北阿拉伯海的光纤海洋观测网络:成功、挑战和机遇

2005年夏,一个先进的海洋观测网络——包括实时的光纤海洋观测系统和内部存储的自动化观测系统——被投放在了阿曼海和北阿拉伯海并运行至今.在2010年初,其中的自动化观测系统被升级到了新的深水光纤观测系统.这个海洋观测网络是在阿曼农业和渔业部的资助下,由美国的Lighthouse R&D公司设计、开发、安装和维护的.这2个观测系统作为一个整体已经连续工作了7年多的时间.所采集数据包括海流、温度、盐度、压力、溶解氧和浊度等.该海区是一个多水团的汇合区,波斯湾的高盐水和阿拉伯海的低盐水在这里汇合并蔓延南下到印度洋.对采集的数据研究表明,这一观测网络对研究该区域的物理和生物过程具有重要价值.在此,将系统介绍整个观测网络,并简要阐述已经完成和接近完成的4个研究主题:①对阿拉伯海有记载以来最强热带气旋“古怒”的海洋响应的研究;②阿曼海北部的季节性缺氧现象的季节及年际变化和成因分析;③深海声散射层的时空演变;④阿曼海和北阿拉伯海的高温高盐现象的成因.该观测网络采集的长期、连续的时间序列对这一地区的海洋动力研究、水文的季节性变化,以及气候的长期变化等研究都有很大帮助.此外,如果观测网络可以完成25年的设定观测目标,这将对验证和改进海洋环流模式和海气耦合模式具有重要意义.     

On the Equation of Position Measurement of XNAV

The equation of position measurement of XNAV (X-ray pulsar-based autonomous navigation) reveals the relation between the time of arrival (TOA) of an X-ray photon and the spacecraft position at this moment. In specific dis- cussions of navigation, the TOA is generally replaced by the difference between the TOA and a preset “time reference”, and the spacecraft position is expressed by the position vector in the barycentric system of the solar system. The “time reference” may be the true TOA at the solar-system barycenter (SSB), or an “equivalent TOA” at SSB. As the true TOA at SSB is difficult to obtain, the “equivalent TOA” is more convenient for navigation. The equations of position measurement based on the two kinds of “time reference” are discussed, the equa- tion based on the “equivalent TOA”, which has the time accuracy of 0.1 ns, is given, and the physical meanings of the every term in which are analyzed.     

Hydrodynamic characteristics of a typical karst spring system based on time series analysis in northern China

In order to study the hydrodynamic characteristics of the karst aquifers in northern China, time series analyses (correlation and spectral analysis in addition with hydrograph recession analysis) are applied on Baotu Spring and Heihu Spring in Jinan karst spring system, a typical karst spring system in northern China. Results show that the auto-correlation coefficient of spring water level reaches the value of 0.2 after 123 days and 117 days for Baotu Spring and Heihu Spring, respectively. The regulation time obtained from the simple spectral density function in the same period is 187 days and 175 days for Baotu Spring and Heihu Spring. The auto-correlation coefficient of spring water level reaches the value of 0.2 in 34–82 days, and regulation time ranges among 40–59 days for every single hydrological year. The delay time between precipitation and spring water level obtained from cross correlation function is around 56 days for the period of 2012–2019, and varies among 30–79 days for every single hydrological year. In addition, the spectral bands in cross amplitude functions and gain functions are small with 0.02, and the values in the coherence functions are small. All these behaviors illustrate that Jinan karst spring system has a strong memory effect, large storage capacity, noticeable regulation effect, and time series analysis is a useful tool for studying the hydrodynamic characteristics of karst spring system in northern China.© 2021 China Geology Editorial Office.     

Full waveform inversion in the time lapse mode applied to CO2 storage at Sleipner

Carbon capture and storage is a viable greenhouse gas mitigation technology and the Sleipner CO₂ sequestration site in the North Sea is an excellent example. Storage of CO₂ at the Sleipner site requires monitoring over large areas, which can successfully be accomplished with time lapse seismic imaging. One of the main goals of CO₂ storage monitoring is to be able to estimate the volume of the stored CO₂ in the reservoir. This requires a parametrization of the subsurface as exact as possible. Here we use elastic 2D time‐domain full waveform inversion in a time lapse manner to obtain a P‐wave velocity constrain directly in the depth domain for a base line survey in 1994 and two post‐injection surveys in 1999 and 2006. By relating velocity change to free CO₂ saturation, using a rock physics model, we find that at the considered location the aquifer may have been fully saturated in some places in 1999 and 2006.     

