Delineation of Geothermal Energy Potentials in Parts of Calabar Flank, Southeastern Nigeria Using Aeromagnetic Data

In tackling energy-related challenges in Nigeria; the exploration of an alternative source of energy (Geothermal Energy) comes to the limelight as it is generated below the earth subsurface. This work focuses on the delineation of geothermal energy potentials in parts of Calabar Flank, in southeastern Nigeria using Aeromagnetic Data. The aim is to understand the geothermal energy potentials and structural of parts of the Calabar flank by studying the various geothermal, geological, and structural parameters using Aeromagnetic Data. The methodologies applied are quantitative for structural analysis and qualitative using spectral analysis and 3D Euler Deconvolution. The study area lies between within Latitude 5°30ʹ00ʺ N - 6°30ʹ00ʺN and Longitude 7°30ʹ00ʺE - 8°30ʹ0ʺE respectively. Results from the 3D Euler analysis revealed the depth range of 0.25 Km to 4.018 Km. the spectral Analysis revealed a depth range of Magnetic source (Zt) is (-)0.564 Km to (-)0.828 Km, the Zo is (-)4.261 Km to (-)5.999Km and the average depth to basement thickness is (-)4.825 Km. The Curie Point Depth, Geothermal Gradient, and Heat flow yield an average depth of (-)9.452 Km, a value of 61.893CKm-1, and 154.983 mWm-2 from the Spectral Analysis. Some structural features such as trending faults, and fractured basements was observed at the NE-SW of the study area and this correlated to the relatively high heat flow and geothermal gradient at the NNE-SSW part is associated with thermal structures, mineralogical and tectonic history from the NE-SW trending fault in the study area is suitable for geothermal energy exploitation.

Subsurface geological model of Malay basin using free air anomaly

The aim of gravity survey is to assist in the detection and delineation of subsurface geological features such as salt domes and faults. In this study, free air anomaly (FAA) data was adopted for mapping and modelling process to delineate subsurface geological features and basement depth in Malay Basin. FAA is the measured gravity anomaly after a free air correction is applied, and it is used for elevation correction. The data of FAA in this study is obtained from Earth Gravitational Model (EGM) 2008 released by the National Geospatial-Intelligence Agency (NGA)-EGM Development Team. Oasis Montaj software was used in the mapping and modelling process whereby the base map which constructed by the Oasis Montaj is used to form the FAA map of Malay Basin. Typically, the positive anomaly is associated with the high-density intrusion at the base of the crust, while in contrast (negative anomaly), it is related to the sedimentary basin in the upper crust. On top of that, the regional-residual anomaly, total horizontal derivative (THD) and 3D Euler Deconvolution enhanced maps were produced and interpreted to acquire comprehensive insight of subsurface geological features. To conclude, this study showed 5% deviation as compared to previous reported works and the deepest basement depth encountered is 14.5 km.

Real magnetic stripping method in unexploded ordnance detection and remediation – a case study from Rohožník military training range in SW Slovakia

In this contribution we present results from a case-study, which was performed in collaboration between geophysicists and explosive ordnance disposal technicians at the Rohožník military training range in SW Slovakia. The aim of this study was to locate a deep-penetrated unexploded Mk-82 aerial bomb using high-definition digital magnetometry. The location where this bomb had entered the ground was known but its final position needed to be determined so that a safe excavation and disposal could be conducted. However, the detection of this unexploded ordnance object was complicated by the presence of intense magnetic interference from a number of near surface ferrous items including non-explosive test bombs, fragmentation and other iron junk. These items contributed a localised, high amplitude of magnetic clutter masking any deeper source. Our strategy was to approach the problem in three stages. First, we used magnetic data to locate the near surface items. After the detection and before the excavation of the searched objects, two quantitative interpretation methods were used. These involved an optimised modelling of source bodies and the application of a 3D Euler deconvolution. Both methods yielded acceptable results, but the former was found to be more accurate. After the interpretation phase, many of the items were then safely excavated and removed individually. A second magnetic mapping was then performed and from this data which was now significantly less cluttered, we were able to identify but not quantify, two deep source items and to confirm that all remaining near surface items were significantly smaller in size than a Mk-82 bomb. As the remaining near surface sources were interpreted as being contained within the surface one metre of soil and being small they could be assured to be non-explosive, it was considered most practical to mechanically excavate and remove this soil and the remaining objects contained.

