NS31A-0149
New Generation of High Sensitivity Airborne Potassium Magnetometers for Mineral Exploration Applications
In the 90's, GEM developed a Potassium magnetometer system that met many of the requirements for "high detail mapping", including high sensitivity, absolute accuracy and minimal orientation errors. Now, this work has been extended with the development of a new Potassium magnetometer / gradiometer for very high sensitivity operation. This paper consists of a short review of two key types of quantum magnetometers (Overhauser and Optically Pumped) – focusing on the development and field testing of a new Potassium system. The new technology provides several advantages over standard optically pumped systems, including high sensitivity, gradient tolerance and bandwidth; "clean" geophysical signal: high speed of operation, high absolute accuracy; ability to combine magnetometer and EM system. Case history data are also provided. Processing methods include corrections, filtering, and gridding. Final conclusions relate these results to the targets shown in original site design records.
NS31A-0150
Analysis of Aeromagnetic data of the NE region of the Indian sub-continent.
The NE region of the Indian subcontinent is a very interesting place for geodynamic and tectonic studies due to the collision of the Indian plate with the Himalayas in the North and with Myanmar in the East. Some of these areas are difficult to access due to the high topography and air borne surveys help map the region efficiently. Further, the NE region including areas of Assam and Bangladesh are drained by the Ganges and Brahmaputra rivers that bring thick sediments with them giving rise to the Bengal Fan that has sediments of up to 20 km in some regions and these mask the underlying crust and pose severe restrictions in constructing the geodynamical history of the region. Geopotential data offer the unique opportunity of addressing some of these issues. We utilize available / published aeromagnetic maps over Bangladesh, Meghalaya, Chachar, parts of North Assam, Brahmaputra valley, Manipur and Nagaland in the North Eastern region of India for understanding the complexities of the tectonics of the Indian sub-continent. All available data are reduced to a common altitude to prepare a composite magnetic anomaly map of the region. We also look at the Grace and Bouguer / isostatic gravity maps of the region; the highest isostatic gravity anomaly of the Indian region lies over Meghalaya with a trough in Sylhet (Bangladesh) to its south. The Eocene hinge zone is a 25-km wide northeast-southwest zone that separates the Precambrian platform in the northwest from the geosynclinal basin to the southeast of Bangladesh. We find that the Hinge Zone in Bangladesh and part of the Dauki fault form a strong divide such that the gravity anomalies and aeromagnetic anomalies show high frequency anomalies to the north of this divide and also most of the magnetic sources as seen from the analytic signal are concentrated to the north and the Euler solutions give shallow solutions to the north and deep solutions to the south of this divide. The tilt derivative of the aeromagnetic data clearly defines the edges of the different geological formations. Within the Meghalaya plateau most of the high frequency anomalies are found associated with near surface ultramafic intrusions, mineralized zones etc. and appear to be controlled by the Dudhuni and Kulsi faults. Most of the shallow magnetic sources in the upper Assam and Brahmaputra valley show NW-SE trend and are restricted by the NW-SE Dhansili-Kopili fault. The Eastern Boundary thrust and the Dhubri fault appears to constrain the Grace anomaly highs in the east and west respectively. Through an analysis of the aeromagnetic data we identify extensions of the known fault below thick piles of sediments and presence of same hitherto unknown faults / lineaments. Results of this analysis and its implication for the geodynamic history of the region will be presented.
