降雨对阳原地震台地电阻率的影响

降雨对阳原地震台地电阻率有较大影响,地电阻率观测曲线下降幅度随着降雨量的增大而增大.经核实,在降雨后,地电阻率的快速下降变化与表层地电阻率下降及大秦铁路有关.地电阻率观测供电时,会有部分电流流向铁轨,造成观测值降低.降雨结束后,随着钢轨与大地的导电性减弱,地电阻率上升恢复.褶积滤波法可有效去除降雨的干扰,显示1991年...     

兴济地电阻率观测精度提高实验

通过增大测量极距和提高供电电流2种实验途径,探讨提高兴济地电台地电阻率观测精度,结果表明,提高供电电流能明显提高兴济地震台地电阻率观测精度,且与供电电流提高倍率相当,而增大测量极距效果甚微。其原因是,提高供电电流是在干扰幅值不变情况下增大人工供电电位差,而增大测量极距使人工供电电位差和干扰幅值同比例增大,二者相抵,对观测精度作用不明显。     

井下地电阻率观测中地表电流干扰影响计算

本文把地电阻率台站地下介质简化为水平三层均匀介质模型,以点电流模拟地表干扰电流,针对对称四极观测装置,计算了在不同电性结构中的不同深度观测时,地表电流干扰源对对称四极装置地电阻率观测的影响,计算得到:地表干扰源对电阻率观测的影响取决于电性结构的类型和层参数、供电电极和测量电极的埋深以及避开干扰源的距离.本文研究结果对实施井下电阻率观测中台址电性结构的选择、电极埋深、干扰源避让距离等有参考意义.     

地电阻率交流观测中的电感性耦合效应特征模拟和分析

地电阻率交流观测方法能够有效抑制测区杂散电流的干扰,包括地铁、轻轨等城市轨道交通的干扰,但是由供电线路与测量线路间磁场的相互作用所引起的电感性耦合效应会对观测结果产生一定的影响.为研究电感性耦合效应的特征,本文建立了地电阻率交流观测的电感性耦合模型,同时计算了供电线路与测量线路间的互感系数.在此基础上,通过正演模拟分析供电信号频率、地电阻率值、供电线和测量线间距离以及平行长度等参数对电感性耦合效应的影响,并将电感性耦合模型应用于台站实际观测数据的分析中,验证模型的合理性.结果表明,电感性耦合效应随供电信号频率、供电线与测量线平行长度的增大而增大,随供电线与测量线间距离的增大而减小,随地电阻率的变化存在峰值;且模型预测的地电阻率值与台站实际观测值有较好的一致性.本文的研究结果为消除地电阻率交流观测值中的电感性耦合效应提供了理论依据和基本模型,为地电阻率交流观测系统的设计和建设提供参考.     

深井电极交叉供电对自然电位的影响

通过实验论证平凉地震台深井电阻率观测自然电位畸变由电极交叉供电造成,讨论分析不同电极供电对自然电位的影响。结果表明:对测量极供电,会影响自然电位,不会影响电阻率测值;单极供电对自然电位的影响表现为阶跃突跳,并以指数形态恢复,恢复时间约10 h,干扰峰值与供电电流、供电时间等有关;对观测数据进行校正,取得较好效果。     

深埋电极的地电阻率观测研究

首先研究了四极观测系统装置系数与电极埋深的关系;然后给出了点电流源在3层地壳模型的地表和第二层时,电源所在层的电位的解析表达式;最后将天津宝坻地区的电性结构简化成一个3层模型,计算给出了当地表层和基岩中的电阻率出现变化时,在地表和基岩上层开展四极地电阻率观测结果与供电极距和深度的关系.     

地表点电流对地电阻率观测的影响

<正>将地电阻率台站地下介质简化为水平层状均匀介质模型,以点电流模拟地表干扰电流源,针对对称四极观测装置,利用转换函数滤波器法计算电流大小、方位角、距离对地电阻率观测的影响。(1)计算方法。地电阻率采用对称四极观测装置,AB为供电电极,MN为测量电极,电极均位于地表,测区采用EW和NS两个相互垂直测向布极,中心点为O。选择理论对称四极装置的奥尼尔滤波器(O’Neill20),依据姚文斌(1989)利用转换函数滤波器法计算地电阻率表达式计算对称四极装置地电阻     

