Magnetic Field Intensity Data Research Articles

Upward continuation of total magnetic field intensity data over Sokoto basin, north-western Nigeria

High-resolution aeromagnetic data covering the Sokoto basin, north-western Nigeria were acquired and analyzed using regional enhancement techniques, upward continuation and second order regional. The objective was to delineate major geological structures. The data, which are in form of a grid, were knitted using Oasis Montaj software to obtain the magnetic total field intensity map. The composite grid was reduced to a magnetic equator to align the peaks of magnetic anomalies over the center of their causative bodies. The reduced grid was continued at various planes of observation to understate anomalies due to shallow features. The planes were at 10 km, 20 km, 30 km, 40 km, 50 km, 70 km, and 100 km above the surface. Upward continuation maps produced greatly enhanced the long-wavelength (low wavenumber) anomalies associated with deeper subsurface regional structures. The continuation revealed a high magnetic intensity anomaly in western parts of the area and low magnetic intensity anomalies in the eastern parts. The low magnetic intensity in the eastern part is interpreted as a sedimentary deposit. The residual field was removed from the total magnetic field data using the second-order least square method to get the second-order regional field. The regional field analysis agrees with the upward continuation result.

Geophysical investigation of the Orle River fracture system in Igarra Township, southwestern Nigeria

The subsurface structure and propagation geometry of the fracture system controlling the Orle River Channel in the Igarra Township, Southwestern Nigeria, have been investigated via a multi-method geophysical survey. The goal was to delineate the nature, distribution, and spatial propagation geometry of the fracture system and evaluate its potential to serve as storage and distribution features for groundwater within adjoining areas. 2D electrical resistivity, total field magnetic, and co-planar loop conductivity measurements were collected along four traverses using an ABEM1000 Terrameter unit, a GEMS Magnetometer, and an EM34 Co-planar loop electromagnetic system. Traverses were established to run across and parallel to the river channel. The observed electrical resistivity field data were inverted for subsurface 2D resistivity structure using a commercially available 2.5D finite element modelling inversion software. Magnetic field intensity data and ground conductivity data were presented against station positions. Three subsurface layers were delineated at the survey site; (1) surficial humus-rich and wet top-soil, (2) a thin poorly developed weathering layer, and (3) the fresh bedrock which occurs as relatively shallow levels and often outcrops. The River channel is controlled by multiple fractures usually located at or near the contracts between contrasting rock types. Fracture dip is usually in the northerly direction but conjugates, dipping southwards also occur in the most northerly extremes of the imaged fracture system. Upstream the fracture path is wider and along with the imaged overburden, isjuxtaposed northwards of the channel axis suggesting a much broader river channel in the geologic past. Low aperture fractures imaged tangential to channel axis likely serve to funnel surface and groundwater from the channels to the surrounding areas. Where such low aperture fractures can be delineated, they offer the best chances of groundwater abstraction within adjoining areas, particularly during the dry season.

The use of anomaly data in identifying faults in Sedoa village, Poso district

A magnitude (Mw) 6.6 Earthquake occurring on 29 May 2017 in Poso shows that there are active faults in these regions. In addition to Palu-Koro fault which is the major fault in the island of Sulawesi, one of secondary faults that may trigger earthquakes is a fault in Lore Utara Subdistrict, Poso District. This fault has been identified using magnetic anomaly in Sedoa Village Lore Utara Subdistrict. A measurement was done to obtain magnetic field intensity data and measurement time at base stations and position data at mobile stations. The data obtained was subjected to IGRF correction and diurnal variation correction. The result of the research shows different susceptibility (k) values with 0.00017 SI for clay rocks, 0.0209 SI for muddy sandstone, and 0.05 SI for granite rocks. This research also shows the existence of subsurface geological structures in Sedoa Village, Poso District, in the form of two normal faults in different directions. The first fault runs from Southeast toward Northwest and the second one runs from Northeast to Southwest.

