Airborne Geophysical Methods Research Articles

Innovative airborne geophysical strategies to assist the exploration of critical metal systems

Critical metals are essential in sustaining the high technology and the green energy transition of modern societies. The future discovery of new critical metal deposits will likely be made at increasing depths and under thick cover sequences. The key roles of the four airborne geophysical exploration methods, gravity, magnetometry, electromagnetism and gamma-ray spectrometry, are reviewed in this article. The measured data from airborne magnetic, gravity and electromagnetic surveys can be inverted to reveal the distribution of underlying mineral prospects in terms of magnetic susceptibility, density and electrical resistivity/conductivity beneath the surface.The interpretation of geophysical data is important in relating geophysical responses to the lithology and geophysical anomalies to potential exploration targets that are concealed under cover. Gamma-ray spectrometry can identify near-surface hydrothermal alteration zones and uranium systems. Structural complexity maps can provide additional key parameters for the exploration targeting of structurally controlled critical metal systems. We briefly discuss the application of airborne geophysical methods to efficiently guide the exploration of concealed critical metal deposits. A robust understanding of the geological setting of the respective mineral prospect is the most relevant factor in choosing the most efficient geophysical exploration strategy. Geophysical tools will likely play an increasingly important role in guiding the future discovery of concealed critical mineral systems.

Detecting and recovering critical mineral resource systems using broadband total-field airborne natural source audio frequency magnetotellurics measurements

Airborne geophysical methods offer a substantial advantage compared to ground-based techniques in exploring territories of different sizes, ranging from entire metallogenic provinces to the deposit scale, including those hosting critical minerals. An airborne method with measurements of natural magnetic field variations, known as audio frequency magnetotellurics (a passive field method), significantly increases the depth of investigation and expands the resistivity detection range compared with some controlled-source primary-field methods. We describe the technical solutions used in an airborne electromagnetic passive system with a mobile sensor of the total magnetic field variations and the stationary sensor of electric field variations, and its applications to recovering the complex geology of hydrothermal-magmatic systems often associated with critical minerals. The system’s ability to explore depths, typically beginning from the near-surface and down to 1–2 km, by recording responses in three orthogonal inductive coils over a broad bandwidth from 22 Hz to 21,000 Hz allows for mapping resistivities across a broad range. This capability is crucial for obtaining more comprehensive exploration models. Field case studies of the natural field system include application in exploring for unconformity uranium mineralization, along with other associated minerals, epithermal gold and polymetallic-bearing structures, and ferromanganese and polymetallic deposits formed in a continental rift valley. An extra case study involving kimberlites was incorporated as a proven example of the natural field system’s capability in conducting near-surface and deep investigations. The case histories illustrate the airborne natural electromagnetic field technology capabilities in recovering geoelectric models and their specific patterns.

Predicted resource assessment of Central Kazakhsta ore districts based on airborne geophysical methods

Purpose. The research is aimed at creating a high-quality geophysical basis for the additional geological study of Central Kazakhstan in determining the geological structure of the study area, identifying promising areas for further geological exploration, as well as analyzing the anomalous distribution of various minerals. Methodology. The research uses the methods of aeromagnetic, airborne gamma-ray spectrometry and gravimetric (ground) surveys. The obtained new airborne geophysical data are used for additional study of the geological structure of the district and the creation of a model of the geological structure of the study area. Gamma-ray spectrometry data analysis is conducted for detailed mapping of intrusive complexes and study of their lithological heterogeneity. Findings. The processing and interpretation of the materials of the conducted field studies in combination with the results of the geological-geophysical data analysis made it possible to refine the geological structure and to present a model of ore-prospecting complexes in the study area. Areas with an anomalous distribution of potassium, uranium, and thorium have been identified, which make it possible to assess the structural heterogeneity of hidden magmatic massifs and to reveal a connection with gold, copper-polymetallic and rare-metal mineralization. Recommendations for further detailed geological exploration, including prospecting-exploration drilling, are given. Originality. Research has shown the high efficiency of airborne geophysical methods, such as aeromagnetic and airborne gamma-ray spectrometry surveys to study the geological structure of ore areas, and in combination with detailed gravimetric exploration to identify lithological heterogeneity of igneous and sedimentary rocks in the study area. New scientific-methodological techniques and the geological structure model based on them gave an opportunity to assess the prospect of the study area for ore mineralization. Practical value. The identified promising areas for mining of minerals are of interest for further research and exploration. The integrated use of geological-geophysical data will make it possible to determine more precisely the nature of radioactive anomalies and to reveal their relationship with ore-prospecting horizons. In general, the research results contribute to increasing the efficiency and reducing the geological survey costs.

