On the determination of marine magnetotelluric receiver orientation using land magnetic observatory data

Marine magnetotelluric measurements using “free‐fall’’ instruments without effective compasses suffer from the problem of unknown orientation of the receivers at the seafloor. While past works indicate that marine magnetotelluric orientation of the instruments can be estimated by reference to land deployments of known orientation using the transfer tensor method, there is limited published information on how this is implemented in practice. We document this method and propose a set of new time‐ and frequency‐domain approaches to solve this orientation problem of the seafloor receivers. We test these methodologies in onshore and offshore magnetotelluric data whose orientations are well known and apply these techniques to marine magnetotelluric data with unknown orientation. For the controlled tests, both time‐ and frequency‐domain approaches produce overall comparable results. To investigate the effects of the subsurface structure distribution on the orientation process, a dimensionality analysis of a controlled dataset is carried out. In subsequent analysis using the available disoriented marine magnetotelluric data from offshore Brazil and from the Vassouras magnetic observatory on the mainland for remote referencing, frequency‐domain methods yield approximate orientation angles among themselves with low standard deviation each. Time‐domain results are consistent for most cases but differ from frequency‐domain results for some situations.     

Application of singularity index mapping technique to gravity/magnetic data analysis in southeastern Yunnan mineral district,China

The fractal/multifractal based singularity index mapping technique efficient in characterizing singular physical or chemical properties is applied for the analysis of gravity and aeromagnetic data in southeastern Yunnan mineral district, China. As follow-up after the introduction of singularity theory to geochemical and geological mapping scenarios, this study extends its application to delineate geophysical potential fields. Based on low gravity and low magnetic properties of granitic intrusions in the study area, singularity mapping technique is used as a high-pass filter to emphasize the geophysical anomalies caused by granitic intrusions in support of future mineral exploration. Comparing with the traditionally used band-pass filtering method, it is shown that the new technique provides an improved and simplified approach in geophysical data analysis with the advantage of scale independence.     

Stability relations of ilmenite and ulvöspinel in the Fe-Ti-O system and application of these data to lunar mineral assemblages

The fO₂ stability relations of ilmenite and ulvöspinel were determined using C-O H-N gas-flow apparatus with fO₂ measured by a solid ceramic (calcia-zirconia) oxygen electrolyte cell. For Fe+TiO₂ + 1/2 O₂ =FeTiO₃ (from 850°–1050°C), 1/2 log fO₂=(−11,250/T) + 0.98 and for Fe+FeTiO₃ + 1/2 O₂ =Fe₂TiO₄ (from 850°–1210°C), 1/2 logfO₂ = (−12,170/T) + 1.93. These curves lie at significantly higher values of ?O₂ than determined by previous investigators (i.e., 3/4 and1/4 order of magnitude for ilmenite and ulvöspinel, respectively). In addition, for Fe+ 2TiO₂ + 1/2 O₂ =FeTi₂O₅ (1210°C), ΔG_r⁰=−45.8 ± 0.6 kcal. The QFI curve crosses the ulvöspinel reduction curve at ∼950°C and is at lower values of fO₂ below this temperature. The occurrences of fayalite reduction to SiO₂ + Fe in lunar rock 14053, as well as a new finding of this assemblage in 14072, are evidence for extreme sub-solidus reduction, whereas ulvöspinel breakdown alone occurs under less reducing conditions. The ‘complete’ reduction of ulvöspinel to TiO₂ + Fe occurs in 2 steps: first, to ilmenite + Fe and then, however more slowly, to rutile + Fe. Thus, the presence of ulvöspinel but lack of ilmenite reduction in lunar rocks cannot be used as evidence that the fO₂ was between the associated curves — only upper limits of fO₂ can be inferred.     

Characterization of the sedimentary cover at the Himalayan foothills using active and passive seismic techniques

The characterization of the sediments, down to bedrock, is very important from the seismological point of view in order to study the possible earthquake effects (site effects). Resonance frequency and shear-wave velocity profile are the main features used to estimate the thickness and stiffness of the sedimentary cover. To map these characteristics different geotechnical, geophysical and seismological methods have been developed and applied over a last few years. In this work, different soil investigation methods have been applied around the Himalayan foothills, focusing on three sites with different soil characteristics that span from the Doon valley to the Ganga foreland basin. Active and passive array experiments were carried out: Multichannel Analysis of Surface Waves (active MASW), Passive Remote MASW and f–k technique. A dispersion curve was estimated for every site covering a wider frequency band rather than if only one method would have been used. Moreover, ambient noise measurements were also recorded in order to apply the H/V method and to estimate the resonance frequencies. Combining the information provided from all methods and using the neighbourhood algorithm, the best suitable shear (S) wave velocity profiles were estimated for each area. In this way, soil sediments were characterized by the resonance frequency, the soil thickness and the mean S-wave velocity. It has been demonstrated that the use of different methods give coherent and more robust results than when only one method is applied. This greatly contributes to the credibility of the results.     