English

Geoscience prospecting for gold mineralisation was conducted in Kwakuti town located within Latitudes 9.362500°N to 9.387500°N and Longitudes 6.920833°E to 6.945833°E in Northern Nigeria. The deployed geosciences techniques comprise surface geological mapping, processing and analysis of aeromagnetic total magnetic field intensity data using Oasis Montaj software and X-ray fluorescence analysis of soil samples. Migmatites and gneiss dominate the rock outcrops in the area. The migmatites occupy high elevations on the north-eastern and eastern portion of the area, where they display low magnetic field intensity values. The schist occupies moderate to low elevation areas and they display high magnetic field intensity values. The migmatites are dotted with quartz veins which constitute the gold mineralisation zone. Oval shaped high magnetic anomalous zones within the schist indicate basic intrusive into the schist. First derivative map of the magnetic field intensity data reveal NE-SW trending lineaments. They likely conducted hydrothermal fluids from the basic intrusive into the migmatites on the northeast, where gold mineralisation occurred by metasomatic ionic exchange. Spatial concentration of TiO2 and MnO2 are the highest within the oval shaped high magnetic anomalous zones in the southern portion of the study area. This strengthens the inference that basic intrusive underlies the area. Gold concentration distribution pattern in the area is skewed NE-SW, thereby suggesting that the NE-SW structures control the mineralisation. Mining activities will be more efficient if directed along the NE-SW structural trend. Key words: Aeromagnetic data, aero-radiometric data, geologic mapping, gold mineralisation, X-ray fluorescence.

Regional Structural Architecture of Part of Northern Nigeria Basement Complex inferred from Upward Continuation of Magnetic Field Intensity Data

2–D upward continuation of total magnetic field intensity data at Katsina area Northern Nigeria Basement Complex, aimed at identifying major regional features, has been carried out and interpreted in this study. The significance is to update the existing geology map of Nigeria and to guide further exploration studies in the area. Data for six aeromagnetic field maps were merged to form the composite data. 2–D upward continuation at various planes of observation was applied to the gridded magnetic field intensity data after coordinates were first converted (from degrees to meters) to understate anomalies due to shallow features. The plane was 15 km above the ground level. The continuation results revealed a high magnetic region at center surrounded by low magnetic parts, especially at the southern part. Corroboration with geology suggested that the high magnetic region could be a central structural uplift of the bedrock, highly rich in magnetic minerals. The low magnetic area at southern part is interpreted as sedimentary deposit, extended from the prominent Bornu Basin. The south – eastern part of the study area is moderately magnetic in nature, and corresponds to the metamorphic rocks of the Kazaure schist belt.

Internal Architecture of Meteorite Impact Crater at Bukit Bunuh, Lenggong – Perak, Malaysia Inferred from Upward Continuation of Magnetic Field Intensity Data

2–D upward continuation of magnetic field data acquired at Bukit Bunuh, Lenggong – Perak, Malaysia, with the objective to ascertain the impact crater and possible rebounds, has be carried out and interpreted in this study. Ground magnetic survey was conducted first as regional study in the entire area followed by a detailed study at the suspected crater region. Data from both studies were compiled, corrected and separated (regional – residual). The residual magnetic data ranged between -272 and +134.2 nT. 2–D upward continuation at various planes of observation was carried out on the gridded residual magnetic field data after coordinates were converted (from degrees to meters) to understate anomalies due to shallow features. The planes were at 250 m, 500 m and 750 m above the ground level. The continuation at 500 m revealed a low magnetic region, believed to be an impact crater, which is now filled with sediments, surrounded by a high magnetic shallow bedrock. Other high magnetic key features interpreted as rebounds (R) also remained after filtering out the ambiguous anomalies.

