Extent Of Aquifers Research Articles

Assessing regional variation in yield from weathered basement aquifers in West Africa and modelling their future groundwater development and sustainability

A data-driven modelling approach was applied to quantify the potential groundwater yield from weathered crystalline basement aquifers in West Africa, which are a strategic resource for achieving water and food security. To account for possible geological control on aquifer productivity, seven major geological domains were identified based on lithological, stratigraphic, and structural characteristics of the crystalline basement. Extensive data mining was conducted for the hydrogeological parameterisation that led to the identification of representative distributions of input parameters for numerical simulations of groundwater abstractions. These were calibrated to match distributions of measured yields for each domain. Calibrated models were then applied to investigate aquifer and borehole scenarios to assess groundwater productivity. Considering the entire region, modelling results indicate that approximately 50% of well-sited standard 60-m-deep boreholes could sustain yields exceeding 0.5 L/s, and 25% could sustain the yield required for small irrigation systems (> 1.0 L/s). Results also highlighted some regional differences in the ranges of productivities for the different domains, and the significance of the depth of the static water table and the lateral extent of aquifers across all geological domains. This approach can be applied to derive groundwater maps for the region and provide the quantitative information required to evaluate the potential of different designs of groundwater supply networks.

Geological investigation in the Bhuleshwari River Basin, Amravati District, Maharashtra

The Bhuleshwari river basin covers an area about 380 Sq. Km. from Bhatkuli and Achalpur Taluka, Amravati district, Maharashtra. It flows over varied geological formation from Deccan trap to Alluvium with groundwater quality varying from fresh water to saline water. The area has been studied to evaluate the impact of geomorphology on groundwater occurrence and its impact on hydrogeochemical evolution of groundwater. Number of field traverses and observation in the bank sections of the river and dug wells has been done in the given basin. The nature and lateral and vertical extent of aquifers are controlled by lithology, stratigraphy and structure of rock formations. Lithological characteristics of rocks are reflected in their storage and yield properties. The present work deals with the regional as well as local geological set of the area and its Stratigraphic position. It also deals with the lithological characteristics of the ‘basalt flows’ and structural features of the area. The detailed geology of the Bhuleshwari river basin has been discussed and presented in chapter two. Based on number of field traverses, in the geological map eight lavaflows has been identified in northern region and alluvium in southern region can be divided into younger and older alluvium. The Deccan trap is separated by alluvium with fault, boulder alluvium is seen to occur in this zone. The geological map of Bhuleshwari river basin modified from geological survey of India map, GSI (2001) is presented. Geologically the study area consists of 420 m thick Alluvium. Only the top 40 m is accessible for direct observation in the bank sections of the river and dug wells.

Evaluation of seasonal catchment dynamic storage components using an analytical streamflow duration curve model

AbstractDynamic storage refers to groundwater storage that is sensitive to rainfall infiltration, streamflow generation, evapotranspiration, and other variables involving groundwater gain or loss. It plays a crucial role in habitat maintenance and the mitigation of environmental impacts on regional hydrological behaviors. Dynamic storage can be separated into direct storage, which contributes to the river channel, and indirect storage, which is insensitive to streamflow. The combination of diverse approaches would provide an estimation of the two storage types. This study estimated optimal baseflow coefficients and direct storage in the wet and dry seasons using an analytical streamflow duration curve model in eight catchments of the Choushui River Basin from 2013 to 2017. The water balance approach was then combined to assess indirect storage for evaluating seasonal dynamic storage components. The model applicability for each catchment of the Choushui River Basin in the wet and dry seasons was assessed using the similarity between observed and simulated flow duration curves, namely Kolmogorov–Smirnov distance. We also applied it to assess the performance difference between model and streamflow recession analysis, which is typically used to estimate baseflow coefficients. The results demonstrated that seasonal differences in baseflow coefficients were related to catchment characteristics as well as the aquifer extent through which groundwater flows. The model utilizing maximum likelihood estimation exhibited superior performance than streamflow recession analysis and was highly applicable in our study area in wet and dry seasons. Dynamic storage components demonstrated a considerable difference in the additional groundwater storage between dry and wet seasons and a loss of direct storage was observed in most catchments during the dry season.

