tom wilson, department of geology and geography 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 Methods (cont.)


Objectives for the day

•Brief discussion of units•Context for AMD problem•Rule of thumb – general utility and limitations•Profiling versus sounding with examples from the Greer mansion site•A couple computer modeling examples•Additional examples illustrating terrain conductivity/resistivity combinations•Around 1:55 or so begin problem discussions•Problem 8.4 (discussion only)•Questions about problems 8.6 – 8.7


(1/)

1 -m 1(-m)-1 or 1 mho/meter or

1000 millimhos/meter

1000 -m (1/1000) mho/meters 1 millimho/meter

Express 15 -m resistivity in terms of mmhos/m conductivity units


Response Functions - & R

Consider the following two-layer problem -

Over this simple two-layer earth model what is the measured apparent conductivity - a – the composite ground

conductivity measured by the conductivity meter be?

How do you compute a?


1=20 mmhos/m

2=2 mmhos/m

3=20 mmhos/m

Z1 = 0.5

Z2 = 1

1 1 2 1 2 3 21 ( ) ( ) ( ) ( )a R z R z R z R z

Given the above diagram, could solve the equation below?

20 1 0.7 2 0.7 0.45 20 0.45a

20 0.3 2 0.25 20 0.45a 15.5mmhos/ma

Review the 3-layer (2 z) earth model problem

Z RV RH

.000 1.000000 1.000000 .200 .9284767 .6770329 .400 .7808688 .4806249 .600 .6401844 .3620499 .800 .5299989 .2867962 1.000 .4472136 .2360680 1.200 .3846154 .2000000 1.400 .3363364 .1732137 1.600 .2982750 .1526108 1.800 .2676438 .1363084 2.000 .2425356 .1231055 2.200 .2216211 .1122055 2.400 .2039542 .1030602 2.600 .1888474 .0952811 2.800 .1757906 .0885849 3.000 .1643990 .0827627 3.200 .1543768 .0776539 3.400 .1454940 .0731363 3.600 .1375683 .0691128 3.800 .1304545 .0655074 4.000 .1240347 .0622578 4.200 .1182129 .0593147 4.400 .1129097 .0566359 4.600 .1080592 .0541887 4.800 .1036061 .0519428 5.000 .0995037 .0498762 5.200 .0957124 .0479660 5.400 .0921982 .0461979 5.600 .0889320 .0445547 5.800 .0858884 .0430231


The in-class AMD problem is based on reclaimed strip mine near Morgantown


How many different conductivity layers need to be considered? How many different values of z are needed?

Does it matter whether d (depth) and s (intercoil spacing) are in feet or meters?

Set up your equation following the example presented by McNeill and reviewed in class, and solve for the apparent conductivity recorded by the EM31 over this area of the spoil.

20’

30’

Pitfloor

60

fee

t to

tal d

ep

th

Subsurface Model

AMD Contamination Zone = 100 mmhos/meter

= 4 mmhos/m

= 4 mmhos/m

= 10 mmhos/m

20 feet

10 feet

30 feet

Let’s make the model a little more complicated


1 1 2 1 2 3 2 3 4 31 ( ) ( ) ( ) ( ) ( ) ( )a R z R z R z R z R z R z

Z RV RH

.000 1.000000 1.000000 .200 .9284767 .6770329 .400 .7808688 .4806249 .600 .6401844 .3620499 .800 .5299989 .2867962 1.000 .4472136 .2360680 1.200 .3846154 .2000000 1.400 .3363364 .1732137 1.600 .2982750 .1526108 1.800 .2676438 .1363084 2.000 .2425356 .1231055 2.200 .2216211 .1122055 2.400 .2039542 .1030602 2.600 .1888474 .0952811 2.800 .1757906 .0885849 3.000 .1643990 .0827627 3.200 .1543768 .0776539 3.400 .1454940 .0731363 3.600 .1375683 .0691128 3.800 .1304545 .0655074 4.000 .1240347 .0622578 4.200 .1182129 .0593147 4.400 .1129097 .0566359 4.600 .1080592 .0541887 4.800 .1036061 .0519428 5.000 .0995037 .0498762 5.200 .0957124 .0479660 5.400 .0921982 .0461979 5.600 .0889320 .0445547 5.800 .0858884 .0430231

