东北大兴安岭多年冻土区工程地质特征及评价

土体在冻结状态具有极高的压缩模量, 具有弹性体的工程地质特征。但是在冻土地温升高过程中, 这种特征急剧衰减, 产生蠕变和流变, 建筑物地基强度降低, 导致建筑物基础破坏。同时土体在冻结过程中产生的冻胀作用也将导致建筑物基础的破坏。东北大兴安岭地区多年冻土为高纬度低海拔多年冻土, 其分布具有明显纬度地带性特点。本文在分析该区多年冻土分布特征及冻土工程地质特点的基础上, 对由于土体的冻胀和融沉导致的建筑物基础的危害进行分类研究, 针对性的提出了用热棒降低土体温度以保证多年冻土稳定及用排水的方法减少水对建筑物地基多年冻土影响的工程病害处理措施。     

Statistical characterization of frost zones: Case of tea freeze damage in the Kenyan highlands

Frequent frost occurrences in the Kenyan highlands have had devastating effects on agricultural productivity. With inadequate management systems to mitigate the impacts, farmers have often had to bear the burden of losses resulting from frost damage. While agriculture in Kenya remains dependent on weather and climate, the agricultural economy of Kenya continues to suffer, underscoring the need for building local knowledge as basis for development of early warning systems. The current paper attempts to delineate frost zones by statistically characterizing them based on known risk factors related to topography (elevation, convexity, aspect, upslope flow length) and Land Surface Temperature (LST) derived from Moderate Resolution Imaging Spectroradiometer (MODIS). Through binary logistic regression, a logistic regression model was developed utilizing observation data (frost occurrence and non-occurrence) as a binary dependent variable to estimate the probability of frost occurrence. Assuming a 0.5 probability cut-off threshold between frost occurrence and non-occurrence, an overall accuracy of 81% with area under Receiver Operating Characteristics (ROC) Curve of 0.88 was obtained. No evidence of lack of model fit was detected. This model outperforms the currently operational model that utilizes MODIS LST alone to detect frost zones in the Kenyan tea plantations. It provides an improved method for effective delineation of frost zones by incorporating local topographic characteristics.     

改进Frost算子在SAR图像斑点噪声抑制中的应用

针对合成孔径雷达(SAR)图像斑点噪声滤波的重要性以及存在的问题,对Frost滤波算法参数进行分析,指出了该算法等权滤波器在处理含有细节结构特征时存在盲目平滑的缺点,进而提出了改进方法。利用灰度差值计算权值,综合考虑滤波窗口内的强度信息,在一定程度上解决了盲目平滑的问题。实验验证了改进算法的有效性,并以斑点噪声指数和边缘保持指数为评价准则,通过与Frost算法比较,对改进算法在去除噪声和边缘保持方面的性能做了客观评价。     

The freeze-thaw weathering regime at a section of the Niagara escarpment on the Bruce Peninsula,Southern Ontario,Canada

The bedrock freeze-thaw and moisture regimes at an actively eroding site on the Niagara Escarpment, Bruce Peninsula, southern Ontario, were monitored between December 1983 and April 1984, and the results compared with amounts of debris collected in an adjacent rock trap. Frost wedging in pre-existing rock fissures is the primary mechanism responsible for the observed rockfall events; hydration effects are negligible. Debris production was more closely related to the duration of the freezing leg of the freeze-thaw cycle than to intensity or to cycle frequency. Release also coincided with periods of high pore saturation (> 60 per cent) and the seepage of water from cracks and fissures. Fewer freeze-thaw cycles were recorded in the air than at 1 and 3.5 cm in the bedrock. The number of cycles that could be declared geomorphologically effective according to established temperature criteria was normally less than half the total number of freeze-thaw cycles recorded in both air and bedrock. Under the current temperature regime at the field site, few effective cycles are capable of penetrating more than 5 cm into the free face.     

Diurnal freeze–thaw depth in rockwalls: Field measurements and theoretical considerations

Rock temperatures were monitored for a year at two alpine rockwalls in the Akaishi Range, Japan, where permafrost is absent. Diurnal frost and thaw penetration depths were evaluated from subsurface isotherms drawn on the basis of the temperature records at 0, 10 and 40 cm depths. The surface of the rockwalls experienced more than 100 diurnal freeze-thaw cycles, most of which accompanied a frost or thaw penetration shallower than 50 cm, and several long duration cycles with deeper frost or thaw penetration. Theoretical frost and thaw depths were also calculated by incorporating the surface freezing indices into the modified Berggren equation, the results from which were then compared with the actual values. The modified Berggren equation provided values that showed a strong correlation with observed depths, despite somewhat underestimating the actual values. Using the modified Berggren equation, the seasonal frost depth in the observed winter was estimated to be about 4·5 m. The frost and thaw depths are considered to give the maximum estimation of the depth to which frost-induced cracking can propagate in the bedrock.     

