Contribution of geophysical surveys to groundwater modelling of a porous aquifer in semiarid Niger: An overview

Subsurface geophysical surveys were carried out using a large range of methods in an unconfined sandstone aquifer in semiarid south-western Niger for improving both the conceptual model of water flow through the unsaturated zone and the parameterization of numerical a groundwater model of the aquifer. Methods included: electromagnetic mapping, electrical resistivity tomography (ERT), resistivity logging, time domain electromagnetic sounding (TDEM), and magnetic resonance sounding (MRS). Analyses of electrical conductivities, complemented by geochemical measurements, allowed us to identify preferential pathways for infiltration and drainage beneath gullies and alluvial fans. The mean water content estimated by MRS (13%) was used for computing the regional groundwater recharge from long-term change in the water table. The ranges in permeability and water content obtained with MRS allowed a reduction of the degree of freedom of aquifer parameters used in groundwater modelling.

Using geophysical surveys to test tracer-based storage estimates in headwater catchments

Acknowledgements The authors are grateful to Stian Bradford, Chris Gabrielli, and Julie Timms for practical and logistical assistance. The provision of transport by Iain Malcolm and Ross Glover of Marine Scotland Science was greatly appreciated. We also thank the European Research Council ERC (project GA 335910 VEWA) for funding through the VeWa project and the Leverhulme Trust for funding through PLATO (RPG-2014-016).

Time-lapse geophysical investigations over known archaeological features using electrical resistivity imaging and earth resistance

Electrical methods of geophysical survey are known to produce results that are hard to predict at different times of the year, and under differing weather conditions. This is a problem which can lead to misinterpretation of archaeological features under investigation. The dynamic relationship between a ‘natural’ soil matrix and an archaeological feature is a complex one, which greatly affects the success of the feature’s detection when using active electrical methods of geophysical survey. This study has monitored the gradual variation of measured resistivity over a selection of study areas. By targeting difficult to find, and often ‘missing’ electrical anomalies of known archaeological features, this study has increased the understanding of both the detection and interpretation capabilities of such geophysical surveys. A 16 month time-lapse study over 4 archaeological features has taken place to investigate the aforementioned detection problem across different soils and environments. In addition to the commonly used Twin-Probe earth resistance survey, electrical resistivity imaging (ERI) and quadrature electro-magnetic induction (EMI) were also utilised to explore the problem. Statistical analyses have provided a novel interpretation, which has yielded new insights into how the detection of archaeological features is influenced by the relationship between the target feature and the surrounding ‘natural’ soils. The study has highlighted both the complexity and previous misconceptions around the predictability of the electrical methods. The analysis has confirmed that each site provides an individual and nuanced situation, the variation clearly relating to the composition of the soils (particularly pore size) and the local weather history. The wide range of reasons behind survey success at each specific study site has been revealed. The outcomes have shown that a simplistic model of seasonality is not universally applicable to the electrical detection of archaeological features. This has led to the development of a method for quantifying survey success, enabling a deeper understanding of the unique way in which each site is affected by the interaction of local environmental and geological conditions.

Geophysical mapping of the sediment architecture in the Orkhon Valley, Mongolia

Ground penetrating radar (GPR) and capacitive coupled resistivity (CCR) measurements were conducted in order to image subsurface structures in the Orkhon Valley, Central Mongolia. The data are extended by information from drill cores to the entire transects distinguishing different sedimentary environments in the valley. The Orkhon Valley is part of the high sensitive Steppe region in Central Mongolia, one of the most important cultural landscapes in Central Asia. There, archaeological, geoarchaeological and sedimentological research aims to reconstruct the landscape evolution and the interaction between man and environment during the last millennia since the first settlement. In May 2009 and 2010 geophysical surveys have been conducted including transects with lengths between 1.5 and 30 km crossing the entire valley and a kilometre-scaled grid in the southern part of the investigation area. The geoelectrical and GPR data revealed the existence of two layers characterized by different resistivity values and radar reflectors. The two layers do not only represent material contrasts, but also reflect the influence of sporadic permafrost which occurs in several areas of Mongolia. The results help to reconstruct the evolution of the braided Orkhon River and therefore give important hints to understand the environmental history of the Orkhon Valley.

