Basement Depth Research Articles

Structural influences on groundwater circulation in the Makgadikgadi salt pans of Botswana? Implications for martian playa environments

Across the surface of Mars, evidence of past lacustrine and evaporitic environments has been found within basins and craters, where often layered sedimentary deposits and hydrated minerals are observed. However, the intensity, duration, and precise phases of aqueous processes during their deposition remain unresolved mostly for our inability to model subsurface structures. Although several geological processes and locations on Earth have been previously proposed as examples to describe these deposits on Mars, we lack a strong visualization of what water activity might have looked like during evaporitic stages within basins and craters. Here we propose to investigate the shallow subsurface of the Makgadikgadi salt pans of Botswana as a potential analog for understanding groundwater upwelling on Mars. The pans are found within the Makgadikgadi Basin, a depression located at the southwestern end of a northeast-southwest set of graben linked with the East African Rift. The Makgadikgadi Pans are evaporitic environment rich in hydrated minerals and groundwater activity. The purpose of this work is to identify buried faults and areas of relative water saturation within the lacustrine sediment of the Makgadikgadi Basin by means of electrical resistivity surveys. This work represents the first electrical resistivity survey of the basin floor which provides a precursory investigation of the relationship between groundwater, faults, basement depth, and the lacustrine sediments. We present four electrical survey lines from different locations in the pans which reveal distinct sedimentary units. Several faults are inferred from the vertical displacement of these units and accompanying low resistivity where displacement is observed. These results provide a framework for visualizing the sedimentary sequences of infilled basins and craters on Mars, which can broaden the ongoing discussion of hydrogeological processes that were active in the planet’s past. We propose Meridiani Planum, as well as Oyama and Becquerel crater of Arabia Terra as locations to establish this framework. Since such processes are still ongoing in the Makgadikgadi Basin, imaging the subsurface of the pans helps explain the formation of layered and salty deposits on the surface of Mars, how they may have interacted with flowing water, and whether they might have hosted life.

Shear wave velocity estimation in the Bengal Basin, Bangladesh by HVSR analysis: implications for engineering bedrock depth

The S-wave velocity (VS) in the Bengal Basin, Bangladesh has not been resolved from the ground surface to an intermediate depth in a regional context despite its importance for seismic hazard and risk evaluation. For this reason, we estimated VS profiles beneath 19 seismic stations in Bangladesh to a depth approximately 2800 m by employing full horizontal to vertical spectral ratio (HVSR) curve inversion under the diffuse field theory for the noise wavefield. The seismic stations are concentrated in three tectonic zones within the basin: the Surma basin (SB, Zone 1), Bengal Foredeep (BF, Zone 2), and Chittagong Tripura Fold Belt (CTFB, Zone 3). Full HVSR analysis (from 0.2 to 10 Hz) allowed us to obtain deep profiles with combined insights from shallow geotechnical boreholes and deep P-wave velocity (VP) information from active seismic surveys. From the resultant VS profiles, engineering bedrock (VS > 760 m/s) depths were also identified throughout the study area for the first time. The VS profiles within the Holocene to Miocene sedimentary sequences showed rapid variations from location to location. This is due to the highly variable near-surface geology caused by the dense and complex river network and tectonic deformation in Bangladesh. Except for three stations, the engineering bedrock depth exceeded 30 m at all stations. These results indicate the existence of deep soft soil in the study area, where VS30 based site characterization is inappropriate. Furthermore, seismic site response was estimated at a station (DHAK) by simulating a subduction zone earthquake. The resulting response spectrum (RS) exhibited ground motion amplification over a longer period, suggesting that multistory buildings at the site may be at risk if subjected to large earthquakes. The outcomes of this study can serve as useful guidelines for seismic risk reduction planning in Bangladesh.

