On the use of the autocorrelation method for estimating the depth to the magnetic basement

The depth to the top of magnetic dykes can be estimated from total field aeromagnetic data using the relation between the depth to magnetic sources and the autocorrelation function of magnetic data. By using synthetic anomalies we show that in the ideal case, depth can be determined to an accuracy of 10% or better, when the anomaly sources are two-dimensional dykes. However, the estimated depths depend on the width of the dykes. The estimated depth is about 0.6 times the actual depth to the top of thin dykes, and around the true depth for thick dykes having width-to-depth ratio around 3. The depth is considerably overestimated for very thick dykes (e.g., contacts, which is a special case of the thick dyke). Thus, the autocorrelation method requires that the width-to-depth ratio of the dyke is estimated independently to correctly estimate the depths. Alternatively, it must be assumed that the width-to-depth ratio for the two-dimensional source body is between 1.5 and 4.     

A method of searching for radial velocity anomalies in the lower mantle using interfering signals

Radial velocity anomalies in the lower mantle that give rise to triplications in the travel-time curve for short-periodP waves will produce arrivals havingdT/d values that differe by roughly 0.2–0.5 s/deg. The first two arrivals associated with such triplications will be separated by less than one second over a distance range of 4°–10° they may not, therefore, be separable visually on single seismograms, so that their presence can only be inferred from some measurable property that depends on their mutual interference. If there are lateral variations in the regions of anomalous velocity gradients, the interfering signals will also have different azimuths of arrival. Using two synthetic wavelets we have investigated the effect of interference on bothdT/d and azimuth measurements at the Yellowknife Array. We found that if the interfering pulses have a dominant frequencyv, there is a range of time separations (0.30/v0.55/v) over which the measureddT/d and azimuth values may fluctuate by much more than the differences indT/d and azimuth between the interfering signals. We have evaluated the following empirically defined functions for three different primary signals, and for three different relative amplitudes of the interfering signals:f (t), the drift function, which expresses how the measured slownesses,p, and azimuths, , differ from the slownesses and azimuths of the primary wavelets; f(), the range function, which describes the behaviour of the upper and lower bounds ofp and as a function of the difference in arrival times of the signals, andf , studied the properties of these functions, and have outlined how these properties provide criteria based on the numerical and statistical characteristics of the arrival vectors, and on the waveform of the signal that will enable small radial velocity anomalies to be more clearly delineated.Contribution No. 863 from the Earth Physics Branch.     

A trial for monitoring temporal variation of seismic velocity using an ACROSS system

Abstract The temporal variation of seismic velocity near the Nojima Fault, which ruptured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake), was detected using an accurately controlled routine-operated seismic source (ACROSS). The source generates elastic waves by a centrifugal force of an eccentric mass rotating around an axis. The mass is driven with an AC servomotor whose angular position is accurately controlled with reference to a very accurate global positioning system (GPS) clock. The error of the mass' position is less than 0.002 radian and does not accumulate. As a result, the source generates sinusoidal waves of very narrow spectral peaks enabling their detection with an excellent signal-to-noise ratio. Although the stability of the rotation is quite excellent, a large daily variation was found, which seems to be caused by changes in atmospheric temperature. The daily variation was 10% in amplitude and 0.1 radian in phase of the signal observed at the 800 m borehole seismometer. A significant variation was found to be due to that of coupling between the rotational source and the foundation made of reinforced concrete in which the source was situated. In order to make a correction on the signal of the 800 m borehole seismometer, the vibration of the foundation was measured and modeled assuming a rigid body movement. The correction successfully reduced the daily variation by approximately 90%, resulting in a variation of 1% in amplitude and 0.01 radian in phase. The phase variation of 0.01 radian corresponds to 100 μs and less than 0.1% in velocity over 1000 m between the source and the receiver.     

On the use of meiofauna in ecological monitoring: Who needs taxonomy?

Taxonomic problems have hindered the widespread use of meiobenthos for the purpose of pollution monitoring despite its potential usefulness. The question whether it is necessary to go down to species level in order to distinguish assemblages and stations was studied using data on meiofauna densities available from a GEEP-workshop held in Norway, August 1986.

Using the complete species list of harpacticoid copepod and nematode species, the six stations could be clearly separated by different multivariate techniques (TWINSPAN, DCA, MDS, Clustering). The abundance data of the genera, the families and even the orders (in the case of nematodes) still allowed a more or less clear separation of the stations, although their relative relationships were sometimes distorted. Thus, from a practical point of view, it may be sufficient to identify the animals to genus or family level.