小波方法在地电场干扰处理中的分析研究

该文在对地电场信号及地电阻率人工供电干扰信号的时频特征进行分析的基础上, 提出了小波域阈值滤波方法, 对地电场秒采样信号中叠加地电阻率人工供电干扰信号进行了分析和处理, 处理产生的误差在地电场观测的允许误差范围之内。     

水平两层均匀介质中井下电阻率观测信噪比的理论计算

在点电源水平两层均匀介质模型下计算了在不同地电断面中观测时地表干扰电流源对观测的影响。得到:地表干扰电流源对地电阻率观测电势差的影响取决于地电断面类型和参数、供电电极和测量电极的埋深以及避开干扰源的距离。本文研究结果对实施井下电阻率观测中台址电性结构选择、电极埋深、干扰源避让有参考意义。     

分形插值法在地电场干扰数据处理中的应用

当地电场和地电阻率同场地观测时,地电场观测会受到地电阻率观测的供电干扰,这类干扰时间短、干扰形态和出现时间固定,影响了地电场观测数据的正常变化形态,给数据分析和地震科学研究造成困难.为解决这一干扰问题,本文在比较分形插值方法与传统插值方法优劣的基础上,采用分形插值方法对受干扰的地电场观测数据进行重建,以提高信号重建的精...     

Limestone solution rates and processes in the Waitomo District,New Zealand

Karst solution processes are investigated on Oligocene limestones in the Waitomo district, west central North Island, New Zealand. Estimates of the inputs, throughputs and outputs of water and dissolved calcium and magnesium in two drainage basins were used to establish the rate of limestone solution by autogenic waters. The best estimate for solution loss from the basins during the study year is 69 m³/km². The potential measurement errors inherent in each parameter used in the erosion rate computations were assessed and the probable maximum and minimum erosion rates were estimated to be 88 and 61 m³/km². In both basins approximately 67 per cent of the annual solute load is transported by flows greater than the mean annual discharge, over 15 per cent being transported by flood flows that are exceeded only 5 per cent of the time. Almost half of the annual load is transported during the three winter months (June-August), but no one month accounts for more than 18 per cent or less than 2.7 per cent of the annual total. Approximately 37 per cent of solution takes place within the soil profile, and most of the remainder is concentrated in 5–10 m of weathered bedrock (the subcutaneous zone) beneath this. Thus, it is likely that at least 85 per cent of the total solutional erosion contributes to the surface lowering of soil and bedrock.     

Effect of saltating sediment on flow resistance and bed roughness in overland flow

The acceleration of saltating grains by overland flow causes momentum to be transferred from the flow to the grains, thereby increasing flow resistance and bed roughness. To assess the impact of saltating sediment on overland flow hydraulics, velocity profiles in transitional and turbulent flows on a fixed sand-covered bed were measured using hot-film anemometry. Five discharges were studied. At each discharge, three flows were measured: one free of sediment, one with a relatively low sediment load, and one with a relatively high sediment load. In these flows from 83 to 90 per cent of the sediment was travelling by saltation. As a result, in the sediment-laden flows the near-bed velocities were smaller and the velocity profiles steeper than those in the equivalent sediment-free flows. Sediment loads ranged up to 87·0 per cent of transport capacity and accounted for as much as 20·8 per cent of flow resistance (measured by the friction factor) and 89·7 per cent of bed roughness (measured by the ratio of the roughness length to median grain diameter). It is concluded that saltating sediment has a considerable impact on overland flow hydraulics, at least on fixed granular beds. Saltation is likely to have a relatively smaller effect on overland flow on natural hillslopes and agricultural fields where form and wave resistance dominate. Still, saltation is generally of greater significance in overland flow than in river flow, and for this reason its effect on overland flow hydraulics is deserving of further study. © 1998 John Wiley & Sons, Ltd.     

地电阻率观测系统稳流电源技术的发展

详尽地分析了地电阻率观测系统的供电电源的发展过程。对几种电源的技术指标和性能作出了评价。在对几种电源的若干技术问题进行了分析和讨论后，指出对地电阻率观测系统的稳流电源的设计应用高频开关电源技术。介绍了作设计的主电路图。     

Solution of marble in the karst of the Pikikiruna range,Northwest Nelson,New Zealand