Three-dimensional direct gravity inversion for Moho and basement depths of the Tuchinh-Vungmay basin, offshore southeast Vietnam, incorporating a lithosphere thermal gravity anomaly correction

This paper presents the determination of the Moho and basement depths of Tuchinh-Vungmay basin (TCVMB) offshore southeastern Vietnam by the three dimension direct gravity inversion. The Moho depth was predicted from the mantle residual gravity anomaly with the lithosphere thermal gravity correction. The downward continuation of the basement residual gravity anomaly is also applied to enhance the basement topography’s resolution. The mean depths of the basement and Moho surfaces were constrained by the power density spectrum (PDS) of the residual gravity anomalies and the oceanic bottom seismic (OBS) data. The predicted Moho depth varies from 13.5 km to 23 km and the basement depth is from some hundred meters to 8.5 km. The gravity basement topography has higher resolution and detail than the National Oceanic and Atmospheric Administration (NOAA) seismic basement.

Generalized Gauss-FFT 3D forward gravity modeling for irregular topographic mass having any 3D variable density contrast

A fully numerical forward model for estimating gravity anomalies in the Fourier domain is presented for irregular topographic mass distributions. Modified Parker’s formula using the Gauss-FFT method is utilized for getting better accuracy than the standard FFT algorithm. Existing frequency domain forward modeling for topographic surfaces uses common analytically derivable density functions for evaluating gravity anomalies. But in the real scenario, subsurface mass distributions can take any functional form of depth. Our presented algorithm provides a generalized approach for evaluating forward gravity anomalies due to any horizontal and vertical variation of density distributions. The computational cost depends on the irregularities of the topographic surfaces and the complexity of the functional form of density distributions. The presented algorithm is compared with analytical solutions and space domain prismatic model approximation. A complex, analytically non-integrable density distribution was also considered and compared with the space domain model. The presented algorithm accurately estimates gravity anomalies in all cases, and the results are close to the space domain prismatic model. Finally, an observed gravity anomaly of the real sedimentary basin and its corresponding inverted basement depth for parabolic density variation is considered to evaluate forward anomalies using our presented algorithm. The estimated gravity anomalies provide close approximations to observed gravity anomalies, revealing the proposed algorithm’s reliability for any 3D topographic forward modeling.

Microtremor array observation in Southern Part of the Bandung basin: deep 1d s-wave velocity structure

Site amplification factor become crucial in seismic hazard microzonation, which depends on local geological condition, therefore identification of bedrock depth is very important related to sediment thickness. One of the Geophysical method that can be applied for subsurface investigation is microtremor array survey method (MAM). This method doesn’t require seismic source, so it is very possible to be applied in the urban area. Bandung basin is a research project for seismic hazard and risk study, where most of this area covered by high population and settlements that becomes the obstacle in this field survey. Geologically, Bandung basin consists of volcanic rock and floodplain, and we can find outcrop of Tertiary rock in some parts the basin. Seismicity in Bandung basin is very active because there are many seismic sources around it, such as shallow crustal and megathrust segment in the southern part. The microtremor array survey was conducted in Southern basin at 3 sites observation represent shallow and deep bedrock depth by using triangular array in shape. The result shows the bedrock depth in BB12 is 139 m, BB13 is 211 m and BB14 is 192 m. This condition represents the feature of bedrock position in Southern Bandung basin.

A Study on Site Specific Ground Response Analysis in Bihar

The influence of local soil conditions on earthquake earthquakes is assessed using seismic site response analysis. Earthquakes are natural phenomena that have claimed the lives and property of millions of people over the centuries. As a result, classifying these strong ground motions, predicting their damage potential, and suggesting mitigation measures become critical. The present study area is Kanti (Muzaffarpur), which is located in north Bihar and covers a geographical area of 3,132 square kilometres and lies between North Latitude 250 54’00”-260 23’00” and East Longitude 84053’00”- 85045’00” which falls in seismic zone IV of IS 1893: 2016. The geotechnical characterization of the site has been investigated using borehole data collected from various locations. The bedrock depth could not be determined due to a lack of deep boreholes, but traces of rock found in the collected data suggest rock outcrops in this region. According to geotechnical characterization, the soil profile in the region is dominated by silty sands, clayey silts, and poorly graded soils. DEEPSOIL software was used to estimate peak ground acceleration. Peak ground acceleration has been estimated using equivalent linear analysis with soil layer depth, bulk density, shear wave velocities, and damping as inputs.

