Bouguer Gravity Research Articles

Geothermal Spatial Potential and Distribution Assessment Using a Hierarchical Structure Model Combining GIS, Remote Sensing, and Geophysical Techniques—A Case Study of Dali’s Eryuan Area

The assessment of geothermal potential has gained prominence among scholars, with a focus on establishing a reliable prediction model to reduce development risks. However, little attention has been given to predicting and evaluating the geothermal potential in Dali’s Eryuan area. This study introduces a novel hierarchical model integrating remote sensing, a Geographic Information System (GIS), and geophysics for the first-ever effective prediction of geothermal potential in Eryuan. The dataset includes lithology, seismic epicenter data, fault distribution, Bouguer gravity anomalies, SRTM-DEM images, and Landsat 8 remote sensing images. These datasets are converted into evidence maps and normalized to generate distinct evidence factor layers. Using the Analytic Hierarchy Process (AHP), a hierarchical model establishes weights for each evidence factor, resulting in a comprehensive prediction map. The results reveal the overall favorable geothermal potential in Eryuan, except the central area. Key hotspots include the Niujie–Sanying–Gromwell Lake and Liantie–Qiaohou, followed by the Youshou, Dengchuan, and Xixiang towns. Validation against known hot springs confirms the model’s accuracy and reliability.

2-Dimensional joint inversion modelling of the Niger – Delta basement using potential field data

Sub-surficial models predict the occurrence and geometry of strata, geological structures, inclusions, tectonic settings and so on. Robust data including geologic information, airborne gravity, and magnetic data, with constraining drill–depth data are here integrated to model the topography of the basement that underlies the Niger Delta sedimentary sequence. The aim was to elucidate the existence of a petroleum system from the unexplored depths of the delta near the basement, hypothetically controlled by the basement structures. Airborne total magnetic intensity (TMI) and Bouguer gravity anomaly (BGA) of the Niger Delta, obtained from the Nigerian Geological Survey Agency that were noise–filtered and corrected are used. The gradient of the geomagnetic field was removed from the magnetic data and the data was reduced to the equator (RTE). As precursory operations, sub-surficial lineament structures were delineated from these grids using the Tilt Derivative filters, and the depths were determined using the Source Parameter Imaging (SPI) technique. Subsequently, basement topography models were generated across four (4) profiles in a gravity-magnetic joint inversion scheme. Consequently, the strata that constitute the Niger – Delta ‘cake’ from the Cretaceous to Recent were modeled. The results showed that the Late Cretaceous Synrift Clastics (LCSC) formation presumed to directly overlie the basement may vary in thickness from 2.15 to 2.74 km. The lineament structures were found to dominantly trend in NE – SW direction. The depth to the basement, from the 2-dimensional models, ranges approximately from 13.18 to 19.35 km. It is postulated that while the Tertiary Niger Delta is characterized by lineaments engendered by diapiric instability of the over–pressured Paleocene – Pliocene marine shales, there exist other lineament structures beneath with NE – SW as the dominant trends, offshoots of Mid – Atlantic fracture zones including Chain and Charcot. Basement undulations, which are relicts of the vast subsidence Passive margins undergo during their formation are observed. These resultant synclinal structures create accommodation for the extensive deposition of the LCSC. The juxtaposition of this reservoir facie, the lineament-laden undulating Passive basement escarpment, and, the Paleocene – Pliocene Marine Shale formation could make for another petroleum system.

