APPLICATION OF GEOPHYSICAL TECHNIQUES

TO MINERALS-RELATED ENVIRONMENTAL PROBLEMS

By Ken Watson, David Fitterman, R.W. Saltus, Anne McCafferty,

Gregg Swayze, Stan Church, Kathy Smith, Marty Goldhaber,

Stan Robson, and Pete McMahon

Open-File Report 01-458

2001

This report is preliminary and had not been reviewed for conformity with U.S.

Geological Survey editorial standards or with the North American Stratigraphic Code.

Any trade, firm, or product names is for descriptive purposes only and does not imply

endorsement by the U.S. Government.

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY
Geophysics in Mineral-Environmental Applications 2

Contents

1 Executive Summary 3

1.1 Application of methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Mineral-environmental problems . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Controlling processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.4 Geophysical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4.1 Electrical and electromagnetic methods . . . . . . . . . . . . . . . 5

1.4.2 Seismic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4.3 Thermal methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4.4 Remote sensing methods . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4.5 Potential field methods . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4.6 Other geophysical methods . . . . . . . . . . . . . . . . . . . . . . 7

2 Introduction 8

3 Mineral-environmental Applications of Geophysics 10

4 Mineral-environmental Problems 13

4.1 The sources of potentially harmful substances . . . . . . . . . . . . . . . . 15

4.2 Mobility of potentially harmful substances . . . . . . . . . . . . . . . . . . 15

4.3 Transport of potentially harmful substances . . . . . . . . . . . . . . . . . 16

4.4 Pathways for transport of potentially harmful substances . . . . . . . . . 17

4.5 Interaction of potentially harmful substances with the environment . . . . 18

5 Processes Controlling Mineral-Environmental Problems 18

5.1 Geochemical processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.1.1 Chemical Weathering (near-surface reactions) . . . . . . . . . . . . 19

5.1.2 Deep alteration (reactions at depth) . . . . . . . . . . . . . . . . . 20

5.1.3 Microbial catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5.2 Flu id flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5.3 Biologic/geologic interactions . . . . . . . . . . . . . . . . . . . . . . . . . 22

5.4 Energy flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.5 Mass movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6 Description of Geophysical Techniques 25

6.1 Electrical and electromagnetic methods . . . . . . . . . . . . . . . . . . . 26

6.2 Seismic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

6.3 Thermal methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

6.4 Remote sensing methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

6.5 Potential fields methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6.6 Other geophysical methods . . . . . . . . . . . . . . . . . . . . . . . . . . 35

7 Glossary of Terms 36

8 References 40
Geophysics in Mineral-Environmental Applications 3

List of Tables

1 Mineral-environmental applications of geophysical techniques . . . . . . . 10

2 Potentially harmful substances related to mineral concentrations and mining 14
Geophysics in Mineral-Environmental Applications 4

1 Executive Summary

Geophysical techniques have many important applications to mineral-environmental problems. This report, the result of a collaborative effort by a team of geophysicists, geologists,

geochemists and hydrologists, focuses on these issues: (1) characterizing environmental

impacts associated with ore bodies, mining, and extraction, (2) the primary physical processes affecting these environmental impacts, and (3) the capabilities and limitations of

geophysical methods with applications to these problems. A glossary of terms used in the

report is included in Section 7.

1.1 Application of methods

Example applications of geophysical techniques to mineral-environmental problems are

summarized in Table 1 (Section 3), which lists application, problems, processes, and

geophysical methods and contains references to the appropriate sections of the report for

details.

1.2 Mineral-environmental problems

We subdivide a typical mineral-environmental problem and discuss the sources, mobility conditions, transport processes, pathways, and ultimate fate of potentially harmful

substances. Geophysical techniques have important applications in all five areas. Concentration of potentially harmful substances can result from natural processes (such as

formation of a metallic sulfide mineral deposit) or from anthropogenic activities (such as

mining). A major factor in the mobilization of potentially harmful substances is contact

with water and thus important parameters are those relating to surface and subsurface

hydrologic flow. Dispersal as airborne particulates is a secondary process that depends

on weather and local topography. Once the potentially harmful substances are mobilized, transport can occur by a variety of agents: water, wind, gravity, and the biota.
Geophysics in Mineral-Environmental Applications 5

Understanding the movement of water requires knowledge of hydrological pathways and

processes. Surface flow is commonly localized by existing rivers and streams; the size of

the channels, depth of fill, and bottom surface conditions are all factors that influence

flow and are also functions of total and peak discharge for a drainage system. Other

forms of surface flow include sheet runoff caused by cloudbursts or rapid melting of snow

and ice pack. Subsurface flow, which is controlled by permeability and hydraulic head,

depends on many factors including porosity, grain size, sorting, cementation, fractures,

and connectivity of pore space. The zones of mixing between surface and subsurface flows

may be critical to the understanding of contaminant flow. The fate of substances that are

of most critical concern to humans depends on several processes: chemical changes of the

substance over time which increase or decrease toxicity (such as natural cyanide breakdown and oxydation/reduction reactions), chemical reactivity of a substance (such as acid

neutralization by a carbonate buffers), inappropriate use of potentially harmful materials

(such as using mine tailings for road base), selective absorption by plants, grazing by

livestock, uptake by biota, and aquifer contamination.

1.3 Controlling processes

Processes controlling mineral-environmental problems are divided into five categories:

geochemical, fluid flow, biologic/geologic, energy flow, and mass movement. Geochemical

reactions are subdivided into chemical weathering and deep alteration. Chemical reactions of weathering range from slow to rapid and may be incomplete or complete, but are

commonly slow and incomplete. Stable end products include quartz, limonite, and kaolinite (Krauskopf, 1979). Oxidation reactions involving many sulfide minerals can produce

acidic waters. Neutralization of these waters can be provided by the presence of many

types of carbonate-bearing rocks. Alteration associated with the interaction of geothermal waters with surface waters can concentrate potentially harmful substances. Fluid
Geophysics in Mineral-Environmental Applications 6

flow is the most significant process contributing to environmental problems because it

provides the medium for the reaction and the mechanism for transport. Both the surface

and subsurface groundwater flows need to be understood in relationship to the geologic

controls and interaction with the mineral deposit and mine-waste materials. Mapping

permeability is an important aspect of modeling subsurface flow; geophysical methods

can be used to complement and extend borehole data. Vegetation can have a profound

effect on the dispersal and concentration of toxic substances by uptake, leaching, and

re-deposition. Two examples of the importance of energy flow are the effects associated

with underground coal fires and the heat generated by sulfide oxidation in mine waste

piles. Mass movement can be a significant process because it can expose fresh material

to weathering, transport hazardous material, or increase the subsurface permeability.

1.4 Geophysical techniques

Geophysical techniques are classified into the following groups: electrical, seismic, thermal,

remote sensing, and potential fields. Their use can be restricted in some environmental

studies depending on how invasive they are; satellite and aircraft data are the least invasive

and borehole methods are the most.

1.4.1 Electrical and electromagnetic methods

Electrical and electromagnetic methods can be used to map subsurface variability in electrical properties caused by changes in lithology, structure, alteration, and contamination

due to mining activity. These methods are sensitive to low resistivity targets and thus

can be used to map the location and migration of drainage waters and mine-waste. The

depth of investigation can range from less than a meter in the case of ground penetrating radar to tens and hundreds of meters for most other methods. Resolution of targets

and detectability tend to decrease with increasing depth of burial. The induced polarization method can differentiate between various metallic sulfide and alteration minerals
Geophysics in Mineral-Environmental Applications 7

and holds promise as a means of characterizing mine-waste deposits. The self-potential

method may be useful in studying waste piles because it responds to the presence of

electrochemical cells and water flow in the ground.