The paper reports the first research on karst solution processes and rates in New Zealand. The study area is an IHD representative basin in the northwest corner of the South Island in a mountain range consisting principally of Ordovician marble. The climate is sunny and warm (17°C) in summer and wet and cool (7°C) in winter. Average precipitation is 2,158 mm of which 525 mm evapotranspires, yielding a discharge of 51–75 l/s/km² in the river basin studied. Almost half of the catchment of 45.1 km² consists of karst which occurs mainly as a doline covered plateau at 600–900 m within which most drainage is subterranean. Water tracing is with fluorescein defined drainage patterns. Marble solution was established by estimating inputs, throughputs and outputs of water and dissolved calcium and magnesium in both autogenic and allogenic karst drainage systems. Particular attention was paid to estimating errors. Water samples for chemical analysis were taken irregularly for approximately one year, and a rating curve relating chemical load to discharge was established. The best estimate of solution loss from the basin yields a mean rate of 100 ±M24 m³/km²/a. Of this 80 per cent is derived from solution of marble by autogenic waters, mostly in the top 10–30 m of the marble outcrop. The remaining 20 per cent is accomplished by allogenic stream solution. Approximately 9.9 per cent of the dissolved calcium and magnesium load leaving the basin originates from non-karst rocks and 4.6 per cent is initially introduced by rainfall. River flows that are exceeded only 5 per cent of the time transport approximately 44 per cent of the annual dissolved load, while mean to low flows that occur for 75 per cent of the time transport 35 per cent of the annual solute load. This confirms the importance of low frequency-high magnitude events, but indicates also that in corrosion systems high frequency events of moderate to low magnitude can also accomplish significant work.     

Tracing sediment movement in interrill overland flow on a semi-arid grassland hillslope using magnetic susceptibility

Experiments were undertaken to determine the feasibility of tracing sediment movement in interrill overland flow. Crushed magnetite was introduced as a source-line 10 cm wide by 8 m long on a runoff plot 18 m wide by 29 m long located in southern Arizona. Initial magnetic susceptibilities along this source line, and along three transects located 0·25, 2·95 and 5 m downslope of the source-line, were measured. Movement of the magnetite in response to three rainfall simulation experiments was monitored. During the first two experiments, overland flow discharge was sampled at miniature flumes located along two cross sections on the plot downslope of the source-line, and at a supercritical flume at the plot outlet. Magnetic susceptibilities along the source-line and transects were measured after all three experiments. Results show that the magnetite moves very early in the experiments and that it reaches one of the flumes 2 m downslope of the source-line in 3 min. Most of the tracer moves a very short distance: 29·7 per cent is deposited within 25 cm of the source-line and only 2·2 per cent is deposited 2·95 m away. The deposition rate appears to decrease exponentially away from the source-line. Very little magnetite is recorded in the flow through the miniature flumes: in general it makes up less than 1 per cent of the total sediment load. No temporal pattern in these percentages is observed. Magnetite appears to be an effective tracer of sediment movement in interrill overland flow, though its higher density than natural soil may affect its detachment and transport.     

Quantifying the effect of a freeze–thaw cycle on soil erosion: laboratory experiments

In this paper we quantitatively test the hypothesis that soil freeze–thaw (FT) processes significantly increase the potential for upland hillslope erosion during run‐off events that follow thaw. We selected a highly frost‐susceptible silt to obtain an upper bound on FT effects, and completed three series of six experiments each to quantify differences in soil erosion and rill development in a bare soil following a single FT cycle. Each series represented a specific soil moisture range: 16–18 per cent, 27–30 per cent and 37–40 per cent by volume, with nominal flow rates of 0·4, 1·2 and 2·4 L/min and slopes of 8° and 15°. Each experiment used two identical soil bins: one a control (C) that remained unfrozen, and another that was frozen and thawed once. Standard soil characterization tests did not detect significant differences between the FT and C bins. We measured cross‐sectional geometry of an imposed straight rectangular rill before each experiment, sediment load during and rill cross‐sections after. Changes in cross section provided detailed measures of erosion at specific locations, while sediment load from time series run‐off samples integrated the rill erosion. Several parameters, including average maximum rill width, average maximum rill depth, rill cross‐section depth measures and sediment load, all followed similar trends. Each was greater in the FT than in the C, with values that generally increased with slope and flow. However, soil moisture was the only parameter that affected the FT/C ratios. Average sediment load grouped by soil moisture provided FT/C ratios of 2·4, 3·0 and 5·0 for low, mid and high moisture, respectively. In contrast, a ‘dry’ experiment at 4–5 per cent soil moisture had FT/C of 1·02 for sediment load. These results show a dramatic increase with soil moisture in the rate and quantity of bare soil eroded due to the FT cycle. As both FT and C results were highly sensitive to initial conditions, minimum differences in soil weight, bulk density and soil moisture through each series of experiments were required to achieve consistent results, indicating that rill erosion may be chaotic. Published in 2005 by John Wiley & Sons, Ltd.     