Using Airborne Geophysical and Geochemical Methods to Map Structures and Their Related Gold Mineralization

Egypt’s mineral resources are an effective means of raising the country’s income. Consequently, searching for deep subsurface ore deposits is essential. In this study, we map the subsurface structure and the occurrence of related gold-ore mineralization deposits in ophiolitic-assemblage rocks around the Wadi El-Saqia area in the Central Eastern Desert of Egypt. Our approach combines airborne geophysical data (aeromagnetic mapping) and geochemical analyses. We enhanced the aeromagnetic data and interpreted them using edge-detection methods, such as the first-order vertical derivative (FVD), the analytic signal (AS), the total horizontal derivative (THD), the tilt derivative (TD), the tilt angle (TA), the theta map, both grid and porphyry analyses from the Centre for Exploration Targeting (CET), and Euler deconvolution (ED) techniques. Utilizing these methods, we located the main structural lineaments/contacts that control the distribution of hydrothermal alteration zones. In addition, our geochemical analyses use the mineral chemistry of pyroxene and plagioclase to describe their tectonic and magmatic evolution. The airborne geophysical results revealed that NW–SE, NE–SW, N–S, and E–W structural orientations are prevalent in the studied area, with depths ranging from less than 50 m to about 600 m. To validate the findings, we carried out geochemical sampling, which indicates that pyrite, galena, pyrrhotite, and electrum contain good percentages of gold (ranging from 0.01–0.09 wt%, 0.03–0.1 wt%, 0.12–0.14 wt%, and 53.55–55.01 wt%, respectively). In this study, we were thus able to find preferred locations for gold mineralization, which highlights the value of combining aeromagnetic and geochemical data for mineral exploration.

GSPy: A new toolbox and data standard for Geophysical Datasets

The diversity of geophysical methods and datatypes, as well as the isolated nature of various specialties (e.g., electromagnetic, seismic, potential fields) leads to a profusion of separate data file formats and documentation conventions. This can hinder cooperation and reduce the impact of datasets researchers have invested in heavily to collect and prepare. An open, portable, and well-supported community data standard could greatly improve the interoperability, transferability, and long-term archival of geophysical data. Airborne geophysical methods particularly need an open and accessible data standard, and they exemplify the complexity that is common in geophysical datasets where critical auxiliary information on the survey and system parameters are required to fully utilize and understand the data. Here, we propose a new Geophysical Standard, termed the GS convention, that leverages the well-established and widely used NetCDF file format and builds on the Climate and Forecasts (CF) metadata convention. We also present an accompanying open-source Python package, GSPy, to provide methods and workflows for building the GS-standardized NetCDF files, importing and exporting between common data formats, preparing input files for geophysical inversion software, and visualizing data and inverted models. By using the NetCDF format, handled through the Xarray Python package, and following the CF conventions, we standardize how metadata is recorded and directly stored with the data, from general survey and system information down to specific variable attributes. Utilizing the hierarchical nature of NetCDF, GS-formatted files are organized with a root Survey group that contains global metadata about the geophysical survey. Data are then organized into subgroups beneath Survey and are categorized as Tabular or Raster depending on the geometry and point of origin for the data. Lastly, the standard ensures consistency in constructing and tracking coordinate reference systems, which is vital for accurate portability and analysis. Development and adoption of a NetCDF-based data standard for geophysical surveys can greatly improve how these complex datasets are shared and utilized, making the data more accessible to a broader science community. The architecture of GSPy can be easily transferred to additional geophysical datatypes and methods in future releases.