1.4.2 Seismic methods

Seismic methods are sensitive to variations in the elastic properties and density of the

subsurface and can be used to map important geologic, hydrologic, and physical interfaces including the base of a surficial weathering layer, the water table, stratigraphic or

structural breaks between contrasting rock units and fractures. Typically, these methods

are most effective in regions with sub-horizontally-layered subsurface interfaces.

1.4.3 Thermal methods

Thermal methods, which use probes to measure subsurface temperature, have potential

for monitoring exothermic or endothermic geochemical reactions in mine tailings and

waste ponds, and application to three-dimensional models of fluid flow.

1.4.4 Remote sensing methods

Thematic Mapper (TM) satellite data have been be used to map surface alteration at 30m

ground resolution and new satellite systems will be launched shortly with spatial resolution capable of detecting features as small as tailings piles. Improved spectral resolution

of satellite data will make it possible to detect individual minerals for more detailed analysis. Aircraft data is now used to map alteration zonation, lithologic differences, surface

temperature, and vegetation type and state. In particular, imaging spectroscopy is used

for mapping a wide variety of minerals (iron-bearing minerals, phyllosilicates, carbonates,

sulfates) as well as vegetation species and stress, and has been shown to be a powerful tool

for mapping ground alteration and detecting lithologies that can act as acid-neutralizing

agents. Multispectral thermal scanners can be used to map igneous lithologies, and silicic
Geophysics in Mineral-Environmental Applications 8

alteration, and may have potential use for detecting heating or cooling caused by chemical

reactions in waste piles. Limitations of remote sensing include masking of surface mineralogy/lithology where extensive vegetation cover is present, residual solar/topographic

heating effects in areas of moderate to high relief, and high sensitivity to surface weathering.

1.4.5 Potential field methods

Gravity and magnetic methods can yield important three-dimensional geologic information. Gravity data, for example, can be used to model the shape of a basin containing

young sedimentary rocks juxtaposed against higher density basement rocks. Magnetic

data can provide complementary information if there is a magnetic contrast between the

sedimentary rocks and the basement. Mapping the sediment/basement interface can be

important for understanding movement of subsurface water. Faults may often be traced

by mapping offsets of both gravity and magnetic source bodies. Magnetic data can also

be useful for finding abandoned wells and buried drums, and locating and mapping waste

dumps. High-resolution repeat gravity measurements may be useful to track changes in

the water table or movement of unstable slopes.

1.4.6 Other geophysical methods

Other geophysical methods with application to mineral-environmental problems include

microwave (active and passive), borehole logging, GPS methods, rock property laboratory

measurements, and radiometrics. Microwave methods have a ground penetration of up to

several meters and can be used to examine near surface heating and for detecting shallow

dielectric property changes related to dry versus wet or frozen versus unfrozen ground.

Borehole logging includes virtually all geophysical methods that can be conducted on

the ground and provide detailed measurements for determining physical properties in a

depth profile. GPS methods can be used to track very small Earth movements between
Geophysics in Mineral-Environmental Applications 9

stations to measure subtle ground movements which might be precursors of landslides

or other hazardous ground movement. Laboratory measurements refers to the empirical

characterization of physical rock properties such as porosity and permeability and their

relationship to other physical properties that are directly measured. Radiometrics includes

the use of airborne surveys to estimate abundances of potassium, thorium, and uranium

for a variety of activities including regional lithologic discrimination, mineral exploration,

and environmental background studies.

2 Introduction

The U. S. Geological Survey (USGS) formed an interdisciplinary team in late 1995 to

examine the role of geophysics in mineral-environmental studies of the USGS Mineral

Resource Program. The team discussions examined USGS technical capabilities, defined

their relevance to mineral-environmental studies with focus on applications, and addressed

such critical issues as interdisciplinary needs, development of new techniques, database

management, and connections to other Geologic Division programs and to cooperative

activities. The overall objective of the task was production of this report. The interdisciplinary composition of the group insured that the report, which resulted from discussions

between geophysicists, geologists, geochemists and hydrologists, would address real problems using the best understanding of the capabilities of the methods. A team approach

was used to identify key issues in our understanding of processes affecting environmental

impacts of mineral concentrations and minerals development, as well as hazards associated with previous mining. A by-product of these discussions was an increased awareness

within the team of both the capabilities and limitations of geophysical methods and the

types of problems that need to be addressed.

This report is intended to provide geophysicists with a background on the problems

and processes that can be important for mineral- environmental problems and to acquaint
Geophysics in Mineral-Environmental Applications 10

geologists and geochemists with the substantial array of powerful geophysical techniques

that can be brought to bear in the study of these problems. There are a great wealth of

texts available on both environmental geochemistry and exploration geophysics but they

are designed for specialists. It is the aim of this report to reach across the discipline

boundaries. The difficulty is elegantly stated in a recent issue of Physics Today ( Herschbach, 1997): ”The typical chemist wants above all to understand why one substance

behaves differently from another; the physicist usually wants to find principles that transcend any specific substances.” With that cultural background in mind geochemists and

geophysicists face a real challenge as they plan to work together to solve the important

mineral-environmental problems that we face.

The most comprehensive study to date of the application of geophysical methods

to minerals-related environmental problems is a Canadian study which focused on the

detection and monitoring of acid mine drainage (Paterson and others, 1994). The study

emphasized the effectiveness of geophysics in recognizing and mapping the movement of

acid effluents in and around mine workings from a variety of measurement platforms,

ranging from satellites and aircraft to ground and waterborne. Patterson and others

(1994) also stressed the importance of expanding the range of geophysical methods and

applications into a greater variety of geologic and topographic settings. They also noted,

as a common theme in the reviewed literature, the importance of integrating geophysical

methods to resolve ambiguities and improve identification. A review of the applicability

of geophysical methods to geoenvironmental studies of ore deposits (Hoover and others,

1995) provides a useful summary of methods with a discussion of how they should be

applied for specific geoenvironmental investigations.
Geophysics in Mineral-Environmental Applications 11

3 Mineral-environmental Applications of Geophysics

This section summarizes, in a tabular form, some example applications of geophysical

methods to mineral-environmental problems. The applications are listed alphabetically

within groups, ordered by our subdivision of a typical mineral-environmental problem (see

Section 4), as indicated by the references to subsequent sections in the Problems column.

The Processes column provides references to subsections of our discussion of physical,

chemical, and biological processes of importance to mineral-environmental problems. Finally, in the Geophysical methods column we list specific geophysical techniques with

reference to the general technique discussions in Section 6. Brief definitions of many of

the geophysical methods listed are also given in the glossary (Section 7).

Table 1: Mineral-environmental Applications of Geophysical Techniques. [Numbers in

parentheses are references to subsequent sections of this report. Abbreviations and

acronyms are defined in the glossary (Section 7).]

