INTRODUCTION

Geo-conductivity meters (GCMs) map the electrical conductivity of the earth. To maintain convenience of use and to avoid lateral inhomogeneity, GCMs explore depths from 1- to 60-m. Thus, GCMs help to distinguish materials at or near the ground surface, and to assess the geological continuity between sample locations.

Of the physical properties of geological materials, which include density, seismic velocity, susceptibility and capacitivity, electrical conductivity has the greatest range of values.[1] For example, crystalline rock can have negligible conductivity, most soils and porous rocks have moderate conductivity, sea-water and graphite are highly conductive, and the conductivity of millerite is one-twentieth that of copper.

The mapping of conductivity yields a great deal of information for many applications. GEOLOGICAL EXPLORATION has been prominent among these since the first electromagnetic instruments for geophysics were developed in the 1920s.[2] GCMs are most frequently applied to exploration for aggregates, aquifers, cavities, faults, and soils.

AGRICULTURE is the basis of second group of applications. GCMs are used to map the characteristics and extent of agricultural soils, manage irrigation, nutrients and yields, and monitor agricultural wastes.

ENVIRONMENTAL MONITORING applications deal with subsurface changes that extend from industrial activity at the surface. Such activity generally increases the conductivity of the subsurface through the introduction or redistribution of chemicals, and these can be spread by water in the ground. GCMs are well suited to indicating the extent, intensity, and change in these effects. The most common applications are the monitoring of contamination by wastes from base-metal mining, chemical manufacturing, coal mining and power generation, ferrous metals and metal processing, forestry and construction, mineral and ceramic mining and manufacturing, oil and gas production, road maintenance, as well as hazardous, miscellaneous and organic wastes.

Other forms of human activity, which are not necessarily related to water in the ground, can have varied effects on conductivity. BURIED-FEATURE DETECTION by GCMs has two typical and often related targets, i.e. the detection of buried metal and of disturbed ground. Applications are most frequent in the areas of archaeology, environmental assessment, and forensic and geotechnical investigation.

Natural materials and artificial features share a broad range of conductivity. Accordingly, the various applications of GCMs have much in common with regard to instrumentation and technique. Thus, examples of one application often are relevant to other applications. Another tie between applications arises from the ability of instrumentation to detect, simultaneously, features of contrasting character.

Some of the common aspects of applications are mentioned in the descriptions that follow. Within each category, endnote references to published examples are ordered, generally, from most- to least-recent.

GEOLOGICAL EXPLORATION

The most common of the geological materials sought in GCM surveys are aggregates, aquifers, cavities, faults, and soils. Compared to other EM instruments, GCMs are used infrequently for mining exploration; exceptions are exploration for kaolin,[3]^,[4] kimberlite,[5] and for oil-sands,[6]^,[7] where bitumen-saturation decreases conductivity.

Aggregates

Sand and gravel are resistive materials, and aggregate deposits are resistive if their pore fluid is resistive. GCMs can define the limits of deposits if these materials are contained in more conductive material such as clay or silt, and if the limits lie well within the depth of exploration. Cases describe exploring:

· a gravel deposit and the thickness of overlying clay,[8]

· a gravel deposit from surface to a 15-m depth,[9]

· shallowly-buried gravel, sand and silt horizons,[10]

· sand and gravel under overburden,[11] and

· reconnaissance surveys for sand and gravel.[12]

Deposits of aggregates are hydraulically conductive, and some cases in Aquifers deal with exploration for aggregates below the water table. Where soil stability is of interest, the cases are found under Soils. Aggregates containing wastes are found under ENVIRONMENTAL MONITORING.

Aquifers

Of the many types of aquifers, GCMs map those with an electrical conductivity that contrasts with surrounding material. Some aquifers are relatively conductive, such as:

· troughs in deeply weathered bedrock, and

· sediments on resistive bedrock.

Resistive aquifers include:

· coarse material bounded by conductive sediments or bedrock, and

· fresh portions of saline aquifers.