Astronomical calibration of the Matuyama-Brunhes boundary: Consequences for magnetic remanence acquisition in marine carbonates and the Asian loess sequences

We have compiled 19 records from marine carbonate cores in which the Matuyama-Brunhes boundary (MBB) has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes. By correlation of the δ¹⁸O data to a timescale based on astronomical forcing, we estimate astronomical ages for each of the MBB horizons. In all but one record the MBB occurs within Stage 19.

Most magnetostratigraphic sections in Asian Loess place the MBB within a loess interval. Since loess deposition is presumed to be associated with glacial intervals, loess horizons should correspond to even-numbered oxygen isotope stages. A glacial age for the MBB is at odds with the results presented here, which firmly place the MBB within interglacial Stage 19. Inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed.

The mean of the Stage 19 age estimates for the MBB is 777.9 ± 1.8 (N = 18). Inclusion of the single Stage 20 age results in a mean of 778.8 ± 2.5 (N = 19). The astronomical age estimate of the MBB compares favorably with an (unweighted) mean of 778.2 ± 3.5 (N = 10) from a compilation of ⁴⁰Ar/39^Ar results of transitional lava flows. Combining the two independent data sets yields a grand mean of 778.0 ± 1.7 (N = 28).

The new compilation shows virtually no trend in placement of the MBB within isotope Stage 19 as a function of sediment accumulation rate. We interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment-water interface.

Separating the cores into two geographic regions (an Indo-Pacific-Caribbean [IPC] Group and an Atlantic Group) results in a significant difference in the position of the mid-point of the reversal with respect to the astronomical time scale. The data presented here suggest a difference of several thousand years between the two regions. This observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest, systematic differences in remanence acquisition, or by genuine differences in the timing of the directional changes between the two regions.     

Chemometrics data analysis of marine water quality and source identification in Southern Hong Kong

Various chemometric methods were used to analyze data sets of marine water quality for 19 parameters measured at 16 different sites of southern Hong Kong from 2000 to 2004 (18,240 observations), to determine temporal and spatial variations in marine water quality and identify pollution sources. Hierarchical cluster analysis (CA) grouped the 12 months into three periods (January-April, May-August and September-December) and the 16 sampling sites into two groups (A and B) based on similarities in marine water-quality characteristics. Discriminant analysis (DA) was important in data reduction because it used only eight parameters (TEMP, TURB, Si, NO(3)(-)-N, NH(4)(+)-N, NO(2)(-)-N, DO, and Chl-a) to correctly assign about 86% of the cases, and five parameters (SD, NH(4)(+)-N, TP, NO(2)(-)-N, and BOD(5)) to correctly assign >81.15% of the cases. In addition, principal component analysis (PCA) identified four latent pollution sources for groups A and B: organic/eutrophication pollution, natural pollution, mineral pollution, and nutrient/fecal pollution. Furthermore, during the second and third periods, all sites received more organic/eutrophication pollution and natural pollution than in the first period. SM5, SM6, SM17, SM10, SM11, SM12, and SM13 (second period) were affected by organic and eutrophication pollution, whereas SM3 (third period) and SM9 (second period) were influenced by natural pollution. However, differences between mineral pollution and nutrient/fecal pollution were not significant among the three periods. SM17 and SM10 were affected by mineral pollution, whereas SM4 and SM9 were highly polluted by nitrogenous nutrient/fecal pollution.     

Comparison of industrial and high-resolution thermal data in a sedimentary basin

Temperature, temperature gradient and heat flow, derived from four wells in the Western Canada Sedimentary Basin have been compared with similar quantities derived from maps constructed from industrial data. Individual industrial temperature data may differ from the high-resolution temperature log by up to 30 K, but linear regression of the collected data agrees within 10 K at all points observed. Some evidence suggests that measured conductivities, using drill-cuttings, are biased toward average values. Derived heat flows show agreement of heat flow within 10% within the Mesozoic section. In the Paleozoic section differences are greater, and more varied, with insufficient data for general conclusions. Both styles of measurement provide opportunities for interpretation, each contributing to thermal analysis of sedimentary basins.     