Spectral depth analysis of some segments of the Bida Basin, Nigeria, using aeromagnetic data

Spectral depth analysis was carried out on ten (10) of the 2009 total magnetic field intensity data sheets covering some segments of the Bida basin, to determine the depth to magnetic basement within the basin. The data was obtained from the Nigerian Geological Survey Agency and used to produce a composite map of the study area which is situated within latitude 8 0 00'N - 10 0 00'N and longitude 4 0 30'E - 7 0 00'E. Sixteen (16) overlapping blocks were generated from the ten sheets and Spectral depth analysis was performed on each of the blocks. The results delineated two prominent magnetic zones of shallow and deeper depths with the depth to the shallow magnetic sources (h 1 ) ranging from 0.254km on Block 9 to 1.719km on Block 2, having an average depth value of 0.968km. The depth to the deeper magnetic bodies, h 2 , on the other hand, varies between 1.830km on Block 10 and 4.615km on Block 15, with an average value of 3.063km. The shallower magnetic sources were attributed to near surface magnetic sources which could be laterite, ironstones or ferruginous sandstones that intruded into the sedimentary basin close to the surface. The places with deeper magnetic sources were ascribed to intrusions of the magnetic basement into the basin at deeper depths, lateral discontinuities in the basement and other features differing in magnetic susceptibilities such as dykes, faults, fractures, horsts and grabens within the study area. Based on these results, the Kainji (1), Fashe (2), Mokwa (4), Egbako (5), Baro (7), Patigi/Baro (14) and Baro/Gulu (15) spectral blocks with sedimentary thickness values above 3.0km show the greatest potential for hydrocarbon accumulation. Keywords: Spectral Analysis, Depth to Magnetic Sources, Aeromagnetic data

Determination of Geothermal Energy Sources in Masu Area Northeastern Nigeria Using Spectral Analysis of Aeromagnetic Data

The incessant power outage and power rationing in Nigeria has affected the economic development, hence the need to seek for other sources of electricity generation such as geothermal energy which is environmentally friendly and renewable. The wide range of geologic formations in the north-eastern Nigeria provides an avenue for exploration of geothermal energy. Hence, this work carries out an investigation of Curie point depth, geothermal gradient as well as heat flow in Masu, which is located within Nigerian sector of Chad Basin (lat. 12°00' to 13°00' N and long. 12°30' to 14°00' E) using spectral analysis. Application of minimum curvature in gridding the total magnetic field intensity data was done using the Oasis Montaj 6.4.2 software. First order polynomial fitting was applied in Regional-residual separation. The Curie point depth obtained ranges from 12.233 to 16.184 km with an average of 13.993 km, the geothermal gradient of the area varies from 35.838 to 47.413°C /km, with an average of 41.821°C /km, while the heat flow ranges from 89 to 117.80 mWm^-2, with an average of 104.551 mWm^-2. The 2D contour maps reveal that the Curie point depth is lowest in the southeast and increases towards the northwest, while the geothermal gradient and heat flow on the other hand are highest in the southeast and decrease towards northwest. The high geothermal gradient and heat flow values in the area are indications that the area might be suitable for geothermal energy generation as an alternative power supply in the area and in the country at large.

Magnetic Base Station Deceptions, a magnetovariational analysis along the Ligurian Sea coast, Italy

