Magnetic Survey Methods Research Articles

Assessing foundation characteristics at the war dam site, lake tana basin, Ethiopia: A geophysical and geotechnical perspective

An integrated geophysical and geotechnical study evaluated the foundation conditions at the War dam site in northwest Ethiopia. This investigation included the classification of rock quality, shallow seismic refraction, and magnetic approaches. The dam's location comprises quaternary soil deposits and rhyolite rock units that have undergone varied weathering and fracturing. The shallow seismic refraction method distinguishes three layers of p-wave velocities that are less than 1.5 km per second with a depth range of 2–6 m, 1.5–2.5 km per second at a depth range of 15–20 m, and 2.5–3.5 km per second ranging from 20 to 40 m, respectively. Magnetic data were used to identify lineaments, and the RQD value acquired from boreholes ranged from extremely poor to excellent. Lineaments were recognized using the tilt angle approach. The results of the permeability tests demonstrated that the rock mass that serves as the dam's foundation had characteristics that are resistant to low permeability. The maximum and minimum lugeon values obtained from the testing were 9Lu and 0.81Lu, respectively. There are weak zones at and below the surface of the dam site, according to the overall findings acquired from seismic refraction, magnetic, and discontinuity surveying. These results were obtained from monitoring the dam site. These significant structures are directed towards a SW-NE, NE-SW, NNW-SSE, and SSW-NNE orientation. The study assessed the geological suitability of a proposed dam site using seismic refraction and magnetic survey methods. Significant geological variations were observed, particularly in the right abutment and valley floor, indicating the need for targeted grouting. The findings suggest that while the site is generally suitable for dam construction, specific areas require further ground improvement to ensure stability.

INVESTIGATION OF MINERAL DEPOSITION IN IJEDA, OSUN STATE SOUTHWESTERN NIGERIA USING GROUND MAGNETIC DATA

A geophysical ground magnetic survey method was used to investigate the presence and depths of magnetic minerals at Ijeda, Oriade Local Government area of Osun-State Nigeria. A G856AX Proton precession magnetometer was used to acquire 122 magnetic data points partitioned into 11 profiles. The station intervals and spacing were 10 m and 5 m respectively between each profile. Regional gradient and the effect of a geomagnetic reference field were removed from the observed survey data, the residual anomalies obtained were plotted against distance using Microsoft excel. Analytic Signal Method (ASM) and Peter’s Half Slope Method (PHSM) were utilized in estimating average Magnetic Source Depths. The Obtained magnetic data were plotted as 1-grid vector map, 2-D contour map and 3-D surface distribution map using surfer application to produce the qualitative analysis. The maximum and maximum magnetic intensity values obtained were 25.0 nT and -41.8 nT respectively, which suggested the area of magnetic mineral occurrences. The depths to the magnetic sources were calculated for 11 profiles and the average depth values ranged from 3.93 m ± 1.22 to 7.34 m ± 1.55 using both the ASM and the PHSM. The results showed that the study area holds a very good potential for magnetic mineral exploration.

An overview of sensing platform-technological aspects for vector magnetic measurement: A case study of the application in different scenarios

Magnetic sensing platform techniques have been used in many years in an attempt to better evaluate the likelihood of recoverable hydrocarbon reservoirs by determining the depth and pattern of sedimentary rock formations containing magnetic minerals, such as magnetite. Utilizing airplanes, large-area magnetic surveys have been conducted to estimate, for example, the depth of igneous rock and the thickness of sedimentary rock formations. In this case, the vector magnetic survey method can simultaneously obtain the modulus and direction information of the Earth’s magnetic field, which can effectively reduce the multiplicity on data inversion, contribute to the quantitative interpretation of the magnetic body and obtain more precise information and characteristics of magnetic field resource, so as to improve the detection resolution and positioning accuracy of the underground target body. This paper presents a state-of-the-art review of the application situations, the technical features, and the development of the vector magnetic sensing platform-technical aspects for different application scenarios, i.e., ground, wells, marine, airborne, and satellites, respectively. The potential of multi-survey sensing platform technique fusion for magnetic field detection is also discussed.

