Correlating Bedrock Folds to Higher Rates of Arsenic Detection in Groundwater,Southeast Wisconsin,USA

Arsenic in private drinking water wells is a significant problem across much of eastern Wisconsin, USA. The release mechanism and stratigraphic distribution of sulfide and iron (hydr)oxide sources of arsenic in bedrock aquifers are well understood for northeastern Wisconsin. However, recent geologic mapping has identified numerous small bedrock folds to the south, and the impact of these geologic structures on local groundwater flow and well contamination has been little studied. This paper examines the hydrologic and structural effects of the Beaver Dam anticline, southeast Wisconsin, on arsenic in groundwater in the region. Multivariate logistic regression shows wells near the Beaver Dam anticline are statistically more likely to detect arsenic in groundwater compared to wells farther away. Structural and hydrologic changes related to folding are interpreted to be the cause. Core drilled near the fold axis is heavily fractured, and many fractures are filled with sulfides. Elevated hydraulic conductivity estimates are also recorded near the fold axis, which may reflect a higher concentration of vertical fractures. These structural and hydrologic changes may have led to systematic changes in the distribution and concentration of arsenic-bearing mineral hosts, resulting in the observed detection pattern. For areas with similar underlying geology, this approach may improve prediction of arsenic risk down to the local level.     

地下水动态多年周期与中强震多发期

地下水动态的短期变化特征已有很深入的研究,本文根据唐山地区6口井1982年－2001年9月的水位动态变化月均值资料探讨地下水的长期变化特征。初步研究表明，从多年较大空间的地下水动态变化看，可视为较平衡的随机过程，存在着10年左右的周期结构，这种周期结构与地质构造环境、水文地质环境、地理地貌、观测层位、补给和排泄条件以及观测井结构关系不密切，这种现象可能与复杂的全息天体运动有关，是一种多种周期的叠加，是一种合力作用，代表着区域应力场。     

Integrated Geophysical Studies in the East-Indian Geothermal Province

Integrated geophysical surveys using vertical electrical sounding (VES), very low frequency (VLF) EM, radiation counting, total magnetic field and self-potential (SP) measurements are carried out to characterize the geothermal area around a hot spring in the Nayagarh district, Orissa, India that lies in the East Indian geothermal province. The study was performed to delineate the fracture pattern, contaminated groundwater movement and possible heating source. VES interpretations suggest a three- to four-layer structure in the area. Resistivity survey near the hot spring suggests that weathered and fractured formations constitute the main aquifer system and extend to 60 m depth. Current flow measured at various electrode separations normalized by the applied voltage suggests that fractures extend to a greater depth. Detailed VLF study shows that fractures extend beyond 70 m depth. VLF anomaly has also very good correlation with the total magnetic field measured along the same profiles. Study results suggest that a gridded pattern of VLF survey could map the underground conductive fracture zones that can identify the movement of contaminated groundwater flow. Therefore, precautionary measures can be taken to check further contamination by delineating subsurface conducting structures. Self potential (SP) measured over the hot spring does not show a large anomaly in favor of the presence of a sulphide mineral body. A small positive (5–15mV) SP anomaly is measured which may be streaming potential due to subsurface fluid flow. A high radiation is measured about four kilometers from the hot spring, suggesting possible radiogenic heating. However, the exact nature of the heating source and its depth is not known in the area. Deep resistivity followed by a magneto-telluric survey could reveal the deeper structures.     

Optimization of spread configuration for seismic data acquisition through numerical modeling in tectonically complex areas: a case study from Badarpur anticline, Cachar, India

Cachar area, a part of the Assam–Arakan frontal fold belt in the eastern part of India is characterized by a series of narrow anticlines which are dissected by a number of faults including thrusts, and broad synclines. Out of several exposed structures, Badarpur anticline is most important because of its major share of hydrocarbon strikes in Cachar. However, the seismic image of this area so far has been extremely poor for deciding locations for exploratory drilling. The problems in obtaining a good seismic image in this area include proper spread configuration including far and near offsets to capture reflected signals from the complex subsurface, variation of charge-sizes and depths along a line, large variation in statics due to rapid changes in elevation and near surface properties. The presence of coherent noise due to scattering and proper migration are major processing problems. This paper deals with the problem of spread configuration. For areas of complex subsurface, spread configuration obtained through field experiment at one spot may be quite different from other spots, requiring a large number of costly experiments. In this work, therefore, we used numerical modeling to derive an optimum spread configuration. For this, we chose a line passing through four wells on Badarpur anticline, and prepared a depth model using available seismic, surface geological and well data. Continuous synthetic common shot records were generated through oblique incidence ray tracing and were processed to obtain stacks with various spread configurations. Comparison of these stacks of synthetic data shows that end-on shooting configuration is inadequate for acquiring a good seismic image of the complex subsurface. From the present study it is evident that a split–spread configuration with near and far offsets of 50 m and 2400 m, respectively, can bring out optimum seismic images in Badarpur and other anticlines in this area.     