Distribution of Active Faults and Lithospheric Discontinuities in the Himalayan-Tibetan Orogenic Zone Identified by Multiscale Gravity Analysis

The lithospheric structure of the Tibetan Plateau and its adjacent area is a hot topic in geodynamic research. It is important to reveal the mechanism of crustal deformation and tectonic evolution of the study area. In this study, the techniques of wavelet multiscale decomposition and field edge detection were used to study the discontinuities of the lithosphere revealed by multilevel Bouguer gravity anomalies. Specifically, we evaluated the depth characteristics of the major active faults in the study area and identified 15 deep major faults that cut through the lithosphere. They are Chaman fault, Shyok suture zone, Altyn-Tagh fault, Karakash fault, Karakoram fault, Talas-Fergana fault, Kashgarr-Yeshgar transfer system, Rushan-Pshart suture zone, Sangri-Nacuo fault, Main Frontal thrust, Burmese fold belt, Yadong-Gulu fault, Gaoligong fault, Sagaing fault and Nujiang fault. We have also elucidated the tectonic mechanisms of two famous geodynamic phenomena in the Pamir Plateau. The first is the intense intermediate depth seismicity beneath Pamir-Hindukush. It cannot simply be described as the rupture of a subducted residual plate, which could be divided into two distinct tectonic units. One belongs to the Indian plate, the other to the Eurasian plate. Secondly, the mechanism of intense seismicity confined to the western upper crust of the Pamir Plateau could be explained as significant fragmentation of crustal material. Finally, and most importantly, we summarized the coupling mechanism between deep geodynamics and horizontal deformation as observed by modern geodetic techniques. In the upper mantle, the leading edge of the subducting Indian plate reached the SW boundary of Tarim basin and forms a closed structure in western Himalaya. Then, the Tibetan Plateau underwent a tectonic escape towards the east under the continuous compression between the Indian and Eurasian plates. During the process of tectonic escape, the role of the N–S direction normal faults in the Himalayan tectonic zone is limited.

Lithosphere density structure of southeastern South America sedimentary basins from the analysis of residual gravity anomalies

We conduct a gravity study of the lithosphere beneath three large sedimentary basins in southeastern South America: Paraná, Chaco-Paraná, and Pantanal. We compile a massive gravity database and estimate the free-air and Bouguer gravity anomalies, resulting in a novel complete Bouguer anomaly map for the study area. To discern the influence of crustal loads with known lithologies, including sediments, basalts, and topography variations of the Moho discontinuity, we calculate their gravity effects and subsequently remove them from the complete Bouguer anomaly, leading to the development of our residual Bouguer anomaly map. This map highlights unknown anomalous masses within the lithosphere. To aid in the interpretation of these residual anomalies, we perform a 2D forward modeling. Based on our results, we propose new boundaries for the Paranapanema block and the Luiz Alves craton. Additionally, we propose that the Ponta Grossa swarm dike has a more substantial impact on the crust and lithosphere than previously considered, and delimit the region of influence of this magmatism in the lithosphere. Moreover, tectonic features such as the São Francisco paleocontinent and the Rio de La Plata craton appear to be associated with negative residual Bouguer anomaly regions. Furthermore, we identify and emphasize the significance of the Western Paraná Suture, which acts as a demarcation between the Paraná Basin region and the Pantanal and Chaco-Paraná basins. Remarkably, this suture appears to play a more important role in shaping the density structure of the southwest of South America than the age and tectonic history of the sedimentary basins.

Integrative Analysis of the Geothermal Structure in Kepahiang: Insights from Magnetotelluric, Gravity, and Remote Sensing Techniques

This research was conducted to determine the structure and depth of the reservoir using remote sensing as an initial survey to assess the geological alignment direction, employing ALOS PALSAR radar imagery. Subsequently, further surveys were conducted using the magnetotelluric and gravity methods to analyze the structure and depth of the geothermal reservoir. The magnetotelluric data were processed using Phoenix software, where the data was transformed from the time domain to the frequency domain using Fourier transformation, and processed to obtain apparent resistivity and phase. The MT data was integrated with gravity data, and the gravity data underwent standard correction procedures to obtain the Complete Bouguer Anomaly (CBA) map. Two-dimensional (2D) inversion using the NLCG algorithm and 2D forward modeling of the gravity data were performed. The dominant alignment pattern obtained was northwest-southeast, with an orientation of 320° NW or 140° SE. Based on the results of geological alignment, a profile is produced that is perpendicular to the straightness. The results from the 2D inversion and gravity forward modeling indicated that the geothermal reservoir is likely located beneath the caprock at an estimated depth of approximately 1800 m, with resistivity values ranging from 32 to 256 Ohm-m and a density value of 2.6 gr/cc.