Undrained sediment loading key to long-runout submarine mass movements : evidence from the Caribbean Volcanic Arc

Long undersea debris runout can be facilitated by a boundary layer formed by weak marine sediments under a moving slide mass. Undrained loading of such offshore sediment results in a profound drop of basal shear resistance, compared to subaerial shear resistance, enabling long undersea runout. Thus large long-runout submarine landslides are not truly enigmatic (Voight and Elsworth 1992, 1997), but are understandable in terms of conventional geotechnical principles. A corollary is that remoulded undrained strength, and not friction angle, should be used for basal resistance in numerical simulations. This hypothesis is testable via drilling and examining the structure at the soles of undersea debris avalanches for indications of incorporation of sheared marine sediments, by tests of soil properties, and by simulations. Such considerations of emplacement process are an aim of ongoing research in the Lesser Antilles (Caribbean Sea), where multiple offshore debris avalanche and dome-collapse debris deposits have been identified since 1999 on swath bathymetric surveys collected in five oceanographic cruises. This paper reviews the prehistoric and historic collapses that have occurred offshore of Antilles arc islands and summarizes ongoing research on emplacement processes.

Mineralogy and Magnetic Properties of Basaltic Substrate Soils: Kaho'olawe and Big Island, Hawaii

Magnetic behavior of soils can seriously hamper the performance of geophysical sensors. Currently, we have little understanding of the types of minerals responsible for the magnetic behavior, as well as their distribution in space and evolution through time. This study investigated the magnetic characteristics and mineralogy of Fe-rich soils developed on basaltic substrate in Hawaii. We measured the spatial distribution of magnetic susceptibility (χlf) and frequency dependence (χfd%) across three test areas in a well-developed eroded soil on Kaho'olawe and in two young soils on the Big Island of Hawaii. X-ray diffraction spectroscopy, x-ray fluorescence spectroscopy (XFCF), chemical dissolution, thermal analysis, and temperature-dependent magnetic studies were used to characterize soil development and mineralogy for samples from soil pits on Kaho'olawe, surface samples from all three test areas, and unweathered basalt from the Big Island of Hawaii. The measurements show a general increase in magnetic properties with increasing soil development. The XRF Fe data ranged from 13% for fresh basalt and young soils on the Big Island to 58% for material from the B horizon of Kaho'olawe soils. Dithionite-extractable and oxalate-extractable Fe percentages increase with soil development and correlate with χlf-and χfd%, respectively. Results from the temperature-dependent susceptibility measurements show that the high soil magnetic properties observed in geophysical surveys in Kaho'olawe are entirely due to neoformed minerals. The results of our studies have implications for the existing soil survey of Kaho'olawe and help identify methods to characterize magnetic minerals in tropical soils.

Constraining complex aquifer geometry with geophysics (2-D ERT and MRS measurements) for stochastic modelling of groundwater flow

Stochastic modelling is a useful way of simulating complex hard-rock aquifers as hydrological properties (permeability, porosity etc.) can be described using random variables with known statistics. However, very few studies have assessed the influence of topological uncertainty (i.e. the variability of thickness of conductive zones in the aquifer), probably because it is not easy to retrieve accurate statistics of the aquifer geometry, especially in hard rock context. In this paper, we assessed the potential of using geophysical surveys to describe the geometry of a hard rock-aquifer in a stochastic modelling framework. The study site was a small experimental watershed in South India, where the aquifer consisted of a clayey to loamy-sandy zone (regolith) underlain by a conductive fissured rock layer (protolith) and the unweathered gneiss (bedrock) at the bottom. The spatial variability of the thickness of the regolith and fissured layers was estimated by electrical resistivity tomography (ERT) profiles, which were performed along a few cross sections in the watershed. For stochastic analysis using Monte Carlo simulation, the generated random layer thickness was made conditional to the available data from the geophysics. In order to simulate steady state flow in the irregular domain with variable geometry, we used an isoparametric finite element method to discretize the flow equation over an unstructured grid with irregular hexahedral elements. The results indicated that the spatial variability of the layer thickness had a significant effect on reducing the simulated effective steady seepage flux and that using the conditional simulations reduced the uncertainty of the simulated seepage flux. As a conclusion, combining information on the aquifer geometry obtained from geophysical surveys with stochastic modelling is a promising methodology to improve the simulation of groundwater flow in complex hard-rock aquifers. (C) 2013 Elsevier B.V. All rights reserved.

