Geophysical Investigation of Fresh‐Saline Water Interface: A Case Study from South Punjab,Pakistan

The importance of the study of fresh‐saline water incursion cannot be over‐emphasized. Borehole techniques have been widely used, but they are quite expensive, intrusive, and time consuming. The electrical resistivity method has proved very successful in groundwater assessment. This advanced technique uses the calculation of Dar‐Zarrouk (D‐Z) parameters, namely longitudinal unit conductance, transverse unit resistance, and longitudinal resistivity has been employed by using 50 vertical electrical sounding points to assess the groundwater and delineate the fresh‐saline water interface over 1045 km² area of Khanewal in Southern Punjab of Pakistan. The x‐y plots and maps of D‐Z parameters were produced to establish a decipherable vision for the occurrence and distribution of different water‐bearing formations of fresh‐saline water aquifers through a complicated situation of intermixing of different resistivity ranges for fresh‐saline water bodies. This technique is useful to reduce the ambiguity produced by the process of equivalence and suppression which cause intermixing in differentiating fresh, brackish, and saline aquifers during interpretation. The fresh‐saline water interface is correlated very well with the previous studies of water quality analysis carried out in Khanewal area. The results suggest that the D‐Z parameters are useful for demarcating different aquifer zones. The behavior and pattern of D‐Z parameters with respect to occurrence and distribution of different water‐bearing formations were effectively identified and delineated in the study area.     

Geoelectrical and hydrochemical investigations for characterizing the salt water intrusion in the Khanasser valley, northern Syria

An integrated approach of geoelectrical and hydrochemical investigation surveys was proposed for indicating contact regions between saline and fresh groundwater in the Khanasser valley region, northern Syria. The qualitative and quantitative interpretations of 34 vertical electrical soundings (VES) enable to characterize the salt water intrusion laterally and vertically. The established iso-apparent resistivity maps for different AB/2 spacings obviously indicate the presence of a lowresistivity (less than 4 Ohm·m) zone related to the salt water intrusion in the Quaternary and Paleogene deposits. The different hydrochemical and geophysical parameters, such as electrical resistivity, total dissolved solids (TDS) and major ions concentrations used to characterize the salt water intrusion gave almost similar results in locating and mapping the different boundaries of the groundwater salinity. The proposed approach is useful for mapping the interface between different groundwater qualities, and can be therefore used to successfully characterize the salt water intrusion phenomenon in other semi-arid regions. The application of such an approach is a powerful tool and can be used for water resource management in the water scarce areas.     

Integrated geophysical and chemical study of saline water intrusion

Surface geophysical surveys provide an effective way to image the subsurface and the ground water zone without a large number of observation wells. DC resistivity sounding generally identifies the subsurface formations-the aquifer zone as well as the formations saturated with saline/brackish water. However, the method has serious ambiguities in distinguishing the geological formations of similar resistivities such as saline sand and saline clay, or water quality such as fresh or saline, in a low resistivity formation. In order to minimize the ambiguity and ascertain the efficacy of data integration techniques in ground water and saline contamination studies, a combined geophysical survey and periodic chemical analysis of ground water were carried out employing DC resistivity profiling, resistivity sounding, and shallow seismic refraction methods. By constraining resistivity interpretation with inputs from seismic refraction and chemical analysis, the data integration study proved to be a powerful method for identification of the subsurface formations, ground water zones, the subsurface saline/brackish water zones, and the probable mode and cause of saline water intrusion in an inland aquifer. A case study presented here illustrates these principles. Resistivity sounding alone had earlier failed to identify the different formations in the saline environment. Data integration and resistivity interpretation constrained by water quality analysis led to a new concept of minimum resistivity for ground water-bearing zones, which is the optimum value of resistivity of a subsurface formation in an area below which ground water contained in it is saline/brackish and unsuitable for drinking.     

Salt-water intrusion mapping by geoelectrical imaging surveys

The geoelectrical imaging method is a tool for mapping the intrusion boundary between fresh water and saline water due to its inherent capability to delineate the lateral changes in pore-water salinity. A new field survey technique that can be used for environmental and geotechnical investigations has been developed. This study evaluates the multiscale survey technique as a tool employed in electrical imaging to detect the salt-water intrusion boundary in Yan, State of Kedah, northwest Malaysia. The technique was incorporated into these surveys, and it has proved to be a robust method for accurately mapping the fresh-water/saline-water boundary. The resulting resistivity sections from these surveys were consistent with both the available geological and borehole information from the area and the previous resistivity surveys conducted by the Geological Survey of Malaysia at those sites.     

