Environmental and Exploration Geophysics I tom.h.wilson* Department of Geology and Geography West Virginia University Morgantown, WV Terrain Conductivity Methods (cont.) *Office Hours TTh, 2:30 – 4, or by appointment

Review Consider the following two-layer problem -

 1 =20 mmhos/m  2 =2 mmhos/m  3 =20 mmhos/m Z 1 = 0.5 Z 2 = 1 Given the above diagram could you set up the equation below? 3-layer (2 z) problem Z R V R H

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. 30’ 40’ Pitfloor

Z R V R H The equation you solved should have looked like this. where -  1 =  3 = 4 mmhos/m  2 = 100 mmhos/m z 1 = (30/12) = 2.5 z 2 = (40/12) = 3.33

Z R V R H The EM31 has a 12 foot intercoil spacing hence - z 1 = (30 feet/12 feet) = 2.5 z 2 = (40 feet/12 feet) = 3.33 Given also that  1 =  3 = 4 mmhos/m  2 = 100 mmhos/m Given the tables of R values at right R V (2.5) ~ (average of R’s for z = 2.4 and 2.6) R V (3.33) ~ (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.

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

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

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

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.

40m 20m 10m 3.7m 60mdepth Midpoint 30m depth 15m depth 5.5m depth ExplorationExploration DepthDepth Coil spacing Sounding EM34 EM31 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 DepthDepth

Individual midpoints Combined profiling and sounding DepthDepth EM34 40m 10m 20m EM313.67m ExplorationExploration 5.5m 15m 30m 60m

Individual midpoints DepthDepth EM34V 40m 10m 20m EM31V3.67m 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.

Survey layout

Examine 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

Grid 1 was re-surveyed about 6 months later. Note reduction in anomaly magnitude

Earlier Survey

Examination of Profile data

Remember how they are constructed? The apparent conductivity measured with a given coil spacing is plotted at the “Exploration Depth.” Profile displays are shown 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 SE NW SW NE

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. The “Pseudo-Section.”

More Profiles

Location of modeled profile shown by gold line Modeled Profile

7.5m 15m 30m EM31 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.

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 A pdf version is available at Another Case Study -

From Carpenter and Ahmed, 2002 Survey Line

From Carpenter and Ahmed, 2002

Infiltration pathways in karstified dolomite subcrop High conductivity region Low conductivity bedrock

From Carpenter and Ahmed, 2002 high resistivity = low conductivity Relatively low resistivity zone Low resistivity = high conductivity

Coal Mine Refuse Pile

Settling/treatment pond

Acidic drainage from the refuse area

Topical lime treatment

Top of refuse pile EM31 magnetometer Magnetic anomaly

Preston County Coal Refuse Area High Conductivity Coal Refuse

Sting and Swift resistivity meter

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? More Review

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

In this case … 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.

Some additional perspectives Also known as the Pitfloor

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 R V (z)

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

Next week we’ll try our luck with the new computers and begin some computer modeling work. Problems discussed in class Tuesday and Thursday will be due next Thursday. Begin reading the resistivity chapter (Chapter 5) in Berger, Sheehan and Jones