Geophysical Exploration of Shallow Groundwater Aquifers in Arid Regions: A Case Study of Siwa Oasis,Egypt

Geophysical and geological studies play a fundamental role in the strategic and sustainable utilization of natural resources, especially that of fossil groundwater, in arid regions. The geophysical exploration of shallow groundwater aquifers is common in arid regions. In this work, a feasibility study of future development plans in the Siwa Oasis, Egypt, was carried out. A land electric resistivity survey was conducted, and approximately 14 vertical electric soundings were measured covering the Siwa Oasis, northwestern desert, Egypt. A detailed surface geology study was also conducted to study the underground water aquifer. Digital filters were applied to the reduced to pole-available magnetic data covering the area. The normalized source strength transformation and tilt depth were calculated and applied to delineate the possible structures that may control the shallow and deep aquifers in the area. The integrated interpretation showed the presence of four main geoelectric layers forming the shallow section of the Siwa Oasis down to 220 m. These layers varied in their resistivity and rock constituents from very low (0.2 Ω m) to very high (6200 Ω m) values. The calculated hydraulic parameters showed that the uppermost central area and the eastern area were the most promising areas for the required water development. Finally, based on the integrated interpretation and the estimated shallow aquifer potentiality, a land use map for the Siwa Oasis was produced to assist future strategic development of the region.

     

Magnetotelluric interpretation of crustal and mantle structure in the Grenville Province

Summary Accurate determinations of depths and conductivities of electrical structures in shield regions are often difficult because of the inhomogeneity of the uppermost crust. A magnetotelluric (MT) station (BAT) in the Grenville Province of the Precambrian Shield in eastern Canada has been in operation since 1975 for time-dependency studies of electrical resistivity changes related to earthquakes. The MT response of the station displays low skew with small to moderate anisotropy. One-dimensional inversion of the apparent resistivity and phase reveals two well-defined conductors in the crust, one at 10 km and the second at the base of the crust. The latter has a resistivity less than 50 Ω m. These results are substantiated by three additional MT stations located up to 40 km distant.
Data from other new MT stations and from stations previously published in the literature are compared with two-dimensional computer model results and with the three-dimensional analogue scale model results of Dosso et al. While additional data for periods less than 100 s would be desirable the results from a number of the MT stations are not inconsistent with a widespread occurrence of a conducting zone at the base of the crust in the Grenville. The inversion analysis also indicates the existence of a conductor at some depth greater than 100 km with a resistivity less than 30 Ω m. This may coincide with a seismic low-velocity zone observed in the mantle under the Canadian Shield.     

应用同位素研究黑河下游额济纳盆地地下水

黑河下游额济纳盆地位于两北内陆干旱区,大部分地区为戈壁沙漠,降水稀少,蒸发强烈,生态环境脆弱。盆地内天然植被的生长发育主要依靠于区域地下水,研究盆地地下水形成和循环机制,对流域治理、生态环境建设具有重要意义。本文采用环境同位素示踪技术,结合水化学分析方法和区域水文地质条件,研究分析了盆地地下水循环特征、地下水与地表水的相互关系和流域不同地区地下水补给来源等,为黑河下游水利工程规划和水量调度提供科学依据。     

Identifying Groundwater Potential in Crystalline Basement Rocks Using Remote Sensing and Electromagnetic Sounding Techniques in Central Western Mozambique

Exploring for groundwater in crystalline rocks in semiarid areas is a challenge because of their complex hydrogeology and low potential yields. An integrated approach was applied in central western Mozambique, in an area covered by Precambrian crystalline basement rocks. The approach combined a digital elevation model (DEM), remote sensing, and a ground-based geophysical survey. The aim was to identify groundwater zones with high potential and to identify geological structures controlling that potential. Lineaments were extracted from the DEM that had been enhanced using an adaptive-tilt, multi-directional, shading technique and a non-filtering technique to characterize the regional fracture system. The shallowness and amount of stored groundwater in the fracture zones was assessed using vegetation indices derived from Landsat 8 OLI images. Then, 14 transient electromagnetic (TEM) survey profiles were taken in different geological settings across continuous lineaments that were considered to be aligned along inferred faults. In the central lineament zones, the TEM soundings gave resistivity values of less than 300 Ωm at a depth of 20–80 m. The values varied with location. Conversely, values greater than 400 Ωm were observed at the sites away from the central zones. This contrast is probably caused by the differences in permeability and degree of weathering along the fractured zones. These differences could be key factors in determining groundwater occurrence. By integrating five water-related factors (lineament density, slope, geology, vegetation index, and proximity to lineaments), high groundwater potential zones were located in the vicinity of the lineaments. In these zones, vegetation remains active regardless of the season.     