Spatial statistics to estimate peat thickness using airborne radiometric data

Soil carbon stores are a major component of the annual returns required by EU governments to the Intergovernmental Panel on Climate Change. Peat has a high proportion of soil carbon due to the relatively high carbon density of peat and organic-rich soils. For this reason it has become increasingly important to measure and model soil carbon stores and changes in peat stocks to facilitate the management of carbon changes over time. The approach investigated in this research evaluates the use of airborne geophysical (radiometric) data to estimate peat thickness using the attenuation of bedrock geology radioactivity by superficial peat cover. Remotely sensed radiometric data are validated with ground peat depth measurements combined with non-invasive geophysical surveys. Two field-based case studies exemplify and validate the results. Variography and kriging are used to predict peat thickness from point measurements of peat depth and airborne radiometric data and provide an estimate of uncertainty in the predictions. Cokriging, by assessing the degree of spatial correlation between recent remote sensed geophysical monitoring and previous peat depth models, is used to examine changes in peat stocks over time. The significance of the coregionalisation is that the spatial cross correlation between the remote and ground based data can be used to update the model of peat depth. The result is that by integrating remotely sensed data with ground geophysics, the need is reduced for extensive ground-based monitoring and invasive peat depth measurements. The overall goal is to provide robust estimates of peat thickness to improve estimates of carbon stocks. The implications from the research have a broader significance that promotes a reduction in the need for damaging onsite peat thickness measurement and an increase in the use of remote sensed data for carbon stock estimations.

Sondagens mecânicas e prospecção geofísica na caracterização de fluidos

Esta dissertação aborda a utilização de métodos geofísicos de superfície e de profundidade na caracterização de fluidos, compreendendo várias abordagens distintas, consoante o tipo de temática em análise. Assim, o assunto proposto para a presente dissertação relaciona-se com a utilização conjunta de sondagens mecânicas e prospecção geofísica na caracterização de fluidos através da exploração de três temáticas distintas, designadamente a Hidrogeologia, a Geotermia e o Ambiente. O presente trabalho de dissertação baseado na experiência profissional da autora, é feito de acordo com o estabelecimento de uma linha condutora, fundamentada e estruturada, para uma visão global do modo como a interpretação integrada das sondagens mecânicas e dos métodos geofísicos dá um forte contributo para a melhoria do conhecimento de uma região, seja ele geológico, hidrogeológico ou ambiental. Os temas a abordar foram seleccionados não só pela sua indiscutível importância, devido ao interesse estratégico que actualmente suscitam, como também pelos diferentes modos de processar e interpretar a informação geofísica adquirida consoante os objectivos da mesma, de forma a salientar particularidades na respectiva interpretação e integração de dados.

Use of electrical resistivity to identify collapsible soils in Brazil

Three soil profiles in Ilha Solteira, Brazil were investigated to establish their potential for collapsible behavior. The soil profiles were identified using terrain evaluation techniques and simple laboratory tests. Geophysical surveys were undertaken as they are quick and relatively cheap. The results were correlated with trial pit descriptions and cone and standard penetration tests. The study has shown that electrical resistivity is a useful tool for the preliminary identification of horizons of collapsible soils, before more expensive intrusive and laboratory work is undertaken.