Combined Magnetic, Transient Electromagnetic, and Magnetotelluric Methods to Detect a BIF-Type Concealed Iron Ore Body: A Case Study in Gongchangling Iron Ore Concentration Area, Southern Liaoning Province, China

The detection and evaluation of concealed mineral resources deep in metallic mines and in the surrounding areas remain technically difficult. In particular, due to the complex topographic and geomorphic conditions on the surface, the detection environments in these areas limit the choices of detection equipment and data collection devices. In this study, based on metallogenic theory and the metallogenic geological characteristics of banded iron formation (BIF)-type iron ores, equipment for surface geophysical surveys (i.e., the high-precision ground magnetic survey method, the transient electromagnetic method, and the magnetotelluric method) and data collection devices capable of taking single-point continuous measurements were employed to detect the concealed iron ore bodies in the transition zone CID-1 between the Hejia and Dumu iron deposits in the Gongchangling iron ore concentration area in the Anshan-Benxi area (Liaoyang, China), a representative area of BIF-type iron ores. The results showed that an optimal combination of these geophysical survey methods accurately determined the anomalous planar spatial locations and anomalous profile morphologies of the concealed iron ore bodies. On this basis, we determined their locations, burial depths, and scales. Two anomalous zones induced by concealed iron ore bodies, YC-1 and YC-2, were discovered in zone CID-1. Two concealed iron-bearing zones, one shallow (0–150 m) and one deep (300–450 m), were found in YC-1. A 100 m scale drilling test showed that the cumulative thickness of the shallow iron-bearing zone was over 23.6 m.

A UAV-based magnetic survey method to detect and identify orphaned oil and gas wells

Recent advances in autonomous unmanned aerial vehicle (UAV) technology, along with successful efforts to miniaturize total field magnetometers, offer a unique opportunity to test low-cost UAV-mounted systems for wide-area high-resolution magnetic surveys. Modern UAV platforms capable of flying at low altitudes and collecting dense aerial surveys, coupled with sensitive and compact instruments, allow identification of anthropogenic targets previously identifiable only in ground magnetometer surveys. We present results of a proof-of-concept study focused on developing and field testing a UAV-based magnetometer system to detect and identify abandoned and unmarked oil and gas wells in an area of historical hydrocarbon exploration and development in New York state. Our results indicate that magnetic anomalies associated with metal casing of vertical wells are pronounced considerably above background levels both at the surface and up to 50 m above-ground elevation. We determine that a detection altitude of 40 m is optimal to avoid any canopy interference while recording magnetic data at the highest signal-to-noise ratio. This methodology makes rapid detection and identification of unmarked wells possible and, in turn, allows for future sustainable development of these areas.

Magnetic detection of archaeological hearths in Alaska: A tool for investigating the full span of human presence at the gateway to North America

Magnetic survey methods have recently shown tremendous potential for the detection of archaeological hearths in Eastern Beringia, ranging from intermittent open-air camp fires to larger heat intensive activity areas. Here we present an overview of the method along with eight supporting case studies from seven U.S. National Park Service units in the U.S. state of Alaska, covering diverse geographic settings and time-periods. Together, these case studies demonstrate the capabilities of magnetic detection instruments in various modes of operation for finding datable archaeological deposits that span the breadth of known human occupation of this region. The examples range from the simplest use of the magnetometer as a single sensor reconnaissance instrument in a “search mode”, to use as a gradiometer in conjunction with other geophysical methods such as ground-penetrating radar, to record precise measurements and inform nuanced interpretations. Examples presented here range from the terminal Pleistocene to historic periods, spanning 12,000 years of human activity in Eastern Beringia, and encompassing the arrival, emergence, and expansion of multiple human groups or cultural traditions in the New World.

Research on Ocean Magnetic Exploration Technology Based on Acoustics

In this paper, based on the analysis of the principle of the ocean magnetic measurement and the underwater acoustic localization, a kind of ocean magnetic measurement method combined with acoustics is proposed. The ocean magnetic survey method overcomes the defect of the positioning of the marine magnetometer probe. Through a two exploration for a submarine pipeline, marine magnetic prospecting technology with improved and no improvement is adopted respectively. After comparison test, it can be seen that the exploration results of the ocean magnetic survey method combined with acoustics are about 33% better than that before the improvement. This method is more practical for the exploration of submarine pipelines.