Application Problems (4) Processes (5) Geophysical methods (6)

Alteration/weathering

mapping

Sources (4.1) Geochemical

(5.1)

Most electrical methods (6.1)

Imaging spectroscopy (6.4)

Magnetics (6.5)

Spectral thermal (6.4)

Geologic/lithologic mapping Sources (4.1) All (5) Multispectral reflectance and

emission (6.4)

Imaging spectroscopy (6.4)

Radiometrics (6.6)

Most electrical methods (6.1)

Gravity and magnetic property mapping (6.5)

3D ore deposit mapping Sources (4.1) Geochemical

(5.1)

Most electrical methods (6.1)

Gravity and magnetics (6.5)
Geophysics in Mineral-Environmental Applications 12

3D waste pile mapping Sources (4.1) Geochemical

(5.1)

Ground-based EM (6.1)

DC resistivity (6.1)

Induced Polarization (6.1)

Magnetics (6.5)

Self-potential, EM (6.1)

Shallow seismic surveys (6.2)

Ground-penetrating radar

(6.1)

Clay content mapping Mobility (4.2) Geochemical

(5.1)

Most EM methods (6.1)

Induced polarization (6.1)

Imaging spectroscopy (6.4)

Climate evaluation and

monitoring

Mobility (4.2)

Transport

(4.3)

Geochemical

(5.1)

Fluid flow (5.2)

Multispectral reflectance and

emission (6.4)

Microwave (active and passive;

6.6)

Borehole thermometry (6.3)

Erosion mapping (nearsurface and sub-aqueous)Mobility (4.2) Geochemical

(5.1)

Aerial photography (6.4)

Thermal infrared (6.4)

Many EM methods (6.1)

Shallow seismic (6.2)

Moisture content mapping Mobility (4.2) Geochemical

(5.1)

Thermal infrared (6.4)

Radar (airborne and ground)

(6.4)

Borehole logging (6.6)

Time-domain reflectrometry (TDR)

(6.6)

Ponding and seepage

mapping

Mobility (4.2) Geochemical

(5.1)

Fluid flow (5.2)

Aerial photography (6.4)

Thermal infrared (6.4)

Many EM methods (6.1)

Shallow seismic (6.2)

Slope stability measurement Mobility (4.2) Mass flow (5.5) Repeat gravity (6.5)

GPS repeat surveying (6.6)

Magnetic tracking of buried

magnets (6.5)

EM mapping of weak clay

zones (6.1)

Remote sensing (6.4)
Geophysics in Mineral-Environmental Applications 13

Water table mapping

and monitoring

Mobility (4.2)

Transport

(4.3)

Geochemical

(5.1)

Fluid flow (5.2)

Ground-penetrating radar (1-

5 meters depth, 6.1)

DC resistivity (6.1)

SP (6.1)

Borehole logging (6.6)

Gravity and magnetic basement mapping (6.5)

Permeability and porosity

determination

Transport

(4.3)

Pathways (4.4)

Fluid flow (5.2) Borehole logging (6.6)

Laboratory measurements

(6.6)

Subsurface water flow

rate measurement

Transport

(4.3)

Fluid flow (5.2) Self-potential method (qualitative) (6.1)

Surface flow modeling Transport

(4.3)

Pathways (4.4)

Fluid flow (5.2) GIS modeling

Fracture mapping Pathways (4.4) Fluid flow (5.2) VLF EM (6.1)

Self-potential EM (6.1)

Seismic tomography (6.2)

Radar tomography (6.1)

Gravity and magnetics (6.5)

Thermal infrared(6.4)

Subsurface plume mapping Pathways (4.4) Fluid flow (5.2) Repeat borehole logging (6.6)

Many electrical and EM methods (6.1)

Ground-penetrating radar

(6.1)

Habitat mapping and

monitoring

Fate (4.5) Biological (5.3) Imaging spectroscopy (6.4)

Multispectral reflectance (6.4)

Thermal infrared (6.4)

SLAR (6.4)

Vegetation mapping and

monitoring

Fate (4.5) Biological (5.3) Imaging spectroscopy (6.4)

Multispectral reflectance (6.4)

Thermal infrared (6.4)
Geophysics in Mineral-Environmental Applications 14

4 Mineral-environmental Problems

Minerals-related geoenvironmental problems are primarily concerned with the movement

of potentially harmful substances (Table 2) from areas of mineral concentration as a result

of both natural and anthropogenic processes. Characterization and understanding of these

problems requires knowledge of:

1. Sources of potentially harmful substances,

2. Conditions under which the substances become mobile,

3. Processes that transport the substances,

4. Pathways available to these transport processes, and

5. Ultimate fates of these substances in the environment.

Geophysical methods can play an important role in studying each of these five areas

(see examples listed in Table 1).

To illustrate this subdivision of a typical geoenvironmental problem, consider acid

rock drainage. This problem is concerned not just with the transport of the acidic waters

themselves, but with the capacity of these waters to carry metals. The exact nature of

the leachates varies with the mineralogy of the source rocks and the composition of the

leaching fluid. The degree to which metals are mobilized is influenced by the acidity and

composition of the leaching fluid. Fluid flow is the typical transport process. The fluid

flow pathways can include a variety of surface and subsurface paths. The ultimate fate of

the metals in the environment can vary from precipitation in a geochemically stable form,

to temporary deposition (in a river bottom, for example, at risk of further mobilization),

to entry into the biosphere and atmosphere (plant and/or animal uptake).
Geophysics in Mineral-Environmental Applications 15

Table 2: Potentially harmful substances related to mineral concentrations and mining.

[L, low mobility; M, moderate mobility; H, high mobility; N, relatively immobile]

Category Substance Conditional Mobility∗

Oxidizing Oxidizing Reducing

(Acidic, pH< 3) (pH> 4) (pH> 5)

(with abundant (Locircumneutral in

iron-rich in presence of

particulates) hydrogen sulfide)

Metals

Ag - silver L N N

Al - aluminum M N N

As - arsenic M N N

Ba - barium L L M

Be - beryllium L N N

Cd - cadmium H M N

Co - cobalt H L N

Cr - chromium L N N

Cu- copper H N N

Hg - mercury M L N

Ni - nickel H L N

Mn - manganese M L M

Mo - molybdenum L N N

Pb - lead L N N

Se - selenium M L N

Sb antimony L L N

Tl - thallium L L L

V - vanadium M N N

Zn - zinc H M N

Radioactive elements

Rn - radon H H half-life 3.8 days

U - uranium M N N

Airborne Contaminants

asbestos

beryllium

dust containing metals

dust containing soluble salts

silica (quartz)

∗Relative mobilities from Smith and Huyck (1999).
Geophysics in Mineral-Environmental Applications 16

4.1 The sources of potentially harmful substances

Potentially harmful substances (Table 2) can be concentrated through either natural or anthropogenic processes. Naturally occurring concentrations of potentially harmful elements

may be located and characterized by standard mineral exploration methods. For example, metal sulfide mineral deposits (a natural source of heavy metals) are a logical target

for a variety of electrical and electromagnetic methods. Evaluation of regional geological,

geochemical, and geophysical data can be used to characterize regions as “favorable” for

various types of mineral concentrations. Anthropogenic enrichments of potentially harmful substances may result from mining, milling, smelting, and disposal practices; examples

include waste piles and abandonded mines.

4.2 Mobility of potentially harmful substances

Mobility is defined as the ease to which a substance is dispersed relative to the other

materials surrounding it. There are chemical, physical, and biological factors which affect

mobility (Rose and others, 1979).

Chemical factors influencing mobility relate to the chemical stability of an element in

a solid phase relative to the coexisting mobile fluid phases. Table 2 summarizes relative

conditional mobility of various potentially harmful substances in oxidizing or reducing

chemical environments (Smith and Huyck, 1999).

A major physical factor affecting the mobility of potentially harmful substances is the

surface area in contact with water because nearly all substances are soluble to some degree.

New material is exposed to flowing water by natural processes such as uplift, fracturing

of rocks, landslides, flooding, weathering (chemical or physical) and mechanical erosion

as well as anthropogenic processes such as mining, dam construction, and road building.

Another physical factor that affects mobility is the availability of potentially harmful

materials to transport by wind.
Geophysics in Mineral-Environmental Applications 17

The overall mobility of a substance depends on the combination of all of these factors. In arid environments precipitation is sporadic, chemical weathering is episodic, and

mechanical erosion may be a key factor in exposing substances to possible transport. A

chemical pulse of dissolved minerals, enriched by weathering during dry periods commonly

is associated with early spring runoff (Jeffries, 1991). This chemical pulse may carry metals in sufficient concentrations to harm aquatic life. Similarly, an intense chemical pulse

can occur in areas of permafrost because concentrated brines that have developed during freeze/thaw cycles are released in the spring (William Miller, pers. comm., 1997).