It is proven that this result is not due to ecological similarities between congeneric or confamiliar species. The species groups typifying the stations in the TWINSPAN analysis did not contain more confamiliar species pairs than expected from random sorting. Randomly assembled ‘families’ allowed a separation of the stations as clear as that based on taxonomic groups.

We conclude that the information contained in species abundances was preserved, at least partially, when the species are grouped, even at random. The grouping in higher taxonomic categories was as good as any, and more convenient than all the others. However, within the (limited) range of habitats studied, ecological adaptation is achieved nearly exclusively on the species level.     

Preliminary results on a near-real-time rock slope damage monitoring system based on relative velocity changes following the September 5, 2022 MS 6.8 Luding,China earthquake

Relative seismic velocity change (dv/v) is important for monitoring changes in subsurface material properties and evaluating earthquake-induced rock slope damage in a geological disaster-prone region. In this paper, we present a rapid damage assessment on three slow-moving rock slopes by measuring dv/v decrease caused by the 2022 ?M_S 6.8 Luding earthquake in Southwest China. By applying the stretching method to the cross-correlated seismic wavefields between sensors installed on each slope, we obtain earthquake-induced dv/v decreases of ～2.1%, ～0.5%, and ～0.2% on three slopes at distances ranging from ～86 to ～370 ?km to the epicenter, respectively. Moreover, based on seismic data recorded by 16 sensors deployed on the rock slope at a distance of ～370 ?km away from the epicenter, a localized dv/v decease region was observed at the crest of the slope by calculating the spatial dv/v images before and after the earthquake. We also derive an empirical in situ stress sensitivity of ?7.29?10^?8/Pa by relating the dv/v change to the measured peak dynamic stresses. Our results indicate that a rapid dv/v assessment not only can help facilitate on-site emergency response to earthquake-induced secondary geological disasters but also can provide a better understanding of the subsurface geological risks under diverse seismic loadings.     

On the use of horizontal acoustic Doppler profilers for continuous bed shear stress monitoring

Continuous monitoring of bed shear stress in large river systems may serve to better estimate alluvial sediment transport to the coastal ocean.Here we explore the possibility of using a horizontally deployed acoustic Doppler current profiler(ADCP) to monitor bed shear stress,applying a prescribed boundary layer model,previously used for discharge estimation.The model parameters include the local roughness length and a dip correction factor to account for sidewall effects.Both these parameters depend on river stage and on the position in the cross-section, and were estimated from shipborne ADCP data.We applied the calibrated boundary layer model to obtain bed shear stress estimates over the measuring range of the HADCP.To validate the results,co-located coupled ADCPs were used to infer bed shear stress,both from Reynolds stress profiles and from mean velocity profiles. From HADCP data collected over a period of 1.5 years,a time series of width profiles of bed shear stress was obtained for a tidal reach of the Mahakam River,East Kalimantan,Indonesia.A smaller dataset covering 25 hours was used for comparison with results from the coupled ADCPs.The bed shear stress estimates derived from Reynolds stress profiles appeared to be strongly affected by local effects causing upflow and downflow,which are not included in the boundary layer model used to derive bed shear stress with the horizontal ADCP.Bed shear stresses from the coupled ADCP are representative of a much more localized flow,while those derived with the horizontal ADCP resemble the net effect of the flow over larger scales.Bed shear stresses obtained from mean velocity profiles from the coupled ADCPs show a good agreement between the two methods,and highlight the robustness of the method to uncertainty in the estimates of the roughness length.     

Identification of hydrological neighborhoods for regional flood frequency analysis using statistical depth function

The adoption of hydrological neighborhoods is one of the common approaches employed for the delineation step in regional frequency analysis (RFA). Traditional methods proposed for building hydrological neighborhoods are mainly based on distance metrics. These methods have some limitations. They are not robust against outliers, they are not affine invariant and they require site characteristics to be normally distributed. To overcome these limitations, the present paper aims to propose a new robust method to identify the neighborhood of a target site. The proposed method is based on the statistical notion of depth function. More precisely, a similarity measure derived from depth functions is used to compute the similarities between the target sites and the gauged ones. A data set from the southern part of the province of Quebec (Canada) is used to compare the proposed method with traditional ones. The obtained results indicate that the depth-based method leads to neighborhoods that are more homogeneous and more efficient for quantile estimation, than those obtained by traditional methods. The triangular shape of neighborhoods obtained by the proposed approach makes it practical and flexible.     