Research progress of geophysical vector magnetic field survey technology

Vector magnetic survey technique is one of the most effective methods for engineering and environmental geophysical exploration. Compared with the traditional geophysical magnetic survey methods, i.e., measurement of the total magnetic field intensity or its gradient, vector magnetic survey can simultaneously obtain the modulus and direction information of the earth magnetic field, which can effectively reduce the multiplicity on data inversion, contribute to the quantitative interpretation of the magnetic body and obtain more precise information and characteristics of magnetic field resource, so as to improve the detection resolution and positioning accuracy of the underground target body. In this paper, based on different application fields, we summarize the application situation, the technical feature and the instrument development of ground vector magnetic survey technique, wells vector magnetic survey technique, marine vector magnetic survey technique, airborne vector magnetic survey technique and satellites vector magnetic survey technique, respectively. As the basic method of the vector magnetic survey technique, ground vector magnetic survey development is the earliest and the technique is the most mature. The accuracy of the instrument system for ground vector magnetic survey is high and the working performance is reliable. But there are also some problems, e.g., the detection depth is shallow and working efficiency is low. Wells vector magnetic survey is one useful tool for the earth’s deep mineral resources exploration. It can finish the detection work that the ground vector magnetic measurement can’t do. But due to the limit of the bore diameter and the high temperature in well, the detection accuracy of the instrument system is lower than the ground magnetic instrument. Marine vector magnetic survey has the irreplaceable effect in the application of military or other ocean engineering. Its working mode changes from the method of ship drag-and-drop to small unmanned underwater vehicle, which can measure more precise underwater magnetic anomaly. Airborne vector magnetic survey has the superiority of fast detection, high efficiency and strong practicality to geophysical environment, etc. But because of magnetic interference and the posture change of the aircraft carrier, detection accuracy reduces. The study of magnetic compensation and data processing methods of aero three-component and full-tensor magnetic gradient is also needed. Satellites vector magnetic survey can obtain high-quality and whole global magnetic field data. Compared with the research abroad, the development of satellites vector magnetic survey in China is later and the technique is relatively backward although lots of exploration work has been done in recent years. We should take full advantage of the opportunity of China aerospace business development to actively carry out research work for the satellites vector magnetic survey. In conclusion, each magnetic survey method has its unique superiority and defect. Any single method has difficulty meeting the requirement of modern geophysical detection application. With the development of magnetic survey technique and instrument system, the future magnetic survey will change from the traditional work mode, i . e ., measuring the total magnetic field intensity or its gradient to the vector information of magnetic field, then to multi-parameter measurements. The magnetic survey application field will change from the traditional ground magnetic survey method with low efficiency and shallow detection depth to high efficient airborne magnetic survey, deep well magnetic survey and abysmal sea magnetic survey, then gradually to the joint detection and interpretation combining with the five kinds of magnetic survey methods mentioned above. Through the combination of a variety of detection methods to realize advantageous complementarities, further improvement of detection accuracy and inversion resolution for earth magnetic field will be implemented.

Integration of Ground Magnetics and Energy Dispersive X-Ray Fluorescence Spectroscopy in Ilmenite Prospection in Magaoni, Kenya

The geology of Magaoni area is associated with the presence of heavy minerals [1]. Magaoni’s neighbours Maumba and Nguluku where ilmenite was discovered by Tiomin Resource Inc. in 1996, using drilling and chemical analysis [2]. Ilmenite mineral is known to be magnetically weak, but provides observable magnetic response [3]. In this study, ground magnetic survey method was carried out to map magnetic anomalies of established stations, associated with ilmenite bearing formations. The magnetic contour map plotted showed weak and shallow magnetic signatures spread throughout the study area. 2D Euler deconvolution solutions revealed presence of magnetised formations from near surface to a maximum depth of about 450 m at some points. The weak magnetic formations of near surface indicated presence of ilmenite. Energy dispersive X-ray spectroscopy was done on soil samples collected randomly from the study area to determine the percentage of iron and titanium oxides. The results showed elevated values of titanium dioxide, ranging from 1.5% to 13% which is way above the global average of about 0.7% [4]. The percentage of iron oxide was low, ranging from 1.5% to 4%, this being the reason for weak magnetisation of the study area.

Sub-surface diagnosis for tantalite in Iseyin SW, Nigeria using magnetic survey method

A magnetic survey was carried out at a site in Iseyin, Oyo state. The study delineates possible structures that may favour the accumulation of tantalite, a magnetic mineral. A ground based technique was employed using proton precision magnetometer G-856 AX to deduce subsurface lithology and structure that may indirectly aid the identification of mineralized rocks. A total of ten geophysical traverses were established in W-E direction in the study area. The acquired magnetic field data were corrected for drift. Qualitative and quantitative interpretations were adopted to obtain the negative peak value and the maximum positive peak value. The contour maps, image maps and the 1-grid maps present the sub-surface picture of the geological structures that are assumed to habour the metallic minerals. The study area is underlain by a rock unit of accumulated minerals. The overburden is relatively thin within the study area and the metallic minerals are in disseminated quantity and at a shallow depth.

The New Magnetic Survey Method for Underwater Pipeline Detection

Magnetic survey has been applied to detect underwater pipeline location, tracking and depth more and more nowadays. In order to detect such smaller targets, besides the application of high-precision magnetometer detector, an effective method especial for underwater pipeline is needed. This paper is to research the magnetic survey method for detecting the underwater pipeline. In terms of its magnetic anomaly characteristic, magnetic sensor array and magnetic anomaly gradient method which can eliminate geomagnetic diurnal variation are adopted to detect the location, tracking and depth of underwater pipeline. Finally, the experiment has shown that this method has good detection accuracy and value in engineering application.