The effect of fault relay and clay smearing on groundwater flow patterns in the Lower Rhine Embayment

Faults strongly impact groundwater flow in the unconsolidated sediments of the Lower Rhine Embayment. Hydraulic head maps show that many individual faults form a barrier to fluid flow whereas relay structures in these faults are sites of hydraulic contact between otherwise separated aquifers. The fluid flow patterns around the Rurrand Fault close to the largest open‐pit mine in the Lower Rhine Embayment is one of the first well‐documented examples of fluid flow around a fault relay zone. The effect of clay smearing could be quantified using the Shale Gouge Ratio (SGR) method that is common in hydrocarbon‐related studies but has not been applied to groundwater flow data so far. The effect of fault relay zones on groundwater flow is analysed using numerical simulations. It is concluded that fault relay needs special consideration in the evaluation of the sealing capacities of faults in sedimentary basins. Moreover, it is demonstrated that the SGR methodology is a promising tool for the estimation of fault zone hydraulic properties in hydrogeological modelling.     

多层抽水试验地下水位动态与同位素浅析——以衡水试验为例

针对不同深度含水层,揭示其富水性,探讨地下水资源可恢复性、可利用性和浅层劣质水改造利用可能性。以国土资源部衡水地下水试验场为依托,在详细调查试验场影响区地下水开发利用状况、监测地下水年动态基础上,开展分层抽水试验和水化学同位素采样。通过对大量抽水水位动态数据过程、局部流场特征和水化学同位素的分析,得出结论:该区农业为影响地下水水位的主控因素;含水层系统承压性、补给强度从下到上分别呈减弱、增强趋势,除浅层外各含水层渗透性较强;各含水层渗流强度及方向不同等。提出实现地下水可持续开发利用建议:对浅层地下咸水适时进行抽咸换淡、混灌轮灌改造利用;170m、300m强含水层作为工农业用水主要开采层,深层地下水主要用于生活用水;按照水量平衡、分质用水原则开发地下水。     

Analyzing the formation cause of Xidatan drinking mineral springs in island permafrost area on north slope of the Kunlun Mountains

The replenishment source of Xidatan drinking mineral springs in island permafrost area on north slope of the Kunlun Mountains are mainly the melting water from the modern glaciers bottom, snow and ice melting water, atmospheric precipitation, and surface water in Yuzhu Peak area on the Kunlun Mountains. This scenario is based on the survey of hydrogeology, water-conducting and water-controlling faults, and water chemistry, and on the EH-4 high-frequency electronic deep exploration. The original water recharges the deep groundwater at fracture zone of active normal faults F3 and F4 , then groundwater enriches at normal faults F2 and F2-1,2 , and then run northward. A water-rich triangle area is formed when groundwater reach the active reverse fault F1 . Groundwater then discharges through fracture zone of F1 , which is the major cause of the Xidatan mineral springs formation.     

