Several Tens Of Millimeters Research Articles

Glacier mass balances (1993–2001), Taylor Valley, McMurdo Dry Valleys, Antarctica

AbstractMass balances were measured on four glaciers in Taylor Valley, Antarctica, from 1993 to 2001. We used a piecewise linear regression, which provided an objective assessment of error, to estimate the mass balance with elevation. Missing measurements were estimated from linear regressions between points and showed a significant improvement over other methods. Unlike temperate glaciers the accumulation zone of these polar glaciers accumulates mass in summer and winter and the ablation zone loses mass in both seasons. A strong spatial trend of smaller mass-balance values with distance inland (r2 = 0.80) reflects a climatic gradient to warmer air temperatures, faster wind speeds and less precipitation. Annual and seasonal mass-balance values range only several tens of millimeters in magnitude and no temporal trend is evident. The glaciers of Taylor Valley, and probably the entire McMurdo Dry Valleys, are in equilibrium with the current climate, and contrast with glacier trends elsewhere on the Antarctic Peninsula and in temperate latitudes.

High-aspect-ratio microdrilling of polymers with UV laser ablation: experiment with analytical model

Systematic experimental studies of KrF laser microdrilling in polymers (PMMA, PET, PS, PC, PI, PEEK) have led to high-aspect-ratio microholes (up to 600) in a final stationary profile. From these results, an original theory is derived, which gives an analytical modeling of the multi-pulse ablation process. In the experiments holes with diameters in the range of 10 to 100 μm and from one to several tens of millimeters in depth, depending on fluence, are obtained for various polymers. The stationary depth increases with fluence and this dependence is well reproduced by the present model. The particular mechanism of radiation propagation and absorption inside the deep laser keyhole is clarified, and does not suggest a significant channeling of the radiation in the forming hole. This mechanism alongside the angular divergence of the beam are important key factors for the mathematical description of high-aspect-ratio laser drilling. As a result (a) the controlling factors of drilling are outlined; (b) final keyhole profile and depth vs. incident fluence are calculated for the rectangular, Gaussian and other spatial distributions of the beam and the comparison with the experiment is given; and (c) the laser drilling is optimized, i.e. the matching conditions for the level and distribution of laser intensity, parameters of the optical focusing scheme and material parameters are derived in an explicit analytical form, allowing us to produce deep keyholes with practically parallel side walls and aspect ratios as high as 300–600.

Global Positioning System Network analysis with phase ambiguity resolution applied to crustal deformation studies in California

A 5‐year campaign is in progress in central and southern California to monitor tectonic motions using observations of the Global Positioning System (GPS) satellites. Evidence from geological and space‐geodetic data indicates that significant horizontal deformations of order 10 mm/yr are occurring west of the San Andreas fault along the California margin. We demonstrate, using data from initial epoch measurements in January 1987, that the goal of measuring horizontal deformations with an accuracy of 5 mm/yr over the duration of the campaign should be readily achievable. The GPS network was designed to span a wide range of baseline lengths within California and across North America to aid in phase ambiguity resolution, an important factor in achieving highest‐accuracy baseline determinations. We present an efficient algorithm for multisession network analysis of GPS data with simultaneous orbit determination and ambiguity resolution. Using this algorithm with the California data, we demonstrate the improvement obtained in station position and satellite orbit estimation when the phase ambiguities are resolved. We demonstrate several millimeters +1 part in 108 short‐term repeatability over five single‐session (daily) analyses for horizontal baseline components and length and several tens of millimeters for the vertical. Comparison with the independently determined very long baseline interferometry baseline between Mojave and Owens Valley suggests that the accuracy is at the same level as the repeatability. We show that simultaneous multisession analyses with ambiguity resolution improve the baseline and satellite orbit estimates compared to the single‐session analyses. With the single‐session analyses we were able to resolve all phase ambiguities on baselines within California. With the multisession analyses we were able to resolve the phase ambiguities for most of the network, over regional and continental scales.

Ultrasonic probing of the fracture process zone in rock using surface waves : Swanson, P L; Spetzler, H In: rock mechanics in productivity and protection (paper to the 25th symposium on rock mechanics, Evanston, Illinois, 25–27 June 1984) P67–76. Publ New York: AIME, 1984

A microcrack process zone is frequently suggested to accompany macrofractures in rock and play an important role in the resistance to fracture propagation. Attenuation of surface waves propagating through mode I fractures in wedge-loaded double-cantilever beam specimens of Westerly granite has been recorded in an attempt to characterize the structure of the fracture process zone. The ultrasonic measurements do not support the generally accepted model of a macroscopic fracture that incrementally propagates with the accompaniment of a cloud of microcracks. Instead, fractures in Westerly granite appear to form as gradually separating surfaces within a zone having a width of a few millimeters and a length of several tens of millimeters. A fracture process zone of this size would necessitate the use of meter-sized specimens in order for linear elastic fracture mechanics to be applicable.

Geodetic evidence for subsidence due to groundwater withdrawal in many parts of the United States of America

Analysis of repeated levelings conducted by the National Geodetic Survey indicates many locations of relative subsidence in the U.S.A. Specific criteria are used to identify those subsidence features which most likely result from sediment compaction due to lowering of water levels in underground aquifer systems. Previously reported areas of such subsidence that seem to be confirmed by this study include: south-central Arizona; Savannah, Georgia; Pecos and Houston-Galveston, Texas; Denver, Colorado; San Joaquin Valley, Santa Clara Valley, Saugus Basin, Los Angeles Basin and Bunker Hill-San Timoteo, California; Milford, Utah; Raft River Valley, Idaho; Baton Rouge and New Orleans, Louisiana; and Las Vegas, Nevada. In addition, our analysis indicates that such subsidence is more widespread than previously reported occurring in a number of locations within the Atlantic and Gulf of Mexico coastal plains and along the Mississippi Valley as well as in many other unconsolidated sedimentary basins from which groundwater has been extracted and for which repeated leveling measurements are available. More than 30 new locations of subsidence possibly due to water withdrawal have been identified. The best documented of these include: Ventura, Ontario and San Pedro-Santa Monica, California; Monroe and Alexandria, Louisiana, and Jackson, Mississippi. Subsidence in newly identified locations ranges from several tens of millimeters to a few hundred millimeters over distances of ∼ 10–60 km and times of 2–48 yr. Subsidence may also have occurred in other groundwater basins where water levels have been drawn down, and remained undetected because of a lack of repeated levelings. It is important to be aware of and to continue to monitor such effects because of increasing rates of utilization of groundwater resources and the potential for significant engineering problems due to surface movements. In addition, identifying cases of subsidence due to water withdrawal is essential in order to use effectively releveling observations for investigating tectonic deformation. Along these lines, our analysis suggests that some of the releveling observations used to define a region of tectonic uplift in southern California more likely reflect subsidence due to water-level declines within sedimentary basins around the periphery of the presumed zone of uplift.