An Assessment of Water Sources for Heritage Listed Organic Mound Springs in NW Australia Using Airborne Geophysical (Electromagnetics and Magnetics) and Satellite Remote Sensing Methods

Discrete phreatophytic vegetation associated with organic mound springs is present in several places in the semi-arid Walyarta Conservation Park (Park) in northern Western Australia. The mound springs are heritage listed, having significant cultural and environmental significance. Increased industrial (mining and agriculture) development in the region, coupled with a growing demand for groundwater to support these developments, requires an enhanced understanding of how the springs operate and the source of water that sustains their presence. The springs are broadly believed to be situated on geological faults and receive groundwater from artesian sources. However, their association with deeper geological structures and aquifer systems, the focus of this study, is not well understood. This study employed regional- and finer-scale airborne geophysical data, including electromagnetics (AEM) and magnetics, to constrain the sub-basin-scale hydrogeology of the West Canning Basin in Western Australia and to detail tectonic deformation, sedimentological and hydrological processes. The AEM data were inverted using 1- and 2D methods to better define structural discontinuities in the Park, and the results identified the location of faults and other geological structures that were coincident with spring locations. A complementary analysis of spatiotemporal patterns of green vegetation was undertaken using remote sensing data. A model for the extent of green vegetation (in percent), calculated using a constrained linear spectral unmixing algorithm and applied to a select Landsat Thematic Mapper ™ image archive, showed the persistence of green vegetation aligned with interpreted fault systems through extended dry periods. These geophysical and remotely sensed datasets demonstrate that in the Park, the sedimentary aquifers and landscapes are highly compartmentalized and that this constrains aquifer distribution, groundwater quality and the location of wetlands and phreatophytic vegetation. Integrating key information from these datasets allows for the construction of a three-dimensional model that predicts the nature and extent of the critical zone which sustains perennial groundwater discharge within mound springs, drainages and wetlands and provides a framework to assess discharge rates, mixing and, ultimately, sensitivity to changed water availability.

Airborne geophysical data interpretation of the southeast portion of Parnaíba Basin, Brazil

Airborne geophysical surveys are widely used in geological prospecting for hydrocarbon reservoirs. The efficiency and fast acquisition of these methods covering large areas, recognize them as a key tool for any prospecting project located in frontier basins, such as Parnaiba Basin, where there is a large amount of technical data. Among the airborne geophysical methods, gravity and magnetic methods, usually called potential field methods, are the most propagated in Oil and Gas projects of this nature. Such methods are used to support the generation of regional geological knowledge, detailed approaches, integrated with seismic, geochemical, and well data. Specifically, in the case of magnetometry, its use in more detailed specifications has been available in the Brazilian market since 1988 with the release of the GPS signal to the market and mainly after the replacement of the proton precession magnetometers by the cesium vapor (1991–1994), which is much more accurate in determining the anomalous magnetic field. In terms of airborne gravity, applications in detailed survey approach were not available to the Brazilian market until 2006. Investments in hydrocarbon prospecting in Brazil were concentrated in the promising Campos Basin, leading to few and almost no investments in other Brazilian sedimentary basins. The Parnaiba Basin is one of the Brazilian sedimentary basins with almost no prospecting investment. The aims of this work is a qualitative interpretation of magnetic lineaments and high gravity and structural lows, to suggest a map of the integrated structural context and of geological and geophysical methods of the southeast portion of Parnaiba basin. Using filters like the First Vertical Derivative and the Analytic Signal maps was possible to highlight the structural magnetic lineaments interpreted and a qualitative interpretation of geological intrusions and sills. On results, the data interpretation showed an important structural complexity and identified graben and horst structures usually favorable to the hydrocarbon trap occurrence, as well as an intrusive rock occurrence usually associated to organic matter maturation, in a higher concentration in the center region of the study area.

Kinematic investigations on the Furggwanghorn Rock Glacier, Switzerland

AbstractMany thermo‐hydro‐mechanical parameters and their interactions influence the creep behaviour of rock glaciers, with the result that the kinematics of rock glaciers are complex and not fully understood. An interdisciplinary project began in 2010 to investigate the physical ground conditions and any climatic dependence of a fast‐moving rock glacier below the Furggwanghorn peak, in the Turtmann Valley of the Swiss Alps. Remote sensing and subsurface instrumentation (in‐place inclinometers and temperature sensors), together with ground and airborne geophysical methods, reveal the creep rate and the hydrological and thermal conditions of the rock glacier. The Furggwanghorn rock glacier exhibits displacements in the order of metres per year and showed substantial surface changes at a minimal ground temperature slightly below 0°C. A detailed three‐dimensional kinematic ground model shows the internal structure and shear surfaces, indicating past creep behaviour and failure mechanisms. The results suggest that observed seasonal changes in the creep rate of the rock glacier did not depend directly on the temperature at depth, but were very likely controlled by hydrological processes.

