Upload
others
View
11
Download
0
Embed Size (px)
Citation preview
Tom Wilson, Department of Geology and Geography
Environmental and Exploration Geophysics I
tom.h.wilson
Department of Geology and Geography
West Virginia University
Morgantown, WV
Terrain Conductivity Lab (cont.)
Main objective for the day
Tom Wilson, Department of Geology and
Geography
• Resolve any questions about IX1D
and cross section EM1-6
• Discuss equivalent solutions and
how to compute them
• Get started on EM7-12
• Wrap up EM lab. Due next Tuesday
Overall learning objectives of the lab
Tom Wilson, Department of Geology and Geography
• Develop terrain conductivity modeling skills
• Apply to specific subsurface problem: evaluate the hypothesis
that the aquifer down gradient from an industrial waste site may be
contaminated
• Constrain your models to be consistent with the local subsurface
geology: i.e., pay attention to the control well.
• Gain some familiarity with forward and inverse modeling
approaches.
• Answer some questions about the analysis you’ve undertaken
(see lab guide and last slide).
Address questions and move into the second
set of data EM7-12
Tom Wilson, Department of Geology and
Geography
Hopefully everyone has the text
We’re heading into chapter 5 on resistivity
Tom Wilson, Department of Geology and Geography
Bring up IX1D while I go over a few ideas related to the
computer lab
Reminder of the problemsee page 19 terrain conductivity lab guide distributed today
Tom Wilson, Department of Geology and Geography
Wastewater lagoon
Community down
gradient from the lagoon
Near surface clay layer
Aquifer
Thick silt layer underlies aquifer
1
2
3
contaminated,
uncontaminated?
Leaky?
Thicknesses do
not vary much
through the area
Resistivity
well
c
Hydraulic gradient
Review program displays
Questions about what you are seeing in this display?
Tom Wilson, Department of Geology and Geography
31H34-10H
34-20H
34-40H
Appare
nt
Conductivity
“Effective” Penetration
Depth
“Conductivity of
individual layers
Depth
(m
)
The problems of an unconstrained inversion -
leave all variables un-fixed
Tom Wilson, Department of Geology and Geography
We will assume that there is
some contamination and
introduce an upper
contaminated layer between
10 and 20 meters subsurface.
This first sounding is located at
a control well, so resistivities/
thicknesses are known. See lab
guide, page 19
Results of an unconstrained inversion
Tom Wilson, Department of Geology and Geography
Doesn’t look much
like the local
geology inferred
from the resistivity
log data.
Tom Wilson, Department of Geology and Geography
After multiple iterations – a near perfect fit (0.34% RMS error)
But – can we believe it? Is it even close?
Inverse models are always
non-unique. There exist
multiple solutions that can fit
the data with low error.
It’s hard to argue in favor of the results within
the context of the background geology
Tom Wilson, Department of Geology and
Geography
One should always retain a healthy
level of skepticism.
This result does not portray what we
know to be the case for this area.
Recall what we know from the control well and local geology.
We have to introduce constraints into the model
Tom Wilson, Department of Geology and Geography
Near surface clay layer
Aquifer
Thick silt layer underlies aquifer
1
2
3
Control well
10m
20m
Uncontaminated reference
Get as much background geological and geophysical data
as possible before undertaking the modeling process.
In all the models developed in this lab exercise, you’ll
start with the same basic set of layers
Tom Wilson, Department of Geology and Geography
Near surface clay layer
Aquifer
Thick silt layer underlies aquifer
1
contaminated
3
Control well
10m
20m
Uncontaminated reference
uncontaminated
Contaminated until proven otherwise
To consider the possibility of contamination, we need
to divide the aquifer into two layers
Tom Wilson, Department of Geology and Geography
Near-surface clay layer
Aquifer
Thick silt layer
1
2
3
Control well
10m
20m
contaminated
4
contaminated
uncontaminated
Local wells indicate the aquifer has roughly constant
thickness of 20 m throughout the area, so we have to
incorporate this as a constraint in the final model.
Your starting model will always look like this
The conductivities of
the upper clay, fresh
water zone (if
present) and
underlying silt are all
known.
So we can fix those
parameters
The thickness of the
upper clay is also
known to be relatively
constant across the
area.
We fix that as well.
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
0.01% errorWe have information about
the subsurface geology that
we can use to constrain the
model: make it more
representative of the local
geology
Well location
EM1&7 are located at log wells & we’re pretty
sure what’s going on there
In both cases (1 and 7) we have an exact fit for
soundings at the borehole
Tom Wilson, Department of Geology and Geography
No contamination. All
fresh water 10 mS/m
Underlying silt
Near
surface
layer
It’s the same for EM7
Comparison of unconstrained and constrained
model results – both have near zero error
Tom Wilson, Department of Geology and Geography
In the end, our solutions are always non-unique.
