Exploration of geothermal structure in Puga geothermal field, Ladakh Himalayas, India by magnetotelluric studies

To understand the crustal electric structure of the Puga geothermal field located in the Ladakh Himalayas, wide band (1000 Hz–0.001 Hz) magnetotelluric (MT) study have been carried out in the Puga area. Thirty-five MT sites were occupied with site spacing varying from 0.4 to 1 km. The measurements were carried out along three profiles oriented in east–west direction. After the preliminary analysis, the MT data were subjected to decomposition techniques. The one-dimensional inversion of the effective impedance data and the two-dimensional inversion of the TE (transverse electric) and TM (transverse magnetic) data confirm the presence of low resistive (5–25 Ω m) near surface region of 200–300 m thick in the anomalous geothermal part of the area related to the shallow geothermal reservoir. Additionally, the present study delineated an anomalous conductive zone (resistivity less than 10 Ω m) at a depth of about 2 km which is possibly related to the geothermal source in the area. A highly resistive basement layer separates the surface low resistive region and anomalous conductive part. The estimated minimum temperature at the top of conductive part is about 250 °C. The significance of the deeper conductive zone and its relation to the geothermal anomaly in the area is discussed.     

Application of the 3D magnetotelluric inversion code in a geologically complex area

The WSINV3DMT code makes the implementation of 3D inversion of magnetotelluric data feasible using a single PC. Audio‐magnetotelluric data were collected along two profiles in a Cu‐Ni mining area in Xinjiang, China, where the apparent resistivity and phase curves, the phase tensors and the magnetic induction vectors indicate a complex 3D conductivity structure. 3D inversions were carried out to reveal the electrical structure of the area. The final 3D model is selected from the inversion results using different initial Lagrange values and steps. The relatively low root‐mean‐square (rms) misfit and model norm indicate a reliable electrical model. The final model includes four types of low resistivity areas, the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone. The second ones are not controlled by magnetotelluric sites and embody little information of the observed data, they are considered as tedious structures. The third one is near to the regional Kangguer fault and should be treated carefully considering the effect of the fault. The last ones are isolated and existing at a limited level as the first ones, they should be paid more attention to.     

Magnetotelluric resistivity modeling for 3D characterization of geothermal reservoirs in the Western side of Mt. Aso, SW Japan

Geothermal reservoirs are usually located at a depth range of 2 to 5 km, so to efficiently utilize such resources an advanced prospecting method is needed to detect these deep geologic structures. This study aimed to three-dimensionally characterize geothermal reservoirs by a combination of Magnetotelluric (MT) survey, inversion analysis of apparent resistivity, and interpolation of the resistivity data obtained. The western side of Mt. Aso crater, southwest Japan, was chosen as the case study area. Three hot springs exist there and a fault is assumed to go in the direction connecting them. A MT survey was carried out at 26 sites and the data processed by a remote reference method to reduce artificial noises. Based on skewness and Mohr circle analyses of the impedance tensor, the local geologic structure at each site could be approximated as horizontally layered and therefore, a one-dimensional inversion analysis was applied to the MT raw data. The resultant resistivity column data were then interpolated by the three-dimensional optimization principle method. The resistivity distributions obtained clarified continuous conductors with especially low resistivity (less than 10 Ω·m) at the hot springs along the fault. Because the resistivity decreases largely with an abundance of clay minerals, the conductors could be considered to correspond with the cap rocks. Thus, two geothermal reservoirs, whose shapes were estimated to be pillar, were detected under the cap rocks in an elevation range of − 1000 to − 3000 m. By comparing the resistivity distributions with the temperature distributions based on fluid-flow calculations at a steady state using FEM, the validity of the location and dimension of the estimated reservoirs were confirmed.     

