Magnetic Method

Basic Concept

The magnetic method was one of the first geophysical methods used for mineral exploration. Though, the scope of applications of the magnetic-survey method has increased with advances in instrumentation that improve data resolution, -quality, and -quantity. Magnetic data can be used to map large geologic structures, characterize bedrock, and aid high-resolution near-surface engineering-, geotechnical-, and environmental investigations (Thompson and others, 1980; Nabighian and others, 2005; Sharma, 2012).

Subsurface materials with high magnetic susceptibility can increase the flux of earth’s magnetic field lines and strengthen the magnetic-field intensity observed at the ground surface. The magnetic method employs a magnetometer to passively measure Earth’s magnetic field at points along the earth’s surface. Anomalies in magnetic data can indicate the presence of subsurface zones with high magnetic susceptibility and, thus, be used for site characterization (Burger, 1992; Telford, 1990).

Theory

Earth’s magnetic field, which is generated by the outer core and protects the planet from solar energy, behaves as if a giant bar magnet ran along Earth’s axis. Because it is composed of two poles (i.e., positive and negative), a bar magnet is called dipolar and forms a closed-field loop. Like poles are repelled from each other while opposite poles attract, and this behavior can generate magnetic fields that can be observed at multiple scales.

All matter consists of moving electrical charges, which create magnetic dipole moments that can influence and be influenced by other magnetic fields. The magnetic susceptibility (Χ_m) of a material is a measure of its response to an applied magnetic field and related to its atomic structure and subsequent magnetic properties. Magnetic susceptibility, which is defined as the ratio of magnetization to magnetizing-field strength, depends on the type(s) and concentration(s) of magnetic materials.

Most earth materials fall within one of three magnetic classifications: ferromagnetic, diamagnetic, or paramagnetic. Ferromagnetic materials (e.g., iron, nickel, cobalt) have permanent magnetic properties. Permanent magnetization forms during material formation wherein groups of internal iron atoms align with (i.e., are magnetized by) Earth’s magnetic field. However, this depends on the formation environment and will only occur if it is below the Curie temperature, above which, materials lose their permanent magnetic properties.

When exposed to an external magnetic field, ferromagnetic materials produce a strong internal field as they become attracted to it (e.g., fridge magnets). The fridge and magnet are ferromagnetic, but the magnet was strongly magnetized during manufacturing. In contrast, diamagnetic and paramagnetic materials lack permanent magnetic properties and respond oppositely to external magnetic fields. Diamagnetic materials (e.g., lead, water) slightly oppose the external field, whereas paramagnetic materials (e.g., magnesium, aluminum) are slightly attracted to it.

The character of alignment of iron atoms determines the object’s magnetic susceptibility and, therefore, how intensely their presence can alter Earth’s magnetic field. Of course, objects containing no iron will have a very low magnetic susceptibility affect Earth’s magnetic field minimally or negligibly (Moskowitz, 2015). The goal of magnetic surveying is to reveal the subsurface locations and variations of ferromagnets such as magnetic minerals or buried utilities.

Compared to Earth’s magnetic field, magnetic fields generated by ferromagnetic materials in the near subsurface are relatively weaker, irregular, and/or anomalous (Burger, 1992). Depending on the orientation of its source object, a magnetic anomaly may have only a single pole (i.e., monopole) or a high- and low-magnetic intensity (i.e., dipole). The polarity of the total magnetic field anomaly allows for the interpretation of the depth, location, and orientation of the anomaly-producing subsurface material (Mussett and Khan, 2000).

For example, consider a magnetic field-producing ferromagnetic material that is oriented so that its field lines are in the direction of the external (i.e., Earth’s) field lines. Such objects can produce a positive (+) anomaly in magnetic data. A data anomaly is negative (-) if the field-producing ferromagnetic is oriented opposite to the direction of the external field. Additionally, objects oriented normal (i.e., perpendicular) to the external field result in no data anomalies.

Applications

Field applications of magnetic surveys require an understanding that Earth’s magnetic field changes daily and can be disrupted by solar storms. Therefore, a base station is required to monitor diurnal magnetic-field fluctuations and can be used to correct data for drift and other required data corrections. The instrument used to meet the objectives of environmental investigations is usually a cesium-vapor magnetometer that measures magnetic-field intensity in nanoteslsa (nT) (i.e., 1 T = 1 newton/amp-meter).