Troughs in Deeply Weathered Bedrock

Intense weathering of bedrock tends to increase its porosity and electrical conductivity.[13] Aquifers can form where weathering is particularly extensive or deep, such as in fracture zones. Published examples describe the use of GCMs to:

· locate aquiferous faults associated with photo-lineaments in Brazil,[14] western[15] and southern Africa,[16]^,[17]

· double the success-rate of drilling wells in fractured bedrock,[18]^,
[19]

· locate aquifers in bedrock and alluvium with reference to modelled conductivity,[20]^,[21]

· site wells in weathered granite, fractured granite and fractured diabase,[22]

· map aquifers of weathered basalt in sedimentary and metamorphic rock in Yemen,[23]

· locate bedrock fracture zones under the Kaduna plain, Nigeria,[24]

· site wells and investigate a correlation between conductivity and water discharge,[25]

· delineate moderate to large faults in granite,[26]

· site 364 productive wells out of 419 boreholes to the top of bedrock,[27]

· increase the success rate of drilling productive wells,[28] and

· determine the depth to bedrock at a well site.[29]

Sediments on Resistive Bedrock

Glacial and alluvial material deposited in lows on the bedrock-surface form aquifers that can be mapped with GCMs, where the aquifer has moderate conductivity and the bedrock is resistive. Examples include:

· fractured metasediments in South Australia,[30]^,
[31]

· tracing crystalline-bedrock fractures beneath a dry river-channel,[32]^,
[33]

· mapping marine clays over fractured granofels, and thin soil over jointed sedimentary rock,³³

· siting a high-capacity well in fractured dolomite,[34]

· mapping the thickness of coarse glacial sediments on crystalline bedrock,[35]

· locating productive wells on fractures in karst,[36]

· locating a well and potential sites in fractured dolomite, sandstone and granite,[37]

· profiling the depths of the water table and fractured gneissic bedrock beneath a surficial aquifer,[38]

· profiling fractures in water-yielding zone of dolomite,[39]

· identifying an aquifer leading down-gradient from a hazardous waste site,[40]

· mapping 10 ha of fractured bedrock near a proposed site for low-level radioactive waste,[41]

· detecting saturated material in an alluvium-filled basin[42] and aquifer,[43] and

· tracing a water-bearing fault in granite.[44]

Coarse Material Bounded by Conductive Sediments or Bedrock

Sands and gravels saturated with freshwater are more resistive than aquitards that are rich in clay or carbon. GCMs can map these conductive aquitards, and thus infer some boundaries of the aquifer, as in the following examples of:

· a coase-grained gap in a riverbed aquitard,[45]

· coarse channels in glacial sediments,⁶

· channel sand surrounded and thinly covered by clay,[46]

· sands and silt bounded by clay,[47]

· fine-grained sediments in a Nevada basin,[48]

· coarse channels in fine-grained overbank deposits[49] over fractured bedrock,[50]

· a sand-filled fracture in clay-bearing limestone,[51]

· coarse glacial material on sedimentary bedrock, on fine-grained glacial material, and within fine-grained glacial material,³⁵

· clay and sand on shale,[52]

· clay and sand in a regional aquifer,[53]

· clay separating two coastal-plain aquifers,[54]

· gravel lenses on the bank of the Red Deer River,[55]

· small channels of alluvium on shale and siltstone,[56]

· buried river-channels beneath the northern plain of Haiti,[57]

· paleochannels on shale bedrock in Abu Dhabi[58] and in river deposits in Niger,[59]

· low-sinuosity channels, meandering channels and floodplain deposits in Nigeria,²⁰

· layering of coarse- and fine-grained material in glacial aquifers,[60]

· sand channels in clay-rich deposits,[61] and

· an area of 375 km², where clay of varying thickness caps aquifers in sandstone and dolomite.[62]

Fresh Portions of Saline Aquifers

As saline water is denser than fresh water, saline aquifers can support lenses of fresh-water near the water-table. Examples describe mapping:

· coastal environments on a barrier island,[63]

· salinization of soil and an aquifer from a flowing artesian well,[64]

· tidal infiltration and flow-channels for saltwater in beach sands,[65]

· seawater intrusion near a canal,[66]

· the freshwater lens beneath Isla de Mona, Pueto Rico,[67]^,
[68]

· reef-facies limestone,[69] and freshwater in 20 km² of reef-facies limestone in south Florida,[70]

· the depth to saltwater in sand under Long Island,[71]

· salinity in the Mississippi delta,[72]

· freshwater lenses beneath atoll islands in Micronesia,[73]

· freshwater lenses under islands composed of sand and oolitic limestone,[74]^,
[75]

· the depth to saltwater in Kent,[76] Cape Cod,[77] and Florida,[78]^,
[79]

· freshwater lenses in salty tidal-sand deposits,[80]

· a plume of brackish water from an artesian well,[81]^{, [82]}

· a channel of gravel saturated with brackish water at a depth of 60 m,[83] and

· freshwater accumulations in alluvial sediments in the Punjab.[84]

Cases of exploration for sand-and-gravel as a mineral resource are found in Aggregates; geotechnical cases of unconsolidated materials are under Soils, and of bedrock are under Cavities and Faults. Aquifers affected by waste are under ENVIRONMENTAL MONITORING.