In tropical environments, high mechanical erosion can expose rocks to intense chemical

weathering.

Important parameters that characterize the mobility of potentially harmful substances

are largely those that relate to surface and subsurface hydrologic flow. Physical parameters include slope, relief, permeability, fracture or fault zones, clay and moisture content,

tectonic regime and changes in tectonic stress, drainage network development, and stream

flow. Chemical parameters include rock lithology/mineralogy, stream/ground water composition (i.e., pH, Eh), alteration, mineralogy of weathering products, and subsurface

contaminant plumes. Biological parameters include the type and extent of biota.

4.3 Transport of potentially harmful substances

If a potentially harmful substance is mobilized, it may be transported by a variety of

agents including wind, water, gravity and biota.

At the Earth’s surface, wind can cause widespread dispersion of fine-grained materials. Very fine particles of asbestos, quartz, volcanic ash, metal-bearing dust, and dust

containing soluble salts can be health hazards (Malcom Ross, pers. comm., 1996). Mineral dust may be transported by sandstorms in desert regions. It may be mobilized by

drought or erosion. Transport can vary from short distances for particles larger than 100
Geophysics in Mineral-Environmental Applications 18

micrometers (saltation), to thousands of kilometers for particles less than 10 micrometers

(Malcom Ross, pers. comm., 1996). Particulates can pose a particular problem in places,

such as open-pit mines, mills, smelters or areas of recent landslide activity, where surficial

materials are disturbed. Additionally, mine-waste materials that have been used locally

in construction and as landfill increase the potential for further dispersal.

Water flow is the most important transport process available for moving potentially

harmful substances. Both surficial and subsurface flows are important. Considerations of

the pathways available to flow are discussed in Section 4.4. Discussion of the fluid flow

itself is deferred to Section 5.2.

Gravity is the driving force behind the movement of surface water, but also plays a

direct role in mass movements such as landslides, mine collapses, and dam failures. These

mass movements may transport potentially harmful substances directly or may increase

their mobility by exposing them to transport by wind or water.

Biologic transport of potentially harmful substances includes uptake and concentration

of chemical elements by rooted plants, and bioaccumulation and magnification up the food

chain by animals. Mobility may be enhanced by bacterial activity (Rose and others, 1979).

4.4 Pathways for transport of potentially harmful substances

Water flow, the most important transport process for contaminants in most situations,

follows both surface and subsurface pathways. Surface pathways include existing rivers

and streams, as well as ephemeral drainages in the form of gullies, arroyos, and washes.

The size of channels, how deeply they are filled, and the conditions of the bottom surface

may all control water flow rates. Under conditions of very heavy rainfall or when infiltration cannot keep up with precipitation, overland (sheet) flow may result in small (less

than 10%) portions of watersheds (Freeze and Cherry, 1979). Subsurface flow pathways

are controlled by permeability and hydraulic head where flow is through porous or frac-
Geophysics in Mineral-Environmental Applications 19

tured media. Zones of well-sorted sand and gravel make excellent aquifers. Clay layers

will retard subsurface water flow. In sedimentary materials, porosity, grain size, degree

of sorting, cementation, fractures, and the presence of clay and alteration minerals are

among the important factors controlling subsurface flow rates (Freeze and Cherry, 1979).

In igneous and metamorphic terranes, fluid flow is usually dominated by fracture flow due

to the lack of interconnected pore space.

4.5 Interaction of potentially harmful substances with the environment

Complex processes beginning at the source of potentially harmful substances and extending through the geologic environment to the biosphere and ultimately to humans provide

multiple pathways for movement of harmful substances. Concentrations of toxic substances may be affected by physical dispersal; dilution; reduced drainage flow caused by

ponding; freeze/thaw cycles in permafrost; interaction with highly weathered soil; changes

in the water chemistry; movement along a geochemical gradient (see Section 5.1), or presence of a geochemical barrier (see Section 5.1). Processes most critical to humans are

those that affect the food chain and drinking water quality including selective absorption

by plants, grazing by livestock, uptake by biota, and contamination of aquifers.

5 Processes Controlling Mineral-Environmental Problems

To gain a complete understanding of a mineral-environmental problem we must examine

the controlling physical, chemical, hydrological, and biological processes. For example, in

the typical acid rock drainage scenario we must understand: (1) the geochemical reactions

taking place in the mine (geochemical processes), (2) microbial processes that encourage

or inhibit these reactions, (3) the fluid flow regime (hydrologic processes) that transports

the acidic water, and (4) the effect of the acidic water on flora and fauna downstream

(biologic/geologic interactions). In the following subsections, we review some critical
Geophysics in Mineral-Environmental Applications 20

aspects of the geologic, geochemical, physical, and biologic processes that affect mineralenvironmental problems. These processes are the link between mineral-environmental

problems and the physical properties measured by the geophysical techniques.

5.1 Geochemical processes

Geochemical reactions control the release of many potentially harmful substances (Table

2) into the environment. The primary processes may be loosely subdivided into categories

of “chemical weathering” and “deep alteration” (Krauskopf, 1979). Chemical weathering

describes the process by which rocks and minerals formed at high temperature and pressure come to chemical equilibrium with water and air at near-surface conditions. Deep

alteration describes the chemical interaction of rocks and ground water that take place at

depth, usually at much higher pressures and temperatures.

Two important concepts in discussion of geochemical processes are geochemical gradients (gradual spatial changes in geochemical environment) and geochemical barriers

(abrupt spatial changes in geochemical environment). Geochemical gradients may occur

as a result of ore-forming (i.e., alteration) processes or as a result of surficial transport

and subsequent deposition by wind or water. Geochemical barriers include mechanical,

physiochemical, biochemical, and anthropogenic types (Smith and Huyck, 1999). These

barriers may localize or confine various chemical weathering or alteration reactions leading

to the accumulation of elements.

5.1.1 Chemical Weathering (near-surface reactions)

The primary processes that control chemical weathering (Krauskopf, 1979, p. 80) are: ionic

dissociation, addition of water and carbon dioxide, hydrolysis, oxidation, and biological

effects. The reactions of chemical weathering may take place slowly or rapidly and may or

may not go to completion. If the reactions go to completion, the ultimate end products of

chemical weathering are typically minerals stable under Earth-surface conditions including
Geophysics in Mineral-Environmental Applications 21

quartz, limonite, and kaolinite in the form of sand and clays.

The simplest form of chemical weathering, dissolution of soluble minerals and the

addition of water, plays a major role in mineral-environmental problems. Since nearly

all substances are soluble in water to some extent, the presence of any of the potentially

harmful substances (Table 2) in rocks in contact with water can lead to finite levels of the

substance in the water. This dissolution may be enhanced if acidic water is present.

Oxidation reactions, involving many sulfide minerals to produce acidic water, are of

particular importance in mineral-environmental problems. A typical reaction is:

2FeS2 +

15

2 O2 + 4H2O → Fe2O3 + 4H2SO4

in which pyrite (FeS2) is oxidized to produce ferric oxide (rust) and sulfuric acid. Oxidation by ferric iron (Fe3+) in solution can enhance acid production, so sulfide-rich mineral

deposits with high percentages of pyrite and ferric iron are particularly prone to generate

acidic water (Plumlee and others, 1995). Bacteria catalyze acid production by converting

ferrous (Fe2+) to ferric iron (Smith, 1994).

Most rocks containing carbonates can provide a natural geochemical buffer for acidic

waters. In contact with a strong acid, most carbonates such as calcite can dissolve and

neutralize the acid. Mechanical erosion can increase the rates of acid production or

consumption by breaking down the rock and increasing surface area available and exposing

fresh mineral surfaces for reaction with the acidic waters.