The magnetic method is used for identifying and mapping anomalous magnetic field intensities. Commonly, these are caused by discrete objects (e.g. underground storage tanks), utilities, or other subsurface objects that may be a contaminant source or related to contaminant transport. Recent research has revealed that magnetic susceptibility alteration can also occur due to biomineralization from natural or active remediation. Though novel, it is suggestive of a new environmental application of the magnetic method (Mewafy and others, 2015).

Typically, magnetic surveys are conducted via field personnel walking in a grid pattern with a handheld magnetometer. Magnetic instruments can be either a cesium-vapor magnetometer, proton-precession magnetometer, or flux-gate magnetometer, each of which has a unique purpose. Grid-pattern surveys render two-dimensional (2-D) maps of the magnetic-field intensity, which can reveal the locations of subsurface ferrous objects with high magnetic susceptibilities. Generally, such objects produce high-magnitude data anomalies (positive and/or negative) as they alter the earth’s magnetic field.

Because the magnetic method only responds to alterations in the earth’s magnetic field, merging multiple methods to achieve a common interpretation is always good practice. For instance, the magnetic survey only identifies ferromagnetic objects, so a complementary electromagnetic induction survey could elucidate the location of other non-ferrous metallic objects. However, the magnetic surveying method has aided the following applications (Thompson and others, 1980):

Locating subsurface or sub-aqueous ferrous utilities or objects (e.g. shipwrecks)
Locating underground storage tanks
Mapping landfills contents
General geologic mapping aiding development of conceptual site models (CSMs)
Magnetic response associated with biomineralization from iron reduction
Locating unexploded ordinance (UXO)

Examples/Case Studies

Canbay, M.M., Aydin, A., and Kurtulis, C., 2010, Magnetic susceptibility and heavy-metal contamination in topsoils along the Izmit Gulf coastal area and IZAYTAS (Turkey), Journal of Applied Geophysics, v. 70, p. 46-57, doi:10.1016/j.jappgeo.2009.11.002.

Abstract: The study on topsoil contamination due to heavy metals was carried out by using the Magnetic susceptibility (MS) measurements in Izmit industrial city, northern Turkey. We attempted to investigate correlations between the concentration of selected heavy metals and the MS from 41 sample sites around Izmit Gulf. These investigations let us quantify and standardize the MS method, which may have consequences for long term monitoring of anthropogenic pollution, especially in urban areas. The MS surfer contour map based on the topsoil measurements was compiled with a randomly ranged distance density. The soil samples collected throughout the industrial areas, the parks, road sides and residential areas were also analyzed by Atomic Absorption Spectrometer. Heavy metals Cu, Ni, Cr and Pb show strong correlations with MS, while Zn and Co show a weak correlation with MS. Moreover, the Tomlinson pollution load index (PLI) shows insignificant correlation with the MS. The MS was examined vertically (0-30 cm) with respect to anthropogenic and/or lithogenic influences at the fourteen sample sites. The maximum values were mostly observed in depths of 2-5 cm and the MS values on the depth profiles vary between 10 × 10- 8 m3 kg- 1 and 203 × 10- 8 m3 kg- 1. The study revealed that MS is an inexpensive, fast and non-destructive method for the detection and mapping of contaminated soils.

Ramalho, E.C., Matias, M.J.S., and Moura, R.M.M., 2015, Surface geophysical methods in the assessment of environmental impacts of landfills: An overview, in Jackson, C.H., ed., Landfills and Recycling Centers: Processing Systems, Impact on the Environment and Adverse Health Effects: New York, Nova Science Publishers, Inc., p. 25-52.

