environmental and exploration geophysics ipages.geo.wvu.edu/~wilson/geol454/labs/reslab1.pdf ·...
TRANSCRIPT
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
Resistivity Lab I
Tom Wilson, Department of Geology and Geography
Objectives for the Day
• Gain familiarity with IX1D resistivity modeling activities (functionality
is very similar to that used for the terrain conductivity model study)
• Get familiar with the geologic background related to the resistivity
data we will model in lab to detect fresh-water aquifers
• resistivity differentiation of glacial drift and pre-glacial channel
deposits
• resistivity changes associated with saline intrusion mask layer
boundaries in many cases
• Qualitative methods used to estimate the number of layers and their
resistivities.
• Nearby boreholes provide ground truth on one sounding
Copy over the resistivity data
Tom Wilson, Department of Geology and Geography
Copy over the folder IX1D-Res and
note that you should have 5 data sets
Also bring up IX1D
Also have a look at this xls file
2LayerReflection.xls
Tom Wilson, Department of Geology and Geography
Linked on the class pages at
http://www.geo.wvu.edu/~wilson/geo252/2LayerReflection.xls
Note inflection point at 10 meters and
estimate 1 and 2.
Tom Wilson, Department of Geology and Geography
Linked on the class pages at
http://www.geo.wvu.edu/~wilson/geo252/2LayerReflection.xls
Let’s take a look at some of the resistivity data
from the Missouri area Bring up IX1D.
Tom Wilson, Department of Geology and Geography
Copy over IX1D-Res folder from the common drive and open sounding S1.
You should have something that looks like that below.
You may have to relabel
axes for the data display
(y) and model display
(x) to show Apparent
Resistivity (ohm-m) and
Resistivity (ohm-m),
respectively.
Resistivity sounding S1
will look something like
this
Bring up S1 from the IX1D-Res folder and
relabel axes if needed
Tom Wilson, Department of Geology and Geography
Axis labels should look similar to those below
Tom Wilson, Department of Geology and Geography
Label height is set to 0.7cm
Worksheet handout-
making an initial guess
Tom Wilson, Department of Geology and Geography
We will undertake a “back-of-the-envelope”
interpretation of this sounding?
Tom Wilson, Department of Geology and Geography
What can we infer
from these variations
in apparent resistivity
about subsurface
resistivity layering?
Without the computer!
We’ll come back to that, but first, some
background on the problem (see Frohlich).
Tom Wilson, Department of Geology and Geography
Some background information about the resistivity lab
Refer to Frohlich
and part 1 of the
resistivity
computer lab
Farmland – gently rolling topography
Tom Wilson, Department of Geology and Geography
S5
S4
S3
S2
S1
Control
well 37
Control
well 16
Tom Wilson, Department of Geology and Geography
Note Drill Hole Locations
along the profile line at left
and below
S1
Glacial outwash overlying pre-glacial channels
that sit on dolomitic limestone bedrock
Tom Wilson, Department of Geology and Geography
Shallow and
deeper aquifers
The bedrock is also a source of water, but has
high dissolved ion concentration
Tom Wilson, Department of Geology and Geography
Bedrock aquifers, my
have increased
concentrations of
dissolved solids and
lower resistivity.
This lowered resistivity
makes it difficult to
“see” the bedrock
interface.
The resistivity contrast
can be minimal.
Tom Wilson, Department of Geology and Geography
Modeling results suggest
this range may be a little
more variable and extend
from ~ 50 to 135 -m
For our purposes, we will say that the basal gravel aquifers
have resistivity between 40-50 m and that bedrock has
resistivity in the range 50-75 m
This is your interpretation template
Tom Wilson, Department of Geology and Geography
S1 S5S4S3S2
Look over discussion of this
section on pages 347 & 348
of Frohlich’s paper
10m
100m
L/2 where L is the current electrode spacing
Back to S1 – developing models for more
complicated soundings
Tom Wilson, Department of Geology and Geography
We will develop a qualitative interpretation of this sounding
using “inflection point” and “extrapolation” rules.
A. Where are the inflection points?
This is a Schlumberger array and the x axis is
usually labeled in terms of AB/2 or l OR L/2
Increasing depth
Tom Wilson, Department of Geology and Geography
B. How many layers are there and
C. What are their depths?
Inflection points
Refer to more recent, less noisy, data set S1
Tom Wilson, Department of Geology and Geography
1 2 3 4 5 6
This is a Schlumberger sounding. AB/2 is one-half the distance
between the source and sink electrodes. A general rule of thumb to
estimate resistivity boundary depth is that depth is about 1/2 the AB/2
distance. Note that AB/2 is l in some representations.
?
Tom Wilson, Department of Geology and Geography
1 2
3
4 5
point AB/2 Depth
1 1.4 0.7m
2 2.7 1.4m
3 8.4 4.2m
4 38 19m
5 64 32m
D. What are the resistivities of these layers?
The rising and falling apparent resistivity trends provide insights
into relative differences of layer resistivity
Tom Wilson, Department of Geology and Geography
1 23
4 5 6
?
1 = 23-m
2 = 32-m
3 = 23-m
4 = 81-m
5 = 49-m6 = 58-m
Note inflection point at 10 meters and
estimate 1 and 2.
