Airborne Electromagnetic Methods Research Articles

Electrical Structure of the Regolith in the Lawlers District, Western Australia

The weathered rock and transported overburden, which covers much of Australia, reduces the effectiveness of electromagnetic (EM) surveys. This material, termed the regolith, is variably conductive, both laterally and vertically, and reduces the sensitivity and penetration of EM systems.AMIRA (Australian Mineral Industries Research Association) project 407 was set up to improve the effectiveness of EM and especially airborne EM (AEM) in Australia. One of the specific aims was to study the electrical structure of the regolith in the expectation that a better understanding might lead to a significant reduction, or even removal, of its effect on deep penetrating AEM methods. As part of that study, several sites around Australia were selected for a detailed study of the electrical structure of the regolith and its effect upon AEM systems. This report discusses the results from a SALTMAP survey over the Lawlers area, which lies within the Eastern Goldfields region of the Yilgarn Craton in Western Australia.To test the accuracy and resolvability of the AEM data, a number of AEM responses were followed up with ground EM surveys. Both sets of data were interpreted using layered earth inversion algorithms. It was appreciated that a stitched set of 1D layered structures would not accurately model the physics of all of the responses, however it was expected that the vast majority could be usefully treated this way. The results show that although there may be up to six or even seven geologically identified layers with distinct electrical properties in the regolith, neither the airborne or surface EM data can effectively resolve more than three layers. However, good fits to three layer models can be obtained and much can be inferred about the nature of the regolith. At Lawlers, these layers can be broadly represented as follows: (1) the (mostly) resistive surface layer of colluvium/alluvium; (2) underlying conductive saprolite, mottled clays, etc.; and (3) resistive basement.

Beyond bump finding – airborne electromagnetics for mineral exploration in regolith dominated terrains

Airborne Electromagnetic (AEM) methods are very good for detecting conductive ore bodies within a resistive host, but have not been as successful for detecting ore bodies located beneath conductive regolith. We believe that in the effort to improve the ability of AEM systems to function as “bump finders” in regolith dominated terrains, the potential of AEM to contribute to other spheres of exploration activity in these settings has been neglected.The evidence available from Australian surveys suggests that AEM could be an effective alternative to aeromagnetics when mapping geology through cover – particularly where the rocks are magnetically quiet and the presence of transported materials precludes the use of multispectral remote sensing data or airborne radiometrics. However, the success of AEM as a mapping tool in regolith dominated terrains is dependent on an adequate contrast in the conductance or conductivity-thickness product of saprolite developed over different rock types. An example where this contrast is present is in the vicinity of the Ida Fault in Eastern Goldfields Province of the Archaean Yilgarn Craton, Western Australia, where AEM has successfully mapped variability in the thickness and conductivity of saprolite developed over felsic and mafic rocks.Contrasts in the conductance of saprolite derived from different lithologies may be obscured by saline ground-water. Saline groundwater may also blur differences in the conductivity of in-situ and transported materials. This is the case in the Black Flag area, Eastern Goldfields Province, Yilgarn Craton, where AEM appears to map total regolith thickness but fails to differentiate between the saprolite and palaeochannel clays.On the other hand, if AEM can provide information on groundwater conditions, then the technology may have a role in mapping the dispersion of solutes within the regolith and in identifying possible sites of supergene enrichment. The potential of AEM in this regard is illustrated with reference to data from the Lawlers area, also in the Eastern Goldfields Province.

Sea ice thickness mapping by airborne and ground EM methods

An experimental campaign was carried out in 1991-1997 in the Baltic Sea, a body of brackish water with resistivities of 3 to 4 ohm-m, to test the ability of airborne and ground electromagnetic methods to map sea ice thickness. The measurements were made as part of the European research projects Baltic Experiment for ERS-1 and Local Ice Cover Deformation and Mesoscale Ice Dynamics.Measurements on sea ice were carried out using a multifrequency, horizontal and vertical coplanar loop-loop system with a coil separation of 10 m. The purpose of these measurements was to obtain detailed information about conductivity properties of the ice, including keel ice rubble.The first interpretation was made by using a layered-earth model. To correct for small variations in coil separation which primarily influences the in-phase component, coil separation was treated as an unknown parameter in the inversion. A three-dimensional model was used to interpret more complicated ice geometry in ridge areas. The results obtained were validated by drilling.Airborne measurements were made with a vertical coplanar coil configuration where the coils were mounted on the wing tips of a Twin Otter aircraft, at a frequency of 3.1 kHz. A laser profilometer was used to map surface topography. Test measurements were made along 1 to 2 km long profiles. In good conditions the thickness accuracy achieved was ±0.2 m for undeformed ice, but worse for deformed ice with complicated geometry.EM measurements on sea ice were found to be a practical tool if the detailed structure of ice and ridges is needed whereas AEM is superior if large coverage is essential.