NS31A-0151
Aeromagnetic survey by a model helicopter at the ruin of ironwork refinement
It is difficult to detect the magnetic anomaly resulting from the small scale of magnetic sources as archeological or historical ruins by a helicopter due to the restraint of the low altitude flights in the narrow area. Although a relatively small unmanned helicopters has been commercialized for agriculture use etc., it is too expensive for aeromagnetic surveys. We have developed a small autonomous unmanned helicopter which modified a model helicopter for aeromagnetic survey. A model helicopter (Hirobo Co.; SF40) with a 40cc gasoline engine, length of 143cm from the nose to the tail and dry weight of 15 kg is selected in this study. The irradiated magnetic field from the bottom-center of skid of SF40 was the total magnetic field (R)=3511 nT, inclination (I)=12 degrees and declination (D)=138 degrees. It was reduced to about 1 nT at 3 m downward from the skid during the hovering. When SF40 was covered with a magnetic shield film (Amolic sheet), the distance to measure 1nT diminished to 2 m. As shielding whole body with the film is not effective for reliable and safety flights, the only servomotors having the strong magnetization were shielded by the film. The autonomous flights based on GPS data succeeded. As the control system was too large and heavy for SF40, we are developing more simple and small navigation system for this project. Magnetometer system consists of a 3-axis fluxgate magnetometer, data logger, GPS and battery, recording every second of x, y and z magnetic fields, latitude, longitude, altitude and satellite number during 3 hours. The total weight of the system is 400g. The system was hanged to 2m lower from the skid by a rope (Bird magnetometer) or 2m front form the nose by a carbon fiber pipe (Stinger magnetometer) in order to avoid the magnetic field of SF40. However, the bird magnetometer was not suitable due to the strong noise resulting from the swing of the sensor. An archeological ruin of the ironwork refinement aged 15th century in western Japan was measured by the stinger magnetometer. The survey area was 70x20m with a gentle slop. The helicopter was controlled by the manual keeping up the roughly same altitude (the 4-8m height from the surface) and speed (1m/s). The result showed the strong anomalies of 500 nT at the NW corner of the area where consists with the refinement. From these viewpoints the model helicopter is useful to find the ironwork refinements instead of the identification based on the feeling and the experience of archeologists.
NS31A-0152
Recent Aeromagnetic Surveys Image Cenozoic Magmatism of the Transantarctic Mountains and Reveal Faults Along the Eastern Margin of the Wilkes Subglacial Basin
We present results from two recent aeromagnetic surveys over two major tectonic elements of the Northern Victoria Land (NVL) sector of East Antarctica: the Transantarctic Mountains (TAM), forming the uplifted flank of the West Antarctic Rift System (WARS), and the Wilkes Subglacial Basin (WSB), in the hinterland of the TAM. The MAGANTER survey was performed during the 2001/02 Italian Antarctic campaign to investigate Cenozoic magmatic patterns over the Admiralty Mountains. Cenozoic magmatic rocks of the TAM provide a unique window on the tectonic and magmatic processes of the WARS. Previous aeromagnetic investigations further south in NVL have delineated Cenozoic volcanic and intrusive complexes assigned to the McMurdo Volcanic Group and Meander Intrusives respectively. Our new aeromagnetic anomaly maps show that the Meander Intrusives are restricted to the coastal region between Malta Plateau and the Daniell Peninsula and trend at high angle to the major Cenozoic strike-slip faults of the region. However, the McMurdo Volcanic Group rocks extend further inland, and may delineate a volcano-tectonic rift zone parallel to offshore rift basins. The newly identified spatial distribution of Cenozoic magmatic rocks may require revisions to recent tectonic models for the region. The WIBEM survey (2003/04) focused on the eastern margin of the enigmatic Wilkes Subglacial Basin. New aeromagnetic maps image subglacial faults systems along the eastern margin of the WSB, which link previously proposed fault zones over Oates Land with those along the Ross Sea Coast. Specifically they reveal a connection between the Matusevich Frature Zone and the Priestley Fault. The new aeromagnetic evidence for structural control on the eastern margin of the WSB contrasts with a purely flexural origin for this part of the WSB, proposed from some previous gravity models.