Electrical resistivity investigations for guiding and controlling fresh water well drilling in semi-arid region in Khanasser Valley,Northern Syria

Electrical resistivity survey is conducted in the Khanasser Valley, considered as a semi-arid region in Northern Syria, to guide and control fresh water well drilling. Vertical Electrical Soundings (VES) are carried out near the existing wells, which were also used to measure salinity and conductivity of water samplings. Resistivity and hydrogeological data combination made it possible to establish empirical relationships between earth resistivity, water resistivity, and the amount of total dissolved solids (TDS). These relationships are used in the present research in order to follow the TDS salinity variations, both in lateral and depth directions, along the longitudinal profile LP3, located in Khanasser Valley. Apparent salinity map under this profile is established for Quaternary and Paleogene deposits, and boundaries of suitable areas for fresh water well drilling in the totality of the Khanasser Valley are drawn. It was shown that excessive fresh-water depletion may affect the groundwater quality by upward seepage of saline water. Therefore, it is important to keep the fresh-salt water interface in a stationary situation. The geoelectrical methods widely contribute in determining the position of such interface, and can be easily applied in similar arid areas.     

Geophysical monitoring and evaluation of coastal plain aquifers

We use time domain electromagnetic (TDEM) soundings to monitor ground water conditions beneath the coastal plain in eastern North Carolina. The TDEM method measures the earth's response to an induced electromagnetic field. The resulting signal is converted, through a complex inversion process, to apparent resistivity values, which can be directly correlated to borehole resistivity logs. TDEM soundings are used to map the interface between fresh and salt water within coastal aquifers, and estimate depth to basement when siting new monitoring wells. Focused TDEM surveys have identified areas of salt water encroachment caused by high volumes of discharge from local supply wells. Electromagnetic sounding, when used in tandem with the state's network of monitoring wells, is an accurate and inexpensive tool for evaluating fresh water/salt water relationships on both local and regional scales within coastal plain aquifers.     

Seawater intrusion and fresh groundwater hydraulics in fjord delta aquifers inferred from ground penetrating radar and resistivity profiles—Sunndalsøra and Esebotn, western Norway

Geophysical surveys have been carried out in two fjord delta aquifers in western Norway. Geophysical methods comprised ground penetrating radar (GPR), shallow reflection seismic and resistivity measurements. The purpose of the investigations was to evaluate hydrogeological properties of the fjord delta aquifers with emphasis on the possibilities of abstracting saline groundwater for use in fish farming. At Sunndalsøra, reflection seismic was helpful in the mapping of the base of the aquifer. Resistivity profiles revealed both the spatial and seasonal variability of seawater intrusion. Penetration depths of ground penetrating radar (GPR-D) revealed the spatial variation of the boundary of the transition zone between saline and fresh groundwater. Maps of GPR-D can be used to indicate direction and magnitude of mean hydraulic gradients and thus to locate optimal sites for both fresh and saline groundwater abstraction.In fjord delta aquifers, both rivers and abandoned river channels constitute important groundwater divides. Optimal locations for saline groundwater abstraction wells are in areas separated from the rivers by abandoned river channels. In areas between the rivers and the abandoned river channels, groundwater abstraction will result in decreasing salinity of the water.     

Distribution of fossil seawater and its role in Neogene sedimentary rock landslides in Niigata,eastern marginal region of the Japan Sea