Correlation of excess pore water pressure ratio on flow liquefaction phenomenon in Sibalaya – Central Sulawesi Province

On September 28, 2018, liquefaction in Sibalaya damaged irrigation canals and hit 51.2 hectares fields and roads. This phenomenon was triggered by an earthquake of magnitude 7.5 that shook Sigi Regency, Central Sulawesi Province. As an increase in soil pore water pressure is one of the causes of liquefaction, the pore water pressure ratio (ru) at the study site was analyzed. Then, this figure is used as a point of reference to determine the liquefaction potential. Standard Penetration Test (SPT) data and laboratory tests from four boreholes resulting from soil investigations in 2021 with a maximum depth of 20 meters were utilized in this study, along with microtremor data down to the depth of bedrock from test results in 2023. Based on the data, the liquefaction potential was assessed using the one-dimensional nonlinear site response approach, the GQ/H + PWP model, and the DEEPSOIL V.7 program. At multiple layers of boreholes, ru is more than or equal to 0.8, indicating that the Sibalaya liquefaction flow area is still susceptible to liquefaction.

Bedrock Depth Calculation of Çanakkale (Turkey) Basin Using Rayleigh Ellipticity and Microgravity Survey

Es un hecho bien conocido que el efecto suelo local juega un papel importante en el daño a las estructuras durante los terremotos. Para definir correctamente la interacción suelo-estructura, se debe conocer bien la geometría de las capas de suelo compactas y sueltas en el suelo. Además, el contraste de impedancia debido a los efectos locales del suelo puede ser más profundo que los primeros 30 m, que se acepta como la profundidad de ingeniería, y en este caso, el parámetro Vs,30 está deshabilitado. Por esta razón, la ubicación del lecho rocoso de ingeniería y el lecho rocoso sísmico cobra importancia. La variación de la velocidad de la onda de corte con la profundidad es un parámetro clave para la estimación del efecto de campo sísmico. Además, la separación de capas de baja densidad cercanas a la superficie y lecho rocoso de mayor densidad se puede realizar fácilmente mediante el método de gravedad. Los niveles de lecho rocoso/cimentación sísmica se determinaron usando estos dos métodos juntos para los suelos de Çanakkale, que se sabe que tienen problemas de licuefacción y espesor aluvial grueso. Se realizaron medidas de microgravedad en 100 puntos y las anomalías obtenidas se modelaron en tres dimensiones. A la luz de los datos obtenidos de este estudio, se ha determinado que el espesor de la unidad de cuenca suelta en el área de estudio está en el rango de aproximadamente 50-300 m. La vibración ambiental se registró con la ayuda de un sismómetro de velocidad en 146 puntos, y los perfiles de velocidad S (Vs) se obtuvieron utilizando la inversión de la curva de elipticidad observada. Además, la estructura de tasa de cambio de profundidad de velocidad de la cuenca se define en función de las velocidades Vs en el modelo de gravedad 2D realizado a lo largo de los dos perfiles que cortan la cuenca en las direcciones N-S y E-W. A lo largo de estos perfiles, el rango de frecuencia se ha determinado de 0,8 a 1,3 Hz en las partes profundas de la Cuenca y de 1,5 a 11,0 Hz en las partes poco profundas.

