Polynomial smoothing of quiet-time magnetic variations for an irregularly spaced array of sites

Both magnetospheric and solid Earth geophysicists often employ two-dimensional arrays of recording variometers to reconstruct the spatial distribution of transient magnetic field variations at the Earth's surface. These discrete data are typically interpolated over a dense grid and the results, for example, are contoured. Few studies, however, have explored the efficacy of employing various polynomial forms for interpolating the same data set, nor have they examined how regional polynomial forms relate to magnetic variation sources on a global scale. The present study addresses some of these concerns. We quantify the characteristics of various smoothed models (i.e., low-order polynomial trend surfaces) for the same data set from a subglobal network of magnetic variometers. Using a relatively quiet interval of undisturbed diurnal variation, we characterize the spatial distribution of the three individual magnetic vector components at a single instant of time—or for what we call a ‘time slice’. We then explore how our model functions are affected by the presence or absence of various site data, i.e., what is the ‘information content’ of a particular site in our array and how important is it to constraining the final smooth model function that we derive? Finally, we explore how such local model functions are affected by including data from outside the array by studying the relation between our local polynomial forms and the global source fields from which they derive.     

Astronomical imaging with InSb arrays

Ten years ago, Forrest presented the first astronomical images with an SBRC 32×32 InSb array camera at the first NASA-Ames Infrared Detector Technology Workshop. Soon after, SBRC began development of 58×62 InSb arrays, both for ground-based astronomy and for SIRTF. By the time of the 1987 Hilo workshop Ground-based Astronomical Observations with Infrared Array Detectors astronomical results from cameras based on SBRC 32×32 and 58×62 InSb arrays, a CE linear InSb array, and a French 32×32 InSb CID array were presented. And at the Tucson 1990 meeting Astrophysics with Infrared Arrays, it was clear that this new technology was no longer the province of IR pundits, but provided a tool for all astronomers. At this meeting, the first astronomical observations with SBRC's new, gateless passivation 256×256 InSb arrays will be presented: they perform spectacularly!In this review, I can only broadly brush on the interesting science completed with InSb array cameras. Because of the broad wavelength coverage (1–5.5 m) of InSb, and the extremely high performance levels throughout the band, InSb cameras are used not only in the near IR, but also from 3–5.5 m, where unique science is achieved. For example, the point-like central engines of AGNs are delineated at L and M, and Br and 3.29 m dust emission images of galactic and extragalactic objects yield excitation conditions. Examples of imaging spectroscopy, high spatial resolution imaging, as well as deep, broad-band imaging with InSb cameras at this meeting illustrate the power of InSb array cameras.     

Infrared astronomy: From one to one million pixels

In less than a decade, infrared array detectors have revolutionized infrared astronomy. Most of us remember using single element photometers in the early eighties, yet today, most of us are using 256×256 pixel arrays. At this meeting we have heard of plans to fabricate 1024×1024 arrays in the near infrared. From one to one million pixels in such a short period of time is amazing. The new array technology has also stimulated the development of many varieties of infrared cameras and spectrometers. At the UCLA Infrared Imaging Detector Lab we have commissioned two near infrared imaging systems (KCam and Gemini) based on 256×256 arrays and a spectrometer design study is in progress for 1024×1024 arrays. Performance of these cameras on the telescope will be reported briefly.     

Experimental Study of Mechanical Behaviour of Rock Joints Under Cyclic Loading

Summary Evaluation of the effects of small repetitive earthquakes on the strength parameters of rock joints in active seismic zones is of interest of the designers of underground constructions. In order to evaluate these effects, it is necessary to study the behaviour of rock joints under dynamic and cyclic loadings. This paper presents the results of a systematic study on the behaviour of artificial rock joints subjected to cyclic shearing. More than 30 identical replicas have been tested using triaxial compression devices under different conditions of monotonic and cyclic loading. At the first stage a few samples have been tested in monotonic loading modes under various confining pressures and rate of displacement. In the second series of tests, small cyclic loads were applied on the samples for increasing number of cycles, frequency levels and stress amplitudes. These were then followed by monotonic loading again. The variations of maximum and residual shear strengths for each test have been studied. The results show increase of shear strength as a result of the increase in confining pressure and they display decrease of shear strength due to the increase of rate of loading, number of cycles, frequency levels and stress amplitudes.     

典型干旱区荒漠戈壁陆面参数的观测研究

根据目前流行的陆面过程模式的需要，利用２０００年５－６月敦煌陆面过程野外观测实验加强期的观测资料，分析了西北典型干旱区荒漠戈壁的一些关键陆面过程和土壤参数的特征和规律。并且利用相对反射为权重加权平均，计算得到典型干旱区敦煌荒漠戈壁的平均反射率为０.２５５±０.０２１；剔除建筑物干扰后，利用对数平均法计算的粗糙度长度平均值为０，００１９±０。０００７１ ｍ；剔除绿洲影响后，用无降水影响的资料确定出土壤湿度影响因子为０．００４５；剔除降水影响后，用观测资料计算的敦煌典型干旱区荒漠戈壁的热容量平均值为１．１２×１０~６ｍ~（－３）Ｋ~（－１），比“黑河试验”在戈壁和在其它沙漠观测的有关值略小一些，但热扩散率和热传导率都比“黑河试验”在戈壁观测的值小一倍左右；观测的敦煌戈壁典型天气条件下的土壤含水量非常小，基本上不超过１％。     