大地电磁三维正演并行算法研究

大地电磁法三维正演算法计算量大,用传统的串行程序计算相当耗时。而三维正演是逐个频率按顺序计算的,并行性好,适合并行运算。结合MPI自身的优越性,在深入分析大地电磁三维正演串行程序实现流程的基础上,确定了并行计算的思路,实现了三维正演的并行计算。通过三个理论模型对实现的三维正演并行程序进行了试算,分析对比了在多种情况下程序的执行效率。测试结果表明,所实现的三维正演并行程序运行结果正确,效率提高明显。此思路可为解决其它地球物理超大计算量问题所借鉴。     

Numerical modeling of geothermal groundwater flow in karst aquifer system in eastern Weibei, Shaanxi Province, China

The quantitative assessment of geothermal water resources is important to the exploitation and utilization of geothermal resources. In the geothermal water systems the density of groundwater changes with the temperature, therefore the variations in hydraulic heads and temperatures are very complicated. A three-dimensional density-dependent model coupling the groundwater flow and heat transport is established and used to simulate the geothermal water flow in the karst aquifers in eastern Weibei, Shaanxi Province, China. The multilayered karst aquifer system in the study area is cut by some major faults which control the regional groundwater flow. In order to calibrate and simulate the effect of the major faults, each fault is discretized as a belt of elements with special hydrological parameters in the numerical model. The groundwater dating data are used to be integrated with the groundwater flow pattern and calibrate the model. Simulation results show that the calculated hydraulic heads and temperature fit with the observed data well.     

Application of 2D magnetotelluric methods in a geological complex area, Xinjiang, China

Magnetotelluric (MT) data along two profiles were collected in Tulargen Cu-Ni mining area, Xinjiang, China. The MT profiles, crossing the known ore-bearing magmatic body, are interpreted to have further understanding of the ore-bearing magmatic bodies' stretch and shape in depth, and to evaluate the prospect in the mining area. The values of phase sensitivity skewness indicate a contaminated two-dimensional (2D) structure (there exist local three-dimensional (3D) structures at shallow level) over 3D structures. G-B decomposition of the impedance tensors indicates the dominant geoelectrical strike in survey area is in East-West direction. To obtain reliable electrical structures, four different 2D inversion codes as REBOCC, NLCG, OCCAM, and ABIC are adopted, inversions of a synthetic 3D model using these codes indicate TM data are more suitable to invert the electrical structures along the profiles in Tulargen Cu-Ni mining area, while TE and TE + TM inversions can be used to verify the transverse variation of electrical structures at shallow level. The 2D electrical structures show high-resistivity basement covered by relatively low-resistivity strata in the mining area, the conductive anomalies are imbedded in the upper covers. The known ore-bearing magmatic body is coincided with the positions of two conductive anomalies in both profiles at shallow level; its extension at depth is deduced according to the shapes of the anomalies. Other conductive anomalies, isolated and existing only at limited depth as the known ore-bearing magmatic body, are presumed of good prospect.     

三维/三维构造下大地电磁相位张量数值模拟

当地表存在三维非均匀电导率分布时,区域大地电磁响应发生畸变. 以往对这种畸变研究多假设近地表为三维,区域构造为一维或二维. 对于更一般的三维/三维构造,为了分析并消除这种畸变影响,真实反映地下三维区域构造信息,本文实现了三维大地电磁相位张量积分方程数值算法,并研究在不同地质模型下相位张量响应. 结果表明,相位张量不仅可以反映一般三维构造信息,亦可有效反映复杂近地表构造下三维区域构造信息,而无须假设区域构造为一维或二维,证明相位张量具有较强抗近地表局部非均匀构造干扰能力,能够保持更为一般的三维区域构造信息. 为了加快正演计算,同时保持一定精度,算法采用了积分方程多网格法.     