Cavities

Detection of air-filled cavities is difficult with EM, as there is no response from the air, and materials that sustain cavities tend to be resistive as well. If the material around the cavity is conductive, EM response will fluctuate, but interpretation remains difficult. Detection becomes easier as the conductivity of the material filling the cavity increases. GCMs have been used to:

· confirm zones of soil piping over sinkholes,[85]^{, [86]}

· identify undocumented excavations in a coal seam beneath about 9 m of clay, shale and siltstone,[87]

· map sinkholes partially filled with clay,[88]

· map conductivity over snake hibernacula,[89]

· map loess-filled fractures in limestone,[90]

· locate sediment-filled cavities in limestone,[91]

· find dolines[92] and a collapse-feature,[93]

· identify unstable cavities in clayey soil,[94]

· delimit active- and incipient-sinkholes in karst[95] and voids in limestone,[96] and

· map prehistoric fireholes in brown coal, now filled with unstable peat and clay.[97]

Cases of cavities or unstable ground created by human activity are found under BURIED-FEATURE DETECTION.

Faults

In resistive bedrock, the increased porosity of fracture-zones can increase conductivity, especially if the pore-fluid is conductive. Fault-mapping helps predict the stability of ground and structures, where the ground will come under stress due to human activity or natural phenomena. GCMs have been used to:

· delineate recently active portions of macroscopic fault zones in Belgium[98] and New Zealand, [99]

· infer the location of a fault between sandy and clayey soil,[100]

· identify fractures[101] and karst fissures[102] near landfills,

· locate faults in siltstone and shale beneath a thin cover of clay,[103]

· locate leaking faults in gypsum bedrock at dam site,[104]

· map seeping faults beneath and adjacent to a dam,[105]

· map vertical fractures in gypsum,⁸² and

· map troughs in bedrock associated with phyllite at a development site.[106]

Cases in which the water-yield of faults is of greatest interest are found under Aquifers, and cases where faults are the conduits of waste are found under ENVIRONMENTAL MONITORING.

Soils

The mapping of soil-types, including frozen ground, is of great importance in geotechnical assessment. Soil conductivities grade through a middle range of values. Soils show a characteristic increase in conductivity with clay-content, and the conductivity of coarse soils is strongly influenced by that of the pore-fluid they contain. Freezing greatly reduces the conductivity of soil. GCMs have been used to investigate:

· salt storage and transport in South Australia,[107]

· water content of Chihuahuan Desert soils,[108]

· alluvial materials and paleochannels,[109]

· distribution of arsenic in deltaic sediments,[110]

· heterogeneity in glacial deposits that affects groundwater flow,[111]

· confining glaciomarine sediments in peatland,[112]

· the depth of dry, residual soil on basalt,[113]

· the extent of salinization in an area of salt scalds,[114]

· the limits of a landslide of gypsum and alpine soil,[115]

· buried paleochannels of sand and clay in the flood plain of the Mississippi River,[116]

· the thickness of clay soil over void-bearing limestone,[117]

· earthquake-induced liquefaction features,[118]

· river-channel sediments,[119]

· a delta of glacial sands,[120]

· clay layers in glacial sediments,[121]

· desert caliche,[122]

· variations in sandy soil at an archaeological site,[123]

· fine- and coarse-tills in forest soils,[124]

· water content of desert soils[125] and the potential for riparian restoration[126] in New Mexico,

· the hydraulic conductivity of coarse soil[127],

· sand and clay at the bottom of the Delaware River shipping channel,[128]

· sand and clay in buried valleys,[129]

· clays, sand and gravel above carbonate bedrock,[130]

· sand and gravel beneath till,[131]

· unconsolidated materials, shales and sandstones under till,[132]

· buried glacial channels on chalk bedrock,[133]

· overburden depth at a limestone quarry,[134] and

· changes in the depth of glacial material on crystalline bedrock.[135]