5.1.2 Deep alteration (reactions at depth)

Alteration processes may play a role in mineral-environmental problems in regions where

geothermal waters interact with surface waters. These geothermal waters may contain elevated concentrations of potentially harmful substances that have arisen from rock/water

interactions that have taken place at higher temperatures and/or pressures than those

typical of surficial weathering.
Geophysics in Mineral-Environmental Applications 22

5.1.3 Microbial catalysis

Microorganisms (principally bacteria) play a fundamental role in controlling the rates

of many important weathering reactions including the principal oxidation reactions that

produce acid rock drainage (Gould and others, 1994). In fact, Gould and others (1994, p.

185) state that acid mine drainage is “primarily the result of biological reactions.” Bacterial growth is dependent on pH; values between 5 and 8 are best (Gould and others, 1994).

Microorganisms (including algae, fungi, and yeasts in addition to bacteria) contribute to

biological processes that are being investigated to ameliorate acid rock drainage (Gould

and others, 1994).

5.2 Fluid flow

Fluid flow is probably the most important process of concern to mineral-related environmental problems because water provides the medium for the reaction and transport of

potentially harmful chemicals from mineral deposits and the mechanism for the spread

of these potentially harmful materials. Therefore, we must understand both surface and

ground-water flows, how they are controlled by geology, and how these flows interact with

each other and with mineral deposits and mine-waste materials.

Surface-water flows are controlled by topographic drainages and channels which dictate

where water will flow and by topographic gradients which influence how fast the water

will move. In addition, the depth of water, which is influenced by precipitation and

infiltration rates, is an important factor. For example. increases in stream depth can

result in flooding which can transport mine waste; these materials may be benign when

dry, but become a contamination source when wet.

Ground-water flow is controlled by hydraulic conductivity and gradient. The former

is a function of the physical properties of geologic materials, the manner in which they

were deposited, and subsequent structural changes, whereas the latter depends upon dif-
Geophysics in Mineral-Environmental Applications 23

ferences in elevation (relief), availability of water (rainfall, infiltration, water levels in

streams, rivers, and lakes), and degree of saturation (Freeze and Cheery, 1979). Variation in subsurface hydraulic conductivity is usually unknown when modeling subsurface

fluid flow. Traditionally, determination of hydraulic conductivity is based on drill hole

information, laboratory measurements, and numerical hydrologic model studies. However, in a hard-rock environment where hydraulic conductivity of the rock matrix is very

low, fractures can dramatically increase fluid flow (Shapiro and Hsieh, 1991). The use of

surface geophysics can greatly add to hydraulic conductivity mapping by correlating measurements to well information, and then using the geophysical measurements to provide

information about the areas between wells.

Understanding the interaction of surficial water and ground water is becoming increasingly important (Nagorski, 1997; Winter and Woo, 1990).

5.3 Biologic/geologic interactions

Vegetation can have a profound effect on the dispersion of potentially harmful substances

in surficial deposits. Elements may be solubilized by complex biogenic processes from

relatively stable mineral phases and ingested into a plant. Plants create an extremely

corrosive environment in the vicinity of their root tips that enables the extraction of

metals and other mineral matter from the soil (Rose and others, 1979, p. 459). Substantial

quantities of inorganic matter may ultimately reside in the body of the plant. As leaves

and other parts of the plant fall to the ground and decay, these elements are susceptible

to leaching or are recycled in the soil for subsequent uptake.

Although plants may absorb small amounts of most elements, several elements which

are toxic at high concentrations, (e.g., Cu, Zn, Mo, and Mn) are required for the health

of most plants at lower levels (Rose and others, 1979).
Geophysics in Mineral-Environmental Applications 24

5.4 Energy flow

In many cases the flow of energy in mineral-environmental problems is studied as part of

other processes such as fluid flow (where flow of heat may be considered), or geochemical

processes (where heats of reaction may be important). In some cases, however, it may

be advantageous to consider the flow of energy (for example, as heat) separately. For

example, the exothermic oxidation of pyrite which is expected in mine waste-rock dumps,

should cause a temperature rise of from 2◦C to 20◦C, depending on the amount of oxygen

transport in the system (Ritchie, 1994); measurement of temperature profiles can be an

effective way to monitor oxidation rate. Another example is underground coal fires in

which energy (thermal and chemical) release can be extreme; such fires can be locally

detected with airborne thermal surveys. Repeat temperature measurement in boreholes

(measurement of energy flow) can be used to monitor rates of certain chemical reactions in

mine-waste piles. In other applications, energy (either heat/temperature or mechanical)

may contribute to an environment that enhances or inhibits important geochemical or

biological processes.

5.5 Mass movement

Several natural and anthropogenically induced processes provide mechanisms for moving

large quantities of mine-waste materials over distances ranging from tens of meters to

several kilometers or more. Mass movement by landslide, avalanche, mine collapse, and

flood can produce physical hazards and can result in placement of mine-waste materials

into new environments. Remediation efforts may also move materials into areas prone to

mass movement.

Mine collapse can cause mass movement through down-dropping of material above a

mine. The processes involved and the form that the collapse takes is dependent upon

the host-rock geology (Lee and Abel, 1983) and the style of mining. Most mine collapse
Geophysics in Mineral-Environmental Applications 25

studies have been concerned with subsidence over coal deposits (Dunrud, 1976; Allen,

1978; Shadbolt, 1978; Carpenter and others, 1995). In hard-rock mining, rock collapse

progresses upward from the mined zone as ore is removed. The collapse will not cause

surface deformation until the upward-stoping process has thinned the intact rock so that

it can no longer support the weight of the overburden (Lee and Abel, 1983). The cave-in

breaches the surface, typically in the form of a circular depression or chimney, with rock

adjacent to the chimney sliding into it. Collapse can take place contemporaneously or

more than a decade after mining activity has stopped (Lee and Abel, 1983; Hobba, 1993).

Mine collapse can have more profound effects on the local hydrologic regime. Mine

collapse can produce fissures in the ground, that eventually reach the surface, providing a

pathway for the migration of surface water to depth (Lee and Abel, 1983; Hobba, 1993).

Likewise, subsurface fracturing increases subsurface hydraulic conductivity. Both of these

actions can greatly increase the mobility of contaminants, due to chemical and physical

effects, as well as modifying regional ground-water flow patterns.

Landslides resulting from slope failure triggered by precipitation, increased groundwater flow, or earthquakes can transport large quantities of material. In addition, the

scar left by the landslide will expose new material to erosion and chemical weathering

processes. Given the right conditions, a buried, benign mineral deposit containing potentially harmful substances, can be quickly transformed into a newly exposed contamination

source. Landslides and avalanches that originate above mine dumps can rapidly transport

mine-waste materials into new areas where they may pose a hazard. In a similar fashion,

floods (caused either by heavy precipitation or dam failure) can transport mine waste,

depositing it far down stream and spreading the contaminants.
Geophysics in Mineral-Environmental Applications 26

6 Description of Geophysical Techniques

In this section we discuss geophysical techniques with known or possible application to

understanding of the processes and problems discussed in the previous two sections. In

each subsection we highlight the following topics:

• Physical property (or properties) that the technique is sensitive to

• Invasiveness of the technique

• A typical application to a mineral-environmental problem

• Sub-specialty techniques

• Typical measurement sensitivities for the technique

• Possible problems or limitations of the technique

• Promising research directions

The use of geophysical methods for environmental studies differs from their use for

exploration because limitations are commonly imposed by the need to preserve environmental conditions at the site and thus minimize or restrict invasion and disturbance of

the site. Remote sensing data from satellite and aircraft are the least invasive and do not

pose a problem in this regard. Low flying aircraft used to acquire magnetic, radiometric,

and electromagnetic data are usually only restricted by special circumstances. Groundbased measurements (gravity, VLF, electromagnetic induction) involve access constraints

similar to those for geologic mapping. Electrical surveys involving the insertion of probes

(DC resistivity, IP, SP) may be a problem if, for example, a clay cap overlaying a mine

dump is to be preserved. Seismic methods requiring an explosive charge are significantly

constrained in some areas (e.g., prohibited in National Parks; not employed where explosive gases such as methane may be present). There are alternative energy sources such as
Geophysics in Mineral-Environmental Applications 27

vibrators or, for local studies, sledge hammers which can be employed. Borehole methods

(heat flow; electromagnetics and hole-hole and hole-surface seismic) are the most invasive

and hence most restricted because they can produce pathways for contaminant transport.