Abstract: The fast development of field, data processing equipment and software has allowed the use of geophysical methods to an ever increasing range of applications. Hence nowadays it is much easier to conduct massive field surveys combining different methods, to obtain more accurate and denser data, so that complex modeling and interpretation at limited costs can be carried out. Landfills have been targeted by geophysical methods in order to investigate their environmental impacts. In fact, landfills have been the classic way to deposit domestic and industrial waste and have generated a large range of negative environmental impacts in groundwater and soils. These problems often persist even after the effective use of the landfills and subsequent recovery processes. Owing to their characteristics, landfills are difficult to access and because of the general lack of accurate information regarding the shape, nature of the refuse, history and development of the landfill, non-invasive, nondestructive methods and sometimes autonomous data acquisition devices must be used to monitor impacts and to investigate and prevent groundwater and soil contamination. Geophysical methods can be applied to investigate a wide range of aspects related with the assessment of the environmental impact of landfills. Problems such as geometry definition, geological settings, contamination plume location and monitoring investigation of internal structure and refuse zoning, determination of fluid flow direction and paths or the determination of sealing conditions and leakage may be more successfully evaluated if a carefully chosen geophysical survey is part of any investigation program. Because of the nature and complexity of the problems to investigate, only multidisciplinary approaches, involving geophysics, hydrochemical, hydrogeological and geological information, can provide meaningful results for a thorough assessment of the landfills impact on the environment. This work intends to demonstrate the application of geophysical methods in the investigation of the environmental impacts, as described above, of industrial and domestic landfills during their life time and after closure. Thus, several examples will be discussed illustrating the use of 2D, 3D and time lapse resistivity, electromagnetic, ground probing radar, self-potential, magnetic, gravity surveys and airborne thermal mapping. Most of the geophysical data will be presented and shortly discussed together with information from boreholes, geology, hydrogeology and hydrochemical data. As it will be shown, it is clear that only a judicious combination of methods and information from different nature can provide tools for the diagnosis and assessment of the impact of landfills in the environment, for the investigation of the best engineering solutions to remediate them and for the possible recovery of refuse with economic interest.

Gamey, T.J., 2008, Magnetic Response of Clustered UXO Targets, Journal of Environment and Engineering Geophysics, v. 13, no. 3, p. 211-221, doi:10.2113/JEEG13.3.211.

Abstract: The objective of many recent UXO Surveys has been described as "wide-area assessment with the purpose of obtaining better definition of a known problem area. The targets of interest are clusters of ordnance. fragments and debris which are all indicators of greater contamination. higher risk of UXO hazard and higher remediation or Construction Costs. This is a different problem from the detection and discrimination of Individual anomalies. This paper provides a definition of a “cluster” based on the amount of overlap between individual dipole Signatures. In total field surveys, Magnetic anomalies overlap significantly and show an Increased amplitude response once the individual Sources are spaced closer than 0.5 times the sensor height. When this condition is extended over a large area, Such as the center of a target Site. the result can be comparable to a horizontal sheet of dipoles. The equations to simulate it horizontal sheet are derived, and from these the relative density of targets may be calculated from the measured data by assuming it nominal target moment. Two field tests Support both the qualitative and quantitative predictions. Extending this concept to field practice, we examine some of the implications for standard operational procedures. For example, if we accept that QA/QC metrics Should represent the targets of interest, then we should require wide-area assessment surveys to create impractically large grids of surface frag. Likewise, for detection of clusters the concepts of detection probability and search radius based oil single items are irrelevant. Discrimination techniques that rely oil dipole flitting will be extremely inaccurate. Instead, QA parameters and models Suitable for horizontal sheets will have to be derived.

Mewafy, F.M., Werkema, D.D., Atekwana, E.A., Slater, L.D., Aal, G.A., Revil, A., and Ntarlagiannis, D., 2013, Evidence that bio-metallic mineral precipitation enhances the complex conductivity response at a hydrocarbon contaminated site, Journal of Applied Geophysics, v. 98, p. 113-123, doi:10.1016/j.jappgeo.2013.08.011.