Antibiotic Resistance and Transferable R Plasmids in Coliforms from a Polluted Estuary

The aim of this work was to find out how far the Nervion Estuary serves as a reservoir of R plasmids and/or for their dissemination. The fecal pollution bacterial indicator population was estimated along the estuary and 167 coliform isolates were assayed for resistance to 16 antibiotics. About sixty-nine per cent of the coliform isolates were antibiotic-resistant and forty-two per cent of these were simultaneously resistant to three or more of 16 antibiotics tested. Furthermore, 31.5% of enterobacterial strains resistant to any of the antibiotics tested could transfer their resistance determinants in conjugal matings. Most of the transferred plasmids confer resistance to ampicillin or to other β-lactamic antibiotics and several codify resistance to tetracyclin, chloramphenicol of fosfomycin. Taking account of the coliforms density found and the ratio of these isolates able to transfer R plasmids, an average of 5 × 10³ coliform strains able to transfer R plasmids in 100 ml of water was obtained. This amount must indicate a significant potential public health risk because these polluted waters get into an area with intensive fishing and recreational use.     

Current stabilizer for the magnetic field of a mass spectrograph

Summary A new automatic current stabilizer is constructed, which can be used for a magnetic field of a mass spectrograph. A stability of the order of 10^–6 is attained for relatively long intervals. The dependence of the magnetic field constancy on the different parameters of the stabilizer is investigated by methods of servomechanism.     

Sediment transport in Swiss torrents

Sediment loads have been measured in six Swiss mountain torrents over several decades. Most of these torrent catchments are situated in the prealpine belt. They have catchment areas of between 0·5 and 1·7 km². Bedslopes at the measuring sites vary between 5 and 17 per cent, and peak discharges up to 12 m³ s⁻¹ have been recorded. Geophone sensors installed in the Erlenbach stream allow bedload transport activity to be monitored and sediment volumes associated with each flood event to be determined. A detailed analysis of the measurements in this stream results in an empirical equation in which the sediment load per flood event is expressed as a function of the effective runoff volume (discharges above the threshold for bedload motion) and of the normalized peak discharge. For the total of 143 investigated flood events in the Erlenbach stream, the deviation of the predicted from the measured value is within a factor of two for more than two-thirds of all events. A distinction can be made between summer and winter events in analysing the bedload transport events. The summer events, mainly caused by thunderstorms, transport comparatively larger sediment loads than the winter events. For the other investigated streams, the periods of the deposited sediment volume surveys cover in general several flood events. An analysis is performed analogous to that for the Erlenbach stream. The sediment loads show a similar dependency on the two factors effective runoff volume and normalized peak discharge. However, the exponents of these factors in the power law expressions differ from stream to stream. A comparison of the investigated stream shows that some of the variation can be explained by considering the bedslope above the measuring site. The inclusion of a bedslope factor is in agreement with laboratory investigations on bedload transport. © 1997 John Wiley & Sons, Ltd.     

A model for grain-size sorting over tidal sand ridges

A model was developed and analyzed to quantify the effect of graded sediment on the formation of tidal sand ridges. Field data reveal coarse (fine) sediment at the crests (in the troughs), but often phase shifts between the mean grain-size distribution and the bottom topography occur. Following earlier work, this study is based on a linear stability analysis of a basic state with respect to small bottom perturbations. The basic state describes an alongshore tidal current on a coastal shelf. Sediment is transported as bed load and dynamic hiding effects are accounted for. A one-layer model for the bed evolution is used and two grain size classes (fine and coarse sand) are considered. Results indicate an increase in growth and migration rates of tidal sand ridges for a bimodal mixture, whilst the wavelength of the ridges remains unchanged. A symmetrical externally forced tidal current results in a grain-size distribution which is in phase with the ridges. Incorporation of an additional external M₄ tidal constituent or a steady current results in a phase shift between the grain-size distribution and ridge topography. These results show a general agreement with observations. The physical mechanism responsible for the observed grain-size distribution over the ridges is also discussed.Responsible Editor: Jens Kappenberg