Delineating mineralization and imaging the regional structure with magnetotellurics in the region of Chibougamau (Canada)

Tensor magnetotelluric (MT) soundings were carried out to delineate the geometry of an extended shallow conductor detected by airborne electromagnetic method in the mining camp of Chibougamau (Canada). Careful distortion analysis of the impedance tensors from eleven sites indicate that the subsurface is reasonably 2-D within the 0.0025–10s range. The recovered regional impedances are corrected for static shifts employing 2-D inversion for minimum structure. The modeled resistivity cross‐sections show a far more complex conductivity structure than anticipated. The sheet‐like shallow conductor lies near the boundary between the conductive southern section located in the Abitibi subprovince and the resistive northern section located in the Opatica subprovince. It extends to a depth of 250 m. Two other limited conductive zones are imaged respectively between 0.4 km and 3 km and between 1.5 km and 4 km. We propose that the conductors are associated with sulfide mineralization and graphite in shear zones. They are delimited by steeply south dipping thrust faults. The northern resistive section does not show clear resistivity changes up to a depth of 15 km. Gravity data collected along the MT profile have provided additional constraints. Both MT and gravity models are consistent with one another and with the known regional geologic structure. They indicate that the Abitibi greenstone belt is 3.5–4.4 km thick in the survey area and that the steeply dipping geology at the surface is flattening at depths of 4 km approximately.

Modeling of broadband airborne electromagnetic responses from saline environments

The removal of vegetation for the development of nonirrigated agriculture and the associated increase in groundwater recharge and discharge has caused significant areas of salinization of surface soil and water resources in Australia. At least three types of salt profiles are known to indicate the relative magnitude of recharge. These profiles may be differentiated by their resistivity structure. Since a broadband airborne electromagnetic (AEM) method offers the possibility of readily obtaining resistivity soundings, modeling was carried out to investigate the ability of a broadband AEM system to distinguish different salt profile types. Salt profile types may be represented by a four‐layer resistivity model. The use of a broadband AEM system to distinguish the relative magnitude of the resistivity of a layer of high salt accumulation and the underlying layer forms the basis for efficiently identifying areas of high or low recharge. Where the resistivity of the underlying layer is greater than that of the salt accumulation, high recharge is indicated, and a lower resistivity of this layer implies low recharge. The response of each of the salt profile models was calculated in the frequency domain and then inverted back to a layered model. With noise added to the calculated responses, the inversion results show that the depth, thickness, and resistivity of a layer of high salt accumulation can be resolved by AEM measurements. Furthermore, the resistivity of this layer can be distinguished from the resistivity of the underlying layer. A high‐recharge profile may therefore be differentiated from a low‐recharge profile with AEM measurements. Since the quadrature component of the AEM response is relatively unaffected by noise caused by the primary field, the effect of using solely the quadrature component of the response was examined briefly as a second part of the AEM modeling investigation. It is found that simultaneous inversion of the quadrature part of the spatial components measured along the line of flight and in a vertical direction gives results similar to those when both the in‐phase and quadrature parts of these components are used in the inversion.

New airborne electromagnetic method provides fast deep‐target data turnaround

Airborne electromagnetic methods have been applied extensively over the last four decades to exploration for base and precious metals, salinity studies, and geologic mapping. They are becoming more favored as rapid reconnaissance methods to cover large tracts of land, particularly in areas of soil cover. Moving source AEM systems have improved much over the last decade such that lower frequencies and more precise measurements can be made thereby increasing their effective depth of investigation. Further improvements hinge on innovations in data collection and processing.