Gravity and electrical resistivity analysis of deep and shallow structures related to aquifers within the Jeffara Plain, southeast Tunisia

The aquifers under the Jeffara Plain in southeastern Tunisia is a complex aquifer system consisting of interconnected shallow and deep aquifers within Triassic and younger sediments. The extent of the aquifers is only roughly known, so a gravity and electrical resistivity analysis was undertaken to aid for determining the aquifers extent. The gravity analysis consisted of creating a residual gravity anomaly map, a power spectrum analysis to determine the depth to density sources and a 3D Euler deconvolution analysis to locate lineaments and the depth of density sources. The gravity analysis was used to infer the location and depth of faults and thicker portions of the aquifer containing sediments The electrical resistivity survey was used to characterize the geometry and the depth of deep and shallow aquifers surrounding Medenine city. The residual gravity anomaly map identified a series of northwest-trending gravity minima related to grabens. The Euler deconvolution determined faults can be classified into deep faults (˃ 2.5 km depth) that these associated with the Medenine fault and nearly determined offshoots that trend north-northeast, shallow (0.5–1.0 km) and near surface faults (<0.5 km). The shallow and the near-surface faults involve Upper Cretaceous and younger sediments and trend in a variety of directions. The electrical resistivity models indicate that the shallow aquifers are approximately 20 m thick and the deep aquifer is found below 25 m in depth and is at least 30 m thick. The shallow aquifers were found on all four models but the deep aquifer was found only on two profiles. The location of both the shallow and deep aquifers were affected by faults inferred by the electrical resistivity data and these faults were correlated with lineaments determined by the gravity analysis. This shows that the shallow/near surface faults are important conduits between the shallow and deep aquifers. The most important fault was the Medenine and its newly deduced offshoots as the electrical resistivity models indicate that these faults act as a barrier for the deep aquifer. Based on both the gravity analysis and the electrical resistivity models, the extent of the deep aquifer can be extended further to the east than was previously known.

Geophysical Resistivity Survey (VES) for Selection of Artificial Recharge of Appropriate Artificial recharge for Augmentation of Ground Water Resources In Paithankheda Village, M. S. India

In this study the data generated while conducting electrical resistivity surveys through Vertical Electrical Sounding (VES) at 22 sites in different location along with a available geological &amp;hydro-geological information of Paithan Kheda village were analyzed. The objective are (i)to understand the nature and extent of aquifer(ii)to find out the location and thickness of unsaturated zones and (iii)to evaluate and extent of appropriate artificial recharge structures at suitable locations across the village. Interpretation of the top sounding curves indicates presence of three to four subsurface geo-electric layers across the study area. The top soil ;layer has range of resistivity values from 2 to 30-ohm m &amp; litho logy comprise clay with kankar,lateritic sand. This is followed by the weathered and jointed Basalt layers with resistivity values 100 300-ohm m could be identified below the depth of 45m.At some places, doleritic dykes were also observed with resistivity values &gt;300-ohms. The interpretation of VES data when correlated with the available lithilog data indicates a prominent water bearing zone between 30 and 45 mbgl.The underlying hard and compact Basalt has very little possibility of occurrence of groundwater. The top unsaturated and unconfined granular zone up to adept of 30mts could therefore be easily recharged artificially through rainwater harvestining measures thereby augmenting the groundwater resources of the existing aquifers. Site specific artificial recharge measures from amongst counter banding ,gully plugs ,check dams, percolation tank. Recharge shafts subsurface dikes have been identified across the village for effective recharge of the aquifer especially in its north eastern &amp;southern blockregions.Considering the deteeoriating groundwater sitution, these initiatives would be significant in catering the needs of future generations.

Insights into the hydrogeological framework of the NW Himalayan Karewas (India)

