Environmental and Exploration Geophysics I

Tom Wilson, Department of Geology and Geography

Environmental and Exploration Geophysics I

[email protected]

Department of Geology and GeographyWest Virginia University

Morgantown, WV

Terrain Conductivity

Phone - 293-6431


Geol454-## (i.e. ## =01, 02, 12, 13, …)

Password is just geol454

Check out contents of the H: (common) drive

And the G: Drive (your personal drive on the network)

Your G drive and the common drive are accessible on any machine hooked into our network.

Copy the Burger files from the H: drive to your G: drive

Store your classwork and models on the G:Drive. That way if you move to another machine those files will still be accessible to you. This also avoide the possibility that someone might inadvertently delete your files from the local C:\Drive.


In this picture an ammeter is connected in the circuit of a conducting loop. When the bar magnet is moved closer to, or farther from, the loop, an electromotive force (emf) is induced in the loop. The ammeter indicates currents in different directions depending on the relative motion of magnet and loop. Notice that, when the magnet stops moving, the current returns to zero as indicated by the ammeter.

http://ww2.slcc.edu/schools/hum_sci/physics/tutor/2220/em_induction/index.html


What would happen if you cut the ring?

What would happen if you put a can of coke inside the coil?

http://ww2.slcc.edu/schools/hum_sci/physics/tutor/2220/em_induction/experiments.html


“Dynamic” Tables 8.1 and 8.2

~15 km (about 9 miles)

< 30 m (about 100 feet)

~1.5 m (about 5 feet)



Clay particles are a source of loosely held cations


Cation clouds provide a source of electrolytes, they can also form a partial barrier to current flow through small pores. In this case their effect is similar to that of a capacitor.

Ion clouds in narrow pore spaces can interfere with current flow


Archie’s Law

The general form of Archie’s law isl

bF

b is the conductivity of the mixture (bulk conductivity)

and l the conductivity of the liquid which we assume is

water.

F is the formation factor, and porosity is related to F as follows

Fl

b

b

l

Note also that

Empirical conductivity porosity relationships

maF

F

1

10


Terrain Conductivity Survey

EM31 EM34

Geonics Limited has specially designed these terrain conductivity meters to take advantage of simple relationships between secondary and primary magnetic fields.

The instrument was designed to operate in areas where the induction number is low.



tiltmeters

Tracer and soil gas monitors

EM Survey

VSP Source Point

CO2 injection well


Marshall Co. WV, coal sequestration pilot


Hunting for Abandoned Wells


Hunting for abandoned Wells


The induction number?

B induction numbers intercoil spacing skin depth depth at which amplitude of the em field drops to 1/e of the source or primary amplitudee natural base - equals 2.71828 ..1/e ~0.37

s

B

In general for a plane wave, the peak amplitude (Ar) of

an oscillating em field at a distance r from the source will drop off as - r

sr eAA


rsr eAA

Oscillating Field

Am

plit

ude

Am

plit

ude

distance travelled

Decaying EM Wave

Am

plit

ude

deca

y

Exponential Decay

~0.37

0.0

0.2

0.4

0.6

0.8

1.0

Am

plit

ude

deca

y

distance traveled

Exponential Decay

equals the skin depth

is an attenuation coefficient. r =1/ is the skin depth .

The distance r= is referred to as the skin depth


rsr eAA

The attenuation factor varies in proportion to the frequency of the electromagnetic wave.

Higher frequencies are attenuated more than lower frequencies over the same distance. Hence if you want to have greater depth of penetration/investigation, lower frequencies are needed.

As a rough estimate, (the skin depth) can be approximated by the following relationship f

1500

We can think of the skin depth as a “depth of penetration”


Low Induction Number

When that assumption is met, there is a simple linear relationship between the primary and secondary fields when subsurface conductivity and the operating frequency of the terrain conductivity meter are confined to certain limits.

Under low induction number conditions the ratio of the secondary to the primary magnetic field is linearly proportional to the terrain-conductivity. Since the secondary and primary fields are measured directly, their ratio is known.