Reliability of high resolution airborne and shipborne magnetic surveys depends on accurate removal of temporal variations from the recorded total magnetic field intensity data. At mid latitudes, one or a few base stations are typ- ically located within or near the survey area and are used to monitor and remove time dependent variations. These are usually assumed to be of external origin and uniform throughout the survey area. Here we investigate the influence on the magnetic base station correction of the time varying magnetic field variations generated by internal telluric currents flowing in anomalous regional 2D/3D conductivity structures. The study is based on the statistical analysis of a data set collected by four magnetovariational stations installed in northwestern Italy. The variometer sta- tions were evenly placed with a spacing of about 60 km along a profile roughly parallel to the coastline. They record- ed the geomagnetic field from the beginning to the end of April 2005, with a sampling rate of 0.33 Hz. Cross-corre- lation and coherence analysis applied to a subset of 125 five hours long magnetic events indicates that, for periods longer than 400 s, there is an high correlation between the horizontal magnetic field components at the different sta- tions. This indicates spatial uniformity of the source field and of the induced currents in the 1D Earth. Additionally, the pattern of the induction arrows, estimated from single site transfer functions, reveals a clear electromagnetic sig- nature of the Sestri-Voltaggio line, interpreted as a major regional tectonic boundary. Induced telluric currents flow- ing through this 2D/3D electrical conductivity discontinuity affect mainly the vertical magnetic component at the closer locations. By comparing this component at near (32 km) and far (70 km) stations, we have found that the mean value of the power spectra ratio, due to the electromagnetic induced field, is about 1.8 in the frequency band rang- ing from 2.5 ×10 −3 to 5.5 ×10 −5 Hz. This energy, folded in the spatial domain of an hypothetical survey in this region produces unwanted noise in the dataset. Considering a fifth of nyquist frequency the optimal tie-line spacing to as- sure complete noise removal would be 1 km and 15 km for a rover speed of 6 knots (marine magnetic survey) and 100 knots (aeromagnetic survey) respectively. Similar power spectra analysis can be applied elsewhere to optimise tie-line spacing for levelling and filtering parameters utlilised for microlevelling.

The INGV tectonomagnetic network

Abstract. The Italian Istituto Nazionale di Geofisica e Vulcanologia (INGV) tectonomagnetic network was installed in Central Italy since the middle of 1989 to investigate possible magnetic anomalies related to earthquakes. The network is part of the INGV L'Aquila Geomagnetic Observatory and is located in an area extending approximately in latitude range [41.6°–42.8°] N and longitude range [13.0°–14.3°] E. Actually the network consists of four stations where the total magnetic field intensity data are collected using proton precession magnetometers. New stations will be added to the network starting from the end of 2007. Here we are reporting the whole data set of the network's stations for the period 2004–2006. No significant anomaly in the local geomagnetic field correlated to the seismic activity has been found. Some considerations about misleading structures present in the data sets are reported.

The development of the INGV tectonomagnetic network in the frame of the MEM Project

Abstract. In the middle of 1989, the INGV (Italian Istituto Nazionale di Geofisica e Vulcanologia) installed in Central Italy a network of magnetic stations in order to investigate possible relationship of the local magnetic field with earthquakes occurrences. Actually the network consists of four stations, where the total magnetic field intensity data are being collected using proton precession magnetometers. Here we are report on the actual state and the future developments of the network. In the frame of the MEM (Magnetic and Electric fields Monitoring) Project, new stations will be added to the network by the end of 2007. The results of the test campaigns carried out in the sites chosen to widen the network are also discussed. Moreover, the 2006 complete data set of the network is also reported. Concerning the data analysis, a new approach is also discussed that takes into account the inductive effects on the local geomagnetic field by means of the inter-station transfer functions time variations analysis.

A tectonomagnetic effect detected in Central Italy

Significant variations in the absolute value of the geomagnetic field intensity related to tectonic events, as earthquakes and volcanic eruptions, have been observed in several cases. To detect such a tectonomagnetic effect related to seismic activity, a seismomagnetic network was installed by the Istituto Nazionale di Geofisica (ING) in the Abruzzi region (CentraI Italy), in July 1989. This area is being uplifting since the Pliocene. A logistic compromise between geophysical requirements and the electrified railway system tracks distribution led to the installation of five total magnetic field intensity data acquisition sites. From July 1989 to September 1992 geomagnetic intensity data were simultaneously recorded at all stations and compared to that recorded at the L'Aquila Observatory, located in the same area. A variation of about 10 nT in the absolute level of the geomagnetic field was measured at two stations located on the eastern side of the network. We suggest that the detected magnetic anomaly could resuIt from aseismic-changes in crustal stress during this time.