The equation you solve should look like this.

where -

1 = 3 = 4 mmhos/m

2 = 100 mmhos/m

4 = 10 mmhos/m

z1 = (20’/12’) = 1.67

z2 = (30’/12’) = 2.5

z3 = (60’/12’) = 5


Z RV RH

.000 1.000000 1.000000 .200 .9284767 .6770329 .400 .7808688 .4806249 .600 .6401844 .3620499 .800 .5299989 .2867962 1.000 .4472136 .2360680 1.200 .3846154 .2000000 1.400 .3363364 .1732137 1.600 .2982750 .1526108 1.800 .2676438 .1363084 2.000 .2425356 .1231055 2.200 .2216211 .1122055 2.400 .2039542 .1030602 2.600 .1888474 .0952811 2.800 .1757906 .0885849 3.000 .1643990 .0827627 3.200 .1543768 .0776539 3.400 .1454940 .0731363 3.600 .1375683 .0691128 3.800 .1304545 .0655074 4.000 .1240347 .0622578 4.200 .1182129 .0593147 4.400 .1129097 .0566359 4.600 .1080592 .0541887 4.800 .1036061 .0519428 5.000 .0995037 .0498762 5.200 .0957124 .0479660 5.400 .0921982 .0461979 5.600 .0889320 .0445547 5.800 .0858884 .0430231

The EM31 has a 12 foot intercoil spacing hence - z1 = (20 feet/12 feet) = 1.67z2 = (30 feet/12 feet) = 2.5z3 = (60 feet/12 feet) = 5

Given also that1 = 3 = 4 mmhos/m2 = 100 mmhos/m & 4 = 10

Given the tables of R values at right RV(1.67) ~ 0.288 (about a third or 0.1 less than 0.298)RV(2.5) ~ 0.197 (average of R’s for z = 2.4 and 2.6)RV(5.0) ~ 0.0995

4 0.712 100 0.091 4 0.097 10 0.0998a

13.34 /a mmhos m

1 1 2 1 2 3 2 3 4 31 ( ) ( ) ( ) ( ) ( ) ( )a R z R z R z R z R z R z

9.12.85 0.39 1a


Recall those “rules of thumb” regarding the optimal sensing depth or exploration depth. For the EM31 operated in the vertical dipole mode the “ROT” says exploration depth is 18feet. Examining the terms in the equation you computed -

How does the middle term - which arises from an average depth of 25 feet - contribute to the apparent conductivity measured at this location. About 70% of the value of ground conductivity comes from the layer centered at depths well beyond (almost twice) the optimal exploration depth.

This is a point to keep in mind especially when trying to locate contamination zones which may have abnormally high conductivity. We might normally exclude use of the EM31 in attempts to detect something at depths greater than 20 feet or so.

4 0.712 100 0.091 4 0.097 10 0.0998

13.4 2.85 0.39 9.1 1a

See 3LayerTCModel.xls


Greer Mine Spoil Terrain Conductivity Study

•The production of acid mine drainage (AMD) from surface and underground coal mines in the Appalachian region has been a major environmental problem since mining began in the region and continues to receive much attention in affected communities.

•Untreated AMD entering surface and ground water degrades the water quality and reduces the value of affected lands.

•The Surface Mining Control and Reclamation Act (SMCRA, 1977) requires that if mining activity contaminates or interrupts the ground water or surface water supply of adjacent users, the mine operator must remediate or replace the water supply.

•Remedial procedures are often set up in response to the need to be in compliance of SMCRA water quality standards and are frequently extensive and costly.

•Lack of site-specific subsurface information often limits the effectiveness and increases the cost of these techniques.

From Fahringer 1999


The water is treated with anhydrous ammonia and calcium hydroxide (lime) in the southwest corner of the site (Sincock, 1998). Treated water collects in settling ponds before being discharged into a tributary of the Cheat River. From Fahringer 1999


Efforts to treat the AMD in-situ have taken place in the last three years and have included injection of sodium hydroxide (NaOH) into the spoil as well as surface

applications of post-treatment alkaline sludge and lime slurry into ditches. From Fahringer 1999


On the surface of the mine three of trenches were dug to dispose of treated sludge and AMD. These trenches are located near a groundwater divide

(Sincock, 1998) and trend northwest-southeast in the western portion of the site.

