Proceedings Template - WORD€¦ · Web viewI. INTRODUCTION. For many ground-penetrating radar applications, the magnetic properties of the sub-surface are often considered unimportant

12th International Conference on Ground Penetrating Radar, June 16-19, 2008, Birmingham, UK

GPR Characteristics of Magnetite-Rich Layered Igneous Bod-ies

Nigel J. Cassidy and Rebecca A. Goodwin

Research Institute for the Environment, Physical Sciences and Applied Mathematics,School of Physical & Geographical Sciences, Keele University, Keele, Staffs, ST5 5BG, UK.

email [email protected]

In submitting this paper for EuroGPR2008 I hereby assign the copyright in it to the University of Birmingham and confirm that I have had the permission of any third party for the inclusion of their copyright material in the paper. The University of Birmingham will license EuroGPR to use this paper for non-commercial purposes. This will be the sole use of this ma-terial.

Abstract - For many geological materials, the magnetic prop-erties of the constituent parts are often considered unimpor-tant when compared to their ‘dielectric’ characteristics (i.e., the permittivity and conductivity). However, if significant amounts of magnetic minerals such as magnetite exist, then the EM relaxation phenomena of these minerals can have a noticeable effect on the GPR wave’s attenuation and propa-gation velocity. Both surface and borehole based GPRs have been successfully used to delineate and map layered igneous ore bodies but subtle, natural, variations in magnetite com-position, grain size and mineral fabric can all have an effect on the nature of the GPR responses. In this paper, we present a measurement and analysis study on the apparent permittivity, attenuation and propagation velocity character-istics of a range of natural occurring, magnetite-rich igneous samples collected from the Upper ore zone of the The Bushveld complex, South Africa; the world's largest mafic-ultramafic layered intrusion. The samples are investigated with a Vector Network Analyser across the common GPR fre-quency range of 40MHz – 1.5GHz and the resultant appar-ent complex permittivities converted into conventional GPR attenuation and velocity characteristics with the aim of de-termining how the magnetic minerals might affect the per-formance of both the surface and borehole GPR systems. The results are also compared to the form of popular dielec-tric-based mixing models (such as the Complex Refractive Index Model – CRIM) with the aim of assessing their practi -cal application for these materials.

Keywords – Magnetite, Attenuation, Velocity, Permittivity Bushveld.

I. INTRODUCTION

For many ground-penetrating radar applications, the mag-netic properties of the sub-surface are often considered unimportant in terms of GPR signal attenuation and propa-gation velocity. Commonly, the magnetic characteristics are equated to that of free space, which as an assumption is generally acceptable if the materials are free of mag-netic minerals such as magnetite, hematite or maghemite. However, many natural mafic igneous bodies do contain significant amounts of magnetite (e.g., ferrogabbros and

magnetite ores), particularly where they form large lay-ered intrusions. In these cases, the magnetite grains crys-tallize out of the original melt fraction and accumulate as magnetite-rich bands in distinct ‘Ore’ layers. Were these occur, significant quantities of economic minerals can be present in the minor constituent phases (e.g., Gold, Plat-inum, Vanadium, etc) and, as such represent a commer-cially viable mining target. Borehole-based GPR investi-gation methods have been used to successfully map such bodies [1], [2] [3] but there has been little published infor-mation on the effect of natural, crystalline, magnetite-rich materials on the GPR signal attenuation/propagation char-acteristics. Recent studies on fine-scaled magnetite-rich mixtures (both natural and simulated) have shown that the magnetically lossy materials can have a significant influ-ence on the measured complex apparent permittivity, ve-locity and attenuation with increasing permittivities (both real and imaginary components) being observed with in-creasing magnetite content [3], [4], [5]. However, there is still some doubt to the extent of this effect in natural, crystalline magnetite ore rocks, particularly where changes in grain sizes, composition, or layered fabric oc-cur within the rocks. In order to provide a degree of quan-titative control on these factors, we present the prelimi-nary findings of a laboratory-based investigation into the apparent permittivity, velocity and attenuation properties of natural magnetite-rich igneous samples collected from the Upper zone of the Bushveld Igneous Complex, South Africa.

II. THE BUSVELD IGNEOUS COMPLEX

The Bushveld Igneous Complex (BIC) is a very large, 9km thick, igneous layered intrusion with a diameter of greater than 350km. Covering an overall area of 65,000km2, the complex is located near Pretoria in South Africa (figure 1) and consists of a ‘bowl’ shaped body of laterally continuous, gently dipping, layered mafic and ul-tra-mafic igneous rocks. It is dated at ~2.06Ga old and

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contains some of the richest platinum-group ore deposits on the planet.