Tom Wilson, Department of Geology and Geography
Asymptote ~ 25m
=2
Inflection
point ~ 10m
a smallest a-
spacing ~ 1=5m
The s are certainly going to change when you go through an
inversion. 2 and 4 may be closer to 100m, for example.
Tom Wilson, Department of Geology and Geography
1 23
4 5 6
?
1 = 23-m
2 = 32-m
3 = 23-m
4 = 81-m
5 = 49-m6 = 58-m
Main objective here is to incorporate relative variations of in your starting model
Your guess may look a little different – that’s
fine – it’s a starting guess!
Tom Wilson, Department of Geology and Geography
point AB/2 Depth Thickness
1 1.4 0.7m 23 0.7m
2 2.7 1.4m 32 0.7m
3 8.4 4.2m 23 2.8m
4 38 19m 81 14.8m
5 64 32m 49 13m
58
A table of our estimates derived from inflection point
and extrapolation approaches to interpretation.
Tom Wilson, Department of Geology and Geography
From EDIT MODEL, input your starting guess for
the resistivity and thickness for each layer.
How well did you do on your guess?
Tom Wilson, Department of Geology and Geography
The computed resistivity variations associated with our guess are shown as the
solid line for comparison to the actual observations of apparent resistivity shown
by the violet squares. The resistivity versus depth model is shown at right.
Click the forward
button
At this point you can run multiple iterations to improve the
agreement between the observations and calculations
Tom Wilson, Department of Geology and Geography
After multiple inversions you’ll obtain a resistivity model similar to that
shown at right above. This particular model has about 2.4% error.
After iteration on S1 what did you get?
Tom Wilson, Department of Geology and Geography
Shallow layer influences
Tom Wilson, Department of Geology and Geography
The influence of a
shallow layer can have
significant impact on
several readings!
Drag to higher
The 30-40 cm shallow layer distorts
observations out to ~10 meters
Tom Wilson, Department of Geology and Geography
Crank it
back down
Model on data view
Tom Wilson, Department of Geology and Geography
We can also show the model on the data.
Model on data
Tom Wilson, Department of Geology and Geography
The derived model parameters can
be viewed in the Edit Model window
Inflect
point
Guess
D
Calc
D
Guess
Calc
1.4 0.7m 0.34m 23 13.4
2.7 1.4m 0.85m 32 61
8.4 4.2m 2.1m 23 10.5
38 19m 7.3m 81 167
64 32m 17m 49 31.5
58 59.9
Useful functionality
Tom Wilson, Department of Geology and Geography
If you click a data point on your data graph, its location
and value will appear in a message box.
Notice how the 5th data point just doesn’t look right. It
may be a bad data point and we should probably
exclude it from the calculations.
If you feel you have a bad data point, you can
mask it without deleting it.
Tom Wilson, Department of Geology and Geography
We can do this through Edit Data. In the Edit Data window, turn
on the mask option for point 5.
The masked data point is represented by an X.
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
With more iterations from the Edit Model window, our error can be
further reduced. Note how the upper layers continue to shallow-out with
increased numbers of iterations.
Tom Wilson, Department of Geology and Geography
In this lab you’ll be carrying out the inversion process for all 5
soundings and putting together a geologic interpretation of the
distribution of strata for each sounding.
Tom Wilson, Department of Geology and Geography
Where are the shallow and the deep gravel aquifers?
Where is bedrock?
Tom Wilson, Department of Geology and Geography
Save your model under another name. You will want to retain derived
model parameters to construct a cross section across this glacial channel.
In the next lab, you may also want to experiment with equivalent models to
help evaluate stratigraphic correlations between adjacent soundings.
Tom Wilson, Department of Geology and Geography
What’s wrong with this model?
Does it honor the drill hole data?
Questions about model development?
Tom Wilson, Department of Geology and Geography
With some adjustments, we can get a pretty good
match to geology inferred from nearby well control
Tom Wilson, Department of Geology and Geography
18-m
90-m
12-m
93-m
44-m
57-m
There is well control
Tom Wilson, Department of Geology and Geography
We haven’t incorporated what we know about the local geology
(the borehole data) at a control point along the profile.
There is well control
Tom Wilson, Department of Geology and Geography
We haven’t incorporated what we know about the local geology
(the borehole data) at a control point along the profile.
clayShallow gravel
Clay and sand
Limestone BR
Bring background geology into the
interpretation
Tom Wilson, Department of Geology and Geography
18-m
90-m
12-m
93-m
44-m
57-m
Clay
Very thin
shallow gravel
Clay
shallow gravel
Drift fill inconsistent with borehole
observation – probably sand
Bedrock
Some tasks to get started on
Tom Wilson, Department of Geology and Geography
Start developing models and interpretations
for soundings S2 through S5.
Look at the section described in Frohlich’s
Figure 3 near sounding 7 (page 344). How
could you test out Frohlich’s interpretation at
S1? Give this a try for Thursday.
Make sure borehole control is incorporated in
the models we develop using the computer
Revisit Frohlich’s paper for additional context on
the lab problem
Questions you will be asked to complete for
the resistivity lab
Tom Wilson, Department of Geology and Geography
Note that you are not required to make a
cross section from these soundings
On the list
Tom Wilson, Department of Geology and Geography
• Problems 5.1-3 due today.
• Don’t forget there’s a mid term exam on Thursday,
September 29th (test review on the Sept. 27th).
… and keep putting models together for soundings S1-5