Flat Spots Within Cenozoic Sediments of the Nansen Basin, Arctic Ocean: Indicators for Serpentinization, Gas Generation and Accumulation Processes

Flat spot anomalies in the Quaternary part of the section of the Nansen Basin are imaged in seismic records and are interpreted to be related to gas-rich fluid accumulations. The flat spots are mainly located above basement highs between magnetic spreading anomalies C20 (~43 Ma) and C12 (~33 Ma). The complex morphometric analysis of flat spots show that serpentinization processes identified from modelling of gravity anomalies could be original gas source. This process also makes smoothing of the basement highs amplitudes. The depth of the top of the flat spots below the seafloor has an almost constant value of ~390 m indicating the ascent of gases from variable basement depths to a common subsurface fluid trap. The depth of the anomalies below the seafloor corresponds to a theoretical thickness of gas hydrate stability zone in the studied region. Gravity modeling along the Arktika-2011-03 section showed the position of the upper mantle blocks with lower (to 2.95 g/cm3) density within the highs of the acoustic basement. The flat spot anomalies occur above basement highs, below which blocks with lower density typical of serpentinized rocks are modeled. Thus, the serpentinization of the upper mantle ultramafic rocks is considered a main geochemical process, which can explain generation and accumulation of gas in oceanic abyss at a 1–3 km thick sedimentary cover, as well as small vertical movements of the basement blocks due to density reduction and expansion of serpentinized rock.

Seismic mapping of shallow bedrock shelves in the hanging wall of the Wasatch fault

The Wasatch fault (WF) has been the subject of intensive study. Adjacent to a large and growing urban corridor, it poses a distinct seismic hazard for ∼2.5 million residents who live on its hanging wall, and has numerous Quaternary scarps and produced a M5.7 quake in March of 2020. Thus, the shallow architecture of the fault is important for understanding its earthquake hazards.Prior work has revealed shallow bedrock shelves in the hanging wall near the fault. To detect and image these shelves, we have employed the HVSR method (horizontal-to-vertical spectral ratio) at three locations along and across the WF. This method is powerful in that data can be collected rapidly in an urban setting. However, this method requires knowledge of the Vs of sediment overlying bedrock in order to estimate bedrock depth. We employed the MASW (multichannel-analysis of surface waves) method to obtain the characteristic Vs of sediment overlying bedrock.At Corner Canyon in Draper, Utah, Lake Bonneville sands and gravels overlie intrusive rocks of the Little Cottonwood Stock. Fault offsets inferred from HVSR correlate to mapped scarps. At Traverse Ridge, an HVSR profile co-located to a prior seismic reflection profile also detected shallow bedrock beneath alluvial fan sediment, including a graben system. However, at this site the geology is more complex. Fan sediment is underlain by altered volcanic rocks with subjacent quartzite. The HVSR method detected and characterized these boundaries.A majority of data were collected across the WF in the vicinity of Rock Canyon, Utah in order to create a 3D surface of the bedrock shelf. This area is heavily developed, but with numerous mapped scarps. A 3D model of the bedrock shelf indicates that it is broad, >1 km from the main trace of the WF. This width was not previously known. The HVSR results conform to mapped fault offsets, but also reveal a large, buried (and previously unknown) graben.One main finding of this study is that HVSR is a useful tool for mapping the shallow architecture of bedrock shelves in the hanging wall of normal faults, as long as a reasonable estimate of the Vs of sediment overlying bedrock can be made. Mapping can be accomplished rapidly in uneven topography or in urban settings as the instrumental footprint is very small. A second main finding is that large normal faults may commonly have concealed bedrock shelves. These shelves may affect engineering inputs like Vs30. However, they may also have other unanticipated effects on the nature of ground shaking, perhaps due to the trapping of energy or reverberation between the ground surface, bedrock shelf, and bedrock in the foot wall.