Karewas which represent the Quarternary sediments deposited in fluvio-lacustrine/fluvio-glacial environment and peripherally surround the valley of the Jhelum river basin in India are highly devoid of surface water resources. The present study is an investigation of the subsurface regime of watersheds of Jhelum basin, encompassing alluvial and Karewa (fluvio-lacustrine) sediments using various software-based techniques with an aim to understand its groundwater scenario. Geological and hydrogeological framework models were developed to infer vertical and horizontal extent of aquifers. The models predict multiple fine sand aquifers in the alluvial sediments, 0.3 to 48 m thick, encountered at 3 to 10 m below the ground level (m bgl); major potential aquifer of 10 to 50 m thickness in Upper Karewa sediments at a depth of 25 to 35 m bgl and about 20 m thick aquifer in Lower Karewa sediments. Water table location obtained by geospatial (stochastic and deterministic) interpolation of static water levels reveals shallow water table at 0.3 to 10 m bgl within almost the entire alluvium region, deepening to 30 m bgl within the Karewa formations. The confining heads vary from a few meters below the ground surface to a few meters above it within alluvium, and much higher within the Karewas attributable to the location of recharge zones within the tall mountains bordering the study area. The study provides a framework for numerical groundwater modelling of the region. The methodology outlined in this study can be applied to diverse geological environments across local or regional scales to understand the hydrogeological conditions therein.

Effects of Multiconstituent Tides on a Subterranean Estuary With Fixed-Head Inland Boundary

While tides of multiple constituents are common in coastal areas, their effects on submarine groundwater discharge (SGD) and salinity distributions in unconfined coastal aquifers are rarely examined, with the exception of a recent study that explored such effects on unconfined aquifers with fixed inland freshwater input. For a large proportion of the global coastline, the inland areas of coastal aquifers are topography-limited and controlled by constant heads. Based on numerical simulations, this article examines the variation of SGD and salinity distributions in coastal unconfined aquifers with fixed-head inland boundaries at different distances from the shoreline (i.e., 50, 100, 150, and 200 m). The results showed that the fluctuation intensity of freshwater input was enhanced as the inland aquifer extent decreased, e.g., the range of tide-induced fluctuations in freshwater input increased by around 5 times as the inland aquifer extent decreased from 200 to 50 m. The frequency spectra of the fluctuations of SGD and salinity distributions showed that the coastal aquifer of a shorter inland aquifer extent smoothed out fewer high-frequency tidal constituents but enhanced interaction among different tidal constituents. The interaction among tidal constituents generated new low-frequency signals in the freshwater input and salinity distributions. Regressions based on functional data analysis demonstrated that the inland freshwater input and salinity distributions at any given moment were related to the antecedent (previous) tidal conditions weighted using the probability density function of the Gamma distribution. The influence of the antecedent tidal conditions depended on the inland aquifer extent.

Investigating the Productivity and Sustainability of Weathered Basement Aquifers in Tropical Africa Using Numerical Simulation and Global Sensitivity Analysis

AbstractGroundwater stored in weathered basement aquifers (WBAs) is a strategic water resource. In this study, we investigate the productivity of WBAs and sustainability of groundwater abstractions using a novel process‐based stochastic modeling approach, which is applied to simulate abstractions in the Precambrian basement aquifer in Ghana. The statistical distribution of the generated synthetic yield data was found in very good agreement with observed yield data from the same Ghanaian aquifer. Further analysis provided robust insights regarding how different hydrogeological parameters of the WBA, and their interplay, control aquifer productivity and sustainability. Results indicate that 97% of the simulated abstractions could sustain the yield of a hand pump (6 L/min), approximately 30% could also sustain yields &gt;60 L/min, while only 1% could sustain yields greater than 300 L/min. The model indicates that an aquifer transmissivity value of approximately 1.4 m2/day is required for a successful hand‐pumped borehole, while a higher yielding source (60 L/min) requires a transmissivity value of at least 9.5 m2/day. A global sensitivity analysis of 13 model input parameters shows that the thickness of the regolith and the maximum hydraulic conductivity developed at the base of the saprolite are the critical factors controlling success and sustainability for low yielding hand‐pumped boreholes. For higher yielding supplies, the net recharge, the depth to groundwater, and the aquifer extent become increasingly significant. Results from this work have important implications for the potential for increased development of groundwater from WBAs in tropical Africa.

