Mise en place de la chaîne volcanique du moyen Atlas (Maroc) : Traitement des données aéromagnétiques. The middle Atlas volcanic orogen setting (Morocco): aeromagnetic data analysis.

International audienceL'analyse de la carte aéromagnétique du Moyen Atlas a permis de caractériser les anomalies magnétiques régionales, de positionner les sources et de les délimiter latéralement et en profondeur. Les laves plio-quaternaires sont les principales sources d'anomalies. Elles s'exposent intégralement à la surface, avec une épaisseur importante au centre de l'aire volcanique et qui diminue vers la périphérie. Il n'y a pas d'évidence de chambre magmatique qui aurait alimenté les appareils volcaniques. Leur mise en place est liée aux accidents hercyniens de direction NE-SW qui parcourent la région. Des évidences de la néotectonique suggèrent que ces accidents sont toujours actifs

Contribution du géomagnétisme à l'étude géologique de la région d'Aguelmous (Est du massif hercynien central marocain) Geomagnetic contribution to the geological study of Aguelmous district (Eastern part of the central Moroccan hercynean Massif)

International audienceLe levé aéromagnétique de la région d'Aguelmous (est du Massif central marocain) présente des anomalies orientées. Leur forme a été simplifiée par le prolongement vers le haut. Les anomalies situées à l'ouest du granite du Ment sont engendrées par les laves plio-quaternaires qui affleurent intégralement. Elles sont alignées suivant la direction NE-SW. En revanche, celles situées à l'est qui sont allongées suivant la même direction, n'ont pas d'évidence de leurs sources à la surface. Nous les attribuons aux laves plio-quaternaires qui ont emprunté les accidents hercyniens. Ces derniers sont néotectoniquement actifs comme l'attestent nos résultats paléomagnétiques préliminaires et ceux de la sismicité naturelle de la région. The aeromagnetic survey of Aguelmous district (Eastern part of the central Moroccan Massif) shows some oriented anomalies. Their form has been simplified by an upward continuation. The anomalies situated west of Ment granite are generated by the Plio-Quaternary lavas that completely outcrop. They are lined up along the NE-SW direction. However, those situated east, elongated along the same direction, do not show any evidence of their source at the surface. We attribute them to the Plio-Quaternary lavas that have followed the Hercynian accidents. The latter are neotectonically active as shown by our paleomagnetic preliminary results and those from the natural seismicity of the district

High-resolution residual terrain model and terrain corrections for gravity field modelling and geoid computation in Niger Republic

In this study, we computed and presented grid maps of high-resolution terrain corrections and residual terrain model (RTM) as short-wavelengths of the gravity field and the geoid in Niger. We constructed RTM elevations from mean elevation surfaces corresponding to ~100 km and ~9 km of  spatial scales and 3 arc-seconds SRTM data. The computations are performed at gravity stations and 1.5 arc-minute regular grid, out to 10 and 200 km for inner and outer zones respectively with the standard density of 2670 kg/m-3. The study area is characterized by low values of terrain effects. The indirect effects are lower than 10 cm for ~9 km and reach 1.8 m for ~100 km. In Niger, 98.44% of indirect effects are lower than 1 cm and 98.2% of direct effect are lower than 5 mgal for ~9 km. For ~100 km, 85.87% of indirect effects are lower than 10 cm and 89.77% of direct effects are lower than 5 mgal for ~100 km, and 98.77% of terrain corrections are lower than 1 mgal. We found out that height discrepancies between gravity stations and SRTM influences the precision of terrain effects. The results are value for applications in geodesy and geophysics that require accurate interpretations

Assessment of recent GOCE-based global geopotential models and EGM2008 in Niger Republic