Gravity map of Kalabsha area, northwest of Aswan Lake, and its structural significance

A detailed gravity survey was carried out in one of the seismo-active areas at the northwestern region of the High Dam Lake (Kalabsha area) to study its subsurface structure. In order to understand the seismicity of the area, the establishment of a geodynamic model from geological and geodetic data is of great importance. After a series of adjustments and corrections of the measured gravity data, free-air and Bouguer anomaly maps were constructed for the Kalabsha area, and several interpretation techniques were applied to analyse these anomalies. The results of the analysis indicate that the Kalabsha area is affected by several faults trending mainly E-W and N-S. The active area located west of Gebel Marawa is bounded by a set of faults striking NE-SW, N-S and E-W. The throws of these faults range from 160 to 370 m.The minimum depth to the basement complex is about 200 m and its maximum depth is around 600 m. The thickness of the sedimentary column (Nubia sandstone) in the Kalabsha area decreases due west and increases toward the southern and eastern parts of the area. The results explain the tectonic framework of the area well.     

Geomorphology of the Alluvial Sediments and Bedrock in an Intermontane Basin: Application of Variogram Modeling to Electrical Resistivity Soundings

The study describes a methodology used to integrate legacy resistivity data with limited geological data in order to build three-dimensional models of the near subsurface. Variogram analysis and inversion techniques more typically found in the petroleum industry are applied to a set of 1D resistivity data taken from electrical surveys conducted in the 1980s. Through careful integration with limited geological data collected from boreholes and outcrops, the resultant model can be visualized in three dimensions to depict alluvium layers as lithological and structural units within the bedrock. By tuning the variogram parameters to account for directionality, it is possible to visualize the individual lithofacies and geomorphological features in the subsurface. In this study, an electrical resistivity data set collected as part of a groundwater study in an area of the Peshawar basin in Pakistan has been re-examined. Additional lithological logs from boreholes throughout the area have been combined with local outcrop information to calibrate the data. Tectonic activity during the Himalayan orogeny has caused uplift in the area and generated significant faulting in the bedrock resulting in the formation of depressions which are identified by low resistivity values representing clays. Paleo-streams have reworked these clays which have been eroded and replaced by gravel–sand facies along paleo-channels. It is concluded that the sediments have been deposited as prograding fan-shaped bodies and lacustrine deposits with interlayered gravel–sand and clay–silt facies. The Naranji area aquifer system has thus been formed as a result of local tectonic activity with fluvial erosion and deposition and is characterized by coarse sediments with high electrical resistivities.     

Tectonic Evolution and Hydrogeological Characteristics of the Khanaser Valley,Northern Syria,Derived from the Interpretation of Vertical Electrical Soundings

The Khanaser Valley was geoelectrically thoroughly surveyed through a grid which consisted of twelve VES profiles. The tectonically-oriented Pichgin and Habibullaev method was enhanced to be applicable in areas of rugged relief and topography. The enhanced profiles were tectonically interpreted and subsurface structures within the Khanaser Valley were delineated. Accordingly, a tectonic evolutional scenario of the valley was established and its hydrogeological characteristics were derived. An approach for groundwater exploration in areas of prominent relief and topography in dry areas such as the Khanaser Valley was established and its validity was estimated.     

四川长宁Ms6.0地震震区上地壳速度结构特征与孕震环境

北京时间2019年6月17日22时55分,四川省宜宾市长宁县发生了Ms6.0地震(28.34°N,104.90°E),四川盆地内部及边缘地带的深部孕震环境和潜在地震危险性再次引起了国内外地震专家和学者们的密切关注.为了揭示长宁Ms6.0震区的深部介质结构特征和孕震环境,综合解译地震活动的构造背景和展布特征,本文充分收集...     

Delineation of Near-Surface Structure in the Southern Part of 15th of May City, Cairo, Egypt Using Geological, Geophysical and Geotechnical Techniques

The integration of geological, geophysical, and geotechnical interpretation at the southern part of 15th of May City, have been used to evaluate the subsurface stratigraphy, especially the clay layer which may cause serious danger to construction. Those techniques have been used to delineate the subsurface structures as normal faults, which play a critical role on the stability of buildings. Geological setting of the area has been evaluated through the construction of a geological map from different geological sections and samples obtained from more than 30 observation points. Geophysical tools such as vertical electrical soundings (12 VESs), 2-D dipole–dipole array (7 sections), P-wave shallow seismic refraction (31 profiles) and multiple channel analysis of surface waves (31 MASW profiles) have been carried out to image the subsurface situation. Geotechnical evaluation using 26 boreholes, samples, laboratory tests and geotechnical parameters has been done at the area of interest. The geological setting demonstrates that the city had been constructed on the second and third members of Qurn Formation (Upper Eocene) composed of argillaceous limestone, marl and shale. Two normal faults are passing through the area were observed. The resistivity (VES and dipole–dipole) and seismic (P-waves and MASW) results reflect the presence of the two normal faults cross the study area, affecting the obtained section of marl, clayey marl and limestone layers. The geotechnical information indicate the presence of the normal faults and the existence of clay layer with swelling ability reaching 140%, which may cause cracks in the upper layers and/or subsidence.     