Subsurface Structure of Unzen-Fugen and Mayu-yama Volcano Inferred from Airborne Electromagnetic Method and Magnetic Survey

Subsurface Structure of Unzen-Fugen and Mayu-yama Volcano Inferred from Airborne Electromagnetic Method and Magnetic Survey

The geoelectrical methods in geophysical exploration : M.S. Zhdanov & G.V. Keller, (Elsevier; Methods in Geochemistry and Geophysics, 31), ISBN (hardback) 0 444 89678 3, price Dfl 375.00 (US$214.25), 1994, 873 pp, index

Part 1 Electrical methods in geophysics: in the beginning the mind set and the future. Part 2 General concepts of electromagnetic field behaviour: Maxwell's equations diffusing electromagnetic fields static electromagnetic fields quasi-stationary electromagnetic fields. Part 3 Properties of rocks and minerals: properties and units properties in a parametric sense properties in an existential sense geoelectrical mesostructures and megastructures electrical properties of the atmosphere and the ocean. Part 4 Electromagnetic environment of planet Earth: Earth currents of external origin industrial noise atmospheric electric sources mechanically generated electric fields extraneous sources of electric fields electromagnetic fields generated by the oceans. Part 5 Direct current and induced polarization methods: point and dipole sources on a uniform Earth electric fields in one-dimensional Earth structures fields of a point source for two- and three-dimensional structures vertical electric sounding and apparent resistivity induced polarization magnetic field measurements. Part 6 Natural-field electromagnetic methods: the Tikhonov-Cagniard approach linearity of relationships between components of the magnetotelluric field electromagnetic fields in a horizontally homogenous medius electromagnetic fields in horizontally inhomogeneous media magnetotelluric and magnetovariational survey methods data reduction interpretation of MT data interpretation of MVP and GDS data. Part 7 Controlled source electromagnetic methods: principles of controlled source electromagnetic methods electromagnetic sounding electromagnetic profiling. Part 8 Modelling and simulation: exact analytical solutions for special cases thin sheet models the integral equation method finite difference modelling finite element method physical and analog modelling other approaches to modelling. Part 9 Insensitivity and ambiguity: insensitivity for a laterally uniform Earth ambiguity for a laterally uniform Earth. Part 10 Practical aspects of data acquisition: sources detection of electric and magnetic fields sampling filtering survey design. Part 11 Interpretation: rules, intuition and pragmatic interpretation pseudo-inversion of soundings by transformation inversion continuation of electromagnetic fields electromagnetic migration comprehensive three-dimensional interpretation. Part 12 Other platforms, other methodologies: airborne electromagnetic (AEM) methods marine electrical surveys bore hole assisted methods spontaneous polarization (SP) method ground penetrating radar (GPR) the piezoelectric method (PEP). Part 13 A baker's dozen of case histories: minerals exploration - Que River Deposit, Tasmania, Australia physical volcanology - Oshima volcano, Japan groundwater exploration - Apodi Valley, Rio Grande do Norte, Brazil. (Part Contents).

The Use of Broadband Electromagnetic Methods to Differentiate Layered Models of Soil Salinity Profiles

The removal of native vegetation for the development of land has caused the salinisation of surface soil and water resources in Australia. It is important to determine the existence of potential salinity hazards so that preventative measures may be taken at an early stage. The current hydrogeological conditions and the topography of a given catchment influence the movement of saline groundwater, and it is of particular importance to identify areas of high- and low-recharge to groundwater. On a catchment scale, the delineation of high-recharge areas by drilling can be prohibitively expensive, and more efficient mapping methods are required. Broadband electromagnetic (EM) sounding methods offer the possibility of obtaining the vertical conductivity distribution in the regolith, to produce three-dimensional maps of the resistivity structure. These maps could be used to interpret the distribution of features related to hydrology, such as recharge and stratification of stored salt in catchments. For salinity-related problems, a regolith depth of about 50 m is relevant. It is therefore necessary to measure the high-frequency content of the EM response to differentiate shallow conductive layers. For both ground-based and airborne EM methods, the response has been modelled in a frequency range of a few hundred Hz to 50 kHz. Salinity profile types have been identified to characterise different types of recharge. These profile types have been simulated with models consisting of horizontal layers of different resistivity. The theoretical modelling has determined that EM methods can be used to distinguish different types of salinity profiles. These results are supported by inversion of EM data collected in the field in a number of catchments. The transient electromagnetic (TEM) method is used for ground-based broadband EM measurements, and modelling shows that both the depth and thickness of the salt accumulation can be resolved when its depth is as little as 6 m and its thickness 10 m. Additionally, a relatively resistive basement below the salt accumulation may be distinguished from a conductive one. Since the former case implies the presence of a high-recharge salinity profile and the latter case a low-recharge profile, this method may be used to differentiate the two types of profiles. Down-hole conductivity logs in catchments near Collie and at East Yornaning, WA, confirm that different salinity profiles can be distinguished with EM methods. At Collie, the TEM method has detected the presence of salt accumulations and has determined the depth of their lower boundary. Inversion of these results shows the presence of a resistive electrical basement below the salt accumulations, and therefore indicates that relatively high recharge conditions are associated with these salinity profiles. Similarly, measurements along traverse lines in the East Yornaning catchment show a correlation between the inversions results and known geological features.

