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1.
Redox hot spots occurring as metal-rich anoxic groundwater discharges through oxic wetland and river sediments commonly result in the formation of iron (Fe) oxide precipitates. These redox-sensitive precipitates influence the release of nutrients and metals to surface water and can act as ‘contaminant sponges’ by absorbing toxic compounds. We explore the feasibility of a non-invasive, high-resolution magnetic susceptibility (MS) technique to efficiently map the spatial variations of magnetic Fe oxide precipitates in the shallow bed of three rivers impacted by anoxic groundwater discharge. Laboratory analyses on Mashpee River (MA, USA) sediments demonstrate the sensitivity of MS to sediment Fe concentrations. Field surveys in the Mashpee and Quashnet rivers (MA, USA) reveal several discrete high MS zones, which are associated with likely anoxic groundwater discharge as evaluated by riverbed temperature, vertical head gradient, and groundwater chemistry measurements. In the East River (CO, USA), widespread cobbles/rocks exhibit high background MS from geological ferrimagnetic minerals, thereby obscuring the relatively small enhancement of MS from groundwater induced Fe oxide precipitates. Our study suggests that, in settings with low geological sources of magnetic minerals such as lowland rivers and wetlands, MS may serve as a complementary tool to temperature methods for efficiently mapping Fe oxide accumulation zones due to anoxic groundwater discharges that may function as biogeochemical hot spots and water quality control points in gaining systems.  相似文献   
2.
当前无人机频率域半航空电磁方法(SAEM)成为地球物理勘探中的新兴技术,该方法通过空中无人飞行器测量地面上单个或多个可控源的垂直磁场.本文为无人机频率域SAEM开发了三维反演程序,正演采用交错网格有限差分.由于无人机采集的数据量巨大,因此使用了有限内存拟牛顿法(LBFGS)实现快速的三维反演,以避免计算和存储巨大的灵敏度矩阵.为了提升无人机频率域SAEM的三维反演效果,本文进一步实现了无人机频率域SAEM与大地电磁(MT)的三维联合反演,并对联合反演实施了地震结构约束,结构约束采用交叉梯度法实现.MT可以增加勘探深度,而结构约束可以提高电磁法反演的分辨率.最后通过建立四个理论模型,证明了本文程序的有效性.  相似文献   
3.
《China Geology》2021,4(4):541-552
The intersection of the Kyushu-Palau Ridge (KPR) and the Central Basin Rift (CBR) of the West Philippine Basin (WPB) is a relic of a trench-trench-rift (TTR) type triple-junction, which preserves some pivotal information on the cessation of the seafloor spreading of the WPB, the emplacement and disintegration of the proto-Izu-Bonin-Mariana (IBM) Arc, and the transition from initial rifting to steady-state spreading of the Parece Vela Basin (PVB). However, the structural characteristics of this triple-junction have not been thoroughly understood. In this paper, using the newly acquired multi-beam bathymetric, gravity, and magnetic data obtained by the Qingdao Institute of Marine Geology, China Geological Survey, the authors depict the topographic, gravity, and magnetic characteristics of the triple-junction and adjacent region. Calculations including the upward continuations and total horizontal derivatives of gravity anomaly are also performed to highlight the major structural features and discontinuities. Based on these works, the morphological and structural features and their formation mechanisms are analyzed. The results show that the last episode amagmatic extension along the CBR led to the formation of a deep rift valley, which extends eastward and incised the KPR. The morphological and structural fabrics of the KPR near and to the south of the triple-junction are consistent with those of the western PVB, manifesting as a series of NNE-SSW- and N-S-trending ridges and troughs, which were produced by the extensional faults associated with the initial rifting of the PVB. The superposition of the above two reasons induced the prominent discontinuity of the KPR in deep and shallow crustal structures between 15°N–15°30′N and 13°30′N–14°N. Combined with previous authors’ results, we propose that the stress produced by the early spreading of the PVB transmitted westward and promoted the final stage amagmatic extension of the CBR. The eastward propagation of the CBR destroyed the KPR, of which the magmatism had decayed or ceased at that time. The destruction mechanism of the KPR associated with the rifting of the PVB varies along strike the KPR. Adjacent to the triple-junction, the KPR was destroyed mainly due to the oblique intersection of the PVB rifting center. Whereas south of the triple-junction, the KPR was destroyed by the E-W-directional extensional faulting on its whole width.©2021 China Geology Editorial Office.  相似文献   
4.
