Subsurface Distribution Research Articles

A geophysical investigation of the urban-expanding area over the seismologically active Dahshour region, Egypt

Western Desert region between Cairo and Fayoum in Egypt is a geologically interesting area due to its active seismicity status and the continuous bidirectional urban expansions ongoing northward and southward growth. This work aims to develop a geological model of the region by combining magnetic intensity data (aeromagnetic) and potential field data (airborne gravity) with magnetotelluric (MT) data to offer decision-makers an additional evaluation tool for long-term future planning. Applying a number of filters to aerial gravity and magnetic data allowed us to calculate the subsurface density distribution and structure of the area between Cairo and Fayoum. In addition, the surface of the crust was imaged down to a depth of 6 km using a 3-D inversion of gravity data. Magnetotelluric data were gathered at eight locations over the NS-extended Cairo-Fayoum Road profile. MT data were inverted in 2-D to generate a resistivity model that defines the subsurface structure of the researched region. The subsurface geometry in the MT-derived resistivity model agrees with the results of airborne potential measurements, and no consideration was given to any faults that may have been undetected from the potential maps. Long-term seismological observations indicated that the likely active sources of earthquakes are limited to the recognized main faults and that the ongoing “1185 buildings” construction project did not and will not cause unanticipated seismicity. The paper concludes that urban growth is secure so long as the earthquake codes are rigorously considered while planning civil projects. The northern study region must be regularly monitored for induced seismicity. On the Cairo-Fayoum Road's western side, the middle sector of the study area is the safest place for future civil developments.

Spatial and temporal differences in surface and subsurface meltwater distribution over Greenland ice sheet using multi-frequency passive microwave observations

Increasingly larger portions of the Greenland ice sheet are undergoing seasonal melting-refreeze cycles due to global climate warming. The cycle begins with the arrival of high temperatures and increased solar radiation in the spring and summer seasons generating meltwater on the ice sheet's surface. Meltwater percolates to deeper ice layers, either refreezing within the firn, creating longer-term meltwater pockets (firn aquifers), or generating peripheral runoff. Depending on the location and climate, the refreeze duration, the depth of infiltration, and meltwater persistence are temporally and spatially complex. Our recent study showed that multi-frequency passive microwave measurements in the 1.4 GHz to 36.5 GHz range effectively distinguished seasonal meltwater between the immediate surface and deeper firn layers at an experiment site in the accumulation zone of the southwestern Greenland ice sheet. Here, we further explored the vertically and horizontally polarized multi-frequency melt response at the pan-Greenland scale. We employed 1.4 GHz brightness temperature (TB) measurements from the NASA Soil Moisture Active Passive (SMAP) satellite and 6.9, 10.7, 18.9, and 36.5 GHz TB measurements from the JAXA Global Change Observation Mission-Water Shizuku (GCOM-W) satellite. The results show that the frequency-dependent response was consistent across the ice sheet. The multi-frequency melt indications match with lasting seasonal subsurface meltwater with delayed refreezing compared to the surface. These results suggest persistent seasonal subsurface meltwater occurrences that are spatially and temporally significant but concealed from the high-frequency observations. Retrieving the meltwater evolution in snow and firn presents a complex problem; this work represents an initial step toward developing an ice-sheet-wide algorithm for more comprehensive retrieval of the meltwater profile.

Selective elimination of sulfonamide antibiotics upon periodate/catechol process: Dominance of quinone intermediates

Natural organic matter, specifically ortho-quinones organics among them, was considered can participate in the transformation of sulfonamide antibiotics (SAs). Herein, based on targeted oxidizing for ortho-dihydroxyl structures (catechol as the model) upon periodate, an efficient approach for SAs elimination was introduced. Results first indicated the generation of ortho-benzoquinone (o-BQ) within periodate/catechol system progresses readily (the energy barriers for 9.6854 kcal/mol). The near-complete eliminations were observed towards sulfamethoxazole (SMX) in periodate/catechol system (with the rate of 0.4229 min−1) as well as other SAs and exhibited unprecedented resistance to operating parameters. Besides, periodate converts little into toxic low-valent iodate species during the reaction process, and both the cytotoxicity and acute toxicity assays revealed a significant decline in antibiotics bioactivity. Mechanistic insight revealed that o-BQ dominated the degradation process, comprehensive analysis further confirmed Michael addition reaction was the first degradation stage, in which electrons flow from o-BQ to SMX and form covalent bonds upon aniline. Furthermore, several catechol derivatives were used to verify the universality of the mechanism, and their wide distribution in both subsurface and wastewater implies the potential applications. Overall, the mechanisms elucidated behind this research proposed an efficient strategy for eliminating trace SAs in aqueous environments and selectively removing SAs from complex wastewater matrices.