Where boreholes already exist at the site the methods may still be employable if the well

construction and geometry are appropriate.

6.1 Electrical and electromagnetic methods

Electrical and electromagnetic (EM) geophysical methods can be used to map subsurface

variability in electrical properties caused by changes in lithology, alteration, temperature,

and fluid content such as contamination due to mining activity. Because silicate minerals

are insulators under near surface temperatures and conditions, electrical current flow in

geologic materials is confined to fluids or clays in the pore space. Therefore, the porosity,

fracture density, pore fluid resistivity, degree of saturation, and clay content, are the

primary factors controlling the bulk resistivity. A more complete explanation of these key

factors affecting resistivity may be found in Keller and Frischknecht (1966).

Drainage water in poorly buffered environments from sulfide ore bodies, mines, and

mine dumps are characterized by high total dissolved solids (TDS) and low pH (2.5 or

less, Plumlee and others, 1992; 1999; Plumlee, 1999). This results in water with very

high specific conductivity (500-38,000 μS/cm, typically 1,000-6,000 μS/cm). Whereas the

response of most electrical geophysical methods are not ion specific, they are very sensitive

to these high conductivity targets. Accordingly, electrical and electromagnetic methods

can be used to map the location and migration of mine drainage waters containing high

TDS.

There are a wide variety of traditional electrical geophysical techniques that can be

used for mapping of mine-waste. These methods are characterized by: 1) the type of source

and sensor (direct current or resistivity, electromagnetic induction, plane wave methods
Geophysics in Mineral-Environmental Applications 28

such as magnetotellurics and VLF resistivity, and radar), and 2) the measurement platform (ground, airborne, or borehole). For a complete summary of the many methods and

their characteristics, references such as Keller and Frischknecht (1966), Grant and West

(1965), Nabighian (1988, 1991), and Wait (1982) should be consulted. Experience with

the use of electrical geophysical methods in mapping of mine waste is limited. However,

literature is growing on the use of these techniques for ground-water and environmental

problems (Ward, 1990).

The user of electrical methods must keep in mind the depth to which information

can be obtained and the resolution of each method. For electrical and electromagnetic

methods the depth of investigation can range from less than a meter in the case of ground

penetrating radar in clayey or very conductive terrains to tens and hundreds of meters

for most other methods, and it can be adjusted for a particular survey need. Resolution

of targets and detectability tend to decrease with increasing depth of burial.

Cultural noise is another consideration for electrical methods. The presence of power

lines and radio transmitters can produce increased electromagnetic noise which reduces

the sensitivity and resolution of many electrical methods. Metallic objects such as fences,

buried pipelines and cables, well casings, and metallic debris from abandoned or buried

mining operations often produce signals that may complicate or confound electrical and

electromagnetic data interpretation due to the highly irregular 3-D response of these

objects. Whereas there is no universal way of predicting or eliminating the noise response

of such bodies, there are means by which their influence can be recognized and reduced.

The foregoing discussion has focused on methods that are primarily sensitive to

changes in bulk resistivity without discriminating between the presence of different minerals. The induced polarization method, developed for disseminated sulfide exploration, can

differentiate under certain circumstances between various metallic sulfide and alteration

minerals. This technique holds promise as a means of characterizing mine-waste deposits.
Geophysics in Mineral-Environmental Applications 29

Likewise, the self-potential method may be of use in studying waste piles because it responds to the presence of electrochemical cells and water flow in the ground. These two

areas deserve further research.

6.2 Seismic methods

Seismic methods are often subdivided into reflection, refraction, and direct-wave categories

based on the path that seismic waves take from source to receiver. For shallow studies,

typical seismic energy sources include sledge hammers, weight dropping devices, small

explosive sources (shotgun shells), or mechanical vibrators. There are two main kinds of

waves used in seismic studies that travel at different velocities: “P” waves and “S” waves

(primary and secondary, or push and shake). Sometimes the ratio of these two velocities

(Poisson’s ratio) can be diagnostic of the material transmitting the seismic energy.

Depending on the strength of the seismic source and geometry of the experiment

(distance from source to receivers, distance between receivers), seismic measurements can

yield very detailed subsurface maps. Depth accuracies can range from ±1% to ±10%.

Seismic methods are sensitive to variations in the elastic properties and density of

the subsurface. Shallow Earth materials display a range of elastic properties, and seismic methods can be used to map important geologic, hydrologic, and physical interfaces

including the base of the surficial weathering layer, the water table, and stratigraphic or

structural breaks between contrasting rock units.

For shallow environmental studies, seismic methods can help define groundwater pathways including faults, fractures, and stratigraphic boundaries. The geometry of the seismic experiment can be tailored to the type of structural information required. Seismic

experiments conducted at the surface are most sensitive to sub-horizontal layers. Seismic

experiments may also be conducted in other geometries, such as between boreholes. This

configuration uses direct-wave travel times and can be interpreted, using tomography, to
Geophysics in Mineral-Environmental Applications 30

give a very detailed reconstruction of the ground, similar to those produced for medical

imaging.

Typical surface-based seismic methods are most effective in regions with sub-horizontallylayered interfaces. Complicated, three-dimensional structure can be very difficult to interpret unless data can be collected using sources or receivers below ground (i.e., in boreholes

or underground mine workings/tunnels). In regions with loose, unconsolidated surficial

material, such as gravel, it can be difficult to transmit seismic energy into the ground.

Accuracy of seismic interpretation is also limited by the presence of ambient noise such

as heavy truck traffic, wind (which affects the ground through the shaking of trees and

other vegetation), or ocean surf.

Areas for research and development in the application of seismic methods to shallow

environmental problems include development of new mechanical seismic sources, experimentation with different source/receiver geometries for different types of targets, and

development of software for processing and imaging seismic data and for integrating seismic data with other subsurface information.

6.3 Thermal methods

Thermal methods detect the production and movement of heat in the Earth. Accurate

measurement of conductive heat flux requires precise measurement of subsurface temperatures (usually in boreholes), and thermal conductivity (usually made in the laboratory

on samples from boreholes). Understanding temporal changes in temperature and heat

flow requires knowledge of the thermal diffusivity. The main sources of heat in the upper

several kilometers of the Earth include the convective movement of hot fluids including

geothermal waters and magma, as well as the radioactive decay of uranium, thorium, and

potassium. At the surface, solar heating dominates and various chemical reactions can

also be important contributors.
Geophysics in Mineral-Environmental Applications 31

Temperature measurements for thermal studies are typically invasive, conducted in

boreholes or by penetration of sediments by a probe. Non-invasive surface-temperature

measurements can be made by aircraft and satellite. Other non-invasive thermal measurements could be made with specialized equipment, either with available instrumentation

or with minimal development. For example, heat flow at the surface could be measured

by spreading a “blanket” of known thermal conductivity and measuring the equilibrium

temperatures above and below.

Temperature is an important variable for a host of geochemical processes, so knowledge of subsurface temperatures may be useful in understanding environmental problems.