The Neogene marine sedimentary rock area in the eastern marginal region of the Japan Sea is an area with some of the highest landslide densities in Japan. Some of the landslides in this area have been known to involve saline groundwater, which can be the cause of these landslides. In order to demonstrate the relationships between landslides and saline water, topographic, geological, groundwater, and electromagnetic surveys were performed in the eastern marginal region of the Japan Sea. Many landslides and gravitational slope deformations with linear depressions and small scarps were recognized in the study area. The resistivity profile obtained by an electromagnetic survey suggests that there is a wide zonal distribution of saline water with salt concentrations equivalent to seawater at depths of 50–100 m or more and that the groundwater shallower than 50 m has an electrical conductivity of less than 100 mS/m. The shallow resistive groundwater is inferred to be meteoric water that replaced the saline groundwater, which likely weakened the bedrock, resulting in landslides. A ridge of competent tuff overlying mudstone has many linear depressions from gravitational slope deformation and low‐resistivity water to a depth of 600 m, which suggests that the mudstone was weakened by water replacement and deformed under the tuff caprock. The saline groundwater is inferred to be fossil seawater trapped in pores during sediment deposition, which is brought near the ground surface along with rocks by tectonic movement in the hills. Thus, the saline water and its fresh water replacement are among the important basic causes of the landslides. The oil well data obtained in the eastern marginal region of the Japan Sea suggest that such saline water replacement has occurred widely and that replacement is likely one of the predispositions for the frequent landslides there.     

电性源瞬变电磁地空逆合成孔径成像

电性源地空瞬变电磁法具有工作效率高、勘探深度大、采集信号信噪比高、适用于地形地质条件复杂地区等优点.但是,到目前为止,由于尚未建立起该方法的解释系统,大大制约了该方法的发展.本文旨在建立起完整的地空电磁探测系统,丰富整个探测系统的理论.本文围绕地空瞬变电磁法全域视电阻率定义、瞬变电磁虚拟波场的克希霍夫偏移成像、逆合成孔径成像方法三个科学问题进行了系统研究.提出了用磁场强度定义全域视电阻率的迭代算法,理论模型试验结果表明计算出的视电阻率曲线首支趋于第一层电阻率,尾支趋于最后一层电阻率,实现了全空域、全时域视电阻率的计算;在先前研究的基础上,实现了适合电性源地空装置的瞬变电磁虚拟波场的克希霍夫偏移成像;采用相关迭加技术,实现孔径内多测点数据合成,将传统的单点处理方式发展成为逐点推移多次覆盖的逆合成孔径处理方法.层状模型试验表明:(1)全域视电阻率能够光滑、完整、渐变地反映出模型的电性信息变化;(2)当改变三层模型中间层电阻率时,全域视电阻率曲线随着参数的改变分异明显,对电性层的识别容易且直观;(3)由于在电阻率计算中同时考虑了接收机高度、偏移距、时间等各参数的影响,全域视电阻率可实现全空域、全时域的视电阻率计算.含水采空区的复杂模型算例表明:(1)根据不同测线的全域视电阻率结果可以看出,在靠近采空区的位置,全域视电阻率断面可以清晰地反映出采空区的空间位置,随着测线离采空区越来越远,采空区异常越来越弱直至消失;(2)波场变换和偏移成像的结果显示存在两个电性差异较大的界面,上界面指示地表,由于空气和大地之间的电性差异较大,故该界面波场信号反映强烈,遍布整个区域,下界面异常信号则主要集中在中部,向外逐渐减弱,指示采空区;(3)逆合成孔径成像结果表明地表界面在合成前后没有变化,而采空区异常合成后范围明显变小,且异常边界清晰,指示的采空区位置与模型吻合很好.本文借助于逆合成孔径雷达成像的基本思想,建立了一套电性源瞬变电磁地空逆合成孔径成像方法.基于反函数思想结合迭代算法提出的电性源地空瞬变电磁法的全域视电阻率定义方法,实现了全空域、全时域的视电阻率计算;借鉴瞬变电磁拟地震偏移成像算法,实现了瞬变场的三维成像;借鉴逆合成孔径雷达的思想,提出电性源瞬变电磁地空逆合成孔径算法,进一步提高了成像的分辨率.采空区模型算例表明相关叠加合成确实具有增强有用信号、提高信噪比、提高分辨率的诸多优点,证实了瞬变电磁地空逆合成孔径成像方法的有效性.     