In this study, we assessed recent GOCE-based Global Geopotential Models (GGMs) and EGM2008 in Niger.  The combined GGMs EIGEN_6C4, GECO and EGM2008 were evaluated up to their maximum degree and order (d/o) 2,190 to select the one for gravity database densification. The following pure satellite GGMs were assessed for the modelling of the long and medium wavelengths in geoid computation: GGM05G, ITU_GGC16, EIGEN_6S4v2 and the fifth releases from direct (DIR5), space-wise (SPW5) and time-wise (TIM5) approaches. The GGMs are compared to terrestrial gravity data and geoid heights from GNSS/Levelling points before and after applying spectral enhancement method (SEM) by residual terrain model (RTM) for combined models and by RTM and the coefficients of selected combined GGM for pure satellite models. The agreements of combined GGMs with terrestrial gravity data and GNSS/Levelling points, in terms of root mean square (RMS) are about 4.88 to 5.02 mGal and 0.14 to 0.16 m, respectively. EIGEN_6C4 was selected as it showed the best performance in terms of geoid height differences and the probability of 3-sigma rule for gravity anomaly differences. At d/o 200, DIR5 showed a good agreement with terrestrial gravity data (5.04 mGal) and GNSS/Levelling points (0.15 m) after applying SEM, it was then retained. All GOCE-based models exhibited a good performance in long and medium wavelengths confirming the good recovery of the gravity field by the spatial gravity mission in these spectral bands

Impact of datum transformation on local variations of geometric geoid in Niger

In this study, we have conducted an investigation on the impact of the coordinates’ transformation on local variations of geometric geoid. The study area is limited by 1°43′12″ to 4°00′37″ East and 13°01′57″ to 14°31′20″ North in the southwest of the Niger Republic. We used 39 network GPS/levelling points es­tablished by the Japan International Cooperation Agency (JICA) and the National Geographic Institute of Niger (IGNN), including the DOPPLER point ANG302/no.65. Using other coordinates of point no. 65 pro­vided by IGNN, we transformed the points into WGS84 and computed a new geometric geoid model. The comparison of the new model with EGM2008 geoid up to d/o 2160 gives the STD of 15 cm and the RMS of 16cm. Local variations of the geometric geoids, were compared to that of EGM2008 geoid. The comparison through basic statistics, trend lines and 3D overlaps, showed a similar trend between the geometric geoid from the transformed coordinates and that of EGM2008. On the contrary, the JICA-IGNN geometric geoid generated an opposite and exaggerated trend. The Jarque-Bera test confirms that the three samples follow a normal distribution at the significance level α = 5%. The equality of variances between EGM2008 and JICA-IGNN geoids has been rejected by the Fisher’s F-Test/two-tailed at α = 10%. However the test confirms the variances equality between EGM2008 and the transformed geometric geoid at α = 5% and α = 10%. The two-tailed Student’s T-Test at α = 5% also confirms the equality of means between EGM2008 geoid and transformed geometric geoid samples

Airborne data processing and 3D modeling of magnetic structures between Nador region and the Chaffarine Islands: Geological significances

The Cap of Trois Fourches-Nador-Chaffarine Islands zone (CTF-N-CI) located in the Moroccan Oriental Rif is recognized by Neogene-Quaternary volcanic outcrops. Those later formed volcanic cones and are considered the main source of the huge Nador magnetic anomaly. This study aims to enhance the magnetic context using geophysical techniques and identify the geometry of magnetic sources by performing 3D modeling using Talwani method. We have achieved the following findings: (1) The elaborated residual magnetic map (Rs) that shows three aligned magnetic anomalies. They are oriented W-E and register a maximum intensity of 380 nT. Applying reduction to the pole to (RTP) the Rs map makes the anomalies on the top of their body sources. Thus, we have found that those anomalies have resulted essentially from the Gourougou volcano and Chaffarine Islands volcanoes outcropping in the study area. Except for the middle Pm anomaly, which is related to volcanic rocks of the Kariate Arekmane drillhole. (2) Performing horizontal derivative (HD) technique leads to establishing the edges of the source bodies. Hence, the inversed map of the magnetization distribution proves that the three bodies are homogeneous and probably splintered from a unique mass. (3) The downward up continuation (DC) to different heights (from 500 to 2000 m) indicates the source's depth. Thus, we have demonstrated that the common source could reach 0.6 km of altitude. (4) Finally, we have constructed a 3D Talwani model of the volcanic edifices. The released 3D model demonstrates that the anomalies are related to a very extensive mass of 80 km in length, 20 km in width, 1.3 km in thickness. This model supports that the Neogene-Quaternary volcanism of the study area was fissural and related to the W-E fracture zone of up to 80 km in length. This volcanism was also affected by the sinistral Nekor fault, which probably made the Trois Fourches Cap body isolated from the other volcanic edifices.Previously mentioned elements probably prove the simultaneous establishment of this large magnetic structure. Moreover, they emphasize the crucial role of the magmatic phenomena which occurred in Northeastern Morocco at the beginning of the Messinian