Hence, the net ground conductivity is also known.


HpSurface

Contamination Plume

Hs

4

20 si

H

H

p

s

TransmitterReceiver

s

- the net ground conductivity is what we are after


HpSurface

Contamination Plume

Hs

4

20 si

H

H

p

s

HS secondary magnetic field at receiver coilHP primary magnetic field = 2f – angular frequencyf = frequencyo = magnetic permeability of free space = ground conductivitys = intercoil spacing (m)i = imaginary number 1


HpSurface

Contamination Plume

Hs

s

f refers to the frequency of the alternating current in the transmitter coil

The operating frequency is adjusted depending on the intercoil spacing

4

)2( 20 sfi

H

H

p

s

Together, the EM31 and EM34 provide 4 different intercoil spacings and two different coil orientations.

The coils can be oriented to produce either the vertical or horizontal dipole field.

EM31

EM34


The operating frequencies for the different intercoil spacings are

Intercoilspacing

frequency

3.66m (EM31) 9800Hz10m (EM34) 6400Hz

20m (") 1600Hz40m (") 400Hz

f 1

500 We could also write this as f

500

is the skin depthf is the frequency of the em wave is the conductivity (in mhos/meter) is the resistivity

Remember

EM31EM34


s

BIn the following table we examine the effect of operating frequency, intercoil spacing and ground conductivity on the induction number.

Instru (spac) g=10 mmhos/m

g=100 mmhos/m

EM31 (3.66m) = 51m B = 0.07 = 16 B = 0.22

EM34 (10m) 63m 0.16 20 0.5

(20m) 125m 0.16 40 0.5 (40m) 250m 0.16 79 0.5

f 1

500Since -

As the frequency and conductivity increase, the depth of penetration decreases

These instruments are designed to work when the induction number is relatively low


In general, for the EM31, operation under the assumption of low induction number is valid for

ground conductivity of about 100 mmhos/meter and less.


The text isn’t very specific, but a little calculation suggests that induction numbers of 0.2 or less are considered to be “low” induction numbers for the EM31.

Perhaps as much as 0.5 or less for the EM34. Generally high ground conductivity is considered 100mmhos/m or greater.

Fortunately, ground conductivity in general tends to be much less than 100 mmhos/meter

Instru (spac) g=10 mmhos/m

g=100 mmhos/m

EM31 (3.66m) = 51m B = 0.07 = 16 B = 0.22

EM34 (10m) 63m 0.16 20 0.5

(20m) 125m 0.16 40 0.5 (40m) 250m 0.16 79 0.5


For example, on the Greer site, terrain conductivities in the darker areas are 22 mmhos/meter and greater. The terrain conductivities in the lighter areas are less than 6 mmhos/meter.

Fahringer (1999)


Vertical Dipole Horizontal Dipole

Changing the dipole orientation changes the depth of penetration and thus the instrument response will provide information about apparent ground conductivity at different depths. McNeill refers to these “depths of investigation” as exploration depths.

The orientation of the dipole is easily controlled by changing the orientation of the coil.

As suggested by the drawing, the vertical dipole will have a greater depth of penetration than the horizontal dipole.


Vertical dipole mode of operation

Exploration Depths “Rule of Thumb”

Intercoil Spacing(meters)

Horizontal Dipole(depth in meters)

Vertical Dipole(depth in meters)

3.67 2.75 5.5

10 7.5 15

20 15 30

40 30 60


Those are easy to remember and useful general relationships. However, the apparent conductivity measured at the surface is a composite response - a superposition of responses or contributions from the entire subsurface medium.

The contribution from arbitrary depths is defined by the relative response function (z), where z is the depth divided by the intercoil spacing.

depthz

s


•Continue reading Chapter 8 – pages 499 to 510 (top).

•Look over the problem I handed out today and ask yourself how you would solve the problem using methods described in pages 514 – 519.

We’ve jumped ahead into some of the technical issues associated with terrain conductivity methods. Next Tuesday we will back up a bit and review some more fundamentals.

Documents

Environmental and Exploration Geophysics I