Extending electromagnetic methods to map coastal pore water salinities

The feasibility of mapping pore water salinity based on surface electromagnetic (EM) methods over land and shallow marine water is examined in a coastal wetland on Tampa Bay, Florida. Forward models predict that useful information on seabed conductivity can be obtained through <1.5 m of saline water, using floating EM-31 and EM-34 instruments from Geonics Ltd. The EM-31 functioned as predicted when compared against resistivity soundings and pore water samples and proved valuable for profiling in otherwise inaccessible terrain due to its relatively small size. Experiments with the EM-34 in marine water, however, did not reproduce the theoretical instrument response. The most effective technique for predicting pore water conductivities based on EM data entailed (1) computing formation factors from resistivity surveys and pore water samples at representative sites and (2) combining these formation factors with onshore and offshore EM-31 readings for broader spatial coverage. This method proved successful for imaging zones of elevated pore water conductivities/salinities associated with mangrove forests, presumably caused by salt water exclusion by mangrove roots. These zones extend 5 to 10 m seaward from mangrove trunks fringing Tampa Bay. Modeling indicates that EM-31 measurements lack the resolution necessary to image the subtle pore water conductivity variations expected in association with diffuse submarine ground water discharge of fresher water in the marine water of Tampa Bay. The technique has potential for locating high-contrast zones and other pore water salinity anomalies in areas not accessible to conventional marine- or land-based resistivity arrays and hence may be useful for studies of coastal-wetland ecosystems.     

Generalized effective medium resistivity model for low resistivity reservoir

With the advancement in oil exploration,producible oil and gas are being found in low resistivity reservoirs,which may otherwise be erroneously thought as water zones from their resistivity.However,the evaluation of low resistivity reservoirs remains difficult from log interpretation.Since low resistivity in hydrocarbon bearing sands can be caused by dispersed clay,laminated shale,conductive matrix grains,microscopic capillary pores and high saline water,a new resistivity model is required for more accurate hydrocarbon saturation prediction for low resistivity formations.Herein,a generalized effective medium resistivity model has been proposed for low resistivity reservoirs,based on experimental measurements on artificial low resistivity shaly sand samples,symmetrical anisotropic effective medium theory for resistivity interpretations,and geneses and conductance mechanisms of low resistivity reservoirs.By analyzing effects of some factors on the proposed model,we show theoretically the model can describe conductance mechanisms of low resistivity reservoirs with five geneses.Also,shale distribution largely affects water saturation predicted by the model.Resistivity index decreases as fraction and conductivity of laminated shale,or fraction of dispersed clay,or conductivity of rock matrix grains increases.Resistivity index decreases as matrix percolation exponent,or percolation rate of capillary bound water increases,and as percolation exponent of capillary bound water,or matrix percolation rate,or free water percolation rate decreases.Rock sample data from low resistivity reservoirs with different geneses and interpretation results for log data show that the proposed model can be applied in low resistivity reservoirs containing high salinity water,dispersed clay,microscopic capillary pores,laminated shale and conductive matrix grains,and thus is considered as a generalized resistivity model for low resistivity reservoir evaluation.     

Interpretation of magnetic resonance soundings in rocks with high electrical conductivity

Magnetic resonance sounding (MRS) is an electromagnetic method designed for groundwater investigations. MRS can be applied not only for studying fresh-water aquifers, but also in areas where intrusion of saline water is rendering the subsurface electrically conductive. In the presence of rocks with a high electrical-conductivity attenuation and a phase shift of the MRS signal may influence the efficiency of the MRS method. We investigated the performance of MRS for allowing us to propose a procedure for interpreting MRS data under these conditions. For numerical modeling, we considered a subsurface with a resistivity between 0.5 and 10 Ω m. The results show that the depth of investigation with MRS depends upon the electrical conductivity of groundwater and surrounding rocks, on the depth of the saline water layer, and on the amount of fresh water above the saline water. For interpreting MRS measurements, the electrical conductivity of the subsurface is routinely measured with an electrical or electromagnetic method. However, due to the equivalence problem, the result obtained with these methods may be not unique. Hence, we investigated the influence of the uncertainty in conductivity distribution provided by transient electromagnetic measurements (TEM) on MRS results. It was found that the uncertainty in TEM results has an insignificant effect on MRS.     

