Magnetic Susceptibility Borehole Logging

Basic Concept

Magnetic susceptibility (MS) borehole logging is a wireline geophysical method that measures the degree to which earth materials become magnetized in an applied magnetic field. MS logging techniques exploit the principles of electromagnetic induction (EMI) and operate similarly to EMI conductivity logging. Magnetic susceptibility data can be used to characterize the type of magnetization exhibited by materials and, by comparison to lab-tested values, differentiate and/or infer formation mineralogy/lithology.

Theory

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

Magnetic susceptibility logging sondes use electromagnetic induction principles to apply a known magnetic field to the formations surrounding the borehole. The magnetization induced within the materials produces a net magnetic field that alters the secondary magnetic field generated by subsurface conductors. The returning signal generated within the receiver coil has a phase shift relative to the transmitted signal and amplitude that are due to the electromagnetic properties of the formation (Mussett and Khan, 2000).

For signal analysis, the returning signal is divided into two components. The in-phase or “real” component has a maximum amplitude that occurs simultaneously with that of the transmitted signal (i.e., no phase shift). The value of the in-phase component represents the bulk Χ_mof the formation. The out-of-phase (i.e., quadrature or “imaginary”) component has a maximum amplitude that lags that of the transmitted signal by some period fraction. The magnitude of the out-of-phase component is related to electrical conductivity (see borehole electromagnetic induction logging for more details).

Applications

Most earth materials fall within one of three magnetic classifications: ferromagnetic, diamagnetic, or paramagnetic. Ferromagnetic materials (e.g., iron, nickel, cobalt) exhibit strong attractions to magnetic fields and can retain their magnetic properties after the field is removed. When in the presence of an external magnetic field, a strong, internal magnetic field is produced as many of the atomic magnetic dipole moments within the ferromagnet become parallelly aligned. Ferromagnetic materials are distinguished by a large positive Χ_m (Arora, 2011).

Diamagnetic and paramagnetic materials both lack permanent magnetic properties and respond opposite of each other under the influence of an external magnetic field. Diamagnetic materials (e.g., lead, water, carbon) oppose the applied field, whereas paramagnetic materials (e.g., magnesium, sodium, aluminum) are realigned with the field. Thus, diamagnetic materials, being slightly repelled from the field, have a weak negative Χ_m, and paramagnetic materials are slightly attracted to a magnetic field and have a small positive Χ_m (Arora, 2011).

As a ratio, the bulk susceptibility measurement (i.e., Χ_m) is dimensionless but can be converted to SI units by multiplying it by four times pi (4π). Additionally, calibrating the MS tool is important and can be done by comparing responses to known/core-sample measurements or at a calibration facility. Calibration is necessary to produce meaningful numbers that allow for the results to be used quantitatively rather than just qualitatively in applications such as:

Determination of stratigraphic changes in minerology/lithology
Distinction between Quaternary and Cretaceous (recent and weathered) sediments
Core-log integration
Ore identification and quality determination
Magnetostratigraphy
Paleoclimate/Paleoenvironment/Paleooceanographic studies
Improvement of natural gamma log interpretation
Identification of alteration zones, basic flows, and diabase rock

Examples/Case studies

Crow, H.L., Hunter, J.A., Olson, L.C., Pugin, A.J.-M, and Russell, H.A.J., 2017, Borehole geophysical log signatures and stratigraphic assessment in a glacial basin, southern Ontario: Canadian Journal of Earth Sciences, v. 55, no. 7, p. 829-845, doi:10.1139/cjes-2017-0016.

Abstract: Over the past two decades, the Geological Survey of Canada has used a standardized suite of slim-hole geophysical tools to log 57 polyvinyl chloride cased boreholes drilled in the glacial sediments of southern Ontario. This article documents downhole tool responses (natural gamma, apparent conductivity, magnetic susceptibility, and seismic velocity) in the context of mineralogical characteristics of the region and grain-size data from 28 of the 57 boreholes. Characteristic geophysical properties and (or) patterns are identified within the units of a regional hydrostratigraphic framework in southern Ontario. The importance of a calibrated suite of tools is emphasized, as stratigraphic units may have variable response from site to site. The use of a high-sensitivity magnetic susceptibility induction probe is shown to be an important tool in the log suite for lithostratigraphic interpretation, and more broadly, for provenance studies of source rock across the region. Ranges of compressional (P) and shear (S) wave velocities and their ratios are provided for each of the hydrostratigraphic units. Case studies are presented to demonstrate how logs may assist in the interpretation of glacial processes at lithological boundaries.