Curie-point depth from spectral analysis of magnetic data in central-southern Europe

The basal depth of the outer layer with internal magnetic sources was calculated from magnetic data available within a roughly 500 km wide and 1200 km long area, running from central Germany to southern Italy. The dataset, deriving from different aeromagnetic surveys, is reduced to the reference altitude of 3000 m a.s.l. and a reference year of 1980.0. The adopted method, which transforms the spatial data into the frequency domain, provides a relationship between the two-dimensional spectrum of the magnetic anomalies and the top and centroid depths of the magnetic sources. The magnetic layer bottom depth (MLBD) thus obtained is 29-33 km deep in the stable areas (central Europe Variscan units, Corsica-Sardinia Variscan block) and corresponds to the Moho, having an average temperature of 560 °C. From the Alps to the Apennines, MLBD ranges between 22 and 28 km and is clearly shallower than the Moho. In these units, the wide variation of MLBD appears to be compatible with the presence of shallow magnetised bodies, consisting of lower crustal rocks (Ivrea-Verbano zone), ophiolitic units (Penninic zone and Voltri Massif) and intrasedimentary basic volcanic bodies (Po Basin). An average value of 25 km can be attributed to MLBD, which corresponds to a temperature of 550 °C. In the peri-Tyrrhenian zone and the Ligurian Sea, MLBD is below the Moho, which ranges from 17 to 20 km depth, and it has a temperature matching approximately to the Curie temperature of magnetite (580 °C).     

Characterization of seasonal local recharge using electrical resistivity tomography and magnetic resonance sounding

A groundwater recharge process of heterogeneous hard rock aquifer in the Moole Hole experimental watershed, south India, is being studied to understand the groundwater flow behaviour. Significant seasonal variations in groundwater level are observed in boreholes located at the outlet area indicating that the recharge process is probably taking place below intermittent streams. In order to localize groundwater recharge zones and to optimize implementation of boreholes, a geophysical survey was carried out during and after the 2004 monsoon across the outlet zone. Magnetic resonance soundings (MRS) have been performed to characterize the aquifer and measure groundwater level depletion. The results of MRS are consistent with the observation in boreholes, but it suffers from degraded lateral resolution. A better resolution of the regolith/bedrock interface is achieved using electrical resistivity tomography (ERT). ERT results are confirmed by resistivity logging in the boreholes. ERT surveys have been carried out twice—before and during the monsoon—across the stream area. The major feature of recharge is revealed below the stream with a decrease by 80% of the calculated resistivity. The time‐lapse ERT also shows unexpected variations at a depth of 20 m below the slopes that could have been interpreted as a consequence of a deep seasonal water flow. However, in this area time‐lapse ERT does not match with borehole data. Numerical modelling shows that in the presence of a shallow water infiltration, an inversion artefact may take place thus limiting the reliability of time‐lapse ERT. A combination of ERT with MRS provides valuable information on structure and aquifer properties respectively, giving a clue for a conceptual model of the recharge process: infiltration takes place in the conductive fractured‐fissured part of the bedrock underlying the stream and clayey material present on both sides slows down its lateral dissipation. Copyright © 2007 John Wiley & Sons, Ltd.     

The provenance analysis of Late Triassic sedimentary sequences in Tethyan Himalaya: The tectonic attribute of materials at the convergent margin

<正>The Late Triassic is one of the key breakup periods of the eastern Gondwana margin and early-stage evolution of the Neo-Tethys, which traversed Laurasia and Gondwana at that time. A series of Late Triassic marine sedimentary sequences, preserved within the Himalaya (e.g. Tethyan Himalaya) of southern Tibet, provides crucial and reliable     

Age modelling of late Quaternary marine sequences in the Adriatic: Towards improved precision and accuracy using volcanic event stratigraphy