降水条件下的典型干旱区陆面特征模拟验证

利用“我国西北干旱区陆－气相互作用观测试验"在敦煌双墩子戈壁站取得的观测资料及最近的一些研究成果对陆面模式中反照率、相似性函数及地表粗糙度(动量粗糙度,标量粗糙度)3个方面的参数化方案进行了改进,然后对一次典型降水过程的陆面特征及近地层的风、温、湿进行了模拟.结果表明：改进的模式能对降水条件下的干旱区陆面特征进行较好的模拟,其中对辐射、地表温度的模拟相当好,而对能量的模拟虽然还有要改进的地方,但总体结果令人满意;另外改进后的模式对近地层的温度和湿度的模拟也有明显的改善.     

Generating basis sets of double differences

When a collection of double differences is used to compute global-positioning-system satellite orbits from a permanent network of receiving stations, linear dependence among the double-differenced observations reduces the number of double differences that contribute new information to the computations. A maximal linearly independent subset of a large collection of double differences contains all the information content of the full set. If r is the number of receivers and s is the number of satellites, the original collection of double differences may have size O(r ² s ²), whereas the linearly independent subset has size no greater than O(rs). Only such a smaller independent subset needs to participate in the more expensive double-precision matrix computations to correctly correlate all double differences, detect cycle slips, resolve ambiguities, and compute satellite orbits and station positions and relative velocities. Dependence among double differences is characterized using vector space methods together with geometric characterizations of Boolean matrices. These characterizations lend themselves to fast, robust algorithms for computing maximal linearly independent sets (bases) of double differences. An algorithm is given for constructing a generating independent set of double differences from the Boolean array of receiving-station/satellite connections. Characterizations of generator equivalence allow alternative generating sets to be identified and selected. An updating algorithm to handle local changes in the satellite–receiver connection matrix is also described. Received: 27 August 1996 / Accepted: 28 January 1999     

Rupture imaging of the 25 April 2015 MW7.9 Nepal earthquake from back-projection of teleseismic P waves

The M_W7.9 Nepal earthquake of 25 April 2015 had over 8, 500 fatalities and was the most destructive earthquake in Nepal since the Bihar-Nepal earthquake in 1934. In this study, we imaged the rupture process of this Nepal event by back-projecting the teleseismic P-wave energy recorded at the three regional networks in Alaska, Australia and Europe. The back-projection images of the three subarrays revealed that the Nepal earthquake propagated along the strike in a southeast direction over a distance of ~ 160–170 km with the duration of ~ 50–55 s. The rupture process was found to be a simple, unilateral event with a near constant velocity of 3.3 km/s. The beam power was mainly distributed in the geographic region just north of Kathmandu and the peak intensity for the source time function curve occurred at about 30 s. The earthquake was destructive due to its occurrence at shallow depth (~ 12–15 km) and the fact that the capital lies in a basin of soft sediment. Additionally, the resonance effect for the longer period waves that occurred in the Kathmandu valley led to destructive aggravation, impacting mainly the taller buildings.     

Modelling of a wave energy converter array with a nonlinear power take-off system in the frequency domain

This paper presents a nonlinear frequency domain model and uses this to assess the performance of a wave energy converter (WEC) array with a nonlinear power take-off (PTO). In this model, the nonlinear PTO forces are approximated by a truncated Fourier series, while the dynamics of the WEC array are described by a set of linear motion equations in the frequency domain, and the hydrodynamic coefficients are obtained with the boundary element method. A single heave absorber is firstly investigated to establish the accuracy of the new model in capturing the nonlinear behaviour of the pumping system. Subsequently, simulations of a 2D array with 18 WECs and a pillar in the centre (representing the tower of a wind turbine) are carried out to understand wave interference effects. Several optimisation strategies are proposed to improve the overall performance of the WEC array. These results demonstrate a computationally effective method for accounting for nonlinear effects in large WEC arrays. The proposed approach may potentially be applied for developing control algorithms for the adaptability of a 2D array to incoming wave excitation.     

Estimation of soil shear modulus softening during strong ground shaking using ground surface and downhole acceleration recordings

An algorithm for estimating the average shear modulus for a soil site containing a downhole accelerometer array is described. For distant or weak earthquakes, the usual procedure for estimating shear modulus is to perform time‐series correlation of two downhole records. The vertical distance between instruments divided by the peak correlation lag time gives the average shear wave velocity. The shear modulus follows easily. This method is not applicable for stronger earthquakes where non‐linear softening effects lead to progressively slower shear wave velocities. The method presented here overcomes the softening effect by compressing the time scale of the upper acceleration record. Time compression is accomplished in such a way that the peak correlation of the two records is maximized. The algorithm steps through the records, maximizing the correlation peak by adjusting the time scale within an active time interval. The resulting compressed upper record can be interpreted as the ground motion that would have occurred had softening not taken place. The summation of the various time scale adjustments shows both the amount of softening and the time at which it occurred. Copyright © 2000 John Wiley & Sons, Ltd.