Visco‐acoustic prestack reverse‐time migration based on the time‐space domain adaptive high‐order finite‐difference method

It is important to include the viscous effect in seismic numerical modelling and seismic migration due to the ubiquitous viscosity in an actual subsurface medium. Prestack reverse‐time migration (RTM) is currently one of the most accurate methods for seismic imaging. One of the key steps of RTM is wavefield forward and backward extrapolation and how to solve the wave equation fast and accurately is the essence of this process. In this paper, we apply the time‐space domain dispersion‐relation‐based finite‐difference (FD) method for visco‐acoustic wave numerical modelling. Dispersion analysis and numerical modelling results demonstrate that the time‐space domain FD method has great accuracy and can effectively suppress numerical dispersion. Also, we use the time‐space domain FD method to solve the visco‐acoustic wave equation in wavefield extrapolation of RTM and apply the source‐normalized cross‐correlation imaging condition in migration. Improved imaging has been obtained in both synthetic and real data tests. The migration result of the visco‐acoustic wave RTM is clearer and more accurate than that of acoustic wave RTM. In addition, in the process of wavefield forward and backward extrapolation, we adopt adaptive variable‐length spatial operators to compute spatial derivatives to significantly decrease computing costs without reducing the accuracy of the numerical solution.     

Seemingly unrelated intervention time series models for effectiveness evaluation of large scale environmental remediation

Large scale environmental remediation projects applied to sea water always involve large amount of capital investments. Rigorous effectiveness evaluations of such projects are, therefore, necessary and essential for policy review and future planning. This study aims at investigating effectiveness of environmental remediation using three different Seemingly Unrelated Regression (SUR) time series models with intervention effects, including Model (1) assuming no correlation within and across variables, Model (2) assuming no correlation across variable but allowing correlations within variable across different sites, and Model (3) allowing all possible correlations among variables (i.e., an unrestricted model). The results suggested that the unrestricted SUR model is the most reliable one, consistently having smallest variations of the estimated model parameters. We discussed our results with reference to marine water quality management in Hong Kong while bringing managerial issues into consideration.     

Earthquake parameters affecting the performance of an RC frame with friction damper

In this paper, correlation between the ground motion parameters and the peak displacement demand of a reinforced concrete single degree of freedom system retrofitted by friction dampers is investigated. The aim is to reveal the best parameters that are used in selecting earthquake records for nonlinear dynamic analysis of the system. Performing numerous nonlinear time history analyses, it is observed that different earthquake parameters play role in peak displacement demand related to the ranges formed by the different periods of a reinforced concrete frame, the strength ratio of total system at slip displacement and the stiff/soft soil profiles. For the both soil types, out of sixteen ground motion parameters, the eight, Spectral Response Acceleration, Peak Ground Acceleration, Effective Peak Acceleration, Maximum Incremental Velocity, Peak Ground Velocity, Arias Intensity, Arias Intensity-based Parameter and Effective Peak Velocity correlate better with peak displacement demand.     

Processes of the displacement field change of the 2009 April 6 M_W6.3 L’Aquila earthquake using persistent scatterer and small baseline methods

Using a time series method that combines both the persistent scatterer and small baseline approaches, we analyzed 9 scenes Envisat ASAR data over the L＇Aquila earthquake, and obtained a Shocke＇s displacement field and its evolution processes. The results show that： （1） Envisat ASAR clearly detected the whole processes of displacement field of the L＇Aquila earthquake, and distinct variations at different stages of the displacement field. （2） Preseismic creep displacement → displacement mutation when faulting → constantly slowed down after the earthquake. （3） The area of the strongest deformation and ground rupture was a low-lying oval depression region to the southeast. Surface faulting within a zone of about 22 km× 14 km, with an orientation of 135°, occurred along the NW-striking and SW-dipping Paganica-S. Demetrio normal fault. （4） In analyzing an area of about 54 km x 59 km, bounded by north-south axis to the epicenter, the displacement field has significant characteristics of a watershed： westward of the epicenter shows uplift with maximum of 130 mm in line-of-sight （LOS）, and east of the epicenter was a region with 220 mm of maximum subsidence in the LOS, concentrating on the rupture zone, the majority of which formed in the course of faulting and subsequence.     