Abstract: The complex conductivity signatures of a hydrocarbon contaminated site, undergoing biodegradation, near Bemidji, Minnesota were investigated. This site is characterized by a biogeochemical process where iron reduction is coupled with the oxidation of hydrocarbon contaminants. The biogeochemical transformations have resulted in precipitation of different bio-metallic iron mineral precipitates such as magnetite, ferroan calcite, and siderite. Our main objective was to elucidate the major factors controlling the complex conductivity response at the site. We acquired laboratory complex conductivity measurements along four cores retrieved from the site in the frequency range between 0.001 and 1000 Hz. Our results show the following: (1) in general higher imaginary conductivity was observed for samples from contaminated locations compared to samples from the uncontaminated location, (2) the imaginary conductivity for samples contaminated with residual and free phase hydrocarbon (smear zone) was higher compared to samples with dissolved phase hydrocarbon, (3) vadose zone samples located above locations with free phase hydrocarbon show higher imaginary conductivity magnitude compared to vadose zone samples from the dissolved phase and uncontaminated locations, (4) the real conductivity was generally elevated for samples from the contaminated locations, but not as diagnostic to the presence of contamination as the imaginary conductivity; (5) for most of the contaminated samples the imaginary conductivity data show a well-defined peak between 0.001 and 0.01 Hz, and (6) sample locations exhibiting higher imaginary conductivity are concomitant with locations having higher magnetic susceptibility. Controlled experiments indicate that variations in electrolytic conductivity and water content across the site are unlikely to fully account for the higher imaginary conductivity observed within the smear zone of contaminated locations. Instead, using magnetite as an example of the bio-metallic minerals in the contaminated location at the site, we observe a clear increase in the imaginary conductivity response with increasing magnetite content. The presence of bio-metallic mineral phases (e.g., magnetite) within the contaminated location associated with hydrocarbon biodegradation may explain the high imaginary conductivity response. Thus, we postulate that the precipitation of bio-metallic minerals at hydrocarbon contaminated sites impacts their complex conductivity signatures and should be considered in the interpretation of complex conductivity data from oil contaminated sites undergoing intrinsic bioremediation.

References

Burger, H.R., Sheehan, A.F., and Jones, C.H., 2006, Exploration Using the Magnetic Method, in Introduction to Applied Geophysics: Exploring the Shallow Subsurface: New York, W.W. Norton & Co., p. 389.

Gamey, T.J., 2008, Magnetic Response of Clustered UXO Targets, Journal of Environment and Engineering Geophysics, v. 13, no. 3, p. 211-221, doi:10.2113/JEEG13.3.211.

Milsom, J. and Eriksen, A., 2011, Field Geophysics: Fourth Edition, in The Geological Field Guide Series: New York, John Wiley & Sons Inc., 304 p.

Morsy, M. and Rashed, M., 2013, Integrated magnetic, gravity, and GPR surveys to locate the probable source of hydrocarbon contamination in Sharm El-Sheikh area, south Sinai, Egypt, Journal of Applied Geophysics, v. 88, p. 131-138, doi:10.1016/j.jappgeo.2012.11.003.

Moskowitz, B.M., Jackson, M., and Chandler, V., 2015, Geophysical Properties of the Near Surface Earth: Magnetic Properties, in Schubert, G., ed., Treatise on Geophysics, Second Edition: Elsevier, v. 11, p. 139-174.

Mussett, A.E., and Khan, M.A., 2000, Magnetic Surveying, in Looking into The Earth: An Introduction to Geological Geophysics: New York, Cambridge University Press, p. 162-180.

Nabighian, M.N., Grauch, V.J.S., Hansen, R.O., LaFehr, T.R., Li, Y., Peirce, J.W., Phillips, J.D., and Ruder, M.E., 2005, The historical development of the magnetic method in exploration, Geophysics, v. 70, no. 6, 111 p., doi:10.1190/1.2133784.

Ravat, D., 1996, Magnetic properties of unrusted steel drums from laboratory and field-magnetic measurements, Geophysics, v. 61, no. 5, p. 1325-1335, doi:10.1190/1.1444056.

Robinson, E.S. and Coruh, C., 1988, Basic Exploration Geophysics: New York, John Wiley & Sons, 562 p.

Sharma, P.V., 2012, Magnetic surveying, Environmental and Engineering Geophysics: Cambridge, Cambridge University Press, p. 65-111, doi:10.1017/CBO9781139171168.

Telford, W.M., Geldart, L.P., and Sherriff, R.E., 1990, Applied Geophysics: Second Edition: Cambridge, Cambridge University Press, 770 p.

Thompson, R.J., Stober, J.C., Turner, G., Oldfield, F., Bloemendal, J., Dearing, J.A., and Rummery, T.A., 1980, Environmental applications of magnetic measurements, Science, v. 207, no. 4430, p. 481-486, doi:10.1126/science.207.4430.481.

Magnetic Method

Basic Concept

Theory

Applications

References

Environmental Geophysics