Multiple-taper spectral analysis of terrestrial free oscillations: part I

We present a new method for estimating the frequencies of the Earth's free oscillations. This method is an extension of the techniques of Thomson (1982) for finding the harmonic components of a time series. Optimal tapers for reducing the spectral leakage of decaying sinusoids immersed in white noise are derived. Multiplying the data by the best K tapers creates K time series. A decaying sinusoid model is fit to the K time series by a least squares procedure. A statistical F -test is performed to test the fit of the decaying sinusoid model, and thus determine the probability that there are coherent oscillations in the data. The F -test is performed at a number of chosen frequencies, producing a measure of the certainty that there is a decaying sinusoid at each frequency. We compare this method with the conventional technique employing a discrete Fourier transform of a cosine-tapered time-series. The multiple-taper method is found to be a more sensitive detector of decaying sinusoids in a time series contaminated by white noise.     

Multiple-taper spectral analysis of terrestrial free oscillations: part II

In Part I of this paper, we derived a set of data tapers designed to minimize the spectral leakage of decaying sinusoids immersed in white noise. Multiplying a long-period seismic record by K of these tapers creates K time series. A decaying sinusoid is fit to these K time series in the frequency domain at a number of chosen frequencies by a least-squares procedure. The fit is tested at each frequency using a statistical F -test. In Part I, we demonstrated that the multiple-taper method is a more sensitive detector of decaying sinusoids than the conventional direct spectral-estimate.
In this paper, we present a number of extensions to the multiple-taper method. We explain how the technique can be modified to estimate the harmonic components of records containing gaps. We discuss how sinusoids at frequencies between FFT bin frequencies can be detected, and how this method can be combined with conventional multi-station stacking procedures.     

Wave propagation in fluid-saturated media: waveform and spectral analysis

A new numerical approach to the solution of waves propagating in a fluid-saturated medium, using Biot's theory as a foundation, has important implications for oil reservoir management and earthquake prediction. A numerical scheme is developed using an exponential transformation that explicitly treats the petrophysical and fluid properties of the medium within the framework of a generalized model. The scheme accounts for wave dissipation and velocity modifications. The numerical solution is used to perform numerical experiments to study the dynamic behaviour of waves in a fluid-saturated medium at well-logging frequencies (15 kHz). The results from the numerical experiments indicate that the degree of saturation by a high-viscosity fluid (HVF) such as oil, the temperature and the porosity of a medium strongly influence the spectral power distribution, frequency content and the velocity of waves propagating through the medium. An increase in HVF saturation causes enhanced attenuation of the low-frequency components, and increases the seismic velocity. An increase in porosity, however, enriches the low-frequency components and decreases the seismic velocity. A spectral quantification procedure is suggested and used to obtain information about the petrophysical and fluid properties of the medium from the spectral characteristics of the transmitted waveform. The procedure involves segmentation of the energy or power distribution of the transmitted waveforms into specified energy bands. The energy or power in these bands is then estimated. The extracted quantification variables are found to have strong correlations with the degree of HVF saturation, and the temperature and the porosity of the medium.     

中国城市化水平的自回归与功率谱分析

利用1949~2000年的城市人口比重数据,开展中国城市化过程的自相关和功率谱分析,建立了ARMA (1,q )模型。中国的城市化过程具有1阶自相关特征:上一年的一个变动直接影响下一年,间接影响则可达10年之久。ARMA (1,q)模型表明中国的城市化过程在趋势性上附加有丰富的随机性。对提取趋势之后的序列进行功率谱分析,发现趋势性和随机性的背后隐含一个长度为30年左右的周期波动。根据上述研究,将中国的城市化过程分解为三种变动:趋势性、周期性和随机性。研究结论对发展更为完善的城市化预测方法,以及对指导具有中国特色的城市化建设,都可能具有一定程度的启示意义。     

Fourier transform infrared spectroscopy,a new cost-effective tool for quantitative analysis of biogeochemical properties in long sediment records

Measurements of Fourier transform infrared spectroscopy (FTIRS) in the mid-infrared (MIR) region were conducted on sedimentary records from Lake El’gygytgyn, NE Siberia, and Lake Ohrid, Albania/Macedonia. Calibration models relating FTIR spectral information to biogeochemical property concentrations were established using partial least squares regression (PLSR). They showed good statistical performance for total organic carbon (TOC), total nitrogen (TN), and biogenic silica (opal) in the sediment record from Lake El’gygytgyn, and for TOC, total inorganic carbon (TIC), TN, and opal in sediments from Lake Ohrid. In both cases, the calibration models were successfully applied for down-core analysis. The results, in combination with the small amount of sample material needed, negligible sample pre-treatments, and low costs of analysis, demonstrate that FTIRS is a promising, cost-effective tool that allows high-resolution paleolimnological studies.     