Density structure of the upper and middle crust in the northeast Dabie Orogen revealed by terrestrial gravity surveys

Moderate to strong earthquakes have occurred frequently in the northeast Dabie Orogen recently. From 2014 to 2017, the Guanmiao Earthquake Swarm (GES), with a maximum earthquake magnitude of M3.9, and the Quanjun Earthquake Swarm (QES), with a maximum earthquake magnitude of M3.7, occurred successively in the area near southeast Jinzhai County, where two main Mesozoic E–W‐striking boundary faults, including the Xiaotian–Mozitan and Xinyang–Shucheng faults, are cut by the Cenozoic NNE‐/NE‐trending dextral strike‐slip faulting system. These earthquake swarms occurred within areas of human activities and have the potential to cause more violent geological hazards. However, the tectonic geneses of the earthquake swarms are still unknown due to a lack of sufficient geophysical evidence. Based on terrestrial high‐resolution Bouguer gravity anomaly data, the density structure of the northeast Dabie Mountains is inverted using a 3D linear gravity inversion method. As a result, the occurrence and cutting depth of the reactivated faults near the earthquake swarms are defined, suggesting that the Shangloufang–Xialoufang Fault (F11) may be the seismogenic fault of the GES. The genesis of the QES is possibly associated with the fault termination of the Yingshan–Quanjun strike‐slip fault (F10). More broadly, F10 is one of the major synthetic faults of the NNE‐/NE‐trending dextral strike‐slip faulting system between the Tanlu and Shangcheng–Macheng (F2) faults, which bound the East Dabie Block together. The synthetic and antithetic faults fully cover the Beihuaiyang belt and North Dabie unit topography. These intraplate strike‐slip faults formed to accommodate vertical and lateral movements of the Dabie Mountains, driven by the oblique subduction of the Pacific Plate, were likely responsible for generating earthquakes in both Huoshan and Jinzhai.

Landslide susceptibility mapping of Penang Island, Malaysia, using remote sensing and multi-geophysical methods

Malaysia is one of the countries in the world experiencing landslides yearly due to natural events and human activities. Penang Island is Malaysia’s second most developed state and the largest by population. It is prone to landslides with devastating environmental impacts. Hence, the need to characterize its near-surface soil-rock conditions. This study uses remotely sensed data via frequency ratio (FR) techniques to identify landslide-prone areas based on different categories of landslide causative factors. To further understand the conditions and hydrodynamics of the soil-rock profiles causing landslides, electrical resistivity tomography and seismic refraction tomography were carried out at a landslide-suspected section in the study area. Also, the satellite-derived Bouguer gravity anomaly modeling was performed to map the varied gravity anomalies associated with landslide-triggering factors in lithologic units. The multi-geophysical models offer strongly correlated results with the causative remote sensed maps and the landslide susceptibility index (LSI) map. The likelihood of landslides occurring in the area, as suggested by the area under curve modeling of LSI data, yielded a high predicted success rate of 83.47%. Hence, prospective landslides were identified at the hilly and elevated sections, while the less susceptible sections were identified on flat reliefs. Landslides may also be triggered, for instance, at steep sections with varied contractive soil bodies and shallow structures. Most importantly, leveraging the LSI map would help the necessary agencies to forestall and mitigate future landslide occurrences in the area.