Sampling frequency for monitoring the actual state of groundwater systems

Sampling frequency is a very important variable in the design of a groundwater monitoring network. Given the objective of sampling as monitoring the actual state of groundwater systems, criteria for the determination of sampling frequency can be based on the trend detectability, the accuracy of estimation of periodic fluctuations and the accuracy of estimation of the mean values of the stationary component of the state variables (such as groundwater heads, temperature, and concentration of hydrochemical constituents). The' criteria are applied to the determination of sampling frequency for monitoring groundwater levels around the Spannenburg pumping station. The analysis and verification of the sampling frequency indicate that the most appropriate sampling frequency is once a month.     

用转换函数方法研究喜马拉雅地区速度结构

利用流动数字地震台网提供的三分量地震波形记录,应用转换函数及快速模拟退火算法对喜马拉雅山脉地区46个地震站下的地壳横波速度结构进行了反演,为进一步揭示青藏高原喜马拉雅山脉地区的动力学演化过程提供了可靠的地球物理证据.根据本文结果可清晰看到,喜马拉雅山脉地区作为当今地壳活动最活跃的地区,物质交换非常活跃,地下结构远远未达到平衡,地壳速度有很大差异,在板块边界处莫霍界面速度间断不是非常明显,自喜马拉雅南坡向高原腹地,地壳厚度大致从55 km增加到80 km;沿经度方向,莫霍面也有一定的起伏.通过研究得到另外一个证据是,在喜马拉雅的主中央逆冲断裂,由大陆碰撞产生的主要构造,其深度可能要大于80 km.     

Global scale patterns of continental fragmentation: Wilson's cycles as a constraint for long-term sea-level changes

We propose to characterize land–ocean distributions over Late Proterozoic to Phanerozoic times from measurement of perimeters and areas of continental fragments, based on paleomagnetic reconstructions. These measurements serve to calculate geophysically constrained breakup and scatter indexes of continental land masses from 0 to 1100 Ma. We then provide quantitative investigation and modelling of relationships between scatter of continental landmasses and mean age of the oceanic lithosphere during Mesozoic times, which appears to range from 56 to 62 Ma over the last 170 My. We then inverse the scatter of continental landmasses in terms of global oceanic crust mean age over the last 600 My, i.e. back in times where no measurement of seafloor accretion history is possible because of subduction. We finally show that the inferred evolution of oceanic lithosphere mean age over the Phanerozoic remarkably correlates in time with long-term sea-level changes since the Cambrian.     

汶川地震区地壳速度相对变化的环境噪声自相关研究

2006年中国地震局地质研究所地震动力学国家重点实验室在川西地区(100°E～105°E, 26°N～32°N)布设了由297个宽频带地震台组成的密集流动地震观测台阵.本文利用川西流动地震台阵29°N以北地区的137个台站2007年1月至2008年10月的连续三分量地震环境噪声记录,研究了汶川地震震前震后地壳速度变化特征.借助川西台阵的密度优势,我们针对单台三分量噪声自相关函数和单台不同分量间的互相关函数,利用互谱移动窗技术,在0.33～1 Hz频带范围内测量了经过50天滑动平均的相关函数与长时间平均参考相关函数的走时变化率,进而求得地壳浅部速度随时间的相对变化,并得到其空间分布特征.本文结果表明,利用环境噪声自相关方法可以得到与互相关方法基本一致的同震速度变化分布图像,同震速度变化分布与同震体应变具有明显相关性.具有更高空间分辨率的噪声自相关研究进一步发现,在鲜水河断裂和龙门山断裂交汇区存在同震速度增加区,这个区域与同震库伦应力变化和地表形变观测预测的周边断层库伦应力增加区一致.我们的研究发现,同震库伦应力增加效应的持续时间大约为2个月左右,在此之后,区域应力场逐渐恢复为普遍的应力下降.     