重力三维聚焦反演在大盐滩深部卤水勘查中的应用

大盐滩地区盐湖企业面临水资源和盐矿资源双重危机,利用重力方法探寻新资源具有重要的实际意义。本文对该区布格重力异常进行三维聚焦反演,获得了大盐滩地区三维剩余密度结构。通过研究发现,大盐滩地区浅地表高低度异常,表征着赋存较为丰富的盐矿资源,但地层中水资源较为贫瘠,浅表水资源主要赋存在山前冲积扇及断裂带内。中深层位不仅具有多个含水层,而且出现多处低密度异常区(T1～T4),可能为深部卤水富集体的反映,是探寻新卤水资源的有利靶区。依此建议在阿尔金山山前冲积带及断裂带内寻找新的淡水资源,并在成矿有利区,开展深部卤水资源勘查工作。     

3-D magnetotelluric inversion and model validation with gravity data for the investigation of flood basalts and associated volcanic rifted margins

20 magnetotelluric (MT) soundings were collected on the Isle of Skye, Scotland to provide a high-resolution three-dimensional (3-D) electrical resistivity model of a volcanic province within the framework of a project jointly interpreting gravity, seismic, geological and MT data. The full 3-D inversion of the MT data jointly interpreted with gravity data reveals upper crustal structure. The main features of the model are interpreted in conjunction with previous geological mapping and borehole data. Our model extends to 13 km depth, several kilometres below the top of the Lewisian basement. The top of the Lewisian basement is at approximately 7–8 km depth and the topography of its surface was controlled by Precambrian rifting, during which a 4.5 km thick sequence of Torridonian sediments was deposited. The Mesozoic sediments above, which can reach up to 2.2 km thick, have small-scale depocentres and are covered by up to 600 m of Tertiary lava flows. The interpretation of the resistivity model shows that 3-D MT inversion is an appropriate tool to image sedimentary structures beneath extrusive basalt units, where conventional seismic reflection methods may fail.     

Polygonal faults-furrows system related to early stages of compaction – upper Miocene to recent sediments of the Lower Congo Basin

A new polygonal fault system has been identified in the Lower Congo Basin. This highly faulted interval (HFI), 700±50 m thick, is characterized by small extensional faults displaying a polygonal pattern in plan view. This kind of fracturing is attributed to volumetric contraction of sediments during early stages of compaction at shallow burial depth. 3‐D seismic data permitted the visualization of the progressive deformation of furrows during burial, leading to real fractures, visible on seismic sections at about 78 m below seafloor. We propose a new geometrical model for volumetrical contraction of mud‐dominated sediments. Compaction starts at the water–sediment interface by horizontal contraction, creating furrows perpendicular to the present day slope. During burial, continued shrinkage evolves to radial contraction, generating hexagonal cells of dewatering at 21 m below seafloor. With increasing contraction, several faults generations are progressively initiated from 78 to 700 m burial depth. Numerous faults of the HFI act as highly permeable pathways for deeper fluids. We point out that pockmarks, which represent the imprint of gas, oil or pore water escape on the seafloor, are consistently located at the triple‐junction of three neighbouring hexagonal cells. This is highly relevant for predictive models of the occurrence of seepage structures on the seafloor and for the sealing capacity of sedimentary cover over deeper petroleum reservoirs.     

民勤盆地地下水地球化学演化模拟

根据稳定同位素分析,民勤盆地地下水在第四系总体补给环境较现代凉。在200m以下的深层地下水为晚更新世补给的古封存水,表现为还原环境。60m—120m左右的浅层水为古地下水与现代降水的混合水,但古地下水占的成分较多,部分水样为氧化环境。民勤盆地地下水地球化学特征主要形成于山区,在沿途运移过程强烈的蒸发浓缩作用占据主导地位,形成了浅层高矿化盐碱水,深层地下水活跃的阳离子交换作用形成高钠浓度水。通过利用PHREEEQC法对民勤盆地地下水化学进行质量平衡模拟,表明民勤盆地地下水水化学沿水流路径以HCO3^-、SO4^2-、Cl^-、Ca^2 、Na^ 升高为主要特征,方解石、白云石的饱和指数随水流路径有减少趋势,而石膏、芒硝和岩盐的饱和指数有增加的趋势：沿水流途径白云石、CO2、石膏、岩盐和芒硝溶解量逐渐增加是常量离子浓度升高的物质来源。     

Hydrogeologic Characterization of a Fault-Related Dome Using Outcrop,Borehole and Electrical Resistivity Data