Gravimetric inversion and three-dimensional geological modelling of the Piquiri Syenite Massif, southern Brazil

The Pelotas Batholith corresponds to the eastern margin of the Dom Feliciano Belt in southernmost Brazil. It comprises multiple intrusions formed by successive tectonic-magmatic processes during the Brasiliano / Pan-African Cycle. One of these intrusive igneous bodies is the Piquiri Syenite Massif (PSM), recently described as a multi-intrusive body formed by three successive pulses, dated by LA-MC-ICP-MS (U-Pb in zircon), with the oldest pulse located at the margins and the youngest in the centre of the intrusion. Pulse 1 (609.3 ± 1.5 Ma) comprises fine- to medium-grained equigranular syenite to quartz syenite and alkali-feldspar quartz syenite, with colour index M′15–30. Pulse 2 (603.4 ± 3.9 Ma) comprises medium- to coarse-grained equigranular alkali-feldspar syenite and alkali-feldspar quartz syenite, with colour index M′ 5–15 and fragments of Pulse 1 varieties. Pulse 3 (588.8 ± 3.1 to 583.2 ± 1.8)comprises medium- to coarse-grained inequigranular quartz syenites, with colour index M′ 2–10, containing fragments of pulses 1 and 2 varieties. As defined by the AMS data, the PSM magnetic fabric is concordant with the magmatic fabric, parallel to the outer edges of the body, dipping towards the centre. However, the construction and emplacement of the body have been the subject of different interpretations. The main objective of this work is to relate the general morphology of the massif and the behaviour of the pulses within the massif with the processes of formation and ascent of the magmas that have built it. For this, terrestrial geophysical surveys were carried out to obtain gravimetric data, with which the Bouguer Anomaly map was generated. Modelling by gravimetric inversion was carried out in nine profiles using tools from the Oasis Montaj software, which was the basis for constructing a three-dimensional geological model using the Leapfrog Geo software. Associating the geological model with the magmatic and magnetic foliations, including the magnetic lineation, it was possible to determine a change in behaviour between the youngest and the two oldest pulses related to the ascension process. In addition, the body’s general shape, with the greatest depths located in the eastern region, allowed us to relate the rise of magma to a structure in the region.

Utilizing Remote Sensing and Satellite-Based Bouguer Gravity data to Predict Potential Sites of Hydrothermal Minerals and Gold Deposits in Central Saudi Arabia

This article aims to aid in exploring and forecasting hydrothermal minerals and gold deposits in Central Saudi Arabia (SA), with a focus on structural contexts. Remote sensing (RS) and satellite-based Bouguer gravity (SBG) data were integrated in order to create a mineral prediction map for the researched location. Data from the Landsat Operational Land Imager (OLI) and Shuttle Radar Topography Mission (SRTM) were transformed and enhanced using a variety of approaches. The delineation of hydrothermal alteration zones (HAZs) and highlighting of structural discontinuities in the OLI data were made possible using band ratios and oriented principal component analysis (PCA). Additionally, the underlying structural features were successfully exposed by processing the SBG using a variety of edge detection techniques, like the analytical signal (AS), total horizontal derivative (THD), tilt angle (TA), horizontal tilt angle (TDX), theta map (TM), horizontal derivative of the tilt derivative (HD_TDR), horizontal gradient of the tilt angle (HGTA), tilt angle of the analytical signal (TAAS), and soft sign function (SF). As a result, more prominent lineaments were found in the NW–SE, NNW–SSE, NE–SW, and NNE–SSW directions than in the N–S and E–W directions. The GIS incorporated surface/subsurface geological structure density maps with zones of hydrothermal alteration. It was found that the lineaments derived from the analysis of the RS and SBG data were more in line with the HAZs, which demonstrated the common connection between alteration zones and deep lineaments. The findings revealed a mineral prediction map with extremely low to extremely high probabilities. Overall, combining RS and SBG data effectively identified probable mineralization sites associated with hydrothermal processes and made it easier to create this study’s final predictive mineralization map.