Statistics of geophysical activity in Nigeria (1975–1984)

Statistics on geophysical activities in Nigeria for the ten-year period (1975–1984) was compiled mainly from questionnaires distributed to government and private agencies, oil and mining companies, and universities which engage in groundwater, petroleum and mineral exploration, engineering and research work. Similar studies had been done worldwide by Epsey (1975, 1976, 1977) and Whitmire (1978). From the statistics, it was deduced that electrical resistivity, magnetic, seismic, radiometric, gravity, airborne and ground magnetic survey methods are the main geophysical techniques used which resulted in the discovery of some of the natural resources (oil, gas, minerals and groundwater) buried a few kilometers below the Nigerian soil. Airborne and ground magnetic surveys have been carried out by at least two government agencies, namely: the Geological Survey of Nigeria and Nigerian Steel Council. The compilation also reveals that a greater part of geophysical and drilling activities of operating oil companies is currently concentrated in the sedimentary basins, mainly the oil-rich Niger Delta and near offshore areas. From the available statistics, at least three companies, the National Steel Council, the Geological Survey of Nigeria, and Kano State Water Resources Engineering and Construction Agency have employed geophysical methods for engineering.

Geophysical anomalies over epithermal systems

The interpretation of most physical anomalies in epithermal systems needs to be integrated with available geological and geochemical data, and incorporated into a conceptual model of the original hydrology of the system. These anomalies result from the passage of hydrothermal fluids through rocks. Pervasive clay minerals associated with argillic-propylitic alteration zones cause low-resistivity anomalies comparable in size to the original geothermal system (typically 10–30 km 2). Resistivities are usually in the range 10–30 Ωm, and decrease to less than 10 Ωm in areas of advanced argillic alteration. These very low-resistivity zones may be caused by the reaction of the rock with acid, steam-heated waters. Most hydrothermal fluids, including relatively cool CO 2-bearing waters, destroy magnetite and cause demagnetisation anomalies. Airborne electrical and magnetic survey methods are therefore ideal for detecting epithermal systems in large exploration areas. Gamma-ray spectroscopy also has potential because of the concentration of potassium minerals (adularia and illite) in high-temperature alteration assemblages. In contrast to system-wide anomalies which have characteristic widths of several kilometres, epithermal ore bodies have characteristic widths of hundreds of metres. The precipitation of quartz and adularia which commonly accompanies gold-silver mineralisation causes an increase in resistivity, with values locally exceeding 1000 Ωm. Although this mineral assemblage causes an increase in apparent resistivity when near-surface, its subsurface extension may be difficult to delineate by electrical methods because of current channelling in adjacent low resistivity zones. The main I.P. anomalies are more likely to be associated with argillic alteration zones, unless quartz-adularia zones have significant sulphide concentrations. Gravity high anomalies of the order of mgals may occur as a result of mineral precipitation, especially in low-density, high-porosity host rocks. In relatively dense host rocks, local fracturing, alteration and late-stage leaching may actually cause gravity low anomalies of several mgal.

On the Spatial Pattern of Magnetic Fluctuations in the Cobar Area, NSW

Five recording magnetometers have been established in the Cobar area of NSW to examine the spatial uniform ity (or otherwise) of natural fluctuations w ith tim e of the geo­ magnetic field. The Cobar area is a particularly suitable test area for such a study because o f its history in the develop­ ment and use o f magnetic survey methods in the search for base metal ore deposits, and because magnetic fluctuations are now a lim iting factor in the production of accurate aeromagnetic survey maps. The magnetic fluctuations in the Cobar area are found to be not uniform , but appear affected by a current concentra­ tion in the vicinity of the east side of the Cobar basin. Thus changes as great as 25% in magnetic fluctuation am plitude are observed over distances o f order 30 km (and may occur over even shorter distances). Some differences are also observed in the amplitude o f the east-west component of the magnetic daily variation. The phenomena are interpreted to indicate a geologic unit of good electrical conductivity (possibly the Cobar slate member o f the Cobar Trough group) which runs along the eastern edge o f the basin, and carries a concentration o f the natural electric current flow induced in the earth.

The location of abandoned mine-shafts in rural and urban environments

Magnetic and resistivity surveys have been carried out over abandoned mine-shafts in a rural area free from background noise. The results are compared with those from a similar survey in an urban area and with anomalies predicted from computer and small-scale models. The magnetic survey method is shown to be the most successful technique although the best results were obtained where the shaft filling contains ferrous material. Seismic and gravity methods were shown to be unsatisfactory.