Geological structures introduce heterogeneity along deformed strata, which in turn exert a strong control on regional groundwater occurrences and water flow. We developed and applied procedures of structural, borehole and direct current resistivity (DCR) data to (1) figure out a reliable structural model of a fault-related dome, (2) visualize the relationship between structural complexity and aquifers geometry and (3) characterize the fault damage zones and its related sealing potentialities. The present approach was applied at the northern Eastern Desert, Egypt, considering its complex deformation history. To alleviate the uncertainties and artifacts in solving the DCR inverse problem, advanced and non-conventional inversion schemes were applied. Moreover, fault seal potentialities were assessed by applying the shale gouge ratio (SGR) estimation to predict the groundwater pathway flow. The main findings include: (a) The DCR measurements can be considered as a complementary tool to visualize the relationship between subsurface structures and aquifers geometry at highly deformed areas; (b) detailed surface structural data analysis combined with sophisticated inversion algorithms application can increase the reliability of the constructed geo-electrical cross sections for deep and large-scale geological surveys; and (c) the structural modeling of geological units and faults distribution contributes to clarify hydrogeological settings and aquifers connectivity. The approach can be readily applied elsewhere, as long as detailed structural analysis, geometric parameters and high-resolution DCR resistivity data are available.

     

黄土塬区苹果种植对深剖面土壤水分和地下水补给的影响

通过对长武黄土塬区不同林龄苹果林地下0~20 m深剖面土壤湿度及土壤水氯离子浓度测定,定量分析了黄土塬区苹果种植对厚深黄土剖面土壤水分及地下水补给的影响。结果表明：随着林龄增加,苹果林地深剖面土壤水分由浅及深逐年降低,深层土壤储水量呈倒“S”曲线趋势下降,27龄之后进入稳定期。丰水年份形成的活塞流是黄土塬区深层渗漏以及地下水补给的主导方式,农田下地下水年均潜在补给量为30.2 mm,占年均降水量的5.2%。农田转换为苹果林地后形成的深厚土壤干层将阻断降水对地下水的补给,减弱地下水补给过程中活塞流的主导作用。需通过政策引导协调农果面积比例,保证地下水资源持续补给,达到可持续利用的目的。     

Geophysical Models for the Cu-Dominated VHMS Mineralization in Katta District, Western Ethiopia

Volcanic hosted massive sulfides (VHMS) have been recognized for many years within the Arabian–Nubian Shield of western Ethiopia. The Shield is a collision belt formed when East and West Gondwana collided during the Neoproterozoic time. It covers NE Africa and the Middle East and is known to have hosted several economic mineral deposits. Situated within the Shield, the Katta VHMS mineralization is primarily made up of chalcopyrite, sphalerite, pyrite, pyrrhotite, and magnetite. These minerals are known to have contrasting values in resistivity, chargeability, and magnetic susceptibility with the enclosing host rocks. These physical attributes make resistivity, IP, and magnetic methods ideal to study and detect the mineralization and possibly delineate the ore body. Dipole–dipole DC resistivity and induced polarization data collected along parallel profiles are inverted to generate both 2D and 3D models of resistivity and chargeability. The chargeability model successfully delineates the zone of sulfide mineralization and the resistivity model gives information on the stratigraphic horizons and the structures. Magnetic data collected along more closely spaced profiles were also inverted to recover the 3D magnetic susceptibility model. The recovered model shows a linear magnetic anomaly that could be associated with the sulfides. This anomaly shows good spatial continuity and is in good agreement with the result obtained in the chargeability model. The geophysical inversion results indicate a NNW oriented mineralized zone of considerable strike length and extending to a depth of about 380 m. This study suggests the presence of exploitable VHMS presence in Katta district and that the inversion methodology is an important interpretation tool in guiding the exploration and exploitation of sulfide mineralization.     