Formation of Australasian tektites from gravity and magnetic indicators

The parent impact crater of Australasian tektites has not been discovered so far, but a consensus has been accepted on its location in a wider area of Indochina. Recently, an alternative location has been suggested in the Badain Jaran Desert (BJD), Northwest China. Employing gravity and magnetic data derived from satellites, possible presence of an impact structure in BJD is investigated. The gravity parameters include the free air gravity disturbance, its vertical derivative component and total horizontal gradient (THG), strike alignment (SA), and Bouguer anomaly with its first vertical derivative and tilt angle. The magnetic parameters include the anomalous total magnetic field (TMF), its reduced to the pole transformation (RTP), the first vertical derivative of the TMF vertical component (Bzz), tilt angle (TA), and logistic total horizontal gradient (LTHG). Both the gravity and magnetic indicators support the presence of the impact structure. Gravity parameters display typical annular gravity highs circumscribing a gravity low. SA analysis reveals preferred parallel directions, implying the susceptibility of special zones to the impact shock waves, both within and beyond the rim. TMF reveals a large magnetic anomaly in the southern part of the proposed crater, and RTP displaces and restricts it further into the rim. Bzz weakens the long wavelength anomalies, amplifies the superficial ones, and separates them horizontally. TA and LTHG delineate the deep-seated and shallow magnetic signals related to the peak and border magnetization, respectively.

Influence of Lateral Plume Channel on the Evolution of Rift Arms of the Afar Triple Junction: Constraints From 3‐D Gravity Interpretation

AbstractThe Afar region presents a unique example of a juvenile R‐R‐R triple junction with the arms evolved to various stages of maturity. The lithosphere in this area has been modified heterogeneously by the nearby Afar mantle plume. To gain a comprehensive and consistent understanding of the link between crustal geometry and geodynamics of the region and facilitate comparisons across different tectonic elements, we developed a high‐resolution crustal thickness map of the region that resolves major tectonic features. The calculation involved a well‐constrained 3‐D inversion of upper mantle‐corrected crustal Bouguer gravity anomalies computed with the help of tomography‐derived densities. Our results match earlier localized crustal thickness estimates, illustrating the method's efficacy. A thinner crust (∼20–26 km) is revealed in the Afar compared to adjacent continental regions. The Main Ethiopian Rift has the thickest crust in the central part of the rift (∼38–40 km) and thinner to the north (∼30 km) and south (∼35 km). The crustal thickness along the Gulf of Aden changes from east (∼5 km) to west (∼17 km) in the west, consistent with a westward transition from seafloor spreading to continental rifting. The southern Red Sea is characterized by asymmetric crustal thickness, with a thicker crust (∼17 km) on the eastern flank compared to the western flank (∼11 km), which we attribute to the influence of a sub‐lithospheric channel from the Afar mantle plume. We propose a model describing the channel leading to asymmetric features of the southern Red Sea, such as crustal accretion, marginal topography, and volcanism.

Application of Seismicity and Gravity Observation-Based Filtering Model for Mapping a Fault Structure in Weh Island, Indonesia

Aceh Province is one of the Indonesian regions prone to earthquakes beneath the Great Sumatran Fault, which has been focused mapped on the land. At the same time, the continuity of the Great Sumatran Fault to the marine and islands is rarely investigated. We apply the Gravity Model Plus which has a resolution of 200 m/px, to map a detailed structure of the the Great Sumatran Fault in Weh Island. The Bouguer anomaly varied between 32 – 78 mGal, which reveals the response of the rocks comprising Weh Island, such as those on the east side with a high-value reaction corresponding to highly dense basalts 2.84 g/cm3 and on the west side with a low-value response representing the density of andesites 2.56 g/cm3. The residual anomaly indicated several local faults, e.g., Sabang and Seuke faults located on the east side, Lhok Jeumpa on the central side, and Pria Laot on the west side. Meanwhile, the derivative data from the Bouguer anomaly could explain the fault structures corresponding to regional geological maps, such as the Sabang fault in Balohan in the northwest-southeast direction and several others controlling the Jaboi volcano, i.e., Ceunohot and Leumomate. The focal mechanism from seismicity analysis figured the fault orientation from the Seulimeum segment in the direction of NW-SE, which is essential to assess an active fault. Based on data processing, we concluded the the Gravity Model Plus data is effective for first mapping the fault structure in developing countries, such as Indonesia.