关于测量大口径HDD环空泥浆固结特性新方法的研究

水平定向钻进（HofizontNDirecfionl Drilling,简称HDD）是采用安装于地表的钻孔设备。对于大直径管道来说,工程竣工后在钻孔和管道之间要留下300mm-400mm的环状间隙。要对泥浆固结后进行应力应变的分析,前提是要知道环空泥浆的固结时间。而传统测量环空泥浆的固结时间的方法存在着很多问题与缺陷,这也导致了时间检测上面误差较大,从而耽误了研究泥浆固结后受力分析的最佳时间。因此,弄清泥浆固结时间的变化规律非常必要,而目前国内在判定泥浆固结时间的方法上仍处于空白阶段。针对这一状况．本人设计采用两种方法进行固结时间的判定,通过湿度传感器测量环空泥浆间隙的水蒸气含量进行泥浆固结时间的判定。     

Weak-mode identification and time-series reconstruction from high-level noisy measured data of offshore structures

The identification of true weak modes buried in high-level, noisy, measured data from offshore structures is a practical but challenging problem because weak modes are typically eliminated as noise and rarely, yield a discrete time series. This study proposes a weak-mode identification and time-series reconstruction method for offshore structures when high-level noise is present. A theoretical development proposed in this study extends the traditional modal analysis to reconstructing the discrete time series of weak modes, thereby removing its previous limitations to only frequencies, damping ratios and mode shapes. Additionally, a second development proposed in this study makes the reconstructed time series not simply a combination of harmonic components from a Fourier transform but rather complex exponentials; the damping of the test structure is thus estimated with a better accuracy. A third theoretical development avoids variations in the results from different original signals by handling multiple signals simultaneously. The proposed approach primarily includes three steps: (1) estimate the poles and corresponding residues of high-level, noisy, measured data by converting high-order difference equations to first-order difference equations; (2) isolate the poles of weak modes by assigning multiple rough-pole windows, and subsequently extract the corresponding residues based on the row number of the isolated pole vector; and (3) identify and reconstruct the time series of the weak modes of interest in the form of complex exponentials. The most primary advantage of the proposed process in engineering applications is that the pole windows can be easily obtained and assigned from the relationship between the frequencies and their poles. Three numerical examples are studied: the first presents the detailed numerical operation of the proposed method, the second extends the proposed method from managing one signal to managing multiple signals, and the third demonstrates the advantage of the approach compared with traditional methods. The numerical results indicate that the original signals can be decomposed into multiple complex exponentials with representative poles and corresponding residues, and that the new signals representing weak modes could be reconstructed by assigning a range of frequencies in terms of their relations with the poles. To study the performance of the proposed method when applied to offshore structures such as offshore platforms and marine risers, the experimental data from the high mode VIV experiments sponsored by the Norwegian Deepwater Programme (NDP) are used firstly. The results show that two dominant frequencies corresponding to the in-line and cross-flow directions can be identified simultaneously even one mode is very weak compared with the other, and the time series of the weak mode could be reconstructed with a rough frequency window. Then sea-test data of two offshore platforms are used: one was collected from the JZ20-2MUQ offshore platform when it was excited by ice, and the other was collected from the WZ11-4D platform when it was excited by waves. The results further demonstrated that a large model order is required to estimate all poles and residues of the original noisy signals, and that the row number corresponding to a weak mode of the isolated pole matrix could be easily determined via finite element analysis or engineering experiences. Therefore, the proposed approach provides not only modal parameters, such as frequencies and damping ratios of true weak modes buried in high-level noise, but also the discrete time series of the weak mode.