Three-dimensional-seismic coherency signature of Niger Delta growth faults: integrating sedimentology and tectonics

This case study of growth faults and associated deltaic sedimentation in the shallow‐offshore Niger Delta uses an integrated analysis of three‐dimensional (3D)‐seismic coherence facies and wireline data that supports an evaluation of the sedimentary response to delta tectonics. The study area comprises four fault blocks bounded by a set of kilometre‐scale, basinward‐dipping, synsedimentary normal faults. Correlation of highly variable growth stratigraphy across faults was achieved by a systematic visualization and interpretation of series of coherence horizon‐slices: the detection and matching of erosive and depositional patterns (e.g. channels) across faults allowed the establishment of sedimentology‐controlled links between diverse footwall and hanging‐wall growth successions. At the same time, this interpretation approach helped to visualize seismic‐sedimentological and seismic‐geomorphological features survey‐wide at all depth levels. The integration of this extensive 3D database with lithology information from wireline logs provides a powerful tool for subsurface sedimentology interpretation. Synoptic analysis of the 3D‐seismic sedimentology interpretation with stratigraphy based fault‐kinematic analysis using throw vs. depth plots (Th–Z plots) enabled a discussion of the relation between delta tectonics and sedimentary‐system development, and the evaluation of the Th–Z method for subsurface‐lithology prediction. The interpretation results document that both motion analysis of synsedimentary deltaic faults and Th–Z‐based lithology prediction are only feasible when supported by detailed 3D information on palaeoenvironment and palaeotopography at and around studied fault systems. We therefore recommend the use of fast‐track fault‐kinematic and subsurface‐lithology predictions based on Th–Z plots only when supported by comprehensive 3D seismic‐sedimentological interpretations.     

Fault‐controlled fluid flow inferred from hydrothermal vents imaged in 3D seismic reflection data,offshore NW Australia

Fluid migration pathways in the subsurface are heavily influenced by pre‐existing faults. Although studies of active fluid‐escape structures can provide insights into the relationships between faults and fluid flow, they cannot fully constrain the geometry of and controls on the contemporaneous subsurface fluid flow pathways. We use 3D seismic reflection data from offshore NW Australia to map 121 ancient hydrothermal vents, likely related to magmatic activity, and a normal fault array considered to form fluid pathways. The buried vents consist of craters up to 264 m deep, which host a mound of disaggregated sedimentary material up to 518 m thick. There is a correlation between vent alignment and underlying fault traces. Seismic‐stratigraphic observations and fault kinematic analyses reveal that the vents were emplaced on an intra‐Tithonian seabed in response to the explosive release of fluids hosted within the fault array. We speculate that during the Late Jurassic the convex‐upwards morphology of the upper tip‐lines of individual faults acted to channelize ascending fluids and control where fluid expulsion and vent formation occurred. This contribution highlights the usefulness of 3D seismic reflection data to constraining normal fault‐controlled subsurface fluid flow.     

巴丹吉林沙漠地下水流场的宏观特征

黑河流域下游盆地与周边地区的水力联系是当地生态建设的关键问题。巴丹吉林沙漠与黑河流域下游盆地存在长达180 km的交界线,由于水文地质调查程度低,其地下水的流场不清楚,黑河流域下游天然水均衡账本中存在不确定因素。针对这个问题,在已有水文地质调查基础上,开展了巴丹吉林沙漠区域尺度的水位调查和钻孔勘探,获得461个已知水位点和63个水位约束点,绘制出巴丹吉林沙漠浅层地下水一级近似等水位线图。结果表明:研究区地下水位具有东南高、西北低的特点,水力梯度0.6‰~4.0‰,沙漠东南部地区的湖泊对地下水流场具有扰动作用,但没有改变宏观的地下水流向;第四系松散砂层是巴丹吉林沙漠浅层地下水的主要含水层,根据现有钻孔和物探资料,第四系沉积物在沙漠腹地较厚、边缘较薄,覆盖厚度普遍超过100 m,联通了沙漠和黑河流域下游地区的地下水;巴丹吉林沙漠的地下水补给黑河下游的古日乃湖、额济纳旗平原和拐子湖地区,地下水侧向出境流量为(0.61~1.97)×10⁸ m³·a^-1,其中进入古日乃湖平原的流量达到(0.33~1.06)×10⁸ m³·a^-1。计算结果仍然受到一些不确定因素的影响。     