Geophysical surveys in Parvati valley geothermal field, Kullu, India

Direct current resistivity surveys and shallow temperature measurements were carried out for geothermal exploration in a part of Parvati valley, goethermal field, Himachal Pradesh, India. At a few places, the Schlumberger soundings pointed to the presence of a relatively low-resistivity shallow layer, which probably represents fractured and jointed quartzite, saturated with hot/cold water. Wenner resistivity profiles indicate the presence of some possible shallow subsurface lateral hot water channels across the valley at Manikaran. Shallow temperature measurements show a good subsurface thermal anomaly near the confluence of the rivers Brahmaganga and Parvati. The results of the survey, together with other available geodata, suggest that an anomalous heat source does not lie beneath the study area.It is postulated that the meteoric water, originating at high elevations after heating as a result of circulation at depth, emerges at the surface in the Parvati valley as hot springs, after mixing in various proportions with near surface cold waters.     

The electrical conductivity of Posidonia black shales — from magnetotelluric exploration to rock samples

We carried out a magnetotelluric field campaign in the South–East Lower Saxony Basin, Germany, with the main goal of testing this method for imaging regional Posidonia black shale sediments. Two‐dimensional inversion results of the magnetotelluric data show a series of conductive structures correlating with brine‐saturated sediments but also with deeper, anthracitic Westphalian/Namurian coals. None of these structures can be directly related with the Posidonia black shale, which appears to be generally resistive and therefore difficult to resolve with the magnetotelluric method. This assumption is supported by measurements of electrical resistivity on a set of Posidonia shale samples from the Hils syncline in the Lower Saxony basin. These rock samples were collected in shallow boreholes and show immature (0.53% Ro), oil (0.88% Ro), and gas (1.45% Ro) window thermal maturities. None of the black shale samples showed low electrical resistivity, particularly those with oil window maturity show resistivity exceeding 10⁴ Ωm. Moreover, we could not observe a direct correlation between maturity and electrical resistivity; the Harderode samples showed the highest resistivity, whereas the Haddessen samples showed the lowest. A similar trend has been seen for coals in different states of thermal maturation. Saturation of the samples with distilled and saline water solutions led to decreasing electrical resistivity. Moreover, a positive correlation of electrical resistivity with porosity is observed for the Wickensen and Harderode samples, which suggests that the electrical resistivity of the Posidonia black shale is mainly controlled by porosity.     

Saltwater intrusion modelling in Jorf coastal aquifer,South‐eastern Tunisia: geochemical,geoelectrical and geostatistical application

Marine intrusion is the most serious problem facing the coastal Jorf shallow aquifer, located in south‐eastern Tunisia on the Mediterranean Sea. Jorf Aquifer is intensively exploited to supply the growing needs of agriculture and domestic sectors. This work proposes a multidisciplinary investigation, involving hydro‐geochemical, geoelectrical survey and geostatistical techniques for modelling the saltwater intrusion. For this purpose, 36 water samples were conducted and analysed. Electric conductivity, pH, total dissolved solids and major ions were measured and analysed. Pie and Durov Diagrams, Q‐mode hierarchical cluster and geostatistical analysis were considered to identify the main groundwater mineralization processes. Results revealed that the Na‐Cl‐Ca‐SO₄ is the dominant water type suggesting that dissolution of halite and gypsum was the main mineralization source of groundwater in the central and southern part of study area. However, saltwater intrusion was shown to control groundwater quality essentially in coastal areas. Variographic analyses were used to select the variographic model that best fits the spatial development of apparent resistivity. Kriged apparent resistivity profiles showed an abnormal decrease of resistivity values in the coastal zone, implying highly saline water because of seawater intrusion. Apparent resistivity values also decrease considerably in the faulted areas, suggesting a contribution of faults to seawater intrusion. Finally, saltwater mixing ratio was computed for each sample, and a refined seawater intrusion map was developed. Copyright © 2012 John Wiley & Sons, Ltd.     

Approximate analysis of upconing

The change of the salinity distribution in coastal aquifers due to pumpage is often described as an upconing of the interface between saline and fresh water. Sea and fresh water are miscible fluids, however. Therefore, dispersion of salinity in the aquifer affects the upconing process. An estimate of the effect of salinity dispersion on the dynamics of the flow as well as on the salinity distribution in the aquifer is presented in this study. The phenomenon is described as a migration of a sharp interface perturbed by small disturbances due to salinity dispersion. The creation of the mixing zone between fresh and saline water is described as a formation of a boundary layer in the vicinity of the sharp interface. This method is primarily recommended for flow fields in which simple representation of the sharp interface migration is obtainable.     