Integrated geophysical interpretation and hydrogeochemistry investigation for groundwater aquifer assessment at El – Farafra depression, Western Desert, Egypt

ABSTRACT The aeromagnetic data is utilised to determine the depth of the crystalline (basement) rocksand subsurface structures where the basement depth ranges from 1777 - 3778 m .and thearea is dissected by several faults with various trends, especially in the NNW-SSE direction. The well-logging data is used to investigate the subsurface stratigraphy where theinterpretation of well-logging data indicates that the area is composed of seven geologicunits, with the third, fifth, and seventh units representing the main aquifer. The first zone ofthe Nubian sandstone aquifer is represented by the sandstone that makes up the thirdgeologic unit with a low gamma ray, which is sandwiched by clay layers. The Nubiansandstone aquifer's second zone is represented by the fifth geologic unit, which is composedof sandstone with minor clay layers. It exhibits low to modest gamma-ray values andmoderate to high resistivity values. The seventh geologic unit is composed of sandstone andrepresents the third zone of the Nubian sandstone aquifer, which is sandwiched by claylayers. It has high resistivity values and a low gamma-ray. The aquifer in the study area hastransmissivity and belongs to a high-potential aquifer.The aquifer in the study area has transmissivity and belongs to a high potential aquiferV.

Sedimentary thickness and basement depths of eastern Niger delta, Nigeria, from aeromagnetic data and implications for hydrocarbon prospects

A knowledge of sedimentary thickness and basement depths of a basin is necessary in understanding the structural set up of formations and evaluation of the hydrocarbon potentials of a region. In this work aeromagnetic data, obtained from the Nigerian Geological Survey Agency (NGSA) Abuja, has been interpreted to obtain sedimentary thickness, structural faults and intrusions including the basement depths of the eastern Niger Delta, Nigeria with the implications for hydrocarbon prospecting. Results from magnetic attribute analysis, total horizontal derivative operations, spectral analysis and source parameter imaging reveal the following: the study area is magnetically heterogeneous; the magnetic intensities range for the region is from – 46.2 nT to 160.3 nT, with residual magnetic intensity values of -15 nT to 13 nT revealing sediments with low magnetic rocks in the southwestern and southeastern parts of the study area and magnetic rocks and basement intrusions in the northwestern, northeastern and central parts of the study area; sedimentary thickness range from 96.5 m to 6601.5 m hosting faulted anticline and syncline structural bodies good enough for hydrocarbon formation and trapping and a maximum depth to basement of 6601.5 m. The sedimentary thickness and structural types, influenced by the basement intrusions, make the region a hydrocarbon potential area and it is recommended that prospecting in the southern and northern parts of the study area should be intensified.

Hydrogeochemical processes and groundwater chemistry in the West Siberia

The present study aimed to identify the formation factors of the current hydrogeochemical character of the Middle-Jurassic aquifer groundwaters (beds YU 2–4) in the Lyaminsky petroleum district of West Siberia in Russia. We hypothesized that the scales of the impact of post-sedimentation processes at the current stage have largely predetermined the character of the Middle-Jurassic aquifer system. To check if the hypothesis is valid, two research problems have been addressed using the lithogenesis theory of the transformation of the groundwater composition during the geologic evolution: we have evaluated factors that governed the genetic type and composition of initial waters (lithologic, paleogeographic), and factors that control subsequent changes in the chemical composition of groundwaters (geotemperatures, neotectonics effect, etc.). To solve the first problem, the formation conditions of groundwaters under study in the continental setting were investigated. To settle the second problem, we obtained the coefficients of correlation between the Middle-Jurassic aquifer groundwater salinity and the current temperature and paleotemperature of the Tyumenian suite, and between the porosity coefficient and the basement depth. We have concluded that the current hydrogeochemical character of the Middle-Jurassic aquifer system is the resultant of the consecutive effects of the whole set of factors.