Grabowski, J., Schnyder, J., Sobień, K., Koptíkovác, L., Krzemiński, L., Pszczółkowski, A., Hejnar, J., and Schnabl, P., Magnetic susceptibility and spectral gamma logs in the Tithonian–Berriasian pelagic carbonates in the Tatra Mts (Western Carpathians, Poland): Palaeoenvironmental changes at the Jurassic/Cretaceous boundary: Cretaceous Research, v. 43, p. 1-17, doi:10.1016/j.cretres.2013.02.008.

Abstract: Upper Tithonian–Berriasian pelagic carbonates in the Central Western Carpathians, Tatra Mts (southern Poland), with well-established bio- and magnetostratigraphy, provide excellent possibilities of testing magnetic and geochemical methods as proxies of palaeoenvironmental changes in the Western Tethys at the Jurassic/Cretaceous boundary. Magnetic susceptibility (MS), field spectral gamma-ray (GRS) as well as CaCO3, total organic carbon (TOC), and elemental analyzes were performed in the Pośrednie III section. MS reveals very good negative correlation with CaCO3 content as well as positive correlation with Al, Zr, Ti and other lithogenic elements and therefore it might be interpreted as a proxy of a detrital input into the basin. Abrupt MS variations correlate well with relative sea-level changes and indicate regressive intervals (MS highs) in the upper Tithonian/lowermost Berriasian (M20r to M19n2n) and upper Berriasian (M16n) and transgressive interval (MS low) in the lower to middle Berriasian (M18r to M17r). Long-term MS variations might be linked to a palaeoclimatic-controlled enhanced continental runoff. Geochemical data (P, Th/U, Mn, Cd, Ni, Mo and TOC content) point to a productivity increase and a slight oxygen deficiency in the lower and middle Berriasian, which corresponds to MS low values and typical calpionellid limestone sedimentation. Timing of major palaeoenvironmental turnovers might be correlated also with general palaeoclimatic trends in the Western Tethys and Western Europe: cooling in the late Tithonian followed by a temperature increase throughout the Berriasian and an important humidity increase in the middle Berriasian (M17n).

Huret, E., Thiesson, J., Tabbagh, A., Galbrun, B., and Collin, P., 2011, Improvement of cyclostratigraphic studies by processing of high-resolution magnetic susceptibility logging: Example of PEP1002 borehole (Bure, Meuse, France): Geoscience Reports, v. 343, no. 6, p. 379-386, doi:10.1016/j.crte.2011.04.002.

Abstract: Logging opens wide paths for cyclostratigraphic studies of sedimentary successions. In clay-dominated rocks, magnetic susceptibility (MS) is very informative and the quality of the results can be further enhanced by deconvolution , where the deformation of the original susceptibility versus depth variation by the instrument is filtered out by its impulse response (thin layer response). This is illustrated by processing the data acquired in the PEP1002 borehole (Bure, Meuse, France) using two coils 0.25 m apart where high-frequency, 0.5 to 0.8 m, period precession cycles (21 kyr) can be identified, as well as the 2.5 to 4 m period eccentricity (95 kyr) ones.

McNeill, J.D., Hunter, J.A., and Bosnar, M., 1996, Application of a Borehole Induction Magnetic Susceptibility Logger to Shallow Lithological Mapping: Journal of Environmental and Engineering Geophysics, v. 1, no. B, p. 77-143, doi:10.4133/JEEG1.B.77.

Abstract: Although magnetic susceptibility logging is used to a certain extent in mineral exploration it does not appear to have been employed extensively for other applications. As a natural outcome of the development of a shallow borehole conductivity logging system (Geonics EM39), a magnetic susceptibility logger, the EM39S, was constructed, specifically for use for shallow lithological studies. This paper offers a brief review of the factors affecting the magnetic susceptibility of soils and rocks, and the desirable features of a shallow susceptibility logger. These include high sensitivity and low‐temperature drift, so as to accurately map low susceptibility materials, and an intercoil spacing small enough to give good vertical resolution but large enough to achieve reasonable radial range of investigation and to be unaffected by the presence of the borehole itself. In highly conductive environments, the response from conductive material can also influence the susceptibility reading; hence, it is necessary to additionally log the hole with a conductivity sonde and to then apply a simple correction to the susceptibility data. Case histories are presented which illustrate the use of the susceptibility probe for delineating the lithology in the following geological sequences: Paleozoic/Precambrian, Holocene/Pleistocene, and Quaternary/Cretaceous. In all cases the desired interfaces were successfully identified by the susceptibility logs. Furthermore, laboratory magnetic susceptibility measurements on selected samples from two boreholes showed excellent agreement with the borehole susceptibility data. On the basis of the results presented here, it is suggested that the magnetic susceptibility probe should be added to the standard suite of geophysical tools as an aid to lithological logging in the near‐surface environment.