Spatio-temporal analysis of tree height in a young cork oak plantation

Cork oak is one of the most valuable natural forest genera in the Mediterranean basin. Modelling cork oak growth has been a challenge for foresters in recent years because of strong site and genetic influences, below-ground competition, management regimes and age effects. Because cork productivity is related to forest height, which is, in turn, related directly with site characteristics, an increase in the accuracy of height prediction implies improved productivity estimation. A Bayesian maximum entropy (BME) geostatistical model was applied to characterize the space–time pattern of height of young cork oak in a forest stand from central Sardinia in the years 2000, 2002, 2003, 2006 and 2008. Cork oak height maps were produced for each of the 5 years. The main goals were to analyse and interpret through time (i) the changes in spatial correlation and (ii) the changes in spatial distribution of cork oak height. The plantation was characterized by an increasing spatial dependence through time, whereas the temporal range was 2 years. Cork oak height was significantly correlated with wind speed (reduced by a neighbouring forest) in all the years implying a single trend. The correlations were larger for 2006 and 2008 than for previous years. Three other environmental variables (shade, elevation and slope) were less significant and their influence restricted to 2 years only. This research has several implications for the management of cork oak in the young phase.     

基于谱间特征和归一化指数分析的城市建筑用地信息提取

以福州市ETM+影像为例,研究了城市建筑用地信息快速准确提取的原理和方法。通过对归一化差异型指数构成原理的分析以及对同名异义和异名同义现象的甄别,选取了归一化差异建筑指数(NDBI)、修正归一化差异水体指数(MNDWI)和土壤调节植被指数(SAVI)来代表城市建成区的三种最主要的土地利用类型--建筑用地、水体和植被。在此基础上进一步对这三个新的指数波段进行谱间特征分析,最后利用基于规则的逻辑判别运算将城市建筑用地信息提取出来。研究表明这一方法可以使繁杂的多波段谱间分析得以简化, 是一种快速准确、未经人工干预的建筑用地信息提取方法。本文还探讨了在城市建成区的研究中采用SAVI指数替代NDVI指数的优点。     

基于不同卫星光谱模拟的土壤电导率估算研究

土壤电导率(Electrical conductivity,EC)是评价土壤盐渍化的重要指标。通过实测新疆艾比湖湿地自然保护区土壤EC及可见光—近红外光谱数据,利用波谱响应技术模拟Landsat 8 OLI、Sentinel 2、Sentinel 3卫星的宽波段数据。构建宽波段模拟数据及其5种预处理后的三维光谱指数(Three-dimensional spectral index,TDSI),采用梯度提升回归树算法(Gradient boosting regression tree, GBRT)建立3种卫星土壤EC估算模型,并比对加入TDSI后模型精度的变化。结果表明:在不同土壤EC条件下,3种卫星具有相似的光谱趋势,均在红、近红外波段附近反射率较高;TDSI与土壤EC相关性基本均在0.4以上,最大程度保留了与土壤EC敏感度高的红、绿、蓝、近红外、短波红外波段信息;GBRT对于土壤EC估算能力表现突出,3种卫星对土壤EC的最佳预测精度R²分别为0.831、0.847、0.903,在加入TDSI后,R²分别提高至0.835、0.857、0.93...     

Combining land cover products using a minimum divergence and a Bayesian data fusion approach

Land cover mapping plays an important role for a wide spectrum of applications that are ranging from climate modeling to food security. However, it is a common case that several and partially conflicting land cover products are available at the same time over a same area, where each product suffers from specific limitations and lack of accuracy. In order to take advantage of the best features of each product while at the same time attenuating their respective weaknesses, this paper is proposing a methodology that allows the user to combine these products together based on a general framework involving maximum entropy/minimum divergence principles, Bayesian data fusion and Bayesian updating. First, information brought by each land cover product is coded in terms of inequality constraints so that a first estimation of their quality can be computed based on a maximum entropy/minimum divergence principle. Information from these various land cover products can then be fused afterwards in a Bayesian framework, leading to a single map with an associated measure of uncertainty. Finally, it is shown how the additional information brought by control data can help improving this fused map through a Bayesian updating procedure. The first part of the paper is briefly presenting the most important theoretical results, while the second part is illustrating the use of this suggested approach for a specific area in Belgium, where five different land cover products are at hand. The benefits and limitations of this approach are finally discussed by the light of the results for this case study.