Subsurface temperature is a function of available heat sources, and the mode and paths

of heat flow through the region. The two primary modes of heat flow in the Earth are

conduction and convection. In a conductive regime, the distribution of temperature with

depth is a function of thermal conductivity and heat production. In a convective regime,

the distribution of temperature with depth (in the shallow subsurface) is a function of the

movement of water through the region; in these systems analysis of borehole temperature

can be used to infer amount and direction of water movement. Subsurface temperatures can change as a result of changing boundary conditions at the Earth’s surface (such

as changes in mean surface temperature or changes in precipitation). The subsurface

changes will lag in time behind changes in the surface conditions. This transient behavior

can be used to measure changes in factors such as average surface temperature or rate

and temperature of water flow.

Temperature versus depth measurements are generally made in boreholes by lowering

a sensitive thermistor on a logging cable. Measurements may be made in unconsolidated sediments using a rigid probe. Ocean-bottom and lake-bottom measurements are

commonly made by penetrating sediments in this way. Temperature versus time measurements can be made on fixed thermister strings installed in a borehole or with a network
Geophysics in Mineral-Environmental Applications 32

of shallow probes spread over an area.

With modern equipment and techniques, temperatures can be measured to millidegree precision. Accuracy of heat flow determination is limited generally by knowledge of

thermal conductivity and by convection of fluid within the measurement borehole.

Possible research directions for thermal methods include development of monitoring

techniques for geochemical reactions in mine tailings ponds, integration of borehole temperature measurements with thermal information from remote sensing data, and application of thermal measurements to three-dimensional models of fluid flow.

6.4 Remote sensing methods

Remote sensing involves the use of images acquired from aircraft and satellites for mapping and monitoring the Earth’s surface. These images typically record electromagnetic

energy, either reflected or emitted. Modern remote sensing began with the LANDSAT

satellites in 1972. These satellites recorded reflected energy in the visible and near infrared parts of the spectrum. Thematic Mapper (TM) data, first collected in 1982 on the

second generation LANDSAT satellites, is of particular interest for mineral-environmental

applications; despite its fairly broad spectral bands (roughly 100nm), it can be used to

group minerals in two broad classes: (1) iron oxides and (2) clays/carbonates at 30m spatial resolution. In addition one channel is used to record emitted energy that is related to

surface temperature. There are also new satellite systems planned for launch shortly with

improved spatial (1-5m) and spectral (10nm) resolution. Greater spatial resolution improves detection of small features (such as tailings piles) and reduces classification errors

due to mixing signals from adjacent objects. Finer spectral resolution makes it possible

to improve identification from groups of minerals (e.g., iron oxides) to individual minerals

(e.g., hematite, goethite, and jarosite).

Aircraft data can now be acquired to map surface mineralogy, lithology, temperature,
Geophysics in Mineral-Environmental Applications 33

and vegetation state in order to support environmental and geotechnical studies. Airborne imaging spectroscopy, a method that acquires reflected radiation from the ground

with a spectral resolution comparable to laboratory spectrometers (10nm) for mapping

a wide variety of minerals (iron-bearing minerals, phyllosilicates, carbonates, sulfates),

vegetation species, and vegetation stress has been shown to be a powerful tool for mapping ground alteration and detecting lithologies that can act as buffering agents. Airborne

multispectral thermal scanners that detect radiation emitted from the ground can be used

to measure both spectral emissivity (analogous to reflectivity) and temperature, and have

been used to map igneous lithologies, silicic alteration, and to detect exothermic reactions

in waste piles as a measure of the presence of pyrite (Watson and Rowan, 1996).

An excellent example of the use of imaging spectroscopy is the mapping of acidgenerating minerals from aircraft at the EPA Superfund site in Leadville, CO (Swayze

and others, 1996). The waste piles were shown to have small zones of jarosite surrounded

by jarosite-goethite zones, which in turn were surrounded by goethite and hematite zones.

Field checking indicated that the small jarosite zones are pyrite-rich, and that oxidized

coatings on fine-grained pyrite was detected by identifying and mapping the sulfate mineral jarosite. Areas covered by jarosite and jarosite-goethite mine waste are believed to

have higher acid generating capacity and 20-100 times higher metal mobility than those

areas covered by goethite and hematite. These results indicate that imaging spectroscopy

can be used to map specific minerals that indirectly indicate pH conditions under which

they were formed and to locate sources of acid rock drainage.

Airborne spectral thermal infrared data are particularly effective for detecting silica

rich rocks and have been used to map silicic alteration (Watson and others, 1990). Broadband thermal data can be used to detect surface temperature differences as small as 0.1◦C

at spatial resolutions as fine as 5m. A recent study has reported that heat associated with

pyrite oxidation and subsequent combustion of bituminous shale in waste piles was de-
Geophysics in Mineral-Environmental Applications 34

tected using an airborne thermal scanner (Kuehn and Hoerig, 1996). There are plans

to acquire comparable spectral data from satellite by 1998, however the initial spatial

resolution will be low (90m); there is also a NASA proposal to acquire 20m resolution

thermal (broadband) data from satellite by 2001.

Limitations of remote sensing include (1) the inability to detect mineralogic/lithologic

changes where vegetation cover is high (causing both a masking effect and spectral contamination), (2) the difficulty of sorting out exothermic heating effects from residual

solar/topographic effects, (3) lack of penetration below the ground surface and thus high

sensitivity to weathering, the (4) limited availability of airborne data, and (5) mixing

effects caused by inadequate ground resolution. There are a wide variety of instruments

and platforms available to address topical to regional scale problems. Cost-effective use

requires knowledge of the capabilities and experience in their use.

New developments include satellite data at spectral and spatial resolutions comparable

to current airborne systems, satellite radar interferometry to measure centimeter-scale

ground movements, and improved spectral resolution for airborne thermal systems that

should improve silicate mineral identification.

6.5 Potential fields methods

Gravity and magnetic methods in exploration geophysics share a common mathematical foundation based on classical potential field theory (Blakely, 1995). Because of this

common basis, these methods share many strengths and weaknesses. Despite their theoretical similarity, however, these methods commonly play very different roles in geological

interpretation because each is sensitive to fundamentally different physical rock properties. The gravity method reveals information about the distribution of density, a bulk

property of Earth materials. The magnetic method, in contrast, is sensitive to the distribution of magnetic minerals (the most important of which are magnetite, Fe3O4, and
Geophysics in Mineral-Environmental Applications 35

its solid solutions with ulvospinel, Fe2TiO4) that are typically trace constituents of rocks

and sediments.

To the extent that the distributions of density or magnetization reflect geologically

significant features, interpretation of gravity and magnetic data can yield important threedimensional information about the distribution and structure of these features. For example, gravity data can be used to model the shape of a basin containing young (typically

low density) sedimentary rocks juxtaposed against (typically higher density) basement

rocks. Interpretation of magnetic data could provide complementary information in this

situation if there is a magnetic contrast between the sedimentary rocks and the basement

rocks, which is commonly the case. Mapping of the sediment/basement interface can be

important in understanding subsurface water movement.

Even in cases when there is insufficient information available to warrant detailed twoor three-dimensional modeling of potential field data, important conclusions can be drawn

about geologic structure, and structural trends can be deduced by examination and analysis of potential field images. For example, faults may often be traced by mapping offsets

of both gravity and magnetic source bodies.

Magnetic data are also sensitive to man-made magnetic objects such as pipelines,

well casings, and buried metal drums. This sensitivity is important for applications such

as finding abandoned wells and buried drums, locating and mapping waste dumps, and

hunting for unexploded metallic ordinance. Gravity data have a more limited application,

but detailed surveys can be used to locate shallow air-filled chambers such as buried

mine shafts or culverts. In theory, closely spaced gravity measurements could be used to

locate shallowly buried empty (or partly filled) drums. In practice, however, the expected

anomalies are so small that they would be difficult to distinguish from noise using present

instrumentation and data reduction methods.

