East-west trending magnetic anomalies in the Southern Hemisphere of Mars: Modeling analysis and interpretation

Maps of the vector components of the martian crustal magnetic field over the strongly magnetized Terra Cimmeria/Sirenum region are constructed using Mars Global Surveyor magnetometer data. Although pronounced east-west trending anomalies are present on the radial and north field component maps at the mapping altitude (∼360-380 km), these trends are much less prominent at the lower aerobraking altitude (∼90-150 km). Comparisons with similar maps produced using artificial data at the aerobraking altitude indicate that elongated sources in this region may have maximum lengths along the martian surface of ∼500 km and maximum aspect ratios of ∼2. Iterative forward modeling of several relatively isolated anomalies in the mapped region yields paleomagnetic pole positions consistent with those estimated in previous studies of other anomalies using mapping phase and science phasing orbit data. On this basis, it is inferred that sources in the studied region are most probably magnetized primarily in northward or southward directions. Using this additional constraint, iterative forward modeling is then applied to determine a magnetization distribution that is consistent with data at both the aerobraking altitude and the mapping altitude. The model magnetization distribution, which includes 41 discrete sources, again indicates no highly elongated sources. An examination of surface geology in the region as well as a consideration of the global distribution of anomalies suggests that magmatic intrusions (e.g., subsurface dike swarms), cooling in the presence of water, are the most likely sources of the magnetic anomalies.     

Validation of very high cloud droplet number concentrations in air masses transported thousands of kilometres over the ocean

The microstructure of orographic clouds related to the aerosol present was studied during the second Aerosol Characterisation Experiment (ACE‐2). Very high cloud droplet number concentrations (almost 3000 cm⁻³) were observed. These high concentrations occurred when clouds formed on a hill slope at Tenerife in polluted air masses originating in Europe that had transported the order of 1000 km over the Atlantic Ocean. The validity of the measured droplet number concentrations was investigated by comparing with measurements of the aerosol upstream of the cloud and cloud interstitial aerosol. Guided by distributions of the ratios between the measurements, three criteria of typically 30% in maximum deviation were applied to the measurements to test their validity. Agreement was found for 88% of the cases. The validated data set spans droplet number concentrations of 150–3000 cm⁻³. The updraught velocity during the cloud formation was estimated to 2.2 m s⁻¹ by model calculations, which is typical of cumuliform clouds. The results of the present study are discussed in relation to cloud droplet number concentrations previously reported in the literature. The importance of promoting the mechanistic understanding of the aerosol/cloud interaction and the use of validation procedures of cloud microphysical parameters is stressed in relation to the assessment of the indirect climatic effect of aerosols.     

Geology and Exploration of the Manteo Prospect off North Carolina

The Manteo Prospect is located about 45 mi northeast of Cape Hatteras, North Carolina. It is a high-risk prospect with world-class potential. The 21-lease unit was approved by the Minerals Management Service (MMS) in May 1990. A suspension of operations (SOO) was issued in October 1992 by the MMS. Chevron was approved for an exploration permit for Block 510 while Mobil's plan for Block 467 was under appeal. The Baltimore Canyon Trough and the Carolina Trough are the two large and deep sedimentary basins of the Atlantic Continental Margin. The Manteo unit is at the juncture of these two sedimentary basins. The Manteo Prospect is interpreted as a reef with its overlying structural high on the seaward edge of a carbonate platform. The structure is approximately30 mi long and 3-5 mi wide. The initial exploration well will be located at the highest point on the structure. Potential source rocks for the prospect are euxinic basinal shales and black micrite as well as interior lagoonal shales associated with the reef. The geothermal gradient projected from wells in the Baltimore Canyon Trough indicates that thermally mature sediments would be encountered below 12,000 ft in the vicinity of the Manteo Prospect. Mobil estimated that the Manteo Prospect may contain as much as 5 trillion ft3 of dry natural gas. A meeting was held between the State of North Carolina, MMS, and Chevron in February 1997 to discuss the proposal for the Manteo Prospect. An additional meeting held in September concentrated on drilling technology. A well could be drilled on Block 467 or Block 510 during the year 2000. Chevron has not decided which type of drilling vessel will be employed. The potential shorebase for operations is Morehead City, North Carolina.     

Bonemapping: a LiDAR processing and visualization technique in support of archaeology under the canopy

For the past decade, archaeologists have been using LiDAR or Airborne Laser Scanning (ALS)-based methods to uncover trace signatures of human civilization in the landscape. A new technique called bonemapping involves processing the ALS data to create a map-like representation of the landscape, which aids in the detection and interpretation of traces of human settlement. The technique is a combination of two methods – the Simple Morphological Filter (SMRF) for ALS processing and the Perceptually Shaded Slope Map (PSSM) for ALS representation – and is used to represent subtle changes in the terrain that are often indicative of previous human settlement. The SMRF algorithm adds value by retaining more “feature” cells than comparable terrain-finding algorithms, and is easy to tune through the use of two intuitive parameters – a slope threshold and a window size. The PSSM visualization is then used to apply a vertical exaggeration-based slope shading, which has proven useful as an aid to rapid feature detection, identification, and interpretation. The findings of two years of field use of bonemapping by archaeologists at El Pilar demonstrate the ways in which the bonemap offers a value-added perspective to archaeology under the canopy.