Aeromagnetic Survey as Reconnaissance Technique for Groundwater Exploration in a Typical Southwestern Nigeria Basement Complex

This study was aimed at investigating the causes of reported boreholes failureparts in some part of the Federal Polytechnic Ado Campus using aeromagnetic geophysical method. The aeromagnetic geophysical data covering the area was processed to enhance shallow anomalies using different enhancement techniques such as derivatives and wavelength filters. Total Magnetic Intensity ranges from -28 to 201 nT after removing regional component. Residual magnetic anomaly ranges from -21 to 127 nT; First vertical derivative ranges from -0.18 to 0.36 nT/m; Magnetic Analytic signal (AS) ranges from 0.03 to 0.36 nT/m. It was observed, by correlating the magnetic responses and geology of the area, that rocks underlain the area have close magnetic susceptibility. Consequently, the variations of earth’s magnetism are mainly controlled by sediment thickness and suspected linear structures which are major factors in basement complex groundwater exploration. The probable depth to basement in the study area ranges from 8.13 to 74.05 m as revealed by AS and spectral method, which implies thicker regolith in southern part of the study area than northern part. However, linear structures are evenly distributed across the study area. Therefore, the groundwater potential of the southern part of the study area is higher due to regolith thickness. The failed groundwater boreholes that are prominent in northern part of the study area are not located on linear structures. Therefore, basement depth and linear structures should be considered during reconnaissance survey in groundwater exploration in a typical Basement Complex terrain.

Magnetic Anomaly Characteristics and Magnetic Basement Structure in Earthquake-Affected Changning Area of Southern Sichuan Basin, China: A New Perspective from Land-Based Stations

The Changning area is located in the southern Sichuan basin and the western Yangtze Plate and is the most abundant shale gas exploration area in China. In recent years, Changning has experienced frequent earthquakes with moderate magnitudes, attracting extensive interest. To investigate the magnetic characteristics in Changning, 952 land-based stations were employed to establish a magnetic anomaly model with a resolution of 2 km, and the subsurface magnetic basement structure was obtained by an iterative algorithm in the Fourier domain. The magnetic anomaly model shows significant distinctions between the northern salt mine area and the southern shale gas area. The magnetic basement includes the crystalline basement and the Sinian sedimentary rock metamorphic basement, which has strong magnetism. The large intracratonic rift that developed in the Sinian–Early Cambrian plays an important role in the evolution of Changning, which also impacts magnetic anomalies and the magnetic basement structure. Finally, by comparing the seismic wave velocity ratio structure, the deeper magnetic basement that corresponds to the higher seismic wave velocity ratio can be explained. This article implies that magnetic anomalies and magnetic basement depth have a certain correlation with earthquakes in Changning, and it provides a geodynamic reference for Changning and the southern Sichuan basin.

Seismic and density signs of mesozoic folding in the earth’s crust of the eastern passive margin of the Siberian craton (profile 3DV)

The results of a complex interpretation of seismic and gravity data along a part (500 km long) of the 3DV profile located in the transition zone from the Siberian Platform to the Verkhoyano-Kolyma fold system are considered. It crosses the Priverkhoyansk advanced trough, the Sette-Daban tectonic zone with access to the folded region [Tectonics…, 2001; Kashubin et al., 2016]. The structure of the crust of the eastern margin of the Siberian craton is interest, which, according to structural and tectonic criteria, belongs to the Mesozoic passive continental margin [Parfenov, 1991; Sokolov, 1991; Tectonics..., 2001] in order to find signs of superimposed folding in the structure of the Earth's crust according to seismic and gravitational data. The depth of the crystalline basement in the Priverkhoyansk trough is also debatable, depending on the nature of the rocks at a depth of 18–23 km at a velocity of 6.2–6.3 km/s, possibly belonging to the Proterozoic.

Aeromagnetic Interpretation of Basement Structures and Geometry in Parts of the Middle Benue Trough, North Central, Nigeria

The research of an analysis of aeromagnetic data collected in the middle Benue Trough in north-central Nigeria is presented. A detailed analysis of basement structures is conducted in order to identify regions with high hydrocarbon potential that is different from those discovered by earlier researchers. Aeromagnetic data were filtered by using the Butterworth and Gaussian filters, transformed by engaging the reduction to the equator technique, and subsequently enhanced. To estimate magnetic basement depths at various places throughout the basin, the Euler deconvolution depth weighting approach was used. Eleven (11) sub-basins with depths ranging from –2000 m to –8000 m were also identified by Euler's findings. The sub-basins trend in the NE-SW direction while the average sediment thickness is found to be more than 3 km. The extracted structural features indicate areas like Kadi Blam and Kado areas in the southeastern part and Ogoja and Obudu in the southern part of the study area as regions with high structural densities. These areas coincide with the areas delineated as the sub-basins. The cross-sections generated reveal depressions caused by the action of some tectonic activities in the area. This study identified undulating basement topography believed to be due to tectonic activities as well as five areas that are possible targets for hydrocarbon exploration.

