NS34A-01 INVITED
The State of the Industry and Research in Airborne Geophysics
Development of airborne geophysical methods has tended to proceed in rushes of energy, when many new systems are developed for the same application simultaneously along many pathways. The tremendous growth of airborne EM through the ‘50s to ‘70s was followed by natural selection in the ‘80s and ‘90s down to two styles: fixed-wing aircraft with high-powered time domain systems (FTEM) offering depth of exploration but poor spatial resolution, and helicopter-borne frequency-domain systems (HFEM) offering the best resolution but poor depth of exploration. At the end of the ‘90s there was an incredible spurt of energy toward helicopter time domain development, spurred technological advances in electronics and materials. By 2007 there were 8 systems operational. Perhaps the most daring current research is toward airborne EM systems utilizing ambient EM fields as sources. Magnetic sensors are almost universally cesium-vapor total field sensors (0.01nT sampled at 0.1s). Because the limitation on target detection is ambient, in-band noise, there is little to gain from producing higher-sensitivity meters. Data quality improvements are being sought by measuring horizontal and vertical gradients more accurately. The new wave of research for magnetic surveys is the measurement of vector or tensor magnetic data with directional sensors, generally either fluxgates or SQUIDS. Magnetometers on autonomous aircraft are newly available. Gamma Ray Spectrometry surveys with sodium-iodide crystal detectors give good performance, and the low cost allows for large volumes to make up for the relatively low sensitivity. The last few years have seen development of new systems in which each crystal in the detector array is monitored, calibrated and stabilized individually using natural radiation. Airborne gravity systems available use the LaCoste zero-length pendulum, or orthogonal accelerometers. Separation of gravity from acceleration is generally done with platforms stabilized for both rotation and translation, and measurement of acceleration. Generally, solutions must be a trade-off between sensitivity and spatial resolution, restricting their application to the large structures of oil exploration. Airborne gravity gradiometry (AGG) achieves higher resolution and sensitivity with meters based on the system of accelerometers on spinning disks, implemented as horizontal gradiometers and as full tensor gradiometers. Putting the sensor on a helicopter improved the data S/N. An airship implementation promises to be a near-ideal platform, restricted by the payload limits. Many projects are on-going to develop new gravity gradiometers toward a goal of 1Eotvös sensitivity at 100m wavelength. Hyperspectral imaging measures the reflected light from the surface across a broad spectrum, originally from near-infrared through visible, but now often including thermal infrared. The research challenges for systems have been to stabilize the system sensitivities, correct for varying ambient light levels reflectance, and improve resolution without degrading signal strength. Data processing requires the determination of the mineral reflectance spectra that best fit the spectrum of each pixel, when each pixel will probably contain many minerals, or be partly covered by vegetation.
NS34A-02 INVITED
Development of a Next Generation Polar Multidisciplinary Airborne Imaging System for the International Polar Year 2007-2009
Key elements in Earth's geodynamic and climatic systems, the polar regions are very sensitive to changing global environmental conditions such as increasing sea surface temperatures and have the potential to trigger significant global sea level rise as large volumes of ice melt. Locked within these icy regions are the records of past global climate shifts and novel ecosystems sealed from open interactions with the atmosphere for millions of years. While satellite missions can image the surface of the polar ice sheet, many of the key processes occur beneath the surface beyond the reach of space based observations. These crucial processes can only be efficiently examined through airborne instrumentation designed to study the vast expanses of snow and ice of the Antarctic continent, the sub-continent of Greenland and the surrounding oceans. The expanding logistical infrastructure associated with the International Polar Year (2007-2009) will enable the scientific community access major new portions of the polar regions. We are developing a state-of-the-art integrated multidisciplinary aerogeophysical instrumentation package for deployment during multi-national expeditions as part of the International Polar Year. This development project brings together the recent developments in radar sounding by the University of Kansas CReSIS (Center for Remote Sensing of Ice Sheets), that now permit the full characterization of the entire ice sheet and the major advances in the accuracy, resolution and efficiency of airborne gravity technology emerging from the private sector. Integrating the full spectrum of ice sheet imaging with high-resolution gravity and magnetics will enable the imaging of the previously invisible world of subglacial hydrodynamics.