Magnetotelluric survey for exploration of a volcanic-rock reservoir in the Yurihara oil and gas field,Japan*

The Yurihara oil and gas field is located on the southern edge of Akita Prefecture, northeastern Japan. In this area, drilling, surface geological surveys and many seismic surveys have been used to investigate the geological structure. Wells drilled into the Nishikurosawa Basalt Group (NBG) of Miocene age found oil and gas reservoirs at depths of 1.5–2 km. Oil and gas are now being produced commercially and further exploration is required in the surrounding areas. However, since the neighbouring areas are covered with young volcanic products from the Chokai volcano, and have a rough topography, the subsurface distribution of the NBG must be investigated using other methods in addition to seismic reflection. According to the well data, the resistivity of the NBG is comparatively higher than that of the overlying sedimentary formations, and therefore the magnetotelluric (MT) method is expected to be useful for the estimation of the distribution of the NBG. An MT survey was conducted along three survey lines in this area. Each line trended east–west, perpendicular to the regional geological strike, and was composed of about 25 measurement sites. Induction vectors evaluated from the magnetic field show that this area has a two-dimensional structure. The evaluated resistivity sections are in agreement with the log data. In conclusion, we were able to detect resistive layers (the NBG) below conductive layers. The results indicate that the NBG becomes gradually less resistive from north to south. In the centre of the northern line, an uplifted resistive area is interpreted as corresponding to the reservoir. By comparison with a seismic section, we prove the effectiveness of the integration of seismic and MT surveys for the investigation of the morphology and internal structure of the NBG. On other survey lines, the resistive uplifted zones are interpreted as possible prospective areas.     

Contribution of the geophysical methods in characterizing the water leakage in Afamia B dam, Syria

The objective of this work was to characterize and understand the origin of the water losses problem in the Afamia B dam located at Al-Ghab basin in Syria. The survey involved various geophysical methods including a superficial electromagnetic (EM), an electrical sounding (ES) and electrical resistivity tomography (ERT). Special considerations were focused on the lake of the dam, which is currently suffering serious water infiltration and leakage through its bedrocks. The application of the ERT technique was more effective and convenient. The analysis of the integrated results of the above mentioned methods revealed a vertical serious leakage which could take place in certain locations via fractures and faults that hit the main valley and pass through the dam lake. Additionally, the presence of an alternating lithological heterogeneity between permeable and impermeable layers may lead to infiltration through the geological formations of the basin. These processes are most likely causing hydraulic connections between the Neogene superficial deposits and the underlying Cretaceous fractured and karstified carbonates rocks. Consequently, such hydraulic connections may lead to water losses and leakage throughout the dam basin. Eventually, the presence of tens of drilled wells which penetrate the Neogene and Cretaceous formations, and the existence of numerous archeological graves and subsurface tunnels complicate the situation in the study area and cause more water leakage.     

Two case studies of the determination of parameters of urban waste dumps

This paper describes a survey carried out on two wastes dumps using VES and the seismic refraction technique. The dumps have different geometrical and technical characteristics and are located on different geological formations. Anomalously low P waves velocities in the dumps allow us to identify the limits and the thickness of the dumps. The VES method contributed to the general reconstruction of the area and indicated the presence of pollution in the surrounding area.     

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

ABSTRACT

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

Editor D. Koutsoyiannis; Associate editor M. Besbes     

Delineation of Shallow Subsurface Structure by Azimuthal Resistivity Sounding and Joint Inversion of VES-TEM Data: Case Study near Lake Qaroun, El Fayoum, Egypt

An azimuthal resistivity survey was conducted at the transition zone between the desert area and the cultivated land near Lake Qaroun, Egypt. This area has been affected by an east-west trending fault system as indicated from the surface geology. Apparent resistivity values were plotted along azimuth on a polar diagram. Resistivity anomalies, for most of the AB/2 values with long axes strike in a direction parallel to the contact between the desert and cultivated lands, indicate the presence of electrical macro-anisotropy, mainly due to the faulting effect, at this area. Vertical electrical soundings (VES) and transient electromagnetic (TEM) measurements were conducted at eight stations along a line that crosses the boundary between the desert and cultivated land. Joint inversion of VES-TEM data was successfully used for identification of the subsurface lithostratigraphic succession and demonstrated the effect of the fault zone on the investigated subsurface medium. Apparent anisotropy coefficients at all current electrode spacings were calculated, plotted against AB/2 values and compared with the geoelectrical cross section. The effect of the fault zone was detected at AB/2 spacings equal to 100 m and extended downward and is largely related to the depth of the fault, as indicated in the constructed cross section.     