Texture-based residual stress analysis of laser powder bed fused Inconel 718 parts.

Although layer-based additive manufacturing methods such as laser powder bed fusion (PBF-LB) offer an immense geometrical freedom in design, they are typically subject to a build-up of internal stress (i.e. thermal stress) during manufacturing. As a consequence, significant residual stress (RS) is retained in the final part as a footprint of these internal stresses. Furthermore, localized melting and solidification inherently induce columnar-type grain growth accompanied by crystallographic texture. Although diffraction-based methods are commonly used to determine the RS distribution in PBF-LB parts, such features pose metrological challenges in their application. In theory, preferred grain orientation invalidates the hypothesis of isotropic material behavior underlying the common methods to determine RS. In this work, more refined methods are employed to determine RS in PBF-LB/M/IN718 prisms, based on crystallographic texture data. In fact, the employment of direction-dependent elastic constants (i.e. stress factors) for the calculation of RS results in insignificant differences from conventional approaches based on the hypothesis of isotropic mechanical properties. It can be concluded that this result is directly linked to the fact that the {311} lattice planes typically used for RS analysis in nickel-based alloys have high multiplicity and less strong texture intensities compared with other lattice planes. It is also found that the length of the laser scan vectors determines the surface RS distribution in prisms prior to their removal from the baseplate. On removal from the baseplate the surface RS considerably relaxes and/or redistributes; a combination of the geometry and the scanning strategy dictates the sub-surface RS distribution.

Subsurface Model Of Mt. Sinabung Using The GGM-Plus Satellite Gravity Data And Deconvolution Euler

Mt. Sinabung in Karo Regency, North Sumatra Province, with an altitude of 2450 meters above sea level, became active again in 2010 after a break in volcanic activity for ± 300 years. Since the eruption in 2010 until now, eruptions are still ongoing periodically. The aim of this research is to obtain a model of the subsurface density distribution of Mt. Sinabung. Modeling was carried out using GGM-Plus 2013 satellite gravity data and ERTM2160 topographical data. Bouguer correction and terrain processing use an average density of 2.67 g/cc. After that, the anomaly was separated by using the moving average. Residual anomalies were then analyzed using Euler deconvolution. The results obtained are the existence of a fault structure on the west and three layers of rock obtained which consist of the basement with a density of 2.8 g/cc 3 – 3.3 g/cc, then Toba pyroclastic deposits with a density 1.8 g/cc – 2.3 g/cc and limestone deposits with a density of 2.4 g/cc – 2.7 g/cc.

Subsurface geometry of the Revell Batholith by constrained geophysical modelling, NW Ontario, Canada

The Revell batholith is located within the Western Wabigoon terrane of the Superior Province, Northwestern Ontario, Canada, and is a potential site for a deep geological repository (DGR). This batholith is considered to have favourable geoscientific characteristics for hosting a DGR, including a sufficient volume of relatively homogenous rock. The subsurface geometry of the batholith plays an important role in determining its volume, as well as assessing regional-scale hydraulics, rock mechanics, and glacial stress disturbances on the bedrock, which are other important features and processes that can impact the batholith over the timeframes of concern for long-term storage of used nuclear fuel. Subsurface geometry is complicated to unravel, and surface mapping alone is inadequate to obtain the information at depth. However, gravity, magnetic, or seismic data can be used to enhance understanding by approximating the geometry.This study aims to refine the subsurface geometry and distribution of the Revell batholith from a constrained forward and inverse geophysical model, incorporating high-resolution geophysical data together with a compilation of historic and recent geological field data. The Revell batholith was previously cited as a flat-based pluton with a depth of 1.6 km, where our findings suggest the batholith is deeper than previously thought, with an uneven contact geometry at its base that extends slightly deeper than 3.5 km. Model uncertainties were assessed by varying probabilistic constraints on volume overlap/commonality and shape within GeoModeller™. Results indicate that overall batholith-greenstone contact is generally unchanged when the geological constraints are varied, providing a high degree of confidence that the Revell batholith has a sufficient volume of relatively homogeneous bedrock.