Use of electrical resistivity to identify collapsible soils in Brazil

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Caracterização geológico-geofísica do meio aqüífero fissural : uma contribuição aos modelos de fluxo e armazenamento de água subterrânea

Northeastern Brazil is mainly formed by crystalline terrains (around 60% in area). Moreover, this region presents a semi-arid climate so that it is periodically subject to drought seasons. Furthermore, ground water quality extracted fromwells usually presents poor quality because of their high salinity contents. Nevertheless, ground water is still a very important source of water for human and animal consumption in this region. Well sitting in hard rocks terrains in Northeastern Brazil offers a mean success index of aboul 60%, given that a successful siting is defined by a well producing at least 0.5 m³/h. This low index reveals lack of knowledga about the true conditions of storage and percolation of ground water in crystalline rocks. Two models for structures storing and producing ground water in crystalline rocks in Northeastem Brazil have been proposed in the literature. The first model,tradnionally used for well sitting since the sixties are controlled by faults or fractures zones. This model is commonly referred, in Brazilian hydrogeological literature, as the "creek-crack" model (riacho-fenda in Portuguese). Sites appearing to present dense drainage network are preferred for water well siting - particularly at points where the drainages cross-cul each other. Field follow up work is usually based only on geological criteria. The second model is the "eluvio-alluvial through" (calha eluvio-aluvionar in Portuguese); it is also described in the literature but it is not yet incorporated in well sitting practice. This model is based on the hypothesis that reclilinear drainages can also be controlled by the folietion of the rock. Eventually, depending upon the degree of weathering, a through-shaped structure filled with sediments (alluvium and regolith) can be developed which can store and water can be produced from. Using severalfield case studies, this Thesis presents a thorough analysis ofthe two above cited models and proposes a new model. The analysis is based on an integrated methodological approach using geophysics and structural geology. Both land (Resitiviy and Ground Penetrating Radar- GPR) and aerogeophysical (magnetics and frequency domain eletromagnetics) surveys were used. Slructural analysis emphasized neolectonic aspects; in general, itwas found that fractures in the E-W direction are relatively open, as compared to fracturas inthe N-S direction, probably because E-W fractures were opened by the neotectonic stress regime in Northeastern Brazil, which is controlled by E-W compression and N-S extension. The riacho-fenda model is valid where drainages are controlled by fractures. The degree of fracturing and associated weathering dictale the hydrogeological potential of the structure. Field work in structural analogues reveals that subvertical fractures show consistent directions both in outcrop and aerophotograph scales. Geophysical surveys reveal subvertical conductive anomalies associated to the fracture network controlling the drainage; one of the borders of the conductive anomaly usually coincide wih the drainage. An aspect of particular importance to the validation of fracture control are the possible presence of relalively deep conductive anomalies wihoul continuation or propagalion to the surface. The conductive nature of lhe anomaly is due to the presence of wealhered rock and sedirnenls (alluvium and/or regolilh) storing ground waler which occur associated to the fracture network. Magnetic surveys are not very sensisnive to these structures.lf soil or covering sedirnents are resislive (> 100 Ohm.m), GPR can ba used to image precisely lhe fracture network. A major limialion of riacho-fenda model, revealed by GPR images, is associated to the fact thal subhorizontal fractures do play a very important role in connecting the fracture network, besides connect shallow recharge zones to relalively deep subvertical frecture zones. Iffractures play just a secondary control on the drainage, however, r/acho-fenda model may have a very limiled validny; in these cases, large portions oflhe drainage do nol coincide wilh frectures and mosl oflhewells localed in lhe drainage surrounding would resull dry. Usually, a secondary conlrol on lhe drainage by Ihefraclure networkcan be revealed only wilh detailed geophysical survey. The calha elClv1o-aluvlonarmodel is valid where drainages are conlrolled by folialion. The degree 01 wealhering 01 lhe lolialion planes dictales lhe hydrogeological polenlial 01 lhe slruclure. Outcrop analysis reveals Ihal lolialion and drainage direclions are parallel and Ihal no Iraclures, orfraclures wilh diflerent directions 01 lhe drainage direclion occur. Geophysical surveys reveal conduclive anomalies in a slab lorm associaled 10 lhe Ihrough 01 lhe wealhered rock and sedimenls (alluvium and/or regolith). Magnelic surveys can ofler a very good conlrol on lolialion direclion. An importanl aspect 10 validale lolialion conlrol are lhe presence 01 conductive anomalies showing shallow and deep portions area which are linked. Illhere is an exlensive soil cover, r/acho-fenda and calha eIClv1o-aluv/onar conlrols can be easily misinlerpreled in lhe absence 01 geophysical conlrol. Certainly, Ihis lacl could explain at leasl a part of lhe failure index in well sitting. The model wealhering sack (bolsllo de Intempertsmo in Portuguese) is proposed to explain cases where a very inlensive wealhering occur over lhe crystalline rock so Ihal a secondary inlerslilial porosity is crealed. The waler is Ihen stored in lhe porous of lhe regolilh in a similar mannerlo sedimentary rocks. A possible example ofthis model was delecled by using land geophysical survey where a relalivelyvery deep isolaled conduclive anomaly, in a slab form, was delected. Iflhis structure does store ground waler, certainly Ihere must be a link 01 lhe deep slructure wilh lhe surface in orderlo provide walerfeeding. This model mighl explain anomalous waler yields as greal as 50 m³/h Ihalsomelimescan occur in crystalline rocks in Northeaslern Brazil