22 meters
7 meters
Perhaps a little
contamination?
Fresh uncontaminated
water indicated by the log
from 10 to 30 meter depths
Tom Wilson, Department of Geology and Geography
Key points about the local geology that help confine
the range of possible solutions
Local well data indicates that
•The near surface clay layer has nearly constant
thickness of 10 meters
•The aquifer also has relatively constant thickness
of 20 meters in the area. It is divided into two
zones: an upper contaminated zone and lower
uncontaminated zone.
•The conductivities of the near surface clay, the
fresh water leg in the aquifer and underlying silt
are 50 mS/m, 10mS/m and 15mS/m respectively.
Remember – you can use the
same starting model for each sounding
Tom Wilson, Department of Geology and Geography
Within the context of the
well data, the geological
constraints allow us to fix
certain parameters during
the modeling processIn this way, we help push the
solution to one that is
geologically reasonable
Examine result and ask if it is consistent with
what we know about the area
Tom Wilson, Department of Geology and Geography
Base of aquifer should
be here at 30 m
Questions about cross section construction?
Tom Wilson, Department of Geology and Geography
We force the base of the aquifer to 30
meters. Remember – equivalent solutions
tell us this doesn’t change error much.
Develop models for EM7-12 but first let’s look at
equivalent solutions
I’ll work through EM3 you can pick one of EM8-12
Tom Wilson, Department of Geology and Geography
Non-uniqueness
of solutions
Accept that results
are uncertain even
when constrained
and consider
equivalent solutions
Generate a model for one of the soundings EM8-12.
Honor constraints and see what you get.
Tom Wilson, Department of Geology and Geography
You can make your
models fit the geology
Save your result
since you can use
it as part of the lab
assignment
Generate
equivalent models
Let’s go through the process of developing equivalent
solutions. Pick any sounding of EM8-12 you’d like
Tom Wilson, Department of Geology and
Geography
Look at the dashed lines
These are equivalent solutions
Tom Wilson, Department of Geology and
Geography
Note that equivalent
solutions (the dashed
lines) indicate that the
base of the aquifer can
be at 19 to 33 meters
subsurface.
We can use this to our
advantage and force
depth to the base of the
aquifer to 30 meters.
Why? We know enough
about the local geology
to place it around 30
meters
You can use the log scale to approximate value
between 10m and 20m and 20m and 30m etc.
Tom Wilson, Department of Geology and Geography
16m17m19m
34m
Base of
contaminated zone
Base of aquifer
This degree of variability comes about with
2.9% error. Try to get around 3% when
estimating equivalent solutions
Tom Wilson, Department of Geology and Geography
If the error is too low, you can move the layers
and conductivities around by clicking and
moving lines on the model
Tom Wilson, Department of Geology and Geography
Pull values away from the initial inversion; do
one forward computation and one inversion;
then run equivalence
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography164.8
25.2
19.4121.35
Change these soundings to EM7 through EM12
Base of contaminated layer
Basal
Silt
The variability you get depends
on the amount of error you
leave in your model before
computing equivalent solutions
I’ll look over work and answer questions re
EM1-6, but for the remainder of the class get
started on EM7-12
Tom Wilson, Department of Geology and Geography
Starting setupApparent
conductivities are
pretty high. Must
be contaminated.
Tom Wilson, Department of Geology and Geography
EM7 EM8 EM9 EM10 EM11 EM12
Continue working through EM soundings 7 through 12
The lab is focused on these soundings
The formal lab requires that you analyze soundings EM7 through 12. Develop models of EM1-6 for practice and discussion today.
Depth to base of aquifer constrained by
local geologic mapping and well control in
the surrounding area?
Can you restrict aquifer
thickness to 20m?
How are contaminants
distributed across the
profile?
?
Let me check your work before leaving today
EM7 EM8 EM9 EM10 EM11 EM12
Practice sheet for EM soundings 7 through 12 – do as much as you can in class today
Depth to base of aquifer constrained by
local geologic mapping and well control
in the surrounding area?
Equivalent solutions justify putting base of
aquifer at 30 meters
Tom Wilson, Department of Geology and Geography
Thickness of
~ 4.6 to 4.9 m
Thickness of
~8m to 15m Low conductivity
intervals are
generally less
well defined.
Note the equivalent solutions. We can
adjust the depth to the base of the
aquifer with very little increase in error.
Tom Wilson, Department of Geology and Geography
Get started on soundings EM7 through 12 and begin filling out the table below
Fill out as much of this table as you can and
let me check it off before leaving today
Name: ______________________
10
10
10
10
?
Activities
Tom Wilson, Department of Geology and Geography
• Check off cross section EM1-6 – Questions?
• Check table EM7-12 before leaving.
• You should look over questions in lab guide (and on
next slide) Due date next Tuesday
•Writing Section should get Chapter summary outline
in this Thursday.