Wear and Subsurface Stress Evolution in a Half-Space under Cyclic Flat-Punch Indentation

Wear is a tremendously important phenomenon, which takes place on the surfaces of two solids in contact under cyclic loads and constitutes one of the most-significant ways of failure for mechanical elements. However, it is not the only source of failure in contacting solids. The subsurface stresses should also be considered, due to the fatigue and crack initiation problems. Nevertheless, these stresses (i.e., their maximum values and distributions) evolve with the solids’ surface wear (i.e., with the load cycles) and also depend on the friction intensity. Therefore, their evolution should be properly computed to predict failures in mechanical elements under wear conditions. This work focused on the study of the evolution of the surface wear and the subsurface stress distributions generated—in an elastic half-space—by a cylindrical flat-ended punch, under cyclic indentation loading (i.e., radial fretting wear conditions). Based on a numerical scheme recently presented by the authors, this is the first time that, for this contact problem, the surface wear and subsurface stress distribution (i.e., maximum value and its location)—and its evolution—were simultaneously analyzed when orthotropic friction and fretting wear conditions were considered. The studies presented in this work were developed for purely elastic contact assumptions.

Assessment of installation suitability of a vertical closed loop ground source heat pump system in regional scale

Installation suitability of a vertical closed loop GSHP system in the Echigo Plain of Japan was assessed considering the regional scale groundwater flow and heat transport analysis. For evaluating and understand the variation of development potential of the system, the necessary length of ground heat exchanger (GHE) was analyzed at different points, where groundwater, geology and subsurface temperature distribution varies. The computation was done for the model case of the Japanese single-family dwelling with reference to standard for energy saving in Japan. The necessary length of GHE ranged from 67 m to 101 m within the plain and found comparatively longer at the central coastal area around the Niigata City. These results showing the variation of the necessary length of GHE in the plain scale is significant and essential for the proper design and site selection of the system in the Echigo Plain, and can also be beneficial for the development of GSHP system in Japan. Influence of the existing groundwater pumping system was also evaluated to determine if the pumping system can enhance heat exchange performance for space-heating. The effect of groundwater pumping was not so significant to improve heat exchange rates due to the natural existence of groundwater flow.

The northeastern Algeria hydrothermal system: gravimetric data and structural implication

The Tell Atlas of Algeria has a huge potential for hydrothermal energy from over 240 thermal springs with temperatures up to 98^circ C in the Guelma area. The most exciting region is situated in the northeastern part which is known to have the hottest hydrothermal systems. In this work, we use a high-resolution gravity study to identify the location and origin of the hot water, and how it reaches the surface. Gravimetric data analysis shows the shapes of the anomalies arising due to structures at different subsurface depths. The calculation of the energy spectrum for the data also showcases the depths of the bodies causing anomalies. 3D-Euler deconvolution is applied to estimate the depths of preexisting tectonic structures (faults). These preprocessing steps assist with assessing signal attenuation that impacts the Bouguer anomaly map. The residual anomaly is used in a three-dimensional inversion to provide a subsurface density distribution model that illustrates the locations of the origin of the dominant subsurface thermal systems. Overall, the combination of these standard processing steps applied to the measurements of gravity data at the surface provides new insights about the sources of the hydrothermal systems in the Hammam Debagh and Hammam Ouled Ali regions. Faults that are key to the water infiltrating from depth to the surface are also identified. These represent the pathway of the hot water in the study area.