Inversion of airborne geophysics over the DO-27/DO-18 kimberlites — Part 3: Induced polarization

The geologically distinct DO-27 and DO-18 kimberlites, often called the Tli Kwi Cho (TKC) kimberlites, have been used as a testbed for airborne geophysical methods applied to kimberlite exploration. This paper focuses on extracting chargeability information from time-domain electromagnetic (TEM) data. Three different TEM surveys, having similar coincident-loop geometry, have been carried out over TKC. Each records negative transients over the main kimberlite units and this is a signature of induced polarization (IP) effects. By applying a TEM-IP inversion workflow to a versatile time domain EM (VTEM) data set we decouple the EM and IP responses in the observations and then recover 3D pseudo-chargeability models at multiple times. A subsequent analysis is used to recover Cole-Cole parameters. Our models demonstrate that both DO-18 and DO-27 pipes are chargeable, but they have different Cole-Cole time constants: 110 and 1160 μs, respectively. At DO-27, we also distinguish between two adjacent kimberlite units based on their respective Cole-Cole time constants. Our chargeability models are combined with the density, magnetic susceptibility and conductivity models to build a 3D petrophysical model of TKC using only information obtained from airborne geophysics. Comparison of this final petrophysical model to a 3D geological model derived from the extensive drilling program demonstrates that we can characterize the three main kimberlite units at TKC: HK, VK, and PK in three dimensions by using airborne geophysics.

Geophysical expression of buried range-front embayment structure: Great Sand Dunes National Park, Rio Grande rift, Colorado

Great Sand Dunes National Park and Preserve (GRSA, Colorado) lies along the eastern margin of the San Luis Basin and the tectonically active Sangre de Cristo fault system that are part of the northern Rio Grande rift. GRSA lies within a prominent embayment in the range front where two separate sections of the Sangre de Cristo fault system intersect. Fault scarps are observed along both intersecting fault zones within older basin-fill alluvium, but have been obscured by the actively migrating dunefield. The dune sand is also strongly magnetic, locally limiting the usefulness of aeromagnetic methods for mapping concealed structure. This study uses airborne geophysical methods, primarily airborne gravity gradient data, along with constraints from geologic mapping and limited subsurface data and groundwater modeling, to interpret the subsurface basin geometry and range-front structure of the embayment. Using forward modeling of the gravity gradient data and locations of faults inferred from gravity gradient and aeromagnetic lineaments, several previously unrecognized tectonic elements are interpreted adjacent to the range front. Some of the largest rift-­related fault offsets are demonstrated to be basinward of the normal fault zones mapped at the surface along the range front of the Sangre de Cristo Mountains, along faults concealed under the dunefield and subparallel to the two fault sections. A fault-bounded structural bench, likely composed of Proterozoic rocks, underlies most of the high dunefield at depths of 500 m to 1 km. The bench is truncated on its southwest margin by a northwest-trending, southwest-dipping normal fault. A northeast-trending, northwest-dipping normal fault with ∼600 m of estimated relief lies under the southern margin of the dunefield and bounds a structurally higher bench of Proterozoic rocks concealed at <400 m depth near the range front. The northwest- and northeast-trending geophysical lineaments generally correspond well with the trends of faults mapped at the surface, and with both pre- and syn-rift structures in the Sangre de Cristo Mountains. Aeromagnetic anomalies are explained by variations in the magnetization of pre-rift rocks, and the strongly magnetic dune sand.