NS34A-03
Airborne Gravity Gradiometry Resolves a Full Range of Gravity Frequencies
Abstract Airborne Full Tensor Gradiometry (Air\-FTGR) was flown at high altitude coincident with Airborne Gravity (AG) flown in 2003 in West Arnhem Land, Australia. A preliminary analysis of two data sets indicates that the Air\-FTGR system has the capability of resolving intermediate to long wavelengths features that may be associated with relatively deeper geological structures. A comparison of frequency filtered slices and power spectral density (PSD) for both data sets using the short (> 5 km), intermediate (10 km) and long (20 km) wavelengths reveals that high altitude Air\-FTGR data show greater response in high frequency anomalies than a conventional Airborne Gravity and matches well with the AG even at the longest wavelengths anomalies. The effect of line spacing and target resolution was examined between the two data sets. Reprocessed gradient and AG data at 2, 4 and 6 km line spacing suggest that Air\-FTGR could be effectively flown at a comparatively wider line spacing to resolve similar targets the AG would resolve with tighter line spacing. Introduction Airborne Full Tensor Gradiometry (Air\-FTGR) data have been available to the mining industry since 2002 and their use for geologic applications is well established. However, Air\-FTGR data has been mostly considered and used in mapping and delineation of near surface geological targets. This is due to the fact that gravity gradiometer measurements are well suited to capture the high frequency signal associated with near\-surface targets ( Li, 2001). This is possible because the gradiometer signal strength falls off with the cube of the distance to the target. Nonetheless, in recent years there has been an increasing demand from the mining, oil, and gas industry in utilizing Full Tensor Gravity Gradiometer as a mapping tool for both regional and prospect level surveys. Air\-FTGR as a Regional Mapping Tool Several, relatively low altitude surveys have been successfully flown in Brazil, Canada and Australia mostly targeting large, regional\- scale crustal structures as well as regional mapping of both lithology and regolith. Air\-FTGR mapping is especially effective in areas of thick lateritic and/or clay cover where other geophysical methods such as airborne magnetics or electromagnetics become less effective. For instance, an Air\-FTGR survey was successfully flown in Brazil in the Province of Minas Gerais, where several crustal\-scale structures associated with iron oxide mineralization were identified ( Mataragio et. al., 2006). In addition, in 2006 Air\-FTGR had good success in the regional mapping of structures associated with Iron Oxide Copper Gold (IOCG) and uranium mineralization in the Wernecke Mountains in the Yukon, and Northwest Territories, Canada. On the basis of these successful surveys, Bell Geospace has initiated a number of high altitude test surveys aiming at evaluating the performance of the Air\-FTGR system in capturing low frequency signal that may be associated with regional\-scale, deeper structures. One of the test surveys was conducted in December of 2006 in Australia, where the performance of Air\-FTGR and the conventional Airborne Gravity were evaluated. Airborne gravity is currently considered well suited for capturing low frequency signal. http://www.bellgeo.com
NS34A-04 INVITED
Advances in Grounded Electrical Source Airborn Transent EM(GREATEM) system
One of the important techniques to challenge on AEM survey is increasing penetration depth to apply for mineral explorations, environmental and disaster prevention issues. AEM systems usually carry a source and a receiver on the airplane and a size of source and/or a source-receiver distance are limited. We have developed the GREATEM system which uses long electric wire source on the ground and measure the induced field in the air at distance to guarantee deeper penetration depth. The penetration depth of the survey depends on how to gain long transient curves in time domain acquisition. We installed high performance magnetometer, A/D converter to obtain full wave of 3 components data and high accuracy fiber optic gyros to monitor motion of the sensor in the bird. We made a motion correction of observed magnetic field using a transformation between measured motion and variation of magnetic field for lower frequency perturbation. Unsteady state noises were suppressed by appropriate filter to delineate the transient curve at later time. We carried out the survey in Aso Volcano in southwest Japan to detect distribution of potential landslide area in an alteration zone and a water reservoir controlling phreatic eruption. The resistivity structures up to 1000m were detected in this survey under suitable conditions.