Geothermal gradient variation in the Büyük Menderes Graben: implications for geothermal potential of the graben, Western Anatolia, Turkey

The Büyük Menderes Graben (BMG) is an E-W oriented active extensional geothermal basin within the Menderes Massif, a metamorphic core complex, in Western Anatolia, Turkey. 1500 
 (megawatts-energy) MWe of installed geothermal capacity for power production exist as of December 2019 in Western Anatolia, mostly generated in the BMG. While the BMG is a vastly producing geothermal resource, it is predicted that it has higher production potential. However, other studies do not include field scale 3D geologic models or geothermal gradient maps. This study aims to first quantitatively test the geothermal gradients in the Aydın-İncirliova-Osmanbükü Geothermal Field (IGF), then map the information. This study also aims to compare the IGF with a neighboring geothermal field. To complete this study, information from stratigraphic columns, bottom hole temperatures, and continuous temperature logs from 13 geothermal wells is utilized with Leapfrog Geothermal to create 3D models of the geology and subsurface temperature distribution. Then, isothermal contour maps of the field are created. The geologic modeling suggests that synextensional deposition has occurred within the graben. The temperature modeling suggests both that thermal breakthrough may have occurred in the field, and that the IGF has a higher geothermal gradient than the nearby Germencik Geothermal Field

Reconstructing archaeological palaeolandscapes using geophysical and geomatic survey techniques: An example from Red Lily Lagoon, Arnhem Land, Australia.

Arnhem Land is a key region for understanding the Pleistocene colonisation of Australia, due to the presence of the oldest sites in the continent. Despite this, conventional archaeological survey has not been effective at locating additional pre-Holocene sites in the region due to a complex distribution of geomorphic units caused by sea level rise and coastal aggradation. This research uses geophysical and geomatic techniques to map the subsurface distribution of the geomorphic units in the Red Lily Lagoon region in eastern Arnhem Land. This reveals a complex Pleistocene landscape, which offers the potential to locate additional archaeological sites and so reveal more about the lifeways of the earliest Australians.

Depth Distribution of Chlorine at Gale Crater, Mars, as Derived From the DAN and APXS Experiments Onboard the Curiosity Rover

AbstractThe subsurface chlorine depth distribution at Gale crater has been studied using measurements from the Alpha Particle X‐ray Spectrometer (APXS) and Dynamic Albedo of Neutrons (DAN) instruments onboard NASA's Curiosity Mars rover. These measurements have been acquired at or near ∼30 drill holes made by Curiosity along its 30 km drive. APXS measured a ∼15 micron surface layer of bedrock and shallow depth of ∼5 cm of drill tailings. DAN is sensitive to the bulk (∼50 cm depth) chlorine abundance and that of other neutron absorbing elements. Joint analysis of APXS and DAN data shows that the chlorine distribution at Gale crater consists of two components: surficial chlorine with concentrations >1 wt.%, possibly controlled by aeolian deposition of Cl‐rich dust and surface alteration, and subsurface chlorine with concentrations <1 wt.%, likely a result of groundwater activity in the past.

Seismic Tomography of Nabro Caldera, Eritrea: Insights Into the Magmatic and Hydrothermal Systems of a Recently Erupted Volcano

AbstractUnderstanding the crustal structure and the storage and movement of fluids beneath a volcano is necessary for characterizing volcanic hazard, geothermal prospects and potential mineral resources. This study uses local earthquake traveltime tomography to image the seismic velocity structure beneath Nabro, an off‐rift volcano located within the central part of the Danakil microplate near the Ethiopia‐Eritrea border. Nabro underwent its first historically documented eruption in June 2011, thereby providing an opportunity to analyze its post‐eruptive state by mapping subsurface fluid distributions. We use a catalog of earthquakes detected on a temporary seismic array using machine learning methods to simultaneously relocate the seismicity and invert for the three‐dimensional P‐ and S‐wave velocity structures (VP, VS) and the ratio between them (VP/VS). Overall, our model shows higher than average P‐ and S‐wave velocities, suggesting the presence of high‐strength, solidified intrusive magmatic rocks in the crust. We identify an aseismic region of low VP, low VS, and high VP/VS ratio at depths of 6–10 km b.s.l., interpreted as the primary melt storage region that fed the 2011 eruption. Above this is a zone of high VS, low VP, and low VP/VS ratio, representing an intrusive complex of fractured rocks partially saturated with over‐pressurized gases. Our observations identify the persistence of magma in the subsurface following the eruption, and track the degassing of this melt through the crust to the surface. The presence of volatiles and high temperatures within the shallow crust indicate that Nabro is a viable candidate for geothermal exploration.