Figure 1. Location of the Bushveld Igneous Complex, South Africa.

The main ore-bearing rocks occur in four zones; the Up-per, Main, Critical and Lower zones with magnetite-rich layers consisting of at least 90% Magnetite, or Titanifer-ous Magnetite [7], [8]. Chromite, Apatite and Ilmenite are common accessory minerals and the layers range in thick-ness from ~0.1m through to several metres in places. In this preliminary study, we are concentrating on the rocks of the Upper Zone (figure 2) where layers of massive Ti-taniferous Magnetite (~10-20% Ti) dominate the upper part of the sequence and contain a significant apatite com-ponent (referred to as Nelsonite). Towards the base, the ore seam mineralogy changes with a reduction in the Ti composition (to less than 10%) and a loss of apatite miner-als [9]. Between the individual ore layers, coarse-grained units of leucogabbronorite, gabbronorite and gabbro occur in the lower sections with a change to norites and olivine diorites with decreasing depth [7]. These units are mag-netite-bearing but have mineral percentages that are much lower than the main ore seams (typically 8-60% Magneti-tie). Pyroxene and feldspars are the dominant minerals of the Norite and Gabbro units and the contact between them and the ore seams varies from being very sharp to grada-tional over the scale of a metre or so. A number of representative samples were taken from Keele University’s Bushveld collection (as core or hand specimens) and cut/polished to provide an appropriate flat, smooth face for the apparent permittivity measurements (figure 3). Where possible, orthogonal faces were cut into the samples in order to measure the permittivity anisot-ropy. Where this was not possible, faces were cut parallel to the grain fabric. A description of all the measured sam-ples is provided in table 1.

Figure 2. Schematic cross-section trough the pre-tectonic Bushveld Igneous Complex and the stratigraphic location of the four selected samples from the Upper Zone (figure adapted from [8] and [9]).


Table 1. Location and petrological description of the measured samples from the Bushveld Upper Zone. Specific examples pro-vided in this paper are shown in grey. Mineral percentages and grain sizes have been obtained trough careful petrographic analysis and subject to and error of +/- 10%. Note that it has not been possible to accurately determine the percentage of ap -atite or ilmenite in the Titaniferous Magnetite samples and, as such, the magnetite percentage composition contains these com -ponents.

Sam-ple No. Description Region Depth Core/hand

specimenPercentage composition Approx

Grain SizeMagnetite Chromite Feld/ProxBS 51 Magnetite Gabbro Ore UZa ~~ Core 63 13 24 4-8mm

BS 51a Magnetite Gabbro UZa ~~ Core 23 9 68 2-5mmBS 51b Magnetite Gabbro Ore UZa ~~ Core 70 8 22 4-8mmBS 51c Magnetite Gabbro Ore UZa ~~ Core 68 7 25 4-8mm

BS 40 Titaniferous Magnetite Main Seam UZc 722m Hand

Specimen 96 4 ~ 3-10mm

BS 50 Titaniferous Magnetite 14th Seam UZc 741m Hand

Specimen 93 7 ~ 3-10mm

BS 49 Norite Below 14th Seam UZc 772m Hand

Specimen 18 ~ 82 <1mm

BS 48 Norite Above 9th Seam Uzb 1381m Hand Specimen 8 ~ 92 0.5mm-

2mm

BS 46 Norite Below 8th Seam UZb 1473m Hand Specimen 7 ~ 93 1-2mm

BS 41 Magnetite Gabbro UZa 1777m Hand Specimen 22 ~ 78 1-3mm

Figure 3. Images of the selected Titaniferous Magnetite Ore, Norite Magnetite Ore and Magnetite Gabbro samples.


III. APPARENT PERMITTIVITY MEASUREMENTS

The apparent permittivity of the Busveld samples were evaluated in the laboratory using a dielectric analysis tech-nique incorporating an Agilent Technologies 8753ES Au-tomated Vector Network Analyser (or ANA), calibrated reflection test probe set and an automated measurement/inversion technique that converts the measured S-parame-ters into a complex apparent permittivity spectrum. The method is well established and has been used to measure a wide range of sub-surface materials including soils and rocks [10], [11]. A detailed account of this measurement technique can be found in a number of general texts [12], [13] whilst appropriate industry-standard testing method-ologies and procedures are covered by [14], [15]. The measurement process includes a calibration routine that reduces the experimental and instrument errors (which are predominantly systematic) to a repeatable minimum. In general, the process is similar to standard, commercial-based dielectric testing techniques but includes an addi-tional repeat measurement and averaging phase in order to reduce random measurement errors to less than 5-20% (across all frequencies) for any specific measurement [16]. The measurements produce a frequency-dependent, com-plex apparent permittivity that includes all the loss effects associated with the analysed material, regardless of the physical attenuation mechanisms (e.g., dipolar permittiv-ity losses and/or magnetic moment spin and domain wall interaction losses). The results can then be used directly with the conventional GPR attenuation and velocity equa-tions (in the form of 1 & 2 below) allowing the electro-magnetic behavior of the materials to be described in a generic form without having to attribute the loss and prop-agation effects to any specific ‘electric’ or ‘magnetic’ phe-nomenon [4].