Magnetic signatures of subsurface faults on the northern upper flank of Mt Etna (Italy)

A ground magnetic study was performed on the northern upper flank of Mt. Etna to provide new insights into subsurface volcano-tectonic structures. The high resolution magnetic survey was focused on the main structures of Piano delle Concazze, a large flat area dominated by the North- East crater and bounded by the rim of the Valle del Leone depression and the extremity of the North- East Rift. More than 2,500 measurements were gathered with a sampling step of about 3 m covering an area of about 0.2 km2. The total-intensity anomaly field shows the presence of intense South- North aligned maxima related to shallow geological structures affecting this area. Filtering techniques and 2.5D modeling have been applied for the determination of the magnetic source parameters. In order to distinguish the near surface structure, filters of the vertical derivatives, Butterworth high-pass and the tilt derivative were used. The 3D Euler deconvolution has been applied to estimate the depth and the structural indices of the causative sources. The calculated structural indices, that express the geometrical nature of the source, are in agreement with forward modeling. They show that the area is mainly affected by subvertical normal fault and the estimated depth of magnetic sources ranges between 10 m and 40 m. Our total field magnetic survey shows that characteristic magnetic anomalies are related to fault zones in the Piano delle Concazze that are well consistent with the local tectonics. The subsurface structures that have been detected allowed to delineate the general structural framework of the area. In particular, it was possible to clarify that these structures seem to be not deep rooted and consequently they can hardly act as preferential pathways for magma ascent.

DELINEATION OF FAULTS AND CAVITIES USING GRAVITY TECHNIQUES: AN IMPLICATION FOR ROAD CONSTRUCTION, SOUTH-SOUTH NIGERIA

This study was carried out using five digitized aerogravity data to delineate near-surface structural faults, cavities, low-density zones and estimate the mass balance unit in foundations. Qualitative and quantitative analysis were performed in order to examines the depths to anomalous bodies, density/mass and stratigraphic features such as faults and cavities. The techniques employed were: Source parameter imaging (SPI), 3D Euler deconvolution, forward and inverse modeling. The results of the SPI shallow values range from -5.62 to -53.74 m and deep values range from 3.33 to 120 m. The 3D Euler deconvolution results range from -1892.2 to -1278.3 m for obscure and -12264 to 644.6 m for superficial formations. The forward and inverse modeling result shows the values of depth ranging from 2.5 to 4.8 km, density/mass range from (0.7 to 2.4) x 10-3 kg/m3 and (27 to 133) x 1010 kg of three profiles which is the parameter contrast of the gravity surveys. This shows sequential depths and density/mass contrast between the body of interest and the surrounding material which depicts the presents of faults, sedimentary basins and rock bearing minerals of shale/marble which comprises of air, water and sediment-filled formations. The information from this study has revealed the true nature of the subsurface and this will serve as a guide during road construction.

Edge Detection and Depth to Magnetic Source Estimation in Part of Central Nigeria

High-resolution aeromagnetic data covering an area of 24, 200 km2 in north central Nigeria has been acquired and analyzed with the aim of carrying out trend analysis, edge detection (structural delineation) and depth to magnetic source estimation using reduce to the pole (RTP), horizontal gradient magnitude (HGM), center for exploration targeting plug-in (CET), 3D Euler deconvolution and source parameter imaging (SPI) techniques. Trend analysis was applied to the RTP data to delineate structures that have dissected the area. The 3D Euler deconvolution and HGM were correlated by plotting the estimated Euler solutions for a structural index of one (SI=1) on HGM map and the resulting map produced have shown that both methods can contribute in the interpretation of the general structural framework of the study area. The structural delineation based on HGM and CET maps showed that two predominant trends (ENE-WSW) and (WNW-ENE) have affected the area. The trend/depth/contacts of these faults were classified into four groups: Faults <150 m, 150 m - 300 m, 300m - 450 m which are the most predominant fault system based on Euler solutions with a structural index of one (SI=1) and those deeper than 450 m while the result of source parameter imaging (SPI) revealed a depth to source varying from 58 m specifically for areas with shallow depth to the magnetic source to those from deeper source occurring at 588.153m depth especially the south-central portion and the south-eastern portion of the study area.