Environmental and Exploration Geophysics I

tom.h.wilson*tom.wilson@geo.wvu.edu

Department of Geology and GeographyWest Virginia University

Morgantown, WV

Terrain Conductivity Methods (cont.)

*Office Hours MW 1-2, after class, or by appointment

Review

Consider the following two-layer problem -

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 you set up 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

Revisiting the problem discussed in class on Tuesday 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

How many different conductivity layers will you actually have to consider? - 3 layer problem

Does it matter whether d (depth) and s (intercoil spacing) are in feet or meters? - No

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.

1 1 2 1 2 3 21 ( ) ( ) ( ) ( )a 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 solved should have looked like this.

where -

1 = 3 = 4 mmhos/m

2 = 100 mmhos/m

z1 = (30/12) = 2.5

z2 = (40/12) = 3.33

1 1 2 1 2 3 21 ( ) ( ) ( ) ( )a 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 EM31 has a 12 foot intercoil spacing hence - z1 = (30 feet/12 feet) = 2.5z2 = (40 feet/12 feet) = 3.33

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

Given the tables of R values at right RV(2.5) ~ 0.197 (average of R’s for z = 2.4 and 2.6)RV(3.33) ~ 0.149 (2/3rds the way from 3.2 to 3.4)

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 35 feet - contribute to the apparent conductivity measured at this location. More than 50% 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.

In 1998 and 1999 Pete Fahringer conducted several geophysical studies over the Greer Mine spoil and two underground room-and-pillar mines for his Masters thesis. The following discussions are taken largely from Pete’s thesis.

His thesis would make interesting reading for anyone interested in conducting similar types of studies or who is generally interested in the possibility of combining geophysical methods with hydrological studies.

The following discussion is restricted to the terrain conductivity surveys conducted over the Greer site.

As a class we will also conduct terrain conductivity and magnetic surveys in a similar environment. This brief overview of Pete’s work at Greer is provided to give you some background on the potential applications of the terrain conductivity method for locating and monitoring the movement of AMD or conductive treatment through mine spoil.

Case Histories

Pete Fahringer, 1999 MS grad, did a thesis part of which covered terrain conductivity studies over the Greer

Mansion site.

A portion of the Greer Mansion study is published in SEG’s The Leading Edge - see

Fahringer and Wilson, 2002, shallow EM investigations of AMD at an abandoned coal mine in northern West Virginia: The Leading Edge, Vol. 21, no 5, pp. 478-481

A pdf version is available at http://www.geo.wvu.edu/~wilson/geo252/fandw.pdf

Greer Mine Spoil Terrain Conductivity Study Revisited

•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) 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 discharge from the spoil drains through six springs on the northern and northwestern sides of the site (S1-S8 in Figure 5a). This discharge enters a shallow ditch 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

•This sludge is a slurry of alkaline metal-rich hydroxide solids formed by lime treatment of AMD seeping from the spoil.

•Application of activated lime and ammonia to AMD at the site results in an increased pH and the associated geochemical reactions cause the metals in solution to precipitate in ponds downstream of the treatment area.

•Because the sludge is moderately alkaline and because its disposal is expensive, it is transported to the top of the mine spoil by vacuum truck as needed and emptied into the southern surface trench.

•The alkalinity left in the sludge is believed to help neutralize AMD in the spoil, increasing its pH.

From Fahringer 1999

The metal-rich sludge appears more conductive than the surrounding spoil and creates a detectable terrain conductivity anomaly at the surface. 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

To summarize the findings of Sincock and others at Greer, a map of well-to-well and well-to-spring observations was generated and is shown below. 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. It is important to note that multiple flowpaths from a single well exist due to advection and dispersion of the salts and that flowpaths are not straight as depicted below. 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 rou n d w ate r D iv id e

F lo w D ire ctio ngroundwater flow d irection

A conceptual model of flow over the site is shown in Figure 5c based on observed potentiometric surfaces and the potentiometric map presented in Sincock's thesis (1998). In this model, both multiple flowpaths and extreme heterogeneity in the spoil are ignored mainly because they are poorly known. The flowpaths shown below should be thought of as ideal paths rather than actual paths which are controlled by heterogeneities in the spoil such as: pitfloor irregularities, variations in sediment size and compaction, variations in porosity, and changes in saturated thickness. Actual flowpaths may vary over time with hydrologic changes in the spoil.

From Fahringer 1999

Before we go further we should note that there are at least three different ways to run a terrain conductivity survey.

1. PROFILING- One can collect data using a single coil spacing over a large area or along a profile. 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 and optimum exploration depth (ROT).

2. SOUNDING - One can also collect data at a point using several 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.

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.