As a traditional method for palaeoseismic studies, trenching can be combined with dating techniques to identify palaeoseismic events and the earthquake recurrence interval. However, when using trenches to study palaeoearthquakes, factors such as the active tectonic background of the earthquake‐caused structure, the lithology on both sides of the fault, the geomorphology location and type and the samples and methods for dating will affect the location of the trench. Thus, trenches should be carefully selected and used to identify the impact of ancient earthquakes. The results have substantial uncertainties and limitations. In recent years, scholars have made considerable progress in using other methods to reveal the palaeoseismic information of faults. Moreover, the history of fault activity may have been recorded in the lacustrine sediment adjacent to the fault. Hasuhai Lake is adjacent to the middle segment of the Daqingshan piedmont fault in Inner Mongolia. Since the Holocene, the region has experienced a temperate continental semi‐arid climate with little interference, and Hasuhai Lake and peripheral waters present weak hydrodynamic conditions that provide an ideal location for the study of palaeoseismic records in lacustrine sediments. Sediment samples and samples for dating were collected from three trenches excavated on the periphery of the Hasuhai Lake. Their variations in grain size and magnetic susceptibility revealed that wind and flowing water jointly produced the sedimentary conditions of Hasuhai sediments. The 14C dating results and variations in the grain size distribution, grain size components and magnetic susceptibility of sediments caused by seismic events obtained in this study were compared with those caused by a series of palaeoseismic events at the middle segment of the Daqingshan piedmont fault reported by previous studies using trenches, knickpoints and palaeosol records. The results identified seven palaeoseismic events recorded near Hasuhai Lake since 12,000 years. The combined use of lacustrine sediment variation characteristics and dating techniques is an effective method for studying palaeoseismic events.  相似文献   
5.
常规的单孔井中三分量磁测能在二维空间上较准确地判断磁性地质体的方位和大致空间位置,该文利用群孔井中三分量磁测实现在三维空间上对磁性地质体赋存状态的准确判定,特别是ΔT磁参量,能更准确、更清晰地反映磁性地质体的三维空间形态。对古林矿区群孔井中三分量磁测数据进行整理后,利用Voxler软件进行井中三分量磁测ΔT磁异常三维立体成图,利用各岩性已知磁性参数特征,圈定可推断为矿致的ΔT三维异常体。通过对圈定的矿致三维异常体的分析,正确的推断了本矿区磁铁矿体的空间赋存状态,为该矿区勘查工作的顺利进行,起到了有力的推动作用。  相似文献   
6.
田振环 《地质学报》2019,93(S1):19-28
莱州市三山岛北部海域金矿区是世界上第一个查明的海域岩金矿。它通过追索延伸至海域的三山岛断裂,圈定破金找矿靶区。利用海域高精度磁测和浅地层剖面测量配合海上钻探,找到了海域大型金矿。以三山岛海域金矿普查为依托,研究出来的这套海域金矿找矿方法和海上钻探平台,可为今后开展海域金及其它与断裂有关的矿产资源找矿,提供指导和借鉴。  相似文献   
7.