Sub-surface foraminifera distribution and preservation in short core samples from the Strait of Malacca

Abstract
 The Strait of Malacca—a shallow and hyposaline strait—is one of the busiest shipping routes in the world. It divides Sumatra and Peninsular Malaysia. This study assessed the sub- surface distribution of benthic foraminifera, their preservation, and their relationship with the sediment characteristics of the Strait of Malacca. One short sediment core sample was collected from the southern part of the Strait of Malacca. The core was then divided into 10 subsamples at every 2 cm interval. The samples were used for foraminifera identification and grain size and organic matter analysis. From the approximately 3000 benthic foraminifera that were selected and identified, 17 species exhibited ≥ 2% occurrence in at least one subsample along the core. Out of the 17 species identified, 4 species (Asterorotalia pulchella (57%), Cavarotalia annectens (13%), Discorbinella bertheloti (7%), and Pseudorotalia schoerteriana (6%)) exhibited an average relative abundance of >5% in all core intervals. Cluster analysis subdivided the core intervals into two groups: BF1 and BF2. Both the groups were dominated by A. pulchella. The PCA biplot indicated that BF1, which represented the upper sub-surface intervals, had relatively more agglutinated species than BF2. Despite being common in surface samples along the Strait of Malacca, species such as Textularia sp. were absent further down the core. This study indicated that in the sub-surface sediment of the Strait of Malacca, calcareous hyaline species, especially those belonging to Rotaliida, were abundant and readily preserved compared to the agglutinated or calcareous porcelaneous groups. The loss of several agglutinated groups down the cores indicated that a more discreet approach to paleoenvironmental interpretation using benthic foraminifera assemblages should be adopted in this region to avoid bias in the interpretation.

Towards a detailed urban structural mapping using outcrop, geophysical and borehole data integration: Implications for sustainable development

Nowadays, geological conditions and geohazards assessment are essential requirements for sustainable urban planning and development. Hence, with lack of outcrop and near-surface geology, additional geohazards can be imposed. The present study is the first such challenge in one of highly deformed areas in Egypt and, accordingly, an integrated approach for detailed urban geological and structural mapping is given. Therefore, the surface and subsurface field measurements converge on the detailed geological conditions characterization including structural settings in a new urban city, as a case study. In this regard, the measured surface structural data are presented on a detailed geological map considering the surface rock units geometry and distributions. In order to unravel the near-surface geology, geophysical direct current resistivity (DCR) sounding survey is performed to represent subsurface layers distribution considering the surface geological mapping. To reduce the limitations of DCR sounding interpretation, conventional and non-conventional inversion schemes are applied with reference to available borehole data. Evidently, the sum of surface and near-surface mapped fault segments are plotted and analyzed to provide a detailed geometrical structural model. Because the fault rupture hazards and shale layer expansion issues are closely related to the urban sustainable development, a preliminary geohazards assessment is illustrated in the form of fault setbacks calculation and the shale free swelling evaluation. The present approach provides a holistic view of the urban geological mapping and introduces a means of recognizing preliminary potential geohazards problems and/or opportunities at an early stage in any proposed sustainable development, which can support site investigation designing strategies.

