A large Luneberg-type antenna with accessories is described as a means of simultaneously monitoring a large number of stars for possible intelligent signals. Various contacting strategies are compared on their probability of success and arguments are advanced favoring a program of transmission at a wavelength of 21 cm to the nearest stars. 相似文献
Black Butte is an early Miocene basaltic volcanic neck that forms a prominent landmark as the highest peak of the Gravelly Range, southwestern Montana. The intrusion cuts mid-Cenozoic and older sedimentary rocks near the eastern margin of the Overthrust Belt.
After erosional removal of the Late Cretaceous Frontier Formation, quartzite-rich detritus from ultimate sources probably far to the west was deposited in the area and now forms a diamicton that rests on striated bedrock. This unit, previously interpreted as a till and as a mudflow deposit, probably represents Upper Cretaceous or lower Tertiary, syntectonic alluvial-fan sediments. These were deposited after the Gravelly Arch had begun to rise and were deformed during overthrusting from the west or possibly during mass movement as the basal part of a landslide.
Scattered cobbles of hard quartzite in the diamicton are crushed. If this crushing occurred within aggregates of coarse clasts that were momentarity in point contact with one another, it does notrequire either overthrusting or mass movement of extremely thick depositional overburden. But if a major thrust sheet did move over the diamicton, the leading edge of the Overthrust Belt must extend considerably further east of where it is currently recognized in this area.
Volcanic and volcaniclastic rocks were deposited in the area throughout much or all of Oligocene time. These include tuffaceous mudstone as much as 265 m thick that contains vertebrate fossils of Chadronian through Whitneyan age. The KAr age of biotite in an airfall tuff within this section at nearby Lion Mountain is 31.4 Ma, and the KAr age of an alkaline basaltic flow at the top of the Lion Mountain section is 30.8 Ma. These tuffaceous rocks and basalt on Lion Mountain correlate with volcaniclastics in Wyoming and as far east as Nebraska and the Dakotas.
Eruptions at Black Butte, dated previously at 22.9 Ma, begun with phreatomagmatic explosions that deposited tuff across an irregular topographic surface cut in the section of tuffaceous mudstone into which the Black Butte plug was emplaced. The alkali basalt magma differentiated to yield the relatively rare rock typetephritic phonolite during fractional crystallization and segregation in situ of potassic late liquids.
Lava flows from Black Butte and the nearby Lion Mountain volcanic center may have covered much of this part of the Gravelly Range but have been mostly removed owing to erodability of the thick blanket of mudstone on which they rested.
Removal of mudstone that contained the Black Butte intrusion involved massive slumping. Mass movement of the diamicton beneath the mudstone is occurring today as an earthflow down the west-dipping structural and topographic slope of the range. 相似文献
Using a slab of Massillon Sandstone, laboratory-scale solute tracer experiments were carried out to test numerical simulations using the Advection–Dispersion Equation (ADE). While studies of a similar nature exist, our work differs in that we combine: (1) experimentation in naturally complex geologic media, (2) X-ray absorption imaging to visualize and quantify two-dimensional solute transport, (3) high resolution transport property characterization, with (4) numerical simulation. The simulations use permeability, porosity, and solute concentration measured to sub-centimeter resolution. While bulk breakthrough curve characteristics were adequately matched, large discrepancies exist between the experimental and simulated solute concentration fields. Investigation of potential experimental errors suggests that the failure to fit solute concentration fields may lie in loss of intricate connectivity within the cross-bedded sandstone occurring at scales finer than our property characterization measurements (i.e., sub-centimeter). 相似文献
Throughout northwest European coastal countries risks associated with coastal erosion are significant but spatially and temporally variable. The level of this risk is largely dependent on the extent of development within the coastal zones and a variety of approaches have been adopted for its management. The decision-making process for responding to erosion risk depends to a large extent on national policy. Coastal protection policy in northwest European countries varies in terms of the level of centralisation and formality of arrangements. In this paper the practical outworking of the informal practice-based system of Ireland, where there is no national policy framework, is compared with the policy-led system of England and Wales where formal national guidelines exist. Using case studies, the strengths and weaknesses of both the bottom-up and top-down approaches are assessed. The findings reveal strengths and weaknesses in both existing types of approach. 相似文献
Loss of life and property caused by landslides triggered by extreme rainfall events demonstrates the need for landslide-hazard assessment in developing countries where recovery from such events often exceeds the country's resources. Mapping landslide hazards in developing countries where the need for landslide-hazard mitigation is great but the resources are few is a challenging, but not intractable problem. The minimum requirements for constructing a physically based landslide-hazard map from a landslide-triggering storm, using the simple methods we discuss, are: (1) an accurate mapped landslide inventory, (2) a slope map derived from a digital elevation model (DEM) or topographic map, and (3) material strength properties of the slopes involved. Provided that the landslide distribution from a triggering event can be documented and mapped, it is often possible to glean enough topographic and geologic information from existing databases to produce a reliable map that depicts landslide hazards from an extreme event. Most areas of the world have enough topographic information to provide digital elevation models from which to construct slope maps. In the likely event that engineering properties of slope materials are not available, reasonable estimates can be made with detailed field examination by engineering geologists or geotechnical engineers. Resulting landslide hazard maps can be used as tools to guide relocation and redevelopment, or, more likely, temporary relocation efforts during severe storm events such as hurricanes/typhoons to minimize loss of life and property. We illustrate these methods in two case studies of lethal landslides in developing countries: Tegucigalpa, Honduras (during Hurricane Mitch in 1998) and the Chuuk Islands, Micronesia (during Typhoon Chata'an in 2002). 相似文献
A detailed set of tests in a wind tunnel investigated the characteristics of flow and sampling efficiency of a low cost sediment trap that can be used in wind tunnels and field applications. The trap is constructed of thin sheet aluminum and has a wedge shape (32° internal inlet angle with a 20 × 500 mm rectangular inlet orifice that extends 40 mm beyond the wedge form. Relatively unimpeded airflow through the trap is achieved by the large exit area (130 × 500 mm) which is covered with fine, stainless steel mesh (62.5 μm openings, 60% porosity), and collected all but the finer (silt) sediment sizes. Despite its angular shape, the trap has good characteristics of flow and a very high sampling efficiency (> 90%) over a wide range of wind velocities because of its near isokinetic characteristics of flow. Results also indicate that this trap, and others that have wide side profiles, are very sensitive to incident wind angle. Incident angles > 5° significantly reduce the sampling efficiency and promote scour around the trap inlet. 相似文献
The distributions of dissolved organic carbon (DOC), phytoplankton biomass (as measured by in vivo fluorescence), total nitrogen and phosphorus, and light extinction were observed on 10 cruises during 1989 and 1990 in the Pawcatuck River estuary located in southern Rhode Island. In the lower estuary, the distance of peak phytoplankton biomass from the head of the estuary was positively correlated with river discharge while the magnitude of the peak increased with decreasing discharge. High light-extinction appeared to limit the accumulation of biomass in the upper estuary. Variability in light extinction was largely (50%) explained by variation in the concentration of DOC. Salinity versus constituent plots suggested that DOC behaved nonconservatively in the estuary. These observational data indicate that the mixing behavior of DOC in the estuary influences light extinction and thus may limit accumulation of phytoplankton biomass in the upper estuary. This interpretation of observational data was supported by experimental work that demonstrated the significant contribution of DOC to light extinction, and by measurements, of phytoplankton productivity that showed greater light limitation in the upper estuary. 相似文献
