Subsurface model development using terrain conductivity measurements Tom Wilson, Department of...

Subsurface model development using terrain conductivity measurements

Tom Wilson, Department of Geology and Geography

The problems in the text provide insights into the practical uses of terrain conductivity measurements to dissect subsurface geological problems.

In all the text examples the geologist and geophysicist have information about some of the variables so they can answer a specific question about the subsurface.

Information used to constrain geophysical problems can come from outcrop, auger and borehole measurements.

Problem 8.5


In problem 8.5, the geologist/geophysicist have information that helps constrain the problem to that of determining 2. In this case a can be interpreted in terms of 2 since the thickness of the surface layer is known.

When you add the air layer you have a 2z, 3 layer problem.

Problem 8.6


In this problem the geologist & geophysicist know there are gravel deposits scattered through the glacial till. The gravel deposits serve to localize fresh water accumulations that could be used as a domestic water supply. Auger holes, for example, could provide information about d1 and the three s so that thickness can be estimated.

1st 2nd layer is the gravel

3rd

Problem 8.7


In this problem, the geologist knows the near surface geology consists of a soil layer with known conductivity (30mS/m) covering granitic bedrock of known conductivity (0.1mS/m). This allows variations in apparent conductivity to be translated into a bedrock depth profile or map.

Developing a subsurface model – some examples: buried druim


Modeling igneous intrusion with gravity and magnetic data


Complex normal faulted structure with syndepositional variations in sediment thickness (grav and mag)



Organize your presentation!

• What are you solving for?

• List given variables associated with the problem

• Show calculations for additional variables needed to solve the problem.

• Show calculations used to calculate requested quantity

• Highlight your results (draw a box around answers sought in the problem).

Some of you are still having difficulty organizing your work. Put some additional effort into preparing your work.

Today – introductory terrain conductivity modeling with IX1D

Chapter 8 problems due this Thursday (Sept. 3) Problems 8.5, 8.6 & 8.7

Begin reading the resistivity chapter (Chapter 5) in Berger, Sheehan and Jones

Writing section should get the chapter summary outline in next Tuesday, September 8th.

The terrain conductivity paper summary will be due on Thursday September 10th.

On your list …


Next we’ll get into the Terrain Conductivity/Resistivity Modeling Software

IX1D


A full set of terrain conductivity measurements obtained from the EM31 and EM34 provides 8 values of apparent conductivity. The variations of apparent conductivity can be used to answer more complicated questions about subsurface geology. However, considerable geological and geophysical information is required to help constrain the solutions or subsurface models that explain apparent conductivity variations.

First – go to the common drive and copy over the file folder


From your programs list > find IX1Dv2


Drop into this folder,Right click and drag the icon to your desktop.

When you lift up, select create shortcut

First - locate and bring up IX1D


You may have to run the program by going directly to the programs folder on your C:\Drive

If needed – Enter Access Code 307609922

Change the default name to Your name. I think it requires 12 characters, so you could add G454

IX1D Software should come up with no problem.

You’ll get a plot that will look something like this. Don’t worry about that right now. We just want to be sure the software comes up.


Two plots: data on left and model on right

Recall that using both the EM31 and EM34 in combination we can collect 8 data points

The 4 vertical dipole apparent conductivity

measurements are shown using purple squares

Computer gets about 18, while our hand

computations gave us about 20 mS/m

log scale

log

scal

e


Effective Penetration DepthsThink of this as depth beneath the surface

Quasi - Depth

The 4 horizontal dipole apparent conductivity

measurements are shown using purple squares

Horizontal

Vertical


The Terrain Conductivity Lab Exercise is designed around the use of data obtained using the EM31 and EM34 terrain conductivity meters

Typical survey yields 8 measurements consisting of one set of measurements made with the vertical dipole orientation and one set with horizontal dipole orientation

Find & open EM1.IXR

Data set EM1.IXR is in folder IX1D-TC

IX1D Display window – basic tour >The data are in the IX1D-TC folder


Vertical Dipole measurements

Horizontal Dipole measurements

Subsurface conductivity model

Open the EM1 data set

50mS/m

10mS/m

15mS/m

Effective Penetration Depths (keep their limited significance in mind)


~4.7m

~12.6m

~25.5m

~51m


5.5m 15m 30m 60m


Remember the skin depth is the depth at which the signal strength drops to 1/e of its source amplitude.

http://www.geo.wvu.edu/~wilson/emvrz.xls

REMEMBER- Although the area under the curve beneath 4.7 meters equals 37% of the total area, intervals beneath that depth can be responsible for more than 37% of the measured

apparent conductivity!


“Effective Penetration Depths” correspond to depths at which remaining area on the relative response function is ~37% of the total area.

~4.7m

~12.6m

~25.5m

~51m


5.5m 15m 30m 60m

3.67m

10m

20m

40m

At this point, make sure you have IX1D open


If some of you are unable to get the program up and running, double up – share a computer.

I’ll note computers and try to get everything up and running before this Thursday.

There should be no problems but …

Did everyone get the IX1D-TC folder copied to their network (N:) drive? Then Bring up IX1D v2

from the start programs window or …


Do a file> open and navigate to your N:\Drive IX1D-TC and open the file EM1.

Note that EM1 –EM6 are for practice and EM7 through 12 are for the lab.

The data display may come up showing resistivity instead of conductivity

Let’s work through some of these soundings.Refer to your handout. All steps and more are illustrated in today’s

“Exploring IX1D …” handout.


Do a file> open and navigate to your G:\Drive IX1D-TC and open the file EM1.

Note that EM1 –EM6 are for practice and EM7 through 12 are for the lab.

Pick a sounding (EM2-6), determine the conductivity and thickness of the contaminated zone and

Turn in before leaving


See page 15 of today’s handout

Fill in the blanks, tear off and submit before leaving.

I’ll spend some time outside the slides showing you how to develop your model.

Today – introductory terrain conductivity modeling with IX1D

Chapter 8 problems due this Thursday (Sept. 3) Problems 8.5, 8.6 & 8.7

Begin reading the resistivity chapter (Chapter 5) in Berger, Sheehan and Jones

Writing section should get the chapter summary outline in next Tuesday, September 8th.

The terrain conductivity paper summary will be due on Thursday, September 10th.


Subsurface model development using terrain conductivity measurements Tom Wilson, Department of...

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