Estruturação litosférica da Província Borborema ao longo da Transecta Angüera (BA) Macau (RN), com base em dados gravimétricos

Since 2005, geophysical surveys have been carried out in the Precambri-an Borborema Province, along two transects with 800 km long each one. A pool of Brazilian public universities and institutions has been acquired deep refrac-tion seismic, gravity and magnetotelluric, with the purpose to model the conti-nental lithosphere of the region. This paper present the gravity survey of the second transect, that crosses the Borborema Province from SW to NE, passing through the São Francisco Craton, Transversal and Meridional zones and Rio Grande do Norte Domain, in the Setentrional Zone. In this way, it cuts some important geologic structures, like the limit of the São Francis Craton and the Borborema Province, Paleozoic and Mesozoic sedimentary basins of Tucano, Jatobá and Potiguar and the extensive Pernambuco and Patos shear zones. Recognition techniques gravity sources in the subsurface, such as spectral analysis and Euler Deconvolution, were applied to the Bouguer anomalies, as well as their regional and residual components. These techniques provided in-formation on possible anomalous bodies, which correlated with pre-existing geological and geophysical data, subsidized a 2.5 D gravity modeling of the lithosphere beneath the Borborema Province and its southern limit with the São Francisco Craton.

Evidências da herança geotectônica pré-cambriana na geração da barcia potiguar: um estudo geofísico multdisciplinar

The 3D gravity modeling of the Potiguar rift basin consisted of a digital processing of gravity and aeromagnetic data, subsidized by the results of Euler deconvolution of gravity and magnetic data and the interpretation of seismic lines and wells descriptions. The gravity database is a compilation of independent geophysical surveys conducted by several universities, research institutions and governmental agencies. The aeromagnetic data are from the Bacia Potiguar and Plataforma Continental do Nordeste projects, obtained from the Brazilian Petroleum Agency (ANP). The solutions of the Euler Deconvolution allowed the analysis of the behavior of the rift main limits. While the integrated interpretation of seismic lines provided the delimitating horizons of the sedimentary formations and the basement top. The integration of these data allowed a 3D gravity modeling of basement topography, allowing the identification of a series of internal structures of the Potiguar rift, as well intra-basement structures without the gravity effect of the rift. The proposed inversion procedure of the gravity data allowed to identify the main structural features of the Potiguar rift, elongated in the NE-SW direction, and its southern and eastern faulted edges, where the sedimentary infill reachs thicknesses up to 5500 m. The southern boundary is marked by the Apodi and Baixa Grande faults. These faults seem to be a single NW-SE oriented fault with a strong bend to NE-SW direction. In addition, the eastern boundary of the rift is conditioned by the NE-SW trending Carnaubais fault system. It was also observed NW-SE oriented faults, which acted as transfer faults to the extensional efforts during the basin formation. In the central part of the residual anomaly map without the gravity effect of the rift stands out a NW-SE trending gravity high, corresponding to the Orós-Jaguaribe belt lithotypes. We also observe a gravity maximum parallel to the Carnaubais fault system. This anomaly is aligned to the eastern limit of the rift and reflects the contact of different crustal blocks, limited by the eastern ward counterpart of the Portalegre Shear Zone