Evaluation of hydrogeological conditions through vertical electrical soundings survey at Mankiganj pourashava, Manikganj, central part of Bangladesh

The Pourashava area of Manikganj, located in the central part of Bangladesh is treated as arsenic hotspot in the country as the outcome of various studies concludes that almost 80% of the domestic tube wells at this area exceed arsenic level of Bangladesh standard (50 ?g/l) for drinking water. To enhance the potable water supply in the area, 15 geoelectric soundings survey using Schlumberger configuration with spreading AB/2 = 300 meters has been executed to decipher the subsurface lithology distribution and aquifers at different depths, to determine the lateral and vertical extent of aquifer at the area. Correlation of the analyzed VES results with primary and secondary borelogs, the subsurface sequence of the area is broadly divided into two geoelectric units. Firstly, the near surface geoelectric unit of resistivity less than 20.0 ?-m represents the top soil composed of silty clay to clay. The thickness of this layer varies from 1 meter to 5 meters. The second geoelectric unit represents the sand layer and is characterized by a resistivity range of 21.0 to 190.0 ?-m. This unit encompasses the very fine sand with variable thickness of 2 to 248m while resistivity is from 21.0 to 30.0 ?-m, very fine to fine sand having thickness from 174-280m and geoelectric value ranges from 31.0 to 40.0 ?-m, fine sand with resistivity from 44.0 to 50.0 ?-m, fine to medium sand having resistivity from 51.0 to 70.0 ?-m, medium sand and medium to coarse sand with resistivity value 75.0 to 110.0 ?-m, and 140.0 to 190.0 ?-m, respectively. The ultimate thickness of this unit could not be determined due to limited spread of survey but at least continues to a depth of 300 m. In the context of groundwater storage, the second geoelectric unit acts as the regional aquifer and is used for groundwater development in the Pourashava area for water supply.Bangladesh J. Sci. Res. 27(2): 109-120, December-2014

Hydrogeological and geophysical study for deeper groundwater resource in quartzitic hard rock ridge region from 2D resistivity data

Electrical resistivity method is a versatile and economical technique for groundwater prospecting in different geological settings due to wide spectrum of resistivity compared to other geophysical parameters. Exploration and exploitation of groundwater, a vital and precious resource, is a challenging task in hard rock, which exhibits inherent heterogeneity. In the present study, two-dimensional Electrical Resistivity Tomography (2D-ERT) technique using two different arrays, viz., pole–dipole and pole–pole, were deployed to look into high signal strength data in a tectonically disturbed hard rock ridge region for groundwater. Four selected sites were investigated. 2D subsurface resistivity tomography data were collected using Syscal Pro Switch-10 channel system and covered a 2 km long profile in a tough terrain. The hydrogeological interpretation based on resistivity models reveal the water horizons trap within the clayey sand and weathered/fractured quartzite formations. Aquifer resistivity lies between ∼3–35 and 100–200 Ωm. The results of the resistivity models decipher potential aquifer lying between 40 and 88 m depth, nevertheless, it corroborates with the static water level measurements in the area of study. The advantage of using pole–pole in conjunction with the pole–dipole array is well appreciated and proved worth which gives clear insight of the aquifer extent, variability and their dimension from shallow to deeper strata from the hydrogeological perspective in the present geological context.

Understanding the origin of salinization of the Plio-quaternary eastern coastal aquifer of Cap Bon (Tunisia) using geochemical and isotope investigations

As in many other semi-arid regions, the Plio-quaternary aquifer of the eastern coast of Cap Bon peninsula (NE Tunisia) shows a parallel increase in overexploitation and mineralization of groundwater resources and so the water quality is deteriorating. Different methods using geochemistry (ions Na+, Cl−, Ca2+, Mg2+, Br−) and stable isotopes (18O, 2H) are compared with the hydrodynamic information for identifying the main processes involved in the increase of salinization. Along the coast, intrusion of seawater resulting from groundwater overexploitation is identified, but is not the only cause of qualitative degradation: the development of irrigation that induces soil leaching and transfer of fertilizers to groundwater over the whole aquifer extent is another major reason for the increase in salinization. A total of 48 groundwater wells were sampled to obtain additional information on the hydrochemical characteristics of the groundwater defined in previous studies.