Detailed 3D seismic imaging of a fault zone beneath Lake Geneva, Switzerland

An efficient high‐resolution (HR) three‐dimensional (3D) seismic reflection system for small‐scale targets in lacustrine settings was developed. In Lake Geneva, near the city of Lausanne, Switzerland, the offshore extension of a complex fault zone well mapped on land was chosen for testing our system. A preliminary two‐dimensional seismic survey indicated structures that include a thin (<40 m) layer of subhorizontal Quaternary sediments that unconformably overlie south‐east‐dipping Tertiary Molasse beds and a major fault zone (Paudèze Fault Zone) that separates Plateau and Subalpine Molasse (SM) units. A 3D survey was conducted over this test site using a newly developed three‐streamer system. It provided high‐quality data with a penetration to depths of 300 m below the water bottom of non‐aliased signal for dips up to 30° and with a maximum vertical resolution of 1.1 m. The data were subjected to a conventional 3D processing sequence that included post‐stack time migration. Tests with 3D pre‐stack depth migration showed that such techniques can be applied to HR seismic surveys. Delineation of several horizons and fault surfaces reveals the potential for small‐scale geologic and tectonic interpretation in three dimensions. Five major seismic facies and their detailed 3D geometries can be distinguished. Three fault surfaces and the top of a molasse surface were mapped in 3D. Analysis of the geometry of these surfaces and their relative orientation suggests that pre‐existing structures within the Plateau Molasse (PM) unit influenced later faulting between the Plateau and SM. In particular, a change in strike of the PM bed dip may indicate a fold formed by a regional stress regime, the orientation of which was different from the one responsible for the creation of the Paudèze Fault Zone. This structure might have later influenced the local stress regime and caused the curved shape of the Paudèze Fault in our surveyed area.     

The diffuse transition between the Zagros continental collision and the Makran oceanic subduction (Iran): microearthquake seismicity and crustal structure

The nature of the transition between the Zagros intra-continental collision and the Makran oceanic subduction is a matter of debate: either a major fault cutting the whole lithosphere or a more progressive transition associated with a shallow gently dipping fault restricted to the crust. Microearthquake seismicity located around the transition between the transition zone is restricted to the west of the Jaz-Murian depression and the Jiroft fault. No shallow micro-earthquakes seem to be related to the NNW–SSE trending Zendan–Minab–Palami active fault system. Most of the shallow seismicity is related either to the Zagros mountain belt, located in the west, or to the NS trending Sabzevaran–Jiroft fault system, located in the north. The depth of microearthquakes increases northeastwards to an unusually deep value (for the Zagros) of 40 km. Two dominant types of focal mechanisms are observed in this region: low-angle thrust faulting, mostly restricted to the lower crust, and strike-slip at shallow depths, both consistent with NS shortening. The 3-D inversion of P traveltimes suggests a high-velocity body dipping northeastwards to a depth of 25 km. This high-velocity body, probably related to the lower crust, is associated with the deepest earthquakes showing reverse faulting. We propose that the transition between the Zagros collision and the Makran subduction is not a sharp lithospheric-scale transform fault associated with the Zendan–Minab–Palami fault system. Instead it is a progressive transition located in the lower crust. The oblique collision results in partial partitioning between strike-slip and shortening components within the shallow brittle crust because of the weakness of the pre-existing Zendan–Minab–Palami faults.     

黄河三角洲土壤氯离子空间变异特征及其控制因素

为了解黄河三角洲地区土壤盐渍化发生与发展的主要控制因素,通过采集土壤样品及潜水样,运用地统计学等方法,分析样品中优势离子Cl^-含量的空间分布特征及成因。结果表明:① 土壤Cl^-含量与潜水Cl^-质量浓度的空间分布表现出高度的相似性和相关性。② 潜水借助包气带自身的水势梯度或植物蒸腾作用向土壤输送水分,进而控制着土壤Cl^-的空间分布与变异。③ 地形地貌通过对水、气、热的再分配控制着包气带土层中Cl^-的迁移方向和富集区域。水文地质条件是对区域"饱和带-包气带"盐分迁移富集起决定作用的内部因素,而地形地貌条件是起控制作用的外部因素。