Locating hidden channels for placer gold exploration in the Cariboo District, British Columbia, Canada: A case study

During the past 150years, most of the modern day creeks were the target of miners roaming the Cariboo Mountains, British Columbia, in the search for placer gold. In these days, the probability to locate new placer gold occurrence in recent river beds is therefore substantially reduced. New, promising exploration targets appear to be channels mostly buried under alluvial cover sediments. It is airborne geophysical methods that can reveal hidden channels fast and cost-effectively as these penetrate the sub-surface contactless and reflect physical properties of the sub-surface, such as electric conductivity and magnetic susceptibility or magnetization, respectively.We applied the airborne geophysical exploration approach on four exploration areas in the Cariboo gold district. Helicopter-borne transient electromagnetic (TEM) and magnetic data were flown using the SkyTEM system. To our knowledge, it has been innovatory to apply high resolution, high density airborne geophysics in the search for placer gold deposited in pre-Holocene sedimentary channel fills of the Cariboo Mountains. A particular effort of our studies aimed at the Mary creek claims which straddle the boundary of the Quesnel and Kootenay terranes of the Canadian Cordillera and include the dormant Toop mine situated in the Mary creek area known for many finds of coarse nugget from the pre-glacial buried Toop channel. Our objective was to locate the southbound extension of the channel buried in Pleistocene sediments of the Toop plateau. Careful analysis of the airborne geophysical data sets provided indications from both the TEM and magnetic data sets favouring the existence of a hidden channel beneath the plateau.The evaluation of seven reverse circulation (RC) drill holes sunk into a promising elongated narrow conductor beneath the plateau was not conclusive as not clearly showing the sedimentary pattern of a channel with gravels typically at its bottom. Only electric conductivity-depth sections compiled from the airborne TEM and 2D direct current (DC) multi-electrode resistivity ground survey data enabled the interpretation of the airborne TEM and magnetic responses recorded over the Toop plateau. The sections suggest that the electric conductor is generated by an upwarp of a conductive layer extending at the bottom of the Pleistocene sediments. Another feature separated by ≤100m from the conductor line is reflected by low electric conductivity, but is rarely prominent through its neat magnetic signature. Fine accumulations of black minerals, i.e. magnetite grains, in sediments of the area are frequently met when panning material from the creeks. We therefore interpret this low conductivity, magnetic feature as expression of a gravel lense hosting accumulations of magnetite grains and possibly indicating the southbound extension of the Toop channel beneath the plateau.Careful analysis of the airborne magnetic data set led to the result in that magnetite is not only wide-spread in present day rivers and creeks, but also in buried channels and palaeo precipitation run-off paths. Magnetic data proved to be very helpful in this project with regard of pursuing not only present day, but buried valleys and channels, in particular.Our experience made on the Mary creek claims is summarized in a straightforward exploration concept for hidden, possibly gold-bearing channels in the Cariboo gold district.

Isidore Zietz (1919–2013)

Isidore “Izzy” Zietz, a leader in airborne geophysical methods, died 9 February 2013 at age 93. During a career that spanned more than half a century, he made fundamental contributions to the early development of potential‐field geophysics for airborne magnetic surveys; led the U.S. effort in compiling state, regional, and national aeromagnetic‐anomaly maps; and promoted their use in understanding upper crustal geology.

Application of frequency-domain helicopter-borne electromagnetics for groundwater exploration in urban areas

Airborne geophysical methods have been used successfully in groundwater exploration over the last decades. Particularly airborne electromagnetics is appropriate for large-scale and efficient groundwater surveying. Due to the dependency of the electrical conductivity on both the clay content of the host material and the mineralisation of the water, airborne electromagnetics is suitable for providing information on groundwater resources, water quality, aquifer conditions and protection levels. Frequency-domain helicopter-borne electromagnetic systems are used to investigate near-surface groundwater occurrences in detail even in rough terrain and populated areas. In order to reveal the subsurface conductivity distribution, the quantities measured, the secondary magnetic fields, are generally inverted into resistivity–depth models. Due to the skin-effect the penetration depths of the electromagnetic fields depend on the system characteristics used: high-frequency data describe the shallower parts of the conducting subsurface and the low-frequency data the deeper parts. Typical maximum investigation depths range from some ten metres (highly conductive saltwater saturated sediments) to several hundred metres (resistive hard rocks). In urban areas there are a number of man-made sources affecting the electromagnetic measurements. These effects on the secondary field values are discussed on the basis of synthetic data as well as uncorrected and corrected field data. The case histories of different hydrogeological setups in Indonesia, The Netherlands and Germany demonstrate that airborne electromagnetics can be applied to groundwater exploration purposes even in urban areas.

