by 5/7/03.
Week
Ten (4/30/01): Discuss upward/downward
continuation of your magnetic data and solve problems
with your EM31
surveys and data. I performed the upward continuation
calculations with FFTFIL.EXE,
one of the programs in the USGS'
Potential Field software package. BOUNDARY*,
part of the USGS package, allows a nice presentation
of the maxima in horizontal gradients in grids and helps
with first guesses at the edge of causative sources. The
USGS DOS software, which includes a large number of useful
gravity and magnetic programs, is currently available on
a USGS ftp site.
Follow the link
(double click the file you want to save to disk - start
with the README) or do an anonymous ftp to: 136.177.80.14/pub/pf.
*See
R. Blakely and R. Simpson, Approximating Edges of Source
Bodies from Magnetic or Gravity Anomalies, Geophysics, v.
51, #7, p. 1494-1498 for a full explanation of Boundary's
approach.
Week
Eleven (5/7/03) (and the last week for 2003)
Introduction to the EG&G
Smartseis 12-channel seismograph.
Read
for 5/14/03: P. J. Wolfe and B. H. Richard, 1996,
Integrated Geophysical Studies of Buried Valley Aquifers,
Journal of Environmental and Engineering Geophysics, V.
1. , #1, p. 75-84.
--no
Week Twelve: Further experimentation with
the EG&G
Smartseis 12-channel seismograph.
Read:
J.
D. Phillips and D. V. Fitterman, 1995, Environmental Geophysics,
Reviews of Geophysics Supplement: U.S. National Report to
International Union of Geodesy and Geophysics 1991-1994.,
p. 185-193.
Finals
Week: We will meet during the scheduled
final time: 3:30, Monday May 14th. The final consists of
two parts:
-
-
In
the field. We will meet in the lab, load the seismograph,
meet again in the field north of the Grizzly Stadium.
There, as a group, you will set up the seismograph and
determine what you can about subsurface seismic velocities.
If we are rained out we will have an oral final.
All
assignments, including the final (part two of 1999 final exam)
must be completed by Wednesday, May 16th.
For
background information, additional problems, and further
study:
-
An
Introduction to Applied and Environmental Geophysics,
J. M. Reynolds, John Wiley and Sons, 1997, 796 p.
-
Environmental
and Engineering Geophysics, P. V. Sharma, Cambridge Univ.
Press, 1997, 475 p.
-
-
Exploration
Geophysics of the Shallow Subsurface, H. R. Burger, Prentice
Hall, 1992, 489 p.
-
Applications
of Geophysics in Environmental Investigations, J. P. Greenhouse,
D. D. Slaine, P. Gudjurgis, CD-ROM, Matrix Multimedia,
1998 - see me if you want to use this.
-
A
Guide to Microsoft Excel for Scientists and Engineers,
B.V. Liengme, John Wiley and Sons, 1997, 207 p.
-
An
Introduction to Error Analysis: The Study of Uncertainties
in Physical Measurements, J. R. Taylor, 2nd edition, University
Science Books, 1997, 328p.
A
note on file/document style:
So far
the most convenient and expedient way to distribute at least
some of the information is to provide the material in Adobe's
.PDF format. Thus several documents are provided as .PDF
files and you need Adobe's
free Acrobat Reader installed in your browser to view
them. If your browser is not currently set up to read and
print such files, download Acrobat Reader from Adobe's
web page, close your browser (preferably version 4.0 or
greater of MS
Internet Explorer or Netscape),
install the reader, restart your browser, click on one of
my links pointing to a set of notes or problems, and Acrobat
Reader should pop up with the .PDF file.
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