(1)

(2)

Where, is the frequency-dependent attenuation co-efficient

in dBm-1.v is the frequency-dependent phase velocity in ms-1. is the free-space magnetic permeability (in Hm -1). ′ is the real component of the frequency-dependent

apparent permittivity (in Fm-1).″ is the imaginary component of the frequency-de-

pendent apparent permittivity (in Fm-1).

o is the permittivity of free space (in Fm-1), real val-ued and constant.

w is the angular frequency (in Radians/s) given by w = 2pf where f is the GPR signal frequency in Hz.

c is the velocity of EM waves in free space.tan is the loss tangent (or loss factor) of the material

and is determined from ″()/′().

In order to obtain the most appropriate value of apparent permittivity for each sample, multiple measurements were made at different positions on the polished face. The full set of results were then averaged to produce a mean value of apparent permittivity for each sample with error limits set to one standard deviation from the mean (in common with [17]). Figures 4, 5 and 6 illustrate the complex, ap-parent relative permittivity measurement, including errors, plus the signal attenuation and velocity characteristics for four representative examples from the full sample set: BS 51a (Magnetite Gabbro ~ 23% Magnetite), BS 51c (Mag-netite Gabbro Ore ~ 68% Magnetite), BS 40 (Titaniferous Magnetite Main Seam ~ 96% Magnetite) and BS40 (Norite ~7% Magnetite).

Figure 4. Complex apparent permittivity of samples BS 51a (Magnetite Gabbro) and BS 40 (Titaniferous Magnetite

Ore).


Figure 5. Complex apparent permittivity of samples BS 46 (Norite) and BS 51c (Magnetite Gabbro Ore).

IV. APPARENT PERMITTIVITY, ATTENUATION AND VELOCITY RESULTS.

The measured apparent permittivities clearly illustrate the differences in EM characteristics of the selected samples across the common GPR frequency range (<1500MHz). As expected, the low-percentage magnetite Norite (BS 46) and Magnetite Gabbro samples have almost constant, low values of permittivity for the real component (~7.7 & 9) whilst the magnetite-rich samples (BS 51c, Magnetite Ore and BS 40 Titaniferous Magnetite Ore) have relatively high permittivity values of about ~18-20. All samples tend to show slightly increasing trends for the imaginary com-ponent of the permittivity but with comparable values (i.e., all values are less than 2). This is consistent with low-loss materials that do not exhibit any significant mag-netic relaxations in the measured GPR frequency range. In fact, the Norite permittivity results are similar in form to dry ‘non-magnetic’ igneous rocks albeit with a slightly elevated real component. This suggests that the relatively low percentage of magnetite in these samples is having lit-tle effect on the bulk EM properties.

Figure 6. Attenuation and velocity characteristics over the common GPR frequency range of 40MHz – 1500MHz for samples BS 46 (Norite), BS 51a (Magnetite Gabbro), BS 51c (Magnetite Gabbro Ore) and BS 40 (Titaniferous Magnetite Ore). Note : error bars have been omitted for visual clarity.

In comparison, the magnetite-rich samples show a signifi-cant increase in the real component of the apparent per-mittivity (which, in turn, will affect the velocity) but no appreciable relaxation response. This is in marked con-trast to a similar study on nano-to-micro scale magnetite mixtures [4] where high permittivities were observed at lower frequencies (r′ > 30 and r″ > 10 below 40MHz) with strong permittivity relaxation responses. Interest-ingly, the Titaniferous Magnetite Ore sample (BS 40), which contains the highest percentage of magnetite (96%), shows a slightly lower value of permittivity than the Mag-netite Gabbro Ore sample BS 51c (68% magnetite). This illustrates the likely effect on the permittivity of the in-creased Ti and ‘non-magnetic’ apatite compositions in the mineral assemblage. In addition, the measurements on the orthogonal cut faces of the individual hand specimens pro-duced apparent permittivity results that were consistently similar across the faces and within the range of measure-ment error. This suggests that, despite the presence of an