Repeat gravity or magnetic measurements can provide low-cost, non-invasive monitor-
Geophysics in Mineral-Environmental Applications 36

ing of subsurface processes. High-resolution repeat gravity measurements may be used to

track changes in the water table or movement of unstable slopes. Magnetic measurements

can be used to monitor movement of placed magnets (buried for the purpose of tracking

settling or instability), or processes that create or destroy magnetic properties (such as

high-temperature fluid injection which could locally raise temperatures above the Curie

point where stable magnetization can exist for a given mineral).

Borehole gravity measurements yield accurate bulk rock densities. Combined with

laboratory measurements on core samples, or with information from other borehole logs,

these bulk densities can be used to deduce bulk porosity, an important hydrologic variable.

Possible research directions for potential field methods include development of improved interpretation techniques, particularly for three-dimensional interpretation and

integration with other data sets, and development of survey techniques for high-resolution

measurements of small areas.

6.6 Other geophysical methods

A number of methods listed in Table 1 are not discussed in the previous sections. These

include microwave (active and passive), borehole logging, GPS repeat surveying, laboratory measurements, and radiometrics. Active microwave techniques including SLAR

(side-looking airborne radar) and SAR (synthetic aperture radar) provide information on

surface texture, moisture, and vegetation cover. Current geographic coverage is limited.

Passive microwave measurements can also be made at comparable wavelengths (cm to

m) much in the same fashion as broad band thermal (both refer to emitted radiation

from the ground). The microwave region provides deeper penetration of the ground (typically meters) that can be useful for examining near surface heating (at depths below the

daily surficial changes) and detecting dielectric property changes that are associated with

dry versus wet ground and frozen versus unfrozen ground. The limitation is low spatial
Geophysics in Mineral-Environmental Applications 37

resolution (passive satellite systems have spatial resolution in the tens of kilometers in

contrast to active systems which can be better than 10m). Borehole logging includes virtually all geophysical methods that can be conducted on the ground and provide detailed

measurements for determining physical rock properties in a depth profile. In addition,

measurements can be made between holes or between the surface and the borehole to

provide a 3-D characterization (LaBrecque and Ward, 1990; Lo and Inderwiessen, 1994).

The method is extremely valuable for exploration, but, because it is highly intrusive,

use is often restricted in environmental studies. Repeat surveys or long-term base-station

monitoring using GPS receivers can be used in differential mode to track very small Earth

movements between stations to measure subtle ground movements which might be precursors of landslides or other hazardous ground movement. Laboratory measurements refers

to the empirical characterization of physical rock properties such as porosity and permeability and their relationship to other physical rock properties that are directly measured.

Radiometrics, commonly referred to as gamma ray spectrometry, is employed from airborne platforms, hand held instruments, or downhole logging to measure the radioactivity

of the ground. Airborne surveys have been used to estimate abundances of potassium,

thorium, and uranium for a variety of activities including regional lithologic discrimination, mineral exploration, and environmental background studies (Nielsen and others,

1990; Greg and Holmes, 1990; Darnley and Ford, 1989; Graham and Bonham-Carter,

1993; Dernley, 1993; Smith, 1985; Raghuwanshi, 1992; Duval, 1989; 1991).

7 Glossary of Terms

Acid rock drainage - Acidic water, often rich in heavy metals, which results from interaction of oxygenated surface water (rainwater, snowmelt, pond water) and shallow

subsurface water with many types of rock that contain sulfur-bearing minerals.
Geophysics in Mineral-Environmental Applications 38

Alteration - A general term to describe a group of geochemical reactions (including

oxidation, hydration, and hydrolysis) that modify the original composition of a

mineral.

DC resistivity method - A geophysical method for determining subsurface resistivity

(to infer geologic structure and stratigraphy) by injecting a direct current into the

ground and measuring the voltage produced at various locations.

Electromagnetic induction method - A geophysical method that uses a time-varying,

controlled, current source and measures the resulting electric and/or magnetic fields

to determine subsurface resistivity (to infer geologic structure and stratigraphy).

The method can be used with a variety of frequency ranges and sources.

Electromagnetic methods (EM) - A class of geophysical methods using measurement

of electric and magnetic fields associated with currents in the subsurface (to infer

geologic structure and stratigraphy). The methods include both controlled and

natural sources.

Geoenvironmental studies - Work that furthers the understanding of environmental characteristics of geological materials, both in natural and man-made settings.

Geographic information system (GIS) - A computer system of software and hardware

that enables the user to store, retrieve, and interact with map-based data.

Gravity method - A geophysical method that uses measurements of the spatial variation

of the force of gravity as a way to infer variations of density in the subsurface.

Ground penetrating radar (GPR) - A high-resolution electromagnetic geophysical technique operating in the megahertz frequency range which propagates electromagnetic

waves into the ground and records the waves reflected by electrical property bound-
Geophysics in Mineral-Environmental Applications 39

aries (dielectric permittivity and conductivity). The depth of investigation is limited

by highly conductive materials such as mineralogical clays.

Induced polarization method (IP) - A geophysical method used to map the presence

of disseminated metallic grans and clays involving the measurement of electrical

polarization which occurs at pore fluid-metallic mineral grain boundaries when an

electrical current is passed through the material. Induced polarization can also be

produced by clay-filled pore spaces. This phenomenon results in a frequency dependent electrical resistivity, which is commonly diagnostic of the minerals present.

Imaging spectroscopy - A technique to map individual minerals using reflected solar light

over many very narrow spectral channels. Current systems have spectral widths of

10nm and 224 channels, operated from aircraft with spatial resolutions ranging from

20m to 3m.

Magnetic method - A geophysical method that employs measurements of the Earth’s

magnetic field to infer the distribution of magnetic minerals (most importantly

magnetite) in the subsurface.

Multispectral scanner - A satellite or aircraft system that measures reflected light in several channels, typically with spectral resolutions of about 100nm. Spatial resolution

of the systems are typically 5-30m. Multispectral scanner systems often include

a thermal infrared channel to measure the emitted ground radiation in order to

estimate surface temperature.

Radiometric method - A geophysical method that involves measurement and interpretation of natural radioactivity produced primarily by the elements uranium, thorium,

and potassium.
Geophysics in Mineral-Environmental Applications 40

Self-potential method (SP) - A geophysical method that measures changes in the natural

voltage between two electrodes in the ground. It can be used to map shallow sulfide

bodies and is also sensitive to fluid flow and subsurface temperature gradients.

Side-looking airborne radar (SLAR) - A radar signal is transmitted from an antenna

on an aircraft and the energy backscattered from the ground is recorded. A SLAR

database exists for parts of the United States at the USGS EROS Data Center.

Spectral thermal method - A multispectral technique which operates in the thermal infrared region of the electromagnetic spectrum to map rock-forming minerals (primarily silicates). Current systems (e.g., NASA-Thermal Infrared Multispectral Scanner)

operate from aircraft with a spectral resolution of 75-100nm and spatial resolutions

of 5-25m.

Synthetic aperture radar (SAR) - A technique used to acquire high-spatial resolution

radar data by adding together multiple signals acquired at different time instants.

Time-domain reflectrometry (TDR) - An electromagnetic measurement technique that

uses electromagnetic pulses transmitted along a probe pushed into the ground to

measure soil moisture.

Tomography - The use of measurements made by passing some form of energy through

an object to make a two- or three-dimensional image of the internal structure of the

object.

Very low frequency electromagnetic method (VLF) - A ground or airborne electromagnetic geophysical method that uses distant radio transmissions (3-30kHz) and measures changes in the tilt of the electromagnetic field to map near-surface conductors.

A variant of this method called VLF-R is used to map subsurface resistivity.
Geophysics in Mineral-Environmental Applications 41

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