Multi-Scale Potential Field Data Integration Using Fuzzy C-Means Clustering for Automated Geological Mapping of North Singhbhum Mobile Belt, Eastern Indian Craton

Fuzzy C-Means (FCM) clustering is an unsupervised machine learning algorithm that helps to integrate multiple geophysical datasets and provides automated objective-oriented information. This study analyzed ground-based Bouguer gravity and aeromagnetic datasets using the FCM clustering algorithm to classify lithological units in the western part of the North Singhbhum Mobile Belt, a mineralized belt in the Eastern Indian Craton. The potential field signatures of clusters obtained using FCM correlate remarkably well with the existing surface geology on a broad scale. The cluster associated with the highest gravity signatures corresponds to the metabasic rocks, and the cluster with the highest magnetic response represents the mica schist rocks. The cluster characterized by the lowest gravity and magnetic responses reflects the granite gneiss rocks. However, few geological formations are represented by two or more clusters, probably due to the close association of similar rock types. The fuzzy membership scores for most of the data points in each cluster show above 0.8, indicating a consistent relationship between geophysical signatures and the existing lithological units. Further, the study reveals that integrating multi-scale geophysical data helps to disclose bedrock information and litho-units under the sediment cover.

Using microgravity techniques in the archaeology case study, the animal cemetery at Saqqara, Egypt

ABSTRACT The use of microgravity technique for the detection of subsurface features is considered a valuable tool in archaeological prospection. The precise and detailed measurements obtained through this method have been crucial for the identification of buried structures. In the animal cemetery of Saqqara, a microgravity survey was conducted using a high-precision relative gravity metre called CG-5 device. This instrument has a standard resolution of 1 microGal (1 microGal = 10−8 m s−2) and a standard deviation of less than 5 microGals. The survey covered the entire animal cemetery, including the Ebbs tunnels, and was processed to calculate the vertical deviations in the area after applying terrain correction (TC). By detecting and delineating local density variations caused by a near-surface void, the microgravity technique was able to identify the presence of a possible void feature, as indicated by the acquired negative anomaly in the residual Bouguer anomalies. The precise gravitation data obtained after correction revealed that both the entrance of the cemetery and the densities of the burial chamber were relatively smaller than the surrounding areas of the animal cemetery in the Saqqara region. The data also showed the presence of two crypts leading to the burial chamber region in the southeast part of the cemetery. However, these preliminary results require further investigation, and the study area needs to be expanded to include a larger area. Additionally, other geophysical methods may be used to confirm the abnormality detected and verify the existence of a burial chamber or other corridors.

Spatial variations of effective elastic thickness of Eastern Arabia: Implications for lithospheric weakening in the Oman-UAE mountain belt

The continental lithosphere of the Arabian Plate has evolved through a long and complex tectonic history and comprises major geological features such as an ophiolite complex, Cenozoic basalt extrusion, and Proterozoic basement outcrops. In eastern Arabia, an NW-SE to N-S oriented orogenic belt is present in Oman and the United Arab Emirates (UAE). The mountain belt formed due to the Late Cretaceous obduction of the Semail ophiolite, where a thick slab of oceanic crust and upper mantle was emplaced on the rifted Arabian continental margin. However, the mechanical strength of the lithosphere over the entire region is poorly known. In this study, we employ Bouguer gravity anomaly and topographic data, along with data on crustal properties obtained from teleseismic analyses, to assess the spatial variations in the effective elastic thickness (Te) of the eastern Arabian Plate. The joint inversion of coherence and admittance yielded Te values ranging from 10 to 50 km and an initial subsurface-to-surface load ratio (F) of 0.1 to 0.8. Te values in the southern part of eastern Arabia are low (< 20 km), while the northeastern regions of Saudi Arabia, the UAE, and Qatar exhibit intermediate to high Te values ranging from 30 to 45 km. The Oman-UAE mountain belt is associated with an intermediate Te of 30 km in the northern segment and a low Te of 20 km in the southern segment. The reduction in Te observed along the strike of the mountain belt suggests weakening of the lithosphere during the obduction of the Semail ophiolite. Te correlates well with both the seismic thickness of the lithosphere (Ts) and the decrease in shear-wave velocity at a depth of 150 km. The seismogenic thickness in eastern Arabia, however, is about 15 km, which is significantly less than Te and shows that both the crust and upper mantle contribute to the strength of the continental lithosphere. The Ts/Te ratio ranges from 1 to 9, with a maximum peak at 3–5, which is similar to the results observed for the oceanic lithosphere.