NS34A-05
Airborne Magnetic and Electromagnetic Data map Rock Alteration and Water Content at Mount Adams, Mount Baker and Mount Rainier, Washington: Implications for Lahar Hazards and Hydrothermal Systems
High resolution helicopter-borne magnetic and electromagnetic (EM) data flown over the rugged, ice-covered, highly magnetic and mostly resistive volcanoes of Mount Rainier, Mount Adams and Mount Baker, along with rock property measurements, reveal the distribution of alteration, water and hydrothermal fluids that are essential to evaluating volcanic landslide hazards and understanding hydrothermal systems. Hydrothermally altered rocks, particularly if water saturated, can weaken stratovolcanoes, thereby increasing the potential for catastrophic sector collapses that can lead to far-traveled, destructive debris flows. Intense hydrothermal alteration significantly reduces the magnetization and resistivity of volcanic rock resulting in clear recognition of altered rock by helicopter magnetic and EM measurements. Magnetic and EM data, combined with geological mapping and rock property measurements, indicate the presence of appreciable thicknesses of hydrothermally altered rock west of the modern summit of Mount Rainier in the Sunset Amphitheater region, in the central core of Mount Adams north of the summit, and in much of the central cone of Mount Baker. We identify the Sunset Amphitheater region and steep cliffs at the western edge of the central altered zone at Mount Adams as likely sources for future debris flows. In addition, the EM data identified water-saturated rocks in the upper 100-200 m of the three volcanoes. The water-saturated zone could extend deeper, but is beyond the detection limits of the EM data. Water in hydrothermal fluids reacts with the volcanic rock to produce clay minerals. The formation of clay minerals and presence of free water reduces the effective stress, thereby increasing the potential for slope failure, and acts, with entrained melting ice, as a lubricant to transform debris avalanches into lahars. Therefore, knowing the distribution of water is also important for hazard assessments. Finally, modeling requires extremely low resistivities (< 20 ohm-m) that laboratory electrical resistivity measurements indicate are most easily explained by sulfuric acid solutions permeating altered rocks. The acid is the result of oxidation of magmatic H2S to sulfuric acid and highlights the continued alteration of volcanoes during periods of relative quiescence. Our results demonstrate that high resolution geophysical and geological observations can yield unprecedented views of the three-dimensional distribution of altered rock and shallow pore water and hydrothermal fluids within active stratovolcanoes.
NS34A-06
Aeromagnetic Constraints on the Subsurface Structure of Stromboli Volcano, Aeolian Islands, Italy
Aeromagnetic surveys were conducted over Stromboli volcano and its surrounding areas in the Aeolian Islands, southern Italy in 2002 and 2004 to better understand the subsurface structure of the area. Observed data of those surveys were merged and aeromagnetic anomalies for Stromboli Island and its vicinity were reduced on a smoothed surface by the method, assuming equivalent anomalies below the observed surface (Nakatsuka and Okuma, 2006). Magnetic terrain corrections (Grauch, 1987) were applied to the anomalies of the study area, assuming a magnetic structure comprised of an ensemble of prism models extending from the ground surface to a depth of 3,000m below sea level: the average magnetization intensity was calculated to be 2.2 A/m by a comparison between the observed and synthetic anomalies and then the synthetic anomalies were removed from the observed. Next, apparent magnetization intensity mapping (Nakatsuka, 1995) was applied to the terrain-corrected anomalies. The apparent magnetization intensity map indicates magnetic heterogeneities among volcanic rocks which constitute the edifice of the volcano. The most obvious characteristics of the magnetization intensities is a magnetization low which occupies the center of the island where the summit craters reside closely, suggesting a demagnetization caused by the heat of conduits and/or a shallow magma chamber in addition to the thick accumulation of less magnetic pyroclastic rocks. By comparison with geologic maps, it can be seen that magnetization highs are distributed on exposures of basaltic-andesite to andesite lavas (Paleostromboli I), shoshonitic shied volcanoes (Neostromboli), at the south, north and west coasts of the volcano, respectively. Those magnetization highs further extend to offshore, implying the continuation of those volcanic rocks from land. An apparent magnetization low lies on an abrasion platform offshore of the northeastern volcano, suggesting that the platform is underlain by the hydrothermally altered center of the dissected edifice of an old volcano. http://staff.aist.go.jp/s.okuma/