Investigations of frozen ground have included:

· the thickness of sea-ice, [136]^{,
[137],
[138],
[139],
[140],
[141]}

· the active zone in river valley sediments,[142]

· shallow unfrozen zones in the Mackenzie delta,[143]

· frozen zones in silt on a highway route,¹⁰

· permafrost at a construction site,[144] and

· 800 km of discontinuous permafrost along a pipeline route.[145]

GCMs have characterized corrosive ground to aid in the design of cathodic protection systems.[146]

A separate section is devoted to agricultural soils. Cases dealing with soil and groundwater are found under Aquifers, cases dealing with soil and industrial wastes are found under ENVIRONMENTAL MONITORING, and disturbed soils and soils containing metal are found under BURIED-FEATURE DETECTION.

AGRICULTURE

In addition to mapping the conductive nature of agricultural soils, GCMs find routine application in assessing the effects of fertilization, irrigation and cropping, and in monitoring agricultural wastes.

Agricultural Soil Mapping

GCMs are sensitive to both the amount and the ionic characteristics of clay and moisture in soils. In agricultural soils, GCMs have been used to research:

· the density[147] and strength[148] of claypan soils,

· compaction,[149] spatial variability,[150]^{,
[151],
[152]} and salinity[153] in fine soils of central California,

· clay content in the lower Macquarie Valley, NSW,[154] at twelve sites across the north-central USA,[155] and in Arizona,[156]

· soil-water content in a vertisol and a clay loam,[157]

· vertisol depth to carbonate bedrock,[158]

· the sampling of soil properties that influence seed-cotton yield,[159]

· soil moisture and salinity at twelve sites through western North America,[160]

· clay content and cation exchange capacity on 4 fields in Illinois and Missouri,[161]

· boundaries between clay loam and sandy loam,[162]

· lateral variability of salinity in a surface-irrigated olive plantation,[163]

· potential crop-management zones in two paddocks,[164]

· depth of salinity in southeast Queensland,[165]

· waterlogging, saline and acid land-degradation,[166]

· the effective cation exchange capacity in an irrigated-cotton field,[167]

· coarse-loamy to fine-loamy soil gradation, related to crop yield,[168]

· variations in soil morphology in the southern Mississippi Valley silty uplands,[169]

· wetland boundaries in South Dakota,[170]

· clay content of soils in the central US,^{[171], [172]}

· depth profiles of conductivity in saline soils in California,[173]

· the clay content of alluvial soils in southeastern Missouri,[174]

· the effect on conductivity of soil water-, clay- and carbonate-content and soil temperature,[175]

· gradational boundaries of moraine soils,[176]

· salinity in reclaimed coastal land,[177]

· salinity in the Tragowel Plains,[178] Wimmera,[179] Kyvalley[180] and Pyramid Hill[181] areas, and the source of subsurface salinity[182]^,
[183] in Victoria, Australia,

· salinity in locally irrigated grazing- and cropping-paddocks in Tasmania,[184]

· clay-content variability in flat, loamy fields in Brandenburg,[185]

· subsurface flow[186]^{, [187], [188]} and leaching[189] in loessial soils,[190]^{, [191]}

· clay content,[192]^, and clay-, moisture- and chloride-content in semiarid soils,[193]

· topsoil depth,[194]

· salinity in somewhat poorly drained flood-plain alluvium,[195]

· aeolian and debris-flow sediments that bear saline scalds,[196]^{, [197]}

· salinity remaining from old geomorphology,[198]

· parna (clay-size aeolian sediment), as an agent of soil salinification,[199]

· subsurface flow in loessial soils,

· smectite-clay and sand in an area of dryland salinity, [200]^,
[201]

· topsoil thickness over claypan,[202]^{, [203],
[204]}

· steppe-region loess thickness over basalt,[205]

· moisture in medium textured soils in Tunisia and Mexico,[206]

· soil conditions that stunt the growth of cotton,¹¹⁸

· the depth of rangeland soils,[207]

· soil-water content and salinity,[208]

· floodplain clay-loam,[209]

· salt-tolerance of trees and grasses in Alberta and Western Australia,[210] and identify areas suitable for small plantations of eucalyptus,[211]

· saline seeps and recharges in deep, well-drained soils in Kansas,[212]

· flood-deposited sand on river-bottom farmland,[213]^,
[214]

· soil-solution partition coefficient in loamy soils,