Lineaments in the Gravity Image of the Border Zone between the Central and Outer Carpathians

The research area covers the border zone between the Central and Outer Carpathians. The purpose of this research was the interpretation of this zone based on a gravitational survey. This survey was integrated with the results of surface mapping, a deep seismic survey, and deep drillings. Three major tectonic units are located in this area: the Outer (Flysch) Carpathians, the Pieniny Klippen Belt (PKB), and the Central Carpathians. All three units contain a significant amount of flysch sequences. The lowering of the Bouguer anomaly value from north to south reflects the dip of the crystalline European Plate; in turn, the renewed increase in value correlates very well with the emergence of the crystalline ALCAPA Plate. The range of variability of the Bouguer anomaly value largely masks smaller anomalies in amplitude originating from smaller geological structures. Only three anomalies with significant horizontal extent and greater amplitudes are visible: two are clearly correlated with the Orava-Nowy Targ Basin and the third anomaly is likely connected with the thicker pile of the Outer Carpathian flysch. To separate the boundaries of geological or tectonic structures (lineaments), a horizontal derivative (THDR) and an analytical signal (ASA) were used. Both methods allowed us to confirm existing geological and tectonic boundaries (lineaments) and to identify new ones.

2D modelling the depth of the southeastern Thrace Basin by using Bouguer gravity anomalies

2D modelling the depth of the southeastern Thrace Basin by using Bouguer gravity anomalies

Crustal structure beneath and surrounding the Appalachian Basin region of Eastern North America imaged by joint inversion of P wave receiver functions and surface wave dispersion measurements

SUMMARY To evaluate the plate flexure model for the formation of the Appalachian Basin, we investigate the extent to which crustal structure beneath and surrounding the basin was modified by the Palaeozoic orogenic events that created the basin. We jointly invert receiver functions and surface wave dispersion measurements to obtain 1-D crustal Vs profiles for 261 seismic stations located within and around the basin. The average crustal thickness for the region is 44 km, and the crust gradually thins to the east, consistent with previous studies. Four areas of anomalous crust are identified with respect to the eastward thinning of the crust. An area of thick crust is found along the Grenville Front on the western side of the Appalachian Basin where the crust thickens by ∼5–10 km. Moho depths of up to 54 km in this region likely result from suture-thickening. The crust is thinner beneath the Neoproterozoic Scranton rift by ∼5–7 km, coincident with a ∼40 mGal Bouguer gravity high. Across the Neoproterozoic Rome Trough, the crust thins by ∼4–5 km, coincident with a ∼10 mGal Bouguer gravity high. Density models for these rifts show that the rift-related crustal thinning is sufficient to explain the gravity anomalies. The Vs models obtained for stations in the rifts indicate little, if any, mafic layering in the mid-crust and only a modest amount of mafic layering in the lower crust. In the northwestern portion of the Appalachian Basin in northeastern Ohio and northwestern Pennsylvania within the Elzevir block, another area of anomalously thick crust (50–52 km) is found. This region is not associated with any known tectonic structures or boundaries or a gravity anomaly. The lower ∼5–10 km of the crust in this region is characterized by high (&amp;gt;3.9 km s−1) shear wave velocities and thus appears to be mafic. The origin of anomalous crustal structure in all four areas is best attributed to Precambrian tectonic events that predate the formation of the Appalachian Basin, indicating that the crystalline crust beneath and surrounding the basin was not significantly affected by the Palaeozoic basin-forming orogenic events, a finding which supports the use of plate flexure models for understanding basin formation.