Hydraulics of a partially penetrating well with skin zone in a confined aquifer

A steady/quasi-steady model is developed for predicting flow into a partially penetrating well with skin zone in a confined aquifer overlying an impervious layer. The model takes into account flow through the bottom of the wellbore, finite skin thickness and finite horizontal and vertical extent of the aquifer. Moreover, the solution can be easily extended to include the mixed-type boundary condition at the well face, where a Dirichlet in the form of a specified hydraulic head and a Neumann in the form of zero flux coexist at the same time at different portions of the well face. The validity of the proposed solution is tested by comparing a few results obtained from the developed model with corresponding results obtained by analytical and numerical means. The study shows that, among other factors remaining constant, both the horizontal and vertical extent of an artesian aquifer, thickness of the skin zone, bottom flow and conductivity contrast of the skin and formation zones, play an important part in deciding flow to a well dug in the aquifer, and hence these factors must be considered while analyzing the problem. The model proposed here can be used to estimate skin thickness as well as hydraulic conductivities of the skin and formation zones of a well with skin zone in an artesian aquifer underlain by an impervious layer by utilizing pumping test data falling in the steady or quasi-steady state of a typical pumping test. As the proposed solution is of a general nature in the sense that it can handle, apart from partial penetration and bottom flow, the finite size skin zone and finite horizontal and vertical extent of an artesian aquifer together with the mixed-type boundary condition at the well face, it is hoped that the predictions coming out of the model will be more realistic than those obtained using solutions developed with more stringent assumptions.

Hydrogeological Investigation and Groundwater Potential Assessment in Haromaya Watershed, Eastern Ethiopia

The paper assesses groundwater quality and productivity in Haromaya watershed, eastern Ethiopia. Continuous pumping test data, collected from seven boreholes was used to determine productivity of the aquifers. 14 water samples were tested for water quality. The aquifers on the basis of permeability, potential and extent of aquifers, are categorized into i) extended and shallow aquifers with intergranular porosity and permeability, and with moderate to high potential (alluvial and lacustrine sediments); ii) limited and shallow aquifers with fracture and/or karstic porosity and permeability, and with moderate potential (sandstone and limestone); and iii) limited and shallow aquifers with intergranular and fracture porosity and permeability, and with low potential (granite). On the basis of chemical data, the water is fresh except in lacustrine and swampy areas. HCO3-, Ca2+, Na+ and Mg2+ are the dominating ions in water from granite, sandstone and alluvium; and Cl and SO4 dominate in water from lacustrine sediments apart from HCO3-, Ca2+, Na+ and Mg2+. Areas suitable for groundwater development in the area are discussed. Keywords: Aquifer, Ethiopia, Groundwater potential, Haromaya, Hydrogeochemistry.

Recovery scenarios for deep over-exploited aquifers with limited recharge: methodology and application to an aquifer in Belgium

Recharge of deep-seated aquifers must be provided through leakage from overlying geological formations and can be very limited. Although hydraulic characteristics of these aquifers may be favorable for extensive exploitation, the renewability of the groundwater resource may be very restricted and only significant on long time scales. Over-exploitation of such aquifers leads to steadily declining piezometric levels and water balancing and steady state conditions are not to be expected on the short term. Recovery of such systems is very difficult, also because of the socio-economic dependencies of the water resource and the long time it requires to replenish the system. Management plans for restoration should be based on the transient hydrodynamical behaviour. A recovery plan should be developed based on results of a groundwater flow model. In a first step must be simulated how long it will take for the aquifer system to fully recover by natural recharge. As this will most often be a very long time it can be cancelled out as a realistic scenario. An alternative objective is to be formulated which could be achieved in the near future on a time scale of years or decades. Using model simulations, reduction schemes of exploitation rates shall be quantified that will allow to reach these defined goals, such as raising piezometric levels above the top of the aquifer itself or above the top of the overlying geological unit. The methodology is illustrated with an example of a deep hard rock aquifer in Belgium where piezometric levels have dropped below the top. The objective here was to raise the levels again above the top within the next 50 years. However, this is not accomplished by balancing aquifer recharge and discharge, but is reached by a lateral redistribution of the water over the aquifer extent. A full recovery of this aquifer would require at least a century with only a very limited amount of exploitation.

