Magnetic Susceptibility of Soils from Eastern Botswana: A Reconnaissance Survey and Potential Applications

Abstract

Soil magnetic properties measurements are relatively fast and inexpensive but have been proved to be sufficient for preliminary investigations in diverse socio-developmental issues. This paper presents results of a reconnaissance study of soil colour and magnetic susceptibility (χ) in eastern Botswana, where ~80% of the population resides. The work is a first step to creating a database of rock and soil magnetic properties and to document spatial variations in magnetic properties in the country. These measurements are important as constraints for interpretation of available aeromagnetic data and can also be exploited for environmental soil research (pollution) and land-use planning (agriculture). The soils sampled include derivatives of varying types and provenance such as Archean gneissic granitoids, metamorphosed rocks (granulites), volcano-sedimentary assemblages, Karoo basalts, and alluvial sediments. A soil colour chart was used since soil colours and magnetic properties are diagnostic of its parent rock sources and weathering profiles. Soil magnetic susceptibilities were measured at both low frequency (0.46 MHz, χlf) and high frequency (4.6 MHz, χhf), thus allowing calculation of frequency-dependent susceptibility (χfd, χfd%) for detecting ultra-fine ferromagnetic minerals.It was found that soils with Hues ranging from 7.5YR to 10YR have appreciable amount of magnetic materials and soils with Hues of 2.5YR are generally nonmagnetic. The results of soil magnetic susceptibility profiles show spatial variation closely related to the variation in basement rocks, which provides excellent evidence that the magnetic susceptibility variation reflects basement rocks or bedrock composition (soil parent material). In relation to the Botswana physiographic units, soils from the hardveld (Precambrian) show the highest susceptibilities, followed by those from the sandveld, with the lowest values being from the alluvial. The frequency dependent magnetic susceptibilities indicate the presence of ultra-fine super-paramagnetic minerals such as magnetite/maghemite. It is suggested that a systematic and continuous programme of rock and soil magnetic measurements would benefit various socio-economic and development priority sectors of Botswana. This also applies to many developing countries in Africa where soil physics and measurement of soil susceptibility in particular, is generally still at an embryonic stage.

Modelling and Inversion of Ground Magnetic and Electromagnetic Data for Delineation of Subsurface Structures at Siloam Hot Spring in the Soutpansberg Basin

“…The diffrent values of magntic susceptibility values for volcanic rocks, obtained using three approaches were 94.7 × 10 −3 , 94.7 × 10 −3 and 100 × 10 −3 , respectively indicating the spontaneous nature of levels of magnetisation and different inversion assumptions and approaches. The magnetic susceptibility values for metamorphic and volcanic rock signatures agree with the geology of the Soutpansberg Basin and Limpopo mobile belt (Barker, Brandl, Callaghan, Eriksson, & van Der Neut, 2006;Ranganai, Moidaki, & King, 2015).…”

“…Magnetic susceptibility in the field can only be measured on outcrops or rock samples (Telford, Geldart, & Sheriff, 1990). In the study of variation of magnetic properties of soil in Botswana including an area overlying the Limpopo Mobile Belt, the magnetic susceptibility values were found to be an indication of the bedrock and identified paramagnetic or antiferromagnetic minerals without magnetite, ferromagnetic minerals with magnetite, ferrimagnetic minerals with iron oxide minerals, as well as diagenetic minerals with negative or very low magnetic susceptibilities (Ranganai, Moidaki, & King, 2015). Magnetic susceptibility is dimensionless in the SI unit system (Cano, Cordova-Fraga, Sosa, Bernal-Alvarado, & Baffa, 2008;Collinson, 2013;Lecoanet, Lévêque, & Segura, P. K. Nyabeze, O.…”

“…SI or cgs unit system, the unit system is specified. A multiplication factor of 1/4π was applied to convert bulk magnetic susceptibility values from SI to cgs units (Ranganai, Moidaki, & King, 2015;Cano, Cordova-Fraga, Sosa, Bernal-Alvarado, & Baffa, 2008). In this study it is the SI unit system that is used.…”

Analisis XRD dan Suseptibilitas Magnetik untuk Identifikasi Karakteristik Endapan Lumpur pada Wilayah Timur Laut Gunung Lumpur X

“…Dari kedua nilai tersebut di hitung perubahan nilainya sehingga diperoleh nilai frequency dependent susceptibility χFD(%) menggunakan persamaan χFD(%)=[(χLF-χHF)/χHF] x 100%. Klasifikasi nilai χFD menurut Dearing et al (1999) (Ranganai et al, 2015). Sampel yang mengandung mineral ultrahalus akan menunjukkan nilai yang sedikit lebih rendah bila diukur pada frekuensi tinggi (Dearing et al, 1999).…”