A physics-informed optimization workflow to manage injection while constraining induced seismicity: The Oklahoma case

A newly developed modelling framework is presented which specifically focusses on the central Oklahoma case and the high-volume injection of wastewater, which led to a surge of induced seismicity. However, the modelling framework is versatile enough to be applied to any anthropogenic subsurface activities and should be seen as a good practice to manage injection while minimizing induced seismicity. The objective is to account for all the available knowledge to deploy the simulation of the flow, induced stress changes and seismicity in the underground. The spatio-temporal pore pressure changes caused by high-volume injection are first determined by using the historical injection rate of the 220 wells at central Oklahoma. From these pressure fields, induced stresses at the basement depth, due to both pore pressure diffusion and poro-elastic inflation of the underground, are computed. The rate-and-state frictional response of the Oklahoma faults is then honored to derive the yearly seismicity rate. After assimilation of the observed seismicity at central Oklahoma, it is demonstrated that our predictions can well explain the historical spatio-temporal evolution of the seismicity at central Oklahoma. Finally, making use of the calibrated predictive model, a constrained optimization approach is used for an efficient screening of multiple injection scenarios. Ultimately, an optimum theoretical scenario is identified which allows the maximization of injection volumes while keeping the seismicity level below a safe cap and, more specifically, would have prevented the dramatical growth of the seismicity rate in 2015. The optimum scenario involves equalizing the injected volumes in all wells and preventing the injection of additional large volumes in the area where most of the wastewater have been already injected prior 2014.

Potential petroleum prospects in the middle Benue trough, central Nigeria: Inferences from integrated applications of geological, geophysical and geochemical studies

The Middle Benue Trough (MBT) is the least well-known of the three major sub-divisions of the Benue Trough (BT) and therefore poorly understood geologically. This paper presents preliminary results of an integrated geological, aeromagnetic, gravity and geochemical studies carried out as a first step approach to petroleum exploration in the central portion (MBT) of the Benue Trough, Nigeria, so as to be able to; 1. Appraise the regional geologic setting of the MBT with a view to understanding the stratigraphy and its oil generation characteristics; 2. Determine the basin configuration and sediment thicknesses and; 3. identify and delineate “leads” or prospect areas for further detailed exploration that could lead to significant commercial discoveries of hydrocarbon in the MBT. Results have revealed the existence of petroleum systems in the MBT with the interbedded shales of the Bima and Yolde formations at the upper portions and the shales of the Asu River Group, Awe, Ezeaku and Agwu formations corresponding to the potential source rocks. The main local potential reservoir rocks are the Bima Sandstones and sandstones of the Yolde Formation coupled with the interbedded arkosic sandstones of the Awe and Keana Formations, while potential regional seals are the shales of Pindiga Formation and the arkosic sandstones overlying the shales of the Asu River Group and the basal shales of the Ezeaku formation and the upper shales horizon of the Agwu formation. Geophysical studies (Aeromagnetic/gravity survey) revealed the variability of the surface to Basement depths within the Middle Benue Trough ranging from 3 – 6 km with the deepest portions of up to 8.2 km towards the Northeastern portion of the MBT. The thick sediment piles coupled with the favorable petroleum systems, provide the conditions necessary for hydrocarbon generation, migration and accumulation. The prominent lineament structure is the NE-SW directional Keana anticline which extends along a strike for more than 100 Km from the south of Makurdi to Zurak in the northern portion. Trans tensional and trans compressional fault systems in the basin have given rise to sub-surface and extensive lineament structures systems which could either constitute major structural traps or pathways for fluid (hydrocarbon) flow. The analysis of a wide range of geochemical data from the MBT revealed TOC values ranging from 0.43 to 3.90wt% for shales/coaly shales and 14.78–79.10wt% for shaly coal/coal facies embedded in the Agwu formation with Tmax values corresponding to immature to mature stages of thermal maturation. Their petroleum potential in terms of HI values ranges from 14 to 267mgHC/gTOC in shale/coaly shale facies and 36–157mgHC/gTOC in shaly coal/coal facies, corresponding to the predominance of type III (gas-prone) in addition to type II (oil and gas-prone) organic matter suggestive of having both oil and gas generating potential. The recent discovery of significant quantity of oil (+gas) in the well Kolmani River-1 in the Gongola Sub-basin, is in strong support of its occurrence in the MBT. The paper has recommended selected prospect areas favorable for further detail hydrocarbon exploration.