Socio-economic constraints of groundwater in Capital La Rioja, Argentina

Historically, the arid conditions of La Rioja, Argentina have been the main controlling factor in its development. The shortage of surface water, which is fully used, makes groundwater a potential source for development. The government encouraged investment in early 1979, resulting in a 20-fold increase of groundwater extraction by 1998 (0.076–1.450 m3/s, respectively) to cover related needs of agriculture, industry and population growth. This extraction created unjustified uncertainties derived from negative results obtained in hydrological balances. However, a 0.5 m lowering of the water-table surface was experienced. A knowledge of groundwater functioning was required to establish a reliable frame of reference for development and, at the same time, to find possible scenarios of feasible economic activities in harmony with accessible water resources and aptitude of the environment. The flow regime was found to be composed of three main systems: a regional, an intermediate and several local. The intermediate system provides water for the extraction boreholes, and discharges naturally in Salina La Antigua. From the chemical perspective the intermediate system has three groundwater groups. Group I has an outstanding fluoride concentration (1.98–3.10 mg/l) defined to the north of the City of la Rioja and the highest temperature (26.8–33.0°C), the lowest lithium content (0.029–0.059 mg/l) and moderate arsenic (≤0.038 mg/l). Group II has the moderate arsenic content (≤0.38 mg/l) detected to the south of the City of La Rioja and high lithium (0.024–0.085 mg/l), Group III has the lowest TDS (456–931 mg/l) and arsenic (0.007–0.012 mg/l) and the highest lithium (0.067–0.141 mg/l). to A regional flow is represented by Group IV with one order of magnitude higher strontium (4.870 mg/l), lead (0.021 mg/l) and uranium (0.362 mg/l) content than the other groups. Results provide evidence to eliminate several well-established hydro-myths such as “the boreholes are getting dry” and “boreholes are getting saline water”. The aquifer (granular Tertiary and Quaternary material) thickness (≈750m) was defined with the aid of the geological framework, geothermometers and Modflow modelling. The aquifer extent extends far beyond the limits of the study area. Several economic activities were found to be feasible with available groundwater resources and without bordening the environment (fish farming, bottled-water marketing, SPA activities and farming of endangered species).

Conceptual hydrochemical model of late Pleistocene aquifers at the Samario–Sitio Grande petroleum reservoir, Gulf of Mexico, Mexico

Carbon-14 concentrations between 0.83 and 11.79 pmC of formation water from the Activo Samaria–Sitio Grande petroleum reservoir in SE-Mexico, extracted from 3500 to 4500 m.b.s.l., indicate a common infiltration event of surface water during the late Pleistocene period. Mixing of two components – meteoric water and seawater, previously evaporated at the surface – explain the widespread mineralization (TDI = 15–257 g/L) of Na–Cl and Na–Ca–Cl type reservoir water. Statistical discrimination by clustering and a heterogeneous chemical–isotopic fluid composition indicate the existence of 4 different water types as part of local aquifer systems, which are separated by normal and thrust faults. Tectonic horst and graben structures show an ambiguous, individual hydraulic behaviour – as permeable conduits and/or as impermeable barriers, causing the local limitation of aquifer extent. The recent increase of water production in petroleum wells is not related to the injection of surface water, but the long-term extraction of oil reserves is modifying the original position and flow direction of the reservoir aquifers. The rise of the initial groundwater level reflects the final stage of an exhausted petroleum reservoir with coning effects of underlying aquifer systems. The flexible change towards superior production intervals could represent a feasible technique to avoid the abrupt closure of invaded production wells.

Impact of geohydrology and neotectonic activity on radon concentration in groundwater of intermontane Doon Valley, Outer Himalaya, India

Radon concentration was measured in 133 water samples from tubewells, handpumps, dug wells and springs of the Doon Valley, Outer Himalaya, India. The observed radon values were found to vary from 10 to 154 Bq/l whereas radium in selected water samples varied from 0.11 to 0.75 Bq/l. Three different clusters of high radon values were observed in the north-western, central and south-eastern parts of the Doon Valley. These clusters were found to be associated with tectonics (thrust/fault) and associated uranium mineralization in the area. In general, radon concentration in groundwater was found to be positively correlated with the depth of the wells, whereas no significant correlation was observed between radon concentration in groundwater and the water temperature, pH value, conductivity and altitude of the water samples. An attempt has also been made to determine the nature and extent of aquifers in the Doon Valley on radon concentration in groundwater. The variation in radon concentration within the groundwater of the study area was found to be controlled by the neotectonic activity and geohydrological processes that occur in the area. The impact of these activities on radon concentration in groundwater are discussed.