Integrated gravity and resistivity investigations of the deep Hammam Bradaa aquifer, Northeast Algeria: Implications for groundwater exploration

Recently, the Guelma region, located northeast of Algeria, has been facing an unusual water shortage. To bypass this situation, a geophysical study using gravity and resistivity has been initiated to enhance the understanding of the geology and hydrogeology of the deep structures of the study area. In order to accomplish this, a residual gravity anomaly map has been constructed by removing from the Bouguer anomaly map a regional gravity gradient using the upward continuation technique. For the purpose of validating the Vertical Electrical Sounding(VES) interpretation, two soundings were conducted near boreholes with known lithology. The Hammam Bradaa deep aquifer of the Guelma region is composed of limestones that belong to the Constantine neritic domain. The Constantine neritic domain is covered by thrust nappes and outcrops in some places of the study area as tectonic windows. The exact geometry such as depth and lateral extent of the aquifer are not well known. Since limestones are characterized by high densities and resistivities relative to the host rock, the combining of gravity and VES was able to accurately map their extension among the other lithologies composing the geological section. Multiscale edge detection of gravity data known as "worms" made it possible to highlight the principal lineaments across the study area which may contribute to the recharge of the Hammam Bradaa aquifer. The 3D Gravity inversion enabled the mapping of the spatial configuration of limestones that are characterized by positive gravity contrast relative to the host rocks. A vertical density slice extracted from the 3D gravity inversion, running from north to south, reveals a significant thickness of limestone beneath Hammam Bradaa which suggests a strong potential for groundwater resources. The geo-electric section, constructed from the interpretation of VES data and calibrated with well lithology, enabled the mapping of the depth to the top of limestone beneath conductive alluvium and marls.

Favourable IOCG and polymetallic environs in Marwar - Khetri, NW Indian Shield imprints from aerogeophysical data analysis

The analysis of aeromagnetic data reveals two significant broad oval-shaped aeromagnetic anomalies oriented in the NE-SW direction, west of Laxmangarh and east of Fatehpur. These significant anomalies identified beneath sand-alluvium-covered areas of Rajasthan's Marwar region are of a low dominance nature. These magnetic signatures are associated with gravity gradients in the regional Bouguer gravity map. Magnetic anomalies and subsurface modeling revealed that the remanence source had a high magnetic susceptibility. The ground gravity profiles across the Laxmangarh anomaly show a high gravity variation of 5 mGal/km. The integration of geochemical distribution patterns, spectrometric elemental concentrations, different ratios, and composite image maps has brought out alteration/shear zones that are favourable for mineralization. The results of selective 2D-profiles, plate modeling, and 3D-inversion modeling of these anomalies reveal that the depth of the causative source varies from 300 to 1000 m. The higher-order magnetic susceptibilities, geochemical dispersion pattern, and spectrometric ratio maps and composite images distinguished hydrothermal alteration zones characterised by potassium enrichment. The results indicate the possibility of concealed source rocks with high magnetic mineral content (magnetite, maghemite, titanomaghemite, pyrrhotite, and franklinite?) and a relatively high density. The geophysical signatures observed around Fatehpur and Laxmangarh, with high magnetic contrast, low dominance, and remanence coupled with high gravity values, as well as hydrothermal alteration zones, appear to have striking similarities with the geophysical and geochemical scenarios of some of Australia's Iron Oxide Copper-Gold (IOCG) prospects in the Gawler, Arunta, Tennant, and Mount Isa regions. Therefore, these anomalous areas may be explored for the possibilities of an IOCG or polymetallic type of mineral.