The thermal evolution of sedimentary basins is usually constrained by maturity data, which is interpreted from Rock-Eval pyrolysis and vitrinite reflectance analytical results on field or boreholes samples. However, some thermal evolution models may be inaccurate due to the use of elevated maturities measured in samples collected within an undetected metamorphic contact aureole surrounding a magmatic intrusion. In this context, we investigate the maturity and magnetic mineralogy of 16 claystone samples from Disko-Svartenhuk Basin, part of the SE Baffin Bay volcanic margin. Samples were collected within thermal contact metamorphic aureoles near magma intrusions, as well as equivalent reference samples not affected by intrusions. Rock-Eval pyrolysis (Tmax), and vitrinite reflectance (Ro) analysis were performed to assess the thermal maturity, which lies in the oil window when 435°C ≤ Tmax ≤ 470°C and 0.6%–0.7% ≤ Ro ≤ 1.3%. In addition, we performed low- (<300K) and high-temperature (>300K) investigations of isothermal remanent magnetization to assess the magnetic mineralogy of the selected samples. The maturity results (0.37% ≤ Ro ≤ 2%, 22°C ≤ Tmax ≤ 604°C) show a predominance of immature to early mature Type III organic matter, but do not reliably identify the contact aureole when compared to the reference samples. The magnetic assemblage of the immature samples consists of iron sulphide (greigite), goethite and oxidized or non-stoichiometric magnetite. The magnetic assemblage of the early mature to mature samples consists of stoichiometric magnetite and fine-grained pyrrhotite (<1 μm). These results document the disappearance of the iron sulphide (greigite) and increase in content of magnetite during normal burial. On the other hand, magnetite is interpreted to be the dominant magnetic mineral inside the contact aureole surrounding dyke/sill intrusions where palaeotemperatures indicate mature to over-mature state. Interestingly, the iron sulphide (greigite) is still detected in the contact aureole where palaeotemperatures exceeded 130°C. Therefore, the magnetic mineralogy is a sensitive method that can characterize normal burial history, as well as identify hidden metamorphic contact aureoles where the iron sulphide greigite is present at temperatures beyond its stability field.  相似文献   
8.
梁山东丁庄铁矿位于东平-汶上铁成矿带西侧的低缓磁异常区,为区内新发现的沉积变质型铁矿床。通过1∶1万高精度磁法面积测量工作,在区内圈定3个磁异常,并进行钻探验证,证实其中2个为隐伏磁铁矿体引起,矿体赋存于泰山岩群山草峪组中。通过研究矿体的地质特征、磁异常特征,并与区域铁矿成矿条件对比分析,认为区内矿致磁异常特征一般表现为在数十纳特的平缓背景场基础上,突然升高至数百纳特的相对高值异常,具突变特征。该类异常一般呈NW走向,与东平-汶上铁成矿带走向基本一致,其形态一般呈规则的长条带状、椭圆状或串珠状。区域上存在较多与研究区具有相似特征的低缓磁异常,且具备有利的成矿地层条件,因此认为区域上低缓磁异常区具有较大的找矿潜力,找矿前景良好。  相似文献   
9.
随着"环日轨道器"(Solar Orbiter, SO)的在轨运行,太阳磁场观测进入了双视角遥测的时代.对利用太阳磁场的双视角观测改正矢量磁图中存在的横场(垂直于视线方向的磁场分量) 180°不确定性进行了模拟,首先模拟了对解析解得到磁图的双视角观测,然后利用"日震学和磁学成像仪"(Helioseismic and Magnetic Imager, HMI)在不同时间观测到的一个老化黑子的磁图模拟了双视角观测.发现要改正一个磁图中横场方向的180°不确定性,在观测上只需要另外一个平行于视线方向的磁场即纵向磁场观测的协助.利用HMI的磁场观测模拟,估算显示30°的张角能够改正50 Gs磁场中的180°不确定性.更大的张角虽然更有利于更弱磁场的改正,但是考虑到投影效应的不利影响, 30°左右的张角应该是未来空间设备进行多视角观测太阳磁场的最佳张角.  相似文献   
10.
The electrical resistivity and induced polarization (IP) methods are widely used in geological mapping, prospecting and exploration of mineral deposits, engineering geology, hydrogeology, archaeology, and geotechnical and environmental applications. Historically, these methods have formed the basis of the electrical prospecting technique. In these methods, a DC or low-frequency AC electrical current is introduced into the earth through a grounded transmitter line. The measured quantity is the electric field. However, if the earth’s resistivity or chargeability changes horizontally, this change gives rise to an anomalous magnetic field, which is studied by the magnetometric resistivity (MMR) and magnetic induced polarization (MIP) methods, respectively. Along with advantages, some shortcomings are inherent in the MMR and MIP techniques. Apparently, the main drawback of these methods is that the magnetic fields of both the transmitter line wire and ground electrodes on the surface are several orders of magnitude greater than the anomalous magnetic field response. This introduces a significant “noise” to magnetic-resistivity data. We investigate the potential of using a circular electric dipole (CED) in magnetometric resistivity techniques. It has been found that the application of a CED, instead of a conventional transmitter line, dramatically enhances the signal-to-noise ratio.  相似文献   
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