Review: “The Hydrogeology of the Chalk of North-West Europe”

The chalk deposits of Late Cretaceous and Early Paleocene age in and around the present North Sea Basin form an important regional aquifer in the shallow subsurface on the basin perimeter, and a major hydrocarbon reservoir in the deeper parts of the basin. Currently, about 8.5 million cubic meters of water are pumped daily from the chalk. About 80 percent of the withdrawal is in southern England and northern France. This book provides an overview of the regional hydrogeology of the chalk, with primary emphasis on the fresh-water reservoir. The first six chapters of the book discuss topics applicable to the aquifer as a whole. Chapter 1, "The Making of an Aquifer," briefly describes the regional structure and stratigraphy and the diagenesis and modification of the chalk into a regionally extensive aquifer. Chapter 2, "The Formation and Diagenesis of the Chalk," poses the question, "Why so much chalk?" It explains how the combination of a rise in sea level, lack of erosion in nearby land areas, and higher temperatures and higher C02 in the atmosphere during Late Cretaceous time led to the extensive accumulation of chalk ooze. The resulting rock consists almost entirely of skeletal fragments of algae composed of lowmagnesium calcite. Chapter 3, "The Chalk as an Aquifer," describes the dual-porosity nature of the chalk, which generally consists of a highly porous matrix of low permeability and a fracture network that contains both tight tectonic fractures and highly permeable karstic fissures. Flow in the unsaturated and saturated zones is described in detail, and downgradient changes in the groundwater chemistry are discussed. The other three introductory chapters discuss simulation of flow and transport, engineering geology, and groundwater pollution in the chalk. The middle section of the book provides descriptions of the chalk aquifer in France, Belgium, the Netherlands, Denmark, Sweden, and the United Kingdom. These six chapters generally describe the aquifer extent, stratigraphy and structure, aquifer properties, groundwater levels and potentiometric surfaces, recharge and discharge characteristics, natural water chemistry and pollution, and current groundwater development in each country. The section on France provides a detailed discussion (15 pages) that relates groundwater levels and flow in the chalk of the Paris Basin to geologic structures, topography, and karstic networks. Ground pollution by nitrates and pesticides also receives a lot of attention. The chapter on Belgium consists largely of descriptions of the three major chalk aquifers: the Hesbaye aquifer, the Mons Basin, and the confined aquifers of northern Belgium, with emphasis on modeling approaches. The chapter describes a complex, 5-layer finite-element model of the Hesbaye aquifer, incorporating pumping from wells and galleries, faults, and various geologic unit boundaries. The stratigraphy and lithology of the chalk in the Netherlands are described for the two separate areas of occurrence in the northern and southern parts of the country. Detailed hydrogeologic information is presented only for the southern part of the Netherlands, where the aquifer is near surface and characterized by a matrix of high porosity and very low permeability; fractures and dissolution openings provide high secondary permeability. The chapter on Denmark contains brief summaries of the structure, stratigraphy, karst features, and aquifer properties; the principal focus of the chapter is on hydrochemistry, including natural chemical constituents and pollution problems. In Sweden, the occurrence ofthe chalk is limited to four areas in the southernmost part of the country. The structural setting, stratigraphy, aquifer properties, karstic features, water chemistry, and groundwater development are described for these areas. The chalk

Deep circulation of groundwater in overpressured subglacial aquifers and its geological consequences

It is generally assumed that meltwater from the base of ice sheets is discharged in a relatively thin subglacial zone. Whereas this may be true for ice sheets resting on impermeable beds, many ice sheets, such as the glacial period ice sheets of North America and Europe, flowed over extensive aquifers. A theory is developed which suggests that high rates of meltwater discharge into these aquifers would have completely reorganised their flow fields, producing intergrated patterns of glacially pressurised flow controlled by the continental-scale form of the ice sheet surface rather than the small-scale topographic basins which determine modern aquifer extent. The theory is applied to the aquifers which underlay the Saalian ice sheet in The Netherlands, where it is shown that potential gradients and groundwater flow velocities would have developed which were two orders of magnitude greater than modern values and that the dominant flow vectors would have been normal to those of the modern flow. Thus, glacial/interglacial cycling in areas which have suffered periodic glaciation during the late Cenozoic may have experienced alternating phases of greater and lesser flow energy. It represents another example of climatically-driven cyclical change in the earth. Under highly energised glacial conditions, potential gradients much larger than modern values may have produced many common features of sediment disruption, such as diapirs, liquifaction structures and pipes, and forms such as glacial ‘doughnuts’ and pock marks, which have hitherto been explained by other processes. Deep and penetrative flushing of aquifers by glacial meltwater may have left a distinctive geochemical signal in them which may be used to test the theory. Gases such as methane, generated at shallow depth, may have been trapped beneath the glacial ‘cap-rock’, and may also have played an important role in these processes.