Listening to the Deep: live monitoring of ocean noise and cetacean acoustic signals

The development and broad use of passive acoustic monitoring techniques have the potential to help assessing the large-scale influence of artificial noise on marine organisms and ecosystems. Deep-sea observatories have the potential to play a key role in understanding these recent acoustic changes. LIDO (Listening to the Deep Ocean Environment) is an international project that is allowing the real-time long-term monitoring of marine ambient noise as well as marine mammal sounds at cabled and standalone observatories. Here, we present the overall development of the project and the use of passive acoustic monitoring (PAM) techniques to provide the scientific community with real-time data at large spatial and temporal scales. Special attention is given to the extraction and identification of high frequency cetacean echolocation signals given the relevance of detecting target species, e.g. beaked whales, in mitigation processes, e.g. during military exercises.     

Bivariate flood frequency analysis using the copula function: a case study of the Litija station on the Sava River

As an alternative to the commonly used univariate flood frequency analysis, copula frequency analysis can be used. In this study, 58 flood events at the Litija gauging station on the Sava River in Slovenia were analysed, selected based on annual maximum discharge values. Corresponding hydrograph volumes and durations were considered. Different bivariate copulas from three families were applied and compared using different statistical, graphical and upper tail dependence tests. The parameters of the copulas were estimated using the method of moments with the inversion of Kendall's tau. The Gumbel–Hougaard copula was selected as the most appropriate for the pair of peak discharge and hydrograph volume (Q‐V). The same copula was also selected for the pair hydrograph volume and duration (V‐D), and the Student‐t copula was selected for the pair of peak discharge and hydrograph duration (Q‐D). The differences among most of the applied copulas were not significant. Different primary, secondary and conditional return periods were calculated and compared, and some relationships among them were obtained. Copyright © 2014 John Wiley & Sons, Ltd.     

Improving the gradient of the image‐domain objective function using quantitative migration for a more robust migration velocity analysis

Migration velocity analysis aims at determining the background velocity model. Classical artefacts, such as migration smiles, are observed on subsurface offset common image gathers, due to spatial and frequency data limitations. We analyse their impact on the differential semblance functional and on its gradient with respect to the model. In particular, the differential semblance functional is not necessarily minimum at the expected value. Tapers are classically applied on common image gathers to partly reduce these artefacts. Here, we first observe that the migrated image can be defined as the first gradient of an objective function formulated in the data‐domain. For an automatic and more robust formulation, we introduce a weight in the original data‐domain objective function. The weight is determined such that the Hessian resembles a Dirac function. In that way, we extend quantitative migration to the subsurface‐offset domain. This is an automatic way to compensate for illumination. We analyse the modified scheme on a very simple 2D case and on a more complex velocity model to show how migration velocity analysis becomes more robust.     

On some of the relations for vertical velocity in the atmosphere

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远程事件次声监测中的信号参数二维子空间计算方法

越来越多的观测发现,在地震、火山爆发、泥石流等重大自然灾害发生前,常产生异常的次声信号,这为地震及其他自然灾害的预报工作增加了一种可能的信息;同时,次声还是监测大气层、浅地表爆炸的有效手段.在自然灾害和爆炸事件次声监测中,慢度和方位角等参数对于源信号传播、定位以及源性质识别等工作具有重要意义.然而,目前的慢度和方位角等参数的算法--频率波数（FK）分析法,尚存在精度和分辨率不高等问题,特别是对多源次声信号的识别能力较差.为提高次声信号的监测精度,基于次声信号和噪声的子空间不相关性,构建了次声信号慢度和方位角二维子空间计算模型,并在此基础上提出了一种高分辨率次声信号二维子空间算法,仿真实验和实际数据的对比分析结果表明：本文提出的方法在精度和分辨率方面明显优于FK法,且能够更好地分离多源次声信号.

     

Relationship between cyclic variations in the velocity and occurrence frequency of coronal mass ejections and cyclic changes in the structure of the large-scale solar magnetic field

This paper investigates the relationship between cyclic variations in the velocity of coronal mass ejections and cyclic changes in the structure of the large-scale solar magnetic field (LSSMF) in solar cycle 23, using the effective solar multipole index as a parameter of the characteristic dimensions of LSSMF structural elements. Cyclic variations in the velocity and frequency of coronal mass ejections are found to resemble cyclic changes in the effective solar multipole index. It is suggested that cyclic changes in the maximum velocity of coronal mass ejections are associated with different conditions for the formation of complexes of active regions connected by coronal arch structures, the energy of which is the main source of energy (velocity) for coronal mass ejections. The study leads to some suggestions about the possible site of initiation of coronal mass ejections.