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

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

LATE QUATERNARY TECTONIC DEFORMATION AROUND THE HUJIATAI ANTICLINE ALONG THE EAST SEGMENT OF THE FODONGMIAO-HONGYAZI FAULT,NORTHERN QILIAN SHAN: AN INSIGHT ON THE SEISMOGENIC PATTERN OF 1609 HONGYAZI M7 1/4 EARTHQUAKE

The Fodongmiao-Hongyazi Fault (FHF)is one of the most active faults of the northern Qilian thrust fault zone. The 1609 Hongyazi M7 1/4 earthquake occurred on the east segment of the FHF, an area with a complex geometry at the Mayinghe River site. The seismogenic pattern of this earthquake revealed by complex surface ruptures remains unclear. In this paper, we focus on active tectonic deformation around the Hujiatai anticline (HA)in the Mayinghe River site. Combining with topographic survey via dGPS across deformed terraces and alluvial fans, a field survey of the geological section across the HA, the characteristics of the active fold and several sub-faults were constrained. Meanwhile, combined with the seismic reflection profiles passing through the anticline, the correspondence relationship between surface expressions of this tectonic and the deep structure was discussed. According to our research, the HA is a result of northward propagation of the range-front thrust fault F₁. At the same time, a thrust fault F₂ with dextral strike-slip motion and a thrust fault F₄ were formed on the east side and north side of the HA, respectively. These two active faults accommodated local deformation. Trench results and ¹⁴C dating reveal that the 1609 Hongyazi M7 1/4 earthquake ruptured the T₁ terrace in the Huangcaoba site. Combined with previous field investigations and literature about the 1609 Hongyazi earthquake, we suggest that this earthquake occurred on the range-front fault F₁, and the depth of the hypocenter may be about 8~22km.     

Direct and indirect methods for groundwater investigations: A case-study of MRS and VES in the southern part of Sweden

Magnetic Resonance Sounding (MRS) has been successfully tested for detecting groundwater in two areas in southern Sweden. Measurements of Schlumberger VES have been conducted in the same place as the MRS and the results are generally consistent. Low resistivity layers interpreted as clay are sometimes identified close to the surface. The MRS result in site 2 is a good example of signals penetrating through the clay and deeper aquifer still being detected. The MRS data suggest aquifers that are not only hosted in soft sediment materials (moraine, sand, and mixed materials), but hosted in basement rocks. Based on the MRS and borehole pumping test data, the results agree with yield, average water content and subsurface geological data.     

A new procedure for the interpretation of VES data: 1.5-D simultaneous inversion method

In the paper the principles of a new inversion method as well as the results of its application are described by using synthetic and field VES data. The basic idea of the method says that the horizontal changes in the layer thicknesses and the resistivities of the 2-D geological structure can be described by (expanding in series) functions of one variable. The coefficients of the functions are determined from the VES data by simultaneous inversion method using a least-squares technique. The local thicknesses and the resistivities of the geological structure are calculated from the coefficients from point to point along the profile. Using this method, the equivalence can considerably be reduced, and by using the functions chosen on the basis of a priori knowledge, the whole geological model can be determined by this inversion method. In the inversion the local 1-D forward modelling is used and this approximation is often applied in the VES practice. To qualify the results of the inversion, the correlation matrix for the coefficients are calculated and the error values for the local model parameters are introduced.     

REGIONAL GEOTHERMAL EXPLORATION IN EGYPT*

Although Egypt is not characterized by abundant Cenozoic igneous activity, its location in the northeastern corner of the African plate suggests that it may possess geothermal resources, especially along its eastern margin. Regional geothermal exploration has been carried out in Egypt using the thermal gradient/heat flow technique and groundwater temperature/chemistry technique. In the thermal gradient/heat flow study, existing oil-well bottom-hole temperature data as well as subsurface temperature measurements in existing boreholes were utilized before special thermal gradient holes were drilled. Groundwater temperature and chemistry data were used to extend the geographic range of the direct subsurface thermal measurements. On a very modest budget, a regional thermal high has been discovered along the eastern margin of Egypt, and a local thermal anomaly has been discovered in this zone. Published geological information suggests that the sandstones of the Nubian Formation may be a suitable reservoir for geothermal fluids. The new data indicate that temperatures of 150°C or higher may be found in this reservoir in the Gulf of Suez and Red Sea coastal zone where it lies at a depth of 4 km and deeper.