Applications of Geophysics to Geologic Models and to Reservoir Description

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose:To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. Summary. Geophysics techniques, and especially seismic techniques, aresimply tools for developing a rock/fluid model. Like well logs, seismictechniques are physical measurements of the reservoir and its container. Theireffectiveness depends on a combination of careful design of the recording andprocessing systems and skilled interpretations. In processing systems andskilled interpretations. In recent years, improvements in the seismic systemhave enhanced its application to reservoir description. Of particular interestarc improved methods of interpretatione.g., seismic stratigraphy and thin-bedanalysis by use of computer techniques to interpret seismic waveforms; andimproved seismic acquisition and processing techniques, resulting inthree-dimensional (3D) surveys and improved resolution of reservoir beds andfault traces. This paper will briefly discuss these and other seismic benefits, all of which can make important contributions to reservoir description if theright questions are posed. Introduction The bread-and-butter application of geophysics is the interpretation ofpatterns of seismic reflections. Used here, "reflections" refers to theindividual cycles on the seismic waveforms:, seen as black-and-white lines onthe seismic section. A sonic pulse at the earth's surface generates reflectionslike an echo from a barn wall. A geophone planted at the surface records aseries of seismic reflection pulse or echoes from progressively deepersedimentary layers. The progressively deeper sedimentary layers. The geophonerecords these pulses as a composite waveform in real time. Each sedimentarylayer has a characteristic acoustic velocity and density, and the product ofthese quantities is the acoustic impedance. product of these quantities is theacoustic impedance. The degree of contrast in acoustic impendance across layersdetermines how much energy is reflected and how much continues to the nextdeeper layer. Sedimentary layers reflect surfaces basically because water andwind tend to spread similar sediment types in relatively thin sheets over abroad area during, periods of similar environmental conditions. Consequently, sediment types, and therefore velocity and density, are much more similarwithin layers than between layers. As a result, reflections show the patternsof strata and their contained fluids in the subsurface. They are like a crosssection of the earth, showing the thicknesses and configurations of bedsdeposited during common time spans. (Naturally, there are exceptions to thissimplified explanation. Most exceptions are not worth mentioning in thisdiscussion, however, an important one will be discussed later.) Strata Patterns in Reservoir Descriptions A primary use of strata patterns is in defining structural closures and thegeometry of the individual reservoir beds and their seals. Typically, severalstructure maps are made during the history of a field. To begin, prospect mapsare made on the basis of a relatively loose grid and with few or no well data. Then, refined maps are made during the evaluation and delineation phase withadditional well and seismic control. Today, 3D surveys are often used to refinethe structural control further. Fault geometry is another key use of seismicreflections. Fault interpretation typically involves the careful integration ofwell data (cores, cuttings, logs, and tests) with seismic correlation to definefault positions and to delineate individual fault drainage positions and todelineate individual fault drainage blocks. Gross interval thickness maps mayrequire seismic reflection input, in addition to log correlations, to completethe areal coverage. These maps assist in pay-thickness mapping and help definethe history of pay-thickness mapping and help define the history of the burialand possible late tilting of a structure. Aquifer extent and geometry can be animportant contribution because Well control is usually sparse away from thefield and because the flanks of the reservoir structure may be deeply buried. In thick reservoirs that encompass several siesmic cycles, seismic reflectionsshow the gross stratification of the field. Seismic examples will be discussedfurther in upcoming Technology Today Series papers. It is important torecognize that seismic resolution is rather limited compared with core and welllog data. Fig. 1 shows a comparison of the scale of the three data sources. Theperiod of a typical single seismic cycle (peak-to-peak, time) is about 30milliseconds. This can represent between 135 and 225 ft [41 and 69 m], depending on the velocity of the sediments transited. At shallow depths, withspecial high-resolution seismic methods, the period may be much shorter, perhaps on the order of period may be much shorter, perhaps on the order of 10milliseconds, with equivalent thicknesses of 45 to 75 ft [14 to 23 m]. Certaincombinations of fluids and rocks allow mapping of fluid contacts. JPT P. 753