From Fahringer 1999


EM 31 field measurements taken around sludge-filled trenches at the Greer site in the fall of 1998 and EM 34 measurements taken in the spring of 1999 show conductivity highs extending from the trenches. These conductivity highs originate at the trench and extend along pathways through the surrounding spoil.

From Fahringer 1999


This map shows the general flowpaths inferred from Sincock's single-salt tracer test, as straight line vectors of flow from 10 wells to 5 springs.

From Fahringer 1999


groundwater flow direction

GR4

GR5

GR8GR11

GR12

GR13

GR14

GR15

GR16

GR19

GR20

GR21

GR22

S1

S2

S3 S4

S7 S8

G R3

G R6

G R7

G R 10

G R 17

G R 18

G R 25G R 27

G R 33

G R 34

G R 35

G R 37

G R 38

G R 39

0 m 5 0 m 1 00 m

W e ll

S p ring T re nc h

N e w T re nc h

0 ft 32 8 ft16 4 ft

G round w ate r D iv ide

F low D ire ction

groundwater flow d irection

A conceptual flow model based on observed potentiometric surfaces and the potentiometric map presented in Sincock's thesis (1998). Multiple flowpaths and

extreme heterogeneity in the spoil are ignored mainly because they are poorly known.

From Fahringer 1999

Let’s talk more about data acquisition and acquisition strategies


1. SOUNDING – Measurements centered at a point using the different intercoil spacings and dipole orientations (vertical or horizontal). This method of surveying is referred to as sounding. A sounding provides information about the variation of conductivity with depth.

2. PROFILING- One can collect data using a single coil spacing over a large area or along a survey line. This is referred to as profiling. Profiling provides information about the variation of conductivity throughout an area at relatively constant depths approximated by the coil separation optimized for the depth of interest.

3. One can also combine these methods to obtain profiles of conductivity variation with depth. The display of such data provide a quasi-cross sectional representation of conductivity variations with depth along a profile.

Using the rule of thumb to provide a rough portrayal of variations of conductivity with depth


The rule of thumb suggests that depths being sensed using a vertical dipole orientation are localized about mean depths

corresponding to 1.5 times the intercoil spacing.

Actually depends on layer conductivity

and weighting function

The rule of thumb helps us display our results in cross section form!

Measure a at different intercoil spacings and display a’s at respective exploration depths


40m20m10m3.7m

60mdepth

30m depth

15m depth

5.5m depth Exploration

Depth

Coil spacing

EM34

EM31EM34

EM34

Surface

Vertical “exploration depths”

What are the horizontal “exploration” depths?


40m20m10m3.7m

60mdepth

Midpoint

30m depth

15m depth

5.5m depth Exploration

Depth

Coil spacing

Sounding

EM34

EM31EM34

EM34

Surface

Vertical “exploration depths”

What are the horizontal “exploration” depths?


“Exploration depth” remains constant and the measured variations in ground conductivity provide a view of relative variations in conductivity at the exploration depth

Profiling

Depth

Target Depth


Individual midpoints

Combined profiling and sounding

Depth

EM3440m

10m20m

EM31 3.67mExplorat ion

5.5m

15m

30m

60m

“exploration depths” at each survey point along the profile


Individual midpoints

Depth

EM34V

40m

10m20m

EM31V 3.67m

Combined horizontal and vertical measurements pseudo cross

section view


Survey layout


Note conductivity anomalies A, B, C and D.

Initial EM31 survey over the trenches – a combination of profile data into a conductivity map

Simple profile – data collected at

points with constant intercoil

spacing


Trench area was re-surveyed about 6 months later. Note reduction in anomaly magnitude from peaks of about 20

mS/m to 14mS/m.


Examination of Profile data


Line B

Extends to pit floor

shallow

All we’re doing here is plotting the data at their exploration depths. These are not computer derived models. They are “pseudo” cross sections.


More Profiles


Reduced conductivity due to Injected sodium

hydroxide?

Something sitting on the pitfloor

AMD?