Urrutia-Fucugauchi, J., Pérez-Cruz, L., Campos-Arriola, S.E., Escobar-Sánchez, E., and Velasco-Villarreal, M., 2014, Magnetic susceptibility logging of Chicxulub proximal impact breccias in the Santa Elena borehole: implications for emplacement mode: Studia Geophysica et Geodaetica, v. 58, p. 100-120, doi:10.1007/s11200-013-0803-0.

Abstract: Magnetic susceptibility logging is used to study the impact breccias in the Chicxulub crater. The basic premise is that the high contrasts in magnetic properties can be used to characterize the breccias. The Santa Elena borehole was drilled 110 km radial distance from crater center and sampled a 172 m thick sequence of impact breccias, between 332 and 504 m depth. Breccia units are distinguished from differences in composition, size, and relative contents of clasts, type of matrix and textural and lithological assemblages, which can be resolved in the susceptibility logs. The whole-core log shows characteristic variation patterns with high, intermediate and low susceptibilities. High resolution logging of matrix and clasts records the heterogeneous nature of impactites, with higher variability at smaller spatial scales. Measurements confirm that diamagnetic susceptibilities characterize the carbonate clasts, high susceptibilities the basement granitic clasts and intermediate values the silicate melt-rich and silicate-poor matrix. Intermediate variable susceptibilities characterize breccias rich in melt particles. Correlation of matrix and clast logs with whole-core log shows that signal is controlled by the matrix. Logs for clast shows a discrete distribution with peaks of intermediate to high values, which correlate with large clast distributions. The ejecta blanket includes the fallback suevites rich in silicate melt particles and shocked minerals, the high temperature vapor deposits from ejecta curtain collapse and high velocity basal flows, and the carbonate rich deposits from lateral basal flows and secondary cratering. Late fallback suevites record minor turbulent conditions resulting from progressive cooling of the ejecta plume.

Zhao, Y., Wu, J., Zhang, P., and Xiao, P., 2012, Approximate relationship of coal bed methane and magnetic characteristics of rock via magnetic susceptibility logging: Journal of Geophysics and Engineering, v. 9, no. 1, p. 98-104, doi:10.1088/1742-2132/9/1/012.

Abstract: In coal bed methane (CBM) exploration, how to improve the accuracy for locating and evaluating the CBM deposits is still a problem due to the rarity of occurrence of CBM. Combined with the distribution of the CBM content in the Huainan coalfield, the approximate relationship between the occurrence of CBM and the magnetic properties of the coal bed and adjacent mudstone have been widely discussed by magnetic logging. Experimental results show that magnetic susceptibility of the coal bed and adjacent mudstone would clearly increase with the CBM content in a coal bed. According to the results of the experiment, the prediction of the CBM content has been accomplished for different coal beds, and the results are consistent with the distribution of the CBM content throughout the whole coalfield. Preliminary data analysis reveals that there is indeed a correlation between the changes of magnetic rock characteristics and the occurrence of the CBM, and this finding may shed some light on the evaluation of CBM.

References

Arora, K., 2011, Magnetic Methods, Principles, in Gupta, H.K., ed., Encyclopedia of Solid Earth Geophysics: Dordrecht, Netherlands, Springer, p. 767-770.

Mussett, A.E. and Khan, M.A., 2000, Paleomagnetism and Mineral Magnetism, in Looking into The Earth: An Introduction to Geological Geophysics: New York, Cambridge University Press, p 139-161.

Urrutia-Fucugauchi, J., Pérez-Cruz, L., Campos-Arriola, S.E., Escobar-Sánchez, E., and Velasco Villarreal, M., 2014, Magnetic susceptibility logging of Chicxulub proximal impact breccias in the Santa Elena borehole: implications for emplacement mode: Studia Geophysica et Geodaetica, v. 58, p. 100-120, doi:10.1007/s11200-013-0803-0.