Contribution of aerogravity data interpretation to the study of the deep structure of Agadem petroleum block (Niger)

The main information provided by gravity maps is the geographical distribution of density heterogeneities in the subsurface. It is an important tool widely used for the mapping of geological structures, especially in the oil industry. Thus, this study based on the interpretation of aerogravity data has for objective, the qualitative description of the characteristics of the gravity anomalies of the study area, interpretation and mapping of the gravity lineaments as well as their depths, knowing that the lineaments constitute potential structural traps favorable to the accumulation of the hydrocarbons. Methods such as horizontal derivative, upward continuation and Euler deconvolution are used to give a geological signifiance to the different anomalies and to highlight deep structures. Thus, the analysis of the residual anomaly map revealed elongated negative and positive anomaly zones, oriented globally NW-SE, considered respectively as horst and graben zones. Gravity lineaments, considered as normal faults, are mapped using the horizontal gradient method. Finally, the depths of the density contrasts are estimated by the Euler deconvolution calculation using the value “1” as structural index. The depths thus determined are highly variable. The shallowest depths vary between 3000 m and 6000 m, while the deepest depths reach 18000 m

Contribution of Python-based BERT software for landslide monitoring using Electrical Resistivity Tomography datasets. A case study in Tghat-Fez (Morocco)

The electrical resistivity tomography (ERT) technique was conducted for the geophysical survey of a landslide on the southern slope of Jbel Tghat, north of the city of Fez, Morocco. Nine electrical resistivity tomography profiles were implemented to: (a) characterize the geometry of the dipping zone; (b) characterize their internal structures; and (c) highlight the faulting zone between the marly deposits and the conglomerate formation.The measured data sets were processed using EarthImager™ 2D (Advanced Geosciences, Inc), and BERT (Boundless Electrical Resistivity Tomography) software packages that offer a simple workflow from data import to inversion and visualization, while offering full control over inversion parameters. Moreover, BERT software is a Python-based open-source inversion software package. Both ERT processing software allows obtaining 2D subsurface electrical models associated with the distribution of the subsurface apparent electrical resistivity property, in Ohm.m units. Those 2D subsurface electrical models are retrieved using the same inversion parameters to determine the distribution of geoelectric layers and their defining parameters (e.g., electrical resistivity, thickness, and depth), giving access to certain characteristics exclusive to one of the two processing techniques, comparing the inversion findings to better understand the process's limits, as well as evaluating the capabilities of the two inversion methods

Integration of Electrical Resistivity Tomography and Seismic Refraction Tomography to Investigate Subsiding Sinkholes in Karst Areas

International audienceOperational and safety issues associated with subsiding sinkholes in karst areas start with the definition of fractures and joints, causing ground weakness. Conventional geotechnical boreholes and geological mapping must be complemented with indirect subsurface exploration techniques to detail those structures. This work aims to use electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) near-surface geophysical techniques to infer the 2D and 3D geometry of sediment-infilled sinkholes formed by the conjunction of fractures and joints in karst areas. Geophysical surveys were performed in a sediment-infilled sinkhole area with two sectors of different subsiding and infilling degrees formed by the conjunction of two fault systems in an experimental research area in the Sierra de Gádor Mountains in southeastern Spain. The ERT survey delimited the geometry of the sinkhole area, including the main fault-boun4ded limits, other minor faults, buried epikarst forms, and the distribution of coarse and clay-rich infilling. The SRT survey corroborated the structure and disambiguated clay-rich and high-moisture-content structures giving similar velocity fields. The integration of the ERT and SRT techniques provides indirect 2D and 3D visualizations of the ground of interest in predicting weakness-triggering mechanisms associated with the regional karst structure. This trial in an experimental uninhabited area, with the possibility of exploring subsiding karst structures is of special interest for designing operational and safety measures in urban areas, where similar karst structures may go undetected and monitoring capability is often more limited. The technological development of the used techniques enables the periodical geophysical monitoring of karst structures, thus making the identification of structural changes modifying the land safety and hydrological mechanisms feasible over time