Retrieval of Subsurface Resistivity from Magnetotelluric Data Using a Deep-Learning-Based Inversion Technique

Inversion is a fundamental step in magnetotelluric (MT) data routine analysis to retrieve a subsurface geoelectrical model that can be used to inform geological interpretations. To reduce the effect of non-uniqueness and local minimum trapping problems and improve calculation speeds, a data-driven mathematical method with a deep neural network was developed to estimate the subsurface resistivity. In this study, a deep learning (DL) inversion technique using a revised multi-head convolutional neural network (CNN) architecture was investigated for MT data analysis. We created synthetic datasets consisting of 100,000 random samples of resistivity layers to train the network’s parameters. The trained model was validated with independent noised datasets, and the predicted results displayed reasonable accuracy and reliability, which demonstrates the potential application of DL inversion for real-world MT data. The trained model was used to analyze MT data collected in the southwestern Athabasca Basin, Canada. The calculated results from the DL method displayed a detailed subsurface resistivity distribution compared to traditional iterative inversion. Since this approach can predict a resistivity model without multiple forward modeling operations after the CNN model is created, this framework is suitable to speed up the computation of multidimensional MT inversion for subsurface resistivity.

Hululais Field Review from Geoscientific Exploration to Exploitation Stage

Geothermal studies in Hululais carried out by Pertamina was started in 1993 with an exploration survey. The detailed geological mapping as the earliest geothermal resource assessment of Hululais was done internally by Pertamina. The geochemical study was done together with the geological study. The geophysical study was carried out using three methods. The gravity survey was run in 1993 for 320 points along 12 trace lines, while the magnetotelluric survey was conducted in 2006 and 2012 for 64 locations with 1-2 km spacing. The microearthquake survey was done in 2012 for 115 days monitoring time by six sensors in 10 km spacing and in 2019. The conceptual model as a product of the integrated study of three geoscientific surveys shows that the upflow zone is interpreted in Suban Agung – Suban Gregok in the southern area. Fumarole, mud pool and steam-heated hot spring are found in this zone. The outflow zone is interpreted towards the north in Semelako area supported by the chloride hot spring. The appearance of these manifestations is possibly controlled by fault structure in Musi Segment zone of Sumatra Fault Zone (SFZ). The heat source comes from granodiorite intrusion below Bukit Beriti Besar – Bukit Gedang Hululais. The cap rock has 500 meters thick in the upflow zone and increases to 800 meters thick towards the outflow zone in the northern part of the study area. The upflow zone was proved by two exploration wells that drilled until 3000 mMD. The first well in Suban Agung has a high temperature reach 300°C with neutral reservoir fluids and good fault structure permeability. The second exploration well that drilled towards outflow zone shows a temperature of 205°C with neutral reservoir fluids but with low permeability. In the exploitation stage, 21 development wells have been drilled for making the update conceptual model. Among them, ten wells are production wells and the remains are reinjection ones. The permeability in the reinjection wells is not so good compared to the production wells. It might be due to secondary mineral filling in the fault structure permeability zone, so this situation causes a problem in injection water to the reservoir. Investigation of the subsurface permeability distribution in the Hululais reservoir is challenging, so further research should be done to answer this challenge.

Spatio-temporal monitoring of leachates dispersion beneath a solid wastes dump in basement complex of southwestern Nigeria

For real-time monitoring and modeling of leachates dispersion beneath Arada wastes dump, Ogbomoso geologically located within Southwestern Nigeria basement complex, an integrated technique involving time-lapse Electrical Resistivity Tomography (ERT), hydrogeology, and geotechnic was used. ERT was performed using Wenner electrodes configuration in a Northeast-Southwest azimuth on the wastes dump monthly from March to September, and twice in March and September, at the control site, during the wet season. The Total Dissolved Solids (TDS), Electrical Conductivity (EC) and static water level of two monitoring wells on the dumpsite were measured during every ERT surveying. Twenty four soil samples were taken at 0.5 m intervals from 2.0 to 3.0 m deep bores on the anomalously low resistivity spots discovered on the subsurface resistivity distribution of the wastes dump, and sieve analysis and permeability tests were performed in the laboratory on these samples. Using the approach of grain-size analysis, the hydraulic conductivity and porosity were calculated empirically. On the wastes dump and control site, the resistivities of spatially distributed lenses of material ranged within 1.45–37.3 Ωm and 22.5–52.6 Ωm, respectively, suggesting leachates and clay anomalies. The wastes dump subsurface might be regarded as a porous medium since its porosity ranged between 38.2 and 44.2%, and the permeability range of (5.4–9.6) x 10−5 m/s showed that water could freely drain from it. Water samples taken from the monitoring wells on the waste dump had TDS and EC that ranged from 520 to 970 ppm and 1.05 to 2.37 mS/cm, respectively, but decreased in the southwest direction of groundwater flow, indicating leachates dispersion. The hydraulic conductivity ranged within 0.00285–0.176 m/s but varied anomalously and spatially indicating heterogeneous hydraulic conductivity field capable of initiating local variation in velocity that caused leachate to disperse longitudinally and transversely. The rate of longitudinal and transverse leachates dispersion as revealed by subsurface resistivity distribution images ranged within 0.000–0.250 m/day and 0.001–0.045 m/day, respectively. The former rate was 5.6 folds the latter rate and hence fell within the stipulated 5–100 for dispersion process that conformed to natural consequence of dispersivity. The conductivity-time curves (CTCs) of the leachates anomalies' steepness and doubly-peaked properties indicated Fickian dispersion, while different proportion times; t-1.588 and t-2.478, which are not equal to t-0.5, showed that the CTCs described non-Fickian dispersion at a significantly later time.