松辽盆地北部地热场深部控制因素研究: 基于大地电磁探测的结果

松辽盆地北部存在中低温地热场,地热场呈现中间高、四周环状降低的特征.松辽盆地内部形成高地热场的主要因素,一是深部热源供给;二是浅部热能储集.通过深部结构特征研究可揭示热源及热储的分布及相互联系,对松辽盆地北部地热场成因研究具有重要意义.为了揭示松辽盆地北部地热场的深部控制因素,本文基于古龙镇至依安县的246 km长大地电磁剖面,对71个宽频测点数据通过傅里叶变换、Robust估计以及相位张量分解等处理手段,在精细分析维性特征及电性主轴的基础上,利用非线性共轭梯度反演获得了剖面40 km深二维电性结构模型.研究发现：电阻率模型具有"纵向分层,横向分块"的特征,以水热性温泉富集的林甸地区为界,剖面南北两侧电性结构存在明显的差异,南侧呈现"低阻-高阻-低阻"的三元电性结构,北侧呈现"低阻"的一元电性结构,这两种结构与地温场分布具有良好的对应关系,林甸以南的三元电性结构区对应高地热异常,以北的一元电性结构区热异常下降明显;林甸地区位于这两种端元的分界区,地表温泉丰富,且发育有基底断裂,为水热型地热发育的"热点"地区;林甸南北两侧深部存在两个高导体C1、C2,这与普遍认识的松辽盆地存在软流圈隆起有关,说明盆地下方具有统一的热源,部分熔融热物质作为深部热源向上传递热量,不同之处在于林甸以南地区,中地壳存在巨厚高阻特征的前寒武纪结晶基底R1,为地热的保存提供了有利条件,而林甸以北地区深部缺少聚集热量的结晶基底,导致地热异常迅速降低.     

面向目标自适应三维大地电磁正演模拟

本文将面向目标的自适应算法应用于三维大地电磁数值模拟.使用基于非结构网格的矢量有限单元法对起伏地表大地电磁正演模拟问题进行求解.使用利用垂向电流密度在物性界面上的连续性对后验误差进行估算的算法指导网格优化.由于全局自适应算法针对观测点优化网格的能力较差,本文通过求解正演问题的对偶问题计算后验误差的加权系数,并对相关加权系数进行改进,从而实现了面向目标的自适应算法.与传统基于结构化网格的电磁正演算法相比,采用非结构网格能够更好地拟合起伏地表和地下不规则异常体.由于使用了面向目标的自适应算法,本文能够使用更少的网格达到较高的计算精度.通过对比本文模拟结果与半空间响应和全局自适应算法计算结果,并通过对比使用改进前和改进后加权系数得到的网格剖分结果验证了本文算法的有效性.

     

Hydrodynamic characterization of a Sahelian coastal aquifer using the ocean tide effect (Dridrate Aquifer,Morocco)

Abstract

In wells tapping coastal aquifers, piezometric fluctuations can be observed in response to the ocean tide. Simultaneous recordings of the ocean tide and of the piezometric variations may provide a basis for characterizing the hydrodynamics of the aquifer. This approach was attempted to characterize the Dridrate aquifer, located on the Atlantic coast of Morocco. This aquifer accounts for most of the regional drinking water resources. However, its hydrodynamic characteristics are very poorly known. The study compares observed and simulated piezometric fluctuations, under various assumptions (confined, semi-confined aquifer). The model, which best explains the hydrodynamic behaviour of this aquifer is a semi-confined and strongly heterogeneous aquifer model (calculated hydraulic diffusivity values vary over several orders of magnitude). This result is new and rather surprising, since to date this aquifer was considered confined in view of its geological setting. Consequently, new questions are raised regarding the protection and management of the groundwater resources of this aquifer.     

大地电磁三维交错网格有限差分数值模拟的并行计算研究

为了更有效的提高大地电磁三维正演的计算速度,引入了并行处理技术.大地电磁三维交错网格有限差分数值模拟是按照不同频率来计算的,各频率之间求取电磁场值的过程是相互独立的.根据这一特点,可以将多个频率的计算任务平均划分为一个或者几个频率的计算子任务,分配到各个计算节点去并行执行,计算完成后将结果汇总.本文通过采用主从并行模式、分频并行计算的方案,在曙光TC5000A高性能并行平台上实现了基于MPI的大地电磁三维正演的并行计算.通过两个理论模型对实现的大地电磁三维正演并行算法进行试算,对比分析了多个节点机下程序的执行效率.测试结果表明,所实现的三维正演并行算法是正确的、高效的,为进一步的大地电磁三维反演并行算法研究奠定了重要基础.     