International workshop on AEM methods

In the 1950s, 60s and 70s, airborne electromagnetic methods (AEM) were the principal reconnaissance tool in mineral exploration, particularly in North America and Scandinavia. Since metal prices collapsed in 1982, the bottom has fallen out of the AEM market; and utilization of the methods has decreased.

Use of airborne electromagnetic methods for resource mapping

Airborne electromagnetic (AEM) methods complement spaceborne remote sensing techniques. AEM surveys carried out from low flying aircraft are capable of detecting geological structures not visible on the surface. The flight height of AEM systems above the ground ranges from 30 to 120 m. Most systems generate primary EM fields by using a loop transmitter; conducting coils are used as antenna to measure the secondary magnetic field caused by conductive inhomogeneities in the ground. The frequency used in AEM surveys (100 Hz to 50 kHz) allows ground penetration in excess of 100 m. At present, two types of AEM systems are widely used: helicopter, frequency-domain, and fixed-wing, towed-bird, time-domain. The most common survey products are apparent conductivity maps. AEM methods are extensively used in prospecting for base and precious metal deposits, kimberlites, uranium, and also in geological mapping, groundwater exploration and environmental investigations.

A Comparison of Airborne and Ground Transient EM Systems in their Resolving Power against Geological Noise

Airborne electromagnetic (EM) methods are often used as a reconnaissance tool in mineral exploration. With the improvement of airborne transient EM (TEM) systems and interpretational techniques, more and more quantitative information can be obtained from airborne EM data. Thus we pose the question is it possible to develop airborne TEM technology to be as effective in orebody detection as ground TEM techniques in the future? Recently, we developed a fast approximate solution of transient EM response to a thin target buried under a thin conductive overburden. Using this solution, the signal-to-geological-noise ratios are computed for an airborne and a ground moving-loop configuration over a range of values of overburden conductance, target conductance and target depths. The results indicate that airborne TEM systems have potentially the same resolving capability as low or moderate power ground TEM systems for discrete targets. The key design features required to achieve this potential capability in airborne TEM systems are large system bandwidth (i.e. range of time windows) and large transmitter moment.

The inversion of time‐domain airborne electromagnetic data using the plate model

Airborne electromagnetic (EM) methods were developed in the early 1950s, mostly by Canadian mining exploration companies as a means of economically carrying out prospecting for sulfide deposits associated with volcanics in resistive shield areas. Present interpretation techniques are based on the use of nomograms but the approach is easily amenable to digital processing. For highly accurate interpretation, however, it is necessary to develop quantitative interpretation techniques that can make full use of all the data available. Inverse theory has been used for interpretation with great success in most geophysical disciplines; however, in airborne EM surveying, inversion has only been used for the interpretation of airborne EM data using half‐space and one‐layer models. By introducing some approximations to the rectangular thin‐plate model, it is now possible to apply inverse theory to the interpretation of time‐domain EM data. This approach provides estimates of the parameter errors, the correlation matrix, and a means of assessing the validity of the model. Synthetic profile data are used to demonstrate the validity of the method. The results of the inversion of real anomalies are compared with ground geophysical survey interpretation and drillhole data. The inversion results agree with the known geology of the area and the ground geophysical survey interpretation.

Airborne electromagnetic measurements of electrical conductivities of seawater and bottom sediments over Shallow Ocean

Abstract Experimental airborne electromagnetic (AEM) survey data collected in Cape Cod Bay are used to derive continuous profiles of water conductivity as well as bottom sediment conductivity. A commercially available Dighem III AEM system was used for the survey without any significant modification. The helicopter‐borne system operated at 385 Hz and 7200 Hz, both in a horizontal coplanar configuration. The interpreted profiles show good agreements with available ground truth data. Where no such data are available, the results appear to be very reasonable. The AEM method can provide a fast survey speed at a reduced cost to measure both seawater and bottom sediment conductivity profiles. Such information carries broad applications to offshore ecological, biological, and geotechnical studies.