Technical Feature: Performance assessment and applications of boom-mounted airborne magnetic and electromagnetic systems

William E. Doll, T.J. Gamey, L.P. Beard, D.T. Bell, J.R. Sheehan, J. Norton and J.S. Holladay report that airborne geophysical methods for investigating the near surface, particularly for identifying unexploded ordnance (UXO), have improved dramatically in recent years and suggest many environmental and engineering applications could also benefit from the emerging technology.

Using HEM surveys to evaluate disposal of by-product water from CBNG development in the Powder River Basin, Wyoming

Production of methane from thick, extensive coal beds in the Powder River Basin of Wyoming has created water management issues. Since development began in 1997, more than 650 billion liters of water have been produced from approximately 22,000 wells. Infiltration impoundments are used widely to dispose of by-product water from coal bed natural gas (CBNG) production, but their hydrogeologic effects are poorly understood. Helicopter electromagnetic surveys (HEM) were completed in July 2003 and July 2004 to characterize the hydrogeology of an alluvial aquifer along the Powder River. The aquifer is receiving CBNG produced water discharge from infiltration impoundments. HEM data were subjected to Occam’s inversion algorithms to determine the aquifer bulk conductivity, which was then correlated towater salinity using site-specific sampling results. The HEM data provided high-resolution images of salinity levels in the aquifer, a result not attainable using traditional sampling methods. Interpretation of these images reveals clearly the produced water influence on aquifer water quality. Potential shortfalls to this method occur where there is no significant contrast in aquifer salinity and infiltrating produced water salinity and where there might be significant changes in aquifer lithology. Despite these limitations, airborne geophysical methods can provide a broadscale (watershed-scale) tool to evaluate CBNG water disposal, especially in areas where field-based investigations are logistically prohibitive. This research has implications for design and location strategies of future CBNG water surface disposal facilities within the Powder River Basin.

As três anomalias elípticas da porção sul do Cráton São Francisco: novos alvos para a mineralização de grafita?

A porção sul do Cráton São Francisco (CSF), Minas Gerais, Brasil, inclui rochas que abrangem desde o Arqueano precoce até o Neoproterozóico tardio. O mapeamento geológico e a interpretação estrutural nessa área estão restritos a informações regionais em função dos poucos afloramentos e impossibilidade de correlações laterais. Neste artigo é apresentado o mapa geológico de três anomalias elípticas na porção meridional do CSF, com base no processamento e interpretação de dados aerogeofísicos (magnéticos e raios gama). A anomalia localizada na porção leste da área hospeda uma das maiores minas de grafite no Brasil. As outras duas anomalias, reveladas a partir desse levantamento aerogeofísico, eram até então desconhecidas. A alta resolução dos dados magnéticos e gamaespectrométricos aerotransportados permitiram a definição das estruturas geológicas, a delimitação de unidades litológicas, bem como a possível ocorrência de anomalias adicionais à noroeste da área. A fusão dos dados geofísicos com imagens de sensoriamento remoto num sistema de informações geográficas forneceu subsídios complementares para o mapeamento geológico da área. Este novo exemplo de aplicação bem sucedida de métodos aerogeofísicos demonstra sua utilidade em mapeamentos regionais, os quais são fundamentais para a indústria mineral.

Stratigraphic and Structural Constraints on Limestone Exploration: A Case Study from Northern New Brunswick, Canada

Industrial-grade limestone is found in both the Lower Silurian La Vieille Formation and Upper Silurian LaPlante Formation of the Chaleurs Group in northern New Brunswick. Currently, between 150 000 and 200 000 tonnes of limestone are produced per year from the proximal facies of the LaPlante Formation at the Sormany quarry of Elmtree Resources Ltd., located west of Bathurst. The proximal facies of the LaPlante Formation was deposited on the margins of tectonically uplifted Ordovician terranes. This facies comprises stromatoporoidal-algal bindstone intercalated with wackestone, packstone, and floatstone in variable proportions. The distal facies comprises calcareous shale and minor limestone deposited deeper offshore. Folding and faulting related to Middle Devonian Acadian tectonism have caused an increase in the apparent thickness of the limestone sequences, especially adjacent to the regional Rocky Brook–Millstream fault. Structural and stratigraphic observations indicate that some of the limestone bodies in the area have been tectonically displaced from their site of deposition. A variety of prospecting techniques was used to locate new limestone resources, including geological mapping, airborne and ground electromagnetic surveys, and satellite remote sensing. Clastic rock units above and below the LaPlante Formation have distinctive properties that help to trace the intervening limestone along strike. Because of water-saturated glacial cover, thick vegetation, and the small size of targets, airborne geophysical methods did not prove effective in delineating limestone beds, but aeromagnetic surveys helped map the underlying clastic unit. The remote-sensing data and especially high-resolution digital elevation models helped in identification of karst topography related to limestone.