NS34A-07 INVITED
Advances in Modeling Aeromagnetic Anomalies Related to Faults in Sedimentary Basins-- Lessons Learned From High-Resolution Surveys in the Central Rio Grande Rift, New Mexico
High-resolution aeromagnetic data acquired over several basins in the central Rio Grande rift, north-central New Mexico, prominently display low-amplitude (5-15 nT), linear anomalies associated with faults that offset basin-fill sediments. The linear anomalies give an unprecedented view of concealed faults, with significant implications for understanding structure and ground-water flow in the top 500 m of sedimentary basins in general. These implications provide the impetus for understanding the sources of the anomalies in greater detail, using the central Rio Grande rift as a case study. Based on geophysical analysis combined with detailed magnetic- property studies, we have established that linear anomalies in the rift can be explained entirely by the tectonic juxtaposition of magnetically differing strata rather than by chemical processes focused at the fault zone. At the scale of ground profiles, tectonic juxtaposition can be adequately modeled by traditional step-like or offset-layers models. However, at a basin-wide scale typical of aeromagnetic surveys, geophysical models of multiple layers stacked on both sides of the fault are more appropriate. Computed curves from these models are complexly determined by differences in magnetic contrasts, stratal thicknesses, and depth to the strata. To better understand the primary geophysical parameters that drive the variability, we translate models of juxtaposed strata into equivalent geophysical models based on lateral magnetic contrast. The equivalent models more simply represent the juxtaposed strata as stacks of magnetic-contrast layers that alternate across the fault with depth. These models can be more easily compared to four end-member models consisting of only one or two truncated layers with variable magnetic-contrast and layer thickness. The end-member models can explain the observed anomaly variability. In particular, they can explain the presence of aeromagnetic lows over a fault zone, a counterintuitive situation that may previously have been mistaken as the expression of altered rocks.
NS34A-08
Terrane Definition From Textural Measures of Aeromagnetic Data
The vertical and horizontal magnetization in the Earth's crust is an anisotropic multifractal distribution, and this results in a horizontal multifractal distribution of magnetic anomalies at and above the Earth's surface. Discreet lithologic terranes are frequently observed to exhibit a characteristic pattern or "texture" of anomalies in aeromagnetic maps. Multifractal measures provide some tools useful in quantifying different textures and the scaling properties of aeromagnetic anomalies in map view can be used to define boundaries between terranes of different magnetic textures. If the source depths are not too large, the magnetic textural measures of lithologic units can be used to map their extent beneath cover. Although magnetic anomaly textures are generally visible to the eye on aeromagnetic anomaly images, the actual boundary between two textures is frequently difficult to determine with certainty. The use of quantitative textural measures provides a more objective framework for the boundary definition problem. A high resolution aeromagnetic survey over an area of highly variable geology has been used as a test area for these studies. Two robust measures that have proven useful for textural analysis are: the number of extrema per unit area; and the surface area per unit area in a window moving over the gridded aeromagnetic data. The former measures the "noisiness" of the data, and the latter depends on anomaly amplitudes and discriminates between large and small magnetizations. Window sizes for texture analysis are typically a few km square because geologic terranes of interest are generally of the order of tens of km in characteristic dimension. Other measures investigated are based on the scaling properties of the field within the window computed from the structure function for various exponents. The minima of the structure function define the characteristic sizes of anomalies, analogous to the power spectrum for a periodic function, and the maxima define characteristic sizes of large amplitude changes within the window. Ratios of the two-dimensional extrema to the total extrema also provides a measure of elongation within the window. Together, maps of these measures provide tools that improve the definition of terrane boundaries in both exposed and covered terranes and constitute a powerful tool for extending bedrock geological maps beneath cover.