Characterization of shallow ocean sediments using the airborne electromagnetic method

Experimental airborne electromagnetic (AEM) survey data collected in Cape Cod Bay are used to derive continuous profiles of water depth, electrical depth, water conductivity, and bottom sediment conductivity. Through a few well-known empirical relationships, the conductivities are used, in turn, to derive density, porosity, sound speed, and acoustic reflectivity of the ocean bottom. A commercially available Dighem III AEM system was used for the survey without any significant modification. The helicopter-borne system operated at 385 and 7200 Hz; both were in a horizontal coplanar configuration. The interpreted profiles show good agreement with available ground truth data. Where no such data are available, the results appear to be very reasonable. Compared with the shipborne electrode array method, the AEM method can determine the necessary parameters at a much higher speed with a better lateral resolution over a wide range of water depths from 0 to perhaps 100 m. The bottom sediment conductivity that can be measured by the AEM method is closely related to physical properties of sediments, such as porosity, density, sound speed, and, indirectly, sediment types that might carry broad implications for various offshore activities.

An overview of the geophysics activity within the canadian nuclear fuel waste management program

The concept of disposal of nuclear fuel wastes in igneous plutonic rocks is being assessed in the Canadian Nuclear Fuel Waste Management Program. The decision on whether to implement this concept will be made only sometime after 1991. Geophysical techniques are being assessed and developed, in order to map igneous plutons, the fractures within them, and also the overburden. Field tests at research areas have shown that gravity and magnetic methods can in most cases map the boundaries of plutons. Airborne EM and surface VLF-EM methods have been particularly useful in mapping near-surface vertical trending fractures. Deeper horizontal trending fractures have been located by reflection seismic and magnetotelluric methods. Shallow seismic, VLF-EM resistivity, radar and, to a certain extent, airborne EM methods can determine the nature of surficial materials. The sonar method provides information on lake bottoms. Several downhole logging methods can detect fractures that intersect a borehole; however, tracing the continuity of fracture zones between holes and determining their hydrogeological characteristics by geophysical means is considerably more difficult. In an underground research laboratory, currently under construction in a granitic batholith, several geophysical methods will be assessed, including radar for mapping fractures and ultrasonics for testing rock quality.

THE AIRBORNE ELECTROMAGNETIC METHOD AS A TOOL OF GEOLOGICAL MAPPING*

AbstractSince its development some thirty years ago, the airborne electromagnetic (AEM) method has been primarily used as a tool for mineral prospecting. However, advanced AEM systems are capable of other tasks, such as geological mapping and groundwater exploration. Excellent correlation between maps of apparent conductivity and geological maps was observed in several regions of Brazil where AEM surveys were performed. The degree of correlation seems to depend on the local climate. In humid and subhumid tropical regions, a weathered layer develops whose thickness and conductivity depend upon bedrock lithology. Therefore certain lithological types can be recognized from their conductivity signature; e.g., granites and Precambrian coarse clastic rocks are resistive, metavolcanic (particularly mafic) and volcanic rocks are conductive, Phanerozoic sediments are generally highly conductive.Two geophysical surveys are analyzed in the paper. The first was conducted with the time‐domain, towed‐bird AEM system in the Itapicuru greenstone belt in the state of Bahia. The apparent conductivity map correlated better with the local lithology than the magnetic map. Results of the AEM survey were successfully used to improve the regional geological map. A helicopter EM system was used in the second survey, which covered a portion of the Precambrian shield of Rio Grande do Sul. Also in this region ground checks confirmed the usefulness of conductivity surveys in geological mapping.The technique outlined in the paper holds great promise for countries of humid tropical climate, where few outcrops exist and access is often difficult. The tests performed in Brazil indicate that by executing AEM/aeromagnetic surveys during initial stages of mapping and exploration programs, time and expenditure required for geological field work can be considerably reduced. The resulting geological maps are more accurate and the inventory of mineral occurrences becomes more complete.