The first part of this paper presents a review of the problems that constrain the reliability of radiocarbon-based age models with particular focus on those used to underpin marine records. The reasons why radiocarbon data-sets need to be much more comprehensive than has been the norm hitherto, and why age models should be based on calibrated data only, are outlined. The complexity of the probability structure of calibrated radiocarbon data and the advantages of a Bayesian statistical approach for constructing calibrated age models are illustrated. The second part of the paper tests the potential for reducing the uncertainties that constrain radiocarbon-based age models using tephrostratigraphy. Fine (distal) ash layers of Holocene age preserved in Adriatic prodelta sediments are analysed geochemically and compared to tephras preserved in the Lago Grande di Monticchio site in southern Italy. The Monticchio tephras have been dated both by radiocarbon and varve chronology. The importance of basing such comparisons on standardised geochemical and robust statistical procedures is stressed. In this instance, both the Adriatic and Monticchio geochemical measurements are based on wavelength dispersive spectrometry, while discriminant function analysis is employed for statistical comparisons. Using this approach, the ages of some of the Adriatic marine ash layers could be estimated in Monticchio varve years, circumventing some of the uncertainty of radiocarbon-based age models introduced by marine reservoir effects. Fine (distal) ash layers are more widespread and better preserved in Mediterranean marine sequences than realised hitherto and may offer much wider potential for refining the dating and correlation of Mediterranean marine sequences as well as marine-land correlations.     

基于Gramian约束与高斯-牛顿算法的重磁三维联合反演

为了降低重力与磁法单独反演中的非唯一性问题,本文实现了一种基于Gramian约束的重磁三维联合反演算法.使用基于模型参数或其空间梯度的Gramian算子,将其添加到目标函数中,约束剩余密度与剩余磁化率模型参数或其梯度更具相关性.采用高斯-牛顿法对目标函数进行最优化求解,对求解方程进行了简化,节省内存并提高了计算效率.反演过程中使用对数法约束模型参数的上下界,使反演结果更符合真实地质情况.提出了新的模型更新步长计算策略,同时考虑了数据拟合与联合约束的影响,使联合反演更加稳定.通过三个数值模型验证了算法的可靠性,并进一步将该算法应用于加拿大McFaulds湖实测航空重磁数据.对比了反演结果切片并展示了交会图,联合反演得到的剩余密度与剩余磁化率的物性、梯度分布都比单独反演的相关性更强.联合反演提供了一个既能拟合数据又在一定程度上满足耦合条件的反演解.验证了所开发的联合反演算法在实际数据上的有效性与适用性.

     

Unravelling the patterns of alluvial fan development using mineral magnetic analysis: examples from the Sparta Basin,Lakonia, southern Greece

Mineral magnetic analysis of B‐horizons of soils developing upon the surfaces of alluvial fans was undertaken in order to: (i) differentiate and rank discrete fan surfaces by order of formation; (ii) establish whether fan surfaces formed simultaneously in adjacent fan systems; and (iii) deduce probable patterns of fan development. The results of the analysis indicate that the greatest concentration of ferrimagnetic and anti‐ferromagnetic minerals occurs within soils which have developed upon the proximal fan surfaces with a progressive reduction in magnetic minerals in soils associated with medial and distal surfaces. The build‐up of magnetic minerals in the proximal fan soils suggests that these surfaces formed first followed by the medial and distal surfaces. With the exception of the Kalivia Sokas fan, the majority of depositional events responsible for fan surface formation occurred simultaneously, suggesting that adjacent fan systems share broadly similar depositional histories. Although the precise timing of depositional events is uncertain, it is probable that by the end of the late Pleistocene, small, largely undissected fans comprising two to three surfaces had formed. At the start of the Holocene, fan systems experienced significant fanhead incision. A net distal extension of the fan trench coupled with a progressive basinward shift of the locus of deposition during the middle and late Holocene resulted in formation of medial and distal fan surfaces. Changes in climate are deemed to be the major control of fanhead incision, fan trenching and fan surface formation. However, the likely effects of long‐term tectonic activity and approximately five thousand years of human occupation upon fan development in the Sparta Basin remain unclear. Copyright © 2000 John Wiley & Sons, Ltd.     