利用大地电磁三维反演方法获得二维剖面附近三维电阻率结构的可行性

大地电磁野外实测数据目前大多为二维剖面数据.如何反演这些二维剖面数据获得较为接近实际地电情况的结果,是多数大地电磁工作者关心的问题.我们通过对理论模型的三维响应进行分析和对合成数据及实测资料的反演结果进行对比研究,讨论了利用三维反演的方法来获得大地电磁二维剖面附近三维电阻率结构的可行性.结果表明:可用三维反演的方法来解释二维剖面数据.对大地电磁二维剖面的张量数据进行三维反演,不仅可以沿剖面获得较好的二维断面结果,还能够得到二维反演所不能获得的剖面附近的三维电阻率结构信息.合成数据的反演算例表明:对二维剖面数据进行三维反演时,对角元素对于圈定剖面附近三维异常体的空间分布具有独特作用,应尽量反演所有的张量元素.     

Two-dimensional magnetotelluric characterization of the El Hito Anticline (Loranca Basin,Spain)

Magnetotelluric surveys were conducted to investigate the structure in the El Hito Anticline in the southern Loranca Basin, Spain. The structure of this anticline is partly known from nearby data (geological cross sections, geological maps, well-logs) from vintage geophysical and geological basin-scale surveys. Unfortunately, these surveys do not have the appropriate resolution to determine certain characteristics of the anticline accurately, such as the thickness or geometry of geological units. To address this deficiency, magnetotelluric data were acquired at 51 sites along three profiles to image the electrical resistivity of the anticline. To identify a geologically reasonable resistivity model, a conceptual model based on previous geological and geophysical information (cross sections and well-log data) was generated. Several inversions were performed using the conceptual model, which played a key role in the interpretation of the magnetotelluric data, and the construction of a suitable initial model was essential in producing geologically meaningful models. Using these results, we obtained 2-D resistivity models that provide information on the main geological units and structures. We also performed sensitivity tests to understand the morphology and structure of the resistive basement better. The results indicate the presence of basement highs that show different structural styles for the basement and its cover, and an increase in the basement depth towards the south. The final 2-D resistivity models provide new information regarding the structure of the anticline and demonstrate a clear correlation between the main geological units and resistivity, as well as correlations with mapped surface faults.     

大地电磁三维快速松弛反演并行算法研究(英文)

为了进一步提高大地电磁三维快速松弛反演的计算效率,在深入分析大地电磁三维快速松弛反演算法的基础上,结合MPI自身的优越性,确定了并行计算的思路,实现了三维快速松弛反演的并行计算。通过理论模型合成数据和实测数据对实现的三维快速松弛反演并行程序进行了试算,分析对比了在多种情况下程序的执行效率。测试结果表明,所实现的三维快速松弛反演并行程序运行结果正确,效率提高明显。此成果在普通微机上实现,推动了大地电磁三维反演技术的实用化,可为其它地球物理三维正反演研究所借鉴。     