Tomographic evidences for hydraulic fracturing induced seismicity in the Changning shale gas field, southern Sichuan Basin, China

Seismic activity near the shale gas fields in the Sichuan Basin of China has risen significantly in the past few years. Due to the lack of industrial stimulation data to the public, it is difficult to directly link the surge in seismicity near the shale gas fields with hydraulic fracturing stimulation. Meanwhile, it is also challenging to better understand the mechanisms inducing/triggering seismicity in these areas. In the absence of stimulation data, understanding subsurface fluid distribution can go a long way toward addressing these problems. Therefore, in this study, we have applied the double-difference seismic tomography method and the focal mechanism tomography method to the monitoring data of a local seismic array in the Changning shale gas field to build high-resolution three-dimensional models of seismic velocity structure and pore pressure field, then used these models to characterize fluid distributions beneath the shale gas field. From the inverted models, the correlation of overpressure regions (>12 MPa) with moderate-to-high Vp/Vs (1.75-1.90) is clearly observed. With the aid of the exploration data (including well log, active and passive seismic data), the overpressure regions are interpreted to be created by pore pressure or fluid diffusion along pre-existing faults from the shale gas reservoirs, while variations of the Vp/Vs model partly indicate water- and gas-bearing rocks. By jointly analyzing the geophysical model features and earthquake spatio-temporal characteristics, we infer that the earthquakes are primarily caused by increased pore pressures on the seismogenic faults due to the presence of water and/or gas. Besides, the poroelastic stress perturbation and aseismic slip produced by hydraulic fracturing also contribute to the generation of certain earthquake swarms.

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This research is focused on the structural setting and subsurface distribution of cretaceous facies in the Calabar Flank, Lower Benue Trough. The aim of this research is to compare the works of Nyong (1995), Reigers and Petters, (1997) in light of new available subsurface data. Materials used are data sets from three wells drilled at different periods within the same acreage (Wells A, B and C). Seismic data, well logs and mud log data have been used to analyze and study the subsurface geology of the study area. Well data from the three wells were viewed and matched with processed seismic data. A composite stratigraphic section produced show correlatable facies, their lateral extents and the subsurface facies distribution in the study area. Well A drilled, south central Calabar Flank encountered shale and sandstone, very thick shale unit with limestone overlying sandstone at the base. Well B also drilled south central Calabar Flank about 0.8km from Well A encountered the same sequence. Whereas Well C drilled about 4.7km from Well B encountered sandstone, shale, marlstone and limestone. This research shows that only well C encountered marlstone. Observations from the well sections, show the penetrated limestone unit appears disconnected, with very high resistive anomaly suggesting that the limestone reservoir penetrated by the three wells are not laterally continuous. These findings tend to modify the earlier published articles on the structural setting and subsurface distribution of Cretaceous sediments SW to NE of the Calabar Flank South Eastern Nigeria in terms of facies distribution, facies lateral extent and structural variability or changes.