地磁数字与模拟观测资料对比分析

“九五”地震前兆数字化台站改造后，数字化的地磁观测仪器在全国各地磁台站推广应用已有两年多时间，两年来，数字化观测仪器和模拟记录仪器在武汉台一直进行并行观测工作，其观测资料稳定可靠。本文通过GM-3磁通门磁力仪与其它仪器进行数字和模拟观测资料的时均值、日均值、月均值、年均值对比分析。分析结果表明数字观测资料优于模拟观测资料，数字观测仪器完全可以取代模拟观测仪器。     

Reconstruction of the near‐offset gap in marine seismic data using seismic interferometric interpolation

In conventional seismic exploration, especially in marine seismic exploration, shot gathers with missing near‐offset traces are common. Interferometric interpolation methods are one of a range of different methods that have been developed to solve this problem. Interferometric interpolation methods differ from conventional interpolation methods as they utilise information from multiples in the interpolation process. In this study, we apply both conventional interferometric interpolation (shot domain) and multi‐domain interferometric interpolation (shot and receiver domain) to a synthetic and a real‐towed marine dataset from the Baltic Sea with the primary aim of improving the image of the seabed by extrapolation of a near‐offset gap. We utilise a matching filter after interferometric interpolation to partially mitigate artefacts and coherent noise associated with the far‐field approximation and a limited recording aperture size. The results show that an improved image of the seabed is obtained after performing interferometric interpolation. In most cases, the results from multi‐domain interferometric interpolation are similar to those from conventional interferometric interpolation. However, when the source–receiver aperture is limited, the multi‐domain method performs better. A quantitative analysis for assessing the performance of interferometric interpolation shows that multi‐domain interferometric interpolation typically performs better than conventional interferometric interpolation. We also benchmark the interpolated results generated by interferometric interpolation against those obtained using sparse recovery interpolation.     

融合多源海洋大地测量数据的南海海底地形多层感知机反演

本文融合SIO(Scripps Institution of Oceanography)发布的垂线偏差、重力异常和垂直重力梯度数据及NCEI(National Centers for Environmental Information)发布的船载测深数据, 利用多层感知机神经网络(Multi-Layer Perceptron, MLP)建立南海海域(108°E—121°E, 6°N—23°N)分辨率为1'×1'的海底地形模型(MLP_Depth).首先, 将642716个船载测深控制点的位置信息与周围4'×4'格网点处的地球重力信息(垂线偏差、重力异常、垂直重力梯度)作为输入数据, 将船载测深控制点处实测水深值作为输出数据, 训练MLP神经网络模型, 训练结束时决定系数R²为99%, 平均绝对误差MAE为39.33 m.然后, 将研究区域内1'×1'格网正中心点处的输入数据输入于MLP模型中, 可得格网正中心点处的预测海深值.最后, 根据预测海深值建立研究区域范围内分辨率为1'×1'的MLP_Depth模型.将MLP_Depth模型预测水深与160679个检核点处实测水深对比, 其差值的标准差STD(75.38 m)、平均绝对百分比误差MAPE(5.89%)与平均绝对误差MAE(42.91 m)皆优于GEBCO_2021模型、topo_23.1模型、ETOPO1模型与检核点实测水深差值的STD(108.88 m、113.41 m、229.67 m)、MAPE(6.11%、6.94%、18.37%)与MAE(47.33 m、52.24 m、130.08 m).同时, 为了研究不同区域内利用该方法建立的海底地形模型的精度, 本文在研究区域内分别建立了A、B区域的海底地形模型(MLP_Depth_A、MLP_Depth_B).经过验证得: MLP_Depth_A、MLP_Depth_B相比于MLP_Depth模型具有更高的精度, 更能反应海底地形的变化趋势.

     

Characterization of groundwater quality in Kinmen Island using multivariate analysis and geochemical modelling

Kinmen is located in the south‐west of Mainland China and mainly has two islands, Large Kinmen and Leiyu. Residents in Kinmen have drunk groundwater over several decades. This work characterized the quality of groundwater in Kinmen using factor analysis (FA), cluster analysis (CA) and geochemical simulation. The factor scores were plotted spatially to illustrate the groundwater quality and were used to discuss the grouped relationship using CA. Salinization, redox and organic matter factors are identified from 17 hydrochemical measurements in 18 wells. Acidic and oxidizing groundwater with nitrate‐N pollution is distributed mainly in the west of Large Kinmen. Saline groundwater is distributed to the north‐east of Large Kinmen and in the south of Leiyu. Groundwater with organic matter is present throughout Leiyu. Five groups of the groundwater quality divided by CA can be interpreted according to one to three factors. The grouped characteristics of the groundwater quality help the local government and industries to plan the use and protection of groundwater resources. Furthermore, a geochemical simulation was used to demonstrate the formation processes of the acidic and oxidizing groundwater properties in granitic sediments. A large amount of the precipitation of the kaolinite and magnetite releases concentrations of hydrogen ion and raises the redox potential in the aquifers. Copyright © 2007 John Wiley & Sons, Ltd.