Using ground and airborne geophysical methods to constrain geologic mapping, and identify new mineral prospective zones in southeast Alaska

Triassic Hyd Group rocks in southeastern Alaska are the most likely target for a volcanogenic massive sulfide (VMS) deposit according to available information. However, because of difficult access and extensive cover, the geology in this region is not well known in detail, and available maps have necessarily been of a reconnaissance nature. Ground and airborne geophysical methods offer perhaps the most cost-effective means for improving the geological mapping, as long as the key geologic units (including the Hyd Group) can be differentiated. This appears to be possible, despite the complication of regional metamorphism which can change both magnetite content as well as resistivity of the key units. Careful selection of ground profiles in areas where the geology is well known has allowed us to develop a geophysical signatures matrix for a large section of southeastern Alaska from Admiralty Island in the north to Prince of Wales Island in the south. We used the ground geophyical signatures with the airborne geophysical data to develop a predictive geophysical model for the key geologic units, and these in turn have been used to substantially modify the earlier geologic maps. This has resulted in an increase of more than 30% in ground now known to be underlain by Triassic Hyd Group rocks, with a concomitant increase in prospective land.

Evaluation of Land-Management Options for Dryland Salinity: The Next Step after Airborne Geophysical Interpretation

Dryland areas affected by salinisation are rapidly expanding in the Murray-Darling Basin. Airborne geophysical methods promise information that is highly valuable to the characterisation of a salinised catchment. This paper presents a modelling approach that is well suited to apply this information. The approach models catchment salt and water balance with land management as an input and thus allows comparison of the effects of different land-management options. A model of vertical soilwater movement and solute transport is coupled with a groundwater model. The approach accounts for the effects of depth to water table and soil salinity on vegetation water use. The groundwater model estimates lateral flow from groundwater recharge areas to discharge areas. The modelling approach is demonstrated using a study area at Kialla East near Shepparton, Victoria. The area has shallow water tables, low aquifer hydraulic conductivity, and an average land surface gradient of 1 m/km. The implications of the results for land salinisation processes are discussed. The results indicate that a location where a net groundwater recharge occurs under annual pasture is likely to become a net groundwater discharge area when trees are planted. In addition, to be effective, the establishment of improved landmanagement options would have to be widespread and would be very expensive in dryland areas where productivity is often low. In future, the use of the modelling approach and airborne geophysical measurements will offer land managers a calibrated evaluation of the effects of different land-management options.

Airborne geophysical surveying for hazardous waste site characterization on the Oak Ridge Reservation, Tennessee

Airborne geophysical methods that were developed for mineral and petroleum exploration can, with some modification, be applied to environmental problems where large areas must be characterized. A helicopter survey that deployed magnetic, electromagnetic, and radiometric sensors carried out one of the first large‐scale airborne environmental surveys at a U.S. government facility at Oak Ridge, Tennessee in 1993–1994. The survey included testing of a new airborne electromagnetic system designed specifically for environmental applications and for controlled field tests of magnetic systems. Helicopter‐borne magnetic measurements were capable of discriminating groups of as few as ten metallic 208-liter (55-gallon) storage drums under representative field conditions. Magnetic and electromagnetic sensors were able to distinguish groups of metal‐filled waste disposal trenches within disposal sites, but were unable to resolve individual trenches. Electromagnetic data proved to be the most effective airborne technique for geological mapping in this portion of the Appalachian fold‐and‐thrust belt and for locating karst features. Radiometric measurements were useful both in geological mapping and in detecting zones of high radiation related to hazardous waste. The Oak Ridge survey proved valuable for quickly screening large areas and for locating anomalies for subsequent ground follow‐up. On‐board video was used to reduce the number of instances of ground follow‐up by allowing the visual screening of anomalous areas.