EXAMPLES OF ELECTROMAGNETIC PROSPECTING FOR KARST AND FAULT SYSTEMS*

Electromagnetic (EM) surveys using the horizontal coplanar loop (‘Slingram’) geometry have been successful in the location of karst aquifer systems in limestone areas of Germany and the Mediterranean. In spite of the rapid changes in shape and resistivity of karst pipes, correlations of significant EM minima can be made by reference to the main structural directions. The EM results have often been verified by drilling. EM prospecting need not be confined to limestone areas. It can be used on all hard rocks where faulting has created vertical zones with a resistivity contrast to the undisturbed rock. EM siting of wells has proved successful in water-supply schemes for several places. This method is now being applied as a routine, fast and low-cost procedure to tap ground-water resources in hard-rock areas.     

Groundwater dynamics of Fongafale Islet, Funafuti Atoll, Tuvalu

Geoelectric and hydrologic surveys during spring tides revealed the spatiotemporal distribution of groundwater quality produced by tidal forcing in Fongafale Islet, Funafuti Atoll, Tuvalu. The observed low resistivity showed that saline water largely immersed the surficial Holocene aquifer, indicating that there is no thick freshwater lens in Fongafale Islet, unlike in other atoll islands of comparable size. Half of the islet was constructed by reclaiming the original swamp with porous, highly permeable coral blocks; this reclaimed area should not be considered as part of the islet width for calculation of the expected thickness of the freshwater lens. The degree of aquifer salinization depends on the topographic characteristics and the hydrologic controls on the inland propagation of the tidal forcing. Large changes in bulk resistivity and the electrical conductivity of groundwater from wells indicate that periodic salinization in phase with the semidiurnal tides was occurring widely, especially in areas at lower elevation than the high-tide level and in reclaimed areas with high permeability. Thin sheets of nearly fresh and brackish water were observed in the surficial aquifer in areas above the high-tide level and in taro swamps, respectively. The thinness of the brackish and freshwater sheets suggests that the taro swamps and the fresh groundwater resources of the islet are highly vulnerable to salinization from anticipated sea-level rise. An understanding of the inherent geologic and topographic features of an atoll is necessary to evaluate the groundwater resources of the atoll and assess the vulnerability of its water resources to climate change.     

The cone penetration test and 2D imaging resistivity as tools to simulate the distribution of hydrocarbons in soil

The purpose of geophysical electrical surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. From these measurements, the true resistivity of the subsurface can be estimated. The ground resistivity is related to various geological parameters, such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity surveys have been used for many decades in hydrogeological, mining and geotechnical investigations. More recently, they have been used for environmental surveys. To obtain a more accurate subsurface model than is possible with a simple 1-D model, a more complex model must be used. In a 2-D model, the resistivity values are allowed to vary in one horizontal direction (usually referred to as the x direction) but are assumed to be constant in the other horizontal (the y) direction. A more realistic model would be a fully 3-D model where the resistivity values are allowed to change in all three directions. In this research, a simulation of the cone penetration test and 2D imaging resistivity are used as tools to simulate the distribution of hydrocarbons in soil.     

Remote Sensing and Geophysical Investigations of Glacial Buried Valleys in Northeastern Kansas

Aquifers found in glacial buried valleys are a major source of good-quality ground water in northeastern Kansas. The extent and character of many of these deposits are not precisely known, so a detailed study of the buried valleys was undertaken. Test drilling, Landsat imagery, shallow-earth temperature measurements, seismic refraction, surface electrical resistivity, and gravity data were used to evaluate two sites in Nemaha and Jefferson Counties. Tonal patterns on springtime Landsat imagery and winter/summer anomalies in shallow-earth temperatures were quick and inexpensive methods for locating some glacial buried aquifers and suggested areas for more intensive field studies. Reversed seismic refraction and resistivity surveys were generally reliable indicators of the presence or absence of glacial buried valleys, with most depth determinations being within 25% of test-drilling results. The effectiveness of expensive test-hole drilling was greatly increased by integrating remote sensing, shallow-earth temperature, seismic, and resistivity techniques in the two buried valley test areas. A gravity profile allowed precise definition of the extent of one of the channels after the other techniques had been used for general information.