郯庐断裂带合肥—宿迁段及邻区大地电磁三维成像

本文利用覆盖郯庐断裂带合肥—宿迁段及邻区的大地电磁阵列数据, 应用大地电磁三维反演技术和印模重构方法, 获得了测区可靠的三维电阻率模型.该电性模型中, 郯庐断裂带东西两侧在浅部均有高阻分布, 在深部电性结构显著不同, 具有西低东高的特征, 而郯庐断裂带则是两侧高低阻明显分界带; 在南北方向, 电性结构也存在较大不均匀性, 嘉山地区呈现低阻特征, 是断裂带上的电性分界点, 郯庐断裂带在此处由北部的四条主断裂变成南部的两条.结合研究区地质、大地测量和地震学等资料, 综合分析结果表明: 郯庐断裂带作为苏鲁造山带和张八岭隆起高阻体陡立的西边界, 切割深度超过莫霍面.嘉山地区相对低阻分割了张八岭高阻条带与苏鲁高阻体, 阻碍了断层上应力的传递, 造成了嘉山北、南两侧断裂带构造变形强、弱的变化, 从电性结构上证实了郯庐断裂带具有分段性的特征; 苏鲁高阻体南端向西偏转穿过郯庐断裂带, 阻碍断层滑动, 致使泗洪—嘉山段存在应力积累和断层闭锁的地震孕育环境, 其地震危险性值得关注.

     

Simulation and consequences of successive anthropogenic activity in the Agua Amarga coastal aquifer (southeast Spain)

Abstract

The Agua Amarga coastal aquifer has been the object of a succession of anthropogenic interventions over the last 90 years: (a) the operation of saltworks from 1925 to 1975; (b) the withdrawal, since 2003, of groundwater from the aquifer along the coast line; and (c) the programme of pouring seawater over the salt marsh, carried out since 2009, to recover the piezometric levels and the soil moisture conditions. For a better understanding of how these past and present human activities have affected the natural groundwater regime, and to validate certain hypotheses concerning the interpretation of experimental data on temperature depth profiles and piezometric and salinity changes, a numerical fluid flow and solute transport model was designed and applied to the period 1925–2010, using SEAWAT. This model reproduces, in a qualitative and quantitative way, the flow and transport processes that operated during this time, as well as the behaviour of the seawater wedge.

Citation Alhama Manteca, I., 2013. Simulation and consequences of successive anthropogenic activity in the Agua Amarga coastal aquifer (southeast Spain). Hydrological Sciences Journal, 58 (5), 1072–1087.

Editor D. Koutsoyiannis     

Impacts of human activities on recharge in a multilayered semiarid aquifer (Campo de Cartagena,SE Spain)

The development of intense agriculture in semiarid areas modifies intensity and spatial distribution of groundwater recharge by summing irrigation return flow to limited rainfall infiltration. Environmental tracers provide key information, but their interpretation is complicated by more complex groundwater flow patterns. In multilayered aquifers, the real origin of the groundwater samples is hard to assess because of local mixing processes occurring inside long‐screened boreholes. We use environmental tracers (¹⁴C, ¹³C, ²H, ¹⁸O, ³H) to investigate the long‐term evolution of recharge in the five‐layer Campo de Cartagena aquifer in South‐Eastern Spain, in addition to high‐resolution temperature loggings to identify the depth of origin of groundwater. Despite the complex background, this methodology allowed a reliable interpretation of the geochemistry and provided a better understanding of the groundwater flow patterns. The tritium method did not give good quantitative results because of the high variability of the recharge signal but remained an excellent indicator of recent recharge. Nonetheless, both pre‐anthropization and post‐anthropization recharge regime could be identified and quantified by radiocarbon. Before the development of agriculture, recharge varied from 17 mm.year^‐1 at the mountain ranges to 6 mm.year^‐1 in the plain, whereas the mean annual rainfall is about 300 mm. In response to the increase of agricultural activity, recharge fluxes to the plain were amplified and nowadays reach up to 210 mm.year^‐1 in irrigated areas. These values are strengthened by global water budget and local unsaturated zone studies. Copyright © 2013 John Wiley & Sons, Ltd.     