The rate of bedrock weathering by frost action: Field measurements and a predictive model

Shattering rate, surface temperature, moisture content, and the physical and strength properties of bedrock were measured in four rockwall sites of the Japanese Alps. Five-year observations revealed that the bedrock shattering rate was usually much higher in the freeze-thaw period from October to next May than in the frost-free period from June to September. This indicates that frost action is the most important shattering process, although unusual heavy rainstorms in summer are also responsible for the shattering. A combination of some empirical relationships derived from recent laboratory experiments leads to a predictive model of the frost shattering rate. This model shows that the annual shattering rate is dependent on the annual freeze-thaw frequency on the rock surface, and the degree of saturation and tensile strength of the rock masses. The coefficients involved in the model were determined using the field data.     

Frost weathering and the saturation of coastal rocks

Measurements were made of the water content in coastal rocks, by simulating tidal oscillations in the laboratory, and by field measurement in eastern Canada. If rapid freezing takes place upon exposure to the air, saturation levels may be high enough to permit frost weathering in fine grained rocks in the lower portions of the intertidal zone. Near the high tidal level, however, it may be dependent upon a supply of water from the ice foot and from melting snow. If freezing is slow, frost action may be inhibited by desorption of the rocks while they are exposed by the ebb tide. There was no evidence of a level of permanent sea water saturation within the intertidal zone. Ambient temperature and humidity may affect the rate of rock desorption.     

Hydrostatic-system palsas at Toolik lake,Alaska: Field observations and simulation

Stratigraphic and chemical data from the ice core of an ‘anthropogenic palsa’ at Toolik Lake, Alaska, indicate that the mound formed as a consequence of hydrostatic pressure developed in an isolated hydrologic system within the active layer. Survey data for five palsas over a three-year period suggest that growth was essentially complete at the time of the initial survey; a net decrease of summit elevation is apparent in all five mounds, but complete degradation of the palsas would require several decades at observed rates. Because accurate field measurements of thermal and hydrologic evolution in such features are extremely difficult, simulation of the environmental conditions and events involved in palsa growth is an important supplement to field observation. Both analytic and finite-element models yield results that are in substantial agreement with inferences drawn from observational data.     

Relict sand wedges in soliflucted London Clay at Wimbledon, London, UK

Exposures on Wimbledon Hill, SW London, in the Palaeogene London Clay Formation, are described. The 3° slopes are mantled by clayey solifluction (head) deposits, comprising two superimposed sheets, each around 1.2 m thick. The lower sheet exhibits relict sand wedges forming polygons 1.0-1.5 m across, up to 50 mm wide and 1.25 m deep. These have no surface expression. The wedges are sand-filled and modify significantly the hydrogeology and properties of the solifluction mantle. There are no sand wedges in the upper solifluction sheet.The mechanics of emplacement of the lower solifluction sheet are explored and the high initial water content estimated. Subsequent desiccation and consolidation have reduced the thickness of this sheet by at least 35%, thus distorting contained features such as the wedges. An active layer about 0.4 m thick is inferred to have formed in the top of the lower solifluction sheet. No direct dating was undertaken but, by analogy with relevant dated sites in southern Britain and the Netherlands it is inferred that the lower solifluction sheet was emplaced during the first part of the Loch Lomond Stadial and frost-cracked during a subsequent, colder and more arid part. On this basis, following filling of the cracks by sand, emplacement of the upper solifluction sheet would have occurred in the later part of the Loch Lomond Stadial.     

Summer season variability of the north residual cap of Mars as observed by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES)

Previous observations have noted the change in albedo in a number of North Pole bright outliers and in the distribution of bright ice deposits between Mariner 9, Viking, and Mars Global Surveyor (MGS) data sets. Changes over the summer season as well as between regions at the same season (L_s) in different years have been observed. We used the bolometric albedo and brightness temperature channels of the Thermal Emission Spectrometer (TES) on the MGS spacecraft to monitor north polar residual ice cap variations between Mars years and within the summer season for three northern Martian summers between July 1999 and April 2003. Large-scale brightness variations are observed in four general areas: (1) the patchy outlying frost deposits from 90 to 270°E, 75 to 80°N; (2) the large “tail” below the Chasma Boreale and its associated plateau from 315 to 45°E, 80 to 85°N, that we call the “Boreale Tongue” and in Hyperboreae Undae; (3) the troughed terrain in the region from 0 to 120°E longitude (the lower right on a polar stereographic projection) we have called “Shackleton's Grooves” and (4) the unit mapped as residual ice in Olympia Planitia. We also note two areas which seem to persist as cool and bright throughout the summer and between Mars years. One is at the “source” of Chasma Boreale (∼15°E, 85°N) dubbed “McMurdo”, and the “Cool and Bright Anomaly (CABA)” noted by Kieffer and Titus 2001. TES Mapping of Mars’ north seasonal cap. Icarus 154, 162-180] at ∼330°E, 87°N called here “Vostok”. Overall defrosting occurs early in the summer as the temperatures rise and then after the peak temperatures are reached (L_s∼110) higher elevations and outlier bright deposits cold trap and re-accumulate new frost. Persistent bright areas are associated with either higher elevations or higher background albedos suggesting complex feedback mechanisms including cold-trapping of frost due to albedo and elevation effects, as well as influence of mesoscale atmospheric dynamics.