Structural analysis of fracture networks controlling geothermal activity in the northern part of the Malawi Rifted Zone from aeromagnetic and remote sensing data

We investigated the relationship between the geothermal activity and fracture networks that control storage and fluid pathways of geothermal systems in the northern part of the Malawi Rifted Zone (MRZ). It is suggested that low to medium temperature geothermal systems in the MRZ are mainly non-magmatic and structurally controlled. However, structural controls and favorable settings of geothermal activity are poorly understood in this region. We used an approach of remote sensing and aeromagnetic data analysis to 1) identify and characterize fracture networks to better understand the structural controls of geothermal activity in the northern part of the rift, 2) quantify and identify permeable areas using topological analysis of fracture intensity and connectivity frequency proxies, 3) understand the role that inherited structures play in the geothermal systems of the northern part of the rift, and 4) build preliminary geothermal conceptual models of selected geothermal systems. Our findings show that fracture networks in the Karonga and Nkhata regions comprise a varying degree of complexity along strike. This structural complexity occurs mainly in favorable structural settings such as 1) two fault segments coalescing to form hard and soft-linked relays, 2) two different oriented fracture segments intersecting each other, and 3) on the tips of major normal faults. The hot springs are located where one or more of these favorable settings occur. The remote sensing analysis shows that Quaternary normal faults are the primary controlling structures of thermal waters at shallow depths in the Karonga (NW-SE to N-S strike) and Nkhata (N-S strike) regions. The aeromagnetic data revealed that permeable NW-striking foliation planes of the Precambrian Mugesse Shear Zone and WNW to ENE-striking foliation planes of the Mwembeshi Shear Zone are important structural controls of geothermal fluids at greater depths in the Karonga and Nkhata region, respectively. The Mwankeja-Mwesia and Chiweta geothermal systems in the Karonga region show favorable structural settings for geothermal fluids related to NNW and NW-striking normal faults segments that coalesce to form hard and soft-linked relay ramps and NW, and NNE -striking faults intersecting each other and NW-striking foliation planes. The Mtondolo geothermal area in the Nkhata region shows that the intersection of N-striking normal faults and ENE-striking foliation planes is the favorable structural setting that controls the emergence of hot springs through the surface. We conclude that high fracture intensity and connectivity are related to the location of the hot springs at the surface and can be used to determine permeable zones and hidden geothermal fluids together with other methodologies. The aeromagnetic data analysis suggested that buried faults and inherited structures (e.g., foliation planes) are controlling the geothermal fluids at depth. Additionally, our aeromagnetic analysis of magnetic basement depth estimation suggests that some of the geothermal reservoirs in the northern part of the MRZ could have an estimated depth of ~ 500 to 1000 m.b.g.l. Finally, the low-cost methodology applied in this study can reduce the risk of drilling non-productive wells in the early exploration stages and become an exploration strategy for similar geothermal systems in countries of the Western Branch of the East African Rift System.