Geophysical Exploration for Precambrian-Related Uranium Deposits: ABSTRACT

Genetic models for uranium mineralization found at Proterozoic unconformities between fluvial sandstones and crystalline lithologies include three general geologic features: (1) structural relief at the unconformity; (2) permeable zones controlled by lithology and faulting; and (3) graphitic and sulfide-bearing zones commonly associated with mantled gneiss domes. Though radiometric surveys have been successfully used where the fluvial rocks are thin or eroded away, non-radiometric geophysical methods are commonly used in the interior parts of the sedimentary basins to detect one or more of the above geologic features to define favorable areas for uranium. In the Athasbasca basin of Canada, airborne and ground electromagnetic (EM) methods as currently used are thought to h ve a depth of exploration of 200 m. Application of these non-radiometric geophysical methods to Precambrian sedimentary basins in the United States is not likely to produce as great a depth of exploration because the fluvial cover rocks are not as geophysically transparent as the Athabasca sandstone. We suggest that two approaches be used to improve the End_Page 786------------------------------ depth of exploration and resolution of non-radiometric geophysical methods. The first approach is to use EM methods that have not been commonly used such as the controlled-source or natural-field audiomagnetotelluric method which can have a depth of exploration of several kilometers. The second approach is to use advanced geophysical interpretation methods to define responses from the alteration halos around the uranium mineralization. Advanced Induced Polarization surveys may detect such halos as evidenced by applications in Tertiary sedimentary basins. End_of_Article - Last_Page 787------------

Effect of tropical weathering on electrical and electromagnetic measurements

Electrical properties of the weathered layer in tropical regions of Brazil were investigated by means of resistivity soundings, airborne, and ground electromagnetic measurements. Five case histories illustrate how changes of climate, lithology, and geomorphology affect geophysical measurements. In humid and subhumid tropical regions (annual rainfall over 650 mm) the weathered layer is between 10 and 80 m thick and moderately conductive. Results from one region (Minas Gerais) indicate that excessive depth of weathering and leaching of massive sulfides, rather than the conductivity of overburden, present the greatest obstacle to effective use of airborne EM methods in mineral exploration. Seasonal variations of precipitation cause changes in soil resistivity, but such changes are not apparent in the underlying weathered layer. In semiarid and temperate regions of Brazil, the weathered layer is 10 to 20 m thick and regional airborne EM surveys are an efficient exploration tool. In all regions, the degree of weathering depends upon lithology and, in several areas, anomaly patterns obtained from airborne EM surveys correlate well with the surface geologic map. However, when comapring electrical properties of similar rock types among regions of the same climatic type, a considerable variation is observed. It seems that also geomorphology plays an important role in weathering. A careful interpretation of airborne EM data is necessary to distinguish anomalies caused by the weathered layer from those due to underlying conductors. Highly conductive, saline alluvia, which cause strong EM anomalies in Australia, were encountered (sporadically) in only one region of Brazil, the semiarid Valley of Curaçá, Bahia.

EARTH CONDUCTIVITY DETERMINATIONS EMPLOYING A SINGLE SUPERCONDUCTING COIL

A low‐frequency airborne electromagnetic prospecting method has been developed which exploits the inherent low resistance of a superconducting coil. Changes in the input resistance of this coil are monitored in the presence of the conducting earth. The response of the system, the change in the input resistance, is proportional to the quadrature secondary magnetic field at the transmitter, although unlike two‐coil systems, the response does not decrease with increasing frequency. This research has demonstrated that superconducting wires, large scale nonmetallic cryostats, the requisite measurement circuitry, and an appropriate data acquisition system are realizable in a practical flight configuration. The unicoil presents the following significant advantages in electromagnetic prospecting: 1) The measurement sensitivity is not limited by the relative coil motion experienced by two‐coil systems. 2) Ample field strength may be supplied to override ambient noise. 3) Optimum frequencies for specific geologic sections are easily implemented in the range of 10 to 2000 Hz. 4) Maps of ground conductivity may be obtained because precise thermal stability is maintained and the measurement, therefore, is absolute. 5) The point source observation minimizes analytic complexity. 6) The combination of the foregoing features with multiple frequency operation, yields a system of potentially high sensitivity and unprecedented depth of exploration. The unicoil system also possesses some disadvantages: 1) An operational complexity results from the cryogenic procedures required in the field, and 2) the heavy sensor requires a large helicopter.