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

Individual midpoints

Combined profiling and sounding

Depth

EM3440m

10m20m

EM31 3.67mExplorat ion

5.5m

15m

30m

60m

Individual midpoints

Depth

EM34

40m

10m20m

EM31 3.67m

Combined horizontal and vertical measurements pseudo cross

section view

•Note that the EM31 and EM34 provide 4 different coil spacings and 2 different dipole orientations. Hence it is possible to collect measurements of 8 different ROT exploration depths.

•Also note that exploration depths for the 20 vertical and 40 meter horizontal dipole provide exploration depths of 30 meters and the 10meter vertical and 20 meter horizontal provide exploration depths of 15m. While the ROT exploration depths are the same, the response is often - if not always - different, as we would expect.

•Although this effectively limits such plots to 6 different ROT exploration depths the coincidence of these two provides additional insight into the earth conductivity structure. The fact that these two measurements often disagree is easily understood to result from the overall differences in the relative response functions of the vertical and horizontal dipole fields. The presence of these differences reinforce the note of caution made earlier that instrument response should not be considered as arising from any single depth but, rather, that it is a cumulative response dominated by the conductivities over a wide range of depths.

Pete Fahringer collected data in the form of sounding profiles and also in the form of single-coil-spacing measurements made along a grid for map display of terrain conductivity variations. The single-coil-spacing measurements were collected over the rectangular grids shown below. The sounding profiles were collected along the lines also shown on the map below.

Because the depth to the pitfloor is approximately 60-70 feet at this site, Pete did not make measurements using the 40 meter coil spacing. (ROT EDs of 100 and 200 feet)

In this brief overview of Pete’s studies at the Greer mine spoil we will look at the EM31 data collected over Grid 1 (red in map below) and along profile lines A, B and C.

Note conductivity anomalies A, B, C and D.

Line A

Conductivity anomalies

A. Coal Refuse Pile

B. Sludge migrating into the spoil from the trench

C. Remains of an old trench that has since been covered

D. Sludge plume migrating into spoil from northern trench?

E. Sludge plume emitted from northern end of the northern trench

Grid 1 was re-surveyed about 6 months later

Earlier Survey

Let’s look at some of Pete’s profiles

Remember how they are constructed? The apparent conductivity measured with a given coil spacing is plotted at the “Exploration Depth” . Pete has presented profile displays of the conductivity variations observed along the line for each coil spacing (A). These data were then plotted and contoured in the figure below (B).

Line B

Line A

Line B

Extends to pit floor

shallow

Let’s look at some of Pete’s profiles

The profile line modeled in the following displays was collected along the gold line shown on the map below.

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

15m

30mEM31

EM34

This line crosses the northeastern trench. Its approximate location is shown by the yellow line in the previous display.

Trench

-21

-18

-15

-12

-9

-6

-3

0D

ep

th (

me

ters

)

Trench

Mine Spoil6 mmho/m

Shale and Sandstone (10 mho/m)

Sludge Plume100 mmho/m

Mine Spoil6 mmho/m

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

Another study that employed terrain conductivity methods to examine the influence of longwall mine emplacement on overburden conductivity was conducted by Carpenter and Ahmed. Their study is also published in SEG’s The Leading Edge - see

Carpenter and Ahmed, 2002, detecting preferential infiltration pathways in soils using geophysics: The Leading Edge, Vol. 21, no 5, pp. 471-473

A pdf version is available at http://www.geo.wvu.edu/~wilson/geo252/carp.pdf

Another Case Study -

From Carpenter and Ahmed, 2002

From Carpenter and Ahmed, 2002

From Carpenter and Ahmed, 2002

Infiltration pathways in karstified dolomite subcrop

From Carpenter and Ahmed, 2002

Preston County Coal Refuse Area

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

A. Evaluate the possibility that the EM31 will be able to detect high conductivity transport zones with depth-to-top of 30feet. Evaluate only for the vertical dipole mode. It may help to draw a cross section.

How many Z’s d we need?

Z1 = ?

Z2 = ?

Z3 = ?

Z4 = ?

Final Chance for Questions

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

Z1 =

Z2 =

2.5

3.33

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

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.

Another modeling problem to work on

27.2613.4

Also known as the Pitfloor

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

In the current application 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

We will meet in Rm 312 next Tuesday for our first computer lab. Work through two of the AMD scenarios illustrated in today’s handout. Bring any questions you might have about these materials to the lab on Tuesday. These two problems will be due next Thursday

Begin reading the resistivity chapter (Chapter 8) in Kearey, Brooks, and Hill. We will begin lecturing on resistivity methods week after next.