Location of modeled profile shown by gold line

Modeled Profile


0

5

10

15

20

25

30

-25 25 75 125 175 225

Line Distance (Feet)

Ap

pa

ren

t C

on

d. (

mm

ho

/m)

10m H obs10m V obs20m H obs20m V obs10m H calc10m V calc20m H calc20m V calc

7.5m

15m, 20 meter horizontal

15m, 10 meter vertical

30mEM31

EM34

This line crosses the northeastern trench.

Trench


The computer will do a lot of this work for you, but you still have to model each sounding, one-by-one.

-21

-18

-15

-12

-9

-6

-3

0

De

pth

(m

ete

rs) Trench

Mine Spoil6 mmho/m

Shale and Sandstone (10 mho/m)

Sludge Plume100 mmho/m

Mine Spoil6 mmho/m

You will learn how to use the computer to model terrain conductivity data next week


Back to the San Juan Basin


11m Massive Sand

1.2m Shale


Coal Mine Refuse Pile

Preston County Coke and Coal Refuse area


Acidic drainage from the refuse area

Acidic water drains out of the spoil


Settling/treatment pond


Topical lime treatment


Top of refuse pile

EM31

magnetometer

Magnetic anomaly

Terrain conductivity and magnetic surveys


0 50 100 150 200 250 300 350 400 450 500 550 6000

50

100

150

200

0 50 100 150 200 250 300 350 400 450 500 550 6000

50

100

100

150

200

250

300

350

400

450

500

750

0

5

10

15

20

25

30

35

40

45

50Terra in Conductiv ity

M agnetic F ie ld


Preston County Coal Refuse Area

High Conductivity Coal Refuse


Sting and Swift resistivity meter


Low resistivity = high conductivity

High Conductivity


A. Can the EM31 detect this AMD zone at more than double the “exploration depth” associated with this instrument when operated in the vertical dipole mode.

How many Z’s do we need?

Opportunities for Questions


How does the setup change for the case shown at left?

Modification to the AMD problem

v(z)0.0 0.2 0.4 0.6 0.8

Z

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Relative Response Function Vertical Dipole Orientation

Dep

th (

feet

)

0

10

20

30

40

50

60

70

Subsurface Model


= 4 mmhos/m

= 4 mmhos/m

= 10 mmhos/m

High conductivity weighted by small

area


….. Review ….

How many different conductivity layers will you actually have to consider?

What does that equation look like that you’d need to solve?

Modification to the AMD problem D

ep

th (

fee

t)

0

10

20

30

40

50

60

70

Subsurface Model


= 4 mmhos/m

= 4 mmhos/m

= 10 mmhos/m


Some additional perspectives

27.2613.4

Also known as the Pitfloor


)()()()()()(1 332211 zRzRzRzRzRzR BRsAMDsa

Solution of this problem requires a simple extension of the approach we’ve developed. We now have 4 conductivity layers & the equation you need to solve will look like this.

In this problem we retain the conductivity of the contaminated regions as AMD = 100

mmhos/m and add a bedrock with conductivity of BR = 10 mmhos/m. S is the conductivity of

uncontaminated spoil (4 mmhos/m)

Computing z’s for depths of 10, 20, 30, 40, and 50 feet using the EM31 vertical dipole configuration we can easily solve for the contribution of the contamination zone to the overall ground conductivity measured at the surface of the spoil.

z RV(z)0.83 0.531.67 0.29 2.5 0.1963.33 0.1494.17 0.119 5 0.1


Relative contribution of the AMD zone to the overall ground conductivity. EM31 vertical dipole mode

In this particular case, the utility goes well beyond the rule of thumb “exploration depth” perhaps down to two times that depth in this particular example.


Recommended homework format -

•write down a sentence explaining what you are going to solve 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

• State your result

Consider 8.4.xls (see class web page) and errors in equations 8.22, 8.25, 8.32 and related equations.

IC pbschap8.xls


Next week we’ll try our luck with the computers and begin some computer modeling work.

Finish up AMD in-class problem Today and hand in

Problems 8.5, 8.6 & 8.7 are due next Thursday.

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

The two terrain conductivity paper summaries will be due two weeks from today on Thursday September 15th.

tom wilson, department of geology and geography environmental and exploration geophysics i...

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