NS31A-0153
Helicopter Electromagnetic and Magnetic Surveys over Volcanoes – Resolution Analysis
Helicopter electromagnetic (HEM) and magnetic surveys over Mt. Adams, Mt. Baker and Mt. Rainier volcanoes were conducted to map altered zones to aid in volcano hazards assessments. As the three volcanoes are covered with ice and have highly magnetic rocks with electrical resistivities spanning several orders of magnitude, inversion of electromagnetic (EM) data to meaningful resistivity values included magnetic susceptibility and dielectric properties of rocks in addition to the standard resistivity vs. thickness parameters. The sensitivity of the HEM data to the approximate conditions on the volcano was examined in order to constrain the inversion. The responses of simple two-layer electrical property models representing the resistivity structure of the volcano were calculated over the full range of HEM frequencies. In all models the top layer resistivity of 106 ohm-m represented dry altered rocks, fresh volcanic rocks or ice. The top layer thickness, bottom layer resistivity, magnetic susceptibility and dielectric permittivity of both layers were varied and the HEM system ppm response was compared to the detectability threshold set at 5 ppm. The HEM system ppm response was modeled at 50 m above ground which was close to the average Mt. Adams survey elevation. The calculations showed that ppm response for a 10 ohm-m bottom layer, representing wet altered rocks, is above 5 ppm up to depths of 200 m indicating that the top of the low resistivity layer could be detected up to this depth. The results showed that the basement resistivities above 1000 ohm-m below 30 m overburden will not be well resolved. The effect of magnetic susceptibility and dielectric permittivity on the measured Hz fields was evaluated for the case of a 30 m thick top layer overlying a 1000 ohm-m bottom layer – a model chosen because it is close to the resolution limit of the system. The susceptibility of 0.025 SI strongly influences the in-phase component by lowering its value by a few ppm along the whole spectrum of frequencies, but has less influence on quadrature response. The in- phase and quadrature of the Hz component calculated for varying relative dielectric permittivity showed that for this resistive model only the highest frequencies, above 10 kHz, are influenced by the dielectric properties of the model, and only for the case of high water permittivity.
NS31A-0154
Mapping Mineralization in the Monitor Pass Mining District
The Leviathan Mine, located in the Monitor Pass mining district in Alpine county California, was previously an open pit sulfur mine that supplied a nearby operation in Yerington, Nevada with sulfur for copper oxide leaching. The area was first mined underground in the mid 1800s and by the 1950s became an open pit operation. A decade later, the sulfur became uneconomic to extract, leaving the company bankrupt. By the 1980s, the EPA had declared Leviathan Mine a Superfund site due to acid mine drainage into Leviathan creek. Remediation continues today, but high sulfur soils and the potential for acid generation is abundant in the Monitor Pass area. This discovery of iron sulfate on Mars motivated the acquisition of airborne and spaceborne data similar to datasets available for Mars. In addition, high spatial resolution mineral mapping could help identify high priority remediation targets within the mine site. Low resolution (30m per pixel) spaceborne data from ALI and ASTER sensors were combined into a 13-filter dataset to locate areas of interest and as an overview of the area. Initial results were promising, showing evidence for iron oxides and clay mineralization, with some areas suggesting sulfates. The combination of thermal infrared and shortwave infrared datasets optimizes the potential for mineral identification. Therefore, we planned dual hyperspectral acquitions. SEBASS thermal data (7-15μm) at 2m per pixel was acquired on July 16, 2007. ProSpecTIR V-S (0.43- 2.45μm) was acquired on August 17, 2007, also at 2m per pixel. Field spectra were collected concurrently with the SpecTIR flight. The high resolution data will be used to confirm and refine the findings from spaceborne results and be presented at the meeting.
NS31A-0155
A large-scale integrated aerogeophysical survey of Afghanistan
A multi-sensor, multidisciplinary aerogeophysical survey of a major portion of Afghanistan was recently conducted by investigators from the Naval Research Laboratory and the U.S. Geological Survey. More than 110,000 line km of data tracks were flown aboard an NP-3D Orion aircraft. Sensor systems installed on the P-3 included dual gravimeters, scalar and vector magnetometers, a digital photogrammetric camera, a hyperspectral imager, and an L-band polarimetric synthetic aperture radar (SAR). Data from all sources were precisely co-registered to the ground by a combination of interferometric-mode Global Positioning System (GPS) and inertial measurements. The data from this integrated mapping mission support numerous basic and applied science efforts in Afghanistan including: resource assessment and exploration for oil, gas, and minerals, development of techniques for sensor fusion and automated analysis, and topics in crustal geophysics and geodesy. The data will also support civil infrastructure needs such as cadastral surveying, urban planning and development, and pipeline/powerline/road routing and construction, agriculture and hydrologic resource management, earthquake hazard analysis, and base-maps for humanitarian relief missions.