observable fabric within the rock samples, the degree of apparent permittivity anisotropy is low at GPR frequen-cies. This is likely to be because the very-low field strengths associated with the measurement system (and also GPRs) are incapable of inducing anisotropic effects in the magnetic domains in the crystalline minerals. This phenomenon has been observed in other natural samples [4] is the subject of further work by the authors. From a practical GPR signal attenuation and velocity per-spective, the differences in the two magnetite-rich Ore samples are minor and unlikely to present any change in the GPR signal properties that could be identified in the recorded GPR data, even with advanced processing. This is despite the significant difference in ore mineral percent-ages. The Norite and Magnetite Gabbro rocks have ap-preciably different velocity values to the Ore samples, re-sulting in a high GPR signal reflectivity at any sharp inter-face boundary (see example in figure 3). With a reflection coefficient value, R, of about 0.2, this level of reflectivity is consistent with ‘high-contrast’ features such dry/satu-rated soils (R ~ 0.3) and rock/soil (R ~ 0.15) and would produce a response that would be easily identified in any GPR sections. More importantly, it would mean that sig-nificant signal scattering would occur if the form of inter-face varied at sub-wavelength scales, either as ‘topo-graphic’ undulations or disaggregated, mixed-flow units.

4.1 Attenuation and Velocity Data from All SamplesThe velocity characteristics from all the Norite and Gab-bro samples appear to follow a simple trend with decreas-ing velocity relating to increasing magnetite content (fig-ure 7). Although shown at a frequency of 1000MHz, the results are almost identical across the GPR frequency range indicating that the velocity response is frequency-in-dependent and, therefore, non-dispersive. With the Titani-ferous Magnetite Ore samples (>90% Magnetite), the ve-locity increases away from the linear trend with values that are approximately 30% faster. Again, this is most likely due to the effect of the ‘non-magnetic’ apatite com-ponent in the samples. The equivalent attenuation characteristics are particularly interesting as it would seem that at low frequencies (<100MHz), all the rocks are effectively low-loss with material attenuations of less than ~2dB/m (figure 7). The attenuation only becomes important at higher frequencies (10-25dB/m at 1000MHz) with a slight increase in attenu-ation associated with increasing magnetite content. How-ever, this level of attenuation can still be considered as low, even at these frequencies, and the rocks can be con-sidered as ‘transparent’ to GPR waves regardless of their composition.The presence of a simple, yet weak, trend in the velocity data (albeit mostly in the Norite and Gabbro samples) sug-gests that dielectric-based mixing models, such as CRIM (Complex Refractive Index Model) or its linear equiva-

lent, could be used on the real part of the apparent permit-tivity to help in the quantitative analysis of the GPR data. Being able to extract compositional percentages of mag-netite from cross-borehole or reflection velocity profiles would be a considerable improvement on current evalua-tion capabilities. Similar approaches have been successful with nano-to-micro scale magnetite mixtures [4] but, un-fortunately, the errors and uncertainty in the composi-tional nature of the samples in this study make it difficult to assess the true worth of such an approach. However, with higher quality data and more accurate petrological/minerological descriptions, dielectric-based mixing mod-els could have applicability to these types of rocks in the future.

Figure 7. Variation in GPR signal attenuation and velocity with increasing magnetite content for all samples at

1000MHz (velocity and attenuation) and 100MHz (attenua-tion).

IV. CONCLUSIONIn this paper, the complex apparent permittivity of natural, crystalline, magnetite-rich igneous rocks has been mea-sured and analysed with respect to the relative degree of GPR signal attenuation and propagation. The results indi-cate that small amounts of magnetite (<10%) have a minor effect on the bulk EM properties of the rocks and that, in


general, the materials can be considered as low-loss from a GPR perspective. At higher magnetite percentages (>60%), a significant reduction in the velocity occurs that can lead to strong reflections at ore body boundaries. However, it is not possible to uniquely identify different ore types (e.g., Magnetite seams or magnetite rich Gabbro ores) from their velocity and attenuation signatures. There is the possibility that traditional, dielectric-based mixing models could be used as a quantitative analysis tools but, unfortunately, the uncertainty in the compositional nature of the samples makes it difficult to determine how suc-cessful this will be. This is the subject of further work by the authors with the aim of providing a method for ex-tracting practical, material property information from the GPR data.

ACKNOWLEDGMENTS

The authors would like to thank Peter Greatbatch and David Wilde for preparing the polished samples. The re-search has been funded, in part, by NERC grant NER/2002/00100 and EPSRC grant EP/004032/1 (IMAGINE project - Inversion, Modelling and Analysis of GPR in Near-Surface Environments).