Study of the Santa Catarina aquifer system (Mexico Basin) using magnetotelluric soundings

A tensor magnetotelluric test survey was carried out in the region of Santa Catarina, located in the Chalco sub-basin of the Mexico Basin. The objective was to define the stratification at depth with an emphasis on the geometry of the main aquifer of that region which is partially known from DC resistivity soundings and drilling. High-quality magnetotelluric soundings could be recorded in the immediate vicinity of large urban zones because the sub-surface is very conductive. Interpretation shows that the solid bedrock is located at a depth of at least 800 m to the south and 1300 m to the north; it could, however, be much deeper. Using complementary DC resistivity sounding and well-logging data, three main layers have been defined overlying the bedrock. These layers are, from surface to bottom, an unsaturated zone of sand, volcanic ash and clay about 10 m thick, followed by a very conductive (1.5 ohm·m) 200 m thick layer of sand and ash with intercalated clay, saturated with highly mineralized water, and finally a zone with resistivity increasing gradually to 60 ohm·m. The investigated deep aquifer constitutes most of this third layer. It consists of a sequence of sand, gravel, pyroclastites and mainly fractured basalts. MT resistivity soundings and magnetic transfer functions also indicate that a shallow resistive structure is dipping, from the northwest, into the lacustrine deposits of the basin. This geologic feature is likely to be highly permeable fractured basaltic flows, which provide a channel by which water contaminated by the Santa Catarina landfill may leak into the basin.     

Forest fire effects on groundwater in a coastal aquifer (Ravenna,Italy)

We examined the fire‐induced changes in groundwater recharge rate. This aspect is particularly important in the case of large forested areas growing over a coastal aquifer affected by saltwater intrusion. In the Ravenna coastal area (Italy), pine forests grow on coastal dune belts, overlying a sandy unconfined aquifer, which is strongly affected by marine ingression. Three groundwater profiles across the forest and perpendicular to the coastline were monitored for groundwater level, physical, and chemical parameters. The aims were to define groundwater quality, recharge rate, freshwater volume, and highlight change, which occurred after a forest fire with reference to pre‐fire conditions. Analytical solutions based on Darcy Law and the Dupuit Equation were applied to calculate unconfined flow and compare recharge rates among the profiles. The estimated recharge rates increased in the partially and completely burnt areas (219 and 511 mm year^?1, respectively) compared with the pristine pine forest area (73 mm year^?1). Although pre‐fire conditions were similar in all monitored profiles, a post‐fire decrease in salinity was observed across the burnt forest, along with an increase in infiltration and freshwater lens thickness. This was attributed to decrease canopy interception and evapotranspiration caused by vegetation absence after the fire. This research provided an example of positive forest fire feedback on the quantity and quality of fresh groundwater resources in a lowland coastal aquifer affected by saltwater intrusion, with limited availability of freshwater resources. The fire provided an opportunity to evaluate a new forest management approach and consider the restoration and promotion of native dune herbaceous vegetation.     

Basin structure and water quality of the Oligocene aquifer in the Sebkhet El Behira basin (central Tunisia)

ABSTRACT

The Sebkhet El Behira–Garaat El Majdoul multilayer aquifer system is composed of Mio-Plio-Quaternary and Oligocene groundwater exploited in Sidi Bouzid and Kairouan governorates. Annual withdrawal volumes from El Behira Oligocene groundwater were about 0.37 hm³ in 2005 and 0.36 hm³ in 2008. The present study of the Oligocene reservoir in the Sebkhet El Behira–Garaat El Majdoul basin is based on various data including 2D seismic sections, petroleum wells, field geological cross-sections and geochemical analysis. The gathered data allowed us to establish the tectonic framework and to define its influence on the structure of the aquifers seated in the deep Oligocene sandstone reservoirs. Three Oligocene sub-basin aquifers are defined showing different depth, thickness and petrophysical characteristics: lower salinity (<1.5 g/L) and higher porosity (30%). Two field sections in the northern and in the southern parts of Cherahil anticline exhibit the presence of four to five sandstone levels. The anticlines limit the Sebkhet El Behira–Garaat El Majdoul basin. In addition, the morphostructural configuration controls the piezometry of underground flows in the Oligocene and Mio-Plio-Quaternary unconfined aquifers.

Editor D. Koutsoyiannis; Associate editor M. Besbes