Environmental and Exploration Geophysics Ipages.geo.wvu.edu/~wilson/geol454/labs/reslab1.pdf ·...

Tom Wilson, Department of Geology and Geography

Environmental and Exploration Geophysics I

tom.h.wilson

[email protected]

Department of Geology and Geography

West Virginia University

Morgantown, WV

Resistivity Lab I


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


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


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.


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.


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


Axis labels should look similar to those below


Label height is set to 0.7cm

Worksheet handout-

making an initial guess


We will undertake a “back-of-the-envelope”

interpretation of this sounding?


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).


Some background information about the resistivity lab

Refer to Frohlich

and part 1 of the

resistivity

computer lab

Farmland – gently rolling topography


S5

S4

S3

S2

S1

Control

well 37

Control

well 16


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


Shallow and

deeper aquifers

The bedrock is also a source of water, but has

high dissolved ion concentration


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.


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


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


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


B. How many layers are there and

C. What are their depths?

Inflection points

Refer to more recent, less noisy, data set S1


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.

?


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


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.


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.


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!


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.


From EDIT MODEL, input your starting guess for

the resistivity and thickness for each layer.

How well did you do on your guess?


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


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?


Shallow layer influences


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


Crank it

back down

Model on data view


We can also show the model on the data.

Model on data


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


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.


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.



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.


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.


Where are the shallow and the deep gravel aquifers?

Where is bedrock?


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.


What’s wrong with this model?

Does it honor the drill hole data?

Questions about model development?


With some adjustments, we can get a pretty good

match to geology inferred from nearby well control


18-m

90-m

12-m

93-m

44-m

57-m

There is well control


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


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


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


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


Note that you are not required to make a

cross section from these soundings

On the list


• 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

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Environmental and Exploration Geophysics Ipages.geo.wvu.edu/~wilson/geol454/labs/reslab1.pdf ·...

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