REFERENCES

[1] M. van Schoor, P. du Pisani and D. Vogt. High-reso-lution, short-range, in-mine geophysical techniques for the delineation of South African orebodies, South African Journal of Science, 102, 355-360 (2006).

[2] P. Du Pisani and D.Vogt. Borehole radar delineation of the Ventersdorp Contact Reef in three dimensions, Exploration Geophysics, 35, 319–324 (2004).

[3] C. M. Simmat, P. Le R. Herselman, M. Rutschlin, I. M. Mason and J. H. Cloete. Remotely sensing the thickness of the Bushveld Complex UG2 platinum reef using borehole radar, Journal of Geophyics and Engineering, 3, 43–49 (2006).

[4] N. J., Cassidy. GPR Frequency-Dependent Attenua-tion and Velocity Characteristics of Nano-to-Micro Scale, Lossy, Magnetite-Rich Materials, Near Surface Geophysics, in press (2008).

[5] E. Mattei, A. De Santis, E. Pettinelli and G. Vanna-roni. Effective frequency and attenuation measure-ments of glass beads/magnetite mixtures by time-do-main reflectometry, Near Surface Geophysics, vol. 5, 77-82 (2007).

[6] A. M. Thomas, M. P. N. Burrow, C. D. F. Rogers, D. N Chapman, N. Metje, G. Dunn and L. Nelder. Elec-tromagnetic Characterisation of a Victorian Railway Embankment Fill Material, Proceedings of the 3rd In-

ternational Conference on Site Characterization 2008, Taipei, Taiwan, 1-6 (2008).

[7] D. M. W. Harney and G. von Gruenewaldt. Ore-form-ing processes in the upper part of the Bushveld com-plex, South Africa, Journal of African Earth Sciences, Vol. 20, No. 2, 77-89 (1995).

[8] F. J. Kruger. Filling the Bushveld Complex magma chamber: lateral expansion, roof and floor interaction, magmatic unconformities, and the formation of giant chromitite, PGE and Ti-V-magnetitite deposits, Min-eralium Deposita, 40, 451–472 (2005).

[9] C. Tegner, R. G. Cawthorn and F. J. Kruger, Cyclicity in the Main and Upper Zones of the Bushveld Com-plex, South Africa: Crystallization from a Zoned Magma Sheet, Journal of Petrology, 47, 11, 2257–2279 (2006).

[10] C. Chassagne, D. Bedeaux and G. J. M. Koper. The Interpretation of Dielectric Spectroscopy Measure-ments on Silica and Hematite Sols, Journal of Colloid and Interface Science, 255, 129–137 (2002).

[11] J. Q. Shang, R. K. Rowe, J. A. Umana and J. W. Scholte. A Complex Permittivity Measurement Sys-tem for Undisturbed/Compacted Soils, Geotechnical Testing Journal, 22, 2, 168-178 (1999).

[12] G. H. Bryant. Principles of Microwave Measurement, IEE Electrical Measurement Series 5, Peter Peregri-nus Ltd. London, U.K. (1988).

[13] J. Baker-Jarvis. Transmission/Reflection and Short-Circuit Line Permittivity Measurements, United States Department of Commerce, National Institute of Stan-dards and Technology, NIST Technical Note 1341, Bolder Colorado, USA (1990).

[14] J. Baker-Jarvis, M. D. Janezic and R. B. Stafford. Shielded Open-Circuited Sample Holders for Dielec-tric and Magnetic Measurements of Liquids and Pow-ders, United States Department of Commerce, Na-tional Institute of Standards and Technology, NISTIR 5001 Report, Bolder Colorado, USA (1993).

[15] I. J. Youngs. NAMAS-accredited microwave permit-tivity/permeability measurement system, IEE Pro-ceedings A: Science Measurement and Technology, 143, 4, pp. 241-253 (1996).

[16] N. J. Cassidy. The Application of Mathematical Mod-elling in the Interpretation of Ground Penetrating Radar Data. Ph.D. Thesis, Keele University (2001).

[17] D. E. Stillman and G. R. Olhoeft, Electromagnetic Properties of Martian Analog Minerals at Radar Fre-quencies and Martian Temperatures. Proceedings of the 37th Lunar and Planetary Science Conference, 2002.pdf, 1-2 (2006).

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Proceedings Template - WORD€¦ · Web viewI. INTRODUCTION